Nitrification rate is a fundamental control on archaeal lipid composition and the TEX86 proxy

Membrane lipids of marine Archaea – known as GDGTs – are the basis of the TEX₈₆ sea surface temperature (SST) paleoproxy. GDGTs are ubiquitous in marine sediments, and their broad distribution and high preservation potential have led to an ever-increasing use of TEX₈₆. The planktonic Thaumarchaeota that are believed to be the major sources of GDGTs to marine sediments are autotrophic nitrifiers, assimilating carbon directly from dissolved CO₂. Therefore the δ¹³C values of GDGTs additionally provide information about the DIC system and paleoproductivity. However, as for all biological proxies, understanding the physiology and biochemistry of the responsible organisms is essential to understanding how the proxies work. From this perspective, the TEX₈₆-SST proxy is uniquely perplexing: How is it possible that multiple approaches to calibration show a good correlation between TEX₈₆ and SSTs, when maximum activity of Thaumarchaeota is near and below the base of the photic zone? Here we show data from two studies that help address this question. Analyses of GDGT δ¹³C values show that the dominant GDGT flux to sediments is not from the sea surface. The data are measured on intact GDGTs purified by orthogonal dimensions of HPLC, followed by measurement of δ¹³C values on a Spooling Wire Microcombustion (SWiM)-IRMS with 1σ precision of ±0.2‰ and accuracy of ±0.3‰. Using this approach, we confirm that GDGTs, generally around ‑19.0‰ to ‑18.5‰, are isotopically “heavy” compared to other marine lipids, and that crenarchaeol in particular is a good tracer of water column GDGT export. In parallel, we investigated the mechanistic underpinning of the TEX₈₆ proxy using isothermal culture studies of the ammonia-oxidizing thaumarchaeonNitrosopumilus maritimus SCM1 to explore the relationship between TEX₈₆ and growth conditions. Evidence suggests that growth rate and electron donor supply are important controls on GDGT ratios and that TEX₈₆scales with the in-situ rate of nitrification. Constraining the physiological basis of the TEX₈₆ proxy and the mechanism by which this signal is transported to the sedimentary record is crucial to the proxy’s application to ancient environments.