Spatial and Temporal Trends in Mineral Dust Provenance in the South Pacific—Evidence From Mixing Models

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

A deep Tasman outflow of Pacific waters during the last glacial period

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Struve, T., Wilson, D., Hines, S., Adkins, J., & van de Flierdt, T. A deep Tasman outflow of Pacific waters during the last glacial period. Nature Communications, 13(1), (2022): 3763, https://doi.org/10.1038/s41467-022-31116-7.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 authors acknowledge financial support from the Grantham Institute of Climate Change and the Environment (T.v.d.F. and T.S.), the Ministry for Science and Culture of the State of Lower Saxony (T.S.), Marie Curie Reintegration grant IRG 230828 (T.v.d.F.), Leverhulme Trust grant RPG-398 (T.v.d.F.), Natural Environment Research Council grants NE/F016751/1 (T.v.d.F.), NE/N001141/1 (T.v.d.F. and D.J.W.), and NE/T011440/1 (D.J.W.), and National Science Foundation grant OCE-1503129 (J.F.A. and S.K.V.H.). Open Access funding is enabled by the DFG open access publication fund and the Carl von Ossietzky University Oldenburg

Sea-ice control on deglacial lower cell circulation changes recorded by Drake Passage deep-sea corals

Financial support to DJW, TS, and TvdF was provided by the Natural Environment Research Council (NE/N001141/1), the Leverhulme Trust (RPG-398), the Grantham Institute for Climate Change and the Environment, and a Marie Curie Reintegration grant (IRG 230828). LFR acknowledges support from the Natural Environment Research Council (NE/N003861/1) and the European Research Council (278705).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 εNd units, with radiogenic values of up to 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.PostprintPostprintPeer reviewe

Neodymium isotopes and concentrations in aragonitic scleractinian cold-water coral skeletons - Modern calibration and evaluation of palaeo-applications

TvdF and TS acknowledge financial support for a bursary by the Grantham Institute of Climate Change and the Environment and a Marie Curie Reintegration grant (IRG 230828), as well as funding from the Leverhulme Trust (RPG-398) and the NERC (NE/N001141/1). Additional financial support was provided to LFR by the USGS-WHOI Co-operative agreement, NSF-ANT grants 0636787 and 80295700, The European Research Council, the Leverhulme Trust and a Marie Curie Reintegration grant. LB was supported by a NOAA/UCAR Climate and Global Change Postdoctoral Fellowship and KJM acknowledges funding from a Marie Curie International Outgoing fellowship (IOF 236962).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.Publisher PDFPeer reviewe

Neodymium isotope analyses after combined extraction of actinide and lanthanide elements from seawater and deep-sea coral aragonite

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 17 (2016): 232–240, doi:10.1002/2015GC006130.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 (εNd = −9.20 ± 0.21 and −13.11 ± 0.21 for 15 and 2000 m water depths, respectively; intercalibration results from 14 laboratories: εNd = −9.19 ± 0.57 and −13.14 ± 0.57). 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.Funding that supported this work was received from the National Science Foundation (NSF 0752402), the Leverhulme Trust (RPG-398), the Natural Environmental Research Council (NE/J021636/1 and NE/N003861/1), the European Research Council (278705), and the Grantham Institute for Climate Change.2016-07-0

Deciphering glacial-interglacial Southern Ocean dynamics with deep-sea corals

Recent observations and past reconstructions have highlighted the significance of the Southern Ocean for modern and past ocean circulation patterns. Deep wind-induced upwelling renders the Southern Ocean unique, such that deep waters are brought directly to the surface where they can exchange with the atmosphere. Moreover, the Antarctic Circumpolar Current (ACC) is the dominant feature of Southern Ocean circulation linking all ocean basins and facilitating the inter-basin exchange of ocean properties. Hence, Southern Ocean dynamics act to (partly) moderate both, zonal and meridional transports, and both were shown to be sensitive to atmospheric forcing on different time scales. However, due to the remoteness and harsh conditions, direct evidence of past Southern Ocean circulation is scarce. This work uses neodymium (Nd) isotopes extracted from the aragonitic skeletons of deep-sea corals collected in the Drake Passage and at the Tasmanian margin. The fidelity of the skeletal Nd isotope signature was tested in a calibration effort and the nature of Nd in coralline aragonite was explored. Based on the promising results, the approach was then confidently applied to uranium-series dated deep-sea corals in order to decipher Southern Ocean water mass mixing across intervals of past climate perturbations. The results show that Drake Passage and Tasmanian margin corals record complimentary features of Southern Ocean circulation. Intervals of high sampling resolution reveal unexpectedly dynamic and abrupt changes of water mass mixing oscillating on millennial to (sub) centennial time scales during the past ~40,000 years. This thesis explores the nature of the recorded signal in the light of available literature, focusing on phases of abrupt change and fitting the processes considered to drive the Southern Ocean Nd isotope signal into a global framework.Open Acces