Weathering, dust, and biocycling effects on soil silicon isotope ratios

Bern, Carleton R.; Brzezinski, Mark A.; Beucher, Charlotte; Ziegler, K.; Chadwick, Oliver A.

doi:10.1016/j.gca.2009.10.046

Cited by 65 publications

(45 citation statements)

References 48 publications

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“…Increased clay genesis during soil formation, including interactions of Si with ferric oxides after the rise of atmospheric oxygen, would result in preferential removal of light Si on particles, and an increase in the d 30 Si of the dissolved load in rivers ultimately delivered to the oceans. Extent of silica depletion as well as wetter versus drier climates can also alter soil d 30 Si (Bern et al, 2010). These processes could also explain the high d 30 Si in mid-Proterozoic cherts, which is a prominent feature in both our dataset and that of Robert and Chaussidon (2006).…”

Section: Toward Understanding the Temporal Variation In The Si Isotopmentioning

confidence: 84%

Si isotope variability in Proterozoic cherts

Chakrabarti

Knoll

Jacobsen

et al. 2012

Geochimica et Cosmochimica Acta

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We report Si-isotopic compositions of 75 sedimentologically and petrographically characterized chert samples with ages ranging from $2600 to 750 Ma using multi-collector inductively coupled plasma mass spectrometry. d 30 Si values of the cherts analyzed in this study show a $7& range, from À4.29 to +2.85. This variability can be explained in part by (1) simple mixing of silica derived from continental (higher d 30 Si) and hydrothermal (lower d 30 Si) sources, (2) multiple mechanisms of silica precipitation and (3) Rayleigh-type fractionations within pore waters of individual basins.We observe $3& variation in peritidal cherts from a single Neoproterozoic sedimentary basin (Spitsbergen). This variation can be explained by Rayleigh-type fractionation during precipitation from silica-saturated porewaters. In some samples, postdissolution and reprecipitation of silica could have added to this effect. Our data also indicate that peritidal cherts are enriched in the heavier isotopes of Si whereas basinal cherts associated with banded iron formations (BIF) show lower d 30 Si. This difference could partly be due to Si being derived from hydrothermal sources in BIFs. We postulate that the difference in d 30 Si between non-BIF and BIF cherts is consistent with the contrasting genesis of these deposits. Low d 30 Si in BIF is consistent with laboratory experiments showing that silica adsorbed onto Fe-hydroxide particles preferentially incorporates lighter Si isotopes.Despite large intrabasinal variation and environmental differences, the data show a clear pattern of secular variation. Low d 30 Si in Archean cherts is consistent with a dominantly hydrothermal source of silica to the oceans at that time. The monotonically increasing d 30 Si from 3.8 to 1.5 Ga appears to reflect a general increase in continental versus hydrothermal sources of Si in seawater, as well as the preferential removal of lighter Si isotopes during silica precipitation in iron-associated cherts from silica-saturated seawater. The highest d 30 Si values are observed in 1.5 Ga peritidal cherts; in part, these enriched values could reflect increasing sequestration of light silica during soil-forming processes, thus, delivering relatively heavy dissolved silica to the oceans from continental sources. The causes behind the reversal in trend towards lower d 30 Si in cherts younger than 1.5 Ga old are less clear. Cherts deposited 1800-1900 Ma are especially low d 30 Si, a possible indication of transiently strong hydrothermal input at this time.

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Section: Toward Understanding the Temporal Variation In The Si Isotopmentioning

confidence: 84%

Si isotope variability in Proterozoic cherts

Chakrabarti

Knoll

Jacobsen

et al. 2012

Geochimica et Cosmochimica Acta

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“…Several studies have noted the typical δ 30 Si of basalt itself of −0.29 to −0.50 ‰ Georg et al, 2007b;Abraham et al, 2008;Bern et al, 2010;Opfergelt and Delmelle, 2012;Pogge von Strandmann et al, 2012). Other studies provide data on the δ 30 Si of DSi produced during the subaerial weathering of basalt, yielding values ranging from −1.0 to +0.4 ‰.…”

Section: Equations For Change In the Isotopic Composition Of Dsi And Bsimentioning

confidence: 99%

Exploring interacting influences on the silicon isotopic composition of the surface ocean: a case study from the Kerguelen Plateau

2014

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Abstract. This study presents six new water column profiles of the silicon isotopic composition (δ 30 Si) of dissolved silicon (DSi) from the Atlantic and Indian sectors of the Southern Ocean and a variable depth box model of silica cycling in the mixed layer that was constructed to illuminate the evolution of surface ocean δ 30 Si over the full course of a year. In keeping with previous observations, δ 30 Si values ranged from +1.9 to +2.4 ‰ in the mixed layer (ML), +1.2 to +1.7 ‰ in Winter Water (WW), and +0.9 to +1.4 ‰ in Circumpolar Deep Water (CDW). These data also confirmed the occurrence of diminished values for ML δ 30 Si at low DSi concentrations in early austral autumn on the Kerguelen Plateau. The box model was used to investigate whether these low, post-growing season values of δ 30 Si were related to input of DSi to the ML from basalt weathering, biogenic silica dissolution (with or without isotopic fractionation), the onset of winter mixing, or some combination of the three. Basalt weathering and fractionation during biogenic silica dissolution could both lower ML δ 30 Si below what would be expected from the extent of biological uptake of DSi. However, the key driver of the early autumn decrease in δ 30 Si appears to be the switch from bloom growth (with net removal of DSi and net accumulation of biogenic silica (BSi) biomass) to steady state growth (when slow but continuing production of BSi prevented significant net increase in DSi concentrations with diffusive input of DSi from WW but not decrease in ML δ 30 Si towards WW values). Model results also indicated that fractionation during dissolution has only a negligible effect on the δ 30 Si of BSi exported throughout the course of the year. However, seasonal changes in export efficiency (e.g. favouring the export of bloom BSi versus the export of BSi produced during other times of the year) should strongly influence the δ 30 Si of BSi accumulating in marine sediments.Finally, the choice for the parameterisation of the mixing between the ML and the WW in terms of δ 30 Si (i.e. constant or allowed to vary with the seasonal migration of the thermocline) is critical to take into account in box model simulations of the silica biogeochemical cycle. Altogether, these results suggest that as a paleoceanographic proxy, δ 30 Si may more reflect the dominant mode of production of the BSi that is exported (i.e. bloom versus steady state growth) rather than strictly the extent of DSi utilisation by diatoms.

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“…Two tracers of the Si cycle in weathering environments have been identified so far: Ge/Si ratios (Derry et al, 2005;Lugolobi et al, 2010) and stable Si isotopes (Ding et al, 2004;Ziegler et al, 2005a;Georg et al, 2006Georg et al, , 2007Georg et al, , 2009aBern et al, 2010;Cornelis et al, 2010a;Engström et al, 2010;Opfergelt et al, 2010). Through the calculation of Si uptake, Si restitution, Si drainage (Table 3), atmospheric Si inputs and phytolith preservation in soils, a Si mass-balance in specific ecosystems allows an estimation of the Si release by silicate weathering and by phytolith dissolution, respectively.…”

Section: Tracing Biogeochemical Si Cycle In the Soil-plant Systemmentioning

confidence: 99%

“…The Si isotope composition of a sample is expressed as δ 30 Si defined as follows: (Ziegler et al, 2005a, b;Opfergelt et al, 2008Opfergelt et al, , 2010Bern et al, 2010;Cornelis et al, 2010a), the plant Si uptake producing biogenic opal (Douthitt, 1982;Ziegler et al, 2005a;Ding et al, 2005;Opfergelt et al, 2006a, b;Ding et al, 2008;Opfergelt et al, 2008), and the adsorption of Si onto Fe-oxides Opfergelt et al, 2009) are three processes favouring the incorporation of light Si isotopes, contributing to enrich rivers in heavy Si isotopes (De La Rocha et al, 2000;Ding et al, 2004;Georg et al, 2006Georg et al, , 2007. Natural environments impacted simultaneously by all these processes will display a bulk Si isotopic signature of DSi.…”

Section: Tracing Biogeochemical Si Cycle In the Soil-plant Systemmentioning

confidence: 99%