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Abstract Climate variability over glacial-interglacial timescales is not well characterized in the tropical Andes, and paleoclimate records are lacking in this region. To offset this gap in knowledge, we analyzed organic compounds from sediment cores from Lake Junin (the Peruvian Andes) to better understand climate variability in the region since the LGM. We measured the δD of long and mid-chain n-alkanes (nC29 – terrestrial vegetation and nC23 – aquatic vegetation) to characterize changes in the intensity of the South American Summer Monsoon (SASM) and evaporative enrichment of lake water. We also measured the δ13C of these compounds to better understand the hydrology of the region and constrain the sources of organic matter through time. Additionally, we used the fractional abundances of brGDGTs to estimate changes in temperature over the same time period. Our results suggest that SASM intensity is controlled by insolation in the southern hemisphere. During the late Pleistocene, the δD of both nC29 and nC23 are relatively D-depleted indicating a wetter time period. This is followed by progressive D-enrichment of both nC29 and nC23 which suggests increasing aridity until the Holocene. The early Holocene is characterized by a decoupling between the δD of nC23 and nC29.The δD of nC23 becomes relatively more D-enriched, matching trends in a carbonate oxygen isotope record from Lake Junin, indicating increased lake-water evaporation during this time. Finally, the late Holocene is characterized by a return to wetter conditions. The δ13C of both nC29 and nC23 further confirms the hydrologic history of this region, while shedding light on vegetation dynamics. During the Pleistocene, the δ13C of both n-alkanes suggests DIC uptake, but at the start of the Holocene they diverge, showing two distinct plant communities, one entirely aquatic and one entirely terrestrial. Our brGDGT-based temperature reconstruction shares similar trends with alkenone-based SST reconstructions off the coast of Peru, indicating a consistent regional climate signal. 

Abstract The South American monsoon is central to the continent’s water and energy cycles, however, the relationships between the monsoon, regional water balance, and global climate change is poorly understood. Sediment records at Lake Junín (11°S, 76°W) provide an opportunity to explore these connections over the last 650 ka. Here, we focus on two interglacials, the Holocene (11.7–0 ka) and MIS 15 (621–563 ka), when sediment proxies suggest rapid regional hydroclimate fluctuations occurred. Clumped isotope distributions of lake carbonates reveal that interglacial water temperatures were similar to present, though analytical limitations preclude detecting the small temperature differences expected in the tropics (<2 °C). Combining the reconstructed water temperatures with carbonate oxygen (δ18O) and triple oxygen (Δ′17O) isotope values, we reconstruct precipitation δ18O values and lake water Δ′17O values. Precipitation δ18O values, a proxy of monsoon strength, range from -18.6 to -12.3 ‰ with lower values reflecting a stronger monsoon. Lake water Δ′17O values are -14 to 43 per meg and indicate the extent of lake water evaporation; lower values reflect a higher proportion of evaporation to inputs (i.e., more negative P-E). The precipitation δ18O and lake water Δ′17O values from both interglacials vary with the pacing of local summertime insolation, which follows an orbital pacing. These data document the close connection between Andean water balance, the South American monsoon, and global climate. Further, we analyze the relationship between precipitation δ18O and insolation, and we find that the relationship is consistent among interglacials, suggesting a similar response of the monsoon to orbital forcings over time. In contrast, while lake water Δ′17O and insolation are also correlated during both interglacials, water balance was overall more positive during MIS 15 than the Holocene. This suggests that either other global forcings or local basin dynamics can also contribute to water balance at Lake Junín. Together, these data provide new evidence of the connections between global climate, monsoon strength, and regional water balance. 

Climate variability at glacial-interglacial timescales is not well characterized in the tropical Andes, and paleoclimate records are lacking in this region. Lake Junin, in the Peruvian Andes, offers a unique and continuous paleoclimate archive that spans the last 700,000 years. Here, we use organic compounds to characterize climate variability in the region since the Last Glacial Maximum. First, we determined the preservation of organic matter in the sediments using the Carbon Preference Index (CPI), which suggests that n-alkanes have not been altered, and their H isotope composition can be used as paleo precipitation proxies. To reconstruct the isotopic composition of lake water, biomarkers from Eustigmatophyte algae (long chain diols) and diatoms (loliolide/isololiolide) have been identified. This will allow us to better understand aridity and evaporation as well as lake water inputs through time. Additionally, we will use the changes in n-alkane chain length distributions to constrain changes in terrestrial plants (long chain n-alkanes) and aquatic macrophytes (mid-chain n-alkanes) as a potential proxy for changes in lake level as a response to climate. Finally, temperature will be reconstructed using the distributions of br-GDGTs (branched glycerol dialkyl glycerol tetraethers). Using these set of proxies, we aim to characterize climate variability during the Holocene and the end of the LGM in the context of teleconnections between the South American Summer Monsoon and global climate patterns 

We present oxygen isotope and charcoal accumulation records from two lakes in eastern Washington that have sufficient temporal resolution to quantitatively compare with tree‐ring records and meteorological data. Hydroclimate reconstructions from tree‐rings and lake sediments show close correspondence after accounting for seasonal‐ to centennial‐ scale temporal sensitivities. Carbonate δ¹⁸O measurements from Castor and Round lakes reveal that the Medieval Climate Anomaly (MCA) experienced wetter November‐March conditions than the Little Ice Age (LIA). Charcoal records from Castor, Round, and nearby lakes show elevated fire activity during the LIA compared to the MCA. Increased multidecadal hydroclimate variability after 1250 CE is evident in proxy records throughout western North America. In the Upper Columbia River Basin, multidecadal wet periods during the LIA may have enhanced fuel loads that burned in subsequent dry periods. A notable decline in biomass burning occurred with Euro‐American settlement in the late nineteenth century.