Tracking eolian dust with helium and thorium: Impacts of grain size and provenance

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: McGee, David, Winckler, Gisela, Borunda, Alejandra, Serno, Sascha, Anderson, Robert F., Recasens, Cristina, Bory, Aloys, Gaiero, Diego, Jaccard, Samuel L., Kaplan, Michael, McManus, Jerry F., Revel, Marie, Sun, Youbin
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Science & Technology; Physical Sciences; EQUATORIAL PACIFIC-OCEAN; CHINESE LOESS PLATEAU; ARCTIC NORTH PACIFIC; LAST GLACIAL PERIOD; RARE-EARTH-ELEMENT; DEEP-SEA SEDIMENTS; EXTRATERRESTRIAL HE-3; ISOTOPIC COMPOSITIONS; RADIOGENIC HELIUM; COSMOGENIC HE-3; Geochemistry & Geophysics; Arctic; Patagonia; Pacific; Argentina
• Online Access: http://ir.ieecas.cn/handle/361006/5840; https://doi.org/10.1016/j.gca.2015.11.023

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 ...