SEDIMENTATION-RATE AS DETERMINED BY RA-226 ACTIVITY IN MARINE BARITE

In recently formed marine barite (BaSO4) separated from two equatorial Pacific cores Ra-226 activity exceeds that of its parent Th-230 by at least an order of magnitude, indicating radium uptake during barite p recipitation. The decay of Ra-226 with depth, in these samples, is exp onential implying that no radium exchange with porewaters occurs after burial. Thus, barite behaves as a closed system below the sediment mi xed layer. The absence of Ra-226(exs) activity in barite samples older than similar to 8000 years, the lack of any detectable Th-228, Ra-228 , and Ra-224 in any of the barite samples analyzed, and the unaltered crystal size and habit with depth in the sediment, further support thi s conclusion. Hence, assuming that oceanic Ra/Ba ratio has not changed throughout the Holocene, the decay of unsupported Ra-226 permits dati ng of marine barite and estimating sedimentation rates. Holocene sedim entation rates calculated using the decay of Ra-226(exs) in barite are consistent with C-14 and delta(18)O derived sedimentation rates of 2- 3 cm/kyr for the same cores. The method could be extendible to carbona te-poor Holocene sediments.