Neodymium Isotopic Composition Research Articles

THE U-PB AGE AND HAFNIUM ISOTOPE COMPOSITION OF ZIRCON FROM METAMORPHOZED ANDESITE OF THE CHORTOMLYK FORMATION AND RHYODACITE HYPABYSSAL INTRUSION OF THE SURA COMPLEX, CHORTOMLYK GREENSTONE BELT

Andesites and felsic volcanic rocks are observed at all stratigraphic levels of the Konka and Bilozerka groups, which comprise greenstone structures in the Middle Dnieper Domain. Their nature and age are still poorly known. The youngest felsic volcanic rocks of the Solone Formation of the Konka Group and comagmatic with the tonalite-trondhjemite-granodiorite (TTG) association of the Sura Complex hypabyssal intrusions were pre-viously dated by the U-Pb zircon SHRIMP method at ca. 3.1 Ga. The purpose of this study is to determine the U-Pb zircon age and geochemical features of i) metamorphosed andesites of the Chortomlyk Formation and ii) low-alkaline metarhyodacite hypabyssal intrusions that cut the rocks of the Sura Formation of the Konka Group. In the Chortomlyk Greenstone Belt, the thickness of volcanogenic rocks of the Chortomlyk Formation (dacite-andesite-tholeiite association) reaches 2000 m. The youngest felsic volcanic rocks of the Solone Formation and comagmatic hypabyssal intrusions are located within three large volcanic fields located near the Novomykolaivka* massif. Using the LA-ICP-MS method, U-Pb ages of two zircon populations from metamorphosed andesites of the Chortomlyk Formation were dated. Twenty-three crystals of transparent colourless zircon crystals yielded a concordant age of (3222 ± 6) Ma. The U-Pb age of the second population of large, brown, opaque zircon crystals is 3132–3073 Ma. Interpretation of the obtained ages is not straightforward and at least two options can be proposed: 1). The studied metaandesites are differentiated mafic magmas and the age of their formation is defined by the older zircon population, and the young population corresponds to the time of superimposed thermal processes during later intrusion of plagioclase granitoids of the Novomykolaivka massif; 2). The age of the metaandesite is defined by the younger population, while the older population is inherited from the protolith. We consider the second option as being far more likely. The first option contradicts the stratigraphic position of the dated rock. The studied metaandesite is low in potassium and belongs to the sodium series. Relative to TTG, they have higher Nb (16.2 ppm) and Y (25.9 ppm). Rare earth elements are weakly differentiated, (La/Yb)N = 3.91 with a strongly negative Eu anomaly, Eu/Eu* = 0.44. The U-Pb zircon age from the low-alkaline rhyodacite hypabyssal intrusion that cuts the Sura Formation of the Konka Group is (3085 ± 6) Ma. It has a highly differentiated REE pattern, (La/Yb)N = 16.2 and a positive Eu anomaly, Eu/Eu* = 1.21. The Nb (6.7 ppm) and Y (10.8 ppm) contents are low. They chemically resemble TTGs of the Sura Complex. Based on our data, the andesites of the Chortomlyk Formation of the Konka Group and the low-alkaline rhyodacite hypabyssal intrusions have the same U-Pb age, but different origins. The former were produced by the melting of older crustal rocks, and the latter were formed due to the partial melting of metabasites with garnet-bearing restite. Hafnium isotope composition in zircon from both samples reveals their juvenile nature, i.e., they crystallized from partial melts of rocks with short crustal residence times. Our isotope data agrees with the neodymium isotope composition of the felsic volcanic rocks of the Sura greenstone belt, which yielded εHf values of +1.8. These values are lower than the depleted mantle isotope composition at this time (3200–3000 Ma). *Novomykolaivka massif was formerly known as Chkalove massif.

Geochemistry of forty‐one eclogitic and pyroxenitic mantle xenoliths from the Central Slave Craton, Canada (Ekati Diamond Mine)

AbstractThis article describes a novel dataset on non‐diamondiferous eclogite and garnet pyroxenite xenoliths from four kimberlite pipes of the Ekati Diamond Mine (Central Slave Craton, Canada). Xenoliths brought to the surface by kimberlite eruptions are direct sources of information on the composition and evolution of the Earth's mantle. Eclogite and garnet pyroxenite xenoliths, specifically, are testimony of subduction into, and metasomatism of, the mantle beneath cratons. Furthermore, these rocks are major hosts for diamond and thus an important part of the deep carbon cycle. The sample suite consists of 41 small xenoliths (2–5 cm) recovered from drill cores. The dataset includes major and trace element concentrations for garnet, clinopyroxene and ilmenite, as well as stable oxygen isotope compositions of garnets. Strontium and neodymium isotopic compositions are reported for garnet and clinopyroxene for four samples which were large enough to allow for analysis. Overall, this dataset significantly expands and complements existing datasets on diamondiferous and non‐diamondiferous xenoliths from the Slave Craton in Canada, furthering our understanding of the composition of the Slave subcratonic lithosphere. The dataset includes several samples with rare mineral assemblages, including an olivine‐bearing eclogite as well as ilmenite and apatite‐bearing garnet‐pyroxenites, and thus provides data shedding light on rarely reported compositional nuances in xenolith suites found in kimberlites.

Neodymium isotope composition and rare earth element distribution of East Antarctic continental shelf and deep waters

Circumpolar Deep Water (CDW) and Antarctic Bottom Water (AABW) play key roles in the Earth's climate system. Both water masses form critical components of the Antarctic Circumpolar Current and Meridional Overturning Circulation and therefore directly influence the large-scale redistribution of heat, nutrients and carbon. Reconstruction of past CDW transport and AABW production and export has been a key target in palaeoceanography. One promising proxy to achieve this has been the neodymium (Nd) isotope composition of seawater. The biogeochemical processes controlling Nd in the ocean, however, remain underconstrained, and modern observations of Nd isotopes in the Southern Ocean are still geographically limited.To overcome this limitation, 61 seawater samples were collected for Nd isotope and rare earth element (REE) analysis at nine stations along the Wilkes Land continental margin and in the Australian-Antarctic Basin (65°S 125°E) near East Antarctica. The results show that the different water masses have the following Nd isotope characteristics: Antarctic Surface Water (AASW), εNd = −9.0 ± 1.0 (2SD; n = 22); Modified CDW (MCDW), εNd = −8.8 ± 0.8 (2SD; n = 22); AABW, εNd = −8.3 ± 0.5 (2SD; n = 17).There is no evidence of continental REE inputs to surface waters on the Wilkes Land margin. Observed zonal variability of Nd isotope composition in AASW can be attributed to seasonal competition between the poleward flow of warm AASW from the AAG and the westward export of cold surface shelf waters by the Antarctic Slope Current.In terms of deep and bottom waters, mixing of upwelled CDW with AASW and AABW exclusively controls the Nd isotope composition of MCDW, with no indication of boundary processes modifying the Nd isotope composition of MCDW as it encroaches the shelf or slope. The regional Nd isotope signature for AABW is intermediate between published data for the Atlantic sector AABW (εNd = −9.1 ± 0.7) and Pacific sector AABW (εNd = −7.4 ± 0.9). In the absence of active convection, this Nd isotope signature can be explained by a mixture of advected AABW from the Adélie Land coast with a more dominant component of local MCDW and CDW.The data presented in this study constitute the first Nd isotope data set in this part of the Southern Ocean and suggest conservative behaviour of Nd in proximity to the old East Antarctic continent.

Roles of changes in land weathering intensity in the Nd cycle of the South China Sea during the past 30 kyr as inferred from neodymium isotopic composition in foraminifera

Neodymium isotopic compositions (εNd) in the marginal sea is influenced by the ocean circulations as well as the lithogenic input from continents, but their relative contributions and the impact of detrital sediments are not well constrained. In this study, Nd isotopic compositions of mixed planktonic foraminifera species and carbonate-free fraction of sediment from two cores collected at 1250–1350 m water depth in the southwestern (core MD01–2393) and northeastern (core MD18–3569) South China Sea (SCS) have been investigated in order to constrain the relative contributions of lithogenic Nd inputs from large Asian rivers and hydrological variations at intermediate depth in the SCS. The foraminiferal εNd values of both cores (−7.3 ± 0.2 to −5.8 ± 0.2 for core MD18–3569 and − 8.1 ± 0.2 to −7.2 ± 0.2 for core MD01–2393) indicate strong modifications of the initial εNd of the North Pacific Intermediate Water (NPIW) (εNd of −4) flowing into the SCS by unradiogenic sediments (−13 to −11) from large Asian rivers. The foraminiferal εNd record of core MD01–2393 displays significantly unradiogenic values from 18 to 8 cal kyr BP which are associated with intensification of monsoon rainfall and river input of detrital material characterized by strongly altered minerals (high illite chemical index and kaolinite/illite ratio) deriving from tropical plain soils of the Mekong River basin. We suggest here that pedogenetic minerals from tropical plain soils are more efficient in terms of Nd exchange with the seawater than primary minerals produced by intense physical erosion during the glacial period. In contrast, the foraminiferal εNd record for the northern core MD18–3569 is characterized by radiogenic εNd values from 18 to 12 cal kyr BP; these values have been linked to an increased intrusion of radiogenic glacial NPIW to the northern SCS and greater ventilation of water masses (deduced from Δ14C) in the northern deep basin of the SCS compared to the southern one. This time interval is associated with newly formed NPIW in the subarctic Pacific Ocean beginning at about 18 cal kyr BP. Overall, our results indicate that the state of chemical weathering of sediment delivered by rivers is the main factor controlling the detrital modification of past seawater εNd distributions in the marginal sea of the SCS.

Depositional age and formation conditions of Archean banded iron formations, Bundelkhand Craton, Central India: Geochemistry, neodymium isotopes and U-Pb zircon geochronology

In this study, we present major and trace element concentrations, U-Pb zircon geochronology as well as neodymium isotopic compositions of the banded iron formations (BIFs) from the Bundelkhand Craton, India. The study aims to constrain the source characteristics, depositional age and evaluate the paleoenvironmental implications of the Meso-Neoarchean seawater from which the BIF precipitated. The presence of a 2898 ± 26 Ma rounded zircon grain in the Girar BIFs indicates their maximum depositional age, which is consistent with the reported age of the metabasalts (2989 ± 190 Ma). Bearing in mind the errors, we consider the age of the studied BIFs to be ca. 2850 Ma. The BIFs from Mauranipur are associated with 2810 Ma dacite and formed at the same time. They have zircon with ages of 2718 ± 22, 2573 ± 66, 2070 ± 29 and 1934 ± 39 Ma. The BIFs from Babina are associated with 2540 Ma felsic volcanics and can be considered of the same age. Our study shows that the sedimentation of BIF in the Girar and Mauranipur greenstone belts took place in the Mesoarchean between ca. 2850 and 2810 Ma, respectively, and in the Babina greenstone belt in the Neoarchean at ca. 2540 Ma.A positive correlation between TiO2 and Zr, Al2O3 and Zr, Zr and Th/U, Y and Zr, Ni and Cr, as well as Hf and Zr, may suggest a detrital component to the BIF derived from terrigenous mafic and felsic sources. The PAAS-normalized REE patterns of the Bundelkhand BIFs are characterized by depleted LREEs, positive La, Eu, Y anomalies and Y/Ho ratios mostly ranging between 18 and 47. The studied BIFs are broadly similar to worldwide BIF occurrences at the time with the exception of the Mauranipur BIFs, which show elevated MnO concentrations (0.92 to 4.82 wt%). The studied BIF samples from the Bundelkhand Craton display a wide range of εNd(t) ranging from −6.57 to + 4.12. The broad range of isotopic values along with the above-noted geochemical signatures suggest that the BIF from the Bundelkhand Craton had significant inputs from submarine hydrothermal sources as well as pre-existing continental crust. The elevated manganese concentrations in BIFs from Mauranipur may imply the availability of free oxygen in seawater on the Bundelkhand Craton during the Mesoarchean.

Strontium and Neodymium Isotopic Signatures in Basalt Glasses of the Mid-Atlantic Ridge, 12°–31° N

This paper presents the new geochemical isotope (Sr–Nd system, H2O, Cl) data obtained for basalt glasses of the Mid-Atlantic Ridge (MAR) sampled from six areas of the MAR axial zone between 31° and 12° N. The data are consistent with the existing ideas about large-scale geochemical segmentation of the MAR. It is shown that samples from predominantly serpentinite segments have a narrower range of variations of strontium isotopic composition (87Sr/86Sr = 0.7027–0.7032) in comparison with samples collected from the areas where the crustal section is dominated by basalts (87Sr/86Sr = 0.7024–0.7041). The variation ranges of the neodymium isotopic composition in these two groups of samples are almost identical (εNd = +4.9 to +10.9 and +5.9 to +11.6 in serpentinite and basalt segments, respectively), although, in general, serpentinite segments have a slightly more enriched composition. The wide variations of the neodymium isotopic composition and increased contents of Cl, H2O, and U, as well as increased K2O/TiO2 and La/Sm ratios, in samples from serpentinites can most probably be related to the participation of different geochemically heterogeneous sources in the magmatism of the MAR axial zone. The influence of enriched plume-type matter cannot be excluded in some segments. The isotopic composition of noble gases may shed light on the subject.

Last Glacial – Holocene variability of the European Slope Current, NE Atlantic

The upper branch of the Atlantic Meridional Overturning Circulation (AMOC) in the mid-latitudes of the Northeast Atlantic remains poorly studied. This study provides a complete overview of the glacial, deglacial and Holocene dynamics of the easternmost portion of the AMOC upper branch, namely the European Slope Current (ESC) and its glacial equivalent known as the Glacial Eastern Boundary Current (GEBC). This is achieved through the study of the sediment core SU81–44 retrieved from the southern Bay of Biscay (BoB) (∼1000 m water depth), and by using a multiproxy approach (i.e. benthic foraminiferal assemblage, grain size proxies, oxygen and carbon stable isotopes, and neodymium isotopic composition of foraminiferal shells). During the glacial period and the onset of the deglaciation, the grain size proxies at SU81–44 fluctuate significantly. These fluctuations are coherent with changes in relative densities of benthic foraminiferal indicator species of current strength and ventilation, thus highlighting significant changes in the GEBC vigor through time. The SU81–44 data confirm the Dansgaard-Oeschger interstadial/faster-stadial/slower flow pattern previously observed in northern BoB. Our results also provide new constraints on the strength of the slope current during the late deglaciation and Holocene period with a significant reinvigoration of the ESC, and by extension the upper branch of the AMOC during the Bølling-Allerød warming. This seems to confirm the crucial role of the ESC in deep water formation at high northern latitudes, as it is the case today. Finally, our data show a progressive weakening of the ESC during the Holocene. We hypothesize a link with a long-term decrease in the density gradient between low and high latitudes that can be attributed to long term changes in insolation and the strength of the subpolar gyre dynamics.

Sr and Nd Isotopes in Mineral Fractions of Ferromanganese Crusts from the Northernmost Pacific

A study of the isotopic compositions of neodymium and strontium in four mineral fractions of the hydrogenous ferromanganese crusts of the northernmost Pacific has been carried out for the first time. The relationship of the 87Sr/86Sr ratio and εNd value of the residual fraction in heterochronous layers of the ferromanganese crusts have shown that the sources of detrital matter are from rock weathering by the Yukon River, the Kuskokwim River, the Anadyr River and the rivers of Kamchatka. The amount of aluminosilicate impurity does not affect the isotopic compositions of the chemogenic (loosely bound, Mn oxides or manganese, hydrous Fe oxides or ferrous) fractions. The decreased εNd value in the ferrous fraction seems to be related to the presence of rare earth element (REE) phosphate complexes in seawater. The increase in those complexes is a consequence of melt waters entering the ocean during warming periods. Our data indicate that the carbonate REE complexes are not the dominant form of the REEs in the northernmost Pacific, which leads to the fact that the neodymium isotopic compositions of the manganese and ferrous fractions of hydrogenous ferromanganese crusts are different. A decrease in εNd value in the ferrous fraction may be a marker of a local increase in the surface water bioproductivity. Our data also show that the increase of the REE in the ferrous fraction relative to the manganese fraction does not reflect the participation of hydrothermal matter in the formation of mixed hydrogenous–hydrothermal crusts but is a consequence of an increase in the REE phosphate complexes.

The spatial and temporal distribution of neodymium isotopic composition within the Rockall Trough

Rare Earth Element (REE) concentrations and Nd isotopic compositions (εNd) of five new seawater profiles scattered across the Rockall Trough (North-East Atlantic) were investigated in order: (1) to establish the seawater εNd distribution in the Rockall Trough, which is poorly known so far; (2) to identify the southward-flowing Wyville-Thompson Ridge Overflow Water (WTOW) originating from the Nordic Seas; (3) to evaluate the potential impact of lithogenic sources on the seawater εNd in the Rockall Trough. Combined with two previously published seawater stations, the results reveal a spatial εNd gradient in the upper layer (roughly the first 900 m) of the Rockall Trough with εNd values ranging from −14.9 ± 0.3 to −11.6 ± 0.2. The influence of Nd lithogenic inputs through seawater-sediment interactions have been found to be likely negligible, so the spatial gradient has been attributed to the relative influence of unradiogenic Subpolar Gyre (SPG) water carried by the North Atlantic Current (NAC) and more radiogenic water derived from the Subtropical Gyre (STG) flowing into the Shelf Edge Current (SEC) along the European margin. Freshwater from the interior seas (Northern UK) is an additional source of radiogenic εNd, although it seems restricted to the shallow continental shelf. Below the upper layer, WTOW has been characterized for the first time with εNd (mean value of −13.8 ± 0.5; n = 9) as a homogeneous intermediate water body between 600 and 1200 m. The results reveal a strong modification of the outflowing water from the Norwegian Sea that cascades through the Wyville-Thompson ridge, as WTOW in the Rockall Trough is composed of more than 80 % Atlantic water. On the western flank of the Rockall Trough, a deep WTOW component has been identified that is responsible for a modification of the Labrador Sea Water (LSW) εNd values (-13.0 ± 0.3 to −12.1 ± 0.3). Two stations separated by only a few kilometers from each other but sampled four years apart (2012 and 2016) show a multi-annual variation of water properties and εNd values (∼1 ε-unit) for the first 800 m, pointing to a change in the relative contribution of both SPG and STG to the composition of water flowing in the Rockall Trough to the Nordic Seas. Overall, these results confirm that εNd can be used to assess the water mass composition in the Rockall Trough, allowing to reconstruct North Atlantic gyre dynamics in paleoceanographic studies.

Continental weathering as the source of iron in Jurassic iron oolites from Switzerland

Iron is extremely insoluble in oxic seawater. The lack of a large aqueous reservoir means that sediments rich in authigenic iron are rare in the modern ocean. In the Middle Jurassic, however, condensed iron-rich sedimentary rocks are widely distributed. Their formation coincides with increased volcanic activity and continental weathering related to the breakup of Pangea, suggesting iron supply through one of these processes. We studied three Swiss shallow-marine iron oolites from Herznach, Windgällen and Erzegg, all from condensed sedimentary sequences of Middle to early Late Jurassic age, to constrain the source of iron to these rocks, combining radiogenic neodymium, strontium and stable iron isotope analyses. Leached authigenic neodymium isotope compositions, which appear to preserve the primary signature, serve as a tracer for the potential involvement of hydrothermal fluids in the formation of the iron oolites. The three iron oolite successions yield crustal Nd isotope compositions (εNd between − 9 and − 7), providing no evidence for the involvement of such fluids. It is, thus, more likely that iron in the sediments derived from detrital fluvial inputs. Strontium isotope compositions, which could potentially support these findings, point to metamorphic overprinting associated with Alpine thrusting. The light iron isotope signatures associated with Middle to early Late Jurassic condensed sequences, δ56Fe between − 1.49 and − 0.57‰, suggest that microbially-mediated iron reduction was also involved in generating these sediments.

Simulating marine neodymium isotope distributions using Nd v1.0 coupled to the ocean component of the FAMOUS–MOSES1 climate model: sensitivities to reversible scavenging efficiency and benthic source distributions

Abstract. The neodymium (Nd) isotopic composition of seawater is a widely used ocean circulation tracer. However, uncertainty in quantifying the global ocean Nd budget, particularly constraining elusive non-conservative processes, remains a major challenge. A substantial increase in modern seawater Nd measurements from the GEOTRACES programme, coupled with recent hypotheses that a seafloor-wide benthic Nd flux to the ocean may govern global Nd isotope distributions (εNd), presents an opportunity to develop a new scheme specifically designed to test these paradigms. Here, we present the implementation of Nd isotopes (143Nd and 144Nd) into the ocean component of the FAMOUS coupled atmosphere–ocean general circulation model (Nd v1.0), a tool which can be widely used for simulating complex feedbacks between different Earth system processes on decadal to multi-millennial timescales. Using an equilibrium pre-industrial simulation tuned to represent the large-scale Atlantic Ocean circulation, we perform a series of sensitivity tests evaluating the new Nd isotope scheme. We investigate how Nd source and sink and cycling parameters govern global marine εNd distributions and provide an updated compilation of 6048 Nd concentrations and 3278 εNd measurements to assess model performance. Our findings support the notions that reversible scavenging is a key process for enhancing the Atlantic–Pacific basinal εNd gradient and is capable of driving the observed increase in Nd concentration along the global circulation pathway. A benthic flux represents a major source of Nd to the deep ocean. However, model–data disparities in the North Pacific highlight that under a uniform benthic flux, the source of εNd from seafloor sediments is too non-radiogenic in our model to be able to accurately represent seawater measurements. Additionally, model–data mismatch in the northern North Atlantic alludes to the possibility of preferential contributions from “reactive” non-radiogenic detrital sediments. The new Nd isotope scheme forms an excellent tool for exploring global marine Nd cycling and the interplay between climatic and oceanographic conditions under both modern and palaeoceanographic contexts.

Weakening of the South Asian summer monsoon linked to interhemispheric ice-sheet growth since 12 Ma

The evolution and driving mechanism of the South Asian summer monsoon (SASM) are still poorly understood. We here present a 12-Myr long SASM record by analyzing the strontium and neodymium isotopic composition of detrital components at IODP Exp. 359 Site U1467 from the northern Indian Ocean. The provenance investigation demonstrates that more dust enriched in εNd from northeastern Africa and the Arabian Peninsula was transported to the study site by monsoonal and Shamal winds during the summer monsoon season. A two-step weakening of the SASM wind since ~12 Ma is proposed based on the εNd record. This observational phenomenon is supported by climate modeling results, demonstrating that the SASM evolution was mainly controlled by variations in the gradient between the Mascarene High and the Indian Low, associated with meridional shifts of the Hadley Cell and the Intertropical Convergence Zone, which were caused by interhemispheric ice-sheet growth since the Middle Miocene.

Multi-elemental and Strontium-Neodymium Isotopic Signatures in Charred Wood: Potential for Wood Provenance Studies

Abstract The chemical composition of the wood reflects the composition of the soil over which the corresponding tree has developed. Multi-elemental and isotopic signatures, which are characteristic of the soil and underlying rock substrates, are potentially powerful tools for determining wood provenance. These tracers are of special interest for charred archaeological wood because they circumvent some limitations of dendrochronological provenancing linked to tree-ring loss. However, thermal degradation may introduce a significant bias in wood chemical and isotopic analyses. This experimental study focused on the effects of carbonization temperature on three geochemical wood markers: elemental signatures and isotopic signatures of strontium and neodymium (86Sr/87Sr and 143Nd/144Nd, respectively). Wood specimens from a variety of oak trees and stand locations were pyrolyzed at four temperatures (ranging from 200°C to 800°C) and analyzed using ICP-MS and μ-XRF (X-ray fluorescence) spectroscopy for elemental composition and with multiple collection ICP-MS (MC-ICP-MS) for strontium (Sr) and neodymium (Nd) isotope composition. The concentration of mineral nutrients generally increased with temperature, but the magnitude of the enrichment depended on the element, wood compartment (sapwood vs. heartwood), and geological substrate. The concentrations of rubidium, strontium, manganese, magnesium, potassium, and, to a lesser extent, calcium, were minimally affected by temperature, wood compartment, and substrate. The ratios between the concentrations of these elements, as well as the 86Sr/87Sr and 143Nd/144Nd isotope ratios, were stable over the entire temperature range. However, only 86Sr/87Sr and selected elemental ratios (calcium or magnesium normalized to manganese) were successful for site discrimination. Therefore, our multi-tracer approach provides promising new information to determine the provenance of charred archaeological wood.

Neodymium isotopes in peat reveal past local environmental disturbances

Over the past decade, the neodymium (Nd) isotope composition of mineral matter from peat cores has seen increasingly common use as a tracer of dust influx associated with major changes in the Holocene atmospheric circulation. However, the incomplete understanding of the local controls on the sources of the sediment supplied to peatlands remains a key difficulty in the interpretation of the archived Nd isotope signals. Here, we used neodymium isotopes to reconstruct environmental disturbances in peatlands. We performed a multi-proxy study of two peatlands that experienced peatland burning and validated the recorded peat Nd signatures using reference surface sampling. Our data show a link between the Nd isotope signals and local environmental disturbances: peat burning, local fire activity and pollution fluxes. Our study illustrates the crucial role of identifying local events that influence the supply of mineral material to peatlands. Insufficient recognition of such local controls may either obscure the large-scale variations in the atmospheric circulation patterns, or introduce artefacts to the Holocene climate record. We also provide recommendations for the use of Nd isotopes in palaeoecological studies of peatlands.

Enhanced weathering input from South Asia to the Indian Ocean since the late Eocene

Enhanced silicate weathering induced by the uplift of the Himalayan-Tibetan Plateau (HTP) has been considered as the major cause of pCO2 decline and Cenozoic cooling. However, this hypothesis remains to be validated, largely due to the lack of a reliable reconstruction of the HTP weathering flux. Here, we present a 37-million-year record of the difference in the seawater radiogenic neodymium isotopic composition (ΔεNd) of Ocean Drilling Program (ODP) sites and Fe-Mn crusts between the northern and central Indian Ocean, which indicates the contribution of regional weathering input from the South Asian continent to the Indian Ocean. The results show a long-term increase in ΔεNd and thus provide the first critical evidence of enhanced South Asian weathering input since the late Eocene. The evolution coincided well with major pulses of surface uplift in the HTP and global climatic transitions. Our foraminiferal εNd record suggests that tectonic uplift and silicate weathering in South Asia, especially in the Himalayas, might have played a significant role in the late Cenozoic cooling.

The late Cenozoic evolution of the Humboldt Current System in coastal Peru: Insights from neodymium isotopes

The Humboldt Current System along the Pacific coast of South America creates one of the most productive ecosystems on Earth. To trace the origin of the water masses in this area, we measured neodymium isotope compositions (ԑNd) in tooth enameloid of two genera of coastal sharks from latest Oligocene to early Pleistocene strata in the Pisco and Sacaco basins in southern Peru. Most ԑNd values range from −4 to −1, with a strong negative excursion in the late Miocene (∼8–7 million years ago [Ma]) with values as low as −9.2. The overall trend of the ԑNd values resembles that of equatorial Pacific deep waters, though with an offset of about +2 ԑNd units until about 6 Ma. With a major input of hinterland weathering considered unlikely, we interpret this pattern as reflecting a modern-type upwelling regime, though with a lower contribution of Antarctic waters than today. Starting about 6 Ma, the contribution of Antarctic waters to the upwelling waters increased approximately to present-day levels, coincident with, and possibly driven by, increased Antarctic glaciation and the Andes reaching their present-day elevation, both of which likely enhanced the counter-clockwise circulation in the South Pacific Ocean. The negative excursion of ԑNd values in the Pisco/Sacaco basins ∼8–7 Ma coincides with a late Miocene biogenic bloom in the Pacific Ocean and elsewhere, and with a strongly increased northward bottom current observed on the Nazca Drift System just offshore our sampling area. Thus, the negative excursion of ԑNd values in the Pisco/Sacaco basins likely resulted from a southern sourced input of nutrient-rich, unradiogenic water, which could have been an important contributor to the biogenic bloom.

Determining provenance of uranium ore concentrates using 143Nd/144Nd

The neodymium (Nd) isotopic composition—particularly 143Nd/144Nd—of a material has long been a source of information utilized in the fields of geochemistry and cosmochemistry due to the radiogenic nature of 143Nd. Uranium ore concentrates (UOCs) often contain Nd in measurable quantities, meaning 143Nd/144Nd can also be employed to determine the provenance of regulated nuclear material early in the fuel cycle. Here, we use an improved chemical separation technique and plasma-source multi-collector mass spectrometry to investigate the 143Nd/144Nd compositions of 28 UOC samples. We show that these UOCs exhibit highly variable 143Nd/144Nd, likely reflecting the geology of the source ore deposit. As such, the 143Nd/144Nd represents a powerful comparative signature in a nuclear forensics investigation involving UOC outside of regulatory control.

Reactive-transport modeling of neodymium and its radiogenic isotope in deep-sea sediments: The roles of authigenesis, marine silicate weathering and reverse weathering

Dissolved Rare Earth Elements (REE) and radiogenic neodymium (Nd) isotope composition (εNd) of seawater are widely used geochemical tools in studying marine processes, but their modern ocean budgets are poorly understood. Recent discoveries of large benthic fluxes of REE with unique εNd signatures from marine sediments, particularly in the deep-sea, have led to a “bottom-up” hypothesis, which suggests that early diagenesis below the sediment-water interface (SWI) controls the ocean's REE and εNd budgets. To investigate such sedimentary processes, we created a reactive-transport model for the biogeochemical cycling of Nd and εNd in marine sediments. Here, we attempt to quantify the roles of authigenesis, marine silicate weathering and reverse weathering in the diagenetic cycling of Nd and εNd at a deep-sea (3000 m) site on the Oregon margin.Our model predicts that, at this site, Nd carried by Fe/Mn oxides into sediments eventually transforms to authigenic Nd-phosphate, during which ∼9% of the incoming solid Nd flux is released as a dissolved benthic flux back to the overlying bottom water. We also find that the classic reversible scavenging formulation applied to Nd co-cycling with Fe/Mn oxides is inconsistent with the data. Rather, a co-precipitation formulation, assuming Nd is structurally incorporated into Fe/Mn oxides, successfully simulates the data. The model also shows that authigenesis alone cannot explain the pore water and authigenic εNd, which are both more radiogenic than bottom water at this site. However, the weathering of volcanic silicates sourced from the local subduction zone can successfully explain εNd. We suggest that, because reverse weathering by authigenic clay formation maintains the under-saturation of primary silicates in pore water, marine silicate weathering can proceed. The processes we model likely affect the sedimentary cycling of many other trace elements and isotopes, with much broader implications for the understanding of ocean biogeochemistry.

Evolution of rare earth element and εNd compositions of Gulf of Mexico seawater during interaction with Mississippi River sediment

A closed-system batch reaction experiment was conducted for 270 days to evaluate the effects of interaction between Gulf of Mexico (GOM) seawater and Mississippi River sediments on the system’s dissolved rare earth elements (REE) concentrations and neodymium isotopic compositions (εNd). This study specifically focuses on geochemical reactions involving silicic sediments derived from weathering of the North American continent as they affect the REEs and εNd of seawater along continental margins, in contrast to previous studies that investigated the influence of basaltic rocks and sediments on REEs and εNd in the ocean. Our results show that the dissolution of labile phases of lithogenic Mississippi River sediments leads to an approximately 100-fold increase in dissolved REE concentrations within the first 33 days of the experiment. Secondary mineral precipitation appears to lower the REE concentrations between days 33 and 270 of the experiment, although seawater REE concentrations remain elevated compared to initial values. The two-way elemental transfer involving dissolution and precipitation results in a net increase by a factor of 24 ± 12 (mean ± 1σ) in the dissolved REE concentrations by the end of the experiment (i.e., day 270). The dissolved REE concentration maxima observed after 33 days of the experiment represent the mobilization of approximately 0.37 % of the REE content of the operationally defined “exchangeable” fraction of the riverine sediments. The εNd values of the reactive lithogenic components were −9.77 and −9.95, which are similar to the GOM value of −9.81 ± 0.36. Because of the similarity between εNd values, changes in the seawater Nd isotope value throughout the experiment were subtle (mean ± std, reacted seawater εNd of −9.87 ± 0.17). The highest REE concentrations coincided with the most radiogenic εNd (-9.65 ± 0.23; day 33), which suggests that REE concentrations and εNd compositions of the GOM may be buffered by fluxes from sediments in the system. Our results are comparable to previous studies involving basaltic rocks and/or sediments of basaltic composition in that they demonstrate that silicic, river sediments are highly reactive in marine environments with regard to REE mobilization. The experimental results further suggest that “boundary exchange” plays an important role in influencing the εNd of seawater along continental margins dominated by large river systems, although the impacts of boundary exchange will be most profound where ambient seawater and river sediments have distinct Nd isotopic compositions (e.g., basaltic, or Precambrian shield material). Finally, our results indicate that the εNd value of GOM seawater is largely controlled by the lithogenic sediment delivered to the basin by the Mississippi River.

Early diagenetic control on the enrichment and fractionation of rare earth elements in deep-sea sediments.

The rare earth elements and yttrium (REY) in bioapatite from deep-sea sediments are potential proxies for reconstructing paleoenvironmental conditions. However, the REY enrichment mechanism and the reliability of this tracer remain elusive because of the lack of key information from ambient pore water. Here, we report high-resolution geochemical data for pore water, bottom water, and bioapatite from deep-sea sites in the western Pacific. Our results reveal that the benthic flux of REY from the deep sea is less substantial than from the shallow marine realm, resulting in REY-rich sediment. The depth distribution of REY in pore water is opposite to that of bioapatite, and REY patterns and neodymium isotopic compositions are not uniformly distributed within bioapatite. These results indicate alteration of REY and neodymium isotopic compositions during early diagenesis. Therefore, we infer that REY from bioapatite are not robust recorders of the deep marine environment through Earth’s history.