Variations in GDGT flux and TEX86 thermometry in three distinct oceanic regimes of the Atlantic Ocean : a sediment trap study

Abstract

TEX86 (tetraether index of tetraethers consisting of 86 carbons) is based on the relative compositions of thaumarchaeotal membrane lipids, glycerol dialkyl glycerol tetraethers (GDGTs), in marine surface sediments, allowing us to estimate sea surface temperature (SST). This proxy is a promising tool for SST reconstructions worldwide. However, it has been recognized that the composition of GDGTs can be altered by non-thermal factors, leading to variable relationships between TEX86 and SST. This thesis shall contribute to a better understanding of TEX86 thermometry and the controlling environmental factors in various oceanic provinces by evaluating the GDGT flux and TEX86 related temperature estimate in sinking particles. In the first part of the thesis, the results in the eastern equatorial Guinea Basin (GBN3) show that TEXH86-derived temperatures correspond to the subsurface water depth ( 50 m), where the nutricline exists, implying the favorable habit at depth of thaumarchaeotal communities. In the coastal upwelling area off Lüderitz (LZ), the results show that the TEXH86-derived temperatures resemble the satellite-derived SSTs with a delay of 26 days during the warmer season while warm-biased estimates occur during the colder season. Relatively higher TEX86 values found imply oxygen-stress. The second part of the thesis in the eastern Fram Strait (79Adegree N; FEVI16), the TEXL86 signal corresponds to water temperature at 30-80 m depth, where nitrification might occur. In the Antarctic Polar Front (50Adegree S; PF3), the TEXL86 -derived temperatures at the shallow trap display cold and warm biases relative to the SSTs with a tendency during periods of relatively low GDGT flux, which may be more dominant in the deep trap. The warm biased TEXL86 signal ( 7 AdegreeC) compared to the SST at the deep trap and in the underlying surface sediment might be due to a contribution of GDGTs derived from Euryarchaeota or a nonliner relationship of TEXL86 with SST in the Southern Ocean. The third part of the thesis focues on the oligotrophic regions. At WAB1, which was located at the fringes of the gyre system, the TEXH86-derived temperatures of the shallow trap resemble the SSTs. The WA9 trap in the more central oligotrophic gyre shows warm biased TEXH86 temperatures due to energy stressed conditions. In the deep traps of both sites, theTEXH86-derived temperatures record subsurface temperature. It is assumed that these signals are caused by a higher relative contribution of colder signal from deep in situ production and a smaller contribution of warmer signal from shallow waters. The last part of the thesis investigates the alkenone-based temperatures. Most of Uk'37-derived temperatures display the SSTs in the tropical regions. It implies that the regional geochemical characteristics (e.g., availability of nitrogen or oxygen), which probably affect the TEX86 thermometry, do not have a profound impact on the Uk'37 thermometry. In the shallow trap of the Antarctic Polar Front, the Uk'37 record show a clear SST seasonality. The Uk'37 in the Fram Strait shows that the applicability of alkenone proxies is limited in low temperature regions that disfavor alkenone producers.