Mervyn Greaves

A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry

The various cleaning steps required for preparation of foraminiferal samples for Mg/Ca (and Sr/Ca) analysis are evaluated for their relative importance and effects on measured elemental ratios. It is shown that the removal of silicate contamination is the most important step for the measurement of Mg/Ca ratios. In an example, bulk sample Mg/Ca decreases from 10.5 to 2.5 mmol mol?1 during clay removal. Oxidation of organic material causes a lowering of sample Mg/Ca in the order of 10% or approximately 1°C when converted to temperature. Use of dilute acid leaching to remove adsorbed contaminants causes partial dissolution of the sample carbonate and a corresponding decrease in Mg/Ca. Reductive treatment also causes dissolution of the sample and a decrease in the Mg/Ca ratio of up to 10–15%. Sample preparation for Sr/Ca analysis does not require the same degree of rigor as is necessary for Mg/Ca work. The “within-run” reproducibility of the method described here for G. ruber in a core-top sample from the Arabian Sea was ±1.8% (mean sample ratio was 4.72 mmol mol?1). When converted to temperature, this becomes 28 ± 0.2°C. The equivalent result for Sr/Ca was ±0.5% (mean ratio = 1.44 mmol mol?1).

Evolution of Ocean Temperature and Ice Volume Through the Mid-Pleistocene Climate Transition

Cycling Down The Mid-Pleistocene Transition, which lasted from approximately 1.25 million to 700 thousand years ago, was a period during which the dominant periodicity of Earths climate cycles inexplicably changed from 41 thousand to 100 thousand years. This change is clearly apparent in the oxygen isotopic composition of many calcifying marine organisms, but changes in both ice volume and temperature affect the signal, and so exactly what the signal means has remained unclear. Elderfield et al. (p. 704; see the Perspective by Clark) separated these two effects by measuring both the oxygen isotopic makeup and the Mg/Ca (a proxy that reflects changes in temperature only) of certain benthic foraminifera. The findings reveal the contributions of ice volume and temperature to glacial cycles, suggest when and why the Mid-Pleistocene Climate Transition occurred, and clarify how carbon is lost from the ocean-atmosphere during deglaciations but also changes because of ocean circulation. The effects of changes in ice volume and ocean temperature during the mid-Pleistocene transition have now been resolved. Earth’s climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.

An intensity ratio calibration method for the accurate determination of Mg/Ca and Sr/Ca of marine carbonates by ICP‐AES

An inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method for the accurate and precise simultaneous measurement of the Mg/Ca and Sr/Ca content of carbonates was established. While a precision of <0.3% (1σ standard deviation (SD)) is easily obtainable for both Mg/Ca and Sr/Ca analysis, a Ca matrix effect complicates achieving similar levels of accuracy with conventional calibration procedures. An alternative ratio calibration procedure is proposed which overcomes the Ca matrix effects and ensures the accuracy of the Mg/Ca and Sr/Ca analysis of marine carbonates to <0.3%, almost an order of magnitude better than conventional calibration methods. The longer-term precision is <0.1% if the batch run average values are corrected for longer-term drift. The method is suitable for analysis of foraminiferal calcite and coral aragonite and can easily be adjusted for the analysis of other carbonates or microsamples.

Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research

Thirteen laboratories from the USA and Europe participated in an intercomparison study of Mg/Ca and Sr/Ca measurements in foraminifera. The study included five planktonic species from surface sediments from different geographical regions and water depths. Each of the laboratories followed their own cleaning and analytical procedures and had no specific information about the samples. Analysis of solutions of known Mg/Ca and Sr/Ca ratios showed that the intralaboratory instrumental precision is better than 0.5% for both Mg/Ca and Sr/Ca measurements, regardless whether ICP-OES or ICP-MS is used. The interlaboratory precision on the analysis of standard solutions was about 1.5% and 0.9% for Mg/Ca and Sr/Ca measurements, respectively. These are equivalent to Mg/Ca-based temperature repeatability and reproducibility on the analysis of solutions of ±0.2°C and ±0.5°C, respectively. The analysis of foraminifera suggests an interlaboratory variance of about ±8% (%RSD) for Mg/Ca measurements, which translates to reproducibility of about ±2–3°C. The relatively large range in the reproducibility of foraminiferal analysis is primarily due to relatively poor intralaboratory repeatability (about ±1–2°C) and a bias (about 1°C) due to the application of different cleaning methods by different laboratories. Improving the consistency of cleaning methods among laboratories will, therefore, likely lead to better reproducibility. Even more importantly, the results of this study highlight the need for standards calibration among laboratories as a first step toward improving interlaboratory compatibility.

The behaviour of the rare earth elements during mixing of river and sea waters

Rare earth element concentrations have been measured in organic-rich Luce river water and coastal sea water. Concentrations (e.g. ∼350−1850 pmol/kg Nd in the Water of Luce and ∼45−350 pmol/ kg Nd in Luce Bay) are related to the presence of particles, with 30–60% of the REE associated with >0.4−0.7 μm particles, and to riverine Fe concentrations. REE fractionation occurs in the river water the submicrometre river water is heavy REE enriched whereas the coarser fraction has a more shale-like REE pattern. Laboratory experiments show that the REE in organic-rich river waters are chiefly associated with Feorganic matter colloids which flocculate during estuarine mixing. Preferential removal of heavy REE (∼95%) relative to light REE (∼60%) occurs, but no Ce anomaly is developed. In contrast, no REE removal occurs during estuarine mixing with organic-poor river water.

Interlaboratory comparison study of calibration standards for foraminiferal Mg/Ca thermometry

An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752-1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSDr%), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSDR%), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases.

Dissolved rare earth elements in the Southern Ocean: Cerium oxidation and the influence of hydrography

Abstract Analysis of dissolved REE distributions in the southeastern Atlantic Ocean has revealed that progressive oxidative removal of dissolved Ce from deepwater ( >1 km) occurs over the mixing time of the oceans. Ce-anomaly (Ce/Ce∗) values measured in deep-waters from the southeastern Atlantic are similar to, but slightly higher than, those from deep-ocean stations throughout the Indian and Pacific oceans, typically falling in the range 0.08Ce ≤ Ce∗ ≤ 0.12. Only deepwater samples from the North Atlantic exhibit values greater than Ce/Ce ∗ ∼ 0.12, with the exception of southeastern Atlantic bottomwaters which are influenced by Antarctic Bottomwater. Covariation of dissolved REE concentrations with salinity and/or dissolved Si concentrations at this site has allowed dissolved REE compositions for various Southern Ocean water masses to be predicted. End-member concentrations for Antarctic Bottom Water (AABW) derived in this way indicate high dissolved REE concentrations, similar to average Pacific Deep Water, but with significantly enhanced dissolved Ce concentrations and less pronounced HREE enrichment. This result is consistent with the proximity of our study site to active AABW deepwater formation and evolution of that water mass during transport to the Pacific Ocean.

Rare earth elements in submarine hydrothermal fluids and plumes from the Mid-Atlantic Ridge

Rare earth element (REE) measurements were carried out on samples from black- and white-smoker vents from the TAG and Snakepit sites at 26° and 23°N on the Mid-Atlantic Ridge. Fluids are substantially enriched in REEs over seawater, by factors of 102 in light-REEs, 103 in Eu, and 101 in heavy-REEs. White-smoker REE patterns appear to reflect the effects of shallow subsurface flow. Samples collected from within 0.5 m above the throats of vents (up to ∼ 10 times dilution) indicate that the REEs behave in a conservative fashion with no evidence of removal at this stage of plume evolution. Higher in the buoyant plume (40–100 m above the vent orifice) where entrainment ratios of seawater to vent fluids are ∼100–700, dissolved REEs fall below the dissolved ambient seawater levels (e.g. seawater: Nd = 21.4 pmol kg−1, Ce = 5.44 pmol kg−, Eu = 1.06 pmol kg−1, Er = 5.47 pmol kg−1; plume waters: Nd = 1.22 pmol kg−1, Ce = 1.12 pmol kg−1, Eu = 0.35 pmol kg−1, Er = 0.45 pmol kg−1). The dissolved REE pool shows a net shortfall of 90–98% but the total REEs fall on conservative mixing lines because of REE uptake by plume particulates. REE/Fe ratios in buoyant plume particles are consistent with a kinetic model for Fe2+ oxidation and coprecipitation of REEs with Fe oxides. The trend in the REE/Fe ratios of the particles indicate that in addition to initial coprecipitation and uptake, scavenging of REEs must occur during dispersion of the particles through the neutral plume. The results of the study demonstrate that scavenging processes, by precipitating Fe-oxyhydroxides, eliminate the impact on seawater of the enrichments of REEs from hydrothermal fluids such that the seawater experiences a net depletion of REEs as a consequence of hydrothermal activity.

Aeolian sources of rare earth elements to the Western Pacific Ocean

Abstract Rare earth element (REE) determinations have been made on 25 surface seawater samples on a longitudinal transect from Asia to North America within a latitudinal band of 24–32°N. REE concentrations are significantly higher close to Asia and decrease to the lowest recorded for surface waters in the worlds oceans at the 180° International Dateline. REE patterns also vary systematically and near Asia are similar to patterns for Chinese Loess. Thus, contrary to previous inferences based on Nd isotopes, atmospheric deposition of mineral dust from the Asian continent significantly affects the REE composition of western Pacific Ocean sea water.

DETERMINATION OF THE RARE EARTH ELEMENTS IN NATURAL WATERS BY ISOTOPE-DILUTION MASS SPECTROMETRY

A method is described for the determination of ten rare earth elements (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu) in natural waters by isotope-dilution mass spectrometry. A 1-l sample is used for sea water, and proportionately less for other natural waters. The rare earth elements are extracted by co-precipitation with hydrated iron (III) oxide and purified on a single cation-exchange column, with hydrochloric and nitric acids as eluents. Final measurements are from a triple Re/ Ta filament in the mass spectrometer, run automatically under computer control. Relative standard deviations are better than 4% for the analysis of standard solutions, with accuracy in the same range. The analytical blank is low ( <0.03 pmol, 4 pg, for Nd) producing a sample/blank concentration ratio greater than 100 for all ten rare earth elements when determined in a 1-l seawater sample. Concentration depth profiles are given for an ocean water and normalised abundance patterns for three natural waters.