REMOBILIZATION OF BARIUM IN CONTINENTAL-MARGIN SEDIMENTS

The rate of Ba release from California continental margin sediments has been measured, using an in situ benthic flux chamber, and the range of values (25-50 nmol cm-2 y-1) is larger than any previously published benthic flux estimate for this element. The magnitude of the Ba flux suggests that a significant fraction of the Ba raining from the euphotic zone is recycled at the seafloor. Ba:Si regeneration ratios from these margin sediments increase with depth, demonstrating that Ba is decoupled from Si during the earliest stages of diagenesis. On the other hand, Ba regeneration rates and CaCO3 dissolution rates covary; the coupling between these two constituents is supported by the observation that the Ba: CaCO3 dissolution flux ratio (1.7 +/- 0.4 x 10(-3)) is independent of bottomwater depth-even in sediments underlying the oxygen minimum zone along the continental margin. Furthermore, this flux ratio is consistent with both the water column Ba:alkalinity ratio for the world's ocean, as well as the Ba:CaCO3 ratio in sediment-trap solid phases from the Equatorial Pacific (1.1-2.2 x 10(-3)). However, the constancy of the Ba:alkalinity ratio over geologic time remains in question, because the mechanism that controls this relationship remains a mystery. Our flux measurements suggest that diagenesis does not significantly influence the Ba:Ca ratio in the upper 0.5 mm of Pacific sediments, thereby supporting the idea of using the Ba concentration in surface-dwelling benthic forams as a proxy for deep-water chemical conditions (Lea and Boyle, 1989, 1990). On the other hand, we predict that if a foraminifer lives 0.5 mm or more below this interface, then diagenetic effects could influence the Ba:Ca ratio that foram species would record. The carrier phase of the particulate Ba reactive during early diagenesis does not appear to be organic matter, oxyhydroxides, or calcium carbonate, but rather a mineral phase related to marine barite or perhaps celestite.