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

The 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

Controls on sediment geochemistry in the Crozet region

Sediment 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

Holocene sediment deposition on a NE Atlantic transect including Feni Drift quantified by radiocarbon and 230Thexcess methods

Radiocarbon 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

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

The 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

Towards the development of a fossil bone geochemical standard: an inter-laboratory study

Ten international laboratories participated in an inter-laboratory comparison of a fossil bone composite with the objective of producing a matrix and structure-matched reference material for studies of the bio-mineralization of ancient fossil bone. We report the major and trace element compositions of the fossil bone composite, using in-situ method as well as various wet chemical digestion techniques.For major element concentrations, the intra-laboratory analytical precision (%RSDr) ranges from 7 to 18%, with higher percentages for Ti and K. The %RSDr are smaller than the inter-laboratory analytical precision (%RSDR; &lt;15–30%). Trace element concentrations vary by 5 orders of magnitude (0.1 mg kg?1 for Th to 10,000 mg kg?1 for Ba). The intra-laboratory analytical precision %RSDr varies between 8 and 45%. The reproducibility values (%RSDR) range from 13 to &lt;50%, although extreme value &gt;100% was found for the high field strength elements (Hf, Th, Zr, Nb). The rare earth element (REE) concentrations, which vary over 3 orders of magnitude, have %RSDr and %RSDR values at 8–15% and 20–32%, respectively. However, the REE patterns (which are very important for paleo-environmental, taphonomic and paleo-oceanographic analyses) are much more consistent.These data suggest that the complex and unpredictable nature of the mineralogical and chemical composition of fossil bone makes it difficult to set-up and calibrate analytical instruments using conventional standards, and may result in non-spectral matrix effects. We propose an analytical protocol that can be employed in future inter-laboratory studies to produce a certified fossil bone geochemical standard. <br/