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Deep-sea Benthic Foraminifera and TOC from Gas Hydrate Sequence of Ocean Drilling Program Holes 991A and 995A, Blake Ridge, NW Atlantic

K. Mohan and Anil K. Gupta
Department of Geology and Geophysics, Indian Institute of Technology
[email protected]; [email protected]

Deep-sea benthic foraminifera are found in different marine settings including gas seepages, gas hydrates, having great potential to reconstruct past climate and oceanic changes owing to their wide distribution, strong sensitivity to different oceanographic variables, high morphological diversity, and well documented fossil record. Organic carbon flux, amount of organic carbon in the sediments and oxygen levels of bottom water as well as pore water oxygenation have been suggested as controlling factors of benthic foraminiferal populations. Thus benthic foraminifera can be used as proxy to identify methane- (highly redox) and bacteria-rich environments. Wefer et al. (1994) examined stable isotope values in benthic foraminifera and arrived at the conclusion that in methane rich settings benthic foraminiferal d13C are typically depleted. Some recent studies suggest that certain species of benthic foraminifera prefer to feed on rich bacterial mats in methane rich environments (Wefer et al., 1994; Sen Gupta et al., 1997; Rathburn et al., 2000; Bernhard et al., 2001).

We have generated a 6.4–Ma record of benthic foraminifera and total organic carbon (TOC) from ODP Hole 991A (32o59.018’N, 75o55.801’W; water depth 2567.5m) and 6.5 Ma record from Hole 995A (31o48.210’N, 75o81.343’W; water depth 2778.5m), located on the Blake Ridge, NW Atlantic Ocean. Cassidulina carinata, Cibicides kullenbergi, C. wuellerstorfi, Globocassidulina obtusa, Pullenia bulloides, Melonis barleeanum, Oridorsalis umbonatus, Quadrimorphina laevigata, Stilostomella lepidula, Uvigerina peregrine and U. proboscidea are the dominant species of benthic foraminifera at both the holes.

The TOC values show quite fluctuating trends at both the holes ranging from 0.6 to 5.6 (Wt %) at Hole 991A and from 0.06 to 3.0 (Wt%) at Hole 995A. The TOC values show a secular trend with low values from 6.5-1.7 Ma and a major jump at 1.7 Ma with abrupt changes during the whole Quaternary interval at Hole 991A. During this time (last 1.7 Ma) the inorganic carbon (IC) also shows a major increase at Hole 991A. At Hole 995A, the TOC values show a different trend. The values are higher in the Pliocene with smaller changes and show continuous decline in the younger interval with abrupt changes during the past 1.7 Ma. Strangely the IC values at Hole 995A are almost similar as those at Hole 991A during the Quaternary. The changes at Holes 991A and 995A appear to have been driven by orbital switches. The study has a great potential in gas hydrate exploration in the marine sediments during the Neogene.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery