Diatom evidence of climatic change in Holsteinsborg Dyb,west of Greenland,during the last 1200 years

Abstract

Diatom assemblages from Holsteinsborg Dyb on the West Greenland shelf were analysed with high temporal resolution for the last 1200 years. A high degree of consistency between changes in frequency of selected diatom species and instrumental data from the same area during the last 70 years confirms the reliability of diatoms (particularly sea-ice species and warm-water species) for the study of palaeoceanographic changes in this area. A general cooling trend with some fluctuations is marked by an increase in sea-ice species throughout the last 1200 years. A relatively warm period with increased influence of Atlantic water masses of the Irminger Current (IC) is found at ad 750–1330, although with some oceanographic variability after ad 1000. A pronounced oceanographic shift occurred at ad 1330, corresponding in time to the transition from the so-called ‘Medieval Warm Period’ (MWP) to the ‘Little Ice Age’ (LIA). The LIA cold episode is characterized by three intervals with particularly cold sea-surface conditions at ad 1330–1350, ad 1400–1575 and ad 1660–1710 as a result of variable influence of Polar waters in the area. During the last 70 years, two relatively warm periods and one cold period (the early 1960s to mid-1990s) are indicated by changes in the diatom components. Our study demonstrates that sedimentary records on the West Greenland shelf provide valuable palaeoenvironment data that confirm a linkage between local and large-scale North Atlantic oceanographic and atmospheric oscillations.

Keywords

diatoms Irminger Current ‘Little Ice Age’‘Medieval Warm Period’sea-ice cover West Greenland shelf

Introduction

During the last few decades, considerable interest has emerged, involving both social and environmental conditions, as to the possible effects of future climate changes. In this context, the growing focus on both magnitude and rate of future climatic change has, in particular, led to an increased recognition of the importance of studies on palaeoclimatic fluctuations and their ecological impacts (e.g. Goosse et al., 2008; Jones and Mann, 2004).

Being located in the climatically highly sensitive Arctic part of the North Atlantic region, West Greenland is an area which is highly suitable for conducting studies on regional climate changes. Palaeoclimatic data from this region are particularly important because of the proximity to both the Greenland ice sheet and Davis Strait, which play important roles in North Atlantic climate dynamics, including both the oceanic and atmospheric circulation patterns in the region (Davis et al., 1998; Williams and Bradley, 1985).

Only a limited amount of data have been published on marine Holocene sediments and palaeoenvironments from West Greenland, and most of these studies have focused on northernmost Baffin Bay (e.g. Knudsen et al., 2008; Levac et al., 2001) and the Disko Bugt area (e.g. Desloges et al., 2002; Krawczyk et al., 2010; Lloyd, 2006; Lloyd et al., 2005, 2007; Moros et al., 2006; Seidenkrantz et al., 2008). Holocene palaeoenvironments in fjords and shelf areas of South and Southwest Greenland have been studied in detail by e.g. Jensen et al. (2004), Lassen et al. (2004), Roncaglia and Kuijpers (2004), Møller et al. (2006), Seidenkrantz et al. (2007) and Ren et al. (2009).

Furthermore, studies of proxies such as pollen, diatoms and macrofossils in lake sediments in West Greenland provide information on climatic changes during the Holocene (Anderson et al., 2000, 2001; Bennike, 2000; Brodersen and Anderson, 2000; Gowan et al., 2003; Ryves et al., 2002).

Diatoms are widely distributed in marine environments and are ecologically extremely sensitive to temperature and salinity changes of the sea-surface waters. The siliceous walls of diatoms are generally well preserved in marine sediments, and changes in abundance, as well as in species composition are, therefore, often sufficiently recorded in the sediments for palaeoenvironmental interpretations. Diatom-based reconstructions of palaeoenvironments are widely applied across the North Atlantic (e.g. Birks and Koç, 2002; Jiang et al., 2002; Witak et al., 2005) and have also proven to be useful in the West Greenland area (Jensen, 2003; Krawczyk et al., 2010; Moros et al., 2006).

Two distinctive climatic intervals during the last millennium have been intensively studied in the North Atlantic region and elsewhere around the Northern Hemisphere, namely the so-called ‘Medieval Warm Period’ (MWP) and the ‘Little Ice Age’ (LIA). Evidence for the MWP and LIA climatic periods has, for instance, been identified in ice-core records (Dahl-Jensen et al., 1998; Johnsen et al., 2001), as well as in Greenland fjord and shelf sediments (e.g. Jennings and Weiner, 1996; Jensen et al., 2004; Knudsen et al., 2008; Lassen et al., 2004; Lloyd, 2006).

The purpose of this study is to reconstruct palaeoceanographic changes off West Greenland for the last 1200 years in high time resolution based on a precisely dated diatom record. As a basis for the interpretation of our fossil record, proxy data from the last 70 years are compared with instrumental data from the same area and with the annual Atlantic Multidecadal Oscillation (AMO) index (cf. Enfield et al., 2001; Kerr, 2000), and the data are discussed in relation to the North Atlantic Oscillation (NAO) index as well (cf. Jones et al., 1997; Osborn, 2011). Climatic and oceanographic changes during the MWP and LIA, as well as the twentieth-century warming, on the West Greenland shelf are discussed and compared with similar marine records from the area and with the general climatic development in the North Atlantic region.

Study area

Holsteinsborg Dyb is a deep submarine valley crossing the West Greenland shelf from the town of Sisimiut towards the southwest (Figure 1). Seismic profiles show that the valley was presumably repeatedly eroded during glaciations and subsequently filled up by a series of Quaternary deposits (Lykke-Andersen and Knudsen, 2007). This deep WSW-trending trough runs for about 60 km towards the middle shelf, before it shallows into a trough mouth fan at the shelf edge (Lykke-Andersen and Knudsen, 2007). Holsteinsborg Dyb coincides with a major thrust line that continues through the Ikertooq fjord (Jensen et al., 2002), one of four large fjords that are located to the east of the trough (Figure 1B).

Figure 1.

(A) Oceanographic surface current system in the northern North Atlantic and around Greenland. EGC, East Greenland Current; WGC, West Greenland Current; B-LC, Baffin-Labrador Current; DS, Davis Strait; DB, Disko Bugt. (B) Location of core site GA306-4 in Holsteinsborg Dyb

Much of the interannual climatic variability in the North Atlantic region can be explained by fluctuations in atmospheric pressure, the NAO, which is defined by the difference in sea-surface pressure over Iceland and over the Azores. It controls temperatures, wind speed and direction, as well as moisture over much of the Northern Hemisphere (e.g. Hurrell, 1995; Hurrell and van Loon, 1997; Osborn et al., 1999; Qian et al., 2000). West Greenland is situated in a climatically sensitive region, and both the atmospheric and oceanographic conditions therefore are influenced by changes in the NAO.

A climatic see-saw pattern between Greenland and western Europe, with warm winters in Europe corresponding to severe winters in Greenland and vice versa (e.g. Dawson et al., 2003; Ribergaard and Buch, 2005), has been demonstrated to be consistent at subdecadal to decadal timescales, although with a few exceptions (Cappelen, 2007; Dawson et al., 2003). Over a longer timescale (c. 140 years), however, Cappelen (2007) found that the temperature patterns co-vary in the two regions.

At the Sisimiut coastal meteorological station, northeast of Holsteinsborg Dyb (Figure 1), the maritime influence is relatively high (Ryves et al., 2002). The annual precipitation is 200–300 mm (cf. Hansen et al., 2004) with maximum rainfall in August (~60 mm), and the mean annual air temperature is approximately −6°C. The coldest month is February (mean −16°C), and the highest temperatures are obtained in July or August with means about +6°C (Hansen et al., 2004).

Davis Strait is a preferred area for outbreaks of cold air blowing southward, as well as for the injection of warm air deep into the Arctic Basin. A typical feature for the arctic West Greenland waters is a low sea-surface temperature all the year round and a summer air temperature below 10°C (Hansen et al., 2004).

The hydrography west of Greenland is governed by the north-flowing West Greenland Current (WGC), which consists of two components of different origin. Closest to the shore, the East Greenland Current (EGC) component brings water masses of Polar origin northward along the West Greenland coast. On its way, this water is diluted by runoff water from the various fjord systems, and it turns westward towards Canada at around 65–66°N. Below and offshore, the WGC consists of the Irminger Current (IC) component originating from the Atlantic Water (Irminger Sea-derived subsurface water). This relatively warm, high-salinity water can be traced all the way along the west coast of Greenland to the northern parts of Baffin Bay (e.g. Andersen, 1981; Tang et al., 2004).

The properties and amounts of the IC water that propagate north in the WGC have been demonstrated to be related to the Subpolar Gyre circulation and the NAO state (Holland et al., 2008; Myers et al., 2007), with the highest salinity and temperature, as well as the largest volumes of Irminger Water inflow in the 1960s and after 1995, apparently related to negative NAO index.

There is an increased mixing of the two components of the WGC, as it flows northward, although still distinguishable as far north as Disko Bugt (Andersen, 1981; Holland et al., 2008). Our core site is located in the present ‘Irminger Mode Water’ with a bottom-water temperature around 4°C and salinity around 34.9 PSU (e.g. Buch, 2000a; Ribergaard et al., 2008).

Sea-ice is an important environmental parameter in Greenland waters. The area west of Greenland is dominated by two types of sea ice. ‘Storis’, is a multiyear ice, which is transported by the EGC around Cape Farewell (the southern tip of Greenland), and continues northward assisted by the WGC. The amount of ‘Storis’ entering Southwest Greenland waters shows great interannual variability, determined by, among other factors, the outflow from the Arctic Ocean and the formation of sea ice, as well as the wind conditions in the Greenland Sea (Buch, 2000a). ‘Westice’ is a first-year ice formed during winter time in Baffin Bay, the Davis Strait, and the western part of Labrador Sea. The waters off Southwest Greenland are normally not affected by the ‘Westice’, only occasionally the ice limit has reached as far south as Kangerlussuaq (Buch, 2000a).

Material and methods

Core material

A 501 cm long gravity core GA306-GC4 and a 35 cm long boxcore GA306-BC4 were both retrieved in the offshore trough, Holsteinsborg Dyb, on the West Greenland shelf at 66°44′41″N, 53°56′25″W (water depth 445 m) during the Galathea 3 expedition in 2006 (Figure 1). In the following, the core site is referred to as GA306-4 and the two cores as GC4 and BC4, respectively. A total of 99 samples (a 1 cm sediment sample slice at every 5 cm) were analyzed for their diatom contents between 500 cm depth and the top of core GC4. In boxcore BC4, a 1 cm sample slice was analysed at every 2 cm (18 samples).

The samples were prepared according to the method described by Håkansson (1984). All samples were treated with 10% HCl to dissolve calcareous matter and with 30% H₂O₂ (3 h in a water bath at 70°C) to oxidize organic material. In order to dilute the remaining acid, test-tubes were filled with distilled water and left to settle for 12–24 h, after which the supernatant was decanted. The procedure was repeated until no acid was left in the sample. An aliquot of shaken suspension was placed on a cover slip and mixed by a pipette in order to settle diatoms evenly on the cover slip. After the material had completely dried up, cover slips were transferred onto permanently labeled slides, mounted with Naphrax (refraction index = 1.73) and heated to 250°C.

A Leica microscope with phase-contrast was employed for diatom identification and counting under oil immersion at a magnification of ×1000. More than 300 diatom valves were counted in random transects for each sample (excluding Chaetoceros resting spores, cf. Koç Karpuz and Schrader, 1990). Diatom percentages were calculated based on the diatom sum, excluding the Chaetoceros resting spores.

Age models

The age model for gravity core GC4 is based on accelerator mass spectrometry (AMS) ¹⁴C age determinations of marine mollusc shells (Table 1, Figure 2C) performed at the AMS ¹⁴C Dating Centre, Aarhus University, Denmark. All ¹⁴C ages are calibrated with the OxCal 4.1.3 software (Ramsey, 2009) using the Marine04 calibration data set (Hughen et al., 2004; Stuiver and Braziunas, 1993) with a standard marine reservoir correction of 400 years (cf. Reimer and Reimer, 2005), which is a reasonable estimation, because the sea floor is swept by Atlantic subsurface waters.

Table 1.

AMS ¹⁴C age determinations from core GA306-GC4 in Holsteinsborg Dyb, West Greenland. The ages are calibrated (cal. yr BP) with the OxCal 4.1.3 software (Ramsey, 2009) using the Marine04 calibration data set (Hughen et al., 2004; Stuiver and Braziunas, 1993) with a standard marine reservoir correction of 400 years (Reimer and Reimer, 2005)

Depth (cm)	Lab. no.	Species	¹⁴C age (BP ±1 σ)	Cal. ¹⁴C yr BP (mean of range)	Cal. range (±1 σ)	δ¹³C	δ¹⁸O
8.5	AAR-11694	Thyasira gouldi	531±27	207	169−245	−3.62	1.44
141.5	AAR-11693	Thyasira gouldi	706±42	419	399−439	−3.57	1.96
152	AAR-11692	Thyasira gouldi	826±30	436.5	417−456	−4.01	2.02
242	AAR-11691	Thyasira gouldi	984±31	581	560−602	−2.33	2.03
291.5	AAR-11690	Thyasira gouldi	1095±33	663	640−686	−5.76	0.73
345.5	AAR-11689	Megayoldia thraciaeformis	1230±37	817.5	795−840	0.31	2.25
384	AAR-11688	Nuculana pernula costigera	1434±35	927	906−948	0.38	2.08
426.5	AAR-11687	Macoma moesta	1497±33	1040	1012−1068	0.21	2.45
452.5	AAR-11686	Thyasira gouldi	1516±32	1109	1075−1143	−2.71	2.15

Figure 2.

(A) Age–depth model for boxcore GA306-BC4 based on ²¹⁰Pb dating. (B) ¹³⁷Cs activity for GA306-BC4. (C) Age–depth model for gravity core GA306-GC4. Calibrated ¹⁴C ages are marked with ±1 σ error bars (see also text). The squares indicate the data points used for the age model

For the age–depth model, the simplest approach has been employed, i.e. sections of straight lines drawn through the ¹⁴C date error bars, assuming a constant sedimentation rate for each section (Figure 2C), but a change at 291.5 cm (663 cal. yr BP; ad 1287). The mean sedimentation rate for each of the two age zones is 620 and 360 cm/ka, respectively, with a mean sedimentation rate for the record of c. 490 cm/ka. Ages are given either as calibrated years BP (before 1950) or years ad.

Samples from boxcore BC4 have been analysed for the activity of ²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs via gamma spectrometry at the Gamma Dating Centre, Institute of Geography, University of Copenhagen, Denmark. The measurements were carried out on a Canberra low-background Ge detector. ²¹⁰Pb was measured via its gamma peak at 46.5 keV, ²²⁶Ra via the granddaughter ²¹⁴Pb (peaks at 295 and 352 keV) and ¹³⁷Cs via its peak at 661 keV.

An age–depth model for BC4 (Figure 2A) was performed applying a modified CRS-modeling approach (Appleby, 2001), even though an almost uniform ²¹⁰Pb activity in the top 20 cm may be caused by some bioturbation of the sediment. This chronology gives a profile of ¹³⁷Cs versus depth (Figure 2B), which is in agreement with the known history of releases of this isotope (cf. Abril, 2004).

Instrumental data

For comparison with the top part of our proxy data, we have used the most complete and well-known oceanographic and atmospheric instrumental data series from West Greenland, which are from the Fylla Bank section west of Nuuk and from the meteorological station in Nuuk, respectively, situated about 300 km to the south of Holsteinsborg Dyb (e.g. Buch, 2002; Buch et al., 2004; Vinther et al., 2006).

The present-day climate of Nuuk is low arctic with a mean annual temperature of −1.4°C and a mean precipitation of 752 mm (Cappelen et al., 2001). A time series of the summer mean air temperature in Nuuk during the period ad 1930–2005 is shown in Figure 3 together with the mid-June mean sea-surface temperature from Fylla Bank (ad 1950–2004), maintained by the Greenland Fisheries Research Institute. The sea-surface temperatures can vary quite drastically from one year to the next, often more than 1°C, reflecting the variability in the atmospheric influence as well as in the inflow of Polar Water. A 3 yr running mean (entered on Figure 3) would smooth out the high-frequency variations and better reflect the large-scale climatic variability (cf. Buch et al., 2004).

Figure 3.

Comparison of selected diatom species and species groups (percentages) for the time interval ad 1934–2006 with summer air temperature in Nuuk (West Greenland) and mid-June sea-surface temperature at Fylla Bank to the west of Nuuk (Buch et al., 2004; Vinther et al., 2006), as well as the annual Atlantic Multidecadal Oscillation (AMO) index (Enfield et al., 2001) and the North Atlantic Oscillation (NAO) index (Jones et al., 1997). Three point running means are shown for the instrumental data (black lines). The limit of grey shadings in the proxy data indicates mean value for each record. WW, Warm Water species; SI, Sea Ice species; A, Arctic species; A-SI, Arctic-Sea Ice species; MW, Mixing Water species (see also Figure 4)

In addition, our data are compared with atmospheric and oceanographic data from the North Atlantic region, i.e. the NAO index (cf. Jones et al., 1997; Osborn, 2011) and the AMO index (cf. Enfield et al., 2001) (Figure 3).

Results

As a background for palaeoenvironmental interpretations of the gravity core GC4 record, diatoms from the boxcore BC4 (ad 1934–2006) were analysed, and the most important diatom species data (species with specific environmental significance) are compared with instrumental data from the area for the same time interval.

Core BC4, ad 1934–2006 (16 to −56 cal. yr BP)

From the mid-1930s to the early 1960s, the diatom assemblage is characterized by relatively high abundances of Thalassionema nitzschioides and Thalassiosira oestrupii and a low proportion of Fragilariopsis cylindrus (Figure 3). Also, the percentages of Bacterosira bathyomphala and Thalassiosira nordenskioeldii are relative low, whereas the amount of Thalassiosira antarctica var. borealis resting spore is fluctuating. There is a decrease in Fragillariopsis oceanica from c. 10% to relatively low values through the time interval. This assemblage indicates relatively high sea-surface influence of Atlantic waters (see below) in the area.

From the early 1960s to the mid-1990s, T. nitzschioides and T. oestrupii decreased markedly and occasionally almost completely disappeared, with generally low abundances of F. oceanica (Figure 3). There is a pronounced increase in F. cylindrus, B. bathyomphala and T. nordenskioeldii, and T. antarctica var. borealis resting spore remains at a relatively high level during this time interval, suggesting cold sea-surface environments as a result of an increase in Polar Water influence.

Relatively high, but fluctuating contents of T. nitzschioides and T. oestrupii are found after ad 1995. F. cylindrus returns to slightly lower, but also fluctuating abundances, and there is a slight decline of B. bathyomphala and T. antarctica var. borealis resting spore, whereas T. nordenskioeldii remains at about the same level. This assemblage implies a return of the influence of Atlantic waters.

Thalassionema nitzschioides is the main species of the warm Atlantic Water (e.g. Jiang et al., 2001, 2004), indicating warm sea-surface conditions influenced both by enhanced Atlantic Water inflow and surface-water heating. This species is described as a widespread neritic species, commonly found in the North Sea and English Channel (Hendey, 1964), and it never occurs in the high Arctic and Antarctic regions (Hasle and Syvertsen, 1997). Abundant occurrences of T. nitzschioides in the Nordic Seas (Koç Karpuz and Schrader, 1990), Skagerrak (Jiang, 1996) and to the south and west of Iceland (Jiang et al., 2001) indicate its affiliation to Atlantic waters (salinities >34.9 PSU and SST >3°C). Thalassiosira oestrupii often occurs in warm-water regions (Hasle and Syvertsen, 1997). It is the main species of the warm Atlantic assemblage in the Nordic Seas (Koç Karpuz and Schrader, 1990), and it reaches the highest abundances south of Iceland as the most important species of the warm-water assemblage (Jiang et al., 2001). The number of the two species T. nitzschioides and T. oestrupii decreases as the Irminger Current waters flow from the Irminger Sea and clockwise around Iceland and southern Greenland. They comprise ~20% of the total assemblage south of Iceland, up to 10% west of Iceland and ~5% north of Iceland, and they almost disappear southeast of Greenland, where no influence of the Atlantic Water is found (Jiang et al., 2001: figure 3A, B). Therefore, these two species are combined here as a measure for the influence of warm Atlantic-derived water masses west of Greenland, although their absolute values are low (Figure 3).

Fragilariopsis cylindrus is a bipolar planktonic species connected to sea ice (Hasle and Syvertsen, 1997; Medlin and Priddle, 1990). It is among the dominant species in the Barents Sea and off Northeast Greenland (von Quillfeldt, 2000), as well as in Disko Bugt, West Greenland (Jensen, 2003). Moreover, F. cylindrus is one of the major components of the Sea Ice assemblage in the Nordic Seas (Koç Karpuz and Schrader, 1990) and Labrador Sea (De Sève, 1999). Fragilariopsis cylindrus is here used as an indicator of the amount of sea ice west of Greenland.

Bacterosira bathyomphala is found in the northern cold-water region (Hasle and Syvertsen, 1997), and it is the main species of the Arctic diatom assemblage around Iceland, indicating the influence of Arctic water masses from the East Icelandic Current (Jiang et al., 2001). Thalassiosira antarctica var. borealis resting spore, an Arctic neritic taxon found in northern cold-water to temperate region (Hasle and Syvertsen, 1997; von Quillfeldt, 2000), is the dominant taxon of the Arctic Water assemblage in the Nordic Seas (Koç Karpuz and Schrader, 1990) and is widely distributed in surface sediments from the Labrador Sea (De Sève, 1999). It is also the chief component of the cold-water diatom assemblage off Iceland, which is strongly affected by the East Icelandic Current (Jiang et al., 2001). Thalassiosira antarctica var. borealis resting spore shows similar temperature preference as B. bathyomphala in surface sediments around Iceland (Jiang et al., 2001), and there is a high degree of similarity between the environmental indication of this species and that of B. bathyomphala. On the West Greenland shelf, these two taxa are used as indicators of the influence of Polar waters.

Thalassiosira nordenskioeldii, a neritic Arctic-boreal species, usually blooms in the spring and may be found in drifting sea ice (Cremer, 1998; De Sève and Dunbar, 1990). Karentz and Smayda (1984) found that it was a part of the winter phytoplankton with a population maximum at temperatures of 2.8±1.8°C. Moreover, diatom assemblages consisting of T. nordenskioeldii and Fragilariopsis spp. were also found in the Chukchi Sea and northern Bering Sea (Sancetta, 1982).

Fragilariopsis oceanica is generally described as a Polar plankton species associated with sea ice (Hasle and Syvertsen, 1997; Jensen, 2003; von Quillfeldt, 1996), and it is therefore normally regarded as a sea-ice indicator with a habitat similar to that of F. cylindrus (Jiang et al., 2001; Koç Karpuz and Schrader, 1990). However, some studies have shown a significant variation in the distribution patterns of these two species (e.g. Jensen et al., 2004; Ren et al., 2009). Heimdal (1989) described F. oceanica as a representative of the mixed water masses of the WGC. Moreover, Jensen et al. (2004) argued that F. oceanica should be interpreted as a proxy for the EGC rather than purely an indicator for sea ice. This would indicate that changes in abundances of F. oceanica at our study site may reflect variations in mixing of the water column rather than sea ice.

The instrumental data from the Nuuk area (Figure 3) show that the West Greenland shelf area generally experienced relatively high air temperatures and sea-surface temperatures from the mid-1930s to the mid-1960s, with the initial rise in temperature occurring already in the early 1920s (Buch, 2000a). This warm period corresponds to an interval with relatively high abundance of warm-water species and low proportions of sea-ice species, coinciding with a period of high AMO index values (Enfield et al., 2001) and generally relatively high, but fluctuating NAO index (Figure 3). The relatively high amounts of F. oceanica at beginning of this period may reflect enhanced mixing of the water column.

The following about 30 years, including three extremely cold spells around 1970, the early 1980s and the early 1990s, which are reflected both in the air temperature and in the sea-surface temperature, correspond to an interval with a decrease in the warm-water indicators T. nitzschioides and T. oestrupii, a remarkable increase of the sea-ice species F. cylindrus and relatively high percentages of Arctic species (Figure 3), including T. nordenskioeldii. A weakly mixed water column is suggested by generally lower abundances of F. oceanica. The most sustained interval of low AMO index occurred during this cold period (Figure 3). The NAO index was also particularly low during the late ad 1960s and early ad 1970s (cf. Belkin et al., 1998; Dickson et al., 1988), followed by an increasing trend.

After the late ad 1990s, there was an increase in temperature, although not reaching the temperature level before ad 1965 coinciding with a generally positive AMO index, whereas the NAO index fluctuated a lot (Figure 3). This corresponds to an interval of relatively high percentages of warm-water species and a slight decline in sea-ice species and Arctic species. The warming trend was interrupted by a short-term temperature decrease shortly after ad 2000 (cf. Buch, 2000a; Buch et al., 2004), indicating a temporary increase in the influence of Polar waters, as also reflected in the diatom assemblages.

In summary, there is a reasonably high degree of similarity between the environmental indication of the diatom species and the instrumental data for the last 70 years, suggesting that the diatom record (particularly sea-ice species and warm-water species) is reliable for the interpretation of palaeoceanography and palaeoclimate off West Greenland in pre-instrumental time (core GC4).

Core GC4, ad 715–1760 (1235–190 cal. yr BP)

The gravity core GC4 record is divided into three assemblage zones (Figure 4) on the basis of the distribution of diatom taxa.

Figure 4.

Percentage distributions of selected diatom species and species groups in cores GA306-GC4 (ad 715–1760) and GA306-BC4 (ad 1934–2006). Five point running means are shown with black lines

Zone I, ad c. 715–1000 (1235–950 cal. yr BP)

The diatom assemblage in zone I is characterized by generally low proportions of the sea-ice species F. cylindrus and relatively high, but fluctuating abundances of the warm-water species T. nitzschioides and T. oestrupii. In general, the percentages of T. nordenskioeldii are low, with the exception of two abundance peaks around ad 835 and ad 915. Fragilariopsis oceanica is relatively abundant throughout the zone.

In zone I, there appears to have been a generally high influence of Atlantic Water, and there is indication of mixing of the water column. An increase in the influence of Atlantic Water may either be a result of generally high inflow of IC waters to the area, mixing of the water column, surface-water heating, or reduced amounts of meltwater from land. Periods with enhanced influence of Polar water masses, on the other hand, may partly be caused by periodically increased amounts of meltwater from the onshore fjord systems to the area.

Zone II, ad c. 1000–1330 (950–620 cal. yr BP)

At the transition to zone II, there is a change to increased fluctuations in the assemblages compared with zone I, reflecting pronounced climate instability. Thus, there is an interval with significant increase in percentages of the sea-ice species F. cylindrus in the lower part of the zone (ad 1000–1100), followed by a decrease to a relatively stable level, although with a further decrease towards the upper part of the zone.

The contents of warm-water species (T. nitzschioides and T. oestrupii) are generally lower than in zone I, but with an increase towards the top. The percentages of T. nordenskioeldii, B. bathyomphala, T. antarctica var. borealis resting spore and F. oceanica remain at about the same level as seen in zone I, but with considerable fluctuation. There is a clear decreasing trend in the abundance of F. oceanica between about ad 1150 and 1330. This presumably indicates a gradual reduction in the mixing of the water column within this time interval.

Zone II is characterized by relatively high influence of Atlantic Water between ad 1250 and 1330, extended sea-ice cover from ad 1000 to 1100 and pronounced fluctuations in the inflow of Polar Water, suggesting increased climate variability in the area.

Zone III, ad c. 1330–1760 (620–190 cal. yr BP)

In the upper part of the core (zone III), F. cylindrus and T. nordenskioeldii become abundant, suggesting considerably colder conditions than for zones I and II. Low contents of F. oceanica presumably indicate stratification of the water column. There is a marked decline of the warm-water species T. nitzschioides and T. oestrupii, indicating decreased influence of Atlantic waters. The abundances of the Arctic species B. bathyomphala and T. antarctica var. borealis resting spore are generally low, but fluctuating, and there is an increasing trend in the percentages of T. antarctica var. borealis resting spore towards the core top indicating increased influence of Polar Water.

The diatom assemblage of zone III thus mainly consists of sea-ice species, indicating an extended sea-ice cover and cold sea-surface conditions with high influence of Polar Water in the area.

Discussion

The diatom assemblages at core site GA306-4 reveal a general cooling trend and a number of changes in the environmental conditions, indicating that significant shifts occurred in the oceanographic and climatic regime off West Greenland during the last 1200 years. In adjacent marine areas, a similar cooling trend has been recorded over the last 1500 years, for instance in western Baffin Bay and offshore Labrador (Keigwin and Pickart, 1999) and in South and East Greenland shelf areas (e.g. Jenning et al., 2002; Roncaglia and Kuijpers, 2004). A comparison of the percentage distribution of the sea-ice species F. cylindricus in core GC4 with a series of atmospheric climatic proxies is shown on Figure 5. F. cylindricus appears to provide the best single proxy for the oceanographic and climatic changes in this area.

Figure 5.

Percentage distribution of the sea-ice species F. cylindrus (ad 715–1760) compared with the total Solar Irradiance variations (Bard et al., 2007), the oxygen isotope record for the Renland ice core (Johnsen et al., 1992; Vinther et al., 2009) and the long-term winter NAO reconstruction (Trouet et al., 2009). The limit of grey shadings indicates mean value for each record. Solar Irradiance minima (e.g. Eddy, 1978; Schröder, 2005) are indicated on the left hand side (M, Maunder; S, Spörer; W, Wolf; O, Oort)

Oceanographic and climatic changes during the MWP

After a pronounced maximum in the sea-ice species F. cylindricus between ad 715 and 800 (Figure 5), relatively warm and stable conditions were established at the core site. The interval ad 800–1330 is initially characterized by relatively high abundance of the warm-water species T. nitzschioides and T. oestrupii (Figure 4), and after a period with decreased values between ad 1000 and 1200, there is a gradual rise to similar high values. This entire interval, ad 800–1330, corresponds to the so-called MWP.

The concept of a MWP was first introduced by Lamb (1965), who found evidence for warm, dry summers and mild winters in western Europe, centered around ad 1000–1200. The MWP has been differently dated in different studies, as various proxy indicators cannot be combined into a global quantitative composite (e.g. Bradley et al., 2003; Jones et al., 2001; Soon and Baliunas, 2003), but the interval ad 800–1300 encompasses most published dates.

Evidence of the MWP can be found throughout the northern North Atlantic region. In Southeast Greenland (Igaliku Fjord), Roncaglia and Kuijpers (2004) identified a period with a decline in cold-water dinoflagellate taxa as evidence of high sea-surface temperatures during the time interval ad 770–960. Sedimentological and foraminiferal evidence of an increase in bottom-water ventilation at ad 960–1285 and ad 900–1200, respectively, in Igaliko Fjord indicates enhanced mixing of the water column, which was ascribed to strong wind and wave action during that time interval by Roncaglia and Kuijpers (2004) and Lassen et al. (2004). This is in accordance with our interpretation of pronounced mixing of the water column at core site GA306-4 during the MWP, as suggested by the relatively high contents of F. oceanica, as well as by the benthic foraminiferal assemblages in the same core (Underbjerg, 2009). Additionally, increased surface-water mixing has been recorded by Moros et al. (2006) from Disko Bugt during the MWP.

It is interesting to note that Jensen et al. (2004) found diatom indication of a cold episode with increased ice cover (ad 960–1140) within the warm MWP (ad 800–1250) in Igaliku Fjord. This interval almost coincides with our short-term maximum in sea-ice species F. cylindrica (ad 1000–1100) in core GC4.

In Skagerrak in the eastern North Atlantic, a similar cold interval has been observed within the MWP, although with large temporal and regional differences (ad 900–1150; Hass, 1996), which may be caused by uncertainties in the chronology. In addition, low percentage values of the warm diatom species Azpeitia nodulifera have been found at ad 1050 in the Gulf of California, indicating a period of cooler sea-surface temperature within the MWP also in the Pacific (Barron and Bukry, 2007). This regionally documented cold interval appears to correspond to the Oort solar irradiance minimum (Figure 5).

Along the West Greenland coast, the MWP is clearly documented as a brief relatively warm event at ad 750–1050 in Disko Bugt (Lloyd et al., 2007), as well as in the North Water Polynya, northern Baffin Bay, where a relatively warm summer (+1°C increase) with reduced sea-ice cover is indicated from ad 1100 to 1400 (Mudie et al., 2005). Additional evidence for a warm MWP is also recorded from the terrestrial regime in South Greenland, where limnological studies by Kaplan et al. (2002) showed a gradual increase of BSiO₂ and loss on ignition in the time interval ad 650–1050.

It should be noted, however, that the generally warm MWP has also been reported as a particularly cold period at some sites in West and South Greenland. For instance, it was reported to be a relatively cold interval in Disko Bugt by Moros et al. (2006) and Krawczyk et al. (2010), and similar results were demonstrated by Møller et al. (2006) from Ameralik Fjord in South Greenland, indicating that climatic anomalies in the Greenland area are characterized by complex patterns.

A study of foraminifera and lithofacies on the East Greenland shelf shows that the influence of the Polar Water was sufficiently reduced from ad 730 to 1100 to allow the enchroachment of Atlantic Intermediate Water on the sea floor throughout this interval (Jennings and Weiner, 1996), and also the North Icelandic shelf was dominated by the influence of Atlantic Water during the MWP, ad 800–1330 (e.g. Eiríksson et al., 2000; Jiang et al., 2002; Knudsen et al., 2004; Ran et al., 2011).

The indication of relatively high sea-surface temperatures and warm climate at core site GA306-4 during the time interval ad 800–1330 almost coincides with a period of high solar irradiance (Figure 5; Bard et al., 2000, 2007; Muscheler et al., 2007), as well as with a relatively warm interval indicated by the oxygen isotopes in the Renland ice-core record (Johnsen et al., 1992; Vinther et al., 2009). Even the cold interval between ad 1000 and 1100 in core GC4 is recorded both in the Renland ice core and in the solar irradiance curve, and it also appears to coincide with a short-term interval with low NAO index (Figure 5). Furthermore, there is a high degree of consistency between the percentage of sea-ice species and the long-term reconstructed NAO index of Trouet et al. (2009) during the time interval ad 1100–1330, in which low proportions of the sea-ice species coincide with high values of reconstructed NAO index (Figure 3).

Documentary and archaeological data show that The Vikings colonized Greenland around ad 982–1200 and two important settlements were present in South and Southwest Greenland during the MWP (e.g. Arneborg et al., 1999; Buckland et al., 1996). One of these, the Norse ‘Western Settlement’, located in the Godthåb district in Southwest Greenland, was abandoned at some time between ad 1341 and 1362 (Grove, 1996; Ogilvie et al., 2000).

Oceanographic and climatic changes during the LIA

The marked increase in sea-ice species indicate an abrupt change to colder sea-surface water conditions after ad 1330 and to the top of core GC4 (ad 1760), encompassing most of the LIA cold episode.

It appears that the percentage of sea-ice species at our core site experienced quasi-periodic changes during the LIA, with three particularly cold intervals at ad 1330–1350, ad 1400–1575 and ad 1660–1710 (Figure 5). There is a generally high degree of consistency between these cold intervals and minima in sun spot activity (Eddy, 1978; Schröder, 2005), such as the Wolf Minimum (ad 1280–1350), the Spörer Minimum (ad 1460–1550) and the Maunder Minimum (ad 1645–1715), indicating that solar forcing may be one of the important contributors to the oceanographic changes.

The detailed mechanism how changes in solar activities result in climate change has not been established, but one theory is that modifications of the Arctic Oscillation/North Atlantic Oscillation are caused by solar irradiance changes (Shindell et al., 2001). This appears to be in overall accordance with the long-term reconstructed NAO index of Trouet et al. (2009), which show generally low values during the LIA, particularly after about ad 1400.

The decreasing trend in the percentage values of F. oceanica towards the end of the MWP (ad 1150–1330), and the low values after ad 1330, marks a transition to a period with relatively low wind stress and strong stratification of the water masses. A similar strong stratification of the water column, as well as persistent sea-ice cover, has also been described for the LIA in Southwest Greenland (Jensen et al., 2004). In accordance with these results, a gradual increase in the sea-ice diatom F. cylindrus in Ameralik Fjord, Southwest Greenland, is suggested to indicate a further cooling of the surface waters and increased sea-ice formation after ad 1450 (Seidenkrantz et al., 2007). Additionally, Lassen et al. (2004) found foraminiferal evidence for culmination of the first LIA cooling at ad 1400 in Igaliko Fjord, Southwest Greenland.

On the East Greenland shelf, Jennings and Weiner (1996) found a marked lithological and foraminiferal faunal change to a pronounced influence of Polar waters at the sea floor at ad 1100. Furthermore, a diatom-based sea-surface temperature reconstruction for the North Icelandic shelf indicates a marked decrease in temperature at around ad 1300 (Jiang et al., 2005; Ran et al., 2011), caused by a relatively high influence of Polar waters from the East Greenland Current to that area. On the North Icelandic shelf, the foraminiferal assemblages also show a pronounced stratification of the water masses during part of the LIA (Knudsen et al., 2004, 2009).

Onshore evidence for a LIA cooling is recorded by a decrease in oxygen-isotope values after ad 1330 in the Renland ice-core record in East Greenland (Figure 5; Johnsen et al., 1992). Also, both the GRIP and DYE-3 ice-core records reveal a cold LIA period with minima at around ad 1500 (Dahl-Jensen et al., 1998; Johnsen et al., 2001). Dahl-Jensen et al. (1998) found this cold interval to span the interval ad 1400–1850, whereas Humlum (1999) identified a 2–4°C mean annual air temperature cooling between ad 1600 and 1775, based on oxygen-isotope analysis of ice from a rock glacier on Disko Island in West Greenland. In the Hudson Strait area, Kasper and Allard (2001) reconstructed the coldest conditions at ad 1450–1850 based on ice wedge studies.

The LIA climate deterioration in Southwest Greenland appears to have had serious implications for the Norse culture in Greenland, and the last Viking settlement, the ‘Eastern Settlement’, near the southernmost tip of Greenland, was abandoned around ad 1450–1500 (e.g. Arneborg et al., 1999; Buckland et al., 1996; Grove, 1996; Ogilvie, 1998; Ogilvie et al., 2000).

Oceanographic and climatic changes during the last 70 years

As already discussed, there is a high degree of consistency between the proxy data from core BC4 and instrumental data from the same area during the last 70 years. The general pattern also corresponds to Northern Hemisphere mean temperatures (e.g. Mann et al., 1999; Osborn and Briffa, 2006). Zweng and Münchow (2006) found a statistically significant warming trend during the last 70 years in most of the Baffin Bay area and demonstrated a significant correlation of the deep subsurface (800–600 m depth) temperature fluctuations in Baffin Bay with the NAO index. They did not, however, find a similar correlation for the upper subsurface waters (600–50 m depth).

The relation between sea-surface temperatures off West Greenland and the NAO index is rather complex. The cold interval between the early 1960s and the mid-1990s at site GA306-4, which is characterized by a marked increase in sea-ice species and Arctic species, includes the effects of Great Salinity Anomaly (GSA) that can be followed all over the North Atlantic area during the late 1960s and 1970s. The GSA was a result of a period of extremely high frequency of northerly winds over the Arctic Ocean and northern North Atlantic in the 1960s (Belkin et al., 1998; Dickson et al., 1988). The anomalously high inflow of Polar Water propagated into the West Greenland waters around 1970 (Buch, 2002; Buch et al., 2004) and is clearly evident in the mid-June sea-surface temperature on Fylla Bank (Figure 3). This inflow was related by Buch (2002) and Buch et al. (2004) to the particularly low NAO index in the late ad 1960s and early ad 1970s, and it coincides with an interval low AMO index values (Figure 3).

The cold period around the early 1980, on the other hand, has been explained by Rosenørn et al. (1985) as a result of the presence of an extremely cold air mass situated over the Davis Strait area, with its center around Disko Bugt in West Greenland. Buch (2000b) also pointed at the 1982–1984 period as one of the coldest episodes ever recorded in West Greenland, although not the coldest (see Figure 3).

A general warming trend, as found at core site GA306-4 from the mid-1990s to the present, has been shown throughout the North Atlantic. Thus Stein (2005) found that a warming in the Subpolar Gyre became gradually more evident from 1997 and onwards. A similar trend was reported by Mortensen and Valdimarsson (1999) south of Iceland, indicating increased strength of the Irminger Current since the mid-1990s, which appears to have influenced the West Greenland waters. The sea-surface temperature rise coincides with an increase in the AMO index (Figure 3), and there is a generally high degree of consistency between the sea-surface and air temperature trend during the last decades in Southwest Greenland.

Although there is a clear increase in sea-surface temperature during the twentieth century compared with the LIA, the environmental indication is not directly comparable with that of the MWP. The oceanographic change in the twentieth century is particularly expressed by a drop in the sea-ice species F. cylindrus, as well as in some of the Arctic species, whereas percentages of warm-water species are fluctuating (Figure 4).

Summary and conclusions

Reconstruction of palaeoceanographic and palaeoclimatic changes during the last 1200 years on the West Greenland shelf is based on diatom assemblages in a high-resolution sedimentary record at site GA306-4 in Holsteinsborg Dyb.

There is a reasonably high degree of similarity between changes in the diatom species of boxcore BC4 (35 cm long) and instrumental air and sea-surface temperature data from West Greenland during the last 70 years, demonstrating the reliability of diatoms (particularly sea-ice species and warm-water species) as a tool for the interpretation of palaeoceanographic and climatic changes through longer pre-instrumental time series in this area.

The decline of the sea-ice species throughout gravity core GC4 (500 cm long) shows that there has been a general cooling trend during the time interval ad 715–1760 on the West Greenland shelf (Figure 4). A relatively warm interval (assemblage zones I and II) with diatom indication of strong influence of Atlantic waters of the Irminger Current is found between about ad 800 and 1330, corresponding in time to the so-called ‘Medieval Warm Period’ (MWP). Increased oceanographic variability is recorded in the upper part of this interval (zone II, ad 1000–1330).

A marked increase in sea-ice diatoms at ad 1330 corresponds in time to the climatic shift from the MWP to the so-called ‘Little Ice Age’ (LIA). Quasi-periodic changes in the diatom assemblages during the LIA (zone III) with three particularly cold intervals during the LIA (ad 1330–1350, ad 1400–1575 and ad 1660–1710) coincide with the minima in sun spot activity (Wolf, Spörer and Maunder).

During the last 70 years (core BC4, 35 cm long), two relatively warm intervals are indicated by generally high contents of warm-water diatom species, the first one from the mid-1930s to the early 1960s, and the second one from mid-1990s to the present (Figure 3). A relatively cold period from the early 1960s to mid-1990s, reflected by the decline of warm-water species and an increase in sea-ice species and Arctic species, is a result of enhanced influence of Polar waters and sea-ice cover west of Greenland. In some respects, the twentieth-century oceanography off West Greenland corresponds to that in the MWP, but the stratification of the water masses appears to have remained as indicated for the LIA.

The palaeoceanographic and climatic changes revealed by the diatom species distribution on the West Greenland shelf coincide with the general pattern of palaeoceanographic and atmospheric changes in the North Atlantic region (Figure 5). The shift in diatom indication to increased sea-ice cover and water-column stratification at the transition from the MWP to the LIA west of Greenland at ad 1330 also coincides with a change from a relatively high reconstructed winter NAO index to a generally low NAO index at the MWP/LIA transition.

During the last 70 years, there is a high degree of consistency between the temperature indication of the diatoms off West Greenland and the AMO index, whereas the relation between sea-surface temperature and the winter NAO index appears to be more complex.

Footnotes

Acknowledgements

The core material was obtained from the Galathea 3 cruise in 2006, and we are grateful to the crew for coring operations onboard R/V Vædderen. We would like to thank Jan Heinemeier, Aarhus University, Denmark, for providing the ¹⁴C age determinations and Thorbjørn Joest Andersen, University of Copenhagen, Denmark, for the lead-210 datings. We are grateful to three anonymous referees for valuable comments.

We acknowledge financial support from the National Natural Science Foundation of China (Grant 40976115, 41076038), the Fund for Creative Research Groups of China (No. 40721004), the European Union 7th Framework project No. 243908 (Past4Future), and the Danish Natural Science Research Council project 09-072321 (GREEN-ICE).

References

Abril

(2004) Constraints on the use of 137Cs as a time-marker to support CRS and SIT chronologies. Environmental Pollution 129: 31–37.

Andersen

OGN

(1981) The annual cycle of temperature, salinity, currents and water masses in Disko Bugt and adjacent waters, West Greenland. Meddelelser om Grønland, Bioscience 5: 1–36.

Anderson

Clarke

Juhler

McGowan

Renberg

(2000) Coring of laminated lake sediments for pigment and mineral magnetic analyses, Søndre Strømfjord, southern West Greenland. Geology of Greenland Survey, Bulletin 186: 83–87.

Anderson

Harriman

Ryves

Patrick

(2001) Dominant factors controlling the major ion composition of West Greenland lakes. Arctic, Antarctic and Alpine Research 33: 418–425.

Appleby

(2001) Chronostratigraphic techniques in recent sediments. In: Last

Smol

(eds) Tracking Environmental Change Using Lake Sediments. Volume 1: Basin Analysis, Coring and Chronological Techniques. Dordrecht: Kluwer Academic Publishers, 171–203.

Arneborg

Heinemeier

Lynnerup

Nielsen

Rud

Sveinbjörnsdóttir

ÁE

(1999) Change of diet of the Greenland Vikings determined from stable carbon isotope analysis and 14C dating of the bones. Radiocarbon 41: 157–168.

Bard

Raisbeck

Yiou

Jouzel

(2000) Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus B 52: 985–992.

Bard

Raisbeck

Yiou

Jouzel

(2007) Comment on ‘Solar activity during the last 1000 yr inferred from radionuclide records’ by Muscheler et al. (2007). Quaternary Science Reviews 26: 2301–2308.

Barron

Bukry

(2007) Solar forcing of Gulf of California climate during the past 2000 yr suggested by diatoms and silicoflagellates. Marine Micropaleontology 62: 115–139.

10.

Belkin

Levitus

Antonov

Malmberg

S-Aa

(1998) ‘Great Salinity Anomalies’ in the North Atlantic. Progress in Oceanography 41: 1–68.

11.

Bennike

(2000) Palaeoecological studies of Holocene lake sediments from west Greenland. Palaeogeography, Palaeoclimatology, Palaeoecology 155: 285–304.

12.

Birks

CJA

Koç

(2002) A high-resolution diatom record of late-Quaternary sea-surface temperatures and oceanographic conditions from the eastern Norwegian Sea. Boreas 31: 323–344.

13.

Bradley

Hughes

Diaz

(2003) Climate in Medieval time. Science 302: 404–405.

14.

Brodersen

Anderson

(2000) Subfossil insect remains (Chironomidae) and lake-water temperature inference in the Sisimiut–Kangerlussuaq region, southern West Greenland. Geology of Greenland Survey, Bulletin 186: 78–82.

15.

Buch

(2000a) A Monograph on the Physical Oceanography of Greenland Waters. Danish Meteorological Institute, Scientific Report 00-12, 1–405.

16.

Buch

(2000b) Air–sea–ice conditions off southwest Greenland, 1981–97. Journal of Northwest Atlantic Fishery Science 26: 1–4.

17.

Buch

(2002) Present Oceanographic Conditions in Greenland Waters. Danish Meteorological Institute, Scientific Report 02-02, 1–39.

18.

Buch

Pedersen

Ribergaard

(2004) Ecosystem variability in West Greenland waters. Journal of Northwest Atlantic Fishery Science 34: 13–28.

19.

Buckland

Amorosi

Barlow

Dugmore

Mayewski

McGovern

. (1996) Bioarchaeological and climatological evidence for the fate of Norse farmers in medieval Greenland. Antiquity 70: 88–96.

20.

Cappelen

(2007) DMI Annual Climate Data Collection 1873–2006, Denmark, The Faroe Islands and Greenland – With Graphics and Danish Summary. Danish Meteorological Institute, Technical Report 07-05, 1–23.

21.

Cappelen

Jørgensen

Laursen

Stannius

Thomsen

(2001) The Observed Climate of Greenland, 1958–99 with Climatological Standard Normals, 1961–90. Danish Meteorological Institute, Technical Report 00-18, 1–154.

22.

Cremer

(1998) The diatom flora of the Laptev Sea (Arctic Ocean). Bibliotheca Diatomologica 40: 5–169.

23.

Dahl-Jensen

Mosegaard

Gundestrup

Clow

Johnsen

Hansen

. (1998) Past temperatures directly from the Greenland ice sheet. Science 282: 268–271.

24.

Davis

Kluever

Haines

(1998) Elevation change of the Southern Greenland Ice Sheet. Science 279: 2086–2088.

25.

Dawson

Elliott

Mayewski

Lockett

Noone

Hickey

. (2003) Late-Holocene North Atlantic climate ‘seesaws’, storminess changes and Greenland ice sheet (GISP2) palaeoclimates. The Holocene 13: 381–392.

26.

De Sève

(1999) Transfer function between surface sediment diatom assemblages and sea-surface temperature and salinity of the Labrador Sea. Marine Micropaleontology 36: 249–267.

27.

De Sève

Dunbar

(1990) Structure and composition of ice algal assemblages from the Gulf of St Lawrence, Magdalen Island area. Canadian Journal of Fisheries and Aquatic Sciences 47: 780–788.

28.

Desloges

Gilbert

Nielsen

Christiansen

Rasch

Øhlenschläger

(2002) Holocene glacimarine sedimentary environments in fiords of Disko Bugt, West Greenland. Quaternary Science Reviews 21: 947–963.

29.

Dickson

Meincke

Malmberg

Lee

(1988) The ‘great salinity anomaly’ in the Northern North Atlantic 1968–1982. Progress in Oceanography 20: 103–151.

30.

Eddy

(1978) ‘The Maunder Minimum’. Science 192: 1189–1202.

31.

Eiríksson

Knudsen

Haflidason

Heinemeier

(2000) Chronology of late Holocene climatic events in the northern North Atlantic based on AMS 14C dates and tephra markers from the volcano Hekla, Iceland. Journal of Quaternary Science 15: 573–580.

32.

Enfield

Mestas-Nunez

Trimble

(2001) The Atlantic Multidecadal Oscillation and its relation to rainfall and river flows in the continental U.S. Geophysical Research Letters 28: 2077–2080.

33.

Goosse

Mann

Renssen

(2008) What we can learn from combining paleoclimate proxy data and climate model simulations of past centuries? In: Battarbee

Binney

(eds) Natural Climate Variablity and Global Warming: A Holocene Perspective. Oxford: Blackwell, 163–188.

34.

Gowan

Ryves

Anderson

(2003) Holocene records of effective precipitation in West Greenland. The Holocene 13: 239–249.

35.

Grove

(1996) The century time-scale. In: Driver

Chapman

(eds) Time-scales and Environmental Change. London: Routledge, 39–87.

36.

Håkansson

(1984) The recent diatom succession of Lake Havgårdssjön, South Sweden. In: Mann

(ed.) Proceedings of the Seventh International Diatom Symposium. Koenigsstein: Otto Koeltz, 411–429.

37.

Hansen

Buch

Gregersen

(2004) Weather, Sea and Ice Conditions Offshore West Greenland – Focusing on New License Areas 2004. Danish Meteorological Institute, Scientific Report 04, 1–31.

38.

Hasle

Syvertsen

(1997) Marine diatoms. In: Tomas

(ed.) Identifying Marine Phytoplankton. San Diego: Academic Press Inc., 5–385.

39.

Hass

(1996) Northern Europe climate variations during late Holocene: Evidence from marine Skagerrak. Palaeogeography, Palaeoclimatology, Palaeoecology 123: 121–145.

40.

Heimdal

(1989) Arctic ocean phytoplankton. In: Herman

(ed.) The Arctic Seas. Climatology, Oceanography, Geology and Biology. Van Nostrand Reinhold Company, 193–222.

41.

Hendey

(1964) An introductory account of the smaller algae of British coastal waters: Part V. Bacillariophyceae (Diatoms). Fishery Investigations, London. Series 4: 1–317.

42.

Holland

Thomas

De Young

Ribergaard

Lyberth

(2008) Acceleration of Jakobshavn Isbrae triggered by warm subsurface ocean waters. Nature Geoscience 1: 659–664.

43.

Hughen

Baillie

MGL

Bard

Beck

Bertrand

CJH

Blackwell

. (2004) Marine04 marine radiocarbon age calibration, 0–26 cal kyr BP. Radiocarbon 46: 1059–1086.

44.

Humlum

(1999) Late-Holocene climate in central West Greenland: Meteorological data and rock-glacier isotope evidence. The Holocene 9: 581–594.

45.

Hurrell

(1995) Decadal trends in the North Atlantic Oscillation regional temperatures and precipitation. Science 269: 676–679.

46.

Hurrell

van Loon

(1997) Decadal variations in climate associated with the North Atlantic Oscillation. Climatic Change 36: 301–326.

47.

Jennings

Weiner

(1996) Environmental changes in eastern Greenland during the last 1300 years: Evidence from foraminifera and lithofacies in Nansen Fjord, 68°N. The Holocene 6: 179–191.

48.

Jennings

Knudsen

Hald

Hansen

Andrews

(2002) A mid-Holocene shift in Arctic sea-ice variability on the East Greenland Shelf. The Holocene 12: 49–58.

49.

Jensen

(2003) Holocene hydrographic changes in Greenland coastal waters: Reconstructing environmental change from sub-fossil and contemporary diatoms. PhD. Thesis, University of Copenhagen. Danmarks Geologiske Undersøgelse, Rapport 58, 1–160.

50.

Jensen

Kuijpers

Koç

Heinemeier

(2004) Diatom evidence of hydrographic changes and ice conditions in Igaliku Fjord, South Greenland, during the past 1500 years. The Holocene 14: 152–64.

51.

Jensen

Hansen

Secher

Steenfelt

Schjøth

Rasmussen

(2002) Kimberlites and other ultramafic alkaline rocks in the Sisimiut–Kangerlussuaq region, southern West Greenland. Geology of Greenland Survey, Bulletin 191: 57–66.

52.

Jiang

(1996) Diatoms from the surface sediments of the Skagerrak and Kattegat and their relationship to the spatial changes of environmental variables. Journal of Biogeography 23: 129–137.

53.

Jiang

Eiríksson

Schulz

Knudsen

Seidenskrantz

M-S

(2005) Evidence for solar forcing of sea-surface temperature on the North Icelandic shelf during the late Holocene. Geology 33: 73–76.

54.

Jiang

Seidenkrantz

M-S

Knudsen

Eiríksson

(2001) Diatom surface sediment assemblages around Iceland and their relationships to oceanic environmental variables. Marine Micropaleontology 41: 73–96.

55.

Jiang

Seidenkrantz

M-S

Knudsen

Eiríksson

(2002) Late Holocene summer sea-surface temperatures based on a diatom record from the north Icelandic shelf. The Holocene 12: 137–147.

56.

Jiang

Zheng

Ran

Seidenkrantz

M-S

(2004) Diatoms from the surface sediments of the South China Sea and their relationships to modern hydrography. Marine Micropaleontology 53: 279–292.

57.

Johnsen

Clausen

Dansgaard

Fuhrer

Gundestrup

Hammer

. (1992) Irregular glacial interstadials recorded in a new Greenland ice core. Nature 359: 311–313.

58.

Johnsen

Dahl-Jensen

Gundestrup

Steffensen

Clausen

Masson-Delmotte

. (2001) Oxygen isotope and palaeotemperature records from six Greenland ice-core stations: Camp Century, Dye-3, GRIP, GISP2, Renland and NorthGRIP. Journal of Quaternary Science 16: 299–307.

59.

Jones

Mann

(2004) Climate over past millennia. Reviews of Geophysics 42: 1–42.

60.

Jones

Jonsson

Wheeler

(1997) Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and south-west Iceland. International Journal of Climatology 17: 1433–1450.

61.

Jones

Osborn

Briffa

(2001) The evolution of climate over the last millennium. Science 292: 662–667.

62.

Kaplan

Wolfe

Miller

(2002) Holocene environmental variability in southern Greenland inferred from lake sediments. Quaternary Research 58: 149–159.

63.

Karentz

Smayda

(1984) Temperature and seasonal occurrence patterns of 30 dominant phytoplankton species in Narragansett Bay over a 22-year period (1959–1980). Marine Ecology Progress Series 18: 277–293.

64.

Kasper

Allard

(2001) Late-Holocene climatic changes as detected by the growth and decay of ice wedges on the southern shore of Hudson Strait, northern Québec, Canada. The Holocene 11: 563–577.

65.

Keigwin

Pickart

(1999) Slope water current over the Laurentian Fan on interannual to millennial time scales. Science 286: 520–523.

66.

Kerr

(2000) A North Atlantic climatic pacemaker for the centuries. Science 288: 1984–1986.

67.

Knudsen

Eiríksson

Jansen

Jiang

Rytter

Gudmundsdóttir

(2004) Palaeoceanographic changes off North Iceland through the last 1200 years: Foraminifera, stable isotopes, diatoms and ice rafted debris. Quaternary Science Reviews 23: 2231–2246.

68.

Knudsen

Eiríksson

Jiang

Jónsdóttir

(2009) Palaeoceanography and climate change off North Iceland during the last millennium: Comparison of foraminifera, diatoms and ice-rafted debris with instrumental and documentary data. Journal of Quaternary Science 24: 457–468.

69.

Knudsen

Stabell

Seidenkrantz

M-S

Eiríksson

Blake

Jr (2008) Deglacial and Holocene conditions in northernmost Baffin Bay: Sediments, foraminifera, diatoms and stable isotopes. Boreas 37: 346–376.

70.

Koç Karpuz

Schrader

(1990) Surface sediment diatom distribution and Holocene paleotemperature variations in the Greenland, Iceland and Norwegian Sea. Paleoceanography 5: 557–580.

71.

Krawczyk

Witkowski

Moros

Lloyd

Kuijpers

Kierzek

(2010) Late-Holocene diatom-inferred reconstruction of temperature variations of the West Greenland Current from Disko Bugt, central West Greenland. The Holocene 20: 659–666.

72.

Lamb

(1965) The early Medieval warm epoch and its sequel. Palaeogeography, Palaeoclimatology, Palaeoecology 1: 13–37.

73.

Lassen

Kuijpers

Kunzendorf

Hoffmann-Wieck

Mikkelsen

Konradi

(2004) Late-Holocene Atlantic bottom-water variability in Igaliku Fjord, South Greenland, reconstructed from foraminifera faunas. The Holocene 14: 165–171.

74.

Levac

de Vernal

Blake

(2001) Sea-surface conditions in northernmost Baffin Bay during the Holocene: Palynological evidence. Journal of Quaternary Science 16: 353–363.

75.

Lloyd

(2006) Late Holocene environmental change in Disko Bugt, west Greenland: Interaction between climate, ocean circulation and Jakobshavn Isbrae. Boreas 35: 35–49.

76.

Lloyd

Kuijpers

Long

Moros

Park

(2007) Foraminiferal reconstruction of mid- to late-Holocene ocean circulation and climate variability in Disko Bugt, West Greenland. The Holocene 17: 1079–1091.

77.

Lloyd

Park

Kuijpers

Moros

(2005) Early Holocene palaeoceanography and deglacial chronology of Disko Bugt, West Greenland. Quaternary Science Reviews 24: 1741–1755.

78.

Lykke-Andersen

Knudsen

(2007) Geologien i Holsteinsborg Dyb. Geoviden: Geologi og Geografi 3: 6–7.

79.

Mann

Bradley

Hughes

(1999) Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759–762.

80.

Medlin

Priddle

(1990) Polar Marine Diatoms. Cambridge: British Antarctic Survey, 190 pp.

81.

Møller

Jensen

Kuijpers

Aagard-Sørensen

Seidenkrantz

M-S

Prins

. (2006) Late-Holocene environment and climatic changes in Ameralik Fjord, southwest Greenland: Evidence form the sedimentary record. The Holocene 16: 685–695.

82.

Moros

Jensen

Kuijpers

(2006) Mid to late Holocene variability in Disko Bugt, central West Greenland. The Holocene 16: 357–367.

83.

Mortensen

Valdimarsson

(1999) Thermohaline Changes in the Irminger Sea. ICES CM 1999/L:16, 11 pp.

84.

Mudie

Rochon

Levac

(2005) Decadal-scale sea ice changes in the Canadian Arctic and their impacts on humans during the past 4,000 years. Environmental Archaeology 10: 113–126.

85.

Muscheler

Joos

Beer

Müller

PSA

Vonmoos

Snowball

(2007) Solar activity during the last 1000 yr inferred from radionuclide records. Quaternary Science Reviews 26: 82–97.

86.

Myers

Kulan

Ribergaard

(2007) Irminger water variability in the West Greenland Current. Geophysical Research Letters 34: 1–6.

87.

Ogilvie

AEJ

(1998) Historical accounts of weather events, sea ice and related matters in Iceland and Greenland, ad c. 1250 to 1430. In: Frenzel

(ed.) Documentary Climatic Evidence for 1750–1850 and the 14th Century. Special issue: ESF Project ‘European Palaeoclimate and Man’ 15, European Science Foundation, Strasbourg, Paläoklimaforschung 23: 25–43.

88.

Ogilvie

AEJ

Barlow

Jennings

(2000) North Atlantic climate c. A.D. 1000: Millennial reflections on the Viking discoveries of Iceland, Greenland and North America. Weather 55: 34–45.

89.

Osborn

(2011) Winter 2009/2010 temperatures and a record-breaking North Atlantic Oscillation index. Weather 66: 19–21.

90.

Osborn

Briffa

(2006) The spatial extent of 20th-century warmth in the context of the past 1200 years. Science 311: 841–844.

91.

Osborn

Briffa

Tett

SFB

Jones

Trigo

(1999) Evaluation of the North Atlantic Oscillation as simulated by a coupled climate model. Climate Dynamics 15: 685–702.

92.

Qian

Corte-Real

(2000) Is the North Atlantic Oscillation the most important atmospheric pattern for precipitation in Europe? Journal of Geophysical Research 105: 910–910.

93.

Ramsey

(2009) Bayesian analysis of radiocarbon dates. Radiocarbon 51: 337–360.

94.

Ran

Jiang

Knudsen

Eiríksson

(2011) Diatom based reconstruction of palaeoceanographic changes on the North Icelandic shelf during the last millennium. Palaeogeography, Palaeoclimatology, Palaeoecology 302: 109–119.

95.

Reimer

(2005) Marine Reservoir Correction Database. Available from http://intcal.qub.ac.uk/marine/ (accessed November 2005).

96.

Ren

Jiang

Seidenkrantz

M-S

Kuijpers

(2009) A diatom-based reconstruction of Early Holocene hydrographic and climatic change in a southwest Greenland fjord. Marine Micropaleontology 70: 166–176.

97.

Ribergaard

Buch

(2005) Oceanographic Investigations Off West Greenland 2004. NAFO Scientific Council Documents 05/019, 1–31.

98.

Ribergaard

Olsen

Mortensen

(2008) Oceanographic Investigations off West Greenland 2007. NAFO Scientific Council Documents 08/003, 1–23.

99.

Roncaglia

Kuijpers

(2004) Palynofacies analysis and organic-walled dinoflagellate cysts in late Holocene sediments from Igaliku Fjord, South Greenland. The Holocene 14: 172–184.

100.

Rosenørn

Fabricius

Buch

Horsted

(1985) Record-hard Winters at West Greenland. NAFO Scientific Council Documents 61, 17 pp.

101.

Ryves

Mcgowan

Anderson

(2002) Development and evaluation of a diatom-conductivity model from lakes in West Greenland. Freshwater Biology 47: 995–1014.

102.

Sancetta

(1982) Distribution of diatom species in surface sediments of Bering and Okhotsk seas. Micropaleontology 28: 221–257.

103.

Schröder

(2005) Case Studies on the Spörer, Maunder, and Dalton Minima. Bremen: Science Edition, 65 pp.

104.

Seidenkrantz

M-S

Aagaard-Sørensen

Sulsbrück

Kuijpers

Jensen

Kunzendorf

(2007) Hydrography and climate of the last 4400 years in a SW Greenland fjord: Implications for Labrador Sea palaeoceanography. The Holocene 17: 387–401.

105.

Seidenkrantz

M-S

Roncaglia

Fischel

Heilmann-Clausen

Kuijpers

Moros

(2008) Variable North Atlantic climate seesaw patterns documented by a late Holocene marine record from Disko Bugt, West Greenland. Marine Micropalaeontology 68: 66–83.

106.

Shindell

Schmidt

Mann

Rind

Waple

(2001) Solar forcing of regional climate change during the Maunder Minimum. Science 294: 2149–2152.

107.

Soon

Baliunas

(2003) Proxy climatic and environmental changes of the past 1000 years. Climate Research 23: 89–110.

108.

Stein

(2005) North Atlantic subpolar gyre warming – Impacts on Greenland offshore waters. Journal of Northwest Atlantic Fishery Science 36: 43–54.

109.

Stuiver

Braziunas

(1993) Modeling atmospheric 14C influences and 14C ages of marine samples to 10,000 BC. In: Stuiver

Long

Kra

(eds) Calibration 1993. Radiocarbon 35: 137–189.

110.

Tang

CCL

Ross

Yao

Petrie

DeTracey

Dunlao

(2004) The circulation, water masses and sea-ice of Baffin Bay. Progress in Oceanography 63: 183–228.

111.

Trouet

Esper

Graham

Baker

Scourse

Frank

(2009) Persistent positive North Atlantic Oscillation mode dominated the Medieval climate anomaly. Science 324: 78–80.

112.

Underbjerg

(2009) Den oceanografiske og klimatiske udvikling gennem Middelalderen og Den Lille Istid i Holsteinsborg Dyb, Vestgrønland, baseret på foraminiferer. Unpublished MSc Thesis, University of Aarhus, 89 pp.

113.

Vinther

Andersen

Jones

Briffa

Cappelen

(2006) Extending Greenland temperature records into the late eighteenth century. Journal of Geophysical Research 111: D11105, doi:10.1029/2005JD00681010.1029/2005JD006810.

114.

Vinther

Buchardt

Clausen

Dahl-Jensen

Johnsen

Fisher

. (2009) Holocene thinning of the Greenland ice sheet. Nature 461: 385–388.

115.

Von Quillfeldt

(1996) Ice algae and phytoplankton in North Norwegian and Arctic waters: Species composition, succession and distribution. PhD. thesis, The Norwegian College of Fishery Science, University of Tromsø.

116.

Von Quillfeldt

(2000) Common diatom species in Arctic spring blooms: Their distribution and abundance. Botanica Marina 43: 499–516.

117.

Williams

Bradley

(1985) Paleoclimatology of the Baffin Bay region. In: Andrews

(ed.) Quaternary Environments: Eastern Canadian Arctic, Baffin Bay and Western Greenland. Boston: Allen & Unwin, 741–772.

118.

Witak

Wachnicka

Kuijpers

Troelstra

Prins

Witkowski

(2005) Holocene North Atlantic surface circulation and climatic variability: Evidence from diatom records. The Holocene 15: 85–96.

119.

Zweng

Münchow

(2006) Warming and freshening of Baffin Bay, 1916–2003. Journal of Geophysical Research 111: C07016, doi:10.1029/2005JC00309310.1029/2005JC003093.