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Paleoclimate records from the tropical Andes are scarce, and the variability of glacial-interglacial cycles is still not well characterized. Lake Junin, in the Peruvian Andes, offers a unique and continuous paleoclimate archive that spans the last 700,000 years. Here, we explore the potential of organic compounds in reconstructing Andean paleoclimate over the last 20,000 years. To address this, we first evaluated the preservation of organic matter in the lake’s sediments. The Carbon Preference Index (CPI) suggests that n-alkanes have not been altered, and their H isotope composition can be used as paleo precipitation proxies. Furthermore, biomarkers from Eustigmatophyte algae (long chain diols) and diatoms (loliolide/isololiolide) have been identified, and can be used to reconstruct the hydrogen isotopic composition of lake water. The contrast between rainfall and lake water will be a good tool for understanding lake water inputs through time as well as evaporation and aridity. Changes in n-alkane chain length will be used to identity the terrestrial plant (long chain n-alkanes) and aquatic macrophyte inputs (mid-chain n-alkanes), with potential implications for interpreting past lake level change as a function of climate. Finally, distributions of br-GDGTs (branched glycerol dialkyl glycerol tetraethers) will be used to reconstruct past temperature changes. With these proxies, we aim to characterize climate variability at the end of the Last Glacial Maximum (LGM) and the Holocene, with a focus on characterizing climate variability in the light of teleconnections between the South American Summer Monsoon and global climate patterns and their relationship with hydroclimate in the Amazon Basin. 

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) have shown great promise in lacustrine temperature reconstructions across different continents. While brGDGTs have been reported from many different regions and global brGDGT-temperature calibrations have been developed with various methods, southern North America remains an understudied area with little available data. In this study, we analyzed 101 lake surface sediment samples across Mexico and Central America and compared their distributions with those in other lacustrine systems. Nine major brGDGTs were found in all samples. We investigated the relationships between the distribution of the fractional abundances of the nine major brGDGTs and temperature and developed regional calibrations for Mean Annual Temperature using three different approaches, including a novel machine learning method – Ridge Regression. All the regional calibrations provide similar results with very close error ranges (RMSE = 3.1 ◦C). The majority of global brGDGT-temperature calibrations tend to reconstruct lower temperatures when it is below 15 ◦C. Interestingly, regional brGDGT calibrations appear to reduce the “cold bias”, but the various global and regional calibrations tested here are not significantly different in their predictive capability. 

Abstract. The primary scientific objective of MexiDrill, the Basin of MexicoDrilling Program, is development of a continuous, high-resolution∼400 kyr lacustrine record of tropical North Americanenvironmental change. The field location, in the densely populated,water-stressed Mexico City region gives this record particular societalrelevance. A detailed paleoclimate reconstruction from central Mexico willenhance our understanding of long-term natural climate variability in theNorth American tropics and its relationship with changes at higher latitudes.The site lies at the northern margin of the Intertropical Convergence Zone(ITCZ), where modern precipitation amounts are influenced by sea surfacetemperatures in the Pacific and Atlantic basins. During the Last GlacialMaximum (LGM), more winter precipitation at the site is hypothesized to have beena consequence of a southward displacement of the mid-latitude westerlies. Itthus represents a key spatial node for understanding large-scalehydrological variability of tropical and subtropical North America and isat an altitude (2240 m a.s.l.), typical of much of western North America. In addition, its sediments contain a rich record of pre-Holocene volcanichistory; knowledge of the magnitude and frequency relationships of thearea's explosive volcanic eruptions will improve capacity for riskassessment of future activity. Explosive eruption deposits will also be usedto provide the backbone of a robust chronology necessary for fullexploitation of the paleoclimate record. Here we report initial resultsfrom, and outreach activities of, the 2016 coring campaign.