Abstract
On Quaternary time scales, the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae (notably diatoms, for which Si is an essential nutrient). Large tropical river systems dominate the export of Si from the continents to the oceans. Here, we investigate variations in Si cycling in the upper White Nile basin over the last 15 ka, using sediment cores from Lakes Victoria and Edward. Coupled measurements of stable O and Si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and Si cycling, while the abundances of lipid biomarkers characteristic of terrestrial/emergent higher plants, submerged/floating aquatic macrophytes and freshwater algae document past ecosystem changes. During the late-glacial to mid-Holocene, 15–5.5 ka BP, orbital forcing greatly enhanced monsoon rainfall, forest cover and chemical weathering. Riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower Si-isotope values. Since 5.5 ka BP, increasingly dry climates and more open vegetation, reinforced by the spread of agricultural cropland over the last 3–4 ka, have reduced dissolved silica inputs into the lakes. Centennial-to millennial-scale dry episodes are also evident in the isotopic records and merit further investigation.
Highlights
The biogeochemical cycles of essential nutrients such as carbon, nitrogen, phosphorus and silicon are major determinants of global climate and ecophysiology on Quaternary time scales (Berner and Berner, 2012; Kump et al, 2013; Schlesinger and Bernhardt, 2013)
The Si cycle is interlocked with the carbon cycle (Conley, 2002; Street-Perrott and Barker, 2008) through several key processes: enhancement of silicate rock weathering by biotic activity (Kelly et al, 1998; Lucas, 2001); pumping of dissolved silica (DSi) by higher plants to form biogenic silica (BSi) particles known as opal phytoliths, which are deposited by litterfall and actively recycled by
Since DSi is an essential nutrient for marine diatoms, which largely control the export of organic carbon to the deep ocean (Smetacek, 1999; Yool and Tyrrell, 2003), Quaternary variations in DSi output from tropical rivers could potentially have had a significant impact on the drawdown of atmospheric CO2 by the marine biological pump
Summary
The biogeochemical cycles of essential nutrients such as carbon, nitrogen, phosphorus and silicon are major determinants of global climate and ecophysiology on Quaternary time scales (Berner and Berner, 2012; Kump et al, 2013; Schlesinger and Bernhardt, 2013). The instantaneous d30Si value of precipitating silica is dependent on the evolving d30Si of the water and the degree of utilization of the available DSi. Theoretically, in a “closed” system such as the oceanic mixed layer or the epilimnion of a large, stratified lake, Si-isotope fractionation by diatoms will follow a Rayleigh model and the d30Si of the accumulated silica (recorded by d30Sidiatom in sediment cores) will rise towards the starting composition of the source water as the DSi supply is exhausted (Alleman et al, 2005; Basile-Doelsch, 2006; De La Rocha, 2006). Si-isotope data have been used as a proxy to reconstruct palaeoproductivity (or more strictly marine silicic-acid use by diatoms relative to initial dissolved silicic-acid concentrations)
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