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

86 pages, 33 figures, 2 tables, 1 appendix.-- Data Availability Statement: The majority of the dissolved data were sourced from the GEOTRACES Intermediate Data Products in 2014 (Mawji et al., 2015) and 2017 (Schlitzer et al., 2018), and citations to the primary data sources are given in the caption for each figure. Data sources for Figure 1 are given below. Figure 1: Iron: Conway & John, 2014a (Atlantic); Conway & John, 2015a (Pacific); Abadie et al., 2017 (Southern). Zinc: Conway & John, 2014b (Atlantic); Conway & John, 2015a (Pacific); R. M. Wang et al., 2019 (Southern). Copper: Little et al., 2018 (Atlantic); Takano et al., 2017 (Pacific); Boye et al., 2012 (Southern). Cadmium: Conway and John, 2015b (Atlantic); Conway & John, 2015a (Pacific); Abouchami et al., 2014 (Southern). Molybdenum: Nakagawa et al., 2012 (all basins). Barium: Bates et al., 2017 (Atlantic); Geyman et al., 2019 (Pacific); Hsieh & Henderson, 2017 (Southern). Nickel: Archer et al., 2020 (Atlantic); Takano et al., 2017 (Pacific); R. M. Wang et al., 2019 (Southern). Chromium: Goring-Harford et al., 2018 (Atlantic); Moos & Boyle, 2019 (Pacific); Rickli et al., 2019 (Southern). Silver: Fischer et al., 2018 (Pacific); Boye et al., 2012 (Southern)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's 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's climate historyThis contribution grew (and grew) out of a joint workshop between GEOTRACES and Past Global Changes (PAGES) held in Aix-en-Provence in December 2018. The workshop was funded by the U.S. National Science Foundation (NSF) through the GEOTRACES program, the international PAGES project, which received support from the Swiss Academy of Sciences and NSF, and the French program Les Envelopes Fluides et l'Environnement. [...] T. J. Horner acknowledges support from NSF; S. H. Little from the UK Natural Environment Research Council (NE/P018181/1); T. M. Conway from the University of South Florida; and, J. R. Farmer from the Max Planck Society, the Tuttle Fund of the Department of Geosciences of Princeton University, the Grand Challenges Program of the Princeton Environmental Institute, and the Andlinger Center for Energy and the Environment of Princeton University. [...] With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-? data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes

Limited impact of eolian and riverine sources on the biogeochemical cycling of Cd in the tropical Atlantic

We present coupled cadmium (Cd) concentrations and Cd isotopic compositions (expressed as δ114Cd) in seawater profiles along the northeast-southwest GEOTRACES GA11 section in the tropical Atlantic Ocean. The GA11 section encompasses three contrasting regions including the Amazon River plume, the North African dust plume, and the Guinea Dome oxygen deficient zone (ODZ). Given the long oceanic residence time of Cd (104 to 105 yr), local inputs such as atmospheric and riverine sources are generally considered to be of little importance for the open ocean Cd budget, and the limited Cd isotope dataset available thus far has prevented any unambiguous conclusions on the importance of these processes. The GA11 section is ideally located for assessing the influence of external, natural and anthropogenic riverine and eolian inputs, as well as internal processes, on the Cd mass balance in the tropical Atlantic Ocean. As in previous Cd isotope studies, this dataset documents that both surface consumption–regeneration of micronutrients within the water column and deep water mass mixing exert the prime control on the cycling of Cd. However, we do observe some near-surface samples with lower-than-expected δ114Cd signatures along the western section of the transect. Surface waters sampled at the margin of the Amazon freshwater plume show no resolvable difference in [Cd] and δ114Cd compared to waters outside of the plume, suggesting that the Amazon River is not an important source of Cd to the open ocean. Although the eastern GA11 transect is directly downwind of the Saharan dust plume, atmospheric Cd deposition accounts for <1% of the inferred upwelling flux, indicating that atmospheric inputs to the surface North Atlantic Ocean, even within the main Saharan dust plume, only have a relatively minor influence on the Cd budget. In the subsurface tropical Atlantic (100–200 m water depth), there is a clear deviation from a tight linear Cd-PO4 correlation toward lower Cd concentrations for a given PO4 concentration. Our new Cd data show that this Cd-PO4 decoupling is likely a feature of the entire tropical Atlantic and may possibly reflect PO4 enrichment via preferential re-mineralization of organically-bound phosphorus. Alternatively, the decoupling may arise from subsurface Cd depletion caused by precipitation of Cd sulphide within sinking organic particulate microenvironments, as has been suggested in recent studies

Particulate cadmium stable isotopes in the subarctic northeast Pacific reveal dynamic Cd cycling and a new isotopically light Cd sink

The nutrient-type distribution of dissolved cadmium concentrations (dCd) reflects a biological control in the global ocean, with uptake of dissolved Cd into biogenic particles in surface waters and regeneration of particulate Cd at depth. Depth profiles of dissolved Cd stable isotope composition (dδ114/110Cd), while sparse in coverage, exist for most of the major ocean basins, with spatial coverage improving through the efforts of the GEOTRACES program. However, a dearth of similarly resolved particulate δ114/110Cd(pδ114/110Cd) distributions limits our ability to use stable Cd isotopes to better understand Cd cycling in the global ocean. Here we present two pδ114/110Cd depth profiles from the subarctic northeast Pacific which demonstrate more complex δ114/110Cd cycling than dissolved profiles would suggest. Surface pδ114/110Cd, while lighter than surface dδ114/110Cd, is heavy relative to Pacific deepwater and crustal pδ114/110Cd components. Surface particulate and dissolved δ114/110Cd distributions are not well explained by closed-system Rayleigh fractionation following a single fractionation factor, in agreement with other recent studies in the Atlantic and Pacific Oceans. These variable fractionation trends in surface waters complicate the potential utility of δ114/110Cd as a paleoproductivity proxy. Particulate δ114/110Cd becomes lighter as particulate Cd is remineralized in the nutricline, reaching a minimum pδ114/110Cdof around −0.5‰, among the lightest values reported in natural telluric samples. This pδ114/110Cd trend within the nutricline might be explained by (1) multiple pools of particulate Cd with different isotopic compositions and labilities, or (2) by fractionation during particulate Cd remineralization. The observed shallow loss of heavy pδ114/110Cd above the winter mixed layer, rather than the formation of especially light surface pδ114/110Cd, may help to maintain the observed surface-to-deep dδ114/110Cd gradient. Below the mid-depth pδ114/110Cd minimum, pδ114/110Cd increases with depth toward the deepwater dδ114/110Cd value, possibly reflecting an isotopic equilibration between the particulate and dissolved phases. Dissolved δ114/110Cd profiles show uniform isotope composition at intermediate depths, while calculated remineralized pδ114/110Cd is isotopically variable and distinct from the bulk dissolved pool. This suggests that one-dimensional particle export and regeneration is not the primary control on dδ114/110Cd in the Pacific Ocean, but rather that regenerated δ114/110Cdis spatially or temporally variable and an advected dδ114/110Cd signal from subsurface Southern Ocean waters controls deep North Pacific dδ114/110Cd. Our results imply that export of isotopically light pδ114/110Cdto shelf sediments may act as an important oceanic sink, helping to balance the known sources and sinks of Cd with the global deepwater dδ114/110Cd

Non-Rayleigh control of upper-ocean Cd isotope fractionation in the western South Atlantic

We present seawater Cd isotopic compositions in five depth profiles and a continuous surface water transect, from 50 degrees S to the Equator, in the western South Atlantic, sampled during GEOTRACES cruise 74JC057 (GA02 section, Leg 3), and investigate the mechanisms governing Cd isotope cycling in the upper and deep ocean. The depth profiles generally display high epsilon Cd-112/110 at the surface and decrease with increasing depth toward values typical of Antarctic Bottom Water (AABW). However, at stations north of the Subantarctic Front, the decrease in epsilon Cd-112/110 is interrupted by a shift to values intermediate between those of surface and bottom waters, which occurs at depths occupied by North Atlantic Deep Water (NADW). This pattern is associated with variations in Cd concentration from low surface values to a maximum at mid-depths and is attributed to preferential utilization of light Cd by phytoplankton in the surface ocean. Our new results show that in this region Cd-deficient waters do not display the extreme, highly fractionated epsilon Cd-112/110 reported in some earlier studies from other oceanic regions. Instead, in the surface and subsurface southwest (SW) Atlantic, when [Cd] drops below 0.1 nmol kg(-1), epsilon Cd-112/110 are relatively homogeneous and cluster around a value of +3.7, in agreement with the mean value of 3.8 +/- 3.3 (2SD, n = 164) obtained from a statistical evaluation of the global ocean Cd isotope dataset. We suggest that Cd-deficient surface waters may acquire their Cd isotope signature via sorption of Cd onto organic ligands, colloids or bacteriallpicoplankton extracellular functional groups. Alternatively, we show that an open system, steady-state model is in good accord with the observed Cd isotope systematics in the upper ocean north of the Southern Ocean. The distribution of epsilon Cd-112/110 in intermediate and deep waters is consistent with the water mass distribution, with the north-south variations reflecting changes in the mixing proportion of NADW and either AABW or AAIW depending on the depth. Overall, the SW Atlantic Cd isotope dataset demonstrates that the large-scale ocean circulation exerts the primary control on epsilon(112/110) Cd cycling in the global deep ocean

Reproduction Data for: Moderately volatile elements in chondrites record chondrule formation, two component mixing and redistribution on parent bodies

Abstract: Most chondrites are depleted in moderately volatile elements (MVE) relative to the bulk solar system composition represented by CI chondrites. Here we present high-precision isotope dilution data for 11 moderately volatile elements (S, Cu, Zn, Ga, Se, Ag, Cd, In, Sn, Te and Tl) together with Cd and Zn stable isotope compositions for carbonaceous, ordinary, enstatite and Rumuruti chondrites complemented by a literature compilation of MVE stable isotope results. Together these data allow new insights into the processes that led to MVE depletion in chondrites and their redistribution within parent bodies. Moderately volatile element abundances in carbonaceous, ordinary and Rumuruti chondrites are best explained by two-component mixing between a CI-like MVE-rich matrix and a MVE-poor refractory component dominated by chondrules. The refractory component is enriched in light MVE isotopes due to kinetic recondensation of a small vapor fraction initially lost from chondrules and chondrule precursor dust. Later, thermal metamorphism redistributed some MVE within chondrite parent bodies which is associated with stable isotope fractionation. Compared to other chondrite classes enstatite chondrites show more complex MVE abundance patterns when the elements are plotted as a function of condensation temperatures. Type 3 and 4 enstatite chondrites are more MVE-rich than expected from their low matrix fractions. Most likely type 3 and 4 enstatite chondrites are MVE-rich, because a larger MVE vapor fraction recondensed after chondrule formation than observed for other chondrite classes and presumably at comparatively high H2 pressures. Because MVE abundances and isotope compositions are fully consistent with chondrule formation, two-component mixing and MVE redistribution on parent bodies, we refute partial condensation from a hot solar nebula as the cause for MVE depletion in chondrite formation regions of the protoplanetary disk

Bioavailable 87Sr/86Sr in different environmental samples — Effects of anthropogenic contamination and implications for isoscapes in past migration studies

87Sr/86Sr reference maps (isoscapes) are a key tool for investigating past human and animal migrations. However, there is little understanding of which biosphere samples are best proxies for local bioavailable Sr when dealing with movements of past populations. In this study, biological and geological samples (ground vegetation, tree leaves, rock leachates, water, soil extracts, as well as modern and archeological animal teeth and snail shells) were collected in the vicinity of two early medieval cemeteries (“Thuringians”, 5–6th century AD) in central Germany, in order to characterize 87Sr/86Sr of the local biosphere. Animal tooth enamel is not appropriate in this specific context to provide a reliable 87Sr/86Sr baseline for investigating past human migration. Archeological faunal teeth data (pig, sheep/goat, and cattle) indicates a different feeding area compared to that of the human population and modern deer teeth 87Sr/86Sr suggest the influence of chemical fertilizers. Soil leachates do not yield consistent 87Sr/86Sr, and 87Sr/86Sr of snail shells are biased towards values for soil carbonates. In contrast, water and vegetation samples seem to provide the most accurate estimates of bioavailable 87Sr/86Sr to generate Sr isoscapes in the study area. Long-term environmental archives of bioavailable 87Sr/86Sr such as freshwater bivalve shells and tree cores were examined in order to track potential historic anthropogenic contamination of the water and the vegetation. The data obtained from the archeological bivalve shells show that the modern rivers yield 87Sr/86Sr ratios which are similar to those of the past. However, the tree cores registered decreasing 87Sr/86Sr values over time towards present day likely mirroring anthropogenic activities such as forest liming, coal mining and/or soil acidification. The comparison of 87Sr/86Sr of the Thuringian skeletons excavated in the same area also shows that the vegetation samples are very likely anthropogenically influenced to some extent, affecting especially 87Sr/86Sr of the shallow rooted plants

Cd isotopes trace periodic (bio)geochemical metal cycling at the verge of the Cambrian animal evolution

No abstract available

Metal cycling in Mesoproterozoic microbial habitats: Insights from trace elements and stable Cd isotopes in stromatolites

Reconstructing the environmental conditions that supported early life on Earth relies on well-preserved geochemical archives in the rock record. However, many geochemical tracers either lack specificity or they are affected by post-depositional alteration. We present a data set of major and trace element abundances and Cd isotope compositions of dome-shaped and conophyton-type stromatolites of the Late Mesoproterozoic Paranoá Group (Brazil), showing distinct values with unprecedented resolution at the lamina scale. The studied stromatolites consist of dolomite with a high purity and a negligible content of immobile elements (e.g., <0.66 ppm Zr), indicating that elemental compositions are not influenced by detrital contamination. Even though the carbonates have experienced different extent of recrystallization, the measured elemental and isotopic compositions do not correlate with fluid mobile elements. The stromatolites thus represent prime archives for geochemical proxies to reconstruct paleo-environmental conditions. Two endmember compositions can be distinguished by multiple proxy analysis, reflecting the contrasting depositional environments of the two types of stromatolites: Shale-normalized rare earth elements including yttrium (REYSN) patterns of domal stromatolites show a light REYSN (LREY) enrichment (YbSN/PrSN < 0.84), slightly super-chondritic Y/Ho ratios (33.6–39.3) and unfractionated Cd isotopes relative to upper continental crust. This indicates that the stromatolites formed in a shallow-water environment that was episodically influenced by seawater. Their REY and Cd compositions are dominated by dissolved elements that were delivered via weathering and erosion processes from the ambient continent. In contrast, REYSN patterns of the conophyta are parallel those of modern seawater with an LREYSN depletion relative to HREYSN (YbSN/PrSN = 2.1 to 3.9), positive GdSN anomalies (1.1 to 1.4) and strong super-chondritic Y/Ho ratios (37.9 to 46.2), suggesting a microbial habitat that was dominated by seawater. Cd isotopes correlate negatively with Cd and U, but positively with Mn and Ce concentrations, reflecting authigenic carbonate formation at different depths within a redox gradient of the ancient microbial mat. ε112/110Cddol values increase from −3.52 at the mat surface to +3.46 in the interior of the mat, due to the effect of kinetic fractionation during Cd-uptake, e.g. by adsorption onto organic matter or by precipitation of sulfides, in addition to incorporation into carbonate minerals. Hence, our multi-proxy approach including Cd isotopes bears a high potential to shed light on environmental conditions in ancient microbial habitats and the activity of microbial life on Early Earth