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

Abstract

International audienceChemical 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 (δ30Si) may be a useful tool to investigate these processes. Here, we present 82 δ30Si 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. δ30Si in dissolved silica (DSi) in surface waters along a 300km transect at near-peak flood show a limited range (0.36-1.19‰), 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 δ30Si (n=30) ranges from -1.49‰ to +0.31‰ and during dissolution, residual BSi tends towards higher δ30Si. The data permit a field-based estimate of the fractionation associated with BSi dissolution, ε30 BSi-DSi=-0.26‰, though it is unclear if this is an artefact of the process of dissolution. Clay δ30Si ranges from -0.97‰ to +0.10‰, (n=15, mean=-0.31‰) 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 δ30Si merit further examination: declining δ30Si in an area roughly corresponding to the permanent floodplains despite net DSi removal, and increasing δ30Si in the area corresponding to the seasonal floodplains. We infer that evaporative enrichment of surface waters creates two contrasting regimes. Chemical weathering of low δ30Si phases releases low δ30Si 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

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…Anthropogenic perturbations of the global biogeochemical Si cycle are due to the gradual aggradation or depletion of the amorphous SiO 2 pool held in continental soils (Barão et al, 2015;Vandevenne et al, 2015) and aquatic sediments (Frings et al, 2014b) in response to these changing environmental forcings (Struyf and Conley, 2012) and river damming (Conley et al, 1993). Our emerging understanding is that DSi inputs from the continents have potentially altered the magnitude and δ 30 Si composition of DSi supplied to the open ocean mostly because of changes occurring on the continent as well as changes of the silica sink on continental margins (Bernard et al, 2010;Frings et al, 2016).…”

“…For example, along the Yangtze River uptake of DSi by grasses in wetlands and rice in paddy fields drives a progressive increase in δ 30 Si DSi due to better phytolith preservation in areas of high phytolith production and/or low phytolith dissolution (Ding et al, 2004). In contrast the net impact of vegetation on δ 30 Si DSi seems to be minimal in the Okavango Delta, Congo and Amazon Basin due to the rapid dissolution/recycling of phytoliths Hughes et al, 2013;Frings et al, 2014b).…”

Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward

“…Street-Perrott and . Secondly, refilling of the lake basin after 15 ka BP (Berke et al, 2012;Johnson et al, 1996;Stager and Johnson, 2008) may have remobilized large amounts of isotopically depleted BSi from newly flooded wetland soils (Frings et al, 2014b;Struyf et al, 2015). Thirdly, Lake Victoria overflowed into the White Nile from~14.2 to 14 ka BP onwards Williams et al, 2006), shortening the residence time of DSi in the waterbody (Beuning and Russell, 2004).…”

“…Si in rivers comprises both dissolved and particulate matter; measurement of both permits assessment of both weathering-and productivity-related fractionation. Modern studies of Si isotopes in large fluvial systems have documented intense biotic Si cycling in the humid tropics (Frings et al, 2014b;Hughes et al, 2012Hughes et al, , 2011Hughes et al, , 2013, which may progressively elevate d 30 Si values downstream in rivers like the Nile that traverse extensive lakes and wetlands, in which DSi is efficiently stripped from the water by both biotic uptake and neoformation of clays (Cockerton et al, 2013;Ding et al, 2004;Fontorbe et al, 2013).…”

“…Major uncertainties still surround the impact of Late Quaternary fluctuations in climate and terrestrial ecosystems on the continental weathering flux of DSi and hence on the oceanic carbon cycle (De La Rocha and Tr eguer, 2012;De La Rocha and Bickle, 2005;Frings et al, 2014a;Froelich et al, 1992;Laruelle et al, 2009;Lupker et al, 2013;von Blanckenburg et al, 2015). Analyses of Si isotopes in diatomaceous lake muds, which are largely immune to recycling by surface processes, have the potential to elucidate the response of the continental Si cycle to past changes in climate, hydrology, vegetation and chemical weathering (Chen et al, 2012;StreetPerrott et al, 2008;Swann et al, 2010).…”