Influence of the Mediterranean Outflow on the isotopic composition of neodymium in waters of the North Atlantic

The isotopic composition of neodymium in the water column of the eastern North Atlantic near the Strait of Gibraltar has been determined for several depths. The data show that the Mediterranean outflow results in a significant shift in ε_(Nd)(0) toward more radiogenic values of ^(143)Nd/^(144)Nd in the water column at a 1000-m depth. This corresponds to a depth in the neighborhood of the salinity maximum associated with the Mediterranean outflow. The core of the Mediterranean outflow gives ε_(Nd)(0) = −9.8, as compared to ε_(Nd)(0) ≈ −12 in overlying and underlying waters, demonstrating that the Mediterranean waters are distinct from the Atlantic. From mixing considerations we estimate that pure Mediterranean waters have ε_(Nd)(0) ≈ −6. Possible sources of this relatively radiogenic Nd could be from drainage of young continental terranes or the injection of remobilized Nd from deep-sea sediments that have a young radiogenic volcanic component. New data from a depth profile in the western Atlantic is presented. Comparison of Nd data for the eastern North Atlantic with that for the western North Atlantic shows fundamental differences in the water column structures for ε_(Nd)(0). While both regions show a pronounced maximum in ε_(Nd)(0), the western basin maximum occurs at the near surface rather than at 1000 m. In addition, deep waters of the eastern basin are found to be more radiogenic than the western basin. These differences indicate several sources of isotopically distinct Nd in the North Atlantic. The deep waters of the North Atlantic (>1000 m) have the lowest values of ε_(Nd)(0) measured in the oceans. We believe that the source of these low ε_(Nd)(0) values, which we associate with North Atlantic deep water, is either from freshwater drainage off the Precambrian shields of North America and Asia into the Arctic Ocean or from the injection of ‘older,’ continentally derived REE from deep-sea sediments. Sm and Nd concentrations are found to increase with depth and ε_(Nd)(0) changes with depth, indicating both vertical and lateral transport processes from different sources. This suggests a surface source of Nd and injection of REE into the water column from deep-sea sediments or large-scale bottom currents with high REE concentrations

Long term records of erosional change from marine ferromanganese crusts

Ferromanganese crusts from the Atlantic, Indian and Pacific Oceans record the Nd and Pb isotope compositions of the water masses from which they form as hydrogenous precipitates. The10Be/9Be-calibrated time series for crusts are compared to estimates based on Co-contents, from which the equatorial Pacific crusts studied are inferred to have recorded ca. 60 Ma of Pacific deep water history. Time series of ɛNd show that the oceans have maintained a strong provinciality in Nd isotopic composition, determined by terrigenous inputs, over periods of up to 60 Ma. Superimposed on the distinct basin-specific signatures are variations in Nd and Pb isotope time series which have been particularly marked over the last 5 Ma. It is shown that changes in erosional inputs, particularly associated with Himalayan uplift and the northern hemisphere glaciation have influenced Indian and Atlantic Ocean deep water isotopic compositions respectively. There is no evidence so far for an imprint of the final closure of the Panama Isthmus on the Pb and Nd isotopic composition in either Atlantic or Pacific deep water masses

Early to middle Eocene history of the Arctic Ocean from Nd-Sr isotopes in fossil fish debris, Lomonosov Ridge

Strontium and neodymium radiogenic isotope ratios in early to middle Eocene fossil fish debris (ichthyoliths) from Lomonosov Ridge (Integrated Ocean Drilling Program Expedition 302) help constrain water mass compositions in the Eocene Arctic Ocean between ∼55 and ∼45 Ma. The inferred paleodepositional setting was a shallow, offshore marine to marginal marine environment with limited connections to surrounding ocean basins. The new data demonstrate that sources of Nd and Sr in fish debris were distinct from each other, consistent with a salinity-stratified water column above Lomonosov Ridge in the Eocene. The 87Sr/86Sr values of ichthyoliths (0.7079–0.7087) are more radiogenic than Eocene seawater, requiring brackish to fresh water conditions in the environment where fish metabolized Sr. The 87Sr/86Sr variations probably record changes in the overall balance of river Sr flux to the Eocene Arctic Ocean between ∼55 and ∼45 Ma and are used here to reconstruct surface water salinity values. The ɛNd values of ichthyoliths vary between −5.7 and −7.8, compatible with periodic (or intermittent) supply of Nd to Eocene Arctic intermediate water (AIW) from adjacent seas. Although the Norwegian-Greenland Sea and North Atlantic Ocean were the most likely sources of Eocene AIW Nd, input from the Tethys Sea (via the Turgay Strait in early Eocene time) and the North Pacific Ocean (via a proto-Bering Strait) also contributed

The History, Relevance, and Applications of the Periodic System in Geochemistry

Geochemistry is a discipline in the earth sciences concerned with understanding the chemistry of the Earth and what that chemistry tells us about the processes that control the formation and evolution of Earth materials and the planet itself. The periodic table and the periodic system, as developed by Mendeleev and others in the nineteenth century, are as important in geochemistry as in other areas of chemistry. In fact, systemisation of the myriad of observations that geochemists make is perhaps even more important in this branch of chemistry, given the huge variability in the nature of Earth materials – from the Fe-rich core, through the silicate-dominated mantle and crust, to the volatile-rich ocean and atmosphere. This systemisation started in the eighteenth century, when geochemistry did not yet exist as a separate pursuit in itself. Mineralogy, one of the disciplines that eventually became geochemistry, was central to the discovery of the elements, and nineteenth-century mineralogists played a key role in this endeavour. Early “geochemists” continued this systemisation effort into the twentieth century, particularly highlighted in the career of V.M. Goldschmidt. The focus of the modern discipline of geochemistry has moved well beyond classification, in order to invert the information held in the properties of elements across the periodic table and their distribution across Earth and planetary materials, to learn about the physicochemical processes that shaped the Earth and other planets, on all scales. We illustrate this approach with key examples, those rooted in the patterns inherent in the periodic law as well as those that exploit concepts that only became familiar after Mendeleev, such as stable and radiogenic isotopes

University of Miami Radiocarbon Dates VIII

This material was digitized as part of a cooperative project between Radiocarbon and the University of Arizona Libraries.The Radiocarbon archives are made available by Radiocarbon and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

University of Miami Radiocarbon Dates IX

This material was digitized as part of a cooperative project between Radiocarbon and the University of Arizona Libraries.The Radiocarbon archives are made available by Radiocarbon and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Strontium and neodymium isotopes in hot springs on the East Pacific Rise and Guaymas Basin

We have determined the concentrations and isotopic compositions of Sr and Nd in hydrothermal fluids from 21°N, East Pacific Rise and Guaymas Basin, Gulf of California. The purest solutions analyzed from 21°N exhibit a small range in Sr concentration between individual vents from 5.8 to 8.7 ppm, close to normal seawater Sr concentrations. They exhibit a small range in ^(87)Sr/^(86)Sr from ε_(Sr)(0) = −13.4 to −17.7, corresponding to ε_(Sr)(0) ≈ −18 ± 2 in the pure hydrothermal end-member. These results indicate extensive but not complete isotopic exchange with Sr in the depleted oceanic crust (ε _(Sr)(0) = −31.8) and suggest that Sr concentrations in these solutions are buffered. In contrast, the concentration and isotopic composition of Nd in solutions show large variations between vents. The concentration of Nd ranges from 20 to 336 pg/g (6–100 times seawater Nd concentrations). The isotopic composition ranges from ε_(Nd)(0) = −3.6 (similar to Pacific seawater) to + 7.9. Many samples show substantial contributions from MORB, but all have ε_(Nd)(0) well below MORB at this locality (ε_(Nd)(0) = + 9.7) in spite of very large enrichments in Nd concentrations. While complete isotopic exchange with water/rock≈ 2 or exchange with anomalous oceanic crust can explain the Sr data, the Nd data require exchange with a reservoir having ε_(Nd)(0) < Pacific seawater. Low-temperature reactions with metalliferous sediments on the ridge flanks may provide such a source. Both Sr and Nd in the Guaymas Basin solution are very different from21°N. ε_(Sr)(0) = + 11.0 and ε_(Nd)(0) = −11.4 and are consistent with the fluid exchanging Sr and Nd with heated sediments having a substantial component of old continental detritus. Some irregularities in the Nd isotopic data reported here indicate that there must be a problem of contamination for some ultra-low-level trace elements during sample collection and processing which requires further attention. Using a simple box model, the estimates for hydrothermal Nd fluxes are compared with fluxes which would be required to maintain the relatively radiogenic value of ε_(Nd)(0) ≈ −3 in the Pacific against the influx of more negative Antarctic waters (ε_(Nd)(0) ≈ −9). It is shown that the hydrothermal flux of Nd from mid-ocean ridges falls far short of that necessary to maintain the isotopic balance. This indicates that weathered material from volcanic terranes (ε_(Nd)(0) ≈ +7) is the most reasonable major source of radiogenic Nd in the Pacific

Rare earth element transport in the western North Atlantic inferred from Nd isotopic observations

The isotopic composition of Nd in the water column from several western North Atlantic sites and formational areas for North Atlantic Deep Water shows extensive vertical structure at all locations. In regions where a thermocline is well-developed, large isotopic shifts (2 to 3 ϵ units) are observed across the base of the thermocline. Regions without a thermocline are characterized by much more gradual shifts in isotopic composition with depth. In general, the data reveal an excellent correlation between the Nd isotopic distribution in the western North Atlantic water column and the distribution of water masses identified from temperature and salinity characteristics. NADW, as identified from T-S properties, is also characterized by a well-defined isotopic composition having ϵ_(Nd)(0) = −13.5 ± 0.5. This signature is associated with waters identified as NADW from high latitudes near formational areas in the Labrador Sea down to the equatorial region. The isotopic signature of NADW would appear to be formed by a blend of more negative waters originating in the Labrador Sea (ϵ_(Nd)(0) < −18) and more positive waters originating in the overflows from the Norwegian and Greenland Seas (ϵ_(Nd)(0) ≈ −8 to −10) and is consistent with classical theories on the formation of NADW. The isotopic signature of NADW is propagated southward to the equator where it is gradually being thinned out by mixing from above and below with more radiogenic Nd associated with northward-spreading Antarctic Intermediate and Bottom Waters. The preservation of the isotopic signature of NADW over these large distances indicate that the REE undergo extensive lateral transport. The isotopic composition of Nd is largely conservative over the time scales of mixing within the Atlantic in spite of the intrinsic nonconservative behavior of neodymium. Nd concentration gradients generally show surface waters to be depleted in Nd relative to deep waters, which must require vertical transport processes. However, isotopic differences in the water column preclude the local downward transport of REE from the surface into underlying deep waters as a simple explanation of the concentration gradient. The apparent decoupling of REE in NADW from overlying (local) surface waters and the increasing concentration with depth provide a conflict with simple vertical transport mechanisms that is not yet resolved