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1. Climate and environmental reconstruction from Lake Junin sediments using terrestrial and aquatic biomarkers

   Lopera_Congote, L; Werne, J; Abbott, M; Woods, A; Rodbell, D (December 2023, American Geophysical Union)

   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 

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2. Paleoclimate Variability of Lake Issyk-Kul from ~14 ka to the Present Based on Organic Biomarkers

   Sun, Heeyeon; Castaneda, Isla S; Salacup, Jeffrey M; Ricketts, Richard Douglas (December 2024, American Geophysical Union)

   Lake Issyk-Kul, located in the glacial mountain ranges in mid-latitude arid Central Asia (ACA), is one of the world’s largest lakes. ACA is projected to face future changes in water supply due to rising temperatures and increased precipitation. These changes pose significant economic and environmental threats, particularly in regions with high population growth. Despite its importance, the mid-latitude continental region has many unknown questions surrounding the history of the westerlies, leaving gaps in our understanding of past climate dynamics. This research examines the paleoclimate of Lake Issyk-Kul from approximately 14,000 years ago to the present, utilizing a suite of organic biomarkers to reconstruct temperature, hydroclimate, and vegetation. We measured several organic geochemical proxies including lacustrine alkenones (U index) and isoprenoid and hydroxylated glycerol dialkyl glycerol tetraethers (GDGTs; TEX86 and RIOH proxies). Additionally, the distribution and isotopic composition of plant waxes (n-alkanes) are analyzed to infer changes in surrounding vegetation and moisture sources. Preliminary results indicate significant and abrupt climate shifts in the Issyk-Kul region during key periods such as the Bølling-Allerød and Younger Dryas. We compare biomarker records from sites on the western and eastern sides of the lake to understand regional variations in climate response in a large lake system that spans a precipitation gradient. These findings enhance our understanding of the sensitivity of Issyk-Kul in regional and global climate regimes, contributing to more accurate predictions of future conditions in ACA. 

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3. Climate Variability in the Central Peruvian Andes Over The Past ca. 22,000 Years

   Lopera_Congote, L; O'beirne, M; Werne, J; Rodbell, D; Woods, A; Abbott, M (December 2024, Geologic Society of America)

   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. 

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4. Temperature and hydroclimate variability in mid-latitude arid Central Asia during the past 13,600 years: a multi-proxy investigation of Issyk-Kul (Kyrgyzstan)

   Castañeda, Isla S; Sun, Heeyeon; Salacup, Jeff (March 2024, Geological Society of America)

   Arid Central Asia (ACA) will be among the places on Earth most strongly affected by future changes in water supply. Rising temperatures and projected increases in inter-annual precipitation variability are expected to bring economic and environmental stress to a region characterized by high population growth. To date, mid-latitude continental locations have received relatively less research attention in comparison to the high-latitude or tropical locations and therefore represent a gap in our understanding of past climate dynamics. Issyk–Kul (Kyrgyzstan), one of the largest lakes in the world, is a unique site to examine mid-latitude climate variability in ACA due to its location in a mountainous region of the Asian continental interior that today is situated outside of the influence of the monsoons to the south or southeast. Here we use previously collected sediment cores from Issyk-Kul to generate new temperature, hydroclimate and vegetation records spanning from 13,600 to 2,000 years ago. We evaluate the use of several organic geochemical temperature proxies including lacustrine alkenones (UK′37 Index), TEX86, and proxies based on hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH-GDGTs). We also examine the distributions and isotopic composition of plant waxes (n-alkanes) to examine changes in the vegetation surrounding Issyk-Kul. Plant wax deuterium isotopes are used to investigate changes in precipitation amount and shifts in the dominant moisture source to ACA, while carbon isotopes are used to examine past shifts in C3 vs C4 vegetation. Our results thus far reveal that multiple proxies indicate a strong response of Issyk-Kul to climate variability during the Bølling–Allerød and Younger Dryas. Improved knowledge of past mid-latitude climate dynamics is needed to predict future conditions in ACA more accurately. 

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5. Lake Dynamics Modulate the Air Temperature Variability Recorded by Sedimentary Aquatic Biomarkers: A Holocene Case Study From Western Greenland

   https://doi.org/10.1029/2022JG007106  

   Cluett, A. A.; Thomas, E. K.; McKay, N. P.; Cowling, O. C.; Castañeda, I. S.; Morrill, C. (June 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Quantitative temperature reconstructions from lacustrine organic geochemical proxies including branched glycerol dialkyl glycerol tetraethers (brGDGTs) and alkenones provide key constraints on past continental climates. However, estimation of air temperatures from proxies can be impacted by non‐stationarity in the relationships between seasonal air and water temperatures, a factor not yet examined in strongly seasonal high‐latitude settings. We pair downcore analyses of brGDGTs and alkenones measured on the same samples through the Holocene with forward‐modeled proxy values based on thermodynamic lake model simulations for a western Greenland lake. The measured brGDGT distributions suggest that stable autochthonous (aquatic) production overpowers allochthonous inputs for most samples, justifying the use of the lake model to interpret temperature‐driven changes. Conventional calibration of alkenones (detected only after 5.5 thousand years BP) suggests substantially larger temperature variations than conventional calibration of brGDGTs. Comparison of proxy measurements to forward‐modeled values suggests variations in brGDGT distributions monotonically reflect multi‐decadal summer air temperatures changes, although the length of the ice‐free season dampens the influence of air temperatures on water temperatures. Drivers of alkenone variability remain less clear; potential influences include small changes in the seasonality of proxy production or biases toward specific years, both underlain by non‐linearity in water‐air temperature sensitivity during relevant seasonal windows. We demonstrate that implied temperature variability can differ substantially between proxies because of differences in air‐water temperature sensitivity during windows of proxy synthesis without necessitating threshold behavior in the lake or local climate, and recommend that future studies incorporate lake modeling to constrain this uncertainty. 

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