Climatic variability and human impact during the last 2000 years in western Mesoamerica : evidence of late Classic ( AD 600 – 900 ) and Little Ice Age drought events

We present results of analysis of biological (diatoms and ostracodes) and non-biological (Ti, Ca /Ti, total inorganic carbon, magnetic susceptibility) variables from an 8.8 m long, high-resolution (∼ 20 yr sample) laminated sediment sequence from Lake Santa María del Oro (SMO), western Mexico. This lake lies at a sensitive location between the dry climates of northern Mexico, under the influence of the North Pacific subtropical high-pressure cell and the moister climates of central Mexico, under the influence of the seasonal migration of the intertropical convergence zone and the North American monsoon (NAM). The sequence covers the last 2000 years and provides evidence of two periods of human impact in the catchment, shown by increases in the diatom Achnanthidium minutissimum. The first from AD 100 to 400 (Early Classic) is related to the shaft and chamber tombs cultural tradition in western Mexico, and the second is related to Post-Classic occupation from AD 1100 to 1300. Both periods correspond to relatively wet conditions. Three dry intervals are identified from increased carbonate and the presence of ostracodes and aerophilous Eolimna minima. The first, from AD 500 to 1000 (most intense during the late Classic, from AD 600 to 800), correlates with the end of the shaft and chamber tradition in western Mexico after ca. AD 600. This late Classic dry period is the most important climatic signal in the Mesoamerican region during the last 2000 years, and has been recorded at several sites from Yucatan to the Pacific coast. In the Yucatan area, this dry interval has been related with the demise of the Maya culture at the end of the Classic (AD 850 to 950). The last two dry events (AD 1400 to 1550 and 1690 to 1770) correspond with the onset of, and the late, Little Ice Age, and follow largely the Spörer and Maunder minima in solar radiation. The first of these intervals (AD 1400 to 1550) shows the most intense signal over western Mexico; however this pattern is different at other sites. Dry/wet intervals in the SMO record are related with lower/higher intensity of the NAM over this region, respectively.


Introduction
Knowledge of past climate and environmental events is critical to understand a wide variety of modern environmental processes and to better estimate causes and effects of future environmental changes.Records from many parts of the world have led to the recognition of climate variability in the late Holocene.However, the magnitude, duration and the possible mechanisms of climate change at each location are not clear, especially at tropical latitudes where there are fewer continental records than at higher latitudes (Maasch et al., 2005;Mann et al., 1999).
In Mesoamerica, the last 2000 years appear to have been a time of substantial climatic change with a complex signal, mostly linked to moisture availability (Metcalfe et al., 2000).At present, the influence of the intertropical converge zone (ITCZ) and subtropical high-pressure cells (SHPCs) varies seasonally, showing latitudinal shifts associated with the tropospheric equator-pole temperature gradient in each hemisphere, and the thermal contrast between the hemispheres (Haug et al., 2001;Nyberg et al., 2002).The seasonal shift in insolation and latitudinal location of the ITCZ and SH-PCs brings shifts in moisture availability that affect most of Mesoamerica, with an intense summer rainfall season associated with the northerly location of the ITCZ and the onset of the North American monsoon (NAM).Past changes in the amplitude, intensity and location of the ITCZ and SHPCs and in the intensity of the NAM are therefore likely explanations for the changes documented in the Mesoamerican geological records (Hodell et al., 2005a;Metcalfe et al., 2010;Haug et al., 2003;Barron et al., 2012).Long-term changes in these climatic systems can be recorded in the sediment archives of lakes in the region, given that lakes are sensitive to climatic variations, which can lead to changes in their stratification patterns, their water chemistry and/or their biological associations.Western Mexico's precipitation is modulated by the position of the ITCZ and the intensity of the NAM, however, most of the available palaeo-limnological records for Mesoamerica are concentrated in the highlands of central Mexico or in the Yucatan region (Metcalfe et al., 2000).
In this paper we present a multi-proxy, high-resolution (∼ 20 yr per sample) study of a laminated sediment sequence from a closed lake basin in western Mexico, Santa María del Oro, Nayarit (Fig. 1), and infer past climatic events that have affected western Mesoamerica during the past 2000 years.This lake currently sits at the intersection between the arid climates of northern Mexico, mostly under the influence of the North Pacific SHPC, and the temperate, sub-humid to humid climates of central Mexico (García-Oliva et al., 2002), under the influence of the ITCZ and the NAM systems.This record gives clear evidence of two intense drought periods, the late Classic (AD 600 to 800) and the onset of the Little Ice Age (AD 1400 to 1550), and a third, less intense dry phase around AD 1690 to 1770.
The climate at SMO is warm and sub-humid, with monthly average temperatures that range from 16.5 to 25 • C (Station 18005-Cerro Blanco -Servicio Meteorológico Nacional).This region receives average annual precipitation of ∼ 1200 mm, mainly during the warm summer months (June to September) when the ITCZ is at its most northerly location and the NAM is active (Liebmann et al., 2008;Berbery, 2001).During summer, tropical storms and hurricanes can also bring moisture from the Pacific (Castro, 2010;García-Oliva et al., 2002).The rest of the year (October to May) is relatively dry, particularly during spring.
SMO is a nearly round, ∼ 2 km diameter lake.It has a maximum depth of 65.5 m and a mean depth of 46 m (Serrano et al., 2002).The crater has steep slopes but there is a small flat and shallow bay (12 m deep) where the core for this study was retrieved (Fig. 1).The lake receives inflows Table 1.Radiocarbon (AMS) dates of core SMO02-V, Lake Santa Maria del Oro, Nayarit (Vázquez-Castro et al., 2008).Calendar years were calculated using the program Calib 5.0 (Stuiver et al., 2005) with the IntCal04 data set (Reimer et al., 2004).directly from precipitation, seasonal run-off and groundwater, and loses water to evaporation and seepage.The lowest point of the crater is on the north-eastern side, where the natural sill has been anthropogenically modified.Today it lies ∼ 5 m above the lake level (755 m a.s.l.).The closed nature of the basin and its relatively unpolluted condition make this lake an ideal palaeo-environmental research site, where lake chemistry depends mostly on the precipitation-evaporation balance.At present SMO is a freshwater, [HCO ], oligotrophic, warm monomictic lake, with a winter mixing period (Caballero et al., 2013); the oxycline lies between 13 and 20 m depth the rest of the year.Currently the lake ostracode fauna is dominated by Potamocypris variegata Brady and Norman, Cypridopsis vidua Müller and Darwinula stevensoni Brady and Robertson and the planktonic diatom flora by Aulacoseira granulta (Ehrenberg) Simonsen, in association with Nitzschia amphibia Grunow.

Methods
Four cores between 4 and 9 m length were recovered from the shallow south-western bay (12 m depth) of SMO in 2002 (Fig. 1c) with a Usinger coring system (Mingram et al., 2007).All cores were split, described, photographed and sampled at the Instituto de Geofísica, Universidad Nacional Autónoma de México (UNAM) and, unless otherwise specified, all analyses were performed there.The longest core (SMO02-V, 8.8 m) was sampled, on average, at 5 cm intervals for magnetic susceptibility (MS), biological (ostracodes, diatoms, pollen) and geochemical analyses (total inorganic carbon, total organic carbon, X-Ray fluorescence elemental analysis).Seven samples for 14 C AMS radiocarbon age determinations were collected at selected levels, dried and sent without further treatment to a commercial dating laboratory (Beta Analytic).The seven dates (Table 1) were calibrated to calendar years with Calib 5.0 (Stuiver et al., 2005) and the IntCal04 data set (Reimer et al., 2004).The age model (Fig. 2) assumed constant, linear sediment accumulation rates between the dates and anchored the top sediment  1. Sedimentation rates are indicated above the line.The stratigraphic position of Toba Jala in the SMO02-V sequences is shown with the black square; grey bars and the grey shaded area represent the range of calibrated ages reported for this volcanic event (Sieron and Siebe, 2008).
to the year of coring ( 2002) (Vázquez-Castro et al., 2008).The sediment stratigraphy, age model, MS and total inorganic carbon (TIC) data were published in Vázquez-Castro et al. (2008).Here we present MS data, expressed in dimensionless international SI units, and TIC expressed as CaCO 3 %, together with new diatom, ostracode, Ca and Ti data from the SMO02-V core.

X-Ray Fluorescence
Thirty nine freeze-dried sediment samples were chosen, on average every 20 cm, for X-Ray fluorescence (XRF) elemental analysis.Samples were homogenized and ground to 200 mesh using an agate mortar.Elemental concentrations were measured at the Institute of Geology, UNAM, using a Siemens SRS 3000 wavelength dispersive XRF spectrometer with a precision of 5 %.Titanium and calcium concentrations are expressed as percentages; Ca was normalized against titanium and expressed as a Ca / Ti mass ratio.

Diatoms
For diatom analysis, 108 samples were chosen, on average at 10 cm intervals, of which 78 contained diatoms.Dry sediment (0.5 g) was cleaned successively with HCl (10 %), H 2 O 2 and HNO 3 .Permanent slides were made with 200 µL of clean sediment material and embedded with Naphrax ® mounting medium.Minimum counts of 300 valves were undertaken in most samples, but 22 had low diatom abundance A. Rodríguez-Ramírez and M. Caballero: Climatic variability and human impact during the last 2000 years and only 100 valves were counted.Diatom counts were done with an Olympus BX50 microscope with interferential phase contrast at 1000x magnification.Species composition is reported as relative abundance (%), and total abundance is reported as valves per gram of dry sediment (v gds −1 ).

Ostracodes
For ostracodes, the same 108 samples were analysed, but only 57 levels contained the remains of ostracodes.Two cubic centimetres from each sample were freeze-dried, from which 1 g of dry sediment was disaggregated by standard methods (Delorme, 1990;Forester, 1988) and wet-sieved (63 µm).Ostracode valves were picked under an Olympus SZX12 stereo microscope, counted, determined to species taxonomic level and stored on micropalaeontological slides.Care was taken to pick whole adult valves to allow species identification, and juvenile valves were counted separately.Ostracode counts are expressed as valves per gram of dry sediment (v gds −1 ).

Core description
Sedimentation rates varied between 0.21 and 1.15 cm yr −1 (Fig. 2) and, according to the age model (Fig. 2), the base of the core (882 cm) is ca.2600 years old.Average sample resolution was about 23 years per sample.The SMO02-V sequence consists of sand/silt laminations with intercalated woody peat layers.Individual silt layers can be of ∼ 1-3 mm up to 60 mm in thickness and either light-brown calcareous, greenish, reddish or dark brown (Vázquez-Castro et al., 2008).A 13 mm thick grey tephra layer at 454 cm depth was identified as the Toba Jala from the nearby Ceboruco volcano; according to our chronology, this tephra dates to AD ∼ 860 (Vázquez-Castro et al., 2008), a date that falls within the range of calibrated radiometric ages reported for this tephra (Fig. 2) (Sieron and Siebe, 2008).The light-brown calcareous silt layers contain either authigenic carbonates or ostracodes, and are more frequent towards the top of the core, whereas woody peat layers are more frequent towards the bottom.There is no evidence for hiatuses, erosion features or slumping structures in the cores.
TIC as its CaCO 3 equivalent varies along the core from 0 to 59 % (Fig. 3), in a pattern nearly opposite to magnetic susceptibility, showing in general higher values (> 20 %) between 530 and 370 cm, 320 and 240 cm, 200 and 150 cm and 120 and 100 cm.The Ca / Ti weight ratio ranges between 1.4 and 175 and shows above average values (> 15) between 530 and 370 cm and at 200 cm.

Diatoms
Diatom abundance shows several intervals of values that are lower than average (< 40 × 10 6 v gds −1 , Fig. 4), but the lowest abundances are recorded between 740 and 690 cm and from 380 to 330 cm.Sixty-four species are recorded in the sequence, nine with relative abundances > 20 % in more than one sample (Fig. 3).Aulacoseira granulata, Nitzschia amphibia and small species of Fragilaria sensu lato (Staurosira construens, Staurosirella pinnata, Pseudostaurosira parasitica, P. brevistriata) are the dominant species in the record.Achnanthidium minutissimum (Kützing) Czarnecki has a preferential distribution between 750 and 650 cm and from 400 to 250 cm, Amphora libyca Ehrenberg and Hippodonta luneburgensis (Grunow) Lange-Bertalot, Metzeltin and Witkowski have higher abundances in the top 500 cm of the core and Eolimna minima (Grunow) Lange-Bertalot and W. Schiller shows a distinctive distribution with maxima between 530 and 420 cm and two less important peaks between 200 and 110 cm and from 75 to 60 cm.

Ostracodes
Ostracode total abundance shows two distinct intervals with high values (> 1000 adult v gds −1 ), the first between 530 and 420 cm and the second between 200 and 110 cm.A smaller peak at 75 to 60 cm is also present, while the rest of the core has very low (< 100 adult v gds −1 ) ostracode concentration (Fig. 3).Six species are recorded in the sequence; Candona patzcuaro Tressler is the most abundant and constant along the core in association with Potamocypris variegata and minor numbers of Chlamydotheca arcuata Sars.Cypridopsis vidua and Darwinula stevensoni are also present along the core, but show a preferential distribution between the 530 and 420 cm interval.Limnocythere verrucosa Hoff is only present as juvenile valves, and shows maximum concentrations between 570 and 530 cm.

Interpretation of sediment variables
MS indicates the abundance of magnetic minerals that reach a lake, mainly by surface run-off from the catchment area or, on occasion, from volcanic activity (Evans and Heller, 2003).Titanium is an element that originates from minerals in the catchment rocks and its abundance in lacustrine Clim.Past, 11, 1239Past, 11, -1248Past, 11, , 2015 www.clim-past.net/11/1239/2015/sediments is also indicative of surface run-off to the lake and erosion (Metcalfe et al., 2010;Sosa-Nájera et al., 2010).Both variables can therefore be used to identify times of higher or lower precipitation and surface transport of sediments to the lake.
On the other hand, given that there are no carbonate rocks in the lake catchment, high values of TIC are indicative of times of higher authigenic carbonate precipitation.This is a process that currently occurs in SMO during the warmer part of the year (Caballero et al., 2013) and that is, in general, favoured by high evaporation rates, leading to increased electric conductivity, alkalinity and pH (Eugster and Hardie, 1978;Gierlowski-Kordesch, 2010).Calcium may originate from the minerals in the catchment but it is also incorporated in authigenic carbonates.Sosa-Nájera et al. ( 2010) demonstrated that in this lake, higher Ca concentrations have corre-  (Caballero et al., 2002); (e) bulk sediment calcite δ 18 O from Lake Aljojuca (Bhattacharya et al., 2015); (f) diatom-based lake level fluctuations from Lago Verde (Lozano-García et al., 2010); (g) biogenic calcite δ 18 O from Aguada X'caamal (Hodell et al., 2005b); (h) sediment density from Lake Chichancanab (Hodell et al., 2005a); (i) biogenic calcite δ 18 O from Punta Laguna (Curtis et al., 1996).(j) titanium record from marine Cariaco Basin (Haug et al., 2001).VPDB represents Vienna Pee Dee Belemnite standard, PBD represents Pee Dee Belemnite standard.Shaded areas correspond to the time intervals of shaded areas in Fig. 3. lated with historical droughts during the last 700 years, supporting the interpretation that in SMO, high calcium values are mostly related with carbonate deposition during times of higher evaporation.Nevertheless, Ca was normalized against Ti to reduce the detrital signal in this element.High TIC and high Ca / Ti values in this record can, therefore, be taken as indicators of increased carbonate deposition during times of lower precipitation and higher evaporation rates, leading to relatively high lake water electric conductivity, pH and alkalinity.Given the contrasting environmental conditions that lead to high MS and Ti versus high TIC and Ca, it is not surprising that they show nearly opposite patterns of downcore variations.Together, these records enable identification of two intervals of intense evaporation and reduced surface run-off to the lake, the first from 530 to 490 cm (AD 600 to 800) and the second from 200 to 150 cm (AD 1400 to 1450).
With respect to the biological variables, three ecological groups can be identified in the diatom record.(1) A planktonic/tychoplanktonic association is identified, dominated by Aulacoseira granulata, Nitzschia amphibia and small Fragilaria spp., which is abundant along most of the sequence (Fig. 3); this association includes the two main diatom species present in the modern lake plankton (Caballero et al., 2013).(2) A benthic/periphytic association is identified in which Amphora libyca, Hippodonta luneburgensis and Eolimna minima, are the main species.These periphytic genera (Amphora) and species (H.luneburgensis, E. minima) are currently distributed in the shallower environments of the lake (Caballero et al., 2013); E. minima in particular is an aerophilic species (Wolfe and Härtling, 1996;Van Dam et al., 1994).Its distinctive distribution along the record (shaded areas in Fig. 3) suggests intervals of particularly shallow lake levels.(3) An eutrophication/human impact association is defined by the presence of Achnanthidium minutissimum, a species that has been considered by some authors as a colonizer in disturbed environments (Peterson and Stevenson, 1992;Hodgson et al., 1997).In Mexico, this species has been recorded in the plankton of tropical lakes in areas that have been severely altered by human activities (Caballero et al., 2006;Vázquez and Caballero, 2013).This interpretation is further supported by the presence of Zea mays pollen grains in the same segments of the core (S.Sosa-Nájera, personal communication, 2013).The ostracode record can be interpreted based on the study of this lake by Caballero et al. (2013), who found that ostracodes were most abundant in shallow water environments with littoral vegetation and a low sediment supply.Given that the dominant species in the SMO record are the same as those that currently live in this lake (P.variegata, D. stevensoni, C. vidua) high ostracode abundances are taken to be indicative of these conditions, which are consistent with lower surface run-off.The ostracode relative abundance record and the distribution of E. minima suggest three zones of lake levels that are lower than average (Fig. 3): 560 to 410 cm (AD 500 to 1000), 200 to 100 cm (AD 1400 to 1550) and 75 to 56 cm (AD 1690 to 1770).The first two periods include the two events of intense evaporation identified in the geochemical record.

Palaeo-environmental history of Lake SMO
The MS, Ti, Ca / Ti, ostracode and diatom records together (Fig. 3) enable reconstruction of the environmental history of Lake SMO during the last 2000 years.Low values of Ca / Ti, TIC and ostracode abundance, together with the absence of benthic/periphytic diatoms in the record prior to AD 500 indicate that Lake SMO was generally deeper and the lake water was more dilute from 100 BC to AD 500.An early phase of human impact is identified by the abundance of A. minutissim from 100 BC to AD 300, possibly related with the local expression of the shaft and chamber tomb tradition (200 BC to AD 600) characteristic of western Mexico (Beekman, 2010;Barrera-Rodríguez, 2006).
After AD 500 the lake experienced a low-level stage defined mainly by an increase in E. minima and ostracode concentrations, by low MS and also by its magnetic mineralogy (Vázquez-Castro et al., 2008).This period of low lake level extended from AD 500 to 1000, with the interval from AD 600 to 800 representing the driest conditions (lowest MS and Ti values and highest TIC and Ca / Ti ratios).This first low lake level phase in SMO correlates with what is known as the late Classic (AD 600 to 900) period in the development of Mesoamerican cultures, characterized by a decline of the Teotihuacan culture in central Mexico after AD 650 (Manzanilla, 2011) and development of Maya cities in the Yucatan region, which ended abruptly between AD 850 and 950.In western Mexico in general and specifically in the state of Nayarit (Barrera-Rodríguez, 2006), this was a time of cultural transition as the shaft and chamber tomb tradition ended by AD 600 (Barrera-Rodríguez, 2006).In the SMO record there is no evidence of human impact during this dry interval.Thus, the inferred dry conditions in this period reflect a true climatic signal, not obscured by anthropogenic activities.
This late Classic drought has been identified in several records from Mexico, particularly from Yucatan (Hodell et al., 2005a;Metcalfe et al., 2010;Caballero et al., 2002;Lozano-García et al., 2010;Curtis et al., 1996;Bhattacharya et al., 2015) (Fig. 1), to the extent that it is referred to as the "Great Maya Drought" (Gill, 2001) and has been put forward as an important factor in the collapse of the Maya culture at the end of the Classic (AD 850 to 950).The exact timing of these droughts varies across sites but in general, it is centred in a similar time interval as in SMO (AD 500 to 1000) (Fig. 4).In some records, the droughts show a recurrent or cyclic pattern (Haug et al., 2003;Hodell et al., 2001;Curtis et al., 1996) which could explain the fluctuating values of the geochemical records of Lake SMO (alternating high and low TIC, Ti and MS) from AD 800 to 1000 and even until AD 1200.
After AD 1000 low abundances of E. minima and ostracode valves suggest that the lake level in SMO recovered.However TIC, Ca / Ti and Ti still fluctuated between high and low values until AD ∼ 1200.After this date, the age model (Fig. 2) also suggests that sedimentation rates increased (from ≤ 0.3 to > 1 cm yr −1 ), which is in accordance with higher surface run-off to the lake.Moister conditions after AD 1000 and especially from AD 1200 to 1400 correlate with moist conditions inferred from the Ti record from Lake Juanacatlán, a site also in western Mexico (Metcalfe et al., 2010).Between AD 1000 and 1400, however, the presence of A. minutissimum points to a time of human disturbance in the system.This trend in human occupation at Lake SMO agrees with recent archaeological findings in Nayarit that show late Post-Classic (AD 1100 to 1400) occupations (Gálvez-Rosales, 2006) and is also in agreement with the late Post-Classic cultural peak reached in western Mesoamerica by groups like the Purepecha or Tarascans (Pollard, 2005).
From AD 1400 to 1550 the increase in TIC, E. minima and ostracode abundance, the decline in MS as well as the magnetic mineralogy (Vázquez-Castro et al., 2008) suggest that the lake level in SMO dropped again.The lowest MS and Ti values and highest TIC suggest that the reduction in moisture was most intense from AD 1400 to 1450.This second dry interval in the Lake SMO record corresponds with the onset of the Little Ice Age.This period has also been recorded as a time of reduced moisture and lower lake levels in other Mesoamerican records (Metcalfe et al., 2010;Hodell et al., 2005b;Cuna et al., 2014) (Fig. 4) and represents the climatic scenario for the Spanish conquest of Mesoamerica in 1521.
Water levels recovered in SMO between AD 1550 and 1690, but a third, less intense period of reduced moisture availability is evident in the records, with a slight increase in E. minima and ostracode concentrations between AD 1690 and 1770.This smaller reduction in moisture is coeval with the end of the Little Ice Age and shows the two periods of cooling in the Little Ice Age, also evident in the record from Lago Verde, eastern Mexico (Lozano-García et al., 2007).

Palaeo-climate implications
The data from Lake SMO enabled us to identify two periods of human impact that are consistent with findings from recent archaeological research in the area (Barrera-Rodríguez, 2006;Gálvez-Rosales, 2006), the first related with the shaft and chamber tomb tradition (100 BC to AD 300) and the second during the late Post-Classic (AD 1100 to 1400).It is interesting to note that these periods of occupation correspond with times of moister conditions, while there is no evidence of human impact in this lake during the drier late Classic (AD 600 to 900).
The biological and non-biological variables in the sediment of Lake SMO provide solid evidence of three intervals that are drier than average, the most intense from AD 600 to 800, correlating mostly with the late Classic, and the other two related with the onset (AD 1400 to 1550) and the late (AD 1690 to 1770) Little Ice Age.There is also evidence of a wetter late Post-Classic period (AD 1200 to 1400).In this region where climate is controlled by the seasonal shift of the North Pacific SHPC, the ITCZ and the onset of the NAM, conditions that are drier than average can be related to an expansion of the North Pacific SHPC, a southward migration of the ITCZ and a less active NAM (Haug et al., 2003;Metcalfe et al., 2010).Other mechanisms that could bring wetter/drier conditions in this region are changes in the frequency or strength of "La Niña"/"El Niño", as today "La Niña" is related with summers that are wetter than average (Magaña et al., 2003;Metcalfe et al., 2010).Changes in the paths or frequency of Pacific tropical storms and hurricanes could also be elements that control the moisture balance in this area.
The data presented in this paper provide further evidence that the late Classic drought (AD 500 to 1000) was the most important one in Mesoamerica during the last 2000 years, affecting the whole region from Yucatan to the Pacific coast (Fig. 4); it was not only a circum-Caribbean feature.In SMO the most intense phase is recorded from AD 600 to 800.The variable nature of the Lake SMO record from AD 800 to 1200 seems consistent with the proposed recurrent nature of droughts, which has been suggested to follow cycles in solar activity (mainly 50 and 200 years), (Hodell et al., 2001(Hodell et al., , 2005a;;Haug et al., 2003).This period of strong fluctuations in the Lake SMO record also correlates with the onset of conditions that are warmer than average over the northern hemisphere from AD 830 to 1100 during the Medieval Climatic Anomaly (PAGES-2k-Consortium, 2013) that locally correlated with warmer sea surface temperatures in the Gulf of California (Barron et al., 2002).
The dry interval from AD 1400 to 1550 which corresponds with the onset of the Little Ice Age, correlates closely with the Spörer minimum in solar activity.The less intense dry period near the end of the Little Ice Age (AD 1690 to 1770) correlates partly with the Maunder solar minimum.A two-phase Little Ice Age cooling, partly following the two solar minima, is also present in the record from Lago Verde, eastern Mexico (Lozano-García et al., 2007).In lakes SMO and Juanacatlán (both in western Mexico, Fig. 1) the Spörer minimum shows the most intense signal.However, in other records in central and eastern Mexico, the Maunder minimum represents the driest and/or coldest period (Haug et al., 2003;Cuna et al., 2014) (Fig. 4).Furthermore, in some records, the two periods seem to merge into a single, longer Little Ice Age drought (Hodell et al., 2005b).These Little Ice Age droughts seem to be related with the Spörer and Maunder solar minima.
The good coherence of the biological and non-biological proxies, the solid chronology and the high-resolution nature of the Lake SMO record yield insights into the environmental and climatic processes that have affected this lake during the last 2000 years, and provide insight into the interplay between climatic and anthropogenic influences on this lake.

Figure 2 .
Figure2.Age model of core SMO02-V, Lake Santa Maria del Oro, Nayarit.Details of dates are listed in Table1.Sedimentation rates are indicated above the line.The stratigraphic position of Toba Jala in the SMO02-V sequences is shown with the black square; grey bars and the grey shaded area represent the range of calibrated ages reported for this volcanic event(Sieron and Siebe, 2008).

Figure 3 .
Figure 3. Biological (diatoms and ostracodes) and non-biological (Ca / Ti, Ti, magnetic susceptibility and TIC) variables from core SMO02-V, Santa María del Oro, Nayarit, Mexico.TIC represents total inorganic carbon.Small Fragilaria spp.includes: Staurosira construens, Staurosirella pinnata, Pseudostaurosira parasitica, P. brevistriata, of which only P. brevistriata had < 20 % abundance in the sequence.Grey shaded areas highlight periods with high Eolimna minima and ostracode valves abundance; dark grey areas highlight periods when high Eolimna minima and ostracode valves abundance correspond with low magnetic susceptibility and high TIC.The black line represents the Toba Jala.Samples with low diatom counts are marked with a cross (+).