GEOTRACES IC1 (BATS) contamination-prone trace element isotopes Cd, Fe, Pb, Zn, Cu, and Mo intercalibration (original) (raw)
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Chemie der Erde - Geochemistry, 2007
Trace elements serve important roles as regulators of ocean processes including marine ecosystem dynamics and carbon cycling. The role of iron, for instance, is well known as a limiting micronutrient in the surface ocean. Several other trace elements also play crucial roles in ecosystem function and their supply therefore controls the structure, and possibly the productivity, of marine ecosystems. Understanding the biogeochemical cycling of these micronutrients requires knowledge of their diverse sources and sinks, as well as their transport and chemical form in the ocean. Much of what is known about past ocean conditions, and therefore about the processes driving global climate change, is derived from trace-element and isotope patterns recorded in marine deposits. Reading the geochemical information archived in marine sediments informs us about past changes in fundamental ocean conditions such as temperature, salinity, pH, carbon chemistry, ocean circulation and biological productivity. These records provide our principal source of information about the ocean's role in past climate change. Understanding this role offers unique insights into the future consequences of global change. The cycle of many trace elements and isotopes has been significantly impacted by human activity. Some of these are harmful to the natural and human environment due to their toxicity and/or radioactivity. Understanding the processes that control the transport and fate of these contaminants is an important aspect of protecting the ocean environment. Such understanding requires accurate knowledge of the natural biogeochemical cycling of these elements so that changes due to human activity can be put in context. Despite the recognised importance of understanding the geochemical cycles of trace elements and isotopes, limited knowledge of their sources and sinks in the ocean and the rates and mechanisms governing their internal cycling, constrains their application to illuminating the problems outlined above. Marine geochemists are poised to make significant progress in trace-element biogeochemistry. Advances in clean sampling protocols and analytical techniques provide unprecedented capability for high-density sampling and measurement of a wide range of trace elements and isotopes which can be combined with new modelling strategies that have evolved from the World Ocean Circulation Experiment (WOCE) and Joint Global Ocean Flux Study (JGOFS) programmes. A major new international research programme, GEOTRACES, has now been developed as a result of community input to study the global marine biogeochemical cycles of trace elements and their isotopes. Here, we describe this programme and its rationale.
Cadmium isotopic composition in the ocean
Geochimica Et Cosmochimica Acta, 2006
The oceanic cycle of cadmium is still poorly understood, despite its importance for phytoplankton growth and paleoceanographic applications. As for other elements that are biologically recycled, variations in isotopic composition may bring unique insights. This article presents (i) a protocol for the measurement of cadmium isotopic composition (Cd IC) in seawater and in phytoplankton cells; (ii) the first Cd IC data in seawater, from two full depth stations, in the northwest Pacific and the northwest Mediterranean Sea; (iii) the first Cd IC data in phytoplankton cells, cultured in vitro. The Cd IC variation range in seawater found at these stations is not greater than 1.5 e Cd/amu units, only slightly larger than the mean uncertainty of measurement (0.8 e Cd/amu ). Nevertheless, systematic variations of the Cd IC and concentration in the upper 300 m of the northwest Pacific suggest the occurrence of Cd isotopic fractionation by phytoplankton uptake, with a fractionation factor of 1.6 ± 1.4 e Cd/amu units. This result is supported by the culture experiment data suggesting that freshwater phytoplankton (Chlamydomonas reinhardtii and Chlorella sp.) preferentially take up light Cd isotopes, with a fractionation factor of 3.4 ± 1.4 e Cd/amu units. Systematic variations of the Cd IC and hydrographic data between 300 and 700 m in the northwest Pacific have been tentatively attributed to the mixing of the mesothermal (temperature maximum) water (e Cd/amu = À0.9 ± 0.8) with the North Pacific Intermediate Water (e Cd/amu = 0.5 ± 0.8). In contrast, no significant Cd IC variation is found in the northwest Mediterranean Sea. This observation was attributed to the small surface Cd depletion by phytoplankton uptake and the similar Cd IC of the different water masses found at this site. Overall, these data suggest that (i) phytoplankton uptake fractionates Cd isotopic composition to a measurable degree (fractionation factors of 1.6 and 3.4 e Cd/amu units, for the in situ and culture experiment data, respectively), (ii) an open ocean profile of Cd IC shows upper water column variations consistent with preferential uptake and regeneration of light Cd isotopes, and (iii) different water masses may have different Cd IC. This isotopic system could therefore provide information on phytoplankton Cd uptake and on water mass trajectories and mixing in some areas of the ocean. However, the very small Cd IC variations found in this study indicate that applications of Cd isotopic composition to reveal aspects of the present or past Cd oceanic cycle will be very challenging and may require further analytical improvements. Better precision could possibly be obtained with larger seawater samples, a better chemical separation of tin and a more accurate mass bias correction through the use of the double spiking technique.
A new methodology for precise cadmium isotope analyses of seawater
Analytical and Bioanalytical Chemistry, 2012
Previous studies have revealed considerable Cd isotope fractionations in seawater, which can be used to study the marine cycling of this micronutrient element. The low Cd concentrations that are commonly encountered in nutrient-depleted surface seawater, however, pose a particular challenge for precise Cd stable isotope analyses. In this study, we have developed a new procedure for Cd isotope analyses of seawater, which is suitable for samples as large as 20 L and Cd concentrations as low as 1 pmol/L. The procedure involves the use of a 111 Cd-113 Cd double spike, co-precipitation of Cd from seawater using Al(OH) 3 , and subsequent Cd purification by column chromatography. To save time, seawater samples with higher Cd contents can be processed without co-precipitation. The Cd isotope analyses are carried out by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The performance of this technique was verified by analyzing multiple aliquots of a large seawater sample that was collected from the English Channel, the SAFe D1 seawater reference material, and several samples from the GEOTRACES Atlantic intercalibration exercise. The overall Cd yield of the procedure is consistently better than 85% and the methodology can routinely provide ε 114/110 Cd data with a precision of about ±0.5 ε (2sd, standard deviation) when at least 20-30 ng of natural Cd is available for analysis. However, even seawater samples with Cd contents of only 1-3 ng can be analyzed with a reproducibility of about ±3 to ±5 ε. A number of experiments were furthermore conducted to verify that the isotopic results are accurate to within the quoted uncertainty.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2016
Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium ( T 1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228 Ra fluxes are combined with TEI/ 228 Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for...
The marine geochemistry of trace metals
1976
The marine geochemical cycles of iron, copper, nickel, and cadmium were studied in order to provide a basis for oceanographic models for trace metals. Copper, nickel, and cadmium can be determined in a 100 ml seawater sample using cobalt pyrrolidine di thiocarbamate chelate coprecipi tat ion and graphite atomizer atomic absorption spectrometry. Concentration ranges likely to be encountered and estimated (10) analytical precisions are copper, 1 to 6 nanomole/kg (10.1); nickel, 3 to 12 nanomole/kg (i 0. 3); and cadmium, o. 0 to 1.1 nanomole/kg (i 0. l). The technique may be applied to freshwater samples with slight modification.-5-ACKNOWLEDGEMENT S I am grateful to John M. Edmond for being my advisor and friend and for providing a stimulating environment in which to do and learn research.
Cadmium: Chemical tracer of deepwater paleoceanography
Paleoceanography, 1988
The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient distributions within ancient oceans. Empirical studies demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D-2.9. This constant is the same for each of the species studied: Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Nuttallides umbonifera, and Uvigerina.spp. Downcore cadmium data from high-quality Pacific and Atlantic sediment cores suggest that the cadmium inventory of the ocean did not change significantly between the most recent glacial maximum and the present. Hence changes in the cadmium content of fossils at a site directly reflect changes in nutrient distributions due to altered oceanic circulation patterns. Studies of cadmium in Pleistocene sediments show that deep ocean circulation patterns were significantly different during the most recent glacial maximum. In the western North Atlantic Ocean, the nutrient content of waters from 2500 to 3500 m was twice as high during glacials as during interglacial periods, signifying an increase in the proportion of waters of Copyright 1988 by the American Geophysical Union.
Biogeochemical cycling of chromium and chromium isotopes in the sub-tropical North Atlantic Ocean
Frontiers in Marine Science, 2023
Chromium (Cr) is a redox-sensitive element and because Cr isotopes are fractionated by redox and/or biological processes, the Cr isotopic composition of ancient marine sediments may be used to infer changes in past seawater oxygenation or biological productivity. While there appears to be a 'global correlation' between the dissolved Cr concentration and Cr isotopic composition of seawater, there is ongoing debate about the relative importance of external sources and internal cycling on shaping the distribution of dissolved Cr that needs to be resolved to validate the efficacy of using Cr isotopes as a paleo proxy. Here, we present full water column depth profiles of total dissolved Cr (Cr(VI)+Cr(III)) and dissolved Cr isotopes (d 53 Cr), together with ancillary data, for three stations along a transect (GEOTRACES GApr08) across the subtropical North Atlantic. Concentrations of dissolved Cr ranged between 1.84 and 2.63 nmol kg-1 , and d 53 Cr values varied from 1.06 to 1.42‰. Although atmospheric dust, hydrothermal vents and seabed sediments have the potential to modify the distribution of Cr in the oceans, based on our observations, there is no clear evidence for substantial input of Cr from these sources in our study region although benthic inputs of Cr may be locally important in the vicinity of hydrothermal vents. Subsurface waters (below the surface mixed layer to 700 m water depth) were very slightly depleted in Cr (by up to~0.4 nmol kg-1), and very slightly enriched in heavy Cr isotopes (by up to~0.14‰), relative to deeper waters and the lowest Cr concentrations and highest d 53 Cr values coincided with lowest concentrations of colloidal (0.02 to 0.2 mm size fraction) Fe. We found no direct evidence for biological uptake of dissolved Cr in the oligotrophic euphotic zone or removal of Cr in modestly oxygen depleted waters (O 2 concentrations~130 mmol kg-1). Rather, we suggest removal of Cr (probably in the form of Cr(III)) in subsurface waters is associated with the formation of colloid aggregates of Fe-(oxyhydr)oxides. This process is likely enhanced by the high lithogenic particle load in this region, and represents a previously unrecognized export flux of Cr. Regeneration of Cr in deeper waters leads to subtly increased levels of Cr alongside decreased d 53 Cr values at individual sites, but this trend is more obvious at the global scale, with d 53 Cr values decreasing with increasing radiocarbon age of deep waters, from 1.16 ± 0.10‰ (1SD, n=11) in deep Atlantic waters to 0.77 ± 0.10‰ (1SD, n=25) in deep Pacific waters. Removal of Frontiers in Marine Science frontiersin.org 01