The silicon isotopic composition of the Ganges and its tributaries

Abstract

International audienceThe silicon isotopic composition (δ30Si) of the headwaters of the Ganges River, in the Himalaya, ranged from +0.49±0.01 to +2.17±0.04 at dissolved silicon (DSi) concentrations of 38 to 239μM. Both the concentration and isotopic composition of DSi in the tributaries increased between the highest elevations to where the Ganges leaves the Himlayas at Rishikesh. The tributaries exhibit a linear correlation between δ30Si and DSi that may represent mixing between a low DSi, low δ30Si (e.g., 40μM, +0.5 ) component potentially reflecting fractionation during adsorption of a small fraction of silicon on to iron oxides and a high DSi, high δ30Si component (e.g., 240μM, +1.7 ) produced during higher intensity weathering with a greater proportional sequestration of weathered silicon into secondary minerals or biogenicsiica. On the Ganges alluvial plain, in the Ganges and the Yamuna, Gomati, and their tributaries, DSi ranged from 122 to 218μM while δ30Si ranged from +1.03±0.03 to +2.46±0.06 . Highest values of δ30Si occurred in the Gomati and its tributaries. In general, the lower DSi and higher δ30Si of DSi in these rivers suggests control of both by removal of DSi by secondary mineral formation and/or biogenic silica production. A simple 1-dimensional model with flow through a porous medium is introduced and provides a useful framework for understanding these results

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

“…By isotope mass-balance: (Cockerton et al 2013;Fontorbe et al 2013;Hughes et al 2012). Confirmation of this effect has been demonstrated in both lake dominated boreal regions (Engstrom et al 2010) and in the tropical Congo river (Hughes et al 2012 Mechanisms to induce variations in lake Si retention efficiency 100-1,000 Year timescales Several centennial to millennial scale ontogenetic mechanisms exist to alter lake Si retention efficiency and thereby the magnitude and d 30 Si of river DSi fluxes.…”

“…The silicon isotopic ratio (d 30 Si) of DSi in rivers is systematically heavier than the ratio in catchment bedrock, since the lighter 28 Si isotope is preferentially incorporated into secondary minerals and BSi , requiring a continental sink to balance river export (Basile-Doelsch et al 2005). d 30 Si of DSi typically (but not always) increases along river longitudinal profiles, implying progressive preferential retention of 28 Si (Hughes et al 2012;Fontorbe et al 2013;Cardinal et al 2010;Cockerton et al 2013). Based on mass-balance De La Rocha et al (2000) concluded that 64 % of weathered silicon is retained in secondary clay minerals, which is in rough agreement with the stoichiometry of weathering reactions.…”

Lithium isotope behaviour during weathering in the Ganges Alluvial Plain

“…It has equally been shown that on average 557 High Himalayan catchments are even more congruent (Fontorbe et al, 2013;558 Kisakürek et al, 2005). This behaviour makes it abundantly clear that dominant 559 secondary mineral formation, and hence isotope fractionation, occurs in the 560 floodplains.…”

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

“…Kinetic limitation refers to a situation where the weathering flux (solute, DSi) is operating at maximum capacity for the conditions; increasing the factors that control the weathering rate (essentially temperature or water supply) will increase the rate of DSi export, because there is an excess of fresh material to be weathered. This could be e.g., the high Himalaya, the Andes, or (sub) glacial catchments (Georg et al, 2007;Fontorbe et al, 2013;Opfergelt et al, 2013) where physical erosion greatly outpaces chemical weathering. At very high erosion rates (low weathering intensity or congruency) river δ 30 Si is low: there is no time, and no thermodynamic driving force, for clays to form, soils to develop or biology to have a meaningful influence on the δ 30 Si measured in a stream.…”

“…Since the first data of DSi in freshwaters by De La Rocha (De La Rocha et al, 2000), δ 30 Si has been analyzed from a number of rivers (and lakes) across the world including the Congo River and tributaries Hughes et al, 2011), the Tana River (Hughes et al, 2012), the Yellow and Yangtze Rivers (Ding et al, 2004(Ding et al, , 2011, the Kalix River in Sweden (Engström et al, 2010), Swiss alpine rivers (Georg et al, 2006), and most recently from the Nile River (Cockerton et al, 2013), the Amazon river and tributaries , areas of central Siberia (Pokrovsky et al, 2013;Panizzo et al, 2017) and the Ganges (Fontorbe et al, 2013;Frings et al, 2015). The range in δ 30 Si from freshwaters, thus far analyzed, now stands at −0.17 to +4.66‰ (Figure 1).…”