Reconstructing Dust Input to the Global Ocean using Thorium Isotope Measurements in Marine Sediments: A High Latitude Perspective

Abstract:

Dust input into the ocean-atmosphere system has significant ramifications for biogeochemical cycles and global climate, yet direct observations of dust deposition in the ocean, especially at higher latitudes, are scarce. The long-lived isotope Thorium-232 is greatly enriched in upper continental crust compared to oceanic crust and mid-ocean ridge basalt (MORB)-like volcanogenic material. In open ocean areas, away from fluvial and ice-rafted sources of continental material, it is assumed to be of eolian origin. In conjunction with ²³⁰Th-flux normalization, ²³²Th measurements in marine sediments are thus a promising new proxy for dust accumulation in the present and past ocean. Here, we present "ThoroMap", a global data compilation of >300 previously published ²³⁰Th-normalized fluxes of ²³²Th, complemented by unpublished data. In the equatorial Atlantic and eastern equatorial Pacific, there is excellent agreement between late Holocene sedimentary dust fluxes and dust deposition estimates derived from 2 recent coupled atmosphere, land, ocean, and sea-ice models (CCSM3, CAM4). At higher latitudes (between 40 and 60 degrees), careful site selection is necessary to avoid areas where eolian contributions to seafloor sediments are overwhelmed by ice rafted material. After addressing this caveat, the new database shows great potential for significantly increasing the spatial coverage of dust deposition records in the high latitudes, especially in the NW Pacific and S-Atlantic.