Molecular signatures for changes in populations of nitrogen-fixing cyanobacteria in response to fluctuations in oxygenation levels during the early Aptian oceanic anoxic event

Author(s): Brassell, Simon C.; Dumitrescu, Mirela
Author Affiliation(s): Primary:
Indiana University, Department of Geological Sciences, Bloomington, IN, United States
ExxonMobil Exploration Company, United States
Volume Title: Geological Society of America, 2007 annual meeting
Source: Abstracts with Programs - Geological Society of America, 39(6), p.15; Geological Society of America, 2007 annual meeting, Denver, CO, Oct. 28-31, 2007. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: 2-Methylhopanoid (2-MH) biomarkers provide a signature for cyanobacterial (cyano) contributions to organic matter (OM) in sedimentary rocks back to 2,700 Ma, although their occurrence is sporadic, unlike that of their hopanoid analogs. 2-MH are prevalent in sediment sequences deposited during oceanic anoxic events (OAEs) and are often associated with low d15N values indicative of nitrogen (N2) fixation by cyanos. Representative low d15N values occur throughout the early Aptian OAE (OAE1a; 120 Ma) at Shatsky Rise (ODP Site 1207) in the Pacific, while the proportions of 2-MH are higher during time intervals characterized by cooler sea surface temperatures and oxygenated waters, determined from the TEX86 proxy and biomarkers, respectively. In modern marine environments higher temperatures favor filamentous non-heterocystous cyanos as the dominant N2-fixing organisms, and exclude heterocystous species, whereas unicellular cyanos are favored by low pO2. Thus, the variations observed within OAE1a suggest that N2-fixing cyano populations changed during these episodes of carbon cycle perturbation. The coupled response of 2-MH, cyano speciation, and O2 levels during OAE bears on their relationships in earlier times, especially during evolutionary diversification of cyanos on the early Earth. The proposition that 2-MH originated in heterocystous cyanos that perform N2 fixation helps explain their occurrence in the late Archean during a global-scale expansion of oxygenated habitats, and their scarcity in anoxic Paleoproterozoic sequences. The presence of isorenieratane in OAE sequences that contain 2-MH suggests changes in populations of green sulfur bacteria and cyanos as environmental conditions alternately favored anoxygenic and oxygenic photosynthesis, respectively. Perhaps changes in phytoplankton communities during OAE may re-enact the dynamic interchange of these organisms that first accompanied the transition to an oxygenated world. This interpretation requires biosynthesis of 2-MH by N2-fixing heterocystous cyanos prior to the initial accumulation of atmospheric O2, and the fact that these heterocystous cyano groups are phylogenetically more evolved members of the clade also argues against the interpretation that specific groups of cyanos post-date the Great Oxygenation Event.
Year of Publication: 2007
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Anaerobic environment; Aptian; Archean; Bacteria; Biologic evolution; Carbon; Carbon cycle; Communities; Cretaceous; Cyanobacteria; Geochemical cycle; Habitat; Isotope ratios; Isotopes; Leg 198; Lower Aptian; Lower Cretaceous; Mesozoic; N-15/N-14; Neoarchean; Nitrogen; North Pacific; Northwest Pacific; ODP Site 1207; Ocean Drilling Program; Organic compounds; Oxygen; Pacific Ocean; Paleo-oceanography; Paleoatmosphere; Paleoenvironment; Paleoproterozoic; Paleotemperature; Photochemistry; Photosynthesis; Precambrian; Proterozoic; Sedimentary rocks; Shatsky Rise; Speciation; Stable isotopes; Upper Precambrian; Variations; West Pacific
Coordinates: N374700 N374800 E1624600 E1624500
Record ID: 2008105931
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