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

Abstract

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 4C state at greater depths. The dominant process influencing pore-water ( 234 U= 238 U) is alpha recoil. In Holocene sediments, the increase of pore-water ( 234 U= 238 U) 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 ( 234 U= 238 U) profiles are offset from one another which constrains the effective diffusivity for U in these sediments to be ³1-2 ð 10 8 cm 2 s 1 . At depths between the Holocene and these reducing sediments, pore-water ( 234 U= 238 U) 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 (š3.4) cm=year. Estimates of flow rate from ( 234 U= 238 U) 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. © 1999 Elsevier Science B.V. All rights reserved. Henderson et al. / Earth and Planetary Science Letters 169 (1999) [99][100][101][102][103][104][105][106][107][108][109][110][111]

Evidence from U–Th dating against Northern Hemisphere forcing of the penultimate deglaciation

“…Overall, the size of the recoil effect is larger than would be expected if U were distributed uniformly within grains of this size (63±250 mm). This observation is consistent with evidence that much of the U in these Bahamas sediments is contained in an organic phase which coats the grains 20 .…”

“…Unlike corals, these samples have never been exposed to air or been in contact with meteoric water. The sediment environment can be directly investigated by measurement of U isotopes and concentrations in the pore waters of Bahamas sediment 20 . Such analyses show that U concentrations in pore waters are more than 1,000 times lower than in the sediments analysed here.…”

Insights into methane dynamics from analysis of authigenic carbonates and chemosynthetic mussels at newly-discovered Atlantic Margin seeps

“…All activity ratios were calculated using λ 230 = 9.17050E−6, λ 234 = 2.82206E−6 (Cheng et al, 2013), λ 232 = 4.93343E−11 (Holden, 1990) (Robinson et al, 2004; Fig. S3 B), meaning that U incorporated in the authigenic carbonates was sourced from seawater, rather than pore waters, which would be comparatively enriched in 234 U (Henderson et al, 1999).…”

The Behavior of U- and Th-series Nuclides in Groundwater

“…For example, studies of the Milk River aquifer in Canada found that reasonable ages could be determined from changes in 234 U/ 238 U ratios along a flowline, as long as significant retardation of U in the aquifer was included in the calculations (Ivanovich et al 1991). Henderson et al (1999) similarly used U isotope variations to determine pore water flow rates. Overall, this raises the hope that a reliable dating method could be found using U isotopes, but also indicated that independent controls on U behavior are required.…”