National Institute Of Oceanography( NIO ) - Bottom water oxygenation history in southeastern Arabian Sea during the past 140 ka: Results from redox-sensitive elements

Bottom water oxygenation history in southeastern Arabian Sea during the past 140 ka: Results from redox-sensitive elements

Marine sediments consist of mixture of components derived from continent, weathering of seafloor rocks, biogeneous, hydrogenous, cosmogenic, and hydrothermal and aeolin source. The elemental composition of sediment reflects this admixture of components since all sediments have contribution more than one source. In the marine system, the depositional environment can be either oxic (marked by presence of O₂ and absence of H₂S), suboxic (dissolved oxygen level ~ 1-2 ml/l and absence of H₂S) or anoxic (absence of O₂ and presence of H₂S). Some of the redox-sensitive elements may be either concentrated or depleted relative to average continental crust under varying redox conditions at the time of deposition. Therefore, the distribution of redox-sensitive elements in marine sediments thus has the potential to provide information on redox conditions of the bottom water. Instead of using single redox-sensitive element to understand depositional conditions it is always better to use multiple redox-sensitive elements which respond differently to the redox state. Recently, we have measured concentration of number of redox-sensitive elements (Re, U, Mo, Cd, V, Sb and Tl) in a sediment core from southeastern Arabian Sea to understand the bottom water oxygenation history throughout the past 140 ka. The enrichment of redox-sensitive elements (Re, U, Cd and Sb) above crustal abundance suggests that Last Glacial Maxima (17.48 ka), stadials of Marine Isotope Stage (MIS) -5 (5b & 5d) and Glacial Termination (GT)-II (~ 133 ka) were all associated with suboxic bottom water conditions. Sediments deposited under this suboxic conditions show highest Re up to 54 ppb (parts per billion) on carbonate free basis and U up to 16 ppm (parts per million) which are highly enriched compared to their average crustal abundance of 0.4 ppb and 0.91 ppm respectively. This suggests that Re is enriched nearly 135 times compared to average crust and appears to be the major sink for global mass balance estimation. The high ratio of Re/Mo (ppb/ppm), Re/TOC (ppb/%) and U/Th (ppm/ppm) are also indicative of suboxic conditions (see Fig.). Marine Isotope Stages 1, 3, 4 and interstadials of MIS-5 (5a, 5c & 5e) were all associated with near-oxic conditions. The sediments of southeastern Arabian Sea never attained anoxic condition during the entire 140 ka evidenced by lack of enrichment of Mo and V above crustal abundance and high Re/Mo ratio (average 18.2 x 10^-3). The changes in bottom water oxygen content can be related to the oceanic circulation pattern during this time.

Ratios Re/Mo, Re/TOC and U/Th (carbonate free) in a sediment core from southeastern Arabian Sea. For more information read: Pattan, J.N.; Pearce, N.J.G. (2009). Bottom water oxygenation history in southeastern Arabian Sea during the past 140 ka: Results from redox-sensitive elements. Palaeogeogr. Palaeoclimatol. Palaeoecol.: 280(3-4); 2009; 396-405.

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