Alain Trentesaux

Clay mineral assemblages in the northern South China Sea: implications for East Asian monsoon evolution over the past 2 million years

Abstract Clay mineral assemblages at ODP Site 1146 in the northern South China Sea are used to investigate sediment source and transport processes and to evaluate the evolution of the East Asian monsoon over the past 2 Myr. Clay minerals consist mainly of illite (22–43%) and smectite (12–48%), with associated chlorite (10–30%), kaolinite (2–18%), and random mixed-layer clays (5–22%). Hydrodynamic and mineralogical studies indicate that illite and chlorite sources include Taiwan and the Yangtze River, that smectite and mixed-layer clays originate predominantly from Luzon and Indonesia, and that kaolinite is primarily derived from the Pearl River. Mineral assemblages indicate strong glacial–interglacial cyclicity, with high illite, chlorite, and kaolinite content during glacials and high smectite and mixed-layer clay content during interglacials. During interglacials, summer enhanced monsoon (southwesterly) currents transport more smectite and mixed-layer clays to Site 1146 whereas during glacials, enhanced winter monsoon (northerly) currents transport more illite and chlorite from Taiwan and the Yangtze River. The ratio (smectite+mixed layers)/(illite+chlorite) was adopted as a proxy for East Asian monsoon variability. Higher ratios indicate strengthened summer-monsoon winds and weakened winter-monsoon winds during interglacials. In contrast, lower ratios indicate a strongly intensified winter monsoon and weakened summer monsoon during glacials. Spectral analysis indicates the mineral ratio was dominantly forced by monsoon variability prior to the development of large-scale glaciation at 1.2 Myr and by both monsoon variability and the effects of changing sea level in the interval 1.2 Myr to present.

Pleistocene forced regressions and tidal sand ridges in the East China Sea

Abstract Tidal sand ridges are common features on modern shelves but only few examples of such preserved sand bodies are described in Pleistocene deposits. In the stratigraphic record, some sand bodies encased in shales, previously interpreted as sand ridges, have been reinterpreted as shoreface deposits. More than 5000 km of high-resolution seismic data from the East China Sea, correlated to geotechnical boreholes and shallow cores, demonstrate the potential of sand ridge preservation and allow reconstruction of the depositional history of Pleistocene fourth order (100-kyr) depositional sequences. A high subsidence rate of about 300 m/Myr allows the preservation of three elementary sedimentary facies, constitutive of a ‘motif’ which was repeated during glacio-eustatic cycles. They consist of (1) regressive marine prodeltaic prograding wedges, (2) estuarine and continental (deltaic) facies, and (3) transgressive sand ridges, similar in shape and orientation to modern sand ridges. Major discontinuities, traceable over the entire outer continental shelf along distances of hundreds of kilometers, are transgressive and regressive surfaces of marine erosion, whereas sequence boundaries formed by fluvial erosion are difficult to identify on this low-gradient shelf. Because of the asymmetry of the Pleistocene glacio-eustatic cycles, most of the preserved sedimentary record (with the exception of sand ridges) corresponds to forced regressive deposits (deposits that formed during a seaward shift of the shoreline due to relative sea-level lowering).

Climatic and tectonic controls on weathering in south China and Indochina Peninsula: Clay mineralogical and geochemical investigations from the Pearl, Red, and Mekong drainage basins

[1] Results of clay mineralogy, major element geochemistry, and Sr and Nd isotopes in 93 argillaceous samples collected from drainage basins of the Pearl, Red, and Mekong rivers reveal different degrees of chemical weathering in Southeast Asia despite similar climate conditions across these regions. The kaolinite/illite ratio, illite chemistry index, and illite crystallinity can be used as indicators of chemical weathering intensity. These mineralogical proxies combined with the K2O/(Na2O + CaO) molar ratio, chemical index of alteration (CIA), and weathering trends observed from major element results indicate intensive silicate weathering in the Pearl River basin, moderate to intensive in the Mekong River basin, and moderate in the Red River basin. Although a significant modification of epsilon Nd(0) values in our riverine sediments during chemical weathering and transport is unlikely, Sr-87/Sr-86 ratios are controlled by various states of chemical weathering of high-Sr minerals such as plagioclase (rich in Na and Ca) with a linear decrease trend from the Pearl, Mekong, to Red river basins. Our results suggest that it is not the warm climate with heavy monsoon precipitation but tectonics playing the most significant role in controlling weathering and erosion processes in south China and Indochina Peninsula. Strong physical erosion caused by tectonic activities and river incision along the eastern margin of the Tibetan Plateau and along the Red River fault system is responsible for high contents of primary minerals in the lowlands of Red and Mekong river basins.

The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity.

The Paleoproterozoic Era witnessed crucial steps in the evolution of Earths surface environments following the first appreciable rise of free atmospheric oxygen concentrations ∼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.

Architecture and long term evolution of a tidal sandbank: The Middelkerke Bank (southern North Sea)

Abstract The internal structure of the Middelkerke Bank (one of the Flemish Banks located in the southern North Sea off the coast of Oostende, Belgium) has been studied in the framework of the Marine Science and Technology (MAST) program co-funded by the European Community. A dense grid of high and very high resolution seismic profiles has been used, as well as several vibrocorings. Seven major seismic units can be identified in the Quaternary sediments, bounded by major discontinuities correlated across the whole study area. The lower units clearly appear as being deposited during periods of relative low sea level (channel infillings, shoreface, estuarine and/or ebb-tidal delta deposits). The present shape of the bank results partly from recent erosional processes, reworking the underlying deposits. Thus, the lower part of the bank as a morphological feature does not consist of “offshore tidal sands”. The master bedding of the upper part of the bank consists of inclined reflectors, dipping at an angle of about 5° in the same direction as the banks “steep” face. These reflectors, very similar to those described by Houbolt (1968), are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms.

Controls on organic accumulation in late Jurassic shales of northwestern Europe as inferred from trace-metal geochemistry

In the Kimmeridge Clay Formation of the Wessex-Weald Basin, five organic-matter-rich intervals (or ORIs), dated from Kimmeridgian-Tithonian times, can be correlated from distal depositional environments in Dorset and Yorkshire (UK) to the proximal environments in Boulonnais, northern France. The ORIs are superimposed on a meter-scale cyclic distribution of organic matter (OM), referred to as primary cyclicity, which is commonly interpreted to result from Milankovitch climate forcing. The present work addresses the distribution of redox-sensitive and/or sulfide-forming trace metals and selected major elements (Si, Al and Fe) in Kimmeridge Clay shales from the Cleveland Basin (Yorkshire) and the Boulonnais cliffs with two objectives: 1) to determine whether the ORIs formed in similar paleoenvironments, and 2) to identify the mechanism(s) of OM accumulation. High-resolution geochemical data from primary cycles in the Yorkshire boreholes (Marton and Ebberstone boreholes), were studied and the results are then applied with lower resolution sampling at the ORI scale in the Flixton borehole and Boulonnais cliff. Good correlations are found between total organic carbon (TOC) vs Cu/Al and Ni/Al, but relationships between TOC and Mo/Al, V/Al and U/Al are more complex. Cu and Ni enrichment is interpreted to have resulted from passive accumulation with OM in an oxygen-deficient basinal setting, which prevented the subsequent loss of Cu and Ni from the sediment. Mo and V were significantly enriched only in sediments where considerable amounts of OM (TOC>7 %) accumulated, the result of strongly reducing conditions and OM burial. At the scale of the Flixton ORIs, the samples with the highest Mo and V concentrations also show relative Fe enrichment, suggesting pyrite formation in the water column (combination of euxinic conditions and presumably low sedimentation rates). Samples from all ORIs were slightly enriched in Si relative to Al, interpreted as reflecting decreased sediment flux during transgressive and early-highstand systems tracts. The data show that in some ORIs, OM accumulation proceeded while productivity was not particularly high and sediments were not experiencing strong anoxia. In other ORIs, OM accumulation was accompanied by widespread anoxia and possibly euxinic conditions in distal settings. Though somewhat different from each other, the ORIs have all developed during episodes of reduced terrigenous supply (transgressive episodes). The common feature linking these contrasted episodes of enhanced OM storage (ORIs) must be the conjunction of productivity coupled with a decrease in the dilution effect by the land-derived supply, in a depositional environment prone to water stratification and, therefore, favorable to OM preservation and accumulation.

Surficial sedimentology of the Middelkerke Bank (southern North Sea)

Detailed surficial investigations over the Middelkerke Bank, a tidal sand bank in the southern North Sea, revealed the relationship between morphology, surficial structures and grain-size parameters. Data from 85 grab samples all over the bank show that on a bank normal profile, the coarser, CaCO3 rich and badly sorted sediments are generally located near the highest point of the bank, seaward at the northern end and landward at the southern end. Sedimentary structures were studied from 239 boxcores sampled on all the morphological units of the bank: crest, flanks and adjacent channels. In the shallower parts, foreset beds are preserved while in the deeper zones, intense bioturbation arises and destroys any structure. The combination between these data and the virtual absence of wave-induced structures indicates that the main agents responsible for the bank shaping are the tidal currents.

Carbonate Grain-Size Distribution in Hemipelagic Sediments from a Laser Particle Sizer

Laser grain-sizer instruments provide the opportunity to study the grain size distribution of sediments across a wide size range in a short time. Automatic measurements can therefore be made, on a routine basis, for a great number of samples. Oceanic studies have proved the utility of these methods in characterizing both climatic changes and changes in sediment provenances. In addition, carbonate content is estimated either directly by CaCO 3 measurement, by visual observations, or by proxies such as sediment color reflectance. Nevertheless, the grain size distribution of the carbonate fraction is still a matter of speculation, and only optical observations can distinguish the nature of each carbonate fraction. Here we present the improvements on a method to study rapidly, with a high resolution, the grain size distribution of the carbonate fraction by use of a laser grain-sizer. We describe the basic methodology and apply it to an example from the Pleistocene of the Northern Atlantic Ocean.

Influence des tempêtes sur la mobilité des dunes tidales dans le détroit du Pas-de-Calais

The present paper deals with dune dynamics in a zone of the Strait of Dover located in the sea lane running into the North Sea. The dunes, widespread in this 35-m depth area, are mobile sedimentary structures (up to 40 m.yr(-1)) that culminate at a maximum of 22 m depth and endanger navigation as well as submarine man-made structures (cables, pipelines). Single- and multibeam bathymetric data, coupled with seismic data, allow us to follow dune displacements over different time scales. A net bedload parting zone has been displayed and divides the area into two parts, SE and NW. However, according to the considered time-scale, dune movements present Variations in intensity and direction. Over a long-term period (decades), sedimentary dynamics fit the regional scheme of the residual tidal currents that induce transport toward the SW and the NE in the respective SE and NW parts of the studied area. Over a medium-term period (several years), meteorological data show that a high frequency of storm winds alters the residual sand transport characteristics by strengthening, slowing or reversing the effect of the tidal currents and can lead to the reversal of dune asymetry.

Carbonate Grain-Size Distribution in Hemipelagic Sediments from a Laser Particle Sizer: RESEARCH METHODS PAPERS

ABSTRACT Laser grain-sizer instruments provide the opportunity to study the grain size distribution of sediments across a wide size range in a short time. Automatic measurements can therefore be made, on a routine basis, for a great number of samples. Oceanic studies have proved the utility of these methods in characterizing both climatic changes and changes in sediment provenances. In addition, carbonate content is estimated either directly by CaCO3 measurement, by visual observations, or by proxies such as sediment color reflectance. Nevertheless, the grain size distribution of the carbonate fraction is still a matter of speculation, and only optical observations can distinguish the nature of each carbonate fraction. Here we present the improvements on a method to study rapidly, with a high resolution, the grain size distribution of the carbonate fraction by use of a laser grain-sizer. We describe the basic methodology and apply it to an example from the Pleistocene of the Northern Atlantic Ocean.