DISTRIBUTION OF THORIUM 232 IN MANGANESE NODULES AND CRUSTS: PALEOCEANOGRAPHIC IMPLICATIONS

Detailed depth profiles of Th-232 in deep-sea ferromanganese nodules and encrustations often show a subsurface maximum followed by a drastic inward decrease to a certain depth before levelling out. Length scales of this feature vary among the profiles but are proportional to growth rates of the specimens, suggesting that the profiles record changes in oceanic Th-232 flux with time. Consideration of the nodule chronology, the history of eolian accumulation recorded in sediment cores, and the marine geochemical behavior and budget of Th-232 leads us to propose that Th-232 in nodules/crust is of eolian origin. The drastic in ward decrease of Th-232 is a reflection of increase in the wind borne dust flux induced by the Northern Hemisphere glaciation beginning 2-3 Ma. The subsurface Th-232 peak may indicate the decline of eolian flux since 0.1-0.3 Ma.