Advancing planktonic foraminifera Mg/Ca thermometry: a microanalytical perspective

Microanalytical techniques, specifically laser ablation - inductively coupled plasma mass spectrometry (LA-ICPMS) and electron microprobe analysis(EMPA), have been used to determine Mg/Ca compositional variation within and between tests of planktonic foraminifera. Micro-scale characterization of this increasingly widely-used seawater temperature proxy has allowed scrutiny of the factors that limit its applicability as well as the development of new microanalysis-based Mg/Ca proxy calibrations for sea surface temperature and thermocline temperatures...This study illustrates the potential for using novel microanalytical techniques to advance Mg/Ca thermometry, by both providing a framework for better understanding the nature of Mg/Ca dependence on seawater temperature and the effects of complicating factors (e.g. seafloor dissolution, seasonality), as well as providing insight into the underlying biomineralization mechanisms that control Mg incorporation into and the formation of foraminiferal calcite.Australian Research Council- via an ARC Discovery Grant DP0450358 to Prof. De Deckker and Dr. Eggins. The Australian National University by a tuition fee scholarship

Intraspecific variation in recent populations of Globigerinoides ruber from the eastern Indian Ocean: evidence from test morphology and geochemistry

Abstrac

The effect of ocean alkalinity and carbon transfer on deep-sea carbonate ion concentration during the past five glacial cycles

Glacial–interglacial deep Indo-Pacific carbonate ion concentration ([CO32−]) changes were mainly driven by two mechanisms that operated on different timescales: 1) a long-term increase during glaciation caused by a carbonate deposition reduction on shelves (i.e., the coral reef hypothesis), and 2) transient carbonate compensation responses to deep ocean carbon storage changes. To investigate these mechanisms, we have used benthic foraminiferal B/Ca to reconstruct deep-water [CO32−] in cores from the deep Indian and Equatorial Pacific Oceans during the past five glacial cycles. Based on our reconstructions, we suggest that the shelf-to-basin shift of carbonate deposition raised deep-water [CO32−], on average, by 7.3 ± 0.5 (SE) μmol/kg during glaciations. Oceanic carbon reorganisations during major climatic transitions caused deep-water [CO32−] deviations away from the long-term trend, and carbonate compensation processes subsequently acted to restore the ocean carbonate system to new steady state conditions. Deep-water [CO32−] showed similar patterns to sediment carbonate content (%CaCO3) records on glacial–interglacial timescales, suggesting that past seafloor %CaCO3 variations were dominated by deep-water carbonate preservation changes at our studied sites

Geochemical imprints of genotypic variants of <i>Globigerina bulloides</i> in the Arabian Sea

Planktonic foraminifera record oceanic conditions in their shell geochemistry. Many palaeoenvironmental studies have used fossil planktonic foraminifera to constrain past seawater properties by defining species based on their shell morphology. Recent genetic studies, however, have identified ecologically distinct genotypes within traditionally recognized morphospecies, signaling potential repercussions for palaeoclimate reconstructions. Here we demonstrate how the presence of Globigerina bulloides cryptic genotypes in the Arabian Sea may influence geochemical signals of living and fossil assemblages of these morphospecies. We have identified two distinct genotypes of G. bulloides with either cool water (type-II) or warm water (type-I) temperature preferences in the Western Arabian Sea. We accompany these genetic studies with analyses of Mg/Ca and stable oxygen isotope (δ18O) compositions of individual G. bulloides shells. Both Mg/Ca and δ18O values display bimodal distribution patterns. The distribution of Mg/Ca values cannot be simply explained by seawater parameters, and we attribute it to genotype-specific biological controls on the shell geochemistry. The wide range of δ18O values in the fossil assemblage also suggests that similar controls likely influence this proxy in addition to environmental parameters. However, the magnitude of this effect on the δ18O signals is not clear from our data set, and further work is needed to clarify this. We also discuss current evidence of potential genotype-specific geochemical signals in published data on G. bulloides geochemistry and other planktonic foraminiferal species. We conclude that significant caution should be taken when utilizing G. bulloides geochemistry for paleoclimate reconstruction in the regions with upwelling activity or oceanographic fronts

Interlaboratory study for coral Sr/Ca and other element/Ca ratio measurements

The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twentyone laboratories took part in an interlaboratory study of coral Sr/Ca measurements. Results show interlaboratory bias can be significant, and in the extreme case could result in a range in SST estimates of 7°C. However, most of the data fall within a narrower range and the Porites coral reference material JCp- 1 is now characterized well enough to have a certified Sr/Ca value of 8.838 mmol/mol with an expanded uncertainty of 0.089 mmol/mol following International Association of Geoanalysts (IAG) guidelines. This uncertainty, at the 95% confidence level, equates to 1.5°C for SST estimates using Porites, so is approaching fitness for purpose. The comparable median within laboratory error is <0.5°C. This difference in uncertainties illustrates the interlaboratory bias component that should be reduced through the use of reference materials like the JCp-1. There are many potential sources contributing to biases in comparative methods but traces of Sr in Ca standards and uncertainties in reference solution composition can account for half of the combined uncertainty. Consensus values that fulfil the requirements to be certified values were also obtained for Mg/Ca in JCp-1 and for Sr/Ca and Mg/Ca ratios in the JCt-1 giant clam reference material. Reference values with variable fitness for purpose have also been obtained for Li/Ca, B/Ca, Ba/Ca, and U/Ca in both reference materials. In future, studies reporting coral element/Ca data should also report the average value obtained for a reference material such as the JCp-1

Mg/Ca composition of benthic foraminifera Miliolacea as a new tool of paleoceanography

The Mg/Ca compositions of benthic foraminifera from the superfamily Miliolacea have been studied to explore the use of these high-Mg foraminifera as a proxy for deep ocean conditions. Taxonomic analyses, relative abundance, and depth distributions of different Miliolacea species were carried out on a collection of core top samples, covering a depth range of 131 m to 2530 m, along the Australian coast of the Timor Sea. Pyrgo sp., composed of Pyrgo sarsi and Pyrgo murrhina, was found to be the most suitable for proxy studies. Mg/Ca values of this group of foraminifera show a strong correlation with bottom water temperatures and carbonate ion saturation described by the linear relationship: Mg/Ca = 2.53(±0.22) × BWT + 0.129(±0.023) × Δ[CO32−] + 4.63(±0.53), within the −1°C to 8°C temperature range. Absolute Mg/Ca values of Pyrgo sp. calcite and their temperature sensitivity are similar to those observed for inorganic calcite, suggesting that Mg composition of Pyrgo sp. calcite is mainly controlled by inorganic processes. The Mg/Ca composition of Pyrgo sp. calcite provides a new tool for reconstructing both water temperature and carbonate ion saturation when combined with other proxies for one of these parameters. A down core record from the Eastern Equatorial Pacific has been generated to illustrate how Mg/Ca values can be used for paleoclimate studies. This down core record shows large changes in Pacific bottom waters [CO32−] across glacial-interglacial transition, implying an increase in [CO32−] during the glacial period

Application of 10(13) ohm Faraday cup current amplifiers for boron isotopic analyses by solution mode and laser ablation multicollector inductively coupled plasma mass spectrometry

RationaleBoron isotope ratios (B-11 values) are used as a proxy for seawater paleo-pH, amongst several other applications. The analytical precision can be limited by the detection of low intensity ion beams from limited sample amounts. High-gain amplifiers offer improvements in signal/noise ratio and can be used to increase measurement precision and reduce sample amounts. Methods10(13) ohm amplifier technology has previously been applied to several radiogenic systems, but has thus far not been applied to non-traditional stable isotopes. Here we apply 10(13) ohm amplifier technology for the measurement of boron isotope ratios using solution mode MC-ICP-MS and laser ablation mode (LA-)MC-ICP-MS techniques. Precision is shown for reference materials as well as for low-volume foraminifera samples. ResultsThe baseline uncertainty for a 0.1 pA B-10(+) ion beam is reduced to <0.1 for a typical measurement period. The external precision is better than 0.2 parts per thousand (2SD) for B-11 measurements for solution samples containing as little as 0.8 ng total boron. For in situ microanalyses with LA-MC-ICP-MS, the external precision of B-11/B-10 from an in-house calcite standard was 1 parts per thousand (2SD) for individual spot analyses, and 0.3 parts per thousand for the mean of 10 replicate spot analyses. Conclusions10(13) ohm amplifier technology is demonstrated to offer advantages for the determination of B-11 values by both MC-ICP-MS and LA-MC-ICP-MS for small samples of biogenic carbonates, such as foraminifera shells. 10(13) ohm amplifier technology will also be of benefit to other non-traditional stable isotope measurements

The effect of ocean alkalinity and carbon transfer on deep-sea carbonate ion concentration during the past five glacial cycles

Glacial–interglacial deep Indo-Pacific carbonate ion concentration ([CO32−]) changes were mainly driven by two mechanisms that operated on different timescales: 1) a long-term increase during glaciation caused by a carbonate deposition reduction on shelves (i.e., the coral reef hypothesis), and 2) transient carbonate compensation responses to deep ocean carbon storage changes. To investigate these mechanisms, we have used benthic foraminiferal B/Ca to reconstruct deep-water [CO32−] in cores from the deep Indian and Equatorial Pacific Oceans during the past five glacial cycles. Based on our reconstructions, we suggest that the shelf-to-basin shift of carbonate deposition raised deep-water [CO32−], on average, by 7.3 ± 0.5 (SE) μmol/kg during glaciations. Oceanic carbon reorganisations during major climatic transitions caused deep-water [CO32−] deviations away from the long-term trend, and carbonate compensation processes subsequently acted to restore the ocean carbonate system to new steady state conditions. Deep-water [CO32−] showed similar patterns to sediment carbonate content (%CaCO3) records on glacial–interglacial timescales, suggesting that past seafloor %CaCO3 variations were dominated by deep-water carbonate preservation changes at our studied site