Oxygen Depletion in Lake Waters May Skew brGDGT‐ Inferred Temperatures by More Than 10°C

Abstract

Abstract Branched glycerol dialkyl glycerol tetraether (brGDGT) paleothermometry is an important tool for paleoclimate research. Oxygen availability has been suggested to skew brGDGT‐inferred temperatures, but the effect is poorly constrained. Here, we examine the influence of dissolved oxygen (DO) on brGDGTs in lacustrine water filtrates. By examini10.1029/2024GL113562ng three subgroups of samples collected within narrow temperature ranges, we more effectively isolate the influence of DO. We find significant correlations between DO and the MBT′5Me temperature index in all three constant‐temperature subgroups. Underlying shifts in key brGDGT abundances indicate the problem is likely to manifest in virtually all existing calibrations. The effects appear at even modest DO depletion, where existing anoxia biomarkers are unlikely to appear. Though creating a quantitative correction is not straightforward, we estimate maximum DO‐driven temperature offsets. These observations highlight a large and hitherto unquantified source of uncertainty in brGDGT‐based paleotemperature reconstructions and underscore the need for further research. Plain Language Summary Branched glycerol dialkyl glycerol tetraether (brGDGT) lipids record the temperature of their environment through their chemical structures. Thus, by measuring their distributions in sedimentary archives, it is possible to reconstruct Earth\u27s temperature back in time. However, these inferred temperatures can be skewed by other environmental parameters, including dissolved oxygen (DO) availability. DO has been shown to affect brGDGT temperatures before, but the effect is not well constrained. Here, we looked at brGDGTs in lake waters from three different locations. To better isolate the influence of DO on brGDGTs, we artificially held temperature constant by grouping samples into three subgroups collected within narrow temperature ranges. We found that, even with temperature held roughly constant in this way, the brGDGT temperature index (MBT′5Me) varied greatly. We also found that this happens even at modest levels of oxygen depletion. We show how this could bias brGDGT reconstructions and highlight the need for more research