The author(s) will give a talk

Assessing new temperature and conductivity calibrations on a 24,000-year record of lacustrine branched glycerol dialkyl glycerol tetraethers in the Polar Ural Mountains

Lovell, Kathryn 1 ; Thomas, Elizabeth K 2 ; Cowling, Owen C 3 ; Castañeda, Isla S 4 ; Svendsen, John Inge 5

1 Department of Geology, University at Buffalo
2 Department of Geology, University at Buffalo
3 Department of Geology, University at Buffalo
4 Department of Geosciences, University of Massachusetts Amherst
5 Department of Earth Science, University of Bergen, Bjerknes Centre for Climate Research

Siberia contains relatively few records of past terrestrial climate, especially when compared to North America, Greenland, and Europe. Yet, this region was last glaciated well before the Last Glacial Maximum (LGM), and therefore contains longer lake sediment records than in much of North America and Europe. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are an emerging proxy for past temperature, pH, and conductivity in lacustrine settings. We apply several new lacustrine brGDGT temperature and conductivity calibrations to a 24,000-year-long record from Bolshoye Shchuchye, a lake in the Polar Ural Mountains, Siberia, and assess the effectiveness of brGDGT-based temperature reconstructions in this lake. The distribution of brGDGTs is similar to that of other Arctic lakes, suggesting a lacustrine source of brGDGTs. The MBT’5Me and Bayesian temperature calibrations result in higher temperatures during the LGM than during the Holocene, which is contradictory to other climate proxies, including glacier extent and pollen, which indicate the LGM was colder than the Holocene in this region. A recently developed calibration that empirically fit fractional abundances of structural subsets of brGDGTs to temperature yields a record with entirely different trends, with the LGM cooler than the Holocene, but the highest inferred temperature in the record from 15 to 10 ka. Pollen, sedimentary ancient DNA, and leaf wax abundance records, show that the Polar Ural Mountains experienced temperature changes between 15 to 10 ka generally in step with those in the North Atlantic region, suggesting that this temperature record inferred using the new brGDGT-based calibration is also not correct. However, we also applied a new brGDGT-conductivity calibration to this record, and find that conductivity was highest from about 11 to 6 ka. Seasonal lake ice cover at Bolshoye Shchuchye was likely at a minimum during this interval, as inferred from elemental ratios and the isotopic difference between long- and mid-chain leaf waxes. Lake water conductivity may have been higher from 11 to 6 ka than the rest of the record due to greater seasonal evaporation of lake water. The varying degrees of success of existing brGDGT-temperature and –conductivity calibrations suggest that brGDGTs distributions are being mainly influenced by other non-thermal factors in this record. We plan to further examine more factors causing brGDGT variability in this record, with the goal of better understanding brGDGTs in Arctic lakes.

Raberg, J. H. et al., 2021, Revised fractional abundances and warm-season temperatures substantially improve brGDGT calibrations in lake sediments: Biogeosciences, preprint.

Regnell, C. et al., 2019, Glacial and climate history of the last 24,000 years in the Polar Ural Mountains, inferred from partly carved lake sediments: BOREAS, v. 48, p. 432-443.

Zhao, B. et al., 2021, Development of an in situ branched GDGT calibration in Lake 578, southern Greenland: Organic Geochemistry, v. 152, p. 1-13.

Russell, J. M. et al., 2018, Distributions of 5- and 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) in East African lake sediment: Effects of temperature, pH, and new lacustrine paleotemperature calibrations: Organic Geochemistry, v. 117, p.56-69.