RATIONALE:Neodymium isotopes are a powerful geochemical tool that has widely been used in terrestrial and extraterrestrial studies. Modern multicollector inductively coupled plasma mass spectrometers (MC-ICP-MS) allow fast, accurate, and precise analysis of the radiogenic Nd isotope ratio 143Nd/144Nd. These analyses comprise relatively high instrumental mass bias that is typically corrected for using the stable 146Nd/144Nd of 0.7219 and an exponential law. The instrument is usually tuned to optimize the operating conditions for isotope analysis, but this tuning is a trade-off primarily between signal intensity, stability, and accuracy. Alternative, more effective approaches for mass bias correction have been proposed that use 145Nd/142Nd as normalizing ratio. However, one drawback of using this ratio is that the efficient removal of Ce from Nd is required to minimize the effect of isobaric interference of 142Ce on 142Nd. METHODS:Here, we analyzed international Nd and rock reference materials using a Thermo Scientific Neptune Plus MC-ICP-MS to evaluate the sensitivity of 145Nd/142Nd-based mass bias correction to varying Ce/Nd and in comparison with the commonly used 146Nd/144Nd-based correction. RESULTS:Our results show that the corrected 143Nd/144Nd of Ce-doped JNdi-1 and Ce-containing USGS BCR-2, NOD-A-1, and NOD-P-1 reference materials are insensitive to Ce/Nd of up ~1. CONCLUSIONS:The correction of instrumental mass bias with 145Nd/142Nd as a normalizing ratio yields, as previously suggested, improved trueness and precision of 143Nd/144Nd data in comparison with 146Nd/144Nd-based corrections, even under high Ce/Nd of to ~1. This allows improved optimization of signal intensity during instrument tuning, which is particularly useful for natural samples with low Nd content.

The isotope composition of dissolved neodymium (Nd) in seawater, expressed as εNd, serves as a valuable geochemical tracer for the provenance and mixing of water masses in the ocean. However, challenges arise in the application due to additional Nd sources and isotope exchange at ocean boundaries and suspended particles of hydrothermal and lithogenic origin, which are not fully understood. In this study, we present seawater Nd concentrations [Nd] and εNd obtained from a deep-water (>1500 m water depth) zonal transect across the oligotrophic South Pacific gyre sampled during R/V Sonne cruise SO245, GEOTRACES process study GPpr09. Across 8 stations, dissolved [Nd] range from 10.8 to 31.1 pmol/kg with a characteristic pattern of increasing [Nd] at all stations, while our εNd results from -3.9 to -8.5 show pronounced zonal differences between the eastern and western part of the transect. We compare our measured data with values predicted from optimum multiparameter analysis (OMPA) of water masses to quantify the non-conservative influence on εNd and [Nd] along our transect. Our detailed analysis demonstrates that the deep water εNd distribution is dominated by conservative mixing of Upper and Lower Circumpolar Deep Water, Pacific Deep Water and Antarctic Intermediate Water, with 66% of all samples falling within ±0.6 epsilon units (i.e., the propagated uncertainty range derived from endmember compositions and analytical uncertainty) of the εNd predicted from conservative water mass mixing. In contrast, our [Nd] results reveal a pronounced deficit relative to concentrations predicted from water mass mixing (up to ∼10 pmol/kg). By excluding other influencing factors such as surface particle rain and boundary scavenging, we ascribe the [Nd] deficit to enhanced scavenging by hydrothermal particles in particular in PDW. This is consistent with the striking similarity of the [Nd] deficit and the distribution of δ3He as an indicator of hydrothermal activity along our transect. These findings highlight the decoupling of [Nd], modulated heavily by non-conservative processes, from the mainly conservative behaviour of εNd in the deep South Pacific.

Abstract Airborne mineral dust is sensitive to climatic changes, but its response to orbital forcing is still not fully understood. Here, we present a reconstruction of dust input to the Subarctic Pacific Ocean covering the past 190 kyr. The dust composition record is indicative of source moisture conditions, which were dominated by precessional variations. In contrast, the dust flux record is dominated by obliquity variations and displays an out‐of‐phase relationship with a dust record from the mid‐latitude North Pacific Ocean. Climate model simulations suggest precession likely drove changes in the aridity and extent of dust source regions. Additionally, the obliquity variations in dust flux can be explained by meridional shifts in the North Pacific westerly jet, driven by changes in the meridional atmospheric temperature gradient. Overall, our findings suggest that North Pacific dust input was primarily modulated by orbital‐controlled source aridity and the strength and position of the westerly winds.

The investigation of triggers causing the onset and intensification of Northern Hemisphere Glaciation (NHG) during the late Pliocene is essential for understanding the global climate system, with important implications for projecting future climate changes. Despite their critical roles in the global climate system, influences of land-ocean interactions on high-latitude ice sheets remain largely unexplored. Here, we present a high-resolution Asian dust record from Ocean Drilling Program Site 1208 in the North Pacific, which lies along the main route of the westerlies. Our data indicate that atmosphere-land-ocean interactions affected aeolian dust emissions through modulating moisture and vegetation in dust source regions, highlighting a critical role of terrestrial systems in initiating the NHG as early as 3.6 Myr ago. Combined with additional multi-proxy and model results, we further show that westerly wind strength was enhanced, mainly at low-to-middle tropospheric levels, during major glacial events at about 3.3 and 2.7 Myr ago. We suggest that coupled responses of Earth’s surface dynamics and atmospheric circulation in the Plio-Pleistocene likely involved feedbacks related to changes in paleogeography, ocean circulation, and global climate. North Pacific dust flux and modelling results provide new evidence for long-term land-atmosphere-ocean interactions associated with the onset and intensification of the Northern Hemisphere Glaciation.

Neodymium (Nd) isotopes have been applied for decades now to trace ocean circulation both in the present and past oceans. Their tracer utility stems from the characteristic Nd isotope signature of different rocks and their imprint on seawater as well as the biological inactivity of Nd and its appropriate residence time in the ocean, allowing for the determination of water mass provenance and flow paths. However, the application of this tracer, especially for the reconstruction of past ocean circulation changes, has been challenged based on uncertainties e. g. in the magnitude of the benthic flux of Nd to deep waters, Nd isotope exchange and input at ocean margins, and diagenetic alterations of the original bottom water Nd isotope signature in sediments.Based on recent studies of dissolved Nd isotope distributions in surface to deep waters we show the power of Nd isotopes for tracing the provenance of currents and water masses particularly within restricted geographic regions. Using additional trace metal and isotope data from marine sediments analyzed alongside authigenic Nd isotopes, we explore the validity and limits of Nd isotopes as tracer of past ocean circulation changes.

Mineral dust is an important component of the Earth system due to its role in oceanic nutrient supply, cloud formation and its radiative properties. Changes in transport pathways and fluxes of mineral dust have attracted increased attention using radiogenic isotope analysis for detailed investigation of changing dust sources through time. However, multi‐isotope studies provide complex datasets of dust provenance, often without exact quantification of source contributions. Here we use Bayesian mixing models and existing radiogenic isotope data to quantify changes in South Pacific dust provenance for the Holocene and the Last Glacial Maximum (LGM; ∼18–24 ka BP). Testing different model configurations showed grouping small source regions to single continental scale end members prior to modeling can lead to biased results, and so we group model outputs post‐modeling. During the LGM, a higher proportion (mean 53%) of dust entering the South Pacific was South American in origin, compared to a Holocene mean of 31%. In contrast, Australian dust contributions were lower during the LGM (mean 38%) than Holocene (mean 55%), with significant spatial gradients for both time slices. In the subpolar South Pacific, the high representation of South American dust during the LGM (up to ∼75%) coincides with larger dust particles; together indicating that far‐traveled dust transport was facilitated by long atmospheric residence times and an accelerated westerly wind circulation during this time. Our study shows how Bayesian mixing models provide valuable constraints for dust source contributions, an approach which may help in the calibration of atmospheric models, using complex isotopic datasets.

The interoceanic exchange of water masses is modulated by flow through key oceanic choke points in the Drake Passage, the Indonesian Seas, south of Africa, and south of Tasmania. Here, we use the neodymium isotope signature (εNd) of cold-water coral skeletons from intermediate depths (1460‒1689 m) to trace circulation changes south of Tasmania during the last glacial period. The key feature of our dataset is a long-term trend towards radiogenic εNd values of ~−4.6 during the Last Glacial Maximum and Heinrich Stadial 1, which are clearly distinct from contemporaneous Southern Ocean εNd of ~−7. When combined with previously published radiocarbon data from the same corals, our results indicate that a unique radiogenic and young water mass was present during this time. This scenario can be explained by a more vigorous Pacific overturning circulation that supported a deeper outflow of Pacific waters, including North Pacific Intermediate Water, through the Tasman Sea.

The input of the soluble micronutrients iron (Fe) and/or manganese (Mn) by mineral dust stimulates net primary productivity in the Fe/Mn-deficient Southern Ocean. This mechanism is thought to increase carbon export, thus reducing atmospheric CO 2 during the Pleistocene glacial cycles. Yet, relatively little is known about changes in the sources and transport pathways of Southern Hemisphere dust over glacial cycles. Here, we use the geochemical fingerprint of the dust fraction in marine sediments and multiisotope mixture modeling to identify changes in dust transport to the South Pacific Subantarctic Zone (SAZ). Our data show that dust from South America dominated the South Pacific SAZ during most of the last 260,000 a with maximum contributions of up to ∼70% in the early part of the glacial cycles. The enhanced dust-Fe fluxes of the latter parts of the glacial cycles show increased contributions from Australia and New Zealand, but South American dust remains the dominant component. The systematic changes in dust provenance correspond with grain size variations, consistent with the circumpolar transport of dust by the westerly winds. Maximum contributions of dust from more proximal sources in Australia and New Zealand (up to ∼63%) paired with a finer dust grain size indicate reduced westerly wind speeds over the South Pacific SAZ during deglacial and peak interglacial intervals. These quantitative dust provenance changes provide source-specific dust-Fe fluxes in the South Pacific SAZ and show how their systematic changes in magnitude and timing influence the Southern Ocean dust-Fe feedback on glacial-interglacial to millennial time scales.

Dataset S2 from "Spatial and temporal trends in mineral dust provenance in the South Pacific – evidence from mixing models", by Jack Longman, Torben Struve, and Katharina Pahnke. Paleoceanography and PaleoclimatologyModel outputs of individual Australian and South American dust sources for open ocean locations (see original publication for details).

The sequence of deep ocean circulation changes between the Last Glacial Maximum and the Holocene provides important insights for understanding deglacial climate change and the role of the deep ocean in the global carbon cycle. Although it is known that significant amounts of carbon were sequestered in a deep overturning cell during glacial periods and released during deglaciation, the driving mechanisms for these changes remain unresolved. Southern Ocean sea-ice has recently been proposed to play a critical role in setting the global deep ocean stratification and circulation, and hence carbon storage, but testing such conceptual and modelling studies requires data constraining past circulation changes. To this end, we present the first deglacial dataset of neodymium (Nd) isotopes measured on absolute-dated deep-sea corals from modern Lower Circumpolar Deep Water depths in the Drake Passage. Our record demonstrates deglacial variability of 2.5 epsilon(Nd )units, with radiogenic values of up to epsilon(Nd) = -5.9 during the Last Glacial Maximum providing evidence for a stratified glacial circulation mode with restricted incorporation of Nd from North Atlantic Deep Water in the lower cell. During the deglaciation, a renewed Atlantic influence in the deep Southern Ocean is recorded early in Heinrich Stadial 1, coincident with Antarctic sea-ice retreat, and is followed by a brief return to more Pacific-like values during the Antarctic Cold Reversal. These changes demonstrate a strong influence of Southern Ocean processes in setting deep ocean circulation and support the proposed sea-ice control on deep ocean structure. Furthermore, by constraining the Nd isotopic composition of Lower Circumpolar Deep Water in the Southern Ocean, our new data are important for interpreting deglacial circulation changes in other ocean basins and support a spatially asynchronous return of North Atlantic Deep Water to the deep southeast and southwest Atlantic Ocean. (C) 2020 Elsevier B.V. All rights reserved.

The response of the Subantarctic Pacific to climate change: Reconstructing dust flux and biological productivity during the last glacial cycle GISELA WINCKLER, ROBERT F ANDERSON, JIWOON PARK, ROSEANNE SCHWARZ, JENNIFER LAMP, ELIZABETH SHOENFELT, KATHARINA PAHNKE, TORBEN STRUVE, GERHARD KUHN, MARC WENGLER, FRANK LAMY 3 11 Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, United States

The increased flux of soluble iron (Fe) to the Fe-deficient Southern Ocean by atmospheric dust is considered to have stimulated the net primary production and carbon export, thus promoting atmospheric CO2 drawdown during glacial periods. Yet, little is known about the sources and transport pathways of Southern Hemisphere dust during the Last Glacial Maximum (LGM). Here we show that Central South America (~24‒32°S) contributed up to ~80% of the dust deposition in the South Pacific Subantarctic Zone via efficient circum-Antarctic dust transport during the LGM, whereas the Antarctic Zone was dominated by dust from Australia. This pattern is in contrast to the modern/Holocene pattern, when South Pacific dust fluxes are thought to be primarily supported by Australian sources. Our findings reveal that in the glacial Southern Ocean, Fe fertilization critically relies on the dynamic interaction of changes in dust-Fe sources in Central South America with the circumpolar westerly wind system.

Neodymium (Nd) isotopes are a powerful proxy tool for reconstructing past changes in water mass mixing, but reliable application of this proxy requires constraints on past changes of source water compositions. A key region of the global deep water circulation system are the Nordic Seas, which provide dense waters fundamental for the formation of North Atlantic Deep Water (NADW). Yet, the Nd isotope evolution of past deep waters in the Nordic Seas is so far poorly constrained. Here we present the first reconstructions of seawater Nd isotope compositions extracted from marine sediments at two locations in the central and northern Nordic Seas covering the period from the last glacial to the present. Further insights into past changes in sediment provenance, weathering inputs and water mass mixing are provided by complementary seawater and detrital strontium (Sr) and lead (Pb) isotope compositions. Our new data reveal that changes in source and magnitude of weathering inputs from the Scandinavian and Svalbard-Barents Ice Sheets (SIS and SBIS, respectively) controlled the Nd and Pb isotope composition of the Nordic Seas' deep waters during the last glacial period. During the Last Glacial Maximum (LGM), deep waters showed distinctly unradiogenic Nd and radiogenic Pb isotope signatures most likely driven by weathering inputs of the SBIS. In contrast, the deglaciation was characterized by enhanced SBIS ice dynamics and/or meltwater release delivering sediments from the distal Eurasian shelves to the Norwegian Sea. Pulses of volcanogenic sediment supply changed the deep water Nd isotope composition during Heinrich Stadials 1, 2 and the Bolling period. As such, the glacial-deglacial Nd and Pb isotope evolution was markedly different in the Nordic Seas, the North Atlantic and the Arctic Oceans, respectively. During the Holocene, Pb isotopes indicate synchronized weathering fluxes around the North Atlantic, whereas the influence of local weathering input on the Nd isotope evolution of the deep Nordic Seas ceased. Instead, the Holocene Nd isotope signal has been driven by the strength of the advection/convection of water masses in the study area. These new constraints on changes in the Nordic Seas provide important endmember information for Nd isotope based reconstructions of NADW export downstream in the Atlantic Ocean. (C) 2019 Elsevier B.V. All rights reserved.

We present the first comprehensive dust provenance and lithogenic flux data set extracted from surface sediments distributed across the polar and subpolar South Pacific. The dataset comprises Th-230-normalized lithogenic fluxes combined with rare earth elements, strontium, neodymium and lead isotope data to determine lithogenic fluxes, spatial distribution and sources of dust (<10 mu m) during the Holocene. We observe the highest lithogenic fluxes in surface sediments near New Zealand which most likely indicate a combination of Australian dust and riverine input from New Zealand. Similarly, high fluxes off West Antarctica are ascribed to sediment input from the nearby continental shelf and margin. In contrast, the lithogenic fluxes in the remote open ocean are derived from dust input. Isotopic and REE data suggest that the Lake Eyre Basin in central Australia is the most prominent potential source area for dust distributed over the South Pacific during the Holocene, with additional input from the Darling Basin in southeast Australia. (C) 2019 Elsevier Ltd. All rights reserved.

Significance Southern Ocean circulation is a central aspect of the climate system influenced by the overlying Southern Hemisphere westerly winds (SWW) and surface buoyancy forcing. Yet, the response of Southern Ocean water mass mixing to SWW changes is largely unconstrained for the Holocene (the last ∼11,700 y). We extracted the fingerprint of ocean chemistry from Drake Passage cold-water corals to trace past water mass mixing. Our data suggest that poleward weakening of the SWW in the Middle Holocene led to increased admixture of CO 2 -rich Pacific-derived water masses into the Southern Ocean. These results indicate that the Holocene circulation reacts more sensitively to atmospheric forcing than previously appreciated, thus providing insight into the Southern Ocean’s possible role during future climate change.

Cold-water corals (CWCs) are unique archives of mid-depth ocean chemistry and have been used successfully to reconstruct the neodymium (Nd) isotopic composition of seawater from a number of species. High and variable Nd concentrations in fossil corals however pose the question as to how Nd is incorporated into their skeletons. We here present new results on modern specimens of Desmophyllum dianthus, Balanophyllia malouinensis, and Flabellum curvatum, collected from the Drake Passage, and Madrepora oculata, collected from the North Atlantic. All modern individuals were either collected alive or uranium-series dated to be < 500 years old for comparison with local surface sediments and seawater profiles. Modern coral Nd isotopic compositions generally agree with ambient seawater values, which in turn are consistent with previously published seawater analyses, supporting small vertical and lateral Nd isotope gradients in modern Drake Passage waters. Two Balanophyllia malouinensis specimens collected live however deviate by up to 0.6 epsilon units from ambient seawater. We therefore recommend that this species should be treated with caution for the reconstruction of past seawater Nd isotopic compositions. Seventy fossil Drake Passage CWCs were furthermore analysed for their Nd concentrations, revealing a large range from 7.3 to 964.5 ng/g. Samples of the species D. dianthus and Caryophyllia spp. show minor covariation of Nd with 232Th content, utilised to monitor contaminant phases in cleaned coral aragonite. Strong covariations between Nd and Th concentrations are however observed in the species B. malouinensis and G. antarctica. In order to better constrain the source and nature of Nd in the cleaned aragonitic skeletons, a subset of sixteen corals was investigated for its rare earth element (REE) content, as well as major and trace element geochemistry. Our new data provide supporting evidence that the applied cleaning protocol efficiently removes contaminant lithogenic and ferromanganese oxyhydroxide phases. Mass balance calculations and seawater-like REE patterns rule out lithogenic and ferromanganese oxyhydroxide phases as a major contributor to elevated Nd concentrations in coral aragonite. Based on mass balance considerations, geochemical evidence, and previously published independent work by solid-state nuclear magnetic resonance (NMR) spectroscopy, we suggest authigenic phosphate phases as a significant carrier of skeletal Nd. Such a carrier phase could explain sporadic appearance of high Nd concentrations in corals and would be coupled with seawater-derived Nd isotopic compositions, lending further confidence to the application of Nd isotopes as a water mass proxy in CWCs.

Isotopes of the actinide elements protactinium (Pa), thorium (Th), and uranium (U), and the lanthanide element neodymium (Nd) are often used as complementary tracers of modern and past oceanic processes. The extraction of such elements from low abundance matrices, such as seawater and carbonate, is however labor-intensive and requires significant amounts of sample material. We here present a combined method for the extraction of Pa, Th, and Nd from 5 to 10 L seawater samples, and of U, Th, and Nd from <1 g carbonate samples. Neodymium is collected in the respective wash fractions of Pa-Th and U-Th anion exchange chromatographies. Regardless of the original sample matrix, Nd is extracted during a two-stage ion chromatography, followed by thermal ionization mass spectrometry (TIMS) analysis as NdO+. Using this combined procedure, we obtained results for Nd isotopic compositions on two GEOTRACES consensus samples from Bermuda Atlantic Time Series (BATS), which are within error identical to results for separately sampled and processed dedicated Nd samples (epsilon(Nd)=-9.200.21 and -13.110.21 for 15 and 2000 m water depths, respectively; intercalibration results from 14 laboratories: epsilon(Nd)=-9.190.57 and -13.140.57). Furthermore, Nd isotope results for an in-house coral reference material are identical within analytical uncertainty for dedicated Nd chemistry and after collection of Nd from U-Th anion exchange chromatography. Our procedure does not require major adaptations to independently used ion exchange chromatographies for U-Pa-Th and Nd, and can hence be readily implemented for a wide range of applications.