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Distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) are frequently employed for reconstructing terrestrial paleotemperatures from lake sediment archives. Although brGDGTs are globally ubiquitous, the microbial producers of these membrane lipids remain unknown, precluding a full understanding of the ways in which environmental parameters control their production and distribution. Here, we advance this understanding in three ways. First, we present 43 new high-latitude lake sites characterized by low mean annual air temperatures (MATs) and high seasonality, filling an important gap in the global dataset. Second, we introduce a new approach for analyzing brGDGT data in which compound fractional abundances (FAs) are calculated within structural groups based on methylation number, methylation position, and cyclization number. Finally, we perform linear and nonlinear regressions of the resulting FAs against a suite of environmental parameters in a compiled global lake sediment dataset (n = 182). We find that our approach deconvolves temperature, conductivity, and pH trends in brGDGTs without increasing calibration errors from the standard approach. We also find that it reveals novel patterns in brGDGT distributions and provides a methodology for investigating the biological underpinnings of their structural diversity. Warm-season temperature indices outperformed MAT in our regressions, with Months Above Freezing yielding the highest-performing model (adjusted R2 = 0.91, RMSE = 1.97 °C, n = 182). The natural logarithm of conductivity had the second-strongest relationship to brGDGT distributions (adjusted R2 = 0.83, RMSE = 0.66, n = 143), notably outperforming pH in our dataset (adjusted R2 = 0.73, RMSE = 0.57, n = 154) and providing a potential new proxy for paleohydrology applications. We recommend these calibrations for use in lake sediments globally, including at high latitudes, and detail the advantages and disadvantages of each.
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Lake Dynamics Modulate the Air Temperature Variability Recorded by Sedimentary Aquatic Biomarkers: A Holocene Case Study From Western Greenland
Abstract Quantitative temperature reconstructions from lacustrine organic geochemical proxies including branched glycerol dialkyl glycerol tetraethers (brGDGTs) and alkenones provide key constraints on past continental climates. However, estimation of air temperatures from proxies can be impacted by non‐stationarity in the relationships between seasonal air and water temperatures, a factor not yet examined in strongly seasonal high‐latitude settings. We pair downcore analyses of brGDGTs and alkenones measured on the same samples through the Holocene with forward‐modeled proxy values based on thermodynamic lake model simulations for a western Greenland lake. The measured brGDGT distributions suggest that stable autochthonous (aquatic) production overpowers allochthonous inputs for most samples, justifying the use of the lake model to interpret temperature‐driven changes. Conventional calibration of alkenones (detected only after 5.5 thousand years BP) suggests substantially larger temperature variations than conventional calibration of brGDGTs. Comparison of proxy measurements to forward‐modeled values suggests variations in brGDGT distributions monotonically reflect multi‐decadal summer air temperatures changes, although the length of the ice‐free season dampens the influence of air temperatures on water temperatures. Drivers of alkenone variability remain less clear; potential influences include small changes in the seasonality of proxy production or biases toward specific years, both underlain by non‐linearity in water‐air temperature sensitivity during relevant seasonal windows. We demonstrate that implied temperature variability can differ substantially between proxies because of differences in air‐water temperature sensitivity during windows of proxy synthesis without necessitating threshold behavior in the lake or local climate, and recommend that future studies incorporate lake modeling to constrain this uncertainty.
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- PAR ID:
- 10441483
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 128
- Issue:
- 7
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Paleotemperature histories derived from lake sediment archives provide valuable context for modern and future climate changes. Branched glycerol dialkyl glycerol tetraether (brGDGT) lipids are a valuable tool in such pursuits due to their empirical correlation with temperature and near ubiquity in nature. However, the relative contributions of terrestrial and lacustrine sources of brGDGTs to lake sediments is site‐dependent and difficult to constrain. Here, we explored the potential for intact brGDGTs—the complete lipids with polar head groups (HGs) still attached—to provide insight into the sources of brGDGTs on the landscape and their contributions to the sedimentary record in a set of Arctic lakes. We measured core and intact brGDGTs in soils, surface and downcore sediments, water filtrates, and sediment traps across five lake catchments in the Eastern Canadian Arctic, with an emphasis on Lake Qaupat (QPT), Baffin Island. Soils were dominated by brGDGTs with a monoglycosyl (1G) HG, while lacustrine samples contained more phosphohexose (PH) brGDGTs, providing evidence for in situ brGDGT production in both settings. Core‐ and PH‐brGDGT‐IIIa were more abundant in sediments than in the soils or water column, implying an additional post‐depositional source of brGDGTs. A hierarchical clustering analysis indicated that core brGDGTs in Lake QPT sediments were largely lacustrine in origin, while 1G‐brGDGTs were primarily soil‐derived. Additionally, we found evidence for preservation of intact brGDGTs—especially 1G‐brGDGTs—downcore on thousand‐year timespans, though in situ production deeper in the sediment column cannot be ruled out. Finally, we explored the possibility of reconstructing 1G‐brGDGT‐derived soil temperatures and core‐brGDGT‐derived lake temperatures in tandem from sedimentary archives.more » « less
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