Antarctic intermediate water circulation in the South Atlantic over the past 25,000years

Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal, and detrital neodymium isotope data from three intermediate-depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate-depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southward shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate-depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.Natural Environment Research Council (Grant IDs: NE/K005235/1, NE/F006047/1), National Science Foundation (Grant ID: OCE -1335191), Rutherford Memorial Scholarship, DFG Research Center/Cluster of Excellence “The Ocean in the Earth System”, FAPESP (Grant ID: 2012/17517-3), CAPES (Grant IDs: 1976/2014, 564/2015

Construction of the Galapagos platform by large submarine volcanic terraces

Author Posting. © American Geophysical Union, 2008. 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 9 (2008): Q03015, doi:10.1029/2007GC001795.New multibeam bathymetric and side-scan sonar data from the southwestern edge of the Galápagos platform reveal the presence of ∼60 large, stepped submarine terraces between depths of 800 m and 3500 m. These terraces are unique features, as none are known from any other archipelago that share this geomorphic form or size. The terraces slope seaward at 3000 m) lava flow fields west of Fernandina and Isabela Islands. The terraces are formed of thick sequences of lava flows that coalesce to form the foundation of the Galápagos platform, on which the subaerial central volcanoes are built. The compositions of basalts dredged from the submarine terraces indicate that most lavas are chemically similar to subaerial lavas erupted from Sierra Negra volcano on southern Isabela Island. There are no regular major element, trace element, or isotopic variations in the submarine lavas as a function of depth, relative stratigraphic position, or geographic location along the southwest margin of the platform. We hypothesize that magma supply at the western edge of the Galápagos hot spot, which is influenced by both plume and mid-ocean ridge magmatic processes, leads to episodic eruption of large lava flows. These large lava flows coalesce to form the archipelagic apron upon which the island volcanoes are built.This work was supported by the National Science Foundation grants OCE0002818 and EAR0207605 (D.G.), OCE0002461 (D.J.F. and M.K.), OCE05-25864 (M.K.), and EAR0207425 (K.H.)

Late Archean to Early Proterozoic lithospheric mantle beneath the western North China craton: Sr–Nd–Os isotopes of peridotite xenoliths from Yangyuan and Fansi

Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Lithos 102 (2008): 25-42, doi:10.1016/j.lithos.2007.04.005.Sr-Nd-Os isotopic analyses are presented for peridotite xenoliths from Tertiary alkali basalts in Yangyuan and Fansi with the aim of identifying and characterizing the relics of ancient lithospheric mantle that survived lithospheric removal in the western North China Craton (NCC). The analyzed samples are residual lherzolites and harzburgites, ranging from fertile to highly depleted (0.36-4.0 wt% Al2O3) composition. Some LREE-enriched samples are characterized by moderate 87Sr/86Sr (0.7044 to 0.7047) and low εNd (–6.9 to –10.6), pointing to an EMI-type signature. This is distinct from the predominant depleted isotopic composition in mantle xenoliths from eastern China. Os isotopic ratios range from 0.1106 to 0.1325. The lower limit is the most unradiogenic value measured so far for Cenozoic basalt-borne xenoliths from eastern China. Two samples show radiogenic Os ratios higher than that of the primitive upper mantle, one sample has an anomalously high Os concentration (>9 ppb). These samples also show high La/Yb, consistent with the addition of radiogenic components during the infiltration of asthenosphere-derived and/or subduction-related melts in the lithospheric mantle. The remaining samples define positive correlations between 187Os/188Os and indices of melt extraction, which yield model ages of 2.4-2.8 Ga. This age of melt extraction overlaps with the Nd model age of the overlying crust, indicating a coupled crust-mantle system in the western NCC. This contrasts with the decoupled nature in the eastern NCC, suggesting distinct mantle domains underneath the NCC. Such a heterogeneous age structure of the upper mantle is compatible with the view that the lithospheric removal was largely limited to the eastern NCC.The authors gratefully acknowledge the financial support from the National Science Foundation of China (40673038; 49925308), the Chinese Academy of Sciences and China Scholarship Council to YGX for a six-month visit to the USA and from Japan Society for the Promotion of Science to KS (Grant-in Aid #14703002)

Isotopic constraints on the genesis and evolution of basanitic lavas at Haleakala, Island of Maui, Hawaii

© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 195 (2016): 201-225, doi:10.1016/j.gca.2016.08.017.To understand the dynamics of solid mantle upwelling and melting in the Hawaiian plume, we present new major and trace element data, Nd, Sr, Hf, and Pb isotopic compositions, and 238U-230Th-226Ra and 235U-231Pa-227Ac activities for 13 Haleakala Crater nepheline normative basanites with ages ranging from ~900 to 4100 yr B.P.. These basanites of the Hana Volcanics exhibit an enrichment in incompatible trace elements and a more depleted isotopic signature than similarly aged Hawaiian shield lavas from Kilauea and Mauna Loa. Here we posit that as the Pacific lithosphere beneath the active shield volcanoes moves away from the center of the Hawaiian plume, increased incorporation of an intrinsic depleted component with relatively low 206Pb/204Pb produces the source of the basanites of the Hana Volcanics. Haleakala Crater basanites have average (230Th/238U) of 1.23 (n=13), average age-corrected (226Ra/230Th) of 1.25 (n=13), and average (231Pa/235U) of 1.67 (n=4), significantly higher than Kilauea and Mauna Loa tholeiites. U-series modeling shows that solid mantle upwelling velocity for Haleakala Crater basanites ranges from ~0.7 to 1.0 cm/yr, compared to ~10 to 20 cm/yr for tholeiites and ~1 to 2 cm/yr for alkali basalts. These modeling results indicate that solid mantle upwelling rates and porosity of the melting zone are lower for Hana Volcanics basanites than for shield-stage tholeiites from Kilauea and Mauna Loa and alkali basalts from Hualalai. The melting rate, which is directly proportional to both the solid mantle upwelling rate and the degree of melting, is therefore greatest in the center of the Hawaiian plume and lower on its periphery. Our results indicate that solid mantle upwelling velocity is at least 10 times higher at the center of the plume than at its periphery under Haleakala.Funding for this project was provided by NSF grants EAR-0001924 and EAR-9909473 to KWWS.2018-08-2

Lithogenic particle transport trajectories on the Northwest Atlantic Margin

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(1), (2021): e2020JC016802, https://doi.org/10.1029/2020JC016802.The neodymium isotopic composition of the detrital (lithogenic) fraction (εNd‐detrital) of surface sediments and sinking particles was examined to constrain transport trajectories associated with hemipelagic sedimentation on the northwest Atlantic margin. The provenance of resuspended sediments and modes of lateral transport in the water column were of particular interest given the energetic hydrodynamic regime that sustains bottom and intermediate nepheloid layers over the margin. A large across‐margin gradient of ∼5 εNd units was observed for surface sediments, implying strong contrasts in sediment provenance, with εNd‐detrital values on the lower slope similar to those of “upstream regions” (Scotian margin) under the influence of the Deep Western Boundary Current (DWBC). Sinking particles collected at three depths at a site (total water depth, ∼3,000 m) on the New England margin within the core of the DWBC exhibited a similarly large range in εNd‐detrital values. The εNd‐detrital values of particles intercepted at intermediate water depths (1,000 and 2,000 m) were similar to each other but significantly higher than those at 3,000 m (∼50 m above the seafloor). These observations suggest that lithogenic material accumulating in the upper two traps was primarily advected in intermediate nepheloid layers emanating from the adjacent shelf, while that at 3,000 m is strongly influenced by sediment resuspension and along‐margin, southward lateral transport within the bottom nepheloid layer via entrainment in the DWBC. Our results highlight the importance of both along‐ and across‐margin sediment transport as vectors for lithogenic material and associated organic carbon transport.This research was funded by the NSF Ocean Sciences Chemical Oceanography program (OCE‐0425677; OCE‐0851350). JH was partly supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (2020R1A2C1008378).2021-06-0

A new method for the determination of low-level actinium-227 in geological samples

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Journal of Radioanalytical and Nuclear Chemistry 296 (2013): 279-283, doi:10.1007/s10967-012-2140-0.We developed a new method for the determination of 227Ac in geological samples. The method uses extraction chromatographic techniques and alpha-spectrometry and is applicable for a range of natural matrices. Here we report on the procedure and results of the analysis of water (fresh and seawater) and rock samples. Water samples were acidified and rock samples underwent total dissolution via acid leaching. A DGA (N,N,N’,N’-tetra-n-octyldiglycolamide) extraction chromatographic column was used for the separation of actinium. The actinium fraction was prepared for alpha spectrometric measurement via cerium fluoride micro-precipitation. Recoveries of actinium in water samples were 80±8 % (number of analyses n=14) and in rock samples 70±12 % (n=30). The minimum detectable activities (MDA) were 0.017-0.5 Bq kg-1 for both matrices. Rock sample 227Ac activities ranged from 0.17 to 8.3 Bq kg-1 and water sample activities ranged from below MDA values to 14 Bq kg-1of 227Ac. From the analysis of several standard rock and water samples with the method we found very good agreement between our results and certified values

Seawater-peridotite interactions : first insights from ODP Leg 209, MAR 15°N

Author Posting. © American Geophysical Union, 2004. 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 5 (2004): Q09F26, doi:10.1029/2004GC000744.We present first results of a petrographic study of hydrothermally altered peridotites drilled during Ocean Drilling Program (ODP) Leg 209 in the 15°20′N fracture Zone area on the Mid-Atlantic Ridge (MAR). We find that serpentinization is extensive at all drill sites. Where serpentinization is incomplete, phase relations indicate two major reaction pathways. One is reaction of pyroxene to talc and tremolite, and the other is reaction of olivine to serpentine, magnetite, and brucite. We interpret these reactions in the light of recent peridotite-seawater reaction experiments and compositions of fluids venting from peridotite massifs at a range of temperatures. We suggest that the replacement of pyroxene by talc and tremolite takes place at temperatures >350°–400°C, where olivine is stable. The breakdown of olivine to serpentine, magnetite, and brucite is favored at temperatures below 250°C, where olivine reacts faster then pyroxene. High-temperature hydrothermal fluids venting at the Logatchev and Rainbow sites are consistent with rapid reaction of pyroxene and little or no reaction of olivine. Moderate-temperature fluids venting at the Lost City site are consistent with ongoing reaction of olivine to serpentine and brucite. Many completely serpentinized peridotites lack brucite and talc because once the more rapidly reacting phase is exhausted, interaction with the residual phase will change fluid pH and silica activity such that brucite or talc react to serpentine. At two sites we see strong evidence for continued fluid flow and fluid-rock interaction after serpentinization was complete. At Site 1268, serpentinites underwent massive replacement by talc under static conditions. This reaction requires either removal of Mg from or addition of Si to the system. We propose that the talc-altered rocks are Si-metasomatized and that the source of Si is likely gabbro-seawater reaction or breakdown of pyroxene deeper in the basement. The basement at Site 1268 is heavily veined, with talc and talc-oxide-sulfide veins being the most common vein types. It appears that the systems evolved from reducing (oxygen fugacity buffered by magnetite-pyrrhotite-pyrite and lower) to oxidizing (dominantly hematite). We propose that this transition is indicative of high fluid flux under retrograde conditions and that the abundance of hematite may relate to the Ca-depleted nature of the basement that prevents near-quantitative removal of seawater sulfate by anhydrite precipitation. At site 1272 we find abundant iowaite partly replacing brucite. While this is the first report of iowaite from a mid-ocean ridge setting, its presence indicates, again, fairly oxidizing conditions. Our preliminary results indicate that peridotite-seawater and serpentinite-seawater interactions can take place under a wider range of temperature and redox conditions than previously appreciated.This research used data and/or samples supplied by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc

Re-Os systematics and platinum-group element distribution in metalliferous sediments from the Troodos ophiolite

Umbers from 8 of 11 localities studied yield 187Os/188Os ratios that range from 0.51 to 0.57. We interpret this range as reflecting the Os isotopic composition of mid-Cretaceous seawater. 187Os/188Os measured in samples from the remaining three localities fall above and below this range and are believed to result from post-depositional redistribution of Os. Extremely large 187Os/188Os ratios occur in hydrothermal sulfides (>200) and in their oxidation products, ochres (3.6–13.2). Measured Re/Os ratios in these samples require open system behavior. Model age calculations indicate that ochre formation occurred over a 1–5 million year interval after sulfide precipitation. In most samples >99% Re loss occurred during sulfide oxidation. Comparison of element ratios in Cyprus umbers to those of modern seawater suggest the following sequence of scavenging efficiency by hydrothermal Fe-oxides: Ir>Pt>Os>Re. Cyprus sulfides exhibit the opposite pattern of element enrichment. These patterns of aqueous platinum-group element (PGE) fractionation are potentially relevant to study of recycled Os in mantle-derived rocks because they give rise to the following phenomena: (1) ephemeral Re enrichment within the basal sediments of the oceanic crust, (2) PGE concentrations in metalliferous sediments (Os up to 1 ppb and Pt up to 22 ppb) that are comparable to those in the mantle, and (3) Pt/Re ratios in Cyprus umbers that exceed those required to explain the coupled 186Os–187Os variations in Hawaiian basalts