Early Holocene to present landscape dynamics of the tectonic lakes of west-central Mexico

Journal of South American Earth Sciences 80 (2017) 120e130

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Early Holocene to present landscape dynamics of the tectonic lakes of west-central Mexico ~ oz-Salinas a, b, Jose  Luis Arce a, Priyadarsi Roy a Miguel Castillo a, b, *, Esperanza Mun a b

noma de M Instituto de Geología, Universidad Nacional Auto exico, Ciudad de M exico, C.P. 04510, M exico noma de M Laboratorio Nacional de Geoquímica y Mineralogía, Universidad Nacional Auto exico, Ciudad de M exico, C.P. 04510, M exico

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 August 2017 Received in revised form 20 September 2017 Accepted 21 September 2017 Available online 22 September 2017

Paleoclimatic reconstructions from lake sediments of central Mexico indicate that the environmental conditions in the Holocene have oscillated from cool-dry to warm-wet, thus, landscape erosion rates have been modified accordingly. The Cenozoic tectonics and volcanic activity of west-central Mexico have produced a set of lakes in warmer and drier conditions compared to lakes of central Mexico. Nevertheless, the Holocene landscape dynamics for this area remains understudied. Using age-depth models, OSL and multi-element chemistry analysis of sediments in the lakes of San Marcos and Sayula we explore the landscape dynamics from early Holocene present of west-central Mexico. Our results indicate that the sedimentation rates in San Marcos Lake notably increased from 240 yr BP to the present. Since AD 1950 the sedimentation rate in Sayula Lake rose fourfold the rates of the last 2000 years. Analysis of OSL and chemistry of major elements of sediments indicates that IRSL/BLSL strongly correlates with Ti/Al (R2 ¼ 0.93) and with the mean monthly rainfall (R2 ¼ 0.70). We propose that the IRSL/ BLSL can be used as a proxy to infer past changes in landscape dynamics. Analysis of climatic data from the 1950s to present indicates that rainfall, and consequently water runoff, is enhanced in summers free of ENSO conditions. Extreme one-day rainfall can, however, exceed mean seasonal rainfall and occur in ~ a. Our results indicate that the all phases of ENSO. Droughts are particularly severe in the phase of La Nin erosion rate in San Marcos Lake was high from ~8000 to ~7000 yr BP in a period coinciding with the advance and recession of glaciers in Central Mexico, however, the erosion rates in the last 165 years have surpassed the rates of the early to mid-Holocene. By constraining the age of sediment and using environmental proxies such as the Ti/Al and IRSL/BLSL from lake sediments of Sayula and San Marcos we present the first model of landscape dynamics of this part of Mexico from the Early Holocene to present times. © 2017 Elsevier Ltd. All rights reserved.

Keywords: West-central Mexico OSL Landscape dynamics San Marcos and Sayula lakes Age-depth model

1. Introduction Analysis of pollen, diatoms, chemistry of sediment and geochronology using 14C, 137Cs and 210Pb of the lakes of central Mexico has yielded valuable information about the oscillation of climate from the Pleistocene to the present (e.g. Caballero-Miranda, 1997; Davies et al., 2005; Leng et al., 2005; Metcalfe and Davies, 2007; Lozano-García et al., 2015). Many interpretations made on past environmental conditions among different lakes, indicate that a generalized paleoclimatic model for central Mexico is

noma * Corresponding author. Instituto de Geología, Universidad Nacional Auto xico, Ciudad Universitaria, 04510, Me xico. de Me E-mail address: [email protected] (M. Castillo). https://doi.org/10.1016/j.jsames.2017.09.024 0895-9811/© 2017 Elsevier Ltd. All rights reserved.

complicated because environmental conditions were different among many lakes for the same period (Metcalfe et al., 2010; Park et al., 2010). Moreover, the steep topography and relief surrounding many of the lakes of central Mexico are factors that influence the distribution of rainfall and precipitation and, consequently, the erosion rates in the landscape. The advance and retreat of glaciers in the high-elevated stratovolcanoes of Central Mexico is of great importance since these provide valuable information of former environmental conditions in the Holocene. The reconstruction of the equilibrium line of altitude (ELA) in the Iztaccíhuatl volcano (~5286 m a.s.l. central Mexico) indicates that the ELA has migrated in altitude ~1030 m since the Last Glacial Maximum (LGM) (V azquez-Selem and Heine, 2011). Retreat of glaciers on this volcano is correlative with the elevation of many moraines of other stratovolcanoes of Central Mexico (Heine, 1994; White, 2002;

M. Castillo et al. / Journal of South American Earth Sciences 80 (2017) 120e130

zquez-Selem and Heine, 2011), thus, this volcano has been used Va to constrain the Holocene glacial evolution of central Mexico. Glaciers in central Mexico started their retreat ~15 ka (Caballero et al., 2010), this event is important because at the onset and during the deglaciation the landscape erosion rates must be high, presumably, by the increase in stream discharge. Lake sedimentary records of non-glaciated landscapes of Central Mexico indicate that the sedimentation rates were high after 15 ka (Ortega et al., 2010), but Quaternary paleoclimatic and landscape dynamics of areas far away of central Mexico have remained understudied. The Plio-Quaternary tectonic and volcanic activity of west-central Mexico has produced rapid changes in the topography resulting in the formation of many lakes. Because these water reservoirs are geologically young and they are free of glacial pulses, these lakes are ideal sites to evaluate changes in past environmental conditions during the Holocene and the consequent response of erosion. So far, the main paleoclimatic reconstruction for west-central Mexico was published by Metcalfe et al., in 2010. These researchers studied a sediment core extracted from the n Lake, which is in the highlands (>1900 m a.sl.) of the Juanacatla volcanic field of Mascota (Luhr et al., 1989). They provided the first runoff curve for the last 2000 years based on Ti concentrations in lake sediments. Their results indicate that dry conditions were from AD 200 to AD 400; from AD 600 to AD 750; from AD 800 to AD 1050 and from AD 1400 to AD 1600; and wetter conditions were from AD 1200 to AD 1400; from AD 1600 to AD 1750 and from AD 1800 to AD 1950. Besides this study, little is known about the past climate and landscape evolution of west-central Mexico. Motivated in understanding the landscape dynamics of westcentral Mexico during the Holocene, we studied the sedimentary record of the lakes of San Marcos and Sayula. We constrain the sedimentation rates by dating sediment with 14C and 137Cs. We use the multi-elements chemistry of sediment to obtain proxies of runoff and we incorporate a novel approach using the optically stimulated luminescence (OSL) to tease out information about the main changes of landscape dynamics. The sediment deposited around 1950s to present was correlated with the climatic data available from meteorological stations. Using these data, we explored the effect of precipitation on the OSL signals of sediments. Our results indicate that ~11,000 yr BP the climate was less arid in San Marcos Lake, but after ~8000 yr BP the climate became drier with a slight increase of runoff at around the time of the Little Ice Age (LIA), since then, the trend to present times is to aridity. We also propose that the luminescence in sediments can yield information of past landscape dynamics supporting our results with a multi-element chemistry analysis. The analysis made in this study allowed us to present the first of landscape dynamics model from the Early Holocene to present times for the lakes of west-central Mexico. 2. Regional setting 2.1. Study area and climate Lakes of Sayula and San Marcos are tectonic lakes of ~1480 km2 and ~1213 km2 of surface area, respectively (INEGI, 2016). They are located west of Chapala Lake (west-central Mexico), which is the greatest lake of Mexico (Fig. 1A). All these lakes are within the hydrologic region of the Santiago-Lerma river basin which has an area of ~135,493 km2 (INEGI, 2016). Catchment area of Lerma River is ~9507 km2. The Lerma river incises on its upper course into the central highlands of Mexico and discharges into Chapala Lake. The Santiago River initiates in the headwaters of the Sierra Madre Occidental (north-western Mexico) and north of Chapala Lake and discharges into the Pacific Ocean (Fig. 1A). The mean water

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discharge for the Santiago River is ~216 m3 s1 and transport ~ oz-Salinas and ~10201  103 tons of sediment annually (Mun Castillo, 2013). Although Sayula and San Marcos lakes are within the Santiago-Lerma river basin, each lake is isolated forming an individual system. Altitude of San Marcos Lake is ~1352 m and ~1346 m for Sayula €eppen's classification), Lake. Climate in this zone is arid (Bw in Ko the mean temperature is ~20  C and annual rainfall is ~600 mm. There are ~15 meteorological stations distributed in the zone of lakes but only few of them contain continuous records of more than 30 years (Fig. 1B). Rainfall is mostly concentrated in summer and induced by the formation of low pressure systems in east Pacific and by the influence of convective clouds formed in the Inter~a tropical Convergence Zone (Cavazos and Hastenrath, 1990; Magan et al., 2003). ~ o Southern Oscillation (ENSO) on the preThe effect of El Nin cipitation and the hydro-climatology for Sayula and San Marcos ~ oz-Salinas lakes are not known in detail, but published data by Mun and Castillo in 2013 confirm an effect of ENSO on the water discharge of Santiago River. These authors found that in summers of ~ a, water discharge increases ~40% (~1637 m3 s1) more than La Nin ~ o. In El Nin ~ o winters, water discharge is in the summers of El Nin ~ a. The ~50% more than the discharge observed during La Nin ~ o also diminishing of stream discharge in the summers of El Nin coincides with the number of droughts recorded on west-central Mexico (Jauregui, 1995). 2.2. Geology and geomorphology Sayula and San Marcos lakes are in the north-eastern sector of the Jalisco Block, this last is a crustal unit moving away from the mainland of Mexico (Luhr et al., 1985). Continental boundaries of the Jalisco Block are the Tepic-Zacoalco Rift (TZR) in the northnortheast, and the Colima Rift (CR) in the east-southeast. The rifts of the Jalisco Block are composed of systems of faults, grabens and half-grabens (Ferrari et al., 2012). San Marcos and Sayula lakes formed in the zone where the TZR and CR converge (Fig. 1B) where they form a triple junction (Allan, 1986). The 1:250000 geologic maps published by the Mexican Geological Service (SGM) indicate that the rocks around the zone of San Marcos and Sayula lakes are Tertiary volcanic rocks of basic to intermediate composition. North of San Marcos lake there are minor edifices and lavas emitted in the Quaternary of basic to intermediate composition. Materials conforming of zone of lakes are mapped as Quaternary alluvium by the SGM. In the compilation of rock ages made by Ferrari et al. (2000) the mean age of rocks (determined from the K/Ar method) around the zone of the lakes is ~3.8 Ma. The composition of the dated volcanic rocks is from basalt to andesite. The youngest dated rock is ~0.9 Ma and the oldest is ~6 Ma. Faulting of rocks is readily visible from the topography (Fig. 1B). San Marcos Lake is bounded to the east by a system of faults oriented from the NW to the SE that belongs to the San Marcos fault (Ferrari and Rosas-Elguera, 2000). Sayula Lake is bounded to the west by a N to S fault system (Fig. 1B). The age of faults has not been determined with radiometric methods but as long the volcanic rocks are ~3.8 Ma it is probable that the faults were formed during the Plio-Quaternary. Moreover, Ferrari and Rosas-Elguera (2000) estimated that the age of San Marcos fault in ~3 to ~1 Ma, and in for the half-graben of Sayula in ~5 to ~3 Ma (Fig. 1B). Thus, the maximum age of these lakes must be not more than 3 Ma. Two contrasting landforms predominate in the landscape of west of Chapala Lake, the alluvial and lacustrine plains of the San Marcos and Sayula lakes and the mountainous landscape surrounding the lakes. In San Marcos Lake, the height between the lacustrine plain and mountains is ~600 m. In Sayula Lake the height

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M. Castillo et al. / Journal of South American Earth Sciences 80 (2017) 120e130

Fig. 1. Location map of west-central Mexico (A) and location of the sample extraction trenches (B). In B, stars indicate the location of trenches, the meteorological stations are marked as dotes with is code above them.

from the lake to the summit of mountains is ~600e~1400. In San Marcos Lake, a narrow piedmont separates the main fault scarp from the lacustrine plain, and in the western side of Sayula Lake many alluvial fans are located at the base of the fault scarp. The length of these fans is ~2e~4 km from the apex to their base. Formation of many valleys at the east of Sayula Lake and the development of alluvial fans are geomorphic evidences supporting an older age for the Sayula half-graben.

3. Materials and methods 3.1. Sampling strategy, sample dating and depositional rates Most the surface of Sayula Lake and whole San Marcos Lake are free of water. Sediments of these lakes are hard and compacted. In the lake of Sayula we dug a trench (Fig. 1B) of ~87 cm of depth. We observed that the pack of sediment was homogeneous along the profile with a predominance of a light brown silty and clayish sediment. We observed a layer of charcoal at ~42 cm up to ~52 cm of depth, just below the charcoal layer we found many fragments of pottery but nothing else below 55 cm of depth. Samples of San Marcos Lake were extracted from a profile of ~218 cm of depth (Fig. 1B). This trench was dug by local developers to build an artesian well but it was abandoned due to high alkalinity of water. The level of ground water in this zone is at ~220 cm of depth. We found shells at ~12 cm of depth and grass roots from the surface to ~23 cm of depth. In this site, the pack of sediment is also a homogeneous silty to clayish sediment of light brown colour. Our sampling strategy consisted in extracting sediment every 2 cm for each profile. Each sample contained a volume of material of ~200 cm3. All samples extracted from the two lakes are rich in organic sediment and suitable for 14C dating. Our main criterion for selecting 14C samples was based on dating only those samples that coincide with major changes of luminescence with depth (see the OSL section for details). A total of 5 samples were selected for San Marcos Lake and 2 for Sayula Lake. Samples were sent to Beta Analytic for their analysis with an AMS. The 14C ages of the two sampled profiles progressively increase with depth (Table 1). The oldest conventional 14C age in the samples of San Marcos is

9620 ± 40 yr BP at 218 cm of depth and for Sayula the oldest age is 2460 ± 30 yr BP at 80 cm of depth. The youngest conventional 14C age is 101 ± 0.3 yr BP which is in San Marcos at ~18 cm of depth. To constrain the 1950 AD in the profiles we measured the 137Cs in the sediment using a beta and gamma spectrometer model AT1315 produced by ATOMTEX®. The gamma channel has a NaI(TL) detector with an error of ±20% reported by the manufacturer. We analysed ~20 gr for each sample of the two profiles starting from the top surface and progressively measuring the samples with depth. We determined the 1950 AD in the last sample where were we detected 137Cs activity (Fig. 2). 137Cs samples were analysed in the Laboratory Geocron-Q that belongs to the Institute of Geology of the National Autonomous University of Mexico (UNAM). In the profile of Sayula Lake we detected two peaks of 137Cs (Fig. 2), one at 2 cm (28 Bq kg1) and a second one at 10 cm of depth (31 Bq kg1). In the San Marcos profile, we only detected one peak at 4 cm of depth (7 Bq kge1; Fig. 2). Based on 137Cs activity in the two profiles we constrained the 1950 at 10 cm in the Sayula profile and 6 cm for the San Marcos profile. We found that the peaks of 137Cs are different between Sayula and San Marcos, we suspect that this dissimilarity is probably caused by the removal of topsoil of San Marcos lake where the sediment was extracted in a trench previously dug by developers using heavy machines. It is highly possible that the few centimetres of topsoil were removed at the sampling site by anthropic actions. For the case of Sayula, we found not evidences of human activities in the sampling site, the only evidence of human activity was the fragmented pottery found at ~52 cm.

3.2. OSL and geochemical analysis Our strategy to extract the OSL samples consisted in inserting plastic tubes of ~10 cm length and with a diameter of ~5 cm for every 2 cm along the sediment profile. Once collected each sample and to avoid its exposition to sunlight, each plastic tubes was wrapped in aluminium foil. Samples were analysed in the Laboratory Geocron-Q using a Photon-Pulsed Stimulated Luminescence unit (PPSL), this equipment was built in the Scottish Universities Environmental Research Centre (UK) (Sanderson and Murphy, 2010). OSL samples were dried at room temperature and placed

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Table 1 List of the age of sediment samples dated with radiocarbon. Sample code

Laboratory code

Site

Depth (cm)

14

2 sigma range (BP)a

Cal. age (BP)a

Material

TJZ16-C3 TJZ16-C3 TJZ16-C3 TJZ16-C3 TJZ16-C3 TJZ16-C2 TJZ16-C2

Beta Beta Beta Beta Beta Beta Beta

San Marcos San Marcos San Marcos San Marcos San Marcos Sayula Sayula

218 160 106 68 18 80 52

9620 ± 40 7240 ± 40 6320 ± 30 2820 ± 30 101 ± 0.3 2460 ± 30 1250 ± 30

10781e11035 8161e7979 7173e7307 2852e3000 226e254 2760e2861 1172e1273

10908 8070 7240 2926 240 2810 1222

Organic Organic Organic Organic Organic Organic Organic

a

0 cm 58 cm 112 cm 150 cm 200 cm 80 cm 52 cm

-

442562 444142 442563 444143 442564 442566 442565

C age (BP)

sediment sediment sediment sediment sediment sediment sediment

Calibration using IntCal13.14C (Reimer et al., 2010) using the code “CLAM” (Blaauw, 2010) writen for R (R Development Core Team, 2010).

Fig. 2. Graphs of 137Cs activity in the sediment samples of the lakes of San Marcos and Sayula. The profile analysed in San Marcos has only one 137Cs peak, removal of topsoil in the sampling site is a plausible explanation of why the second peak is missing in the profile which is present in Sayula profile.

in aluminium petri dishes of ~10 cm of diameter and covering the whole surface of the petri. Samples were stimulated for 60 s in the red wavelength and 60 s in the blue wavelength. The IRSL and BLSL signals were plotted versus depth (Fig. 3). In the profile of Sayula the IRSL and BLSL are well correlated with depth (R2 ¼ 0.70 for the IRSL and R2 ¼ 0.61 for the BLSL; Fig. 3A). In the profile of San Marcos, the IRSL is not well correlated with depth (R2 ¼ 0.16) but the BLSL has a fair correlation (R2 ¼ 0.58). Nevertheless, in San Marcos Lake the BLSL abruptly changes at ~150 cm where there is a reversal in the expected pattern of luminescence (Fig. 3B). We also analysed the relationship between the IRSL and BLSL for the Sayula and San Marcos profiles (Fig. 3C and D). We observed that in Sayula Lake the IRSL weakly correlates with the BLSL (R2 ¼ 0.48; Fig. 3C) and we observed almost no correlation (R2 ¼ 0.17) between the IRSL and BLSL for the case of San Marcos Lake. Elemental concentrations were determined in oven dried at 40  C, homogenized and ground sediment samples using a Thermo Scientific Niton FXL 950 X-ray Fluorescence (XRF) analyzer. Approximately ~5 g dry and powdered sediment was placed in a plastic capsule and it was covered using a thick polypropylene Xray film. Samples were analyzed in four different filters of mining Cu/Zn mode and the obtained data were corrected using the linear regression equations generated after comparing results of Niton FXL and traditional Siemens XRF as per Roy et al. (2012). Elements analyzed were Zr, Sr, Rb, Zn, Cr, Fe, Mn, Ti, Ca, K, Al, P, Si and Mg. This

analysis was made in the Laboratory of Paleoenvironments and Paleoclimates of the Institute of Geology (UNAM). Because the concentration of elements like Ti, Si and K are indicators of erosion and runoff (Lopez et al., 2006; Metcalfe et al., 2010; Roy et al., 2012; Lozano-García et al., 2015), these were plotted together with the IRSL and BLSL ratio (Fig. 4). We noticed that the major changes in the concentration of Ti, Si and K coincide with changes in the IRSL/ BLSL. Since the concentration of Ti and Al in sediment are common in volcanic rocks here we also used the Ti/Al as proxy of runoff or fluvial input (e.g. Perez et al., 2016).

3.3. Analysis of climatic data We used the available data of precipitation and evaporation and ~ o (warm phase), related these with the three phases of ENSO: El Nin ~ a (cold phase) and non-Enso (also termed here as normal La Nin conditions). Meteorological data were downloaded from CLICOM platform compiled and managed by the CICESE (http://clicom-mex. cicese.mx). In this platform are available the meteorological data of the main stations of Mexico. Most of the stations are ruled by the n Nacional del Agua Mexican government office of the Comisio (CONAGUA). There are ~15 stations in the zone of the Sayula and San Marcos lakes, but most of them have few years of measurements or contain incomplete records of meteorological data. We n”), selected the stations 14018 (“Atoyac”), 14146 (“Teocuitatla

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Fig. 3. Profiles of IRSL and BLSL with depth (A and B) and scaling between the IRSL and BLSL (C and D) for the lakes of Sayula and San Marcos. The weak scaling of IRSL and B suggest changes in the pattern of sedimentation. The scaling of C and D denotes a different bleaching of grains.

n de Ju 14077 (“Jocotepec”), 14002 (“Acatla arez”) and 14168 (“Zacoalco de Torres”) because these contain about 40e50 years of almost continuous meteorological data (Fig. 1B). Phases of ENSO ~ o Index (ONI) series published by were based on the Oceanic Nin National Weather Service (http://www.cpc.ncep.noaa.gov) and the National Oceanic and Administration (NOAA). The NOAA is an office of the government of the United States of America. The main advantage in using the ONI series is that the information of the phases of ENSO are updated monthly and available up to 1950s. The analysis of the climatic data and the phases of ENSO was done per season. Winter season runs from November to April and summer is from May to October. We used the data of mean monthly rainfall, maximum rainfall intensity, mean monthly evaporation, maximum evaporation in 24 h and the aridity index which is the ratio between the mean monthly rainfall and the mean monthly evaporation (Table 2).

4. Results Using the 14C calibrated and 137Cs ages we constructed two agedepth models (Fig. 5A and B). Based on these models we estimated different phases of sedimentation (Fig. 5C). The age-depth model of San Marcos Lake indicates that the sedimentation rate increased from 0.24 to 0.59 mm yr1 from ~7240 yr BP to ~6230 yr BP (Fig. 5C). After 6230 yr BP and before 101 yr BP the sedimentation rates drop to ~0.08e0.11 mm yr1, respectively. The sedimentation rates sharply increased from 101 yr BP to 1950 AD to ~1.2 mm yr1 and decreased to ~0.6 mm yr1 from 1950 AD to 2015. The agedepth of Sayula spans from 2460 yr BP to 2015 AD only (Fig. 5A). In Sayula Lake the sedimentation rates were ~0.2e~0.3 mm yr1 from 2460 yr BP to 1950 AD. After 1950 AD, the sedimentation rate

has been ~1.2 mm yr1, meaning an increase of ~70% in the last 65 years (Fig. 5C). Fragments of pottery in Sayula Lake at ~52 cm fall in the age of 700 AD, corresponding to the Epiclassic Mesoamerican period under the architecture of the Teuchitlan tradition (Beekman, 2000). Using the age constrains and sedimentation rates of the San Marcos and Sayula profiles (n ¼ 9; Fig. 5A and B) we estimated the mean value of the Ti/Al and IRSL/BLSL for each period and we estimated the uncertainty at the 95% confidence level. We found a strong correlation between the IRSL/BLSL and Ti/Al (R2 ¼ 0.92; Fig. 6A). The relationship between the sedimentation rates and Ti/ Al was also explored (Fig. 6B). The regression of the sedimentation rates and Ti/Al in Sayula Lake resulted in a strong correlation (R2 ¼ 0.89) but not significant because of the reduced number of cases (p-value >0.2). In the lake of San Marcos, the regression between the sedimentation rates and Ti/Al results in almost no correlation (R2 ¼ 0.18; p-value >0.3). Based on the apparent deviation of the Ti/Al concentrations from the trend observed for the sedimentation rates older than cal. 240 yr BP the regression for the data of San Marcos was repeated but in this case, we only removed the sedimentation rates from cal. 240 yr BP to the present (Fig. 6B). The regression yielded a good correlation (R2 ¼ 0.79) between the sedimentation rates and the Ti/Al, this however, was not statistically significant (p-value >0.1). Results of the seasonal analysis indicate that the rainfall in all normal summers (i.e. non-ENSO conditions) is ~40% more than in ~ o and La Nin ~ a with exception of station 14168, summers of El Nin where the rainfall during normal summers only contributes to ~20% ~ a summers. In non-ENSO summers occurs in comparison to La Nin the highest rates of evaporation, with exception of station 14146 that occurs in non-ENSO winters. Nevertheless, the difference

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Fig. 4. Variation of IRSL/BLSL and multi-elements with time. The Ti concentration in San Marcos Lake indicates less arid conditions around the Younger Dryas, ~1000 yr BP and around the LIA (lines in diagonals). The concentration of Si/Al and K/Al are inversely related to the Ti concentration. Changes in the TI concentration are also related with variation in the IRSL/BLSL. The thick line indicates the main trend for the profile and the grey area marks the 95% confidence level for the trend.

between of summer and winter with non-ENSO conditions is less than 16% and it is only ~10e20% more than in summers and winters ~ o and La Nin ~ a. Maximum rainfall intensity recorded on of El Nin each station occurs in all the phases of ENSO (Table 2). Such extreme events exceed, in most of the cases, the amount of the mean rainfall registered during non-ENSO summers. In stations 14018, 14168 and 14146, which are 50% of the stations analysed in this study, extreme rainfalls occurred in winters and summers ~ o. The index of aridity in the study area is enhanced having El Nin ~ a, but it is extreme in during the winters and summers of La Nin ~ a conditions. winters under La Nin Stations 14018 and 14168 contain almost continuous records of climatic data from the 1950s to present. We used data of the mean monthly rainfall of these stations to explore their relationship with the IRSL/BLSL ratio. The different rainfall periods were based on the age-depth models of the Sayula and San Marcos lakes and are presented in Fig. 7A and B. The IRSL/BLSL were estimated by averaging the two values marking each period and including the data of San Marcos and Sayula. We performed a linear regression of the IRSL/BLSL against the mean monthly rainfall (Fig. 7C). This last

analysis strongly suggest that the mean monthly precipitation is related with changes in the IRSL/BLSL meaning that the difference between the IRSL and BLSL occurs where there is a drop in the amount of rainfall in the landscape.

5. Discussion and conclusions 5.1. Holocene landscape dynamics Our age-depth models confirm that sedimentation rates in San Marcos Lake have been variable during the Holocene (Fig. 5B). The concentration of Ti, which is a proxy runoff (Metcalfe et al., 2010), was high from ~10900 to ~10000 yr BP. The highest records of Ti concentrations are within the timing of the Younger Dryas. For this period the Ti/Al was high while the Si/Al and K/Al ratios were low as well as the IRSL/BLSL (Fig. 4). The IRSL/BLSL is expected to change with the abundance of either Si/Al or K/Al since Si and K are major components of quartz and feldspars, respectively. Based on the high Ti concentrations and low IRSL/BLSL values we interpret that this period was dominated by runoff but with relatively low erosion

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Table 2 Seasonal statistics of rainfall, evaporation and aridity index during the phases of ENSO. Uncertainty of the mean montly rainfall and evaporation is at the 95% confidence level. Bold numbers indicate extreme seasonal values. ~o El Nin

Data

Station 14018 (Atoyac) Mean monthly rainfall (mm) Maximum rainfall intensity (mm/24 Mean monthly evaporation (mm) Maximum evaporation (mm/24 h) Mean montly Rainfall/Evaporation Station 14002 (Acatlan de Ju arez) Mean monthly rainfall (mm) Maximum rainfall intensity (mm/24 Mean monthly evaporation (mm) Maximum evaporation (mm/24 h) Mean montly Rainfall/Evaporation Station 14077 (Jocotepec) Mean monthly rainfall (mm) Maximum rainfall intensity (mm/24 Mean monthly evaporation (mm) Maximum evaporation (mm/24 h) Mean montly Rainfall/Evaporation Station 14168 (Zacoalco de Torres) Mean monthly rainfall (mm) Maximum rainfall intensity (mm/24 Mean monthly evaporation (mm) Maximum evaporation (mm/24 h) Mean montly Rainfall/Evaporation Station 14146 (Teocuitatlan) Mean monthly rainfall (mm) Maximum rainfall intensity (mm/24 Mean monthly evaporation (mm) Maximum evaporation (mm/24 h) Mean montly Rainfall/Evaporation

h)

h)

h)

h)

h)

~a La Nin

Normal

Winter

Summer

Winter

Summer

Winter

Summer

16 ± 8 71.0 e e e

51 ± 9 83.3 e e e

5±3 57.4 e e e

49 ± 9 60.0 e e e

6.2 ± 3 55.9 e e e

89 ± 10 76.3 e e e

18 ± 9 85.0 122 ± 10 9.8 0.15

67 ± 12 77.3 140 ± 8 1.2.2 0.48

6±3 92.8 139 ± 11 13.5 0.04

58 ± 13 68.0 146 ± 9 13.7 0.40

7±3 49.0 153 ± 13 12.7 0.05

105 ± 14 101.0 167 ± 9 14.5 0.63

11 ± 4 63.0 119 ± 17 10.5 0.09

61 ± 12 98.8 138 ± 14 12.7 0.44

6±3 71.0 140 ± 18 12.6 0.04

58 ± 13 120.0 150 ± 14 16.0 0.39

9±3 45.0 132 ± 16 11.9 0.07

102 ± 15 109.0 156 ± 14 16.6 0.65

8 ± 5.3 60.0 e 12.9 e

35 ± 7 127.0 e 13.2 e

4±3 45.0 e 15.0 e

45 ± 11 84.0 e 12.9 e

2±1 25.0 e 11.6 e

58 ± 10 93.0 e 13.6 e

13 ± 7 70.0 116 ± 11 11.1 0.11

46 ± 11 183.0 125 ± 9 14.7 0.37

5±3 69.0 131 ± 11 13.9 0.04

35 ± 9 78.0 132 ± 8 13.4 0.27

9±5 55.0 159 ± 14 13.3 0.06

66 ± 10 123.0 155 ± 11 17.7 0.43

Fig. 5. Age depth model and sedimentation rates for Sayula (A) and San Marcos lakes (B).

rates since the lake deposition was ~0.24 mm yr1 (Fig. 5B). From ~10000 to ~8000 yr BP it was a progressive declining of Ti concentrations but with a sharp fall from ~8500 to ~7200 yr BP

(Fig. 4). The rapid fall of the Ti coincides with an increase in Si/AL and K/Al ratios and with high IRSL/BSL values (Fig. 4). We interpret that this trend results from a change in the environmental

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Fig. 6. In A is shown the correlation between the IRSL/BLSL and Ti/Al for each of the period of ages constrained with the 14C data (Fig. 5). Horizontal and vertical bars is the uncertainty is a 95% confidence level. This scaling strongly suggest that the bleaching of quartz and feldspars result from changes in the environmental conditions. In B are plotted the sedimentation rates against the Ti/Al. The deviation of Ti/Al of the most recent samples of San Marcos from apparent trend observed from other samples suggest that the erosion in the landscape has increased due to processes unrelated to environmental conditions.

conditions where climate became more arid (low Ti values) and the mobilization of sediment was, perhaps, in episodes characterized by a massive transport of sediment. This seems a plausible explanation for the increase in the Si/Al, K/Al and IRSL/BLSL ratios. Erosion rates increased in these periods since the lake depositional rate was ~0.59 mm yr1. From ~7200 to ~2000 yr BP the Ti concentrations were at the lowest and the Ti/Al declined while the Si/ Al and K/Al reached the highest values and the IRSL/BLSL notably increased for the same period (Fig. 5B). Low concentrationS of Ti and Ti/Al suggest that the environmental conditions reached the maximum level of dryness in the Holocene. Erosion rates were relatively low since the deposition rate was ~0.11 mm yr1 and the continuous high Si/Al and K/Al suggest that the transport sediment was probably in episodic events that mobilized bulky packs of sediments. We discard an important effect of the depositional rates caused by volcanic activity since most of the volcanic rocks in the area range from 1 to 5 Ma and the most recent volcanism (Quaternary) is mostly concentrated outside the lakes, on the north. Also, the fact that the lakes are in a tectonically active area does not seem to relate with changes in the depositional rates since most of the faults bounding the lakes formed in the Pliocene and Pleistocene (Ferrari et al., 2000). Thus, the sediments stored in the lakes of San Marcos and Sayula are highly likely to result from the erosion of the surrounding landscape. Increase in the Ti concentration from ~2000 to ~900 yr BP coincides with a drop in the Ti/Al, Si/Al and K/Al ratios, and with a sudden increase in the IRSL/BLSL (Fig. 4). Our interpretation is that in this period there was a returning of less arid conditions and runoff dominated the landscape dynamics like occurred ~8000 yr BP. This period was, however, a short-lived event and the erosion rates were low since the rate of sediment deposition in San Marcos was ~0.08 mm yr1. Interestingly, the IRSL/BLSL sharply increased during this period, which resulted from more supply of K in the

sediments. The limited record obtained for Sayula Lake makes difficult the correlation with the results of San Marcos, however, in Sayula the Ti concentrations are also related with changes in the Ti/ Al, Si/Al, K/Al and the IRSL/BLSL (Fig. 4) and the amount of precipitation is about the same of San Marcos (Fig. 7), thus, it is reasonable to assume that the changes in climate and landscape dynamics at San Marcos can be extrapolated to the case of Sayula. It is important to note that the rates of erosion/deposition of Sayula are greater than in San Marcos, recalling that Sayula has more area (~103 km2) than San Marcos (~24 km2) and that the number of rivers discharging into Sayula exceeds those of San Marcos (Fig. 1). This explains why we could only cover two millennials in Sayula at a depth of ~80 cm. Thus, the profile of San Marcos contains a complete record of the changes in the environmental conditions of the zone. From ~900 yr BP to the present the Ti concentrations in San Marcos have been reducing (Fig. 4), interrupted by a sudden increase of Ti at ~200 yr BP (19th Century). The same occurs in Sayula where the Ti increased ~230 yr BP (Fig. 4). This period is within the LIA which in central Mexico was characterized by a high climatic variability with dry and humid conditions (Lozano-García et al., 2007; Metcalfe et al., 2010). For the case of Sayula and San Marcos the resolution of our analysis does not allow to evaluate the LIA in detail but the notorious increase in Ti concentrations (Fig. 4) suggest that the runoff was particularly enhanced during this period which also coincides with high Ti values obtained by Metcalfe et al. (2010) in Laguna de Juanacatl an for the same period.

5.2. Historical climate records Results of the instrumental period indicate that from the 1950s to the present there has been a slight decrease in the amount of rainfall recorded on stations 14018 and 14168 (Fig. 7). Our multi-

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Fig. 7. In A and B are shown the mean values of monthly rainfall for the instrumental period. The time periods are based on the age-depth model. In C is shown the relationship between the IRSL for each of the instrumental period in A and B against the monthly rainfall. The positive correlation suggests the amount of rainfall influences the beaching of quartz and feldspars.

element analysis of San Marcos also suggests that there is a tendency to aridity towards the present since there is a notably drop in the Ti and Ti/Al after the LIA period (Fig. 4). The correlation found between the IRSL/BLSL with the mean monthly rainfall (Fig. 7C) confirms our idea that the OSL of sediments can be indicative of changes in landscape dynamics. Changes in IRSL/BLSL are due to grain sensitivity (Aitken, 1998), variation of grain sizes (e.g. Kenworthy et al., 2014), bleaching of minerals during their transport in the water column (e.g. Wallinga, 2002; e.g. Duller, 2008; Bishop et al., 2011), changes in the erosion patterns in the catch~ oz-Salinas et al., 2014; Portenga et al., 2016) and the ment (e.g. Mun supply of different minerals in the catchment (Aitken, 1998). For the case of the lakes of San Marcos and Sayula the effect of grain size on the OSL signals is negligible since the sediment fraction analysed ranges between clay and silty loam. Changes in the IRSL/BLSL caused by the supply of different types of minerals into the lakes is also unlikely since the volcanic activity ceased ~0.9 Ma. We do not discard that ash-fall and pyroclastic products from the Colima Volcano, which is more than 50 km to the south have fallen in the zone of lakes, but most the volcanic deposits of this volcano are distributed on its southern flank (Saucedo et al., 2005) so changes in the IRSL/BLSL must be minimal. Luhr et al. (2010) reported a tephra fall unit (their Unit Y) of ~1 m thick with a maximum age of 5320 ± 293 cal. BP. Deposits of this event were found by Luhr and co-workers close to Ciudad Guzman, a town located in the north-western flanks of the volcano. We did not find

this event in the sedimentary record of San Marcos Lake. We suspect that tephra layer was thin in the surroundings of San Marcos and Sayula lake. In 1913 an important eruption modified the summit of the volcano and produced the pumice and ash fall of ~35 m thick. Deposits of this event have been recognized more than 700 km to the NW of the volcano (Saucedo et al., 2005). We did not find deposits of this event in the profiles of Sayula and San Marcos, thus, it is highly possible that the volcanic materials have mostly distributed NW of the volcano and with minimal cover on the N where are located the lakes studied here. Based on the positive correlation of Fig. 7C we interpret that the variation in the IRSL/ BLSL results from the different bleaching of grains during their transport. Our interpretation is that when the amount of rainfall increases, the runoff produces a continuous bleaching of quartz (BLSL) and feldspars (IRLSL), but when the rainfall drops, the mobilization of sediments occurs in episodic floods (cf. Bull, 1997) where minerals are rapidly transported in bulky sediment. Since quartz bleaches faster than feldspars (Godfrey-Smith et al., 1988), the drop in the IRSL/BLSL of Fig. 7C (i.e. the increase in the BLSL) can be attributable to the rapid bleaching of quartz grains during their transportation and their further deposition. This behaviour partially explains the long-term variation of IRSL/BLSL, Si/Al and K/ Al when contrasted with the Ti (Fig. 4). Low Ti concentrations correspond to peaks of Si/Al (quartz) and K/Al (feldspars), meaning that erosion increases during the dry periods where intense and episodic rains produce the mobilization of bulky sediments, in this

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case the IRSL/BLSL values remain relatively low. When the Ti increases, the IRSL/BLSL increases as well but the K/Al and Si/Al values drop. The similar shape of the K/Al and Si/Al curves indicates that the concentrations of these elements vary as a function of runoff (Ti concentration values), and because these elements are major components of feldspars and quartz, respectively, part of the variation of the IRSL/BLSL values probably result from changes in the environmental conditions. The seasonal analysis of the meteorological data confirms that summers having non-Enso conditions, concentrate most of rainfall in the zone of lakes, however, extreme rainfall events (i.e. 24 h rainfall), which occur in any of the phases of the ENSO, and these events can, indeed, exceed the mean monthly rainfall recorded on the summer (Table 2). Evaporation surpasses rainfall in the zone of lakes, confirming the prevailing arid conditions. Evaporation is particularly high in summer with non-Enso conditions but this is not significantly higher than the evaporation recorded on other phases of ENSO and from the evaporation recorded on winter (Table 2). The rainfall and evaporation ratio confirms that in either ~ a, the aridity in the landscape the winters or summers of La Nin reaches its maximum values (Table 2). We suspect that the erosion ~ a where extreme rainfalls, and the subis enhanced during La Nin sequent runoff, can produce the mobilization of loose sediment. Our climatic data for the instrumental period indicate that the climate in the zone of lakes is highly arid. Having in mind the concentration of Ti for the instrumental period and observing the whole trend from the early Holocene to the present we infer that the aridity in the landscape in the early Holocene was less that in the present (Fig. 4), but with a remarkable period of dryness from ~7000 to 2000 yr BP. 5.3. Final remarks Our study of the sediment extracted from the lakes of Sayula and San Marcos (west-central Mexico) based on age depth-models, measurement of OSL using a PPSL unit and the multi-element chemistry allowed us to detect the changes in landscape dynamics in the zones of lakes of west-central Mexico. The Ti concentration from San Marcos lake indicates that the climate has increased its aridity since the timing of the Younger Dryas but the wetter period was from ~10000 to ~8000 yr BP. Arid conditions in San Marcos Lake were particularly high from 72000 to ~2000 yr BP, the onset of this dry period started after the advance and retreat of glaciers Central Mexico. At around the LIA there was a less arid period but this was less wet than that observed in the early Holocene. Interestingly, in Sayula and San Marcos the deposition/ erosion rates in the last 165 years has increased significantly. We found evidences of human activities in this zone from 700 AD based on the position of pottery found in Sayula Lake at ~50 cm. Even though human activities may have started earlier in this area, the rates of deposition/erosion before the last 200 years were lower than in present times. We suspect that the clearance of vegetation for agriculture and cattle activities in the last 200 years is related to soil erosion which in turn, has increased the rates of sedimentation in the lakes. For the instrumental period, we found that the IRSL/BLSL positively scales with the mean monthly rainfall. Using the IRSL/BLSL in depositional settings is a promissory tool for understanding the landscape dynamics, but testing this approach in other areas is needed. Our data indicate that the climate around the lakes of westcentral Mexico has been arid during the most of the Holocene. Present environmental conditions in the lakes of Sayula and San Marcos are arid but this is particularly enhanced in both winters ~ a conditions. The lake of San Marcos and summers under La Nin provided us a continuous record of from the early Holocene to the

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present and we present the first long-term landscape dynamics for the zone of the lakes of west-central Mexico. Further work requires extending the analysis to the Pleistocene, for that purpose the lake of Sayula is an ideal site to extend the analysis of climate and landscape dynamics since this is probably the oldest lake of westcentral Mexico. Acknowledgments This research benefited from the funds provided by the DGAPAPAPIIT grant (Ref. IA103506) and the CONACYT grant (CB-2013-01, Ref. 221803). We thank the undergraduate students A. Valoix and A. Godínez for their help during fieldwork and J. Zavala for his help in the preparation and processing of samples for the geochemical analysis. We kindly thank to two anonymous reviewers whose comments improved the final version of this manuscript. References Aitken, M.J., 1998. An Introduction to Optical Dating. The Dating of Quaternary. Oxford University Press, New York, United States of America. Allan, J.F., 1986. Geology of the northern Colima and zacoalco grabens, southwest Mexico: late cenozoic rifting in the mexican volcanic belt. Geological society of America bulletin. Geol. Soc. Am. 97, 473e485. Beekman, C.S., 2000. The correspondence of regional patterns and local strategies in formative to Classic Period west Mexico. J. Antropological Archaeol. 19, 835e412. ~ oz-Salinas, E., MacKenzie, A.B., Pulford, I., McKibbin, J., 2011. The Bishop, P., Mun character, volume and implications of sediment inpounded in mill dams in Scotland: the case of the Baldernock mill dam in east Dumbartonshipre. Earth Environ. Sci. Trans. R. Soc. Edinb. 101, 97e110. Blaauw, M., 2010. Methods and code for ‘classical’ age-modelling of radiocarbon sequences. Quat. Geochronol. 5, 512e518. Bull, W.B., 1997. Discontinuous ephemeral streams. Geomorphology 19, 227e276. Caballero-Miranda, M., 1997. The last glacial maximum in the basin of Mexico: the diatom record between 34,000 and 15,000 years BP from Lake Chalco. Quat. Int. 43, 125e136. zquez-Selem, L., Ortega, B., 2010. Evidencias de Caballero, M., Lozano-García, S., Va tico y ambiental en registros glaciales y en cuencas lacustres del cambio clima xico durante el último ma ximo glacial. Bol. la Soc. Geol. Mex. 62, centro de Me 359e377. Cavazos, T., Hastenrath, S., 1990. Convection and rainfall over Mexico and their modulation by the Southern Oscillation. Int. J. Climatol. 10, 377e386. Davies, S.J., Metcalfe, S.E., Bernal-Brooks, F., Chacon-Torres, A., Farmer, J.G., MacKenzie, A.B., Newton, A.J., 2005. Lake sediments record sensitivity of two hydrologically closed upland lakes in Mexico to human impact. AMBIO A J. Hum. Environ. 34, 470e475. Duller, G.A., 2008. Single-grain optcial dating of Quaternary sediments: why aliquot size matter in luminescence dating. Boreas 37, 589e612. Ferrari, L., Orozco-Esquivel, T., Manea, V., Manea, M., 2012. The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone. Tectonophysics 522e523, 122e149. , G., Venegas-Salgado, S., Romero-Ríos, F., 2000. Geology of the Ferrari, L., Pasquare western mexican volcanic belt and adjacent Sierra Madre occidental and Jalisco block. In: Delgado-Granados, H., Aguirre-Díaz, G., Stock, J. (Eds.), Cenozoic Tectonics and Volcanism of Mexico. Geological Society of America (Special Paper), pp. 65e83. Ferrari, L., Rosas-Elguera, J., 2000. Late Miocene to Quaternary extension at the northern boundary of the Jalisco block, western Mexico: the Tepic-Zacoalco rift revised. In: Delgado-Granados, H., Aguirre-Díaz, G., Stock, J. (Eds.), Cenozoic Tectonics and Volcanism of Mexico. Geological Society of America (Special Paper), pp. 1e23. Godfrey-Smith, D.I., Huntley, D.J., Chen, W.H., 1988. Optical dating studies of quartz and feldspar sediment extracts. Quat. Sci. Rev. 7, 373e380. Heine, K., 1994. The late-glacial moraine sequences in Mexico: is there evidence for the Younter Dryas event? Palaeogeogr. Palaeoclimatol. Palaeoecol. 112, 113e123. ficas. Available from. INEGI, 2016. Simulador de flujos de agua de cuencas hidrogra http://www.inegi.org.mx. Jauregui, E., 1995. Rainfall fluctuations and tropical storm activity in Mexico. Erdkunde 49, 39e48. Kenworthy, M.K., Rittenour, T.M., Pierce, J.L., Sutfin, N.A., Sharp, W.D., 2014. Luminescence dating without sand lenses: an application of OSL to coarse-grained alluvial fan deposits of the Lost River Range, Idaho, USA. Quat. Geochronol. 23, 9e25. Leng, M.J., Metcalfe, S.E., Davies, S.J., 2005. Investigating Late Holocene climate variability in central Mexico using carbon isotope ratios in organic materials and oxygen isotope ratios from diatom silica within lacustrine sediments. J. Paleolimnol. 34, 413e431. ~ ez, J., Caputo, L., Armengol, J., 2006. Elemental Lopez, P., Navarro, E., Marce, R., Ordon

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