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https://doi.org/10.5194/bg-2019-507
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-507
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Feb 2020

Submitted as: research article | 04 Feb 2020

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This preprint is currently under review for the journal BG.

Decoupling of water and air temperature in winter causes warm season bias of lacustrine brGDGTs temperature estimates

Jiantao Cao1,2, Zhiguo Rao3, Fuxi Shi4, and Guodong Jia1 Jiantao Cao et al.
  • 1State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
  • 2Key Laboratory of Western China's Environmental Systems, Ministry of Education, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, 730000, China
  • 3College of Resources and Environmental Sciences, Hunan Normal University, Changsha, 410081, China
  • 4Jiangxi Provincial Key Laboratory of Silviculture, College of Forestry, Jiangxi Agricultural University, Nanchang, 330045, China

Abstract. It has been frequently found that lacustrine brGDGTs-derived temperatures are warm season biased relative to measured annual mean air temperature (AT) in the mid to high latitudes, the mechanism of which, however, is not very clear. Here, we investigated the brGDGTs from catchment soils, and suspended particulate matter (SPM) and surface sediments in the Gonghai Lake in north China to explore this question. Our results showed that the brGDGTs 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. 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, hence suggesting that the apparent warm season bias of lacustrine brGDGTs-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 follows AT during warm seasons when ice disappears. Therefore, we believe what lacustrine brGDGTs actually reflected is 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 mid to high latitudes, we propose this phenomenon could be also the reason for the widely observed warm season bias of brGDGTs-derived temperatures in other lakes, especially the shallow lakes.

Jiantao Cao et al.

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Jiantao Cao et al.

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Short summary
BrGDGTs distribution in Gonghai Lake is different from surrounding soils, and its-derived temperature reflects mean annual lake water temperature (LWT) that is higher than mean annual air temperature (AT). The higher annual mean LWT is due to ice formation in winter that prevents thermal exchange between lake water and air. So, in mid- to high-latitude shallow lakes the brGDGTs thermometer should not be used to reflect AT.
BrGDGTs distribution in Gonghai Lake is different from surrounding soils, and its-derived...
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