Pierre Debrabant

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. 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 (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.

Latitudinal control of astronomical forcing parameters on the high‐resolution clay Mineral distribution in the 45°–60° N range in the North Atlantic Ocean during the past 300,000 years

The clay mineralogy of four 5.5- to 13.5-m-long cores sampled between 45° and 60°N in the North Atlantic Ocean has been investigated at high latitudes within a well-constrained chronostratigraphic scale. Cross-correlation spectral analyses have been performed on both clay mineral and δ18O planktonic records. Detrital clay minerals display strong signals which are coherent with the δ18O record, within the three main Milankovitch frequency bands (eccentricity, obliquity, and precession). The climatic control on clay mineral sedimentation largely depends on the latitudinal location of the sediment cores. The 100,000-year signal occurs as a uniformly acting factor, whereas the 41,000-year signal dominates clay sedimentation at high latitudes and the 23,000-year signal dominates at midlatitudes. We suggest that the latitudinal variations of the orbital forcing on the detrital clay mineral distribution in the North Atlantic Ocean not only result from climatic control of the intensity of physical and chemical weathering, but also from latitudinal control on the detrital clay supply linked to influences of the high-latitude wind-driven and midlatitude ocean-driven transportation processes, respectively.

Neogene to Quaternary clay mineral fluxes in the Central Indian basin

Abstract The late Cenozoic clay sedimentation in the Central Indian Basin was investigated by mineralogical analyses of five very long cores (33–49 m long), which were recovered between 1° and 10°S along a transect at about 80°E and compared with data from adjacent ODP and piston cores. Progressive changes occur from North to South, and are especially marked by a decrease of illite and chlorite amounts balanced by increased contents of smectite and kaolinite. These changes are attributed to a southwards reduction of Himalaya-derived detrital fluxes, and to a correlative augmentation both of smectite reworked from a coastal Indian, Indonesian and volcanic submarine sources, and of kaolinite supplied from Australia and blocked by the equatorial divergence. The stratigraphic changes are more diversified than the geographic changes and develop discontinuously. High fluxes of illitic and chloritic materials during the middle-late Miocene reflect major activity of the Himalayan Main Central Thrust. A tectonic relaxation developed during the latest Miocene-early Pliocene, allowing alternating supply of illite-rich and smectite-rich sediments, which suggests periodical changes from distant to more local supply partly controlled by climate and by Indian intraplate deformations. The more abundant and constant amounts of smectite in late Pliocene-early Pleistocene sediments are chiefly attributed to erosion of Indian coastal zones favored by a worldwide sea-level drop. The middle to late Quaternary increase of the illite group resulted from the resumption of the tectonic activity in the Himalaya-Tibet range associated with a general climatic cooling.

Paleoenvironmental history of the North Atlantic region from mineralogical and geochemical data

Abstract Principles and applications of inorganic mineralogy and geochemistry in reconstructing past environments in marine basins are summarized by means of two examples from North Atlantic deep-sea drilling sites. The first approach is chronological and permits, by comparing the Hatteras and Cape Verde basins since the Oxfordian, to identify the effects of continental and marine influences, land climate and morphological evolution, alternation of tectonically active and stable periods, ocean deepening and widening stages and latitudinal migration of lithospheric plates. The second approach, more recently developed, concerns the geographic distribution of paleoenvironments during Albian time, particularly the role of the Bermuda area as a submarine barrier separating the western and eastern detrital input, and the contrast between the stable American margin that favored pedologic processes on land and the unstable African margin characterized by strong erosion, resedimentation and volcanic processes.

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.

Continental and marine influences expressed by deep-sea sedimentation off Japan (Kaiko project)

Abstract A total of about 100 sediments and sedimentary rocks, composed of various clays to muddy silstones of late Cenozoic age, and limestones and sandstones of early Cretaceous age, were sampled by the submarine Nautile during the Kaiko project at 3000 to 6000 m water depths in the trench areas South and East of Japan. A total of 70 samples were selected and studied mainly by optical and electronic microscopy, X-ray diffraction, atomic absorption spectrometry and microprobe analysis. The dominant terrigenous influences allow us to identify two main mineralogical provinces, to recognize the sedimentary expression of an early Pleistocene tectonic event southeast of Japan, and to propose an indirect stratigraphic use of clay assemblages in poorly fossiliferous deep-sea sediments. The Cretaceous sedimentary environments in Kashima Seamount area were submitted to various terrigenous and volcanogenic influences, with possible early to late diagenetic modifications. Only very minor post-sedimentary changes (oxidations, metallic coating, barite and zeolite formation, opal dissolution) affect most of the late Cenozoic deposits cropping out on the sea-floor, included those charaterized by the development of deep-sea benthic communities (e.g. clams). Important early diagenetic changes occur only in volcanic-rich sediments of South Zenisu Ridge and South Kuril Trench. In those peculiar deposits very abundant smectites develop with various shapes and chemical compositions, pointing to the local peculiarities of volcanic glass morphology, chemistry and alteration, of permeability and of fluid migration processes.