Change in provenance of branched glycerol dialkyl glycerol tetraethers over the Holocene in the Baltic Sea and its impact on continental climate reconstruction

Lisa Warden,Matthias Moros,Yuki Weber,Jaap S Sinninghe Damsté

doi:10.1016/j.orggeochem.2018.03.007

Abstract

The Baltic Sea is an enclosed basin that experienced a number of different salinity phases during the Holocene corresponding to the establishment of a connection with the North Sea. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) in surface sediments and Holocene sedimentary successions from the Gotland and Arkona Basins were analyzed to examine their potential applicability as indicators for soil organic matter input, as well as their suitability for paleoclimate reconstructions. Our results show a marked change in brGDGT distributions and the branched and isoprenoid tetraether (BIT) index. The transition of the Ancylus Lake (fresh) to the Littorina Sea (brackish) phase is revealed by a large drop in the BIT index and an increase in the MBT′5Me, which reveals a large shift in provenance of the sedimentary brGDGTs. During the lake phase, brGDGTs are presumed to be primarily produced by in situ production in the water column. During the brackish phase, in situ brGDGT production in the alkaline pore waters of the surface sediments (as revealed by their high degree of cyclization) was predominant although there was evidence of occasional input of soil-derived brGDGTs. The predominant aquatic autochthonous production of brGDGTs does not allow the use of brGDGTs for continental air temperature reconstructions but they can be used for bottom water and lake temperature reconstructions during some intervals. The results from this study demonstrate that geological changes and concomittant salinity variations can be revealed by the provenance of brGDGTs.

Highlights

Investigating climatic variability is important for predicting future climate change as well as how ecosystems, organisms and human society could be affected
Their total organic carbon (TOC) content varied from ca. 3 to as high as 16% in the recent brackish phase, while it was substantially lower (i.e.
Marked changes in Branched glycerol dialkyl glycerol tetraethers (brGDGTs) distribution and the branched and isoprenoid tetraether (BIT) index occurred concomitantly with the re-establishment of the connection between the Baltic Sea (BS) and the North Sea. These shifts were likely caused by the change in the brGDGT sources from water column in situ production during the lower salinity phases to in situ production by benthic microbes during the brackish phase

Summary

Introduction

Investigating climatic variability is important for predicting future climate change as well as how ecosystems, organisms and human society could be affected. Be restricted to soils and peats (Hopmans et al, 2004; Weijers et al, 2006b). In marine and lacustrine sediments the bacterial brGDGTs were initially assumed to originate predominantly from continental soil erosion and terrestrial runoff, whereas the isoprenoid GDGTs (isoGDGTs; including crenarchaeol, the GDGT specific to Thaumarchaeota; Sinninghe Damsté et al, 2002) were thought to be mainly produced by aquatic ammonia-oxidizing archaea.

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Discussion

Conclusion