Paleoenvironmental study of the Late Preclassic period in the Northern Mesoamerican Frontier

Abstract

The Northern Mesoamerican Frontier was a complex multicultural region characterized by frequent human settlement changes and shifts in agricultural conditions during the Late Preclassic period (~400 BCE-150 CE). Here, we report a high-resolution paleoenvironmental record from the varved sedimentary sequence of the crater maar La Alberca which spans the Late Preclassic (~400 BCE-150 CE) to part of the Early Classic period (~150 CE-250 CE) corresponding to Late Chupicuaro phase (400 BCE-100 CE) and Mixtlan phase (0–250 CE). Our work aims to study the paleoenvironmental conditions during the rise of agriculture in the Northern Mesoamerican Frontier and provide insights related to landscape alteration by human activity. To reach these aims, a multiproxy investigation was conducted by means of varve counting, high-resolution XRF scans, magnetic susceptibility, pollen data and fecal stanol biomarkers as a proxy for human population change. Our results reveal two varve type. Type 1 is characterized by the alternation of detrital-organic layers and aragonite layers, type 2 by alternating detrital-organic layers with an organic layer formed by diatom frustules and aragonite layers. This study suggest that the increase of erosion by human activity during the Late Chupicuaro phase (400 BCE-100 CE) and the start of the Mixtlan phase (0–250 CE) coincide with a high percentage of Amaranthaceae pollen, a rise of sedimentation rates, increase in nutrient content and the increase of human waste flux interpreted with the (Coprostanol + epi) :((Coprostanol + epi)+cholestanol biomarker. Moreover, a wetter period (~137 BCE-37 CE) interpreted during the Late Chupiacuaro phase and the start of the Mixtlan phase could suggest favorable environmental conditions for the establishment of agriculture.

Keywords

agriculture environmental change Northern Mesoamerican Frontier varve sediments

Introduction

The Northern Mesoamerican Frontier (NMF) was characterized by its dynamic and social complexity and pluricultural nature. Hunters-gatherers occupied the NMF and the emergence of stratified societies was delayed compared to other Mesoamerican regions (Viramontes, 2008). During the Late Preclassic period (~400 BCE-150 CE), the Chupicuaro culture, considered to be the most influential culture of the region (Gorenstein, 1985), started to migrate from the Acambaro Valley toward the northern region of NMF following the track of the Lerma, Turbio and Laja rivers` traces (Viramontes, 2008).

Based on regional archeological evidence, Armillas (1969) interpreted that after 600 CE, populations expanded northward to the Lerma River until 1000 CE, thus describing this period as the maximum northern extension. Afterward, severe drought conditions around 1100 CE provoked a general exodus of ancient farmers, reducing the number of permanent settlements (Figure 1a). These paleoenvironmental conditions resulted in the retraction of the NMF south of the Lerma River Valley. The consequence of this retraction provoked political and social instability in the NMF. O'Hara et al. (1994) proposed that long-term agriculture in the NMF was not viable due to climate variability, concluding that flexibility in the cultivation methods in the driest places and out-migration were typical practices of the ancient cultures of this region. Wogau et al. (2019) support Armilla`s hypothesis that severe paleoenvironmental conditions triggered social instability in the NMF around ~700–880 CE.

The Valle de Santiago, Guanajuato lies at the NMF where the start of incipient agricultural activities is indicated by the pollen records of San Nicolas and Parangueo lakes around 3700 BCE (Park et al., 2010) (Figure 1). Based on La Alberca Lake’s pollen record, Conserva (2003) suggests an anthropogenic landscape disturbance between 400 BCE-150 CE, correlating with the Chupicuaro occupation period (Gorenstein, 1985). However, several questions regarding paleoenvironmental conditions and their impact on the human dynamics during the Late Preclassic in the NMF remain unsolved.

Varved lake sediments are valuable archives for studying paleoenvironmental variability and human impact (Lamoureux et al., 2001; Lotter and Birks, 1997). Furthermore, these types of sediments potentially provide an independent, absolute dating method with decadal, annual and sometimes seasonal resolution (Corella et al., 2014; Ojala et al., 2012; Zolitschka et al., 2015). This characteristic makes them suitable for the study of the complex relationship between landscape changes and human activities.

Few systematic varve counts have been conducted in the Mesoamerican region. Kienel et al. (2009) using the varved sedimentary sequence of La Alberca (AD 1852-1973) and Rincon de Parangueo (AD 1939-1943), demonstrated that the varve formation is related to the strong seasonal climate regime in the region. The study suggested the occurrence of drought periods with a duration of 3–7 years, centered around the 1850s, 1865, 1880, 1895, 1905, 1915, 1920s, produced by ENSO activity and positive SST anomalies. Using a laminated sequence of Laguna Minucua, Goman et al. (2018) discussed the potential of this natural archive for studying the impact of paleoclimate and paleoenvironmental oscillations on Mesoamerica, especially in the Mixteca region, south of the NMF.

Here we present a high-resolution paleoenvironmental record from a laminated sedimentary sequence of crater maar La Alberca. Our record spans the Late Preclassic period (~400 BCE-150 CE) to part of Early Classic period (~150 CE-250 CE) which encompasses the Late Chupicuaro phase (400 BCE-100 CE) and Mixtlan phase (0–250 CE) (Figure 2). Multiproxy analyses were processed based on varve counting, high-resolution XRF scans, magnetic susceptibility, pollen data and fecal stanol biomarkers as a proxy for human population change. The study objectives are to: (A) Describe the lamination properties to evaluate the main paleoenvironmental process involved in their formation, (B) Produce a high-resolution paleoenvironmental record of the NMF between the Late Preclassic (~400 BCE-150 CE) to part of Early Classic (~150 CE-250 CE) periods, (C) Explore the possible landscape alteration by human presence and activity during at NMF between the Late Preclassic period (~400 BCE-150 CE) period to part of Early Classic period (~150 CE-250 CE) (Figure 2).

Archeology associated to the Northern Mesoamerican Frontier

Socially the NMF is described by the multicultural character of its population, a dynamic region with continuous social change, inhabited by stratified societies and hunter-gatherers. Chupicuaro was one of the most influential regional pre-Hispanic cultures in the NMF. Its occupation period along the Rio Lerma, southeast of Guanajuato and Cuitzeo Basin was dated between 650 BCE-250 CE (Gorenstein, 1985). This culture is known for its high-quality polychrome ceramics and figurines with unique forms. Most of the Chupicuaro artifacts were found near the Lerma River basin, there is evidence of its ceramics and vessels in Central Altiplano and La Quemada (Zacatecas) archeological sites (Figure 1). Chehuayo and El Cenicero, both located in the Cuitzeo Basin could be the main sites of this culture, nevertheless their origin still an enigma. While some authors refer that colonization of the Lerma region was conducted by Central Mesoamerica groups (Porter, 1956), some others suggest that they came from the Western Mesoamerica (Braniff and Lowe, 1998; Florance, 2000).

The study of the Chupicuaro culture has been subdivided into three main phases: Early Chupicuaro phase (600-400 BCE), Late Chupicuaro phase (400 BCE-100 CE) and Mixtlan phase (0–250 CE). Early Chupicaro was characterized by the presence of rudimentary villages near to Lerma River and low social development. During the Late Chupicuaro phase, the increase of habitants in the Lerma Basin was accompanied by the establishment of a hierarchical state and the constant northwards migrations following the Lerma, Turbio and Laja rivers (Florance, 2000). Also, a characteristic of this place is the construction of circular buildings (e.g. Guachimontones) and the construction of sunken patios started. The Mixtlan phase, in general, is represented by the evolution in the architectural style of the sunken patios. The expansion to the north continued, evidenced by the archeological site Morales with a chronology between 300 BCE to 100 CE. Most of the Chupicuaro settlements based their economy on agricultural activities (Florance, 2000).

Around 300–700 CE, during the Bajio Tradition stage, the cultures in the NMF evolved into more complex societies characterized by towns and cities with monumental and ceremonial architecture (Braniff, 2000). This phase is also represented by the large number of archeological sites such as: Cerro Barajas, Plazuelas, Peralta, Cañada de La Virgen and El Coporo (Figure 1). This period also describes the rise of inhabitants, constant migrations, the establishment of commercial networks, the abandonment and the formation of different pre-Hispanic towns. The growing number of inhabitants in the NMF led to the colonization of new regions and the intensification of agricultural activities (Zamora Ayala, 2004).

Study region

Local and geological settings

The La Alberca maar is in the Valle de Santiago volcanic region (VSVR) (Figure 1). The VSVR comprises a series of east-west trending valleys covered by alluvial and lacustrine sediments (Aranda-Gómez et al., 2013). The valley depression is filled by sediments where vertisols developed in poorly drained areas, after the Late Tertiary and Quaternary volcanism modifying the original drainage system (Butzer and Butzer, 1997). According to Murphy (1986), the volcanic structures in the VSVR are younger than 0.07 Ma. The VSVR is part of the Mexican Volcanic Belt, which is related to the subduction of the Cocos/Rivera plate during the last 17 Ma (e.g. Ferrari, 2000). The chemical composition varies from calci-alkaline rocks, typical for subduction zones, to alkaline magmas (Aranda-Gómez and Carrasco-Núñez, 2014; Luhr et al., 2006). The VSVR comprises diverse volcanic structures such as cinder cones, maars and calc-alkaline shields, older than 6.8 Ma (Murphy, 1986).

La Alberca maar has a quasi-circular shape with a crater diameter of 700 m and a surface area of 0.15–0.69 km². Five units make up the stratigraphic column of the Alberca crater. The basal unit is andesite lava flow. Using Ar/Ar method, this unit was dated to 0.24 ± 0.02 Ma (Rincón, 2005). The next unit is a paleosol which separates the pre-maar unit from the pyroclastic products of the phreatomagmatic eruptions. This unit represents a lack of volcanic activity. Above the paleosol, a scoria cone was formed, indicating a dry magma eruption. The final unit is a pyroclastic deposit directly related to the phreatomagmatic activity.

The present climate conditions in Valle de Santiago are as follows: average annual temperature of 19–20°C, average annual precipitation of 715–737 mm with maximum precipitation of 962–1192 mm between May-October and minimum precipitation of 369 and 452 mm between November-April. Other important characteristics regarding the climate are: (A) The present climate is marginaL for rain-fed agriculture; (B) The evaporation exceeds precipitation and the rain quantity may vary from year to year. These conditions may trigger the possibility of drought seasons; (C) The region of Valle de Santiago is located in the northern margin of the tropical summer rain belt (Alcocer et al., 2000; Kienel et al., 2009).

La Alberca limnology

La Alberca lake is a crater maar, and its genesis is related to phreatomagmatic activity. Based on its mixing state La Alberca was classified as a monomictic lake. Vertical mixing occurs once per year in monomictic lakes, and they remain stratified during the rest of the year (Lewis, 1983).

The water at the Alberca Lake was hyposaline with conductivity values up to 2300 μScm-1 and the principal salt content is NaHCO₄ followed by Na₂CO₃ > NaCl>NA₂SO₄ (Alcocer and Hammer, 1998). Reported pH values are between 9.6 and 9.7. These pH and conductivity values were measured in October 1999 (Armienta et al., 2008). The presence of HCO₃ and SO₄ are derived from CO₂ and H₂S, respectively; Cl is leached from the volcanic rocks and NO₃ proceeds from pollutants related to the intensive pasture (Orozco and Madinaveitia, 1941). The low concentration of Ca²⁺ and Mg²⁺ is due to their precipitation as carbonates.

Methods

Fieldwork and sampling

Maar lake La Alberca was sampled at the central part. Two parallel cores were retrieved. The cores have lengths of 8.69 m and 6.39 m, named Alberca Azul (AZ) and Alberca Rojo (AR), respectively (Figure 1b). The coring device employed was developed in the Paleomagnetism Laboratory of Geoscience Center of the National University of Mexico (CGEO). Both cores were transported and stored in the cold room of Centro de Geociencias, UNAM, Campus Juriquilla, at 4º C before the analysis.

Both cores were cut into halves and photographed. One half was used for non-destructive core scanning (XRF) and then archived in a cold room. The second half was sub-sampled for different analyses. For measurement of magnetic susceptibility, small plastic samples with a volume of 8 cm³ were taken at every ~2.3 cm. These samples were measured in the CGEO. Macro fossil plant remains were selected for radiocarbon dating.

Cores were correlated using magnetic susceptibility, and stratigraphic markers. We selected AZ as a master core, which was the longest and better-preserved record. Moreover, pollen data were compiled from a third core called Hoya Alberca (Appendix 1, available online) drilled in 2001 in the same location by a joint team of the CGEO, UNAM and UC Berkley. The correlation between the earlier Hoya Alberca core and AZ was based on magnetic susceptibility curves, isotopic data, XRF Ca/Ti ratio, and an independent age model (Appendix 1, available online).

Laboratory methods

XRF scanning, magnetic susceptibility, pollen, and radiocarbon dating

High-resolution XRF scans (ITRAX XRF Core Scanner, COX, e.g. Croudace et al., 2006) were conducted using step sizes between 200 and 500 μm using the AZ core. Volume-specific magnetic susceptibility (k, in dimensionless SI units) was measured with a Kappabridge magnetic susceptibility system, using a field of 400 A/m with a frequency of 976 Hz. Pollen identification was performed using 400 grains and fern spores on 44 samples, according to Faegri and Iversen (1989) standard procedures (Conserva, 2003). Specifics of high-resolution XRF scans and pollen counts can be found in Wogau et al. (2019, 2021).

The age model was based on twelve ¹⁴C AMS ages using gyttja and woody remains samples. All analyses were done in Beta Analytic Laboratory using the atomic mass spectrometry method (AMS). Further age model details can be found in Wogau et al. (2019). Nevertheless, the age model was updated using the calibration curve IntCal20 for this publication (Reimer et al., 2020).

Biomarkers

The samples were subsequently freeze-dried to remove water. Dried sediment samples were ground, weighed, added to a PTFE tube and extracted using a CEM MARS 6 microwave extractor with 10 ml of 9:1 dichloromethane:methanol. This ratio of solvents was selected after testing various methods for their extraction efficiencies using lake sediment samples (Battistel et al., 2015; Kornilova and Rosell-Melé, 2003). The MARS 6 was heated to 80^oC and held at that temperature for 20 min. The contents of the PTFE tube were then transferred to a centrifuge vial, centrifuged and the Total Lipid Extract (TLE) was transferred to an evaporating vial. 9:1 DCM:Methanol was added twice more to the centrifuge tube in order to ensure complete removal of extracted material. The TLE was evaporated and split into two fractions (a non-polar fraction and a fraction containing the neutral and polar fractions) using silica gel chromatography. The pipette columns consisted of 5 cm of silica gel, and 1 cm of sodium sulfate. 15 ml of hexane was eluted to collect the non-polar hydrocarbon fraction and 15 ml of methanol was eluted to collect the remaining neutral and polar fractions. The solution containing the neutral and polar fractions was saponified using KOH (potassium hydroxide) and separated into a sterol and fatty acid fraction. The sterol fraction was then derivatized with BSTFA (bis-trimethyl silyl trifluoroacetamide) to replace the hydrogen with the less exchangeable trimethylsilyl (TMS) group.

The neutral (sterol) fraction was analyzed using gas chromatography with a flame ionization detector (GC-FID) with a TRACE TR-5 GC Column (60mx0.25mm) at McGill University in sequence with known standards for coprostanol, epicoprostanol, cholestanol, cholesterol, stigmastanol and stigmasterol (Sigma-Aldrich) to quantify these compounds. A set of representative samples were analyzed using an Agilent 7890B GC with an Agilent 5977B MSD at Concordia University to confirm compound identification. Because of the similar retention time of coprostanol and epicoprostanol it was not possible to consistently resolve these molecules. We followed the approach of White et al. (2018) and reported the sum of these two compounds.

Microfacies

We selected a laminated sediment interval of core AZ between 100 and 289 cm. Our age model indicates that this section lies between Late Preclassic (1800 BCE-250 CE) to Classic (150–650 CE) archeological periods (Wogau et al., 2019) (Figure 2). Twenty-three overlapping thin sections of embedded epoxy-resin-impregnated sediment blocks (10 x 2 x 0.5 cm) were prepared following standard procedures (Brauer and Casanova, 2001). For microfacies characterization and varve counting, an Olympus BX-53 microscope was employed. Detailed thickness measurements of every seasonal sublayer that forms different varve types were performed using the 50x magnification objective. Varves were counted three times to estimate the standard counting error. The average age of massive layers was inferred by interpolating the sedimentation rates from neighboring varves.

Results

Sediment profile

We focus our analysis on stage III (~175.5–236 cm) of the La Alberca lake sedimentary sequence previously described by (Wogau et al., 2019, 2021). This stage is described by the presence of alternations of dark and light laminations interbedded with massive layers. According to the stratigraphic characteristics and the degree of laminae preservation, six principal sub-stages were distinguished (Figure 3). Well-preserved laminate sections characterize the sub-stages III.a, III.c, and III.f. Sub-stage III.b is formed by alternations between well-preserved laminations and thick massive layers. Finally, the sub-stages III.e and III.d comprise a less preserved laminae sequence.

Laminae structure and composition

The study sequence contains three main laminations. These were classified as detrital organic layers (DOL), aragonite layers (AL) and organic layers (OL). Microscopic inspection reveals two typical successions of such layers: DOL-AL (Type 1) and DOL-OL-AL (Type 2) (Figure 3). Type 1 seems to predominate during substages III.a, III.b, III.d and III.e. Type 2 predominates in substages III.c and III.f. Massive layers (ML) and turbidite layers were also distributed along the stage (Figure 5).

The DOL layers are red-brown color and are formed by clastic detritus, organic detritus, and plant fragments supported in organic mud. The thickness of this layer oscillates between 0.88 and 0.49 mm with flat and erosive contacts. The largest thickness is observed at stages III.f and III.b. (Table 1). The clastic fraction consists of feldspars (anorthite, anorthoclase, albite), quartz, pyrite crystals with angular shapes, rounded carbonate crystals and rounded andesitic lithic fragments no bigger than 3 mm. The organic mud is composed of highly preserved valves of planktonic diatoms such as Nitzschia palea, Nitzschia ovali, Nitzschia incospicua, Nitzschia palea, Navicula cryptotenella and Cymbella muelleri. Ostracode shells rarely occur (Figure 4).

Table 1.

Mean thickness of detrital organic layers, aragonite layers and varves.

Sediment unit	Depth (cm)	Mean thickness DOL layer (mm)	Mean thickness OL layer (mm)	Mean thickness AL layer (mm)	Total varve thickness (DOL + OL + AL)	% Varve type 1	% Varve Type 2
IIIa	236–227.5	0.49	0.44	0.30	1.23	89.4	1.51
IIIb	227.5–217	0.71	0.24	0.28	1.23	78.95	0
IIIc	217–192.6	0.65	0.55	0.36	1.56	39.39	46.22
IIId	192.6–183.1	0.66	0.49	0.34	1.49	69.4	12.28
IIIe	183.1–175.5	0.63	0.20	0.30	1.13	60	0
IIIf	175.5–168.1	0.88	0.61	0.31	1.8	54.13	40.62

Percent of distribution of varve Type 1, varve Type 2 and massive layers.

AL laminae have a minimum thickness of 0.28 mm and a maximum thickness of 0.36 mm (Table 1). These layers are dominated by the presence of aragonite crystals with rice shapes forms, up to 0.01 mm length. The dominance of aragonite is corroborated by SEM images (Figure 4). Calcite is rare in these layers.

The thickness of OL oscillates between 0.61 mm and 0.21 mm. Well-preserved and broken diatom valves entirely form these layers. These are present in sub-stage III.a, III.c and III.f (Figures 3 and 4). Nitzschia palea species dominate the assemblage. However, some other of its variants (Nitzschia ovali, Nitzschia incospicua) and Cymbella muelleri species are present as well (Figure 4).

The massive layers are of red-brown color. At naked eye they seem to be homogeneous, nevertheless, microscopic observations revealed that they are mainly composed by reworked carbonate crystals, clastic detritus such as feldspars crystals, fragments of diatoms valves and a low proportion of reworked plant debris (Figure 5). The maximum thickness is 20 mm (Figure 4).

Turbidite layers are graded, ranging from fine sand to clay size at the top, and with an erosive contact at the base (Figure 5). The presence of angular clastic detritus dominates these layers and a high content of organic debris, such as plant remains. These were observed in all the substages.

XRF element scanning results

Accordingly, to Wogau et al. (2019) the PCA 1 reflects detrital delivery during high runoff periods, variation in calcium carbonate precipitation is described with the Ca/Ti ratio, and oscillation between anoxic/oxic conditions in the lake hypolimnion is given by Mn/Fe ratio (Wogau et al., 2019). PCA 1 reveals a high detrital input during stages III.a, III.c and an increasing trend between III.e and III.f sub-stages (Figure 3). The high (low) concentration of carbonates (Ca/Ti) correlates well with the increase on oxic(anoxic) conditions suggested by the Mn/Fe ratio. Both ratios describe a decreasing trend between substages III.e and III.f. High amplitude variations in the three parameters occur on the sub-stage III.e (Figure 3).

Magnetic susceptibility

The magnetic susceptibility curve reveals a high concentration of ferrimagnetic minerals in the sub-stages III.a, III.c and III.f. The high(low) presence of these minerals correlates well with the increase(decrease) of the thickness of DOL and PCA 1, suggesting a relation between detrital input and the magnetic susceptibility (Figure 3b).

Biomarkers

Fecal stanols have been shown to scale with human population (Keenan et al., 2022) and have been quantified as a proxy for human population in several instances (Keenan et al., 2021). Fecal stanol concentration are discussed as a ratio of coprostanol(+epi-coprostanol) to 5α-cholestanol, a stanol commonly found in lake environments (coprostanol + epi-copro):((coprostanol+epi-copro.)+cholestanol)) (White et al., 2018). The ratio increases from 0.24 at236 cm to 0.31 at 211 cm and peaks at 0.325 at 191 cm. The ratio decreases to 0.15, the lowest value of the record, at 168 cm (Figure 2). Coprostanol (5β-cholestan-3β-ol) is produced during metabolic reduction of cholesterol in the intestinal tract of most mammals and is the major sterol present in human feces (Bethell et al., 1994; Bull et al., 2002; Prost et al., 2017; Zocatelli et al., 2017). We interpret the changing ratio as increasing flux of human waste to the lake, reflecting changing human populations.

Discussion

Varve interpretation

Two lamination patterns are observed in the record: Type 1 (DOL-AL) and Type 2 (DOL-OS-AS).

Type 1(DOL-AL)

The origin of DOL describes a combination of mechanical and biological processes. First, the mechanical process is interpreted by the occurrence of reworked detrital fraction (lithics-crystals), broken diatoms valves, amorphous organics matter, and basal erosive contact at the base. We interpret that this layer was deposited during the rainy season of the year, between May and October (Figure 6), generating a surface runoff that carries detritus and organic material into the lake. Second, with the onset of the rainy season, sediment influx increases the content of nutrients, raising the lake’s productivity (e.g. Wetzel, 2001), thus producing a diatom bloom. This diatom bloom forms the organic debris settling down together with the larger detrital particles and plant remains.

The AL is formed during the final part of the dry season, when maximum temperatures are registered (March to May), strong evaporation raises the saturation of carbonates in the lake and the precipitation of these minerals occurs as soon as its solubility limit is reached (Zolitschka et al., 2015). The rainwater isotopic values of Valle de Santiago are between −11 and −7.7 ‰. Kienel et al. (2009) argued that the oxygen isotopic composition of the carbonates fraction during 1852–1973 AD in La Alberca lake is enriched to values around +1.6‰ and concluded that the evaporation process mainly rules precipitation of the carbonate sublayer. In the study section, oxygen isotopic values are around +1.1 ‰, confirming that evaporation is the main process for the precipitation of the AL (Wogau et al., 2019). However, biological processes like diatom blooms may also be involved, as these consume CO₂ and increase the pH and thus reduce the solubility of carbonates (Stabel, 1986).

Type 2 (DOL-OL-AL)

The formation of the Type 2 pattern follows the same mechanism proposed for pattern Type 1. However, we argue that the observed increase of detrital input displayed by PCA 1 in sub-stages III.c and III.f could be related to an increase of soil erosion, leaching high amounts of nutrients and thus resulting in eutrophic conditions in the lake (e.g. Striewski et al., 2009).

We conclude that the formation of each lamination pattern (Type 1 and Type 2) represents an annual climate cycle (Figure 5), however, the formation of Type 2 is also influenced by the onset of anthropogenic activity in the study area (see discussion below).

The verification of varve counts by an independent chronology method is needed due to stratigraphic hiatuses (Ojala et al., 2012). The two weighted mean ages that anchor the floating study section lies between 1746 ± 76 cal yr BP and 2241 ± 132 cal yr BP covering a time interval of 491 years (Figure 7). The average of the triple counts resulted in a deviation of 52 varve years or 10.59%. The varve counts seem to be systematic younger than ¹⁴C age model (Figure 7), but always remain within the confidence limits. Varve counts in general reflect minimum chronologies, due to lost varves by bioturbation, erosion of the varve surface or complete erosion of the varve sequence (Mingram et al., 2018). The presence of massive layers or turbidites could be responsible for the apparent age divergence. Turbidites were removed from the section and a correction factor was applied in the massive layers. This factor was estimated by inferring sedimentation rates from the nearest neighboring varve. The corrected varve counting resulted in deviations of 21 years or 4.1% (Figure 6). This supports our interpretation regarding the annual nature of varve Type 1 and Type 2.

Paleoenvironment variability during the Late Preclassic period

Sub-stage III.a (300–190 BCE) is characterized by well-preserved Type 1 varves, constant input of detrital fraction indicated by the PCA 1 and k parameters, continuous and low calcium carbonate precipitation, a constant anoxic state indicated by the Mn/Fe ratio, and the small thickness of massive layers. These conditions imply a relatively stable and stratified water column. Based on these observations, we interpret the dominance of moist conditions during this sub-stage. Additionally, the good preservation in Type 1 varves reflects a strong seasonal contrast (Figure 8).

The substage III.b (~190-96 BCE) is described by the decrease in the preservation of Type 1 varves, the reduction of detrital flux, low sedimentation rates (Figure 8), the rise of carbonate concentration and the increase of oxic conditions. We interpret a lake level reduction due to strong evaporation conditions. The strong evaporative conditions in the lake resulted in a high concentration of dissolved carbonates in the hypolimnion. The drop in the lake level and high physical mixing thus produced an unstable lake stratification (Anderson, 1985). This interpretation is strongly supported by an increase in Mn/Fe and Ca/Ti ratios (Figure 8). This evidence reveals a predominantly arid environment.

The sub-stage III.c (~96 BCE-90 CE) reveals the increase of detrital input (PCA 1 and k), a rise in the sedimentary rates, and a change between Type 1 and Type 2 varves (Figure 7). Therefore, we argue that the rise in the sedimentary clastic input increased the nutrient-loading in the lake. In consequence, with the overload of nutrients, a rise of primary production occurs during the mixing state of the lake during the autumn season via the recycling of nutrients. This high productivity is represented by the OL that is mainly formed by Nitzschia paleacea (Type 2 Varve), a diatom species tolerant to high nutrient availability (Voigt et al., 2008). This assumption is supported by paleo and modern limnological studies (e.g. Urrea and Sabater, 2009; Voigt et al., 2008). The high preservations in both varves types, reduction in the endogenic calcium carbonate precipitation and dominant anoxic state indicated by the Mn/Fe ratio suggest stable lake levels (Figure 8). These lake conditions can be linked with the establishment of wetter conditions and the start of human impact in the area (see discussion below)

The sub-stage III.d (~90–150 CE) displays a decrease in varve preservation, low detrital input indicated by the PCA1 and magnetic susceptibility (k) (Figure 8). Moreover, the rise of calcium carbonate and the dominant oxic conditions prevailed during the stage. Together these characteristics are interpreted as the establishment of an arid environment. A notable feature occurs in the lower part of the stage where varve Type 2 is still prevalent (Figure 8). Diatoms quickly react with the increase of nutrients in the lake systems; however, these changes can permanently modify the diatom community. We interpreted that modification in the nutrients budget of the lake system continued for some years after the high erosion described in sub-stage III.c resulting in the prevalence of diatoms blooms.

Sub-stage III.e (~150–204 CE) is characterized by the alternation of massive detrital layers and well-preserved varves (Figure 8). The short periods of high detrital input described by the PCA 1 parameter coincide with alternations between oxic-anoxic conditions revealed by the Mn/Fe ratio and the alternations of endogenic calcium carbonate precipitation (Figure 8). Together these observations are interpreted to be the result of a variable precipitation regime.

The substage III.f (~204–250 CE) reveals a well-preserved varve section, an increase in catchment input indicated by PCA1, high sedimentation rates, high magnetic susceptibility, and the highest mean varve thickness in all the record (Figure 8). Moreover, the Mn/Fe ratio reveals a progressive increase of the anoxic lake state, and the presence of varve Type 2 predominates over varve Type 1 (Figure 8). These pieces of evidence suggest high lake levels and wetter conditions.

Human imprint in La Alberca sedimentary sequence

We interpreted that the increase of erosion enhanced the nutrient availability to the lake system, producing diatoms bloom reflected in the predominance of varve Type II during substages III.c and III.f lies. The substage III.c comprises the Late Chupicuaro (400 BCE–100 CE) and Mixtlan phases (0–250 CE), while the substage III.f encloses the Mixtlan phase (Darras and Faugère, 2007) (Figure 8). The archeological evidence reveals the expansion of human settlements and the increase of the social organization by the Chupicuaro culture in the study area (Darras and Faugère, 2007).

We propose that the increase of erosion and the subsequent eutrophication interpreted in both sub-stages could be a consequence of two mechanism: (A) A strong human impact via the intensification of agricultural practices in the area increasing the sedimentation rate (B) A rise of detrital input during intense rainy seasons.

The application of coprostanol as a proxy for human population delineates that human population increased from 290 BCE, peaked at 25 CE and declined toward 250 CE. The results indicate that population grew, peaked and declined during the Late Chupiacuaro (400 BCE–100 CE) and the Mixtlan phase (0–250 CE). Increases in the ratio coprostanol:(coprostanol + cholestanol) are co-eval with an increase of varve thickness, increased detrital input with the anomalous increase of nutrients indicative of increased human presence and activity around the lake. This suggests that the input of nutrients in the form of fecal waste triggered phytoplankton blooms, as indicated by the presence of two diatom species tolerant to high nutrient loading, and presence of organic laminae. Furthermore, erosion associated with human presence contributed to this nutrient and detrital input a direct link between human-derived nutrient input and paleo-eutrophication was recorded in Lake Amatitlán, Guatemala (Waters et al., 2021). Eutrophication may have been a stressor for inhabitants around the lake as the food or water source was impacted, and perhaps point to a role for the decrease in coprostanol input, and inferred decline in human population, following the period of highest coprostanol input.

Biomarkers findings agree with the rise in pollen percent of Amaranthaceae (Conserva, 2003; Wogau et al., 2019) and archeological evidence, such as the Mixtlan phase (0–250 CE) (Figure 7). Moreover, we suggest that predominant wetter conditions interpreted in the substage III.c produced a favorable natural environment to the establishment of agriculture practices. These data show the first clear signal of human occupation in the Valle de Santiago region and the rise of productive practices such as agriculture, which resulted in the alteration of the natural environment.

Concluding remarks

Varve analysis of La Alberca lake sediments revealed complex relations between anthropogenic land use by ancient agriculturists and climatic fluctuations during the Late Preclassic (400 BCE-100 CE) in the Northern Mesoamerica Frontier. Type 1 varves are comprised by the alternation of detrital organic layers and aragonite layers. Type 2 varves are composed by an alternation of a detrital organic layer with an organic layer which is exclusively formed by post-mortem diatom frustules and aragonite layers. Therefore, the formation of both varve types represents an annual hydrological cycle, however, the formation of Type 2 is also influenced by the rise of anthropogenic activities.

The varve record and the multiproxy approach indicate the occurrence of wetter periods during sub-stage III.a (300–190 BCE), sub-stage III.c (~96 BCE-90 CE) and substage III.f (~205–250 CE). Conversely, arid events occur during substages III.b (~190–96 BCE) and sub-stage III.d (~90–150 CE). We interpreted a growth, peaked and decline of the population during the Late Chupiacuaro (400 BCE-100 CE) and the start of Mixtlan phase (0–250 CE). The intensification of human activities in the area is interpreted by the presence of Type 2 varves during the sub-stage III.c (~96 BCE–90 CE), which resulted from the increase of soil erosion, leaching high amounts of nutrients, and the development of eutrophic conditions in the lake. This interpretation is supported by the biomarkers (Coprostanol + epi): ((Coprostanol + epi)+cholestanol) signal and the high percentage in Amaranthaceae pollen. Finally, wetter climatic conditions could support the start of diverse human practices during the Mixtlan phase, such as agriculture.

Supplemental Material

sj-docx-1-hol-10.1177_09596836231185828 – Supplemental material for Paleoenvironmental study of the Late Preclassic period in the Northern Mesoamerican Frontier

Supplemental material, sj-docx-1-hol-10.1177_09596836231185828 for Paleoenvironmental study of the Late Preclassic period in the Northern Mesoamerican Frontier by Kurt H. Wogau, Benjamin Keenan, Helge W. Arz and Harald N. Böhnel in The Holocene

Footnotes

Acknowledgements

We thank the Programa de Becas Postdoctorales UNAM and Instituto de Investigaciones Antropologicas. We further thank Dra. Nadine Dräger, Prof. Dr. Achim Brauer and Dr. Jens Mingram for their technical support and for providing full access to Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ laboratory facilities.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by project PAPIIT (code 1N110117). This work is dedicated to the memory of Dr. Roberto S. Molina. Thank you for everything Roberto, I will always miss you.

ORCID iDs

Kurt H. Wogau

Benjamin Keenan

Supplemental Material

Supplemental material for this article is available online.

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