Ice formation on lake surfaces in winter causes warm-season bias of lacustrine brGDGT temperature estimates

Abstract

Abstract. It has been frequently found that lacustrine branched glycerol dialkyl glycerol tetraethers (brGDGT)-derived temperatures are warm-season-biased relative to measured mean annual air temperature (AT) in the middle to high latitudes, the mechanism of which, however, is not very clear. Here, we investigated the brGDGTs from catchment soils, suspended particulate matter (SPM) and surface sediments in different water depths in Gonghai Lake in northern China to explore this question. Our results showed that the brGDGT distribution in sediments resembled that in the SPM but differed from the surrounding soils, suggesting a substantial aquatic origin of the brGDGTs in the lake. Moreover, the increase in brGDGT content and decrease in methylation index with water depth in sediments suggested more contribution of aquatic brGDGTs produced from deep or bottom waters. Therefore, established lake-specific calibrations were applied to estimate local mean annual AT. As usual, the estimates were significantly higher than the measured mean annual AT. However, they were similar to (and thus actually reflected) the mean annual lake water temperature (LWT). Interestingly, the mean annual LWT is close to the measured mean warm-season AT, thus suggesting that the apparent warm-season bias of lacustrine brGDGT-derived temperatures could be caused by the discrepancy between AT and LWT. In our study region, ice forms at the lake surface during winter, leading to isolation of the underlying lake water from air and hence higher LWT than AT, while LWT basically follows AT during warm seasons when ice disappears. Therefore, we think that lacustrine brGDGTs actually reflected the mean annual LWT, which is higher than the mean annual AT in our study location. Since the decoupling between LWT and AT in winter due to ice formation is a universal physical phenomenon in the middle to high latitudes, we propose this phenomenon could be also the reason for the widely observed warm-season bias of brGDGT-derived temperatures in other seasonally surface ice-forming lakes, especially in shallow lakes.

Temperature and water depth effects on brGDGT distributions in sub-alpine lakes of mid-latitude North America

“…Lacustrine and soil brGDGT distributions in mid-latitude, temperate regions are seasonally biased towards late summer or fall [Buckles et al, 2014a, Loomis et al, 2014a, Miller et al, 2018, Dang et al, 2018Crampton-Flood et al 2020]. Our lakes also remain frozen for most of the year (October to early June) and, during the ice-cover season, lake water temperatures remain near 4 °C while air temperatures can drop well below 4 °C [Musselman 1994;Liefert et al, 2018; see also Cao et al, 2020].…”

“…Previous studies suggest that brGDGT distributions may be seasonally biased towards late summer or fall in temperate regions [Buckles et al, 2014a, Loomis et al, 2014a, Miller et al, 2018, Dang et al, 2018 and that their concentration increases with increasing water depth [Sinninghe Damsté et al, 2009, Buckles et al, 2014a, Miller et al, 2018. However, the relatively short summer season in high-elevation, cold environments of mid-latitude North America where snow and ice cover persist from early November to late June [Musselman, 1994, Liefert et al, 2018 can potentially influence the seasonality of lacustrine production of brGDGTs [Cao et al, 2020]. Given these considerations, the existing MBT' 5Me calibration for lakes in East Africa may not be universally applicable [Russell et al, 2018].…”

Comment on bg-2021-16

“…All other water chemistry parameters were averaged across all depths and seasons before being used in calibrations. Previously published data from 36 lakes in Central Europe (Weber et al, 2018), 65 lakes in East Africa (Russell et al, 2018), and 38 lakes in China (Dang et al, 2018;Ning et al, 2019;Qian et al, 2019;Cao et al, 2020), were added to the dataset for a total of 182 data points (Fig. 1).…”

“…A possible explanation for the success of the MAF temperature index is that the activity of brGDGTproducing microbes may be heavily depressed under lake ice and/or in frozen soils (Pearson et al, 2011;Shanahan et al, 2013;Peterse et al, 2014;Cao et al, 2020). However, studies employing sub-seasonal sampling of sediment traps and suspended particulate matter in two mid-latitude lakes have shown that brGDGTs are produced within the water column throughout the year, despite the presence of ice cover (Woltering et al, 2012;Loomis et al, 2014b).…”

“…However, it was recognized early on that brGDGTderived temperatures in cold regions may more accurately reflect warm-season temperatures (Pearson et al, 2011;Sun et al, 2011), an hypothesis that was strongly supported in high-latitude lake sediments (Shanahan et al, 2013;Peterse et al, 2014;Foster et al, 2016). Since the methodological advances that allowed for the separation of 5-and 6-methyl isomers (De Jonge et al, 2014a) and the development of new calibrations, both modern (Hanna et al, 2016;Dang et al, 2018;Cao et al, 2020) and paleo (Super et al, 2018;Thomas et al, 2018;Crump et al, 2019;Harning et al, 2020) studies have continued to support a warm-season bias. Additionally, a recent Bayesian calibration found the mean temperature of Months Above Freezing (MAF) to be the only mode to significantly correlate with brGDGT distributions in a global lake sediment dataset (Martínez-Sosa et al, 2020).…”