Millennial hydrological variability in the continental northern Neotropics during MIS3-2 inferred from sediments of Lake Petén Itzá, Guatemala

We inferred hydrological changes in Lake Petén Itzá (Guatemala) during Marine Isotope Stages (MIS) 3-2 using geochemical (Ti, Ca/Ti+Al+Fe ratio and Mn/Fe) and mineralogical (carbonates, gypsum, quartz, clay) data from sediment core PI-2 to reconstruct changes in runoff, lake evaporation, organic matter sources and potential oxic/anoxic conditions associated with variations in water-level during the last ~59 cal ka BP. Early MIS3 (57.0-52.5 cal ka BP) was dominated by relatively wet conditions, higher lake primary productivity and anoxic waters, which persisted into 5 the subsequent interval (52.5-39.0 cal ka BP), except for two periods of possible low water-level at 52 and 46 cal ka BP when our data suggest higher evaporation, high terrestrial organic matter input and persistent oxic conditions. Towards the end of MIS3 and start of MIS2 (39.0-23.0 cal ka BP), lake evaporation increased considerably, as did inputs of terrestrial organic matter, and waters became more oxic as water-levels dropped and the site moved from the hypolimnion to the epilimnion. These conditions reversed during the Last Glacial Maximum (23.0-18.0 cal ka BP) 10 when runoff and lake productivity increased and waters again became anoxic as a result of rising water-levels. Refining the age-depth model for the Site PI-2 also allowed the correlation to Greenland Interstadials (GI14-2), Greenland Stadial (GS14-2) and Heinrich Stadials (HS5-1). HS and GS were characterized by increases in Ca/Ti+Al+Fe ratios and gypsum content generally indicative of drier conditions. GS13, 9 and 5 showed the driest conditions associated with the contemporaneous establishment of HS5-3, respectively. In contrast, GI show high Ti values which suggests 15 relatively greater runoff and overall wetter conditions compared with GS and HS, with the most marked GI peaks between 40 and 30 cal ka BP. This runoff variability is in accord with shifts in the average position of the Intertropical Convergence Zone and strength of the Atlantic Meridional Oceanic Circulation during the Late Pleistocene. study abrupt climate change for stadial-interstadial events during MIS3-2. studies of the Lake Petén Itzá’s sediments, the identification of these events based on the presence of gypsum and clay layers Hodell et al., (2008) and et al., (2010). Subsequent work has been conducted including stable isotope geochemistry et al., 2012; Hodell et al., 2012; Grauel et al., 2016; Mays et al., 2017), as well as biological indicators of 40 climate change such as ostracods al., 2018, 2020; Pérez al., 2021), pollen and charcoal al., 2012). During HS, a drop-in mean annual air temperature between 6 and 10°C compared with modern values has been reconstructed in the region al., 2012). Lake Petén Itzá have a reduction of up to 3°C in the mean water-temperature according with recent ostracods-based transfer function (Cohuo et al. 2018; Pérez et al., 2021). High salinities associated with a shallow lacustrine environment have been reconstructed based on ostracods 45 data suggesting drier conditions. Moreover, stable isotope geochemistry reveals low lake levels during HS that caused Site PI-6 (water depth 71 m) to be within the epilimnion where oxygen concentrations are high (Escobar et al., 2012). These cold and dry the establishment of savanna vegetation such as Poaceae and Acacia, as well as littoral ostracods species as Cytheridella ilosvayi and Paracythereis opesta the assemblage. GS, air is characterized the Eastern Equatorial Pacific 2007) a more northerly position of the ITCZ this interval 59-53 BP, TOC/TN ratios sediment organic of origin Lake Petén Itzá, the the lake our Mn/Fe ratios are used to infer variations in redox conditions in lakes (Wersin et 1991; Naeher 2013; Friedrich 2014; Ortega-Guerrero 2020). Lower values of Mn/Fe generally indicate low concentration of O 2 in the water column, that is explained by a more rapid reduction of Mn than Fe under anoxic conditions favoring the preferential Mn deposition. Whereas, higher values of Mn/Fe suggest high levels of O 2 , as Fe oxidizes quicker than 285 Mn under an oxic environment. During this unit, low values of the Mn/Fe ratio (mean -3.7±0.3) in the sediments indicate anoxic conditions in the bottom waters of Lake Petén Itzá. Anoxia may have been associated with high water level as today, which prevented the wind-driven mixing of the water column. Correa-Metrio et al., (2012) provided a high-resolution record of pollen and carbonized material in the Site PI-6 record (~200 year-resolution), which demonstrated an increase in forest fires in this region 480 during GS. This is consistent with high evaporation that is in response to dry conditions in the northern Neotropics. The latter is more apparent for GS13, GS9 and GS5, whose presence in the record is synchronous with the North Atlantic IRD deposits associated with HS5-3, respectively. The enhanced Ca/Ti+Al+Fe ratios during GS13, GS9 and GS5 in Lake Petén Itzá may be a response to dry conditions that dominated during HS. In addition, higher abundances of gypsum were found during GS13 and GS9 suggesting high evaporation in the lake. Similar increases in 485 Ca/Ti+Al+Fe ratios and gypsum content are observed during GS8, GS7, GS5, GS4 and GS3. We did not observe an increase in gypsum content during the GS11, GS10 and GS6, which is likely due to the low sampling resolution in this interval, but in which the Ca/Ti+Al+Fe ratio suggests an increase in evaporation. Samples putatively corresponding to GS14, GS12 and GS2 cannot be identified unequivocally based on the gypsum content. This may be caused by the following reasons: First, GS14 and 12 are located in a depth interval with a relatively high uncertainty of the Petén 490 Itzá age-depth model, which makes the identification of both GS difficult. Second, GS2 is linked to the beginning of the LGM which, as discussed above, is characterized by high humidity in Petén Itzá. This likely suppressed the GS2 signal in the sediments.