Accretion rates in coastal wetlands of the southeastern Gulf of California and their relationship with sea-level rise

Abstract

Sea-level rise (SLR) is one of the most conspicuous examples of the environmental impact of recent climate change. Since SLR rates are not uniform around the planet, local and regional data are needed for proper adaptation plans. ²¹⁰Pb-dated sediment cores were analyzed to determine the trends of sediment accretion rates (SARs) at three tropical saltmarshes in the Estero de Urias lagoon (Gulf of California, Mexico), in order to estimate the SLR trends during the past ~100 years, under the assumption that these ecosystems accrete at a similar rate to SLR. A chemometric approach, including multivariate statistical analysis (factor analysis) of geochemical data (including δ¹³C; δ¹⁵N; C/N ratios; and Br, Na, and Cl as proxies for marine transgression) was used to identify the marine transgression in the sediment records. Based on core geochemistry, only one of the three cores provided a long-term record attributable to marine transgression. SLR trends, estimated from SARs, showed increasing values, from a minimum of 0.73 ± 0.03 mm yr⁻¹ at the beginning of the 20th century and up to 3.87 ± 0.12 mm yr⁻¹ during the period 1990–2012. The estimated SLR trend between 1950 and 1970 was comparable to the tide gauge records in Mazatlan City for the same period. Results showed the caveats and strengths of this methodology to reconstruct decadal SLR trends from the sedimentary record, which can be used to estimate long-term SLR trends worldwide in regions where monitoring data are scarce or absent.

Keywords

factor analysis Gulf of California sea-level rise sediment accretion rates tropical saltmarsh

Introduction

Mean sea level (MSL) has decreased from 200 m above present levels at 100 Myr BP (Haq et al., 1987) to nearly present levels at 3 Myr BP, and during the last million years MSL has oscillated with an amplitude of about 100 m following glacial–interglacial cycles (Donoghue, 2011). During the last interglacial (c. 120 kyr BP), MSL was about 5–9 m above present levels (Anderson et al., 2014), and coral records in the Yucatan Peninsula (Mexico) indicated a 2- to 3-m abrupt increase at 121 kyr BP (Blanchon et al., 2009). During the early Holocene (about 6 kyr ago), MSL was about 20 m below present levels (Donoghue, 2011). In certain intervals, during the last deglaciation, the sea-level rise (SLR) rate was close to 45 mm yr⁻¹ (Blanchon and Shaw, 1995).

Today, climate-induced SLR is one of the most noticeable outcomes of recent global change, and it is mainly caused by the thermal expansion of seawater and melting of land-based ice (Nicholls, 2011, and references therein), although other human-induced processes, such as the reduction of liquid water storage on land (e.g. groundwater depletion), might also contribute to global SLR (Wada et al., 2012). The analysis of sea-level data from a global network of tide gauges (Hay et al., 2015) indicated a global mean SLR of 1.2 ± 0.2 mm yr⁻¹ from 1901 to 1990 and of 3.0 ± 0.7 mm yr⁻¹ from 1993 to 2010. For the worst case scenario (Representative Concentration Pathway (RCP) 8.5), it is projected that at the end of the 21st century, the global mean SLR rate will range from 8 to 16 mm yr⁻¹ (Church et al., 2013). However, because of non-uniform variations in temperature and salinity, changes in the ocean circulation, local or regional vertical crustal movements, differential resistance to erosion, varying wave climates, and changeable longshore currents, the sea-level change is not geographically uniform (Gornitz, 1991). Considering that 23% of the world’s human population is established in the coastal zones (<100 km distance from the coast and <100 m above MSL) and population densities are three times higher than the global average (Small and Nicholls, 2003), it is important to increase knowledge of the spatial variability of SLR trends around the world and to improve the assessment of potential impacts in order to plan adaption strategies at the local and regional scale.

Dated sediment cores collected from low-energy microtidal coastal wetlands (e.g. mangroves, saltmarshes, and flood plains) have been successfully used to track sea-level changes over different timescales (Lynch et al., 1989, and references therein; Chmura et al., 2001; Cundy and Croudace, 1996; Smoak et al., 2013; Smoak and Patchineelam, 1999; Teasdale et al., 2011) under the assumption that the continued existence of these ecosystems in place depends on their ability to maintain a vertical accretion rate, sustained by the accumulation of allochthonous and autochthonous sediments, at least equal to the SLR rate.

Tropical saltmarshes are usually found along semi-sheltered low-energy coastlines, protected from the open ocean, located behind the mangrove fringe, in the highest possible topographic position within the tidal range, so they are intermittently inundated by medium to high tides. The high evaporation and relatively infrequent flooding favor the formation of hypersaline soils that can be colonized by halophyte ‘glassworts’ vegetation (i.e. Batis maritima and Salicornia pacifica) that can tolerate inundation with seawater and high soil salinity (Costa et al., 2009; Flores-Verdugo et al., 2007). Saltmarsh sediments originate from upland runoff, organic production within the wetland, and suspended marine particles transported by the tide water that settle out and accumulate around plant stems (USFSW, 2014) or following microtopography.

The study of buried foraminiferal and ostracode assemblages in dated sediment cores is commonly used as evidence of marine transgression and to determine regional SLR rates (Cearreta et al., 2003; Murray-Wallace and Woodroffe, 2014). However, this methodology can be problematic in tropical intertidal environments because of poor preservation of tests of agglutinated foraminifera which dominate these areas (Perry et al., 2008). Since intertidal sediments are made of terrestrial and marine sources, the sediments display physical and chemical characteristics of both domains; thus, marine transgression can be identified in the sedimentary record using geochemical proxies for marine organic matter, such as δ¹³C and δ¹⁵N (Wilson et al., 2005), and the concentrations of elements associated with saltwater inundation, such as Br (Ziegler et al., 2008) and Na and Cl (Chagué-Goff et al., 2011).

²¹⁰Pb sediment dating is a widely used method to determine core chronologies during the last 100–150 years, and it is amply used to reconstruct the trends of global change impacts in the coastal environment, such as heavy-metal contamination and effects of land use change (Ruiz-Fernández et al., 2012), the incidence of harmful algae blooms (Sanchez-Cabeza et al., 2012), the variability of mangrove organic carbon burial rates (Breithaupt et al., 2012), or variations in sediment accretion rates (SARs) in response to hydrological changes, including SLR (Lynch et al., 1989).

The objective of this study was to evaluate the accretion rate trends recorded in ²¹⁰Pb-dated sediment cores, collected from three tropical saltmarshes at the Estero de Urias Lagoon (EUL), with the purpose to understand sea-level changes during the past 100 years and contribute data that can be used for the estimation of future SLR at the regional scale. The ²¹⁰Pb-derived accretion trends were compared with the reported SLR rate from a local tide gauge record in order to validate the usefulness of the proposed methodology, which could be replicated at locations lacking long-term SLR historical information, but where undisturbed sediments are available from tidal wetlands not affected by major anthropogenic influence.

Study area

The EUL is located on the Pacific Coast of Mexico, at the entrance of the Gulf of California (Figure 1). It is a shallow and narrow inner shelf barrier-type lagoon (Lankford, 1977) with a free and permanent exchange with the sea (<2 m up to ~12 m depth, <1 km width, 17 km in length, total surface of ~18 km²; salinity range of 25.8–38.4 psu; Páez-Osuna et al., 1990). EUL has a semidiurnal tidal cycle, with maximum spring tides of 1.8 m and minimum neap tides of 0.3 m (Cardoso-Mohedano et al., 2015). The climate in the study area is warm and humid with a mean temperature of 25°C (extreme values ranging from 7.5°C to 39°C; SMN, 2014) and summer rains (precipitation range of 700–1300 mm; INEGI, 2009). Most of the year, the wind flows from the NW, although during the summer winds come from SE. The EUL coastal plain is composed by Quaternary alluvial soils (predominantly regosol) overlying Tertiary igneous rocks, for example, andesite, rhyolite, dacite, trachyte, rhyodacite, basalt, and tuff (Alba Cornejo et al., 1978). There is no information available about vertical crust velocities in the study area, although it is reported as a low seismicity zone (SSN, 2014).

Figure 1.

Location of sampling sites (•) in saltmarshes of the Estero de Urias Lagoon, Gulf of California.

The EUL is adjacent to Mazatlan, a medium-sized city (438,434 inhabitants) which has an economy mainly based on fisheries, tourism, and port activities. Mazatlan has the most important shrimp fleet of the Mexican Pacific; it hosts one of the largest tuna canneries and cold storage points in Mexico; it is ranked as the sixth most popular beach destination of the country; and it is a relevant cabotage and overseas port in the Mexican Pacific, predominantly for the carriage of petroleum products. Studies in EUL report contamination by trace elements, pesticides, and polychlorinated biophenyls (Raygoza-Viera et al., 2014, and references therein) which have been associated with point discharges of untreated waste waters from the human settlements, the industrial and port facilities, and shrimp farms established around the EUL.

The inner area of EUL is surrounded by a mangrove forest colonized by Rhizophora mangle, Laguncularia racemosa, and Avicennia germinans which include tropical saltmarshes (locally known as ‘marismas’) at the high intertidal zone, which are intermittently inundated during high tide, and are characterized by hypersaline conditions (>80 psu) and scarce halophyte vegetation (Salicornia spp and Batis maritima).

Materials and methods

Sampling

Three sediment push cores (EUI, EUII, and EUIII), ranging from 47 to 50 cm in length, were collected with polyvinyl chloride (PVC) tubes (10 cm in diameter) in May 2012. The sampling sites were in flat and unvegetated saltmarsh areas (Figure 1), far away from human and cattle access. The elevation (a.s.l.) of the sampling sites was determined with a total station Topcon GTS-105N-1, with a resolution of 1 mm; the baseline was established with a GPS station Topco Hiper GGD-1, relative to the MSL in Mazatlan City, established by the National Sea Level Service of the Universidad Nacional Autónoma de Mexico. The elevations determined were EUI = 0.197 m, EUII = 1.029 m, and EUIII not available (although it was collected from a damp area colonized by benthic algae, which suggested more frequent inundation and therefore lower elevation, than EUI).

Laboratory analysis

The sediment cores were cut in 1-cm-thick sections. Neither laminations nor evidences of sediment disturbance (sediment cracks, gas bubbles, plant roots, and infaunal burrows) were observed. Sediment samples were freeze-dried and ground to powder with a porcelain mortar (except samples for grain size analysis). Grain size distribution (percentage of sand, silt, and clay fractions) was determined by laser diffraction with a Malvern Mastersizer 2000 in sediments treated with 30% H₂O₂ to destroy organic matter. Organic matter (OM) and CaCO₃ percentages were estimated by loss on ignition at 550°C and 950°C (LOI₅₅₀ and LOI₉₅₀), respectively. Magnetic susceptibility (MS, SI units) was analyzed to identify changes in sediment provenance along the core, and it was determined with a Bartington MS2G sensor.

Elemental composition and the isotopic ratios δ¹³C and δ¹⁵N of OM were used to identify the fingerprint of marine transgression in the accumulated sediments. For elemental composition analysis, samples were placed in low-density polyethylene cells (bottom covered with Prolene^® film), compressed manually using a Teflon^® rod, and analyzed by energy dispersive X-ray fluorescence spectrometry (EDXRF) using a secondary target instrumentation (XEPOS, Spectro) under He atmosphere. The elemental concentrations and isotopic composition of C and N were determined on acidified sediment samples (6% H₂SO₄) loaded into silver capsules, using a Carlo Erba NA1500 Series 2 elemental analyzer, coupled to a Finnigan Delta Plus isotope ratio mass spectrometer, via a Finnigan Conflo II open split interface (Stanford University Stable Isotope Laboratory). Replicate analysis carried out on EUIII untreated samples, treated with 1N HCl or 6% H₂SO₄, showed that most results were not sensitive to the acidification method. Results are presented in standard delta notation, with δ¹³C reported relative to the Vienna Pee Dee Belemnite (VPDB) carbonate standard and δ¹⁵N relative to air.

For foraminiferal and ostracod taxonomic analysis of the EUI and EUII cores, sediment samples were sieved through 2-mm (to remove large organic fragments) and 63-µm meshes and dried at room temperature. All the available tests and valves were picked and examined under a stereoscopic binocular microscope using reflected light.

²¹⁰Pb was determined through its radioactive descendant ²¹⁰Po by alpha spectrometry (Ortec Ametek 576A spectrometer), according to the method described by Ruiz-Fernández and Hillaire-Marcel (2009). The ²¹⁰Pb-derived chronologies were calculated using the constant flux (CF) model (Robbins, 1978; Sanchez-Cabeza and Ruiz-Fernández, 2012), and uncertainties were estimated with a Monte Carlo simulation with 30,000 iterations (Sanchez-Cabeza et al., 2014). Results were expressed in years of the Common Era (CE). SARs (mm yr⁻¹) were computed according to Lynch et al. (1989); thus, sediment depth was corrected for consolidation by normalizing each section’s bulk density to the average bulk density in the deeper sections, characterized by lower organic and water content and higher bulk density. The ²¹⁰Pb chronology was corroborated using the radiometric markers ¹³⁷Cs and plutonium isotopes. ¹³⁷Cs was determined by γ-ray spectrometry using an Ortec HPGe well detector (Ruiz-Fernández et al., 2014), and plutonium isotopes (²³⁹Pu and ²⁴⁰Pu) were determined by low-energy accelerator mass spectrometry (AMS) according to the method described by Chamizo et al. (2008).

Analytical quality control included the evaluation of analytical blanks, the assessment of precision by replicate analysis (n = 6), and the assessment of accuracy through the analysis of reference materials: IAEA-158, IAEA-405, and IAEA-433 for XRF; IAEA-300 for ²¹⁰Pb and ¹³⁷Cs; Bartington G-039 for MS; and Malvern QAS3002 for grain size. Results were within the reported range of the certified values. The coefficients of variation of the replicate analysis were <8% for XRF, LOI₅₅₀, and LOI₉₅₀; <5% for ²¹⁰Pb and grain size; <3% for MS; 0.05% for δ¹³C; 0.14% for C_org; 0.20% for δ¹⁵N; and 0.01% for N_org.

Factor analysis (FA) was performed to simplify the interpretation of the geochemical variables and infer sediment provenance. FA is a multivariate analysis technique useful to reduce the number of variables in a dataset by grouping them into a latent variable or factor (i.e. not directly measured) based on the correlation among the variables. Each factor becomes a latent variable that can be used to explain why these variables are grouped. FA of each sediment core involved 40 variables, including sediment characteristics (texture, MS, LOI₅₅₀ and LOI₉₅₀, and δ¹³C and δ¹⁵N) and sediment elemental composition.

Results

Sediment chronology

²¹⁰Pb_xs depth profiles showed decreasing trends with depth (Figure 2a–c). The departure of the ²¹⁰Pb activity profiles from exponential decay was attributed to variations in the sediment flux, according to the basic assumptions of the CF dating model. The ²¹⁰Pb fluxes determined from the cores ranged between 69 ± 2 and 83 ± 2 Bq m⁻² yr⁻¹; these values were consistent with the ²¹⁰Pb flux reported for the Culiacan River Estuary (only 200 km up north from EUL; 82 ± 9 Bq m⁻² yr⁻¹; Ruiz-Fernández et al., 2002) and with the ²¹⁰Pb atmospheric flux predicted by Turekian et al. (1977) for western North America at the same latitudinal region (83 Bq m⁻² yr⁻¹). These data supported that atmospheric deposition is the dominant pathway of ²¹⁰Pb supply to the sampling areas and the absence of erosion in the core. The maximum ages for each core were EUI = 127 ± 5 years, EUII = 78 ± 5 years, and EUIII = 125 ± 7 years. The activity ranges of ¹³⁷Cs (1–6.5 Bq kg⁻¹) in the three cores were very low, and the depth profiles did not show the peak value usually used to identify the maximum fallout period (1962–1964). However, in all sediment cores, the ¹³⁷Cs onset layer corresponded to the early 1950s, in agreement with the beginning of the nuclear tests in 1951 (Fehner and Gosling, 2006). The ^239+240Pu activities, only measured in core EUIII, were also very low (0.09–0.23 Bq kg⁻¹), and the depth profile (not shown) displayed trends decoupled from those of ¹³⁷Cs activities, so they were not useful to corroborate the ²¹⁰Pb-derived chronology. These low activities and decoupled trends could be explained by the low radioactive atmospheric fallout in the zone and the high ¹³⁷Cs solubility in seawater (Ruiz-Fernández and Hillaire-Marcel, 2009). The ²⁴⁰Pu/²³⁹Pu atom ratios (0.18 ± 0.01) were almost constant along EUIII core and comparable to the average global fallout ratio (0.17–0.19) which confirmed the atmospheric (and not local) origin of the Pu isotopes. The ¹³⁷Cs/^239+240Pu activity ratios (Figure 2d) were used to validate the ²¹⁰Pb chronology in sediment core EUIII, under the assumption that differences in geochemical behavior and provenance are responsible for the changes observed in Cs and Pu activities. Before and shortly after the Limited Test Ban Treaty (1963), the main source of both radionuclides was atmospheric fallout, but as it rapidly decreased (within months), the contaminated seawater became the main source of radionuclides to coastal sediments. As Cs is much more soluble than Pu in seawater, after 1963, the ¹³⁷Cs/²³⁹Pu ratio in seawater (and hence the supply to sediment) increased with time. The constant value of the ¹³⁷Cs/²³⁹Pu ratio observed below 15 cm depth (Figure 2d; 14 ± 3, N = 4) can be considered representative of the local bomb fallout ratio. The inflexion point, from which the ¹³⁷Cs/²³⁹Pu ratios started to increase toward the core surface, corresponded to 1966 ± 10 years and was compatible with the expected change of contamination pathways, thus confirming that sediment mixing was negligible.

Figure 2.

²¹⁰Pb dating in saltmarsh sediment cores EUI, EUII, and EUIII from the Estero de Urias Lagoon, Gulf of California: (a–c) ²¹⁰Pb activities depth profile, (d) ¹³⁷Cs/²³⁹Pu+²⁴⁰Pu ratio depth profile in core EUIII, and (e) ²¹⁰Pb-derived sediment accretion rates chronology.

The mass accumulation rate (MAR; g cm⁻² yr⁻¹) obtained for the cores ranged from 0.02 to 0.20 (EUI), 0.05 to 0.38 (EUII), and 0.02 to 0.15 (EUIII), and the consolidation corrected SAR (mm yr⁻¹) ranged from 0.75 to 6.5 (EUI), 0.32 to 2.7 (EUII), and 0.55 to 3.6 (EUIII). The SAR in the three cores (Figure 2e) showed little changes during the early 20th century (~1 mm yr⁻¹ up to the 1950s). Afterward, core EUII exhibited a stable SAR of ~2 mm yr⁻¹ up to the present and the SAR in EUI increased steadily up to a maximum of 6.5 ± 1.3 mm yr⁻¹, whereas core EUIII showed more variability, with the highest SAR values between the decades of 1960–1970 and 1990s.

Sediment provenance

Sediments in the EUI and EUIII cores were mostly silty-clayey (>54% of fine grained particles), had moderate C_org concentrations (between 11% and 18%), and low MS values (<8 SI). Core EUII sediments were mostly silty-sandy (25–60% of sand), had lower C_org content (<6%, most likely because of the higher sand content), and higher MS values (6–25 SI; Figure 3).

Figure 3.

(a) Depth distribution profiles of grain size; (b) magnetic susceptibility (MS); (c) organic carbon (C_org), organic nitrogen (N_org), and the dashed line represents modeled C_org concentrations; (d) δ¹³C and δ¹⁵N; and (e) C/N ratio in saltmarsh sediment cores EUI, EUII, and EUIII from the Estero de Urias Lagoon, Gulf of California.

The C_org concentration profile in core EUI showed a decreasing trend toward the surface of the core, whereas in cores EUII and EUIII, the profiles showed decreasing trends with depth. The early diagenetic processes of organic material in bottom sediments is a function of time, and in steady-state conditions, the amount of organic molecules decreases exponentially with time after deposition; hence, a model can be applied to describe the OM degradation, as well as to elucidate the OM concentration at the time of deposition (Berner, 1980). The use of the simple exponential decay model showed that in the three cores, the surface C_org concentrations were lower than expected if the C_org accumulation and decay had been at steady state, suggesting a recent diminution of C_org fluxes in the three cores (Figure 3). The range of C/N (10–40), δ¹³C (−26‰ to −20‰), and δ¹⁵N (4–8‰) was similar among the cores (Figure 3), with higher δ¹³C and δ¹⁵N, and lower C/N ratios toward the surface of the three cores. Terrestrial OM shows C/N ratios ⩾12 (Kukal, 1971), δ¹³C ratios lower than −27‰, and δ¹⁵N ratios from 0‰ to 10‰ (Gearing, 1988; Létolle, 1980), whereas marine derived OM is characterized by C/N ratios ⩽10 (Parsons, 1975), δ¹³C ratios from −17‰ to −24‰ (Gearing et al., 1984), and δ¹⁵N between 4‰ and 10‰ (Létolle, 1980). In addition, a general trend of heavier δ¹³C values in particulate organic carbon with increasing salinity seawards has been described (Yu et al., 2010, and references therein). In estuarine areas, OM results from the mixture of autochthonous (e.g. primary production by surrounding vegetation, phytoplankton, microphytobenthos, and vegetal litter) and allochthonous sources (e.g. OM transported predominantly by tidal currents or rivers). Therefore, it is expected that δ¹³C values range between those observed for marine and terrestrial OM (i.e. −17‰ to −27‰). In general, C and N isotopic composition is less affected by the OM diagenesis than C/N ratios (Meyers, 1994); however, OM mineralization might account for small changes of both isotopic ratios (up to 5‰; Macko, 1981). Anomalously heavier δ¹⁵N values not only might be a consequence of ammonia volatilization and subsequent nitrification (Macko and Ostrom, 1994) but also can be explained as a result of the contamination from sewage or agriculture wastes. In fact, other studies elsewhere (e.g. Voss et al., 2000) have suggested that the δ¹³C and δ¹⁵N values increasing upcore are because of increased primary production caused by eutrophication.

The range of C/N, δ¹³C, and δ¹⁵N values along the cores reflected a mixture of OM from terrestrial and marine sources. As the C_org concentration profiles in the three sediment cores showed a recent diminution of C_org fluxes, eutrophication was ruled out to explain the increasing values of the C and N isotopic ratios. Thus, we concluded that the C/N, δ¹³C, and δ¹⁵N trends indicate a gradual transition to more marine conditions in the recent sediments.

Efforts were made to confirm the marine influence in the sedimentary record by determining the foraminiferal and ostracode assemblages in the cores. A few number of calcareous and agglutinated foraminiferal species (Ammonia tepida, Ammotium salsum, Arenoparrella mexicana, Cribroelphidium gunteri, Miliammina fusca, Triphotrocha comprimata, and Trochammina inflata), and only two species of ostracodes (Cyprideis castus and Perissocytheridea sp) were identified, as expected for a marginal environment at the upper limit of the tidal influence (Benson and Kaesler, 1963; Murray, 1991). The number of foraminiferal tests was also too scarce to be relevant for this study (0–20 tests in 35- to 50-g samples, in total only 325 tests were found in the 97 samples analyzed). The scarcity of foraminiferal tests was most likely because of post-mortem destruction of the agglutinated foraminifera during burial, as they are highly dominant in the relatively abundant surface assemblages (unpublished results). However, calcareous tests, although scarce, are well preserved in the core samples. Ostracode shells were common in the upper (0–4 cm) part of EUII core (up to 20 valves/g of sediment) and sporadic in the rest of the down-core samples.

Na, Cl, and Br are conservative major constituents of seawater and have been used as indicators for marine intrusion and paleosalinity (Chagué-Goff, 2010, and references therein). Al, Ti, Rb, and Zr are lithophile elements which are commonly used as indicators for terrigenous contribution (Zabel et al., 2003). The Na and Cl concentrations were comparable in the three cores, but the Br concentration range in core EUII was lower than in the other two cores. While cores EUI and EUII showed increasing trends for Na, Cl, and Br from 20 cm depth toward the surface, core EUIII showed relatively constant concentrations, comparable to the highest concentrations in EUI and EUII. The concentrations of the terrestrial indicators Al, Ti, Rb, and Zr were comparable in the three sediment cores and showed decreasing trends in the upper sections of cores EUI and EUII (where Na, Cl, and Br concentrations increased) but remained almost constant along core EUIII. The trends observed in Na, Cl, Br, Al, Ti, Zr, and Rb concentrations were unrelated to changes in the grain size distribution throughout the cores. In addition, the concentrations of Na, Cl, and Br in cores EUI and EUII showed significant correlations (p < 0.05) with the values of C/N (negative) and δ¹³C (positive). Thus, upward increasing trends of Na, Cl, and Br concentrations were interpreted as an indication of increasing marine conditions toward the surface of both cores.

FA was performed with the geochemical dataset (40 variables) of each sediment core. The two main factors accounted for 66% (EUI), 65% (EUII), and 56% (EUIII) of the dataset variability. The FA biplots (Figure 5) showed a similar grouping of geochemical variables in cores EUI and EUIII, with a main factor related to two processes of element mobilization in the environment (identified as ‘element mobilization’; 37% and 34% of the total variance explained, respectively); it included the positive loadings of δ¹⁵N and trace metal such as Cu, Ni, and V, which are most likely associated with the various anthropogenic inputs reported in EUL (e.g. municipal, port, and industrial activities; Raygoza-Viera et al., 2014, and references therein); and the negative loadings of S, As, Fe, which are usually related owing to coprecipitation in soils under sulfate-reducing conditions, but further mobilized with soil flooding (Burton et al., 2008). In core EUII, the factor identified as ‘grain size distribution’ explained 34% of the total variance and included the percentage of fine grained fractions (clay and silt); the concentrations of some lithophilic elements such as Al, Ga, and Th with positive loadings; and the sand content with negative loading.

In the three cores, the second main factor included the concentrations of Br, Na, and Cl (marine indicators) that showed opposite factor loadings to the concentrations of Si, K, Ti, Rb, Zr, and Ba (terrigenous indicators; Figure 4). Therefore, this factor was interpreted as ‘marine transgression’ and represents the 29% of the total variance of the dataset in core EUI, 42% in core EUII, and 22% core in EUIII. The depth profile of the ‘marine transgression’ factor scores (Figure 5) showed that marine conditions were established more than 100 years ago in cores EUI and EUIII, collected at lower topographic elevation (EUI: 0.197 m a.s.l., EUIII possibly lower), whereas in core EUII (1.029 m a.s.l), the marine signal is more recent (since the 1970s). Furthermore, the “marine transgression” depth profile showed a clear upward increasing trend in cores EUI and EUII, signaling a still higher marine influence. In contrast, core EUIII showed a decreasing trend since late 1980s to the present, suggesting that this sampling site is presently subject to a process of ‘terrestrialization’.

Figure 4.

Elemental composition in saltmarsh sediment cores EUI, EUII, and EUIII from Estero de Urias Lagoon, Gulf of California: (a) Si and Ti, (b) K and Al, (c) Zr and Rb, and (d) Cl, Na, and Br.

Figure 5.

Factor analysis of the geochemical data for the saltmarsh sediment cores EUI, EUII, and EUIII from the Estero de Urias Lagoon, Gulf of California. (a) Loading biplots of the two principal factors at each core and (b) score depth profiles of the ‘marine transgression’ factor at each core.

Discussion

Accretion rates and SLR

Tropical marshlands are periodically flooded during high tides, which transport seawater-borne particles that favor sediment accretion. Indeed, the geochemical proxies for marine transgression (δ¹³C, δ¹⁵N, and C/N ratios and Br, Na, and Cl concentrations) supported that accretion in the three sites has been influenced by the accumulation of particles transported by seawater since more than 100 years ago in cores EUI and EUIII and since at least 40 years in core EUII. Assuming that there is an equilibrium between sediment supply and erosion and that the influence of the wetland surface and shallow subsurface processes (e.g. biotic contribution, OM decomposition, and autocompaction; Webb et al., 2013) are negligible and/or taken into account by the sediment consolidation corrections performed, the ²¹⁰Pb-derived SARs were used to estimate the SLR trends in the study area.

Figure 6 and Table 1 present the chronology of the mean SLR trends (relative to 2012, sampling year) estimated from the linear least squares trends of the SAR values with time. SLR trends recorded in cores EUI and EUIII were 1.26 ± 0.05 and 0.73 ± 0.03 mm yr⁻¹, respectively, during the period 1886–1940; 1.79 ± 0.06 and 1.68 ± 0.19 mm yr⁻¹, respectively, between 1940 and 1970; and 2.00 ± 0.07 and 2.54 ± 0.14 mm yr⁻¹, respectively, during the interval 1970–1990. Core EUII (which started to show marine influence only since the 1970s) recorded a lower SLR trend of 1.11 ± 0.06 mm yr⁻¹ during the period 1970–1990, comparable to those values recorded in EUI and EUIII several decades before. Since 1990 to the present, the SLR trend in cores EUII (1.93 ± 0.05 mm yr⁻¹) and EUIII (2.77 ± 0.10 mm yr⁻¹) were comparable, but lower than in core EUI (3.87 ± 0.12 mm yr⁻¹).

Figure 6.

Reconstruction of SLR during the past century (relative to 2012), according to ²¹⁰Pb-dated sedimentary records from saltmarshes around the Estero de Urias Lagoon, Gulf of California.

Table 1.

Sea-level elevation trends in Mazatlan, according to the accretion rates from saltmarsh sediment cores in the Estero de Urias Lagoon, Gulf of California.

Sea-level period	EUI (mm yr⁻¹)		EUII (mm yr⁻¹)		EUIII (mm yr⁻¹)
1990–2012^a	3.87	±0.12*^a,b	1.93	±0.05*^a,b	2.77	±0.10
1970–1990^b	2.00	±0.07	1.11	±0.06	2.54	±0.14*^b,c
1940–1970^c	1.79	±0.06*^c,d	n.a.	n.a.	1.68	±0.19*^c,d
1886–1940^d	1.26	±0.05	n.a.	n.a.	0.73	±0.03

n.a.: not available.

The trends are significantly different between periods. Letters ‘a–d’ identify the period included in the comparison.

The differences observed in SLR trends among the cores suggest that there are other factors, besides eustatic SLR (e.g. hydrological changes derived from infrastructure building) that affect differently each sampling site. EUI and EUIII recorded a similar SLR trend during the past century (especially between 1940s and 1970s) but were decoupled after the 1980s, with EUI displaying a higher trend. One possible explanation is that because of faster sedimentary accretion between 1970s and 1980s, the EUIII site grew higher and the number and duration of tidal flooding decreased, therefore receiving less marine particles. In fact, according to the FA, the core EUIII is receiving a higher terrestrial contribution since the 1980s, (a) likely because of the accumulation of soil particles, delivered to the lagoon from the earthen ponds of the shrimp farms established in the vicinity of the EUIII collection site since 1988 (Figure 1); or (b) owing to a former faster accretion (higher than SLR) and a consequent less frequent tidal inundation, this wetland is currently receding landward. The most recent accretion rates of core EUIII (1990–2012: 2.77 ± 0.10 mm yr⁻¹), although falling within the interval of the global SLR trend (2.8 ± 0.8 mm yr⁻¹) reported by Church and White (2011), are likely no longer representative of recent SLR at EUL.

The core EUII showed marine transgression only since the 1970s, and the SAR trend during the period 1970s–1990s was lower than in the other two cores, likely because this site is the highest topographically among the three collection sites, and therefore, tidal flooding is less frequent. This observation emphasized the relevance to measure with precision the collection site elevation with respect to MSL in order to improve the comparability of the SLR trends within the sampling areas. We concluded that the EUII sedimentary record underestimated the SLR at EUL, but it is expected that the EUII site should better record the SLR rate in the future, when MSL will be closer to the EUII elevation.

The SLR trends in the EUI sedimentary record are similar to those estimated from tidal gauge measurements and satellite altimeter data (Church and White, 2006, 2011) that indicate that sea level has risen by 1.1 ± 0.7 mm yr⁻¹ between 1880 and 1935, 2.4 mm yr⁻¹ between 1967 and 1982, and 2.8 ± 0.8 mm yr⁻¹ (tide gauges) or 3.2 ± 0.4 mm yr⁻¹ (satellite data) between 1993 and 2009. The SLR trend estimated from core EUI for the period 1950–1970 (1.99 ± 0.07 mm yr⁻¹) was also in agreement with the mean SLR trend obtained from tide gauge measurements in Mazatlan City for the same period (1.9 ± 3.3 mm yr⁻¹; Zavala-Hidalgo et al., 2010). Regrettably, there are not continuous tide gauge data available between 1970 and 2008 (SMN, 2015) to make further comparisons. However, our results disagree with the regional sea-level trends obtained from satellite altimetry (Church et al., 2013) which indicate negative SLR trends in the Eastern Pacific (including the Mexican Pacific coast) for the period 1992–2009. This contradiction highlights the need for intensive local and regional research to better understand the non-uniform sea-level change around the world coastal areas.

The core EUI is the only one that provided both a long-term record and clear geochemical signatures of marine transgression. The consistency of EUI results with global and regional tide gauge data supports the fundamental assumptions that (a) mixing of the sediment core was negligible, (b) the ²¹⁰Pb profile is the result of sediment flux changes with time, and (c) elemental composition and other geochemical proxies of sediment provenance (terrestrial vs marine) can be used with confidence to identify marine transgression in sedimentary records from coastal environments where the micropaleontological remains are not well preserved. In addition, we learnt that accurate topographic measurements are crucial to choose the core collection sites, in order to obtain comparable time windows and geochemical signals indicative of SLR.

Therefore, based on the EUI SLR record (relative to current sea level; Figure 6), the overall SLR in Mazatlan was 24 cm during the past 127 years. Furthermore, the SLR during the past 27 years (10.5 cm, 1995–2012) was almost three times faster than during the first 100 years of the record (13.5 cm, 1885–1985). These observations imply that making the conservative estimation that the current SLR trend in core EUI (3.87 ± 0.12 mm yr⁻¹) remains constant, we might expect inundation of the lowlands surrounding the EUL 34 cm above present MSL by the end of the 21st century.

Conclusion

Based on the analysis of geochemical proxies and SARs derived from ²¹⁰Pb-dated sediment cores collected from tropical salt marshes, we reconstructed SLR during the 20th century until 2012 in Mazatlan, Mexico, southern Gulf of California. All sedimentary records evidenced marine transgression, with SLR trends varying from a minimum of 0.73 ± 0.03 mm yr⁻¹ between 1886 and 1940 to 3.87 ± 0.12 mm yr⁻¹ for the past 20 years (1990–2012). Our results contrast with the satellite altimetry records that reported a marine regression for the Mexican Pacific coast during the past 20 years but are in agreement with the global SLR trends derived from tide gauges, and with the local tide gauge record available for the period 1950–1970. These observations highlight the importance of obtaining local and regional measurements of the SLR trends for more accurate adaptation plans and showed that the methodology used is reliable and can be reproduced in tropical and subtropical areas where long-term records of SLR trends are scarce or non-existing.

Footnotes

Funding

This work has been supported by the grants CONACyT CB2010-153492, PAPIIT-IN203313, CONACYT PDCPN2013-01/214349, and the CONACYT fellowship to JLSP. Technical support was provided by G Ramírez Reséndiz, C Suárez Gutiérrez, D Oviedo (database and figures), JA Galaviz Solís, V Carnero-Bravo (sampling), H Bojórquez-Leyva (chemical analysis), H Quintas Mendizabal (foraminiferal analysis), A Rodriguez-Ramírez and XA Nava Fernández (ostracode processing), and M Vargas-Cárdenas from Ingeniería Civil de Sinaloa (topographic measurements).

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