Luc André

CLAY SUPPLIES IN THE CENTRAL INDIAN BASIN SINCE THE LATE MIOCENE - CLIMATIC OR TECTONIC CONTROL

Abstract Mineralogical (X-ray diffraction, differential thermal analysis), geochemical [microprobe, inductively coupled plasma (ICP)-atomic emission spectrometry, ICP-mass spectrometry] and Sr-Nd isotopic analyses have been carried out on the clay size fraction of Late Miocene to Pleistocene sediments from the Central Indian Basin. The samples were taken from five giant cores recovered between 1° and 10°S on a transect along 80°E. The clay assemblages are homogeneous and characterized by an alternation of illite- and smectite-rich levels. Most of the clays are detrital and were derived from a unique source: the weathering of the Indo-Gangetic Plain supplied most of the eroded material. Temporal clay mineralogical fluctuations in the depositional basin reflect environmental changes in the provenance. On the basis of spectral analyses of a mineralogical parameter (peak height ratios), the fluctuating smectite-illite clay sedimentation is controlled by periodic Late Miocene climatic changes. During the Late Pliocene, an irregular, probably tectonic, control appeared.

Multiple seawater-derived geochemical signatures in Indian oceanic pelagic clays

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. Srue5f8Nd 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 (LaN/YbN = 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.

Advective excess Ba transport as shown from sediment and trap geochemical signatures

Abstract We report the results of a geochemical study of sediment and trap material. Major and trace elements (Zr, Ba, rare earth elements, and Th) were analyzed on bulk sedimentary material collected along the NE Atlantic margin. Our aim is to test the widespread use of Ba-barite as a proxy for paleoproductivity in a continental margin area. This environment is of great interest because atmospheric–oceanic exchanges are important. In sediments, the geochemical signatures remain close to an upper crust reference, with flat shale-normalized rare earth elements patterns and constant elementary ratios. The calculated biogenic fraction of Ba or excess Ba (20–45%) remains lower than the excess Ba record in trap material (80–99%). The evolution of the geochemical signature along the margin reflects variable dilution of a detrital Post Archean Australian Shale-like component by a biogenic carbonaceous seawater-derived component. The trap material displays a wide range of variation in its trace element content (e.g., Ba ∼150–3000 ppm, Zr ∼2–100 ppm), except for the abyssal site, which is characterized by constant signature. In the two other sites, all of the trace element contents increase with water depth and present pronounced seasonal changes at each sampled water depth. The amount of excess Ba also increases in the deepest traps, and its evolution throughout the year mimics the change of the other analyzed trace elements. In contrast, its relationships with particulate organic carbon are not obvious. In term of fluxes, two periods of enhanced excess Ba fluxes are observed: (1) excess Ba flux increases with the detrital-like elements like Th especially during winter, and (2) excess Ba flux is enhanced without any change for the other trace elements during spring. To explain the first case, a supply through lateral advection is proposed. Such transient input of significant excess Ba flux will have a great impact on the yearly averaged estimation of the export production. Indeed, only the second case reflects a bloom in the biological productivity of the water column and must be taken into account in a mean calculation of the export production. Finally, a normalization of the excess Ba by detrital-like element like Th will help to discriminate between a real increase of the excess Ba due to local productivity change (high excess Ba and high excess Ba/Th ratio ≥ 10,000) and any input due to advection process (high excess Ba but low excess Ba/Th ratio

Clay sedimentation in the Japan Sea since the Early Miocene : influence of source-rock and hydrothermal activity

Abstract X-ray diffraction analyses have been carried out on 128 samples of Miocene to Quaternary sediments from ODP Sites 794, 795 and 797. Some clay fractions of samples from Site 797 have also been studied for rare earth elements and by Nd isotopic analyses. These three sites display similar lithological and clay assemblages (with dominant chlorite, illite and smectite) showing that the sedimentation was homogeneous throughout the whole Japan Sea Basin. Three mineralogical zones are recognized. The first zone (Lower Miocene sandy clay of Sites 794 and 797) is mainly composed of chlorite resulting from hydrothermal transformation of arc-derived smectite, due to sill injections during the initial oceanic spreading stage. The second zone (Lower Miocene to Lower Pliocene siliceous claystone and diatomaceous silty clay) is dominated by arc-derived smectite; the abundance of this mineral decreases upwards while illite and chlorite increase. This trend reflects a change of detrital source, from an eastern arc-derived source (ϵNdt > −3.3; variable LREE enrichment) to a western continental crust source ( ϵ Nd t ; shale-like REE patterns); climatic modifications in the current dynamics are proposed as a cause for this change. The third zone (Upper Pliocene to Recent silty clay with minor diatom oozes) is characterized at Site 797 by increasing amounts of illite and chlorite. This reflects a more and more important western supply which is assumed to be related to tectonic rejuvenations of the Asian margin or climatic modifications affecting the alteration conditions or the current dynamics. At Sites 794 and 795, the more or less sharp supply of chlorite seems to be driven by the incipient subduction zone on the eastern margin of the Japan Sea.