Lack of steady-state in the global biogeochemical Si cycle: emerging evidence from lake Si sequestration

Abstract

Weathering of silicate minerals releases dissolved silicate (DSi) to the soil-vegetation system. Accumulation and recycling of this DSi by terrestrial ecosystems creates a pool of reactive Si on the continents that buffers DSi export to the ocean. Human perturbations to the functioning of the buffer have been a recent research focus, yet a common assumption is that the continental Si cycle is at steadystate. However, we have no good idea of the timescales of ecosystem Si pool equilibration with their environments. A review of modelling and geochemical considerations suggests the modern continental Si cycle is in fact characterised in the longterm by an active accumulation of reactive Si, at least partially attributable to lakes and reservoirs. These lentic systems accumulate Si via biological conversion of DSi to biogenic silica (BSi). An analysis of new and published data for nearly 700 systems is presented to assess their contribution to the accumulating continental pool. Surface sediment BSi concentrations (n = 692) vary between zero and [60 % SiO 2 by weight, apparently independently of lake size, location or water chemistry. Using sediment core BSi accumulation rates (n = 109), still no relationships are found with lake or catchment parameters. However, issues associated with single-core accumulation rates should in any case preclude their use in elemental accumulation calculations. Based on lake/reservoir mass-balances (n = 34), our best global-scale estimate of combined lake and reservoir Si retention is 1.53 TMol year -1 , or 21-27 % of river DSi export. Again, no scalable relationships are apparent, suggesting Si retention is a complex process that varies from catchment to catchment. The lake Si sink has implications for estimation of weathering flux generation from river chemistry. The size of the total continental Si pool is poorly constrained, as is its accumulation rate, but lakes clearly contribute substantially. A corollary to this emerging understanding is that the flux and isotopic composition of DSi delivered to the ocean has likely varied over time, partly mediated by a fluctuating continental pool, including in lakes.

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

“…Frings et al (2014a) highlight the effectiveness of lacustrine environments in retaining DSi in sediments via the bSiO 2 pump, which on a global scale can translate to a net export of between 21 and 27% of river DSi export. This equates to an estimation of 1.53 Tmol yr −1 (Frings et al, 2014a) (Figure 1). These data highlight the challenges in underpinning global estimates of lake biogeochemical cycling (particularly for large lakes and reservoirs) to better constrain the evolution of the global biogeochemical cycling of Si.…”

“…Lakes, in addition to the soil-vegetation system, play an important role in buffering continental DSi export to the oceans. Previous assumptions were that this buffering capacity of the continental Si cycle was in steady-state; however, this has now been challenged (Frings et al, 2014a). Frings et al (2014a) highlight the effectiveness of lacustrine environments in retaining DSi in sediments via the bSiO 2 pump, which on a global scale can translate to a net export of between 21 and 27% of river DSi export.…”

“…Previous assumptions were that this buffering capacity of the continental Si cycle was in steady-state; however, this has now been challenged (Frings et al, 2014a). Frings et al (2014a) highlight the effectiveness of lacustrine environments in retaining DSi in sediments via the bSiO 2 pump, which on a global scale can translate to a net export of between 21 and 27% of river DSi export. This equates to an estimation of 1.53 Tmol yr −1 (Frings et al, 2014a) (Figure 1).…”

How big is the influence of biogenic silicon pools on short-term changes in water-soluble silicon in soils? Implications from a study of a 10-year-old soil–plant system

“…Furthermore, evidence arises that BSi pools are in disequilibrium at decadal timescales due to disturbances and perturbations by humans, e.g., by changes in forest management or farming practices (Barão et al, 2014;Keller et al, 2012;Vandevenne et al, 2015). As a consequence, BSi accumulation and BSi dissolution are not balanced, which influences Si cycling in terrestrial biogeosystems, not only on decadal but also on millennial scales (Clymans et al, 2011;Frings et al, 2014;Sommer et al, 2013;Struyf et al, 2010). Sommer et al (2013), for example, found the successive dissolving of a relict phytogenic Si pool to be the main source of dissolved Si in soils of a forested biogeosystem.…”

The contribution of tephra constituents during biogenic silica determination: implications for soil and palaeoecological studies

“…changes in productivity, climate, precipitation and nutrient supply). In lacustrine sediment cores, BSi content can range from the detection limit (0.01 wt %) to > 70 wt % SiO 2 (Frings et al, 2014a). The downcore variations in BSi through time vary from as little as 2 wt % SiO 2 (Adams and Finkelstein, 2010;Ampel et al, 2008) to a high of 10-40 wt % SiO 2 (Johnson et al, 2011;Prokopenko et al, 2006;Van der Putten et al, 2015) and depends on several interacting factors such as mineral matter or organic matter accumulation, diatom productivity and preservation/dissolution processes.…”