Sample preparation for determination of comminution ages in lacustrine and marine sediments

Abstract

The uranium isotope composition (234U and 238U) of detrital matter has become an essential tool for evaluating the response of erosion and catchment dynamics to climate variability on geological time scales. Relative variability of the (234U/238U) activity ratio can be used as an estimate of the time that has elapsed since physical and chemical weathering has formed grains <63μm, termed comminution age, as a result of ongoing depletion of 234U in detrital matter. However, as non-detrital (authigenic, endogenic) components are commonly enriched in 234U, sequential extraction methods are required to carefully remove any non-detrital matter from the sediments. Herein, we tested different methods, which use similar chemical reactants but differ in terms of applied heat and time. Based on the results, it can be inferred that an ultrasound-assisted approach enables the removal of non-detrital matter from the sediments without alternating the isotope ratio of the detrital grain, and provides a high reproducibility of the (234U/238U) activity ratios. Moreover, it is the fastest, and thus, most cost effective method tested. Fast and cost-effective treatment methods are a prerequisite for high-resolution studies on long paleoenvironmental records, and thus, these results are fundamental for the further development of U isotope analyses as a tool for the reconstruction of past erosion and catchment dynamics in response to climate variability.Combined U isotopes and gas adsorption surface area analyses on two sediment samples from the Mediterranean Sea yield comminution ages up to 5 times older after sample treatment, which highlights the importance of a comprehensive removal of non-detrital matter from the bulk sediment composition. Moreover, gas sorption analysis allowed determining whether a fractal correction for calculation of the recoil fraction should be applied. Precise estimates of the recoil fraction are crucial for calculating the comminution ages, as it governs the loss of 234U from detrital matter. Samples analyzed in this study display Type II isotherms suggesting a non-porous or macroporous surface. Micro- and mesopores, which increase the surface area during gas adsorption analyses but do not contribute to the loss of 234U are absent. Thus, a fractal correction to account for micro- and mesopores is not required.