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1. Investigating OH-GDGTsand GMGTs in lacustrine sedimentsand potential relationships toenvironmental conditions

   Castañeda, Isla S; Daniels, W; Kocecky, Bronwen; Sae-Lim, Jarunter Nadia; Salacup, Jeff; Schneider, Tobias; Sun, Heeyeon; Wilk, Alexander; Zhao, Boyang (December 2024, American Geophysical Union)

   Numerous temperature and environmental proxies are based on glycerol dialkylglycerol tetraethers (GDGTs), which are membrane lipids commonly found in thewater columns and sediments of lakes. The TEX86 temperature proxy is based onisoprenoid GDGTs, which are produced by members of the archaea, and is used toreconstruct lake surface temperature. Branched GDGTs are lipids produced bybacteria and form the basis of the MBT′5ME temperature proxy. Although manyoutstanding questions still exist regarding proxies based on isoprenoid and branchedGDGTs, both compound classes have been relatively well-studied in lakes. Morerecently, other types of GDGTs and related compounds are increasingly beingreported from lacustrine sediments including hydroxylated GDGTs (OH-GDGTs) andglycerol monoalkyl glycerol tetraethers (GMGTs). In the process of generating lacustrine TEX86 or MBT′5ME temperature records, we noted that OH-GDGTs orGMGTs (or both) are frequently present. The RI-OH, based on OH-GDGTs, recentlyhas been proposed as a temperature proxy in lakes while GMGTs are associatedwith oxygen-deficient environments. Here we examine distributions of OH-GDGTs and GMGTs in a variety of lakes that also have existing TEX86 or MBT′5ME temperature reconstructions. These lakes range from small to large, shallow to deep,tropical to arctic, differ in oxygenation state, and have sedimentary records coveringtimespans from the Holocene to multiple glacial-interglacial cycles. Study lakesinclude El’gygytgyn (arctic Russia), Malawi (tropical southeast Africa), Issyk Kul(Kyrgyzstan), Lake 578 (Greenland), and high elevation lakes in the central Andes (South America). We explore the presence/absence of these compounds incontrasting depositional environments and examine their GDGT distributions inrelationship to temperature variability, oxic/anoxic conditions, hydroclimatefluctuations, and other geochemical/environmental parameters. 

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2. The Branched GDGT Isomer Ratio Refines Lacustrine Paleotemperature Estimates

   https://doi.org/10.1029/2024GC012069  

   Novak, Joseph B; Russell, James M; Lindemuth, Emma R; Prokopenko, Alexander A; Pérez‐Angel, Lina; Zhao, Boyang; Swann, George_E A; Polissar, Pratigya J (March 2025, Geochemistry, Geophysics, Geosystems)

   Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane-spanning lipids synthesized by bacteria in numerous substrates. The degree of methylation of the five methyl brGDGTs in both soils and lake sediments, described by the MBT′5Me index, is empirically related to surface atmospheric temperature. This relationship in lakes is generally assumed to reflect lake surface temperatures captured by brGDGT production in the water column and exported to lake sediments, and the MBT′5Me index has been applied to brGDGTs in lake sediment successions to reconstruct changes in temperature through time. We analyzed the relationship between MBT′5Me and the isomerization of brGDGTs (IR6Me) in globally distributed surficial lake sediments and demonstrated that the relationship, and calibrations, of MBT′5Me and temperature in middle and high latitude lakes are sensitive to incompletely understood factors related to IR6Me. IR6Me does not appear to track a non-thermal influence of brGDGT methylation in tropical lakes, but this could change as the data set is expanded. We address ongoing challenges in the application of the MBT′5Me paleothermometer in middle and high latitude lakes with new MBT′5Me-temperature calibrations based on grouping lakes by IR6Me. We demonstrate how IR6Me can distinguish samples with a significant non-thermal influence on MBT′5Me by targeting anomalously warm temperatures during the Last Glacial Maximum from newly analyzed piston and gravity core samples from Lake Baikal, Russia. 

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3. A 1,800-year Air Temperature Record from the Tropical Andean Highlands based on Branched GDGTs from Lake Chacacocha, Peru

   Jarunetr (December 2022, Transactions American Geophysical Union)

   Late Holocene air temperature of the tropical Andean highlands is poorly constrained. Most inferences of past temperature from this region are either qualitatively inferred from environmental proxies such as pollen, moraines, and XRF, or derived from proxies with multiple climatic drivers such as ice core oxygen isotopes. Historical temperature records are either short or nonexistent. Here we present a quantitative reconstruction of air temperature based on branched Glycerol Dialkyl Glycerol Tetraethers (br-GDGTs) derived from sediments of Lake Chacacocha, southeastern Peru (13.96 S, 71.08 W; 4,860 m asl). Chacacocha’s catchment and lake remain above freezing for most of the year. Thus, we interpret calibrated temperatures to reflect mean annual air temperature, though potentially with a bias towards months with more water column mixing. The Chacacocha br-GDGT record suggests a cooling trend after the first millennium, beginning prior to the cooling recorded in global temperature reconstructions. However, a multi-centennial cooling event between 400-600 CE appears to be the strongest centennial-scale cooling of the Common Era, unlike global temperature but in agreement with regional pollen and other paleoenvironmental proxies. The Chacacocha br-GDGT record exhibits striking similarity to a diatom-inferred lake stratification reconstruction and bulk carbon isotopes from the same sediment core, suggesting that the aquatic ecosystem and the surrounding environment responded to air temperature changes. We also compare and contextualize our lacustrine br-GDGT record using br-GDGTs from nearby peatlands and soil, isoprenoid GDGTs from nearby Lake Sibinacocha, and the hydrogen and carbon isotope composition of leaf waxes from the same sediment core. Our results help to illuminate long-term temperature change and its impacts on tropical Andean environments. 

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4. 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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5. 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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