A sampling artefact affecting the uranium content of deep-sea porewaters obtained from cores

Abstract

A wide range of uranium concentrations (1.3–65 µg l⁻¹) have been reported in deep-sea porewaters extracted from cores^1–5, values which contrast with the accepted constant figure of 3.3 ± 0.2 µg l⁻¹ for the uranium content of open ocean seawater^4,6–8. In seawater the constant level of uranium is believed to be due to the conservative behaviour of the anionic uranyl carbonate complex^9,10. In a recent review Cochran¹¹ concludes that porewater uranium contents higher than the seawater value are found in association with an anoxic condition in the sediments, while most oxic sediments have lower values in the range 2–2.5 µg l⁻¹. This latter observation caused Cochran and Krishnaswami⁵ to suggest that uranium uptake by some sediment solid phase must occur and ought to be included in the oceanic uranium balance^4,7. We present here detailed uranium porewater profiles for two contrasting eastern Atlantic sediments from an in situ porewater sampler and from squeezed corer sediment. These indicate that lower values in oxic sediments are almost certainly caused by a sampling artefact related to core decompression.