Measurement of adsorbed and total 232Th/230Th ratios from marine sediments

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doi: 10.1016/j.chemgeo.2008.02.015
Author(s): Robinson, Laura F.; Noble, Taryn L.; McManus, Jerry F.
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Marine Chemistry and Geochemistry, Woods Hole, MA, United States
Volume Title: Chemical Geology
Source: Chemical Geology, 252(3-4), p.169-179. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541 CODEN: CHGEAD
Note: In English. 42 refs.; illus., incl. 3 tables
Summary: The predictable in situ production of 230Th from the decay of uranium in seawater, and its subsequent removal by scavenging onto falling particles, provides a valuable tool for normalizing fluxes to the seafloor. We describe a new application, determination of the 232Th that dissolves in the water column and is removed to the seafloor. 232Th is supplied to the ocean in continental minerals, dissolution of which leads to a measurable standing stock in the water column. Sedimentary adsorbed 232Th/230Th ratios have the potential to provide a proxy for estimating the amount of dissolved material that enters the ocean, both today and in the past. Ten core top samples were treated with up to eight different leaching techniques in order to determine the best method for the separating adsorbed from lattice bound thorium. In addition, separate components of the sediments were analyzed to test whether clay dissolution was an important contribution to the final measurement. There was no systematic correlation between the strength of acid used in the leach and the measured 232Th/230Th ratios. In all cases clean foraminifera produced the same ratio as leaches on bulk sediment. In three out of five samples leaches performed on non-carbonate detritus in the <63 µm size fraction were also identical. Without additional water column data it is not yet clear whether there is a simple one to one correlation between the expected deep-water 232Th/230Th and that produced by leaching, especially in carbonate-rich sediments. However, higher ratios, and associated high 232Th adsorbed fluxes, were observed in areas with high expected detrital inputs. The adsorbed fraction was ∼35-50% of the total 232Th in seven out of ten samples. Our 230Th normalized 232Th fluxes are reasonable by comparison to global estimates of detrital inputs to the ocean. In nine cases out of ten, the total 230Th-normalized 232Th flux is greater than predicted from the annual dust fall at each specific location, but lower than the average global detrital input from all sources. Abstract Copyright (2008) Elsevier, B.V.
Year of Publication: 2008
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Actinides; Adsorption; Atlantic Ocean; Carbonate sediments; Chemical composition; Clastic sediments; Clay minerals; Deposition; Dust; Geochemistry; Indian Ocean; Isotope ratios; Isotopes; Lattice; Leg 115; Leg 177; Leg 183; Marine sediments; Measurement; Metals; North Atlantic; North Pacific; ODP Site 1090; ODP Site 1142; ODP Site 716; Ocean Drilling Program; Ocean floors; Pacific Ocean; Radioactive decay; Radioactive isotopes; Sampling; Sea water; Sediments; Sheet silicates; Silicates; Solutes; Southern Ocean; Th-232/Th-230; Thorium; Uranium
Coordinates: N045600 N045600 E0731700 E0731700
S425449 S425449 E0085359 E0085359
S321354 S321354 E0970730 E0970730
Record ID: 2008130101
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands