Tracing silicon cycling in the Okavango Delta, a sub-tropical flood-pulse wetland using silicon isotopes

摘要

Chemical weathering of silicate minerals releases elements into solution whereas the neoformation of secondary minerals works in the opposite direction, potentially confounding estimates of silicate weathering rates. Silicon isotopes (delta Si-30) may be a useful tool to investigate these processes. Here, we present 82 delta Si-30 measurements from surface waters, pore waters, biogenic silica (BSi), clays, sand and vegetation from the Okavango Delta, Botswana, a freshwater sub-tropical, flood-pulse wetland. Hydrologically, the Okavango is dominated by evapotranspiration water losses to the atmosphere. It receives an annual pulse of water that inundates seasonal floodplains, while river baseflow is sufficient to maintain a permanent floodplain. delta Si-30 in dissolved silica (DSi) in surface waters along a 300 km transect at near-peak flood show a limited range (0.36-1.19 parts per thousand), implying the Delta is well buffered by a balance of processes adding and removing DSi from the surface water. A key control on DSi concentrations is the uptake, production of BSi and recycling of Si by aquatic vegetation, although the net isotopic effect is necessarily small since all BSi re-dissolves on short timescales. In the sediments, BSi delta Si-30 (n = 30) ranges from 1.49 parts per thousand to +0.31 parts per thousand and during dissolution, residual BSi tends towards higher delta Si-30. The data permit a field-based estimate of the fractionation associated with BSi dissolution, epsilon(30)(BSi-DSi) = -0.26 parts per thousand, though it is unclear if this is an artefact of the process of dissolution. Clay delta Si-30 ranges from -0.97 parts per thousand to +0.10 parts per thousand, (n = 15, mean = -0.31 parts per thousand) and include the highest values yet published, which we speculate may be due to an equilibrium isotope effect during diagenetic transformation of BSi. Two key trends in surface water DSi delta Si-30 merit further examination: declining delta Si-30 in an area roughly corresponding to the permanent floodplains despite net DSi removal, and increasing delta Si-30 in the area corresponding to the seasonal floodplains. We infer that evaporative enrichment of surface waters creates two contrasting regimes. Chemical weathering of low delta Si-30 phases releases low delta Si-30 DSi in the relatively dilute waters of the permanent floodplains, whereas silicon removal via clay formation or vegetation uptake is the dominant process in the more enriched, seasonal floodplains. (C) 2014 Elsevier Ltd. All rights reserved.