TY - JOUR A1 - McGee, David A1 - Winckler, Gisela A1 - Borunda, Alejandra A1 - Serno, Sascha A1 - Anderson, Robert F. A1 - Recasens, Cristina A1 - Bory, Aloys A1 - Gaiero, Diego A1 - Jaccard, Samuel L. A1 - Kaplan, Michael A1 - McManus, Jerry F. A1 - Revel, Marie A1 - Sun, Youbin T1 - Tracking eolian dust with helium and thorium: Impacts of grain size and provenance JF - Geochimica et cosmochimica acta : journal of the Geochemical Society and the Meteoritical Society N2 - Reconstructions of the deposition rate of windblown mineral dust in ocean sediments offer an important means of tracking past climate changes and of assessing the radiative and biogeochemical impacts of dust in past climates. Dust flux estimates in ocean sediments have commonly been based on the operationally defined lithogenic fraction of sediment samples. More recently, dust fluxes have been estimated from measurements of helium and thorium, as rare isotopes of these elements (He-3 and Th-230) allow estimates of sediment flux, and the dominant isotopes (He-4 and Th-232) are uniquely associated with the lithogenic fraction of marine sediments. In order to improve the fidelity of dust flux reconstructions based on He and Th, we present a survey of He and Th concentrations in sediments from dust source areas in East Asia, Australia and South America. Our data show systematic relationships between He and Th concentrations and grain size, with He concentrations decreasing and Th concentrations increasing with decreasing grain size. We find consistent He and Th concentrations in the fine fraction (<5 mu m) of samples from East Asia, Australia and Central South America (Puna-Central West Argentina), with Th concentrations averaging 14 mu g/g and He concentrations averaging 2 mu cc STP/g. We recommend use of these values for estimating dust fluxes in sediments where dust is dominantly fine-grained, and suggest that previous studies may have systematically overestimated Th-based dust fluxes by 30%. Source areas in Patagonia appear to have lower He and Th contents than other regions, as fine fraction concentrations average 0.8 mu cc STP/g and 9 mu g/g for He-4 and Th-232, respectively. The impact of grain size on lithogenic He and Th concentrations should be taken into account in sediments proximal to dust sources where dust grain size may vary considerably. Our data also have important implications for the hosts of He in long-traveled dust and for the He-3/He-4 ratio used for terrigenous He in studies of extraterrestrial He in sediments and ice. We also investigate the use of He/Th ratios as a provenance tracer. Our results suggest differences in fine fraction He/Th ratios between East Asia, Australia, central South America and Patagonia, with ratios showing a positive relationship with the geological age of source rocks. He/Th ratios may thus provide useful provenance information, for example allowing separation of Patagonian sources from Puna-Central West Argentina or Australian dust sources. He/Th ratios in open-ocean marine sediments are similar to ratios in the fine fraction of upwind dust source areas. He/Th ratios in mid-latitude South Atlantic sediments suggest that dust in this region primarily derives from the Puna-Central West Argentina region (23-32 degrees S) rather than Patagonia (>38 degrees S). In the equatorial Pacific, He/Th ratios are much lower than in extratropical Pacific sediments or potential source areas measured as a part of this study (East Asia, South America, Australia) for reasons that are at present unclear, complicating their use as provenance tracers in this region. (C) 2015 Elsevier Ltd. All rights reserved. Y1 - 2016 U6 - https://doi.org/10.1016/j.gca.2015.11.023 SN - 0016-7037 SN - 1872-9533 VL - 175 SP - 47 EP - 67 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Martinez-Garcia, Alfredo A1 - Rosell-Mele, Antoni A1 - Jaccard, Samuel L. A1 - Geibert, Walter A1 - Sigman, Daniel M. A1 - Haug, Gerald H. T1 - Southern Ocean dust-climate coupling over the past four million years JF - Nature : the international weekly journal of science N2 - Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean(1,2). Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p. p. m. v.) of the decrease (80-100 p. p. m. v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles(3-7). So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles(5,8,9), providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history. Y1 - 2011 U6 - https://doi.org/10.1038/nature10310 SN - 0028-0836 VL - 476 IS - 7360 SP - 312 EP - U141 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Studer, Anja S. A1 - Martinez-Garcia, Alfredo A1 - Jaccard, Samuel L. A1 - Girault, France E. A1 - Sigman, Daniel M. A1 - Haug, Gerald H. T1 - Enhanced stratification and seasonality in the Subarctic Pacific upon Northern Hemisphere Glaciation-New evidence from diatom-bound nitrogen isotopes, alkenones and archaeal tetraethers JF - Earth & planetary science letters N2 - Coincident with the intensification of Northern Hemisphere Glaciation (WIG) around 2.73 million years (Ma) ago, sediment cores from both the open subarctic North Pacific and the Antarctic indicate a rapid decline in diatom opal accumulation flux to the seabed, representing one of the most abrupt and dramatic changes in the marine sediment record associated with the development of Pleistocene glacial cycles. In the North Pacific, bulk sediment nitrogen isotope data and alkenone-derived sea surface temperature (SST) estimates suggest that the productivity decline was driven by reduced exchange between surface and deep water, due to weaker wind-driven upwelling and/or a strengthening of the halocline (i.e. "stratification"). In this study of the 2.73 Ma transition at Ocean Drilling Program (ODP) Site 882 in the western subarctic North Pacific, diatom-bound nitrogen isotopes (delta N-15(db)), alkenone mass accumulation rate, and alkenone- and archaeal tetraether-based SST reconstructions support the stratification hypothesis, indicating perennially lower export production, generally higher nitrate consumption, and greater inter-seasonal variation in SST after the 2.73 Ma transition. In addition, the delta N-15(db) of large and small size fractions of Coscinodiscus spp. suggest that these diatoms grew mostly during the spring bloom during the late Pliocene, switching to their current fall-to-winter growth period at the 2.73 Ma transition; this view is consistent with their decline in dominance and provides further evidence for increased stratification (reduced vertical exchange) in the North Pacific after 2.73 Ma. The delta N-15(db) data indicate that, over the similar to 100 kyr period after the 2.73 Ma transition studied here, nitrate consumption did not reach late Pleistocene ice age levels and that nitrate consumption in post-2.73 Ma warm stages was similar to that before the transition, even though productivity was greatly reduced. We tentatively attribute this to relatively weak dust-borne iron inputs in the early post-2.73 Ma period. KW - subarctic Pacific stratification KW - Plio-Pleistocene transition KW - Northern Hemisphere Glaciation KW - nutrient status KW - diatom-bound nitrogen isotopes KW - sea surface temperature reconstructions Y1 - 2012 U6 - https://doi.org/10.1016/j.epsl.2012.07.029 SN - 0012-821X VL - 351 SP - 84 EP - 94 PB - Elsevier CY - Amsterdam ER -