Program
 
Special Session 3: Size matters or not, particles export in marine environments
 

 
 
1030
Stable carbon isotope ratios of archaeal GDGTs in the marine water column and surface sediments
Tuesday 10th @ 1030-1050
Room 4
Ann Pearson* , Harvard University
Sarah J. Hurley, Harvard University
Hilary G. Close, University of Miami
Presenter Email: pearson@eps.harvard.edu
Archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids are ubiquitous throughout the marine environment and are preserved in sediments and sedimentary rocks on million-year timescales.  Variations in the number of ring-containing GDGT isomers are formalized in the TEX86 sea surface temperature proxy.  Ammonia-oxidizing Thaumarchaeota are believed to be the major sources of these GDGTs, implying that the greatest production and export of GDGTs from the water column should be associated with the maximum expression of ammonia monooxygenase (amoA) genes and maximum number of thaumarchaeal cells, both of which occur in the subsurface NO2- maximum near a depth of ca. 80-250 m.

To examine the relationship between production and export of GDGTs, we measured the concentrations and d13C values of GDGTs in suspended particulate matter (SPM) of the western South Atlantic Ocean and compared them to values from pure thaumarchaeal cultures and from available sediment core-tops from other locations. The depth distribution of GDGTs was consistent with our current understanding of ammonia-oxidizing Thaumarchaeota: maximum GDGT concentrations occurred at the base of the primary NO2- maximum. Core GDGTs dominated the structural distribution in surface waters, while intact polar GDGTs – thought to potentially indicate live cells – were more abundant at all depths at and below the maximum NO2- concentration, and > 98% of all GDGTs were present in waters at and below the depth of the primary NO2- maximum.

Thaumarchaeota are believed to fix the majority of their carbon directly from dissolved inorganic carbon (DIC).  However, both the SPM and core-top d13C values in some cases are more 13C-depleted than would be predicted based on the 13C content of local DIC and the previously-published biosynthetic isotope fractionation. This indicates that the average metabolism of the planktonic archaeal community either is mixotrophic (≥ 25% organic carbon assimilation) or that the published e value for the model organism Nitrosopumilus maritimus may not be representative of the total autotrophic community.  In addition to this offset, d13C values of GDGTs in SPM inversely mirror DIC profiles, with lowest values in the nitrite maximum and higher values in the deeper water column, similar to the overall trends for bulk SPM and for bacterial fatty acids. Finally, while individual GDGTs in SPM samples have similar or identical d13C values, in many sediments measured to date, the isotopic composition of individual GDGTs varies.  This implies that multiple sources of GDGTs are necessary to explain core-top d13C distributions, especially on continental margins.