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

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doi: 10.1016/S0012-821X(02)00556-3
Author(s): Henderson, Gideon M.
Author Affiliation(s): Primary:
Oxford University, Department of Earth Sciences, Oxford, United Kingdom
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 199(1-2), p.97-110. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. 55 refs.; illus., incl. 3 tables
Summary: Constraining the history of seawater (234U/238U) 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 (234U/238U) for the last 800 kyr. For the last 360 kyr, U/Th dating of these sediments provides ages and initial (234U/238U) values. Sixty-seven samples, largely from marine highstands, have initial (234U/238U) which scatter somewhat about the modern seawater value (∼1.145) but neither this scatter nor the average value increases with age of sample. These data contrast with published coral data and suggest that seawater (234U/238U) has remained within 15 per mil of the modern value for the last 360 kyr. This confirms the rejection of coral U/Th ages where the initial (234U/238U) is significantly different from modern seawater. Data from older highstands, dated with δ18O stratigraphy or by the presence of the Brunhes/Matuyama (B/M) reversal at 780 kyr, allow seawater (234U/238U) to be assessed prior to the range of the 230Th 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 (234U/238U) with age. High U samples from close to the B/M reversal show less diagenetic scatter and an initial (234U/238U) that averages 1.102. This lower value can be explained by lower seawater (234U/238U) at the time of the B/M reversal, or by progressive loss of 234U from the sediment by α-recoil. A simple box model is presented to illustrate the response of seawater (234U/238U) to variations in riverine input, such as might be caused by changes in continental weathering. Comparison of the Bahamas (234U/238U) data with model results indicates that riverine (234U/238U) has not varied by more than 65 per mil 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. Abstract Copyright (2002) Elsevier, B.V.
Year of Publication: 2002
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 24 Surficial Geology, Quaternary Geology; Absolute age; Actinides; Atlantic Ocean; Bahamas; Caribbean region; Cenozoic; Chemical weathering; Cores; Dates; Experimental studies; Geochemistry; Great Bahama Bank; Hydrochemistry; ICP mass spectra; Isotope ratios; Isotopes; Leg 166; Marine environment; Marine sediments; Mass spectra; Metals; North Atlantic; Ocean Drilling Program; Paleo-oceanography; Physical weathering; Quaternary; Radioactive isotopes; Residence time; Sea water; Sediments; Slope environment; Spectra; Th/U; Thermal ionization mass spectra; U-238/U-234; Upper Quaternary; Uranium; Weathering; West Indies
Coordinates: N220000 N260000 W0760000 W0792000
Record ID: 2002047493
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands