The quantification of silicon isotopic fractionation by biotic and abiotic processes contributes to the understanding of the Si continental cycle. In soils, light Si isotopes are selectively taken up ...by plants, and concentrate in secondary clay-sized minerals. Si can readily be retrieved from soil solution through the specific adsorption of monosilicic acid (
H
4
Si
O
4
0
) by iron oxides. Here, we report on the Si-isotopic fractionation during
H
4
Si
O
4
0
adsorption on synthesized ferrihydrite and goethite in batch experiment series designed as function of time (0–504
h) and initial concentration (ic) of Si in solution (0.21–1.80
mM), at 20
°C, constant pH (5.5) and ionic strength (1
mM). At various contact times, the δ
29Si vs. NBS28 compositions were determined in selected solutions (ic
=
0.64 and 1.06
mM Si) by MC–ICP–MS in dry plasma mode with external Mg doping with an average precision of ±0.08‰ (±2
σ
SEM). Per oxide mass, ferrihydrite (74–86% of initial Si loading) adsorbed more Si than goethite (37–69%) after 504
h of contact over the range of initial Si concentration 0.42–1.80
mM. Measured against its initial composition (δ
29Si
=
+0.01
±
0.04‰ (±2
σ
SD)), the remaining solution was systematically enriched in
29Si, reaching maximum δ
29Si values of +0.70
±
0.07‰ for ferrihydrite and +0.50
±
0.08‰ for goethite for ic 1.06
mM. The progressive
29Si enrichment of the solution fitted better a Rayleigh distillation path than a steady state model. The fractionation factor
29ε (±1
σ
SD) was estimated at −0.54
±
0.03‰ for ferrihydrite and −0.81
±
0.12‰ for goethite. Our data imply that the sorption of
H
4
Si
O
4
0
onto synthetic iron oxides produced a distinct Si-isotopic fractionation for the two types of oxide but in the same order than that generated by Si uptake by plants and diatoms. They further suggest that the concentration of light Si isotopes in the clay fraction of soils is partly due to
H
4
Si
O
4
0
sorption onto secondary clay-sized iron oxides.
Biological productivity in the ocean directly influences the partitioning of carbon between the atmosphere and ocean interior. Through this carbon cycle feedback, changing ocean productivity has long ...been hypothesized as a key pathway for modulating past atmospheric carbon dioxide levels and hence global climate. Because phytoplankton preferentially assimilate the light isotopes of carbon and the major nutrients nitrate and silicic acid, stable isotopes of carbon (C), nitrogen (N), and silicon (Si) in seawater and marine sediments can inform on ocean carbon and nutrient cycling, and by extension the relationship with biological productivity and global climate. Here, we compile water column C, N, and Si stable isotopes from GEOTRACES‐era data in four key ocean regions to review geochemical proxies of oceanic carbon and nutrient cycling based on the C, N, and Si isotopic composition of marine sediments. External sources and sinks as well as internal cycling (including assimilation, particulate matter export, and regeneration) are discussed as likely drivers of observed C, N, and Si isotope distributions in the ocean. The potential for C, N, and Si isotope measurements in sedimentary archives to record aspects of past ocean C and nutrient cycling is evaluated, along with key uncertainties and limitations associated with each proxy. Constraints on ocean C and nutrient cycling during late Quaternary glacial‐interglacial cycles and over the Cenozoic are examined. This review highlights opportunities for future research using multielement stable isotope proxy applications and emphasizes the importance of such applications to reconstructing past changes in the oceans and climate system.
Plain Language Summary
The ability of marine phytoplankton to fix carbon—and hence influence the air‐sea partitioning of the greenhouse gas carbon dioxide—highlights the potential for these organisms to influence global climate in the past and future. In addition to C, phytoplankton require nutrients including inorganic N and for certain groups, Si. Because nutrients fuel phytoplankton growth, tracing past nutrient uptake can inform on important aspects of past biological production. Phytoplankton preferentially incorporate the light isotopes of C, N, and Si into their cells and metabolic products. These isotopic signatures can be preserved in marine sediments, providing a means to reconstruct past changes in biological activity. Here we use new data to illuminate processes driving the stable isotopic composition of C, N, and Si in the water column and in marine sediments. We evaluate the processes that lead to changes in the concentration of these elements and their isotopes in the ocean. We discuss scientific caveats and the extent of uncertainty relevant for interpreting past records of these isotopes. We then discuss examples of representative geochemical reconstructions using sediment records from the last ice age and over the last 70 million years. We use this knowledge to highlight directions for future research.
Key Points
Review of oceanic distribution, controlling processes, and sedimentary archives of carbon (C), nitrogen (N), and silicon (Si) isotopes
Late Quaternary C, N, and Si sedimentary isotope records demonstrate coupling between ocean carbon and nutrient cycling and atmospheric CO2 levels
Cenozoic C, N, and Si sedimentary isotope records indicate large‐scale changes in nutrient sources, concentrations, and the carbon cycle
In order to determine the origin of silicon (Si) in banded iron formation (BIF), we have undertaken a multi-tracer study combining REE+Y data, Ge/Si ratios and Si isotopes (δ30Si) on ...stratigraphically resolved layers from a ∼2.95Ga BIF from the Pongola Supergroup, South Africa. Si in both Si-rich and Fe-rich layers has a common origin, represented by a seawater reservoir strongly influenced by continent-derived freshwaters (∼10%) and very limited (<0.1%) high-T hydrothermal fluids as indicated by Eu anomalies and Y/Ho ratios. The coevolution of δ30Si signatures of Si- and Fe-rich layers of the BIF coupled with similar Eu and Y anomalies in both types of layers is in accordance with a common silica precipitation promoted by Si adsorption onto Fe-oxyhydroxides from Archaean seawater. An increase in δ30Si values from −2.27‰ to −0.53‰ stratigraphically upwards in the BIF is inferred to be the result of two successive isotopic fractionation processes during (1) silicon adsorption onto the Fe-oxyhydroxide precursor and (2) silica precipitation at the sediment–water interface from pore fluid triggered by the local silica saturation consecutive to an early diagenetic Si desorption from the precursor Fe-oxyhydroxide. The first fractionation process depleted the parental water in 28Si while the second released 30Si back into the parental water, resulting in an increase of the δ30Si value of the parental water reservoir over time.
► Silicon in both Si-rich and Fe-rich layers has a common origin. ► Parental fluid is seawater with about 10% freshwater and 0.1% hydrothermal fluids. ► Si- and Fe-rich layers have a common siliceous ferric oxyhydroxides precursor. ► δ30Si variations in Si-rich layers are controlled by two successive fractionation processes.
We present here the first large-scale study of riverine silicon isotope signatures in the Amazon Basin. The Amazon and five of its main tributaries were studied at different seasons of the annual ...hydrological cycle. The δ30Si signature of the dissolved silicon (DSi) exported to the estuary (weighted for DSi flux) for the period considered is estimated at +0.92‰. A river cross-section shows the homogeneity of the Amazon River regarding DSi concentration and isotope ratio. The biogenic silica (BSi) concentration measured in surface water from all rivers is generally small compared to the DSi reservoir but large variations exist between rivers. Very low isotope signatures were measured in the upper Rio Negro (δ30Si=+0.05±0.06‰), which we explain both by an equilibrium between clay formation and dissolution and by gibbsite formation. The Si isotope fractionation in the Andean tributaries and the Amazon main stem can be explained by clay formation and follow either a Rayleigh or a batch equilibrium fractionation model. Our results also suggest that the formation of 2:1 clays induces a fractionation factor similar to that of kaolinite formation.
A massive diatom bloom is observed each year in the surface waters of the naturally Fe-fertilized Kerguelen Plateau (Southern Ocean). We measured biogenic silica production and dissolution fluxes ...(ρSi and ρDiss, respectively) in the mixed layer in the vicinity of the Kerguelen Plateau during austral spring 2011 (KEOPS-2 cruise). We compare results from a high-nutrient low-chlorophyll reference station and stations with different degrees of iron enrichment and bloom conditions. Above the plateau biogenic ρSi are among the highest reported so far in the Southern Ocean (up to 47.9 mmol m−2 d−1). Although significant (10.2 mmol m−2 d−1 on average), ρDiss were generally much lower than production rates. Uptake ratios (ρSi : ρC and ρSi : ρN) confirm that diatoms strongly dominate primary production in this area. At the bloom onset, decreasing dissolution-to-production ratios (D : P) indicate that the remineralization of silica could sustain most of the low silicon uptake and that the system progressively shifts toward a silica production regime which must be mainly supported by new source of silicic acid. Moreover, by comparing results from the two KEOPS expeditions (spring 2011 and summer 2005), we suggest that there is a seasonal evolution of the processes decoupling Si and N cycles in the area. Indeed, the consumption of H4SiO4 standing stocks occurs only during the growing stage of the bloom when strong net silica production is observed, contributing to higher H4SiO4 depletion relative to NO3−. Then, the decoupling of H4SiO4 and NO3− is mainly controlled by the more efficient nitrogen recycling relative to Si. Gross Si : N uptake ratios were higher in the Fe-rich regions compared to the high-nutrient low-chlorophyll (HNLC) area, likely due to different diatom communities. This suggests that the diatom responses to natural Fe fertilization are more complex than previously thought, and that natural iron fertilization over long timescales does not necessarily decrease Si : N uptake ratios as suggested by the silicic acid leakage hypothesis. Finally, we propose the first seasonal estimate of the Si biogeochemical budget above the Kerguelen Plateau based on direct measurements. This study points out that naturally iron-fertilized areas of the Southern Ocean could sustain very high regimes of biogenic silica production, similar to those observed in highly productive upwelling systems.
The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H
4SiO
4
0), thereby influencing ...Si concentration in soil solution. Here we study the impact of H
4SiO
4
0 adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0–72
h) and initial concentration (ic) of Si in solution (0.61–1.18
mM) at 20
°C, constant pH (5.5) and ionic strength (1
mM). After various soil-solution contact times, the δ
30Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σ
SEM). The quantitative adsorption of H
4SiO
4
0 by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ
30Si
=
+0.02
±
0.07‰ (±2σ
SD)), the solutions were systematically enriched in
30Si reaching maximum δ
30Si values ranging between +0.16‰ and +0.95‰ after 72
h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H
4SiO
4
0 adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.
Although the Southern Ocean is considered a high-nutrient, low-chlorophyll (HNLC) area, massive and recurrent blooms are observed over and downstream of the Kerguelen Plateau. This mosaic of blooms ...is triggered by a higher iron supply resulting from the interaction between the Antarctic Circumpolar Current and the local bathymetry. Net primary production, N uptake (NO3− and NH4+), and nitrification rates were measured at eight stations in austral spring 2011 (October–November) during the KEOPS 2 cruise in the Kerguelen Plateau area. Natural iron fertilization stimulated primary production, with mixed layer integrated net primary production and growth rates much higher in the fertilized areas (up to 315 mmol C m−2 d−1 and up to 0.31 d−1 respectively) compared to the HNLC reference site (12 mmol C m−2 d−1 and 0.06 d−1 respectively). Primary production was mainly sustained by nitrate uptake, with f ratios (corresponding to NO3−-uptake / (NO3−-uptake + NH4+-uptake)) lying at the upper end of the observations for the Southern Ocean (up to 0.9). We report high rates of nitrification (up to ~ 3 μmol N L−1 d−1, with ~ 90 % of them < 1 μmol N L−1 d−1) typically occurring below the euphotic zone, as classically observed in the global ocean. The specificity of the studied area is that at most of the stations, the euphotic layer was shallower than the mixed layer, implying that nitrifiers can efficiently compete with phytoplankton for the ammonium produced by remineralization at low-light intensities. Nitrate produced by nitrification in the mixed layer below the euphotic zone is easily supplied to the euphotic zone waters above, and nitrification sustained 70 ± 30 % of the nitrate uptake in the productive area above the Kerguelen Plateau. This complicates estimations of new production as potentially exportable production. We conclude that high productivity in deep mixing system stimulates the N cycle by increasing both assimilation and regeneration.
The terrestrial biogenic Si (BSi) pool in the soil–plant system is ubiquitous and substantial, likely impacting the land-ocean transfer of dissolved Si (DSi). Here, we consider the mechanisms ...controlling DSi in forest soil in a temperate granitic ecosystem that would differ from previous works mostly focused on tropical environments. This study aims at tracing the source of DSi in forest floor leachates and in soil solutions under various tree species at homogeneous soil and climate conditions, using stable Si isotopes and Ge/Si ratios. Relative to granitic bedrock, clays minerals were enriched in
28Si and had high Ge/Si ratios, while BSi from phytoliths was also enriched in
28Si, but had a low Ge/Si ratio. Such a contrast is useful to infer the relative contribution of silicate weathering and BSi dissolution in the shallow soil on the release of DSi in forest floor leachate solutions. The δ
30Si values in forest floor leachates (−1.38‰ to −2.05‰) are the lightest ever found in natural waters, and Ge/Si ratios are higher in forest floor leachates relative to soil solution. These results suggest dissolution of
28Si and Ge-enriched secondary clay minerals incorporated by bioturbation in organic-rich horizons in combination with an isotopic fractionation releasing preferentially light Si isotopes during this dissolution process. Ge/Si ratios in soil solutions are governed by incongruent weathering of primary minerals and neoformation of secondary clays minerals. Tree species influence Si-isotopic compositions and Ge/Si ratios in forest floor leachates through differing incorporation of minerals in organic horizons by bioturbation and, to a lesser extent, through differing Si recycling.
As part of the GEOTRACES Bonus-GoodHope (BGH) expedition (January–March 2008) in the Atlantic sector of the Southern Ocean, particulate organic carbon (POC) export was examined from the surface to ...the mesopelagic twilight zone using water column distributions of total 234Th and biogenic particulate Ba (Baxs). Surface POC export production was estimated from steady state and non steady state modelling of 234Th fluxes, which were converted into POC fluxes, using the POC/234Th ratio of large, potentially sinking particles (> 53 μm) collected via in situ pumps. Deficits in 234Th activities were observed at all stations from the surface to the bottom of the mixed layer, yielding 234Th export fluxes from the upper 100 m of 496 214 dpm m−2 d−1 to 1195 158 dpm m−2 d−1 for the steady state model and of 149 517 dpm m−2 d−1 to 1217 231 dpm m−2 d−1 for the non steady state model. Using the POC/234Thp ratio of sinking particles (ratios varied from 1.7 0.2 μmol dpm−1 to 4.8 1.9 μmol dpm−1) POC export production at 100 m was calculated to range between 0.9 0.4 and 5.1 2.1 mmol C m−2 d−1,assuming steady state and between 0.3 0.9 m−2 d−1 and 4.9 3.3 mmol C m−2 d−1, assuming non steady state. From the comparison of both approaches, it appears that during late summer export decreased by 56 to 16% for the area between the sub-Antarctic zone and the southern Antarctic Circumpolar Current Front (SACCF), whereas it remained rather constant over time in the HNLC area south of the SACCF. POC export represented only 6 to 54% of new production, indicating that export efficiency was, in general, low, except in the vicinity of the SACCF, where export represented 56% of new production. Attenuation of the POC sinking flux in the upper mesopelagic waters (100–600 m depth interval) was evidenced both, from excess 234Th activities and from particulate biogenic Ba (Baxs) accumulation. Excess 234Th activities, reflected by 234Th/238U ratios as large as 1.21 0.05, are attributed to remineralisation/disaggregation of 234Th-bearing particles. The accumulation of excess 234Th in the 100–600 m depth interval ranged from 458 633 dpm m−2 d−1 to 3068 897 dpm m−2 d−1, assuming steady state. Using the POC/234Thp ratio of sinking particles (> 53 μm), this 234Th accumulation flux was converted into a POC remineralisation flux which ranged between 0.9 1.2 mmol C m−2 d−1 and 9.2 2.9 mmol C m−2 d−1. Mesopelagic particulate biogenic Ba has been reported to reflect bacterial degradation of organic matter and to be related to oxygen consumption and bacterial carbon respiration. We observed that the highest Baxs contents (reaching up to > 1000 pM), in general, occurred between 200 and 400 m. Depth-weighted average mesopelagic Baxs (meso-Baxs) values were converted into respired C fluxes, which ranged between 0.23 and 6.4 mmol C m−2 d−1, in good agreement with 234Th-based remineralisation fluxes. A major outcome from this study is the observed significant positive correlation between POC remineralisation as estimated from meso-Baxs contents and from 234Th excess (R2 = 0.73; excluding 2 outliers). Remineralisation of POC in the twilight zone was particularly efficient relative to POC export resulting in negligible bathypelagic (> 600 m) POC export fluxes in the sub-Antarctic zone, the Polar Front zone and the northern Weddell Gyre, while the subtropical zone as well as the vicinity of the SACCF had significant deep POC fluxes.
We present the first study of the Si isotopic composition of dry season river waters from the Tana River, Kenya. Data encompasses the entire river basin, with samples collected from headwaters to the ...estuary, thereby capturing a salinity gradient. In the headwaters, the isotopic signature is affected by climate, as a result of its control on soil drainage and weathering. The δ30Si signatures in the basin range from +0.69‰ up to +2.23‰. Signatures are clearly affected by dams: an increase in δ30Si ratios of 0.54‰ and a decrease in the dissolved Si (DSi) concentration by 41% were observed downstream of the Masinga dam, the largest of a succession of 5 hydroelectric dams. This reduction in Si load is most likely due to increased diatom productivity as the corresponding change in δ30Si signature is consistent with the known fractionation by these organisms. The δ30Si composition of waters entering the estuary is ca. +2‰ and DSi concentrations are 349μM. In the estuary, the DSi concentrations decrease linearly following the salinity, while the δ30Si ratio remains stable, indicating the absence of processes affecting the DSi pool for the studied range of salinity.
► First study of Si isotopes in river water at the basin scale. ► In headwaters, the isotopic signature seems linked to soil drainage. ► The impact of diatoms in dam reservoir is clearly observed. ► δ30Si ratio remains stable in the estuary.
PDF
