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Abstract South American summer monsoon (SASM) strength tracks insolation on orbital timescales, linking global climate and continental hydrology. However, whether local water availability also responds to global climate forcings is unclear. Here, we present water balance records from Lake Junín, an Andean lake within the SASM domain. Local water balance and SASM strength is inferred from triple oxygen isotopes of lake carbonates during two interglacial periods (Marine Isotope Stage (MIS) 15, 621–563 ka; the Holocene, 11.7–0 ka). We find SASM strength and water balance both follow the precession‐pacing of local summer insolation, with the driest conditions occurring at Lake Junín under weakened SASM conditions (and vice versa). Further, the largest variations occurred during MIS 15, when insolation was more variable than the Holocene. These results suggest that global climate influences South American hydrology on both the local and continental scales, with implications for tropical water resources, the atmospheric greenhouse effect, and ecosystem dynamics. 

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. 

Atmospheric water vapor is predominately sourced from the tropics, such that characterizing the link between the tropical water cycle and global climate is of critical importance. Studies of central Andean climate from Lake Junín (11 °S, Peru) show that tropical glacial extent tracks global ice volume at a ~100 ka periodicity for the last 6 glacial cycles, indicating a tight coupling between tropical water balance and high latitude climate. However, it can be difficult to decouple temperature, precipitation, and water balance histories from records of glacial extent, especially for older intervals. In this work, we focus on one such interval, MIS 15 (621–563 ka), when the connections between tropical Andean water balance and global climate seem different than the last glacial cycle. Globally, MIS 15 was a weak interglacial, with cool temperatures and low GHG concentrations, however, the Lake Junín glacial record suggests an amplified hydroclimate response to this interglacial, stronger than any other over the last 700 ka. Causes for this apparent tropical amplification may be due to large, precession-paced changes in meridional insolation gradients that exceed other interglacials owning to enhanced orbital eccentricity. Given that the role of precession on South American monsoon strength over the last glacial cycle is well established, we hypothesize that monsoon strength may have been highly variable during MIS 15 and forced changes in central Andean water balance and glacial extent. To test this, we reconstructed temperature and evaporation histories using carbonate clumped and triple oxygen isotopes of Lake Junín sediments. Preliminary results suggest temperatures were relatively stable, but possibly lower than both the present and Holocene, consistent with cool global climate at that time. Triple oxygen isotope values vary substantially, indicating massive swings in lake hydrology, between open and (nearly?) closed basin hydrology on a ~12 ka cycle that exactly match insolation variations. From this work, we conclude that hydrologic change in the central Andes was rapid and extreme during MIS 15, owning to profound changes in monsoon strength. Given that monsoons in other sectors are also sensitive to insolation changes, our work could suggest pervasive hydrologic variability throughout the tropics at this time.