Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES-Era Data

Phytoplankton productivity and export sequester climatically significant quantities of atmospheric carbon dioxide as particulate organic carbon through a suite of processes termed the biological pump. Constraining how the biological pump operated in the past is important for understanding past atmospheric carbon dioxide concentrations and Earth\u27s climate history. However, reconstructing the history of the biological pump requires proxies. Due to their intimate association with biological processes, several bioactive trace metals and their isotopes are potential proxies for past phytoplankton productivity, including iron, zinc, copper, cadmium, molybdenum, barium, nickel, chromium, and silver. Here, we review the oceanic distributions, driving processes, and depositional archives for these nine metals and their isotopes based on GEOTRACES-era datasets. We offer an assessment of the overall maturity of each isotope system to serve as a proxy for diagnosing aspects of past ocean productivity and identify priorities for future research. This assessment reveals that cadmium, barium, nickel, and chromium isotopes offer the most promise as tracers of paleoproductivity, whereas iron, zinc, copper, and molybdenum do not. Too little is known about silver to make a confident determination. Intriguingly, the trace metals that are least sensitive to productivity may be used to track other aspects of ocean chemistry, such as nutrient sources, particle scavenging, organic complexation, and ocean redox state. These complementary sensitivities suggest new opportunities for combining perspectives from multiple proxies that will ultimately enable painting a more complete picture of marine paleoproductivity, biogeochemical cycles, and Earth\u27s climate history

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES-Era Data

Phytoplankton productivity and export sequester climatically significant quantities of atmospheric carbon dioxide as particulate organic carbon through a suite of processes termed the biological pump. Constraining how the biological pump operated in the past is important for understanding past atmospheric carbon dioxide concentrations and Earth\u27s climate history. However, reconstructing the history of the biological pump requires proxies. Due to their intimate association with biological processes, several bioactive trace metals and their isotopes are potential proxies for past phytoplankton productivity, including iron, zinc, copper, cadmium, molybdenum, barium, nickel, chromium, and silver. Here, we review the oceanic distributions, driving processes, and depositional archives for these nine metals and their isotopes based on GEOTRACES-era datasets. We offer an assessment of the overall maturity of each isotope system to serve as a proxy for diagnosing aspects of past ocean productivity and identify priorities for future research. This assessment reveals that cadmium, barium, nickel, and chromium isotopes offer the most promise as tracers of paleoproductivity, whereas iron, zinc, copper, and molybdenum do not. Too little is known about silver to make a confident determination. Intriguingly, the trace metals that are least sensitive to productivity may be used to track other aspects of ocean chemistry, such as nutrient sources, particle scavenging, organic complexation, and ocean redox state. These complementary sensitivities suggest new opportunities for combining perspectives from multiple proxies that will ultimately enable painting a more complete picture of marine paleoproductivity, biogeochemical cycles, and Earth\u27s climate history

Survey of natural cadmium isotope fractionation by double spike thermal ionisation mass spectrometry

International audienc

Natural mass-dependent Cd isotopic variations determined by TIMS

International audienc

Mass independent fractionation of cadmium isotopes during thermal ionization

International audienc

Origin and temporal evolution of Koʻolau Volcano, Hawaiʻi: Inferences from isotope data on the Koʻolau Scientific Drilling Project (KSDP), the Honolulu Volcanics and ODP Site 843.

International audienceThe “Koʻolau” component of the Hawaiian mantle plume represents an extreme (EM1-type) end member of Hawaiian shield lavas in radiogenic isotope space, and was defined on the basis of the composition of subaerial lavas exposed in the Makapuʻu section of Koʻolau Volcano. The 679 m-deep Koʻolau Scientific Drilling Project (KSDP) allows the long-term evolution of Koʻolau Volcano to be reconstructed and the longevity of the “Koʻolau” component in the Hawaiian plume to be tested. Here, we report triple spike Pb isotope and Sr and Nd isotope data on KSDP core samples, and rejuvenation stage Honolulu Volcanics (HV) (together spanning ∼2.8 m.y.), and from ∼110 Ma basalts from ODP Site 843, thought to be representative of the Pacific lithosphere under Hawaiʻi.Despite overlapping ranges in Pb isotope ratios, KSDP and HV lavas form two distinct linear arrays in 208Pb/204Pb–206Pb/204Pb isotope space. These arrays intersect at the radiogenic end indicating they share a common component. This “Kalihi” component has more radiogenic Pb, Nd, Hf, but less radiogenic Sr isotope ratios than the “Makapuʻu” component. The mixing proportions of these two components in the lavas oscillated through time with a net increase in the “Makapuʻu” component upsection. Thus, the “Makapuʻu” enriched component is a long-lived feature of the Hawaiian plume, since it is present in the main shield-building stage KSDP lavas. We interpret the changes in mixing proportions of the Makapuʻu and Kalihi components as related to changes in both the extent of melting as well as the lithology (eclogite vs. peridotite) of the material melting as the volcano moves away from the plume center. The long-term Nd isotope trend and short-term Pb isotope fluctuations seen in the KSDP record cannot be ascribed to a radial zonation of the Hawaiian plume: rather, they reflect the short length-scale heterogeneities in the Hawaiian mantle plume.Linear Pb isotope regressions through the HV, recent East Pacific Rise MORB and ODP Site 843 datasets are clearly distinct, implying that no simple genetic relationship exists between the HV and the Pacific lithosphere. This observation provides strong evidence against generation of HV as melts derived from the Pacific lithosphere, whether this be recent or old (100 Ma). The depleted component present in the HV is unlike any MORB-type mantle and most likely represents material thermally entrained by the upwelling Hawaiian plume and sampled only during the rejuvenated stage. The “Kalihi” component is predominant in the main shield building stage lavas but is also present in the rejuvenated HV. Thus this material is sampled throughout the evolution of the volcano as it moves from the center (main shield-building stage) to the periphery (rejuvenated stage) of the plume. The presence of a plume-derived material in the rejuvenated stage has significant implications for Hawaiian mantle plume melting models

Ẏoung formation age of a mantle plume source

The Ninetyeast Ridge hotspot track displays strontium, neodymium and lead isotope variations over time that reflect simple radioactive decay in the plume source rather than a change in the mantle components present. The lead isotope variations indicate that the time spent by the plume source in a non-convecting mantle boundary layer was only a few hundreds of millions of years, contrary to the conventional view of individual plume sources as old (1-3 Gyr) or persistent features.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Correlated trace element-Pb isotope enrichments in Indian MORB along 18–20°S, Central Indian Ridge

The Central Indian Ridge (CIR), between 18° and 20°S, shows topographic and chemical characteristics, which suggest interaction of the ridge with a mantle plume. In order to investigate the previously postulated input from the Réunion plume (presently located 1000 km off-axis to the west) on the CIR, we present chemical and isotopic compositions of basalts, collected on and off the CIR axis between 18° and 20°S. We distinguish two geographical groups of samples, called On-Axis and Gasitao, respectively. The On-Axis group is characterized by unradiogenic Sr and Pb isotope ratios and high εNd values. Gasitao group basalts have lower SiO2, are more depleted in incompatible elements and have more radiogenic Sr–Pb isotope ratios and lower εNd values than the On-Axis group. The two groups form two distinct, subparallel linear arrays in 207Pb/204Pb–206Pb/204Pb space. While the Gasitao array trends towards Réunion plume compositions, and therefore appears to contain some Réunion-type plume material, this is not the case for the On-Axis array. Along the ridge axis, Pb isotopes become more radiogenic from south to north, and incompatible trace elements become more enriched, but the compositional field of Réunion lavas is not a suitable end member for the Pb isotope and highly incompatible element trends (e.g. Ba/Nb). This indicates that the geochemical enrichment seen in the On-Axis region is not related to Réunion-type plume material. Basalts from both groups show both positive and negative Eu anomalies, which are strongly correlated with Sr/Nd ratios, thus indicating both gains and losses of feldspar phenocrysts. However, this has little effect on ratios of other trace elements. The trace element enrichment patterns are strongly correlated with Pb isotope ratios, with the most E-MORB-like samples having the most radiogenic Pb isotopic compositions. Using the trace element (TE)/Pb ratios versus 206Pb/204Pb correlations, and by extrapolating these linear correlations to TE/Pb = 0, we constrain possible 206Pb/204Pb ratios of the enriched and depleted endmembers. These lie at 18.3 ≤ 206Pb/204Pb ≤ 18.8 for the depleted and enriched components, respectively, and not very far outside the range of the actual data. We infer that the CIR MORB, between 18° and 20°S are generated by partial melting of a heterogeneous source consisting of an enriched component and a normal, depleted upper-mantle peridotite. The nature of the enriched component is a matter of speculation. As noted, its composition is different from known Réunion plume compositions. Instead, it may represent recycled (oceanic) crustal material, perhaps derived from a subducted oceanic island. It could also be formed by a “metasomatic” enrichment process similar to that modeled by Donnelly et al. [K.E. Donnelly, S.L. Goldstein, C.H. Langmuir, M. Spiegelman. Origin of enriched ocean ridge basalts and implications for mantle dynamics. Earth Planet. Sci. Lett. 226 (2004) 347–366] to explain “E-type” MORB compositions. In either case, the location of the enriched anomaly on the CIR near the intersection with the Gasitao Ridge appears to be coincidental, because the Gasitao enrichment can be traced to the Réunion plume, whereas the On-Axis group enrichment cannot. We speculate that the Réunion plume flow might be deflected towards the South by the hot upwelling E-MORB mantle, because the southernmost On-Ridge sample does fall on the Gasitao-Réunion trend

Modulation of the Southern Ocean cadmium isotope signature by ocean circulation and primary productivity

The High Nutrient Low Chlorophyll (HNLC) Southern Ocean plays a key role in regulating the biological pump and the global carbon cycle. Here we examine the efficacy of stable cadmium (Cd) isotope fractionation for detecting differences in biological productivity between regions. Our results show strong meridional Cd isotope and concentration gradients modulated by the Antarctic Fronts, with a clear biogeochemical divide located near 56°S. The coincidence of the Cd isotope divide with the Southern Boundary of the Antarctic Circumpolar Current (ACC), together with evidence for northward advection of the Cd signal in the ACC, demonstrate that Cd isotopes trace surface ocean circulation regimes. The relationships between Cd isotope ratios and concentrations display two negative correlations, separating the ACC and Weddell Gyre into two distinct Cd isoscapes. These arrays are consistent with Rayleigh fractionation and imply a doubling of the isotope effect due to biological consumption of Cd during water transport from the Weddell Gyre into the ACC. The increase in magnitude of Cd isotope fractionation can be accounted for by differences in the phytoplankton biomass, community composition, and their physiological uptake mechanisms in the Weddell Gyre and ACC, thus linking Cd isotope fractionation to primary production and the global carbon cycle.

San Antonio's Save for Tomorrow Energy Plant (STEP)

Here we report the first ever observations of a strong correlation in ocean surface waters of the dissolved δ114Cdwith dissolved CO2. This is observed in the Southern Ocean along the 0°W meridian in both the AntarcticCircumpolar Current and the Weddell Gyre, as well as in the Weddell Sea proper, near the Antarctic Peninsulaand in Drake Passage. This uniform trend in several surface water masses hints at a uniform biochemical mechanismwithin the Southern Ocean. One hypothesis for the underlying mechanism would be a role of Cd in thecarbonic anhydrase function for conversion of bicarbonate ion [HCO3−] into CO2, the latter being required byRuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) that only accepts CO2. At low ambient [CO2] thealgae maintain growth by also operating a Carbon Concentrating Mechanism (CCM) for utilization of [HCO3−]and its conversion to CO2. For this the algae need more enzyme carbonic anhydrase that normally has Zn as itsco-factor, but Cd may substitute for Zn and there also are Cd-specific carbonic anhydrases known for somephytoplankton species. Indeed in incubations of the local plankton communities it is shown that the phytoplanktonhave a very strong preferential uptake of CO2, such that the uptake ratio {[CO2]/[HCO3−]} is muchhigher than the dissolved ratio {[CO2]/[HCO3−]} in ambient seawater. Therefore the here reported observationsin the Southern Ocean are also expressed for δ114Cd as function of the ratio {[CO2]/[HCO3−]} in ambientseawater. Future research of local phytoplankton in unperturbed natural waters of the Southern Ocean is recommendedto be able to verify the hypothesis of a function of Cd in carbonic anhydrase in Antarctic phytoplankton