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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. 

Numerous temperature and environmental proxies are based on glycerol dialkyl glycerol tetraethers (GDGTs), which are membrane lipids commonly found in the water columns and sediments of lakes. The TEX86 temperature proxy is based on isoprenoid GDGTs, which are produced by members of the archaea, and is used to reconstruct lake surface temperature. Branched GDGTs are lipids produced by bacteria and form the basis of the MBT′5ME temperature proxy. Although many outstanding questions still exist regarding proxies based on isoprenoid and branched GDGTs, both compound classes have been relatively well-studied in lakes. More recently, other types of GDGTs and related compounds are increasingly being reported from lacustrine sediments including hydroxylated GDGTs (OH-GDGTs) and glycerol monoalkyl glycerol tetraethers (GMGTs). In the process of generating lacustrine TEX86 or MBT′5ME temperature records, we noted that OH-GDGTs or GMGTs (or both) are frequently present. The RI-OH, based on OH-GDGTs, recently has been proposed as a temperature proxy in lakes while GMGTs are associated with 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 covering timespans from the Holocene to multiple glacial-interglacial cycles. Study lakes include 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 in contrasting depositional environments and examine their GDGT distributions in relationship to temperature variability, oxic/anoxic conditions, hydroclimate fluctuations, and other geochemical/environmental parameters. 

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. 

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. 

Abstract. Four adjacent lakes (Arco, Budd, Deming, and Josephine) within Itasca State Park in Minnesota, USA, are reported to be meromictic in the scientific literature. However, seasonally persistent chemoclines have never been documented. We collected seasonal profiles of temperature and specific conductance and placed temperature sensor chains in two lakes for ∼1 year to explore whether these lakes remain stratified through seasonal mixing events and what factors contribute to their stability. The results indicate that all lakes are predominantly thermally stratified and are prone to mixing in isothermal periods during spring and fall. Despite brief, semi-annual erosion of thermal stratification, Deming Lake showed no signs of complete mixing from 2006–2009 and 2019–2022 and is likely meromictic. However, the other lakes are not convincingly meromictic. Geochemical data indicate that water in Budd Lake, which contains the most water, is predominantly sourced from precipitation. The water in the other three lakes is of the calcium–magnesium–bicarbonate type, reflecting a source of water that has interacted with the deglaciated landscape. δ18OH2O and δ2HH2O measurements indicate the lakes are supplied by precipitation modified by evaporation. Josephine, Arco, and Deming lakes sit in a valley with likely permeable sediments and may be hydrologically connected through wetlands and recharged with shallow groundwater, as no streams are present. The water residence time in meromictic Deming Lake is short (100 d), yet it maintains a large reservoir of dissolved iron, indicating that shallow groundwater may be an additional source of water and dissolved ions. All four lakes develop subsurface chlorophyll maxima layers during the summer. All lakes also develop subsurface oxygen maxima that may result from oxygen trapping in the spring by rapidly developed summer thermoclines. Documenting the mixing status and general chemistry of these lakes enhances their utility and accessibility for future biogeochemical studies, which is important as lake stratification and anoxia are becoming more prevalent due to changes in climate and land use.