Chemistry of surface sediment along a north–south transect across the equator in the Central Indian Basin: an assessment of biogenic and detrital influences on elemental burial on the seafloor

Abstract

Nineteen surface sediment samples collected along a north–south transect across the equator in the Central Indian Basin (CIB) were analysed for their major, minor, and rare earth element (REE) composition. In absence of the element-flux data, we demonstrate here the association of various elements and their preferential sinks using inter-elemental associations in sediment exhibiting latitudinal dissimilarity. Biogenic sedimentation north of the equator (1° to 4°N latitudes) is dominated by carbonate ooze, whereas south of the equator (4° to 14°S latitudes) is dominated by siliceous ooze. Carbonate ooze deposition (>60% CaCO 3) above 4400 m and its drastic reduction (to <10%) below 4700-m water depths place the carbonate lysocline and the compensation depth (CCD) in a narrow depth zone of around 300-m thickness in the CIB. The siliceous ooze deposition is evident only below 5050-m water depth. This equatorial biogenic sediment domain is interrupted by a detrital clay zone (0° to 3°S latitudes) probably due to a combined effect of reduced dilution of distal Bengal fan sediment below CCD and focusing of suspended detritus drawn from the Bay of Bengal during vigorous southwest monsoon equatorial circulation. The latitudinal trends in the Element XS (detrital unsupported or non-terrigenous element) composition of the sediment and inter-elemental correlations yield four main carrier phases responsible for burial of several elements; (a) carbonate phase burying almost all Ca and Sr; (b) detrital phase burying the majority of Al, Fe, Mg and all of Ti and Sc; (c) oxide phase burying bulk of Mn,Co,Cu and Ba; and (d) biogenic phase burying the Al XS. Ba exhibits latitudinal similarity and strong positive association ( r=∼0.8 at 99.9% confidence level) with Mn, Co and Cu in both Element bulk and Element XS correlation matrices. The proportion of Ba XS, Mn XS, Co XS and Cu XS is >80% of their respective total content in all the three sediment zones. These observations suggest a close link between Ba and a Mn-oxide phase. On the other hand, nearly overlapping latitudinal trends of Al XS with carbonate in areas north of the equator and opal south of the equator; strong positive correlation of Al XS with opal ( r=+∼0.8), similar ratios between carbonate-free opal and Al XS (∼16) in both biogenic sediments suggest a close link between productivity and Al XS. The Al XS proportion in both biogenic sediment types is around 28% of the total Al suggesting nearly the same level of primary productivity along the transect (4°N to 14°S) which supports the estimates of Berger [Berger, W.H., 1989. Global maps of the ocean productivity. In: W.H. Berger, V.S. Smetacek, G. Wefer (Eds.), Productivity of the Ocean: Present and past. Dahlem workshop reports, Wiley, Chichester, pp. 429–455] and Kabanova [Kabanova, J.G., 1968. Primary production of the northern part of the Indian Ocean. Oceanology, 8: 214–255] (i.e., ∼70 g Cm −2 y −1). The REEs exhibit mutual coherence in the sediment ( r=>+0.7). However, they display, an increasing affinity towards Mn (` r' increases from +0.6 to +0.8) and decreasing affinity towards Ti (` r' decreases from +0.8 to +0.3) with increasing atomic number. Such a subtle but systematic variation within REE group suggests preferential burial of heavy-REE by Mn-oxide phase and light-REE by detrital phase.