CO2-dependent carbon isotope fractionation in Archaea, Part II: The marine water column

TitleCO2-dependent carbon isotope fractionation in Archaea, Part II: The marine water column
Publication TypeJournal Article
Year of Publication2019
AuthorsHurley, SJ, Close, HG, Elling, FJ, Jasper, CE, Gospodinova, K, McNichol, AP, Pearson, A
Date PublishedSEP 15

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 (delta C-13) of dissolved inorganic carbon (DIC). The premise for paleo-delta C-13(DIC) reconstructions from GDGTs is based on observations of relatively constant delta C-13(GDGT) 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 (delta C-13(GDGT)) should vary in parallel with water column delta C-13(DIC) 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 delta C-13(GDGT) increase with depth in the water column, in contrast to the characteristic decrease in delta C-13(DIC) values. These divergent trends imply a decrease in the observed total biosynthetic isotope effect (epsilon(Ar)) with depth, i.e., the offset between delta(13)(DIC) and delta C-13(GDGT) is not constant. Instead, we find that values of epsilon(Ar) 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 epsilon(Ar) should be sensitive to growth rate (mu) and CO2 availability under conditions of atmospheric pCO(2) < 4 times the pre-anthropogenic Holocene level. Further tests of the sensitivity of epsilon(Ar) to u and CO2 in the modern marine environment will be essential to exploring the potential for a new, archaeal lipid-derived pCO(2) paleobarometer. (C) 2019 Elsevier Ltd. All rights reserved.