Rapid growth rates of syndepositional marine aragonite cements in steep marginal slope deposits, Bahamas and Belize

Growth rates of marine botryoidal aragonite cements from steep (35-45[degree]) marginal slope deposits in the Bahamas and Belize have been determined by accelerator mass spectrometer radiocarbon dating of samples taken at the base and top of individual botryoids. The pore-filling cements, which range from approximately 11,000-13,000 years old, grew at average rates of 8-10mm/100 yr with maximum rates > 25mm/100 yr. Radiocarbon dating of coexisting skeletal components indicates that cementation was syndepositional. Microsampling transects across individual botryoids for stable-isotope analyses show little variation in [delta][sup 31]C and [delta][sup 18]O, supporting the conclusion that cementation was extremely rapid. Although the cements show a progressive depletion in isotopic composition of approximately 1[per thousand]([delta][sup 13]C) and 2[per thousand]([delta][sup 18]O) from 13 ka to 11 ka, the average variation ([delta][sub 1]) within individual pore-filling cements, ranging in size 2 mm to 32 mm (bottom to top), was 0.11[per thousand]([delta][sup 13]C) and 0.14[per thousand]([delta][sup 18]O). Results of this study provide the first quantitative data on growth rates of marine carbonate cements in a marginal slope environment. The data indicate that marginal slope deposits may lithify within several tens of years and suggest that geologically instantaneous cementation may be critical in stabilizing steep carbonate slope deposits at or above angles of repose

Sea-level related resedimentation processes on the northern slope of Little Bahama Bank (Middle Pleistocene to Holocene)

Middle Pleistocene to Holocene sediment variations observed in a 26 metre long core taken during a cruise of the RV Marion Dufresne are presented. Core MD992202 was retrieved from the northern slope of Little Bahama Bank and provides an excellent example for sedimentation processes in a mid-slope depositional environment. The sediment composition indicates sea-level related deposition processes for the past 375 000 years (marine isotope stages 1 to 11). The sediments consist of: (i) periplatform ooze (fine-grained particles of shallow-water and pelagic origin) with moderate variations in carbonate content, carbonate mineralogy and grain-size; and (ii) coarser intervals with cemented debris consisting of massive, poorly sorted, mud-supported or clast-supported deposits with an increased high-magnesium calcite content. During interglacial stages (marine isotope stages 1, 5, 7, 9 and 11) periplatform oozes (i) are characterized by higher aragonite contents, finer grain-size and higher organic contents, whereas during glacial stages (marine isotope stages 2 to 4, 6, 8 and 10), increased low-magnesium and high-magnesium calcite values, coarser grain-size and lower organic contents are recorded. These glacial to interglacial differences in mineralogy, grain-size distribution and organic content clearly show the impact of climatically controlled sea-level fluctuations on the sedimentation patterns of the northern slope of Little Bahama Bank. The coarser deposits (ii) occur mainly at the transitions from glacial to interglacial and interglacial to glacial stages, and are interpreted as redeposition events, indicating a direct link between sediment properties (changes in mineralogy, grain-size distribution, variations in organic contents) and sea-level fluctuations. Changes in hydrostatic pressure and the wave base position during sea-level changes are proposed to have triggered these large-scale sediment redepositions

Controls on microbial activity and tidal flat evolution in Shark Bay, Western Australia

Microbial deposits at Shark Bay constitute a diverse living microbial carbonate system, developed in a semi-arid, highly evaporative marine setting. Three tidal flats located in different embayments within the World Heritage area were investigated in order to compare microbial deposits and their Holocene evolution. The stressing conditions in the intertidal–subtidal environment have produced a microbial ecosystem that is trapping, binding and biologically inducing CaCO3 precipitation, producing laminated stromatolites (tufted, smooth and colloform), non-laminated thrombolitic forms (pustular) and cryptomicrobial non-laminated forms (microbial pavement). A general shallowing-upwards sedimentary cycle was recognized and correlated with Holocene sea-level variations, where microbial deposits constitute the younger (2360 years BP) and shallower sedimentary veneer. In addition, sediments have been documented with evidence of exposure during the Holocene, from 1040 to 940 14C years BP, when sea-level was apparently lower than present.Filamentous bacteria constitute the dominant group in the blister, tufted and smooth mat types, and coccus bacteria dominate the pustular, colloform and microbial pavement deposit types. In the subtidal environment within colloform and pavement structures, microbial communities coexist with organisms such as bivalves, serpulids, diatoms, green algae (Acetabularia), crustaceans, foraminifera and micro-gastropods, which are responsible for exoskeleton supply and extensive bioturbation. The internal fabric of the microbial deposits is laminated, sub-laminar, scalloped, irregular or clotted, depending on the amount of fine-grained carbonate and the natural ability of microbial communities to trap and bind particles or induce carbonate precipitation. Nilemah tidal flat contains the thickest (1.3 m) and best-developed microbial sedimentary system; its deposition pre-dated the Rocky Point and Garden Point tidal flats, with the most positive isotope values for δ13C and δ18O, reflecting strong microbial activity in a highly evaporative environment. There is an evolutionary series preserved within the tidal flats reflecting relative ages and degree of salinity elevation

Occupational asthma and extrinsic alveolitis due to isocyanates: current status and perspectives.

Isocyanates are used for the large scale production of polyurethane polymers, which have an almost endless variety of applications in the manufacture of flexible and rigid foams, elastomers, adhesives, and surface coatings. Acute or chronic exposure to high concentrations of isocyanates can result in respiratory health hazards through a direct irritant effect. Isocyanates are of special interest, however, because, in some exposed workers, they can cause occupational asthma or extrinsic alveolitis through an apparently sensitising mechanism.Because of their wide industrial use, isocyanates are the principal cause of occupational asthma which is now the most common respiratory disease linked to the working environment. This review focuses on recently studied aspects of occupational asthma and extrinsic alveolitis related to exposure to isocyanates