Preprints
https://doi.org/10.5194/cp-2022-19
https://doi.org/10.5194/cp-2022-19
 
15 Mar 2022
15 Mar 2022
Status: this preprint is currently under review for the journal CP.

Evaluation of the distributions of hydroxylated isoprenoidal GDGTs in Holocene Baltic Sea sediments for reconstruction of sea surface temperature: The effect of changing salinity

Jaap S. Sinninghe Damste1,2, Lisa A. Warden1, Carlo Berg3,a, Klaus Jürgens3, and Matthias Moros3 Jaap S. Sinninghe Damste et al.
  • 1Department of Marine Microbiology and Biogeochemistry, NIOZ Netherlands Institute for Sea Research, PO Box 1790 AB 59, Den Burg, The Netherlands
  • 2Faculty of Geosciences, Department of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands
  • 3Departments of Biological Oceanography and Marine Geology, Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Seestraße 15, D-18119 Rostock, Germany
  • apresent address: Public Health Agency of Sweden, Nobels väg 18, Solna 17182 Stockholm, Sweden

Abstract. Hydroxylated glycerol dibiphytanyl glycerol tetraethers (OH-GDGTs) produced by both marine and freshwater thaumarchaea are increasingly used for the reconstruction of past sea surface temperature (SST). They occur throughout the modern Baltic Sea, but it is unknown if their OH-GDGTs can be used for assessing past SST in this area, where salinity has changed considerably over the Holocene. Three commonly applied OH-GDGT for SST reconstruction, i.e., the OH-GDGT%, RI-OH, and RI-OH´ indices, were tested using a Thaumarchaeotal culture enriched from the Baltic Sea grown at 4 and 22 °C, and 12 surface sediments from the Baltic Sea and the adjacent Skagerrak. In the culture experiments all three proxies showed the expected response with the rise of temperature, but their absolute values were not always in line with existing marine core top calibrations, especially for the OH-GDGT% index. Of the two proxies based on the distribution of OH-GDGTs, the RI-OH index shows no increase with increasing mean annual SST, whilst the RI-OH´ index shows a linear correlation with SST when the sediments from the Bothnian Sea and Bay area, with a reduced salinity and increased lateral sediment influx, are omitted. Two sedimentary Holocene records from the Arkona and Gotland basins were studied, the latter in high resolution. In the brackish phase of the Baltic Sea (the Littorina Sea stage), the RI-OH´ index shows a good correlation with the TEX86L, an established temperature proxy in the Baltic Sea, and can be used to identify important climatic events. However, during the preceding Ancylus Lake phase the RI-OH´ (and RI-OH) index records far too high values, resulting in anomalously high SST estimates. This is probably because freshwater thaumarchaea adjust their OH-GDGTs differently, as has been shown for thaumarchaea in Lake Lugano’s water column. In the Littorina Sea, the Ancylus Lake and the Yoldia Lake phases of the Baltic basin, the record of the RI-OH´ index, thus, most likely reflects both changes in temperature and salinity. Overall, our study indicates that a reduced salinity increases the values of the RI-OH´ (and RI-OH) indices substantially and this should be considered when applying these proxies in other settings.

Jaap S. Sinninghe Damste et al.

Status: open (extended)

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  • RC1: 'Comment on cp-2022-19', Anonymous Referee #1, 18 Apr 2022 reply

Jaap S. Sinninghe Damste et al.

Jaap S. Sinninghe Damste et al.

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Short summary
Reconstruction of past climate conditions is important for understanding current climate change. These reconstructions are derived from proxies enabling reconstructions of e.g. past temperature, precipitation, vegetation, sea surface temperature (SST). Here we investigate a recently developed SST proxy based on membrane lipids of ammonium-oxidizing archaea in the ocean. We show that low salinities substantially effect the proxy calibration by examining Holocene Baltic Sea sediments.