408 resultados para Adenosine A1


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Strontium/calcium (Sr/Ca) ratios in bulk and foraminiferal calcite have been used to constrain the history of Sr/Ca in the oceans and to evaluate calcite diagenetic alteration. However bulk Sr/Ca records also may be influenced by differences in Sr uptake and/or in the diagenetic susceptibility of different calcium carbonate sedimentary components. We present data on the sediment size fraction and calcium carbonate distribution in bulk samples, Sr/Ca in a range of sedimentary size components, and Sr/Ca in bulk sediments. Ocean Drilling Program samples from sites on Ontong Java Plateau and Ceara Rise (in the western equatorial Pacific and Atlantic, respectively) and from sites in the eastern equatorial Pacific were selected to represent progressive stages in the diagenetic pathway from the sea floor through a range of burial depths equivalent to sediment ages of ~5.6, ~9.4, and ~37.1 Ma. Samples were subdivided by size to produce a unique data set of size-specific Sr/Ca ratios. Fine fraction (<45 ?m) Sr/Ca ratios are higher than those of all corresponding coarse fractions, indicating that fine nannofossil-dominated calcite has a Sr partition coefficient 1.3-1.5 times greater than that of coarse foraminifera-dominated calcite. Thus, absolute values of bulk Sr/Ca in contemporaneous samples reflect, in part, the ratio of fine to coarse calcite sedimentary components. Sr/Ca values in fine and coarse components also behave differently in their response to pre-burial dissolution and to recrystallization at depth. Coarse size components are sensitive to bottom water carbonate ion undersaturation, and they lose original Sr/Ca differences among contemporary samples over not, vert, similar10 my. In contrast, fine components recrystallize faster in more deeply buried samples. Interpretation of the historical Sr/Ca record is complicated by post-depositional diagenetic artifacts, and thus our data do not provide clear evidence of specific temporal changes in oceanic Sr/Ca ratios over the past 10 million years. This paper represents the first systematic attempt to examine trends in calcite Sr/Ca as a function of sediment size fraction and age.

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The technical details of drilling and coring at the Kirchrode I and II sites are presented. At these sites, a sequence of claystones and marlstones from an Albian shelf basin was recovered. Constraints on the ages of the sediments in the two boreholes are provided by the occurrence of the inoceramid bivalve Actinoceramus sulcatus, the first appearance of which is used to define the Middle/Upper Albian boundary and by observed facies changes that can be correlated to the established lithostratigraphy. The cores from the two boreholes provide a rather complete, 285-m-long sequence of the Upper Albian, with a 155.5-m-long overlap. Analysis of the tectonic structures showed considerable shortening in the Middle and Lower Albian part of the sequence due to normal faulting. Of the Upper Albian, only the lowermost part is affected by faults. The increase in sedimentation rates of terrigenous detritus and of marine biogenic carbonate, which occurs in the basal part of the C. auritus Subzone, is interpreted to reflect a regional change to a more humid climate and regional tectonic movements (uplift of the Rhenish Bohemian massif, subsidence of the Lower Saxony basin intensified locally by halokinetic movements). The further increase in marine productivity in the latest Albian may be related to upwelling of more nutrient-rich deep water along submarine relief in this shelf sea. Identification of Milankovitch cyclicity documented by the fluctuating CaCO3 contents of the sediments is used (i) to constrain the minimum time represented by the Upper Albian deposits, and (ii) to determine the duration of the sea level cycles (Cycle V: >=1.6 Ma, Cycle VI: >=2 Ma), and (iii) to establish the duration of the Late Albian ammonite subzones (e.g. Callihoplites auritus Subzone: 2.1 Ma). Average sedimentation rates determined from the identified 100-ka eccentricity cycles show a stepwise increase in sedimentation rates from 1-2 cm/1000 a in the Lower Albian dark claystones to 7-13 cm/1000 a in the late Late Albian. In addition to the general deepening trend through the Late Albian, two, nearly completely documented 3rd-order sea-level cycles in the Upper Albian of Kirchrode I were recognised, plus another one, cut short by faulting, at the base of the Upper Albian (documented in Kirchrode II). These global sea-level cycles were identified on the basis (a) of the sequence of the abundance maxima of selected benthos and plankton groups, (b) of trends in the fluctuations of the CaCO3 content, and (c) of the abundance of glauconite. The transgression periods in this Upper Albian deep shelf-basin are characterised by intensified circulation. This intensified circulation is found to have affected first the surface-near waters, resulting e.g. in an increase in the abundance of immigrant plankton and nekton species from the Tethys. At a later stage the deep water was affected, supporting then an increased population of suspension-feeding benthos, and causing condensation and erosion in the sediment at the sea floor.

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In comparison to other sectors of the marine system, the palaeoceanography of the subarctic North Pacific Ocean is poorly constrained. New diatom isotope records of d13C, d18O, d30Si (d13Cdiatom, d18Odiatom, and d30Sidiatom) are presented alongside existing geochemical and isotope records to document changes in photic zone conditions, including nutrient supply and the efficiency of the soft-tissue biological pump, between Marine Isotope Stage (MIS) 4 and MIS 5e. Peaks in opal productivity in MIS 5b/c and MIS 5e are both associated with the breakdown of the regional halocline stratification and increased nutrient supply to the photic zone. Whereas the MIS 5e peak is associated with low rates of nutrient utilisation, the MIS 5b/c peak is associated with significantly higher rates of nutrient utilisation. Both peaks, together with other smaller increases in productivity in MIS 4 and 5a, culminate with a significant increase in freshwater input which strengthens/re-establishes the halocline and limits further upwelling of sub-surface waters to the photic zone. Whilst d30Sidiatom and previously published records of diatom d15N (d15Ndiatom) (Brunelle et al., 2007, 2010) show similar trends until the latter half of MIS 5a, the records become anti-correlated after this juncture and into MIS 4, suggesting a possible change in photic zone state such as may occur with a shift to iron or silicon limitation.