EGU2020-12592
https://doi.org/10.5194/egusphere-egu2020-12592
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Biomarker (brGDGT) degradation and production in lacustrine surface sediments: Implications for paleoclimate reconstructions.

Cindy De Jonge1, Annika Fiskal2, Xingguo Han2, and Mark Lever2
Cindy De Jonge et al.
  • 1Geological Institute, Department of Earth Sciences, ETH Swiss Federal Institute of Technology, 8092 Zurich, Switzerland
  • 2Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, ETH Swiss Federal Institute of Technology, 8092 Zurich, Switzerland

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a class of biomarker lipids that can be conserved over long timescales in lake sediments. Produced throughout the lake water column before settling and incorporation in the sedimentary archive, they are used to reconstruct lake water temperature changes through time. However, it is not clear how degradation and/or production of these compounds in the surface sediments influences the brGDGT signal and the reconstructed temperature record.

Here we present the core lipid (“fossil”) and intact polar lipid (“recently produced”) signal of brGDGT lipids in 8 short cores collected in 4 Swiss lakes, covering a eutrophic gradient. In eutrophic conditions (Lake Baldegg), a clear subsurface (20-35 cm blf) maximum in intact polar lipids is observed (15-20%), whereas the most surficial sediments (0-2 cm blf) show the lowest percentage of IPL lipids (<5%). Our data indicates that tetramethylated brGDGT lipids are produced in the subsurface. As the bacterial community has been reconstructed in all cores, using 16S rRNA gene distribution, we observe that this production is coeval with an increase in the relative abundance of OTUs in the phyla Acetothermia, Aminicenantes, Caldiserica and Spirochaetes. Hexamethylated brGDGTs are encountered in increased amounts in most surficial sediments (0-2 cm bsf), but are degraded further downcore. Both degradation and in-situ production cause the reconstructed temperatures based on the surface sediments to be 2 ℃ colder than those from the subsurface.

In sediments where degradation and subsurface production of brGDGT lipids occurs, this has the potential to impact paleoclimate reconstructions. A colder MBT’5ME signal in surface sediments has indeed been observed in several studies (i.e. Tierney et al., 2012; Miller et al., 2018, Martin et al., 2020). Furthermore, a distinct brGDGT signal in surface sediments has a possible impact on existing lacustrine calibration datasets, as these are based on surface sediments.

References:

Tierney et al. (2012), GCA 77, p561-581. Miller et al. (2018), CoP 14 (11), p1653-1667. Martin et al. (2020), QSR 228, 106109.

How to cite: De Jonge, C., Fiskal, A., Han, X., and Lever, M.: Biomarker (brGDGT) degradation and production in lacustrine surface sediments: Implications for paleoclimate reconstructions. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12592, https://doi.org/10.5194/egusphere-egu2020-12592, 2020

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Presentation version 1 – uploaded on 01 May 2020
  • CC1: Comment on EGU2020-12592, Camille Thomas, 04 May 2020

    Very nice work. This is the kind of work that i feel has been missing before all the GDGT-based paleothermometer studies kicked in, although many are probably correct. What would be your feeling about lake-based use of GDGT-paleothermometer. My point is that lakes so often differ from each others in terms of OM inputs and lipid compositions (because of stratification, water temperatures, sulfate/methane chemistry, salinity) that is quite complex to have a GDGT thermometer model for them. Should people systematically undergo an IPFL and 16S rRNA gene survey of the shallow sediments and water column before hunting for GDGTs in the sedimentary archives ? thanks a lot!

    • AC1: Reply to CC1, Cindy De Jonge, 06 May 2020

      Hi Thomas, thanks for your comment,

      this dataset fits in an effort to determine which environmental factors change the brGDGT distribution within lakes, combined with insights in bacterial community changes. I'm hoping this will allow us to determine the effect on brGDGTs of all the different drivers in the lake you mention.

      An offset in brGDGT-derived temperatures in surface sediments has been observed in a few different studies (references in abstract). This effect does not seems to affect older sediments (although there is no clear ‘age cut-off’ for this. For using brGDGTs to reconstruct recent environmental changes I would currently advise to use low temperature extraction and separate the IPLs, based on the results from these 2 first cores, at least for a few samples. Expanding on our results in different lake systems can still teach us a lot about their production!