Seawater (234U/238U) during the last 800 thousand years

Constraining the history of seawater (U-234/U-238) is important because this ratio is used to assess the validity of U/Th ages, and because it provides information about the past rate of physical weathering on the continents. This study makes use of U-rich slope sediments from the Bahamas in an attempt to reconstruct seawater (U-234/U-238) for the last 800 kyr. For the last 360 kyr, U/Th dating of these sediments provides ages and initial (U-234/U-238) values. Sixty-seven samples, largely from marine highstands. have initial (U-234/U-238) which scatter somewhat about the modern seawater value (similar to1.145) but neither this scatter nor the average value increases with age of sample. These data contrast with published coral data and suggest that seawater (U-234/U-238) has remained within 15parts per thousand of the modern value for the last 360 kyr. This confirms the rejection of coral U/Th ages where the initial (U-234/U-238) is significantly different from modem seawater. Data from older highstands, dated with delta(18)O stratigraphy or by the presence of the Brunhes/Matuyama (B/M) reversal at 780 kyr, allow seawater (U-234/U-238) to be assessed prior to the range of the Th-230 chronometer. Unfortunately, diagenetic scatter in the data between the B/M reversal and 360 kyr is rather large, probably relating to low U concentrations for these samples. But there is no indication of a trend in seawater (U-234/U-238) with age. High U samples from close to the B/M reversal show less diagenetic scatter and an initial (U-234/U-238) that averages 1,102. This lower value can be explained by lower seawater (U-234/U-238) at the time of the B/M reversal, or by progressive loss Of U-234 from the sediment by alpha-recoil. A simple box model is presented to illustrate the response of seawater (U-234/U-238) to variations in riverine input, such as might be caused by changes in continental weathering. Comparison of the Bahamas (U-234/U-238) data with model results indicates that riverine (U-234/U-238) has not varied by more than 65parts per thousand for any 100 kyr period during the last 360 kyr. It also indicates that the ratio of physical to chemical weathering on the continents has not been higher than at present for any extended period during the last 800 kyr. (C) 2002 Elsevier Science B.V. All rights reserved.