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Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) are membrane lipids of Archaea. Organic biomarker proxies associated with these lipids, such as the TEX 86 paleothermometer and Branched and Isoprenoid Tetraether (BIT) index, are often used in paleoenvironmental reconstructions for the marine environment, but their general applicability in lacustrine settings is hampered by limited understanding of the biological sources and environmental drivers influencing isoGDGT production. To validate the use of isoGDGT proxies in lakes, we studied the occurrence of isoGDGTs in Lake Chala, a permanently stratified (meromictic) crater lake in equatorial East Africa. We analyzed the abundance and distribution of isoGDGTs in 17 depth profiles of suspended particulate matter (SPM) collected monthly between September 2013 and January 2015, and compared this with the abundance and composition of archaea based on 16S rRNA gene and quantitative PCR analysis. Both isoGDGTs and archaeal abundance in the SPM were exceptionally low throughout the study period. In the oxygenated part of the water column, higher fractional abundances of crenarchaeol are matched by a predominance of the ammonia-oxidizing Thaumarchaeota I.1b that are known to produce this GDGT, whereas deep anoxic water layers are characterized by high fractional abundances of GDGT-0 as well as the anaerobic heterotrophic Group C3 MCG Bathyarchaea and specific euryarchaeotal methanogens. Analysis of intact polar lipid (IPL) isoGDGTs using SPM depth profiles from three months representing distinct seasons during the study period revealed the presence of several IPLs of GDGT-0 in the anoxic lower water column, which are rarely found in natural settings. IPL GDGT-0 with a phosphatidylglycerol (PG-), monohexosephosphatidylglycerol (MH-PG-) and dihexose-phosphatidylglycerol (DH-PG-) head-group was typically only present just above the lake bottom at 90 m depth and probably reflect specific communities of anaerobic archaea. We also determined the flux and distribution of isoGDGTs in settling particles collected monthly between November 2006 and January 2015 from a sediment trap suspended at 35 m water depth to assess seasonal and inter-annual variability in surface-water isoGDGT production, and compared this with the temporal distribution of isoGDGTs in the 25,000-year long sediment record from Lake Chala. Monthly variation of isoGDGTs in the 98-month settling-particles record did not show a strong annual pattern related to seasonal water-column mixing and stratification, likely because the oxycline was regularly situated below sediment-trap depth. Episodes of high GDGT-0 concentrations relative to crenarchaeol in the settling particles can therefore be linked to periods of exceptionally shallow oxycline depth, which suppresses the thaumarchaeotal bloom. During such intervals, TEX86 based paleotemperatures are not reliable because isoGDGT input from other archaeal sources disproportionally influences TEX86 values and creates a cold-temperature bias. Additionally, the abundance of the crenarchaeol isomer relative to crenarchaeol (f[CREN]) gradually increases during such episodes of high GDGT-0/crenarchaeol ratio, suggesting increasing dominance of Group I.1b over Group I.1a Thaumarchaeota, and might prove a good marker for prolonged shallow-oxycline conditions. Most importantly, the associated near-absence of crenarchaeol during times of strong upper-water-column stratification results in high BIT-index values. We propose that this suppression mechanism may be the principal driver of BIT-index variation in the sediment record of Lake Chala, and the main source of observed congruence between the BIT index and climate-driven lake-level variation on long time scales. (C) 2021 The Authors. Published by Elsevier Ltd.

Landscapes in high northern latitudes are assumed to be highly sensitive to future global change, but the rates and long-term trajectories of changes are rather uncertain. In the boreal zone, fires are an important factor in climate–vegetation interactions and biogeochemical cycles. Fire regimes are characterized by small, frequent, low-intensity fires within summergreen boreal forests dominated by larch, whereas evergreen boreal forests dominated by spruce and pine burn large areas less frequently but at higher intensities. Here, we explore the potential of the monosaccharide anhydrides (MA) levoglucosan, mannosan and galactosan to serve as proxies of low-intensity biomass burning in glacial-to-interglacial lake sediments from the high northern latitudes. We use sediments from Lake El'gygytgyn (cores PG 1351 and ICDP 5011-1), located in the far north-east of Russia, and study glacial and interglacial samples of the last 430 kyr (marine isotope stages 5e, 6, 7e, 8, 11c and 12) that had different climate and biome configurations. Combined with pollen and non-pollen palynomorph records from the same samples, we assess how far the modern relationships between fire, climate and vegetation persisted during the past, on orbital to centennial timescales. We find that MAs attached to particulates were well-preserved in up to 430 kyr old sediments with higher influxes from low-intensity biomass burning in interglacials compared to glacials. MA influxes significantly increase when summergreen boreal forest spreads closer to the lake, whereas they decrease when tundra-steppe environments and, especially, Sphagnum peatlands spread. This suggests that low-temperature fires are a typical characteristic of Siberian larch forests also on long timescales. The results also suggest that low-intensity fires would be reduced by vegetation shifts towards very dry environments due to reduced biomass availability, as well as by shifts towards peatlands, which limits fuel dryness. In addition, we observed very low MA ratios, which we interpret as high contributions of galactosan and mannosan from biomass sources other than those currently monitored, such as the moss–lichen mats in the understorey of the summergreen boreal forest. Overall, sedimentary MAs can provide a powerful proxy for fire regime reconstructions and extend our knowledge of long-term natural fire–climate–vegetation feedbacks in the high northern latitudes.

The Amazon basin is a major component of the global carbon and hydrological cycles, a significant natural source of methane, and home to remarkable biodiversity and endemism. Reconstructing past climate changes in the Amazon basin is important for a better understanding of the effect of such changes on these critical functions of the basin. Using a novel biomarker proxy, based on the membrane lipids of soil bacteria with a new regional calibration, we present a reconstruction of changes in mean annual air temperatures for the Amazon catchment during the last 37 kyr B. P. Biomarkers were extracted from Ocean Drilling Program sediment core ODP942 recovered from the Amazon fan. The Amazon fan is a major depository for terrestrial sediments, with the advantage that the terrestrial material captured reflects a regional integration of the whole river catchment. The reconstructed tropical Amazonian temperatures were similar to 5 degrees C cooler at the Last Glacial Maximum (similar to 21 degrees C) compared to modern values (similar to 26 degrees C). This is in agreement with previous estimates of tropical continental temperatures in the tropical Amazon basin and tropical Africa during the Last Glacial Maximum. Moreover, we also illustrate how the soil bacterial membrane lipid record reveals major changes in basin dynamics and sediment provenance during the glacial-Holocene transition, impacting the biomarker reconstructions from similar to 11 kyr onward.

The role of the tropics in global climate change is actively debated, particularly in regard to the timing and magnitude of thermal and hydrological response. Continuous, high-resolution temperature records through the Last Glacial Maximum (LGM) from tropical oceans have provided much insight but surface temperature reconstructions do not exist from tropical continental environments. Here we used the TEX₈₆ paleotemperature proxy to reconstruct mean annual lake surface temperatures through the Last Glacial Maximum (LGM) in Lake Malawi, East Africa (9°–14°S). We find a ∼3.5°C overall warming since the LGM, with temperature reversals of ∼2°C during the Younger Dryas (12.5 ka BP) and at 8.2 ka BP. Maximum Holocene temperatures of ∼29°C were found at 5 ka BP, a period preceding severe drought in Africa. These results suggest a substantial thermal response of southeastern tropical Africa to deglaciation and to varying conditions during the Holocene.