Stable carbon isotope ratios of archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids have been proposed as a proxy to infer past changes in the carbon isotope composition (δ13C) of dissolved inorganic carbon (DIC). The premise for reconstructing paleo-δ13CDIC from GDGTs is based on observations of relatively constant δ13CGDGT values in recent depositional environments. Marine Thaumarchaeota, thought to be the dominant source of GDGTs to marine sediments, fix inorganic carbon using the 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB) pathway, which is specific to HCO3- as the substrate. Bicarbonate-dependent autotrophy has been the basis for predicting that the stable carbon isotopic composition of GDGTs (δ13CGDGT) should vary in parallel with water column δ13CDIC values, because HCO3- is by far the dominant fraction of DIC in modern seawater. However, this relationship has never been systematically tested. Here we examine the carbon isotopic composition of GDGTs from four water column profiles in the Southwest and Equatorial Atlantic Ocean. Values of δ13CGDGT increase with depth in the water column, in contrast to the characteristic decrease in δ13CDIC values. These divergent trends imply a decrease in the observed total biosynthetic isotope effect (𝜀Ar) with depth, i.e., the offset between δ13CDIC and δ13CGDGT is not constant. Instead, we find that values of 𝜀Ar specifically correlate with oceanographic variables associated with extent of organic remineralization, decreasing as CO2 concentration increases. This observed relationship is consistent in both magnitude and direction with the results of an isotope flux-balance model for Thaumarchaeota that suggests 𝜀Ar should be sensitive to growth rate (µ) and CO2 availability under conditions of atmospheric pCO2 < 4 times the pre-anthropogenic Holocene level. Further tests of the sensitivity of 𝜀Ar to µ and CO2 in the modern marine environment will be essential to exploring the potential for a new, archaeal lipid-derived pCO2 paleobarometer.
