Major, trace, and rare earth elements in the sediments of the Central Indian Ocean Basin: Their source and distribution

A large number of surface sediments as well as short sediment cores collect ed in the Central Indian Ocean Basin have been subjected to various geochem ical investigations during the last one and a half decade. The studies vari es, covering different aspects of sediments and resulting in a number of pu blications. In the present article, we have put together the data from 82 s urface sediments and 14 short sediment cores, including 25 new analyses, to study the trend of their distribution and source at large. The distributio n maps of elements show that highest concentrations of Mn, Cu, Ni, Zn, Co, and biogenic opal in the surface sediment occurs between 10 degreesS and 16 degreesS latitude, where diagenetic ferromanganese nodules rich in Mn, Cu, Ni, and Zn are present. The studies highlight that the excess element conc entration (detrital unsupported) such as Mn, Cu, Ba, Ni, Co, Pb, and Zn hav e contributed > 80% of their respective bulk composition. These excess elem ents exhibit strong positive correlation with each other suggesting their a ssociation with a single authigenic phase such as Mn oxide. Biogenic opal c ontributes 30-50% of the total silica in the siliceous sediment. Aluminium, Fe, and K have contributed > 60% from terrigenous detrital source compared to their bulk composition. In calcareous ooze, Ca, and Sr excesses contrib ute > 95% while, in siliceous ooze it is only 50% of their bulk composition . Nearly 35% of structurally unsupported Al in the sediment raises doubt of using Al as a terrigenous index element to normalize the trace and minor e lements. Biogenic apatite is evident by the positive correlation between Ca (<1%) and P, Calcium, Sr, and P depict a common source such as biogenic. B ulk element concentration such as Li, V, Cr, Sc and Zr are positively corre lated with Ti indicating their terrigenous detrital source. Rare earth elem ent (REE) concentration increases from calcareous ooze to siliceous ooze an d reaches a maximum in the red clay. Presence of positive Eu-amonaly in the se sediments has been attributed to aeolian input. REE in these sediments a re mostly carried by authigenic phases such as manganese dioxide and biogen ic apatite. Based on the distribution of transition elements in the sedimen t cores, three distinct zones - oxic at top, suboxic at intermediate depth, and a subsurface maxima - have been identified. Oxic and suboxic zones are incidentally associated with high and low micronodule abundance in the coa rse fraction (> 63 mum) respectively. Ash layers encountered at intermediat e depth between 10 to 35 cm are correlative with the Youngest Toba eruption of similar to 74 ka from Northern Sumatra. This ash is mainly responsible for the high bulk Al/Ti ratio up to 48.5 (three times higher than Post Arch ean Australian Shale) other than scavenging of dissolved Al by biogenic com ponents.