Comparison of TEX₈₆ and U<super>K</super>₃₇ temperature proxies in sinking particles in the Cariaco Basin

The Cariaco Basin, a silled, permanently anoxic basin on the continental shelf of Venezuela with a dynamic chemocline (-240-350 m), has been subject of > 20 years of oceanographic observation and sediment trap studies. We evaluated U-37(K') and the TEX86 temperature proxies using sinking particles collected in shallow sediment trap samples at 275 m (Trap A) and 455 m (Trap B) (within and below the chemocline). The organic geochemical temperature proxies, U-37(K'). (based on coccolithophorid alkenone lipids) and TEX86 (based on archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids), use observed relationships between the ratio of specific lipids and measured sea surface temperature to hindcast past sea surface temperatures. In this study, both U-37(K') and TEX86 temperature proxies record seasonal temperature variations, including the cooling associated with upwelling events. U-37(K')-based temperatures are colder than measured sea surface temperatures, and better correlated temperature at the chlorophyll maximum. In sediment trap material collected below the chemocline (Trap B),U-37(K') values are higher than those in Trap A. Warmer subchemocline U-37(K') based temperatures may be related to autooxidation of sinking particles, either by small amounts of available oxygen or by alternate electron acceptors concentrated in the biologically dynamic chemocline (e.g. intermediate sulfur compounds). The absolute flux weighted TEX86 temperature values measured in sinking particles from Trap A match the measured SST well. The differences in the TEX86 values between Traps A and B are small and reflect less impact of degradation. Overall, the TEX86 temperatures in sinking particles in the Cariaco Basin reflect annual SST

Comparison of TEX86 and UK'37 temperature proxies in sinking particles in the Cariaco Basin

The Cariaco Basin, a silled, permanently anoxic basin on the continental shelf of Venezuela with a dynamic chemocline (-240-350 m), has been subject of > 20 years of oceanographic observation and sediment trap studies. We evaluated UK'37 and the TEX86 temperature proxies using sinking particles collected in shallow sediment trap samples at 275 m (Trap A) and 455 m (Trap B) (within and below the chemocline). The organic geochemical temperature proxies, UK'37. (based on coccolithophorid alkenone lipids) and TEX86 (based on archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids), use observed relationships between the ratio of specific lipids and measured sea surface temperature to hindcast past sea surface temperatures. In this study, both UK'37 and TEX86 temperature proxies record seasonal temperature variations, including the cooling associated with upwelling events. UK'37-based temperatures are colder than measured sea surface temperatures, and better correlated temperature at the chlorophyll maximum. In sediment trap material collected below the chemocline (Trap B), UK'37 values are higher than those in Trap A. Warmer subchemocline UK'37 based temperatures may be related to autooxidation of sinking particles, either by small amounts of available oxygen or by alternate electron acceptors concentrated in the biologically dynamic chemocline (e.g. intermediate sulfur compounds). The absolute flux weighted TEX86 temperature values measured in sinking particles from Trap A match the measured SST well. The differences in the TEX86 values between Traps A and B are small and reflect less impact of degradation. Overall, the TEX86 temperatures in sinking particles in the Cariaco Basin reflect annual SST

Biomarkers, chemistry and microbiology show chemoautotrophy in a multilayer chemocline in the Cariaco Basin

The Cariaco Basin is the world's largest truly marine anoxic basin. We have conducted a comprehensive multidisciplinary investigation of the water column (42-750 m) bracketing the redox boundary (a 250-m thick "chemocline") of the Cariaco Basin to evaluate linkages between lipid biomarkers, distributions of major dissolved chemical species, and the microbial community and associated redox processes. Our multidimensional data set includes: hydrography, water column chemistry, microbial distributions and rates, and lipid biomarkers. Multivariant statistical analysis of this data set partitions the investigated water column into 5 distinct zones, each characterized by different chemistries, microbiologies and biomarker compositions. The core of this chemocline is a 25-m thick suboxic zone where both dissolved oxygen and sulfide were below detection limits, bacterial and archaeal cell numbers and the rate of chemoautotrophic (dark) carbon fixation are elevated, and dissolved chemical species and bacterial and archaeal lipid biomarkers are indicative of tightly coupled cycles of carbon, nitrogen, and sulfur through chemoautotrophy