Fluid flow through carbonate platforms; constraints from 234U/238U and Cl- in Bahamas pore-waters

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doi: 10.1016/S0012-821X(99)00065-5
Author(s): Henderson, Gideon M.; Slowey, Niall C.; Haddad, Geoffrey A.
Author Affiliation(s): Primary:
Lamont-Doherty Earth Observatory, Palisades, NY, United States
Other:
Texas A&M University, United States
Rice University, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 169(1-2), p.99-111. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Lamont-Doherty Earth Obs., Contrib. No. 5900. 41 refs.; illus., incl. 1 table, sketch map
Summary: The geometry, timing, and rate of fluid-flow through carbonate margins and platforms is not well constrained. In this study, we use U concentrations and isotope ratios measured on small volumes of pore-water from Bahamas slope sediment, coupled with existing chlorinity data, to place constraints on the fluid-flow in this region and, by implication, other carbonate platforms. These data also allow an assessment of the behaviour of U isotopes in an unusually well constrained water-rock system. We report pore-water U concentrations which are controlled by dissolution of high-U organic material at shallow depths in the sediment and by reduction of U to its insoluble 4+ state at greater depths. The dominant process influencing pore-water (234U/238U) is alpha recoil. In Holocene sediments, the increase of pore-water (234U/238U) due to recoil provides an estimate of the horizontal flow rate of 11 cm/year, but with considerable uncertainty. At depths in the sediment where conditions are reducing, features in the U concentration and (234U/238U) profiles are offset from one another which constrains the effective diffusivity for U in these sediments to be ≈1-2×10-8 cm2 s-1. At depths between the Holocene and these reducing sediments, pore-water (234U/238U) values are unusually low due to a recent increase in the dissolution rate of grain surfaces. This suggests a strengthening of fluid flow, probably due to the flooding of the banks at the last deglaciation and the re-initiation of thermally-driven venting of fluid on the bank top and accompanying recharge on the slopes. Interpretation of existing chlorinity data, in the light of this change in flow rate, constrain the recent horizontal flow rate to be 10.6 (±3.4) cm/year. Estimates of flow rate from (234U/238U) and Cl- are therefore in agreement and suggest flow rates close to those predicted by thermally-driven models of fluid flow. This agreement supports the idea that flow within the Bahamas Banks is mostly thermally-driven and suggests that flow rates on the order of 10 cm/year are typical for carbonate platforms where such flow occurs. Abstract Copyright (1999) Elsevier, B.V.
Year of Publication: 1999
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Actinides; Atlantic Ocean; Bahamas; Carbonate platforms; Carbonate sediments; Caribbean region; Concentration; Continental margin; Fluid dynamics; Geochemistry; Great Bahama Bank; Isotope ratios; Isotopes; Lee Stocking Island; Leg 166; Marine sediments; Measurement; Metals; North Atlantic; Northwest Atlantic; ODP Site 1005; ODP Site 1009; Ocean Drilling Program; Pore water; Radioactive isotopes; Sediments; Solution; U-238/U-234; Uranium; West Indies
Coordinates: N243000 N244000 W0791000 W0792000
N234700 N234800 W0760500 W0760500
Record ID: 1999040237
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands