Alkaline‐extractable silicon from land to ocean: A challenge for biogenic silicon determination

Abstract

The biogeochemical cycling of silicon (Si) along the land-to-ocean continuum is studied by a variety of research fields and for a variety of scientific reasons. However, there is an increasing need to refine the methodology and the underlying assumptions used to determine biogenic silica (BSi) concentrations. Recent evidence suggests that contributions of nonbiogenic sources of Si dissolving during alkaline extractions, not corrected by standard silicate mineral dissolution correction protocols, can be substantial. The ratio between dissolved Si and aluminum (Al) monitored continuously during the alkaline extraction can be used to infer the origin of the Si fractions present. In this study, we applied both a continuous analysis method (0.5 M NaOH) and a traditional 0.1 M Na 2 CO 3 extraction to a wide array of samples: (1) terrestrial vegetation, (2) soils from forest, cropland and pasture, (3) lake sediments, (4) suspended particulate matter and sediments from rivers, (5) sediments from estuaries and salt marshes and (6) ocean sediments. Our results indicate that the 0.1 M Na 2 CO 3 extraction protocol can overestimate the BSi content, by simultaneously dissolving Si fractions of nonbiogenic origin that may represent up to 100% of the Si traditionally considered as biogenic, hampering interpretation especially in some deeper soil horizons, rivers and coastal oceanic sediments. Moreover, although the term amorphous Si was coined to reflect a growing awareness of nonbiogenic phases we show it is actually inappropriate in samples where silicate minerals may account for a large part of the extracted Si even after linear mineral correction.

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…Human activities such as deforestation, eutrophication, agriculture, and damming have altered weathering rates, amounts of terrestrial BSi production, and the amount of amorphous Si retained and/or released from soils and freshwater systems (Struyf et al, 2010). Our recent work demonstrates the gradual aggradation or depletion of the amorphous Si pool held in continental soils (Barão et al, 2015) and aquatic sediments (Frings et al, 2014) in response to changing environmental forcings (Struyf and Conley, 2012). Our emerging understanding is that DSi inputs from the continents have potentially altered the magnitude and isotopic composition to an extent great enough to impact wholeocean isotopic signatures on the timescale of Quaternary glacial cycles (Bernard et al, 2010).…”

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).…”

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

“…The k parameter reflects how fast a Si-bearing fraction reaches complete dissolution in an alkaline media, and depends on bonding strengths and specific reactive surface areas. Here, relative differences of k values between modelled fractions are used to classify high and low reactive fractions in alkaline media, where nanocrystalline and absorbed Si fractions are suggested to be more rapidly released as compared to biogenic Si fractions (Barão et al, 2015). The number of fractions (x) in the first-order model was determined by consecutively allowing an extra fraction to obtain an optimal model fit (i.e.…”

“…phytoliths, diatoms), inorganic forms such as Al-Si precipitates, volcanic glass shards, adsorbed Si on amorphous iron oxides, and nanocrystalline fractions such as allophanes and imogolite can comprise a substantial portion of the nonlinearly dissolving Si. Partial dissolution of clays can also rapidly release Si (Barão et al, 2015;Koning et al, 2002). We introduce a procedural term "Si Alk " (alkaline extracted Si) to refer to the full range of Si-bearing phases that dissolve non-linearly under normal experimental conditions.…”

“…nanocrystalline minerals and secondary clays) could affect Si Alk measurements, as their dissolution characteristics in alkaline solutions can overlap with the biogenic fraction (Barão et al, 2015;Hashimoto and Jackson, 1960;Sauer et al, 2006). Discrete volcanic ash deposits, composed of shards, minerals, pumice and rock fragments, also known as tephra, are common in sedimentary archives.…”