Analysis of Semi-Steady State Productivity of a Hyadraulically Benthic Foraminiferal and Geochemical Evidence for the Formation and Destabilization of Gas Hydrate at Blake Ridge Plateau, NW Atlantic

Ajoy K. Bhaumik¹ and Anil K. Gupta^2
1Department of Applied Geology, Indian School of Mines University, Dhanbad, Jharkhand, India
²Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal, India

Blake Ridge is the earliest recognized marine gas hydrate province having huge amount of disseminated gas hydrates above the BSR. So far, numerous studies have been performed to quantify the gas hydrate reserves as well as their formation and destabilization history. Some of the previous studies suggest that hydrates in that region are biogenic in origin where as others linked destabilization of gas hydrates to the bottom water warming as well as by the creation of gas migration pathways by the formation of sediment waves and related erosion. Benthic foraminiferal and geochemical analyses were performed on samples from two Ocean Drilling Program Holes 994C and 997A, Leg 164, at Blake Outer Ridge to understand the genesis and destabilization history of gas hydrates in this region. On the basis of the previous studies and observations from the present study, benthic foraminifera were subdivided into two groups as seep-related taxa and high-organic carbon taxa. High-organic carbon taxa dominate over the free methane and hydrate rich zone (7 – 2.5 Ma), which is also rich in organic carbon. Enriched d¹³C and high organic carbon values indicate that gas hydrate in this zone is biogenic in origin. High porosity, high opal concentration, low temperature and high pressure provide the favorable conditions for the formation of gas hydrates. On the other hand the seep-related taxa dominate over the methane free zone (2.95 Ma – Recent), which is quite exiting. Species of the seep-related taxa are already reported from the different seep environments from different ocean basins, hence it indicates the methane seepages in this zone during the geological past. Low organic carbon, relatively depleted d¹³C values indicate that methane originated by the dissociation of gas hydrates. Bottom water warming effect is not a cause of gas hydrate dissociation here due to the greater water depth (2799m). But the reduction of hydrostatic pressure owing to lowering of the sea level due to strengthening of the Northern Hemisphere Glaciation could be a driving force for the dissociation of gas hydrates.

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