Multiple seawater-derived geochemical signatures in Indian oceanic pelagic clays

Abstract

This paper reports a geochemical study of oceanic clays. Major and trace elements were analyzed on smectite-rich, clay size (<2 μm) samples, bulk sediments, and leachate residues from the Central Indian Basin. SrNd isotopes were also studied to investigate their geochemical evolution during transport in the water column, sedimentation, and diagenesis. The region is of special interest because the sedimentation records the interaction between the detrital supply from the Bengal Fan in the north and the biosiliceous input associated with the equatorial divergence in the south. The clay size fractions display extremely variable trace element contents, e.g., [Ba] = 100–5000 ppm, [Sr] = 20–200 ppm, Ce/Ce ∗ = 0.9–3.3, [Nd] = 10–50 ppm. Although in the argillaceous samples, clay size fractions have a similar trace element imprint to the bulk sediment, some major fractionations occur in the biosiliceous samples between the clay and the bulk sediment, especially for Sr and rare earth elements (REE). Three major components may account for the variable geochemical signatures of these pelagic clays. The first component (component A), already identified by Fagel et al. (1994), is characterized by a homogeneous geochemical signature (La N/Yb N = 1.03–1.05; Th/Ta = 12.8–21.1; Ba/Th ∼ 28) and a nonradiogenic Nd isotopic composition ( 143Nd/ 144Nd ∼ 0.511880): it traces a detrital Himalayan-derived origin. The two other components display a seawater-derived isotopic composition with global Sr ( 87Sr/ 86Sr ∼ 0.709060) and regional Indian Ocean Nd ( 143Nd/ 144Nd ∼ 0.512200) signatures. Both components are enriched in Sr and Ba (Sr ∼ 150 ppm, Ba/Th ∼ 500), and they are either enriched in light rare earth elements (LREE, e.g., Nd ∼ 50 ppm) in the argillaceous sediments (component B) or LREE-depleted (Nd < 20 ppm) in the biosiliceous sediments (component C). The frequent occurrence of micrometric (<5 μm) Sr-REE-Th enriched barite grains showing three major habits (rhombic, rounded, dendritic) suggests that these biologically-derived mineral phases had a major role in the genesis of components B and C. A strong clay-barite equilibration is deduced from the Post Archean Australian Shales PAAS-like REE patterns of these barites and the Ba enrichment of the clays. We suggest that it results from two successive mechanisms of exchange. First, at the top of the oxygen minimum zone, the microbial-induced decay of organic matter is proposed to trigger a series of trace element transfers between the various particulate-forming components (clays, barites, and decaying organic coatings). This is proposed as the origin of the clay component B: the barite-derived components (Ba, Sr) and the organic-derived positive Ce anomaly are imported to the clay particles while the PAAS signature of the clays is retained by the remaining barite crystals. Second, after settling, the barites are believed to partly dissolve and recrystallize, especially in the anoxic part of the sedimentary column. This diagenetic barite dissolution is proposed as the origin of the clay component C.