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Sulfur Cycling Mediates Calcium Carbonate Geochemistry in Modern Marine StromatolitesModem marine stromatolites forming in Highborne Cay, Exumas (Bahamas), contain microbial mats dominated by Schizothrix. Although saturating concentrations of Ca2+ and CO32- exist, microbes mediate CaCO3 precipitation. Cyanobacterial photosynthesis in these stromatolites aids calcium carbonate precipitation by removal of HS+ through CO2 use. Photorespiration and exopolymer production predominantly by oxygenic phototrophs fuel heterotrophic activity: aerobic respiration (approximately 60 umol/sq cm.h) and sulfate reduction (SR; 1.2 umol SO42-/sq cm.h) are the dominant C- consuming processes. Aerobic microbial respiration and the combination of SR and H2S oxidation both facilitate CaCO3 dissolution through H+ production. Aerobic respiration consumes much more C on an hourly basis, but duel fluctuating O2 and H2 depth profiles indicate that overall, SR consumes only slightly less (0.2-0.5) of the primary production. Moreover, due to low O2 concentrations when SR rates are peaking, reoxidation of the H2S formed is incomplete: both thiosulfate and polythionates are formed. The process of complete H2S oxidation yields H+. However, due to a low O2 concentration late in the day and relatively high O2 concentrations early in the following morning, a two-stage oxidation takes place: first, polythionates are formed from H2S, creating alkalinity which coincides with CaCO3 precipitation; secondly, oxidation of polythionates to sulfate yields acidity, resulting in dissolution, etc. Vertical profiles confirmed that the pH peaked late in the afternoon (greater than 8.8) and had the lowest values (less than 7.4) early in the morning. Thus, the effect of this S-cycling through alkalinity production, followed by acidification during H2S oxidation, results in a six times stronger fluctuation in acidity than photosynthesis plus aerobic respiration accomplish. This implies that anaerobic processes play a pivotal role in stromatolite formation.
Document ID
20040053476
Acquisition Source
Ames Research Center
Document Type
Other
Authors
Visscher, P. T.
(Connecticut Univ. Groton, CT, United States)
Hoeft, S. E.
(Connecticut Univ. Groton, CT, United States)
Bebout, B. M.
(NASA Ames Research Center Moffett Field, CA, United States)
Reid, R. P.
(Miami Univ. FL, United States)
Date Acquired
August 21, 2013
Publication Date
January 1, 2004
Subject Category
Oceanography
Funding Number(s)
PROJECT: RTOP 839-88-20
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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