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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. 

The Common Era history of effective moisture in the Central Andes is poorly understood, as most Andean proxy records reflect large-scale atmospheric circulation over the South American lowlands rather than localized precipitation vs. evaporation. Here we present 1800-year leaf wax hydrogen and carbon isotope sedimentary records from Lake Chacacocha (13.96°S, 71.08°W, 4,860 m asl.) in the Central Andes. Leaf wax δ2H from different chain lengths offers information about large-scale atmospheric conditions and local-scale effective moisture. Our leaf wax δ2H data record a gradual intensification of the South American summer monsoon (SASM) beginning around ~1250 CE, prior to the external forcings of the Little Ice Age (LIA). Despite peak SASM intensification, our leaf wax δ13C data reveal a locally arid interval between ca. 1600 and 1800 CE. The arid interval was most likely driven by enhanced evaporation and reduced local precipitation, as indicated by the hydrogen isotope fractionation between mid- and long-chain n-alkanes as well as by climate model simulations. Our results help to reconcile conflicting interpretations of the SASM, glacial, and lake-level histories in the Central Andes during the Common Era. 

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 Understanding eastern African paleoclimate is critical for contextualizing early human evolution, adaptation, and dispersal, yet Pleistocene climate of this region and its governing mechanisms remain poorly understood due to the lack of long, orbitally-resolved, terrestrial paleoclimate records. Here we present leaf wax hydrogen isotope records of rainfall from paleolake sediment cores from key time windows that resolve long-term trends, variations, and high-latitude effects on tropical African precipitation. Eastern African rainfall was dominantly controlled by variations in low-latitude summer insolation during most of the early and middle Pleistocene, with little evidence that glacial–interglacial cycles impacted rainfall until the late Pleistocene. We observe the influence of high-latitude-driven climate processes emerging from the last interglacial (Marine Isotope Stage 5) to the present, an interval when glacial–interglacial cycles were strong and insolation forcing was weak. Our results demonstrate a variable response of eastern African rainfall to low-latitude insolation forcing and high-latitude-driven climate change, likely related to the relative strengths of these forcings through time and a threshold in monsoon sensitivity. We observe little difference in mean rainfall between the early, middle, and late Pleistocene, which suggests that orbitally-driven climate variations likely played a more significant role than gradual change in the relationship between early humans and their environment. 

Abstract The meridional variability of the Subtropical Front (STF) in the Southern Hemisphere, linked to expansions or contractions of the Southern Ocean, may have played an important role in global ocean circulation by moderating the magnitude of water exchange at the Indian‐Atlantic Ocean Gateway, so called Agulhas Leakage. Here we present new biomarker records of upper water column temperature (and) and primary productivity (chlorins and alkenones) from marine sediments at IODP Site U1475 on the Agulhas Plateau, near the STF and within the Agulhas retroflection pathway. We use these multiproxy time‐series records from 1.4 to 0.3 Ma to examine implied changes in the upper oceanographic conditions at the mid‐Pleistocene transition (MPT, ca. 1.2–0.8 Ma). Our reconstructions, combined with prior evidence of migrations of the STF over the last 350 ka, suggest that in the Southwestern Indian Ocean the STF may have been further south from the Agulhas Plateau during the mid‐Pleistocene Interim State (MPIS, MIS 23–12) and reached its northernmost position during MIS 34–24 and MIS 10. Comparison to aGloborotalia menardii‐derived Agulhas Leakage reconstruction from the Cape Basin suggests that only the most extreme northward migrations of the STF are associated with reduced Agulhas Leakage. During the MPIS, STF migrations do not appear to control Agulhas Leakage variability, we suggest previously modeled shifting westerly winds may be responsible for the patterns observed. A detachment between STF migrations and Agulhas Leakage, in addition to invoking shifting westerly winds may also help explain changes in CO₂ventilation seen during the MPIS.