1,720,968 research outputs found
Controls on sediment geochemistry in the Crozet region
No full textSediment composition to the north (M10), east (M5), and south (M6) of the Crozet Plateau in the Southern Ocean Polar Frontal Zone (PFZ) reflects the dual control on input from the locally elevated upper water-column export production and the proximity to the volcanic islands. The magnitude of the major sedimentary components (biogenic and lithogenic) has been constrained down core. The major lithogenic input to the three sites is identified as Crozet island basalts based on linear extrapolation of sedimentary major element data to known island basalt domains. High-resolution X-ray scanning of cores identified significant heterogeneity in the distribution of biogenic and lithogenic phases. All sites are characterised by a significant heavy mineral fraction that has been identified by SEM as biogenic barite and a mixed titanium–iron oxide of volcanic origin. There are multiple mass flow or turbidite events in the sediment records at each site that are inferred to be generally associated with sea-level rise during deglaciation. 230Thexcess-corrected Holocene mass accumulation rates have been estimated for the sedimentary constituents (calcium carbonate, biogenic silica, lithogenic material, organic carbon, authigenic U, and Baexcess). The core-top proxy records indicate that there is a significant enhancement of biogenic silica export both north and south of the Plateau and lower values east of the Plateau. Organic carbon export is highest to the north of the Plateau, lower within the High Nutrient Low Chlorophyll (HNLC) waters to the south of the Plateau and indistinguishable from the background PFZ values to the east of the island. The presence of the Crozet Plateau has a significant impact on organic carbon production and export throughout the Holocene, corroborating shipboard and remote observations of enhanced productivity in this region
Optimization of an inductively coupled plasma-optical emission spectrometry method for the rapid determination of high-precision Mg/Ca and Sr/Ca in foraminiferal calcite
No full textThe ability to measure Mg/Ca and Sr/Ca in foraminiferal calcite has increased in importance following their identification as palaeoceanographic tracers with significant potential to constrain past changes in ocean chemistry and temperature. However, in order to tackle palaeoceanographic problems, it is essential to develop methods for the determination of these ratios that have both rapid sample throughput and high analytical precision. Here, we demonstrate the importance of simultaneous measurement for high-precision Mg/Ca and Sr/Ca analysis. Our method is readily applicable to automated analysis over a wide range of Ca concentrations and is not subject to significant matrix effects. Using a “dual view? inductively coupled plasma–optical emission spectrometer (ICP-OES), supported by a dedicated autosampler, we have investigated three nebulizers and two spray chambers and determined optimum analytical parameters. We show that there are consistent relationships between signal-to-noise ratio and gas flow rates and that the working sample dilution range has an important effect on linear response. Further, for this instrument it is important to manually constrain the timing of measurements to obtain truly simultaneous readings. Using the method developed, precisions of better than 0.21% are obtained for Mg/Ca and Sr/Ca in solutions containing between 1 and 4 ppm Ca with no significant matrix effect, using a Glass Expansion “Conikal? nebulizer coupled with a “Tracey? spray chamber, in autosampler mode
The effect of plume processes on the Fe-isotope composition of hydrothermally derived Fe in the deep ocean as inferred from the Rainbow vent site, Mid-Atlantic Ridge, 36,14'N
No full textThe Rainbow hydrothermal vent site, which is the largest known point source for dissolved Fe delivered to the deep North Atlantic ocean, has remained invariant in its Fe isotope composition over at least the past 16,000 years, based on analysis of metalliferous sediments beneath the plume. Because of the conservative behavior of Fe in the Rainbow plume, 56Fe values of particles in the neutrally buoyant plume (?0.18±0.05‰) and underlying sediments (?0.19±0.05‰) are indistinguishable from the 56Fe values of the high-temperature fluid sources (?0.23±0.04‰). Particles from the near-vent, buoyant stage of the plume, however, have higher 56Fe values (+0.15‰ to +1.20‰) relative to the original vent fluid, consistent with fractionation during oxidation of Fe(II)aq to Fe(III)aq. Isotope compositions become invariant in the plume once all Fe(II)aq is fully oxidized, preserving the original composition of the vent fluid. The constant Fe isotope compositions of the vent fluids over time implies that changes in seawater Fe isotope composition of the North Atlantic ocean, as they are recorded in Fe–Mn crusts, requires changes in the relative fluxes of Fe to the ocean
Hydrothermal plume processes in the Indian Ocean (abstract of paper to be presented at: AGU Ocean Sciences Meeting, 26-30 January 2004, Portland, Oregon)
No full textHolocene sediment deposition on a NE Atlantic transect including Feni Drift quantified by radiocarbon and 230Thexcess methods
No full textRadiocarbon and 230Thexcess measurements were undertaken on the sediments from fifteen box cores recovered between 48–58°N and 12–22°W at water depths of 1100–4500 m in the northeast Atlantic. Eight of the cores were from Feni Drift in the Rockall Trough at depths of 1700–2500 m and the remaining seven formed an approximate north/south transect at various water depths from the abyssal plain north on to Rockall Plateau. Mean Holocene sediment accumulation fluxes for all cores were established from profiles of bulk sediment radiocarbon age against depth to be in the range 3–7 cm ky? 1 off the Drift and 4–23 cm ky? 1 on the Drift. When compared with these radiocarbon-based sediment accumulation rates, precision measurements of 230Th reveal that the 230Thexcess levels present in the sediments match or slightly exceed the potential supply from the vertically overlying water column in a majority of the cores. One core from the open abyssal plain that had the lowest radiocarbon-based accumulation rate in the entire set also had a close balance between predicted and measured 230Thexcess values. With the assumption that only 230Thexcess produced in the overlying water column is supplied to the sediments, the 230Thexcess values in this core and the other six containing carbonate ooze sediments all imply a rather constant regionally averaged sedimentation flux of 2.0 ± 0.2 (1?, n = 7, min. 1.8, max. 2.3) g cm? 2 ky? 1 rather than matching the 230Thexcess fluxes implied by the radiocarbon data. This singular value is similar to the mean Holocene flux reported for the northeast Atlantic by previous work that utilized the 230Thexcess method, although it is somewhat lower than estimates for the Holocene based on oxygen isotope stratigraphy or radiocarbon methods. The current transport processes that result in the high sediment accumulation rates on Feni Drift have also resulted in a compositional fractionation of the sediments. All sediments from Feni Drift have lower CaCO3 contents than the remainder of the sediments studied, and these are accompanied by lower 230Thexcess specific activities than are found at comparable water column depths elsewhere on the transect. Consistently higher and more variable regional sediment accumulation fluxes are therefore calculated from the Feni Drift 230Thexcess data with the constant flux assumption (average 2.8 ± 0.4 g cm? 2 ky? 1; n = 8, min. 2.3, max. 3.3), some 40% higher than the constant value measured in the other cores. It seems likely that the mean sediment accumulation flux at Feni Drift inside Rockall Trough is consistently higher than on the open ocean margin of the basin, so that the regionally averaged sedimentation fluxes indicated for the Drift by the measured 230Thexcess data are also consistently higher than the singular value measured elsewhere in the northeast Atlantic
Chemistry of newly-discovered hydrothermal vents in the East Scotia Sea (abstract of paper presented at Goldschmidt 2010 - Earth, Energy, and the Environment, 13-18 June 2010, Knoxville, TN)
No full textPaleogene record of elemental concentrations in sediments from the Arctic Ocean obtained by XRF analyses
No full textWe present a high-resolution X-ray fluorescence (XRF) core scanner record for the expanded middle Eocene section from Integrated Ocean Drilling Program (IODP) Expedition 302 (ACEX) drilled on the Lomonosov Ridge, central Arctic Ocean. The division of the middle Eocene into two units (subunit 1/6 and unit 2) is seen in the cyclical behavior of the elements as well as the changing interelemental correlations and their relationship to physical property measurements of bulk sediment. Al, Ti, and K strongly correlate throughout the record, while the behavior of Fe, Mn, and Si is more complex. Utilizing sediment geochemistry calibration to ground truth the XRF data, we suggest the middle Eocene Arctic Ocean was predominately euxinic, although periodic oxygenation of bottom waters must have occurred during unit 2 (49.7–45.4 Ma). Initially, the sediments are rich in biogenic silica (unit 2), but there is a pronounced shift to terrigenous dominated sediment accumulation in subunit 1/6. We report changes in the elemental concentrations of these elements and investigate the relationship between Fe content and pyrite. Additionally, we explore the potential change in paleoenvironmental conditions across the unit boundary
Hydrothermal sediments as a potential record of seawater Nd isotope compositions: The Rainbow vent site (36°14?N, Mid-Atlantic Ridge)
Full text linkGeochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater Nd values. Nd isotope data from these cores span the period of 4–14 ka (14C ages) and yield Nd values for North East Atlantic Deep Water (NEADW) that are higher (?9.3 to ?11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (?12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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