Biogenic Silica In Sediments Research Articles

Potential deposition of biogenic silica source sediment in the Paleo-Yangtze Grand Underwater Delta estimated with satellite remote sensing

ABSTRACTBiogenic silica sediment constitutes one of the critical sources of particle income in the oceans. In this paper, satellite remote sensing of primary production, sinking flux, and molar ratio of Si/C were utilized to elucidate potential biogenic silica sediment in the Paleo-Yangtze Grand Underwater Delta. Primary production retrieval showed that daily primary production in the Paleo-Yangtze Grand Delta was 3.3–10.8 Mt d−1, with 5.8 Mt d−1 on average. Sinking flux, retrieved with in situ observations and satellite remote sensing, was within the range of 109–657 mg Cm−2 d−1, or about 23% of total primary production, on average. A molar ratio of Si/C of 0.11–0.45 interval was used in the area, for the Paleo-Yangtze Grand Delta was similar to coastal water and affected by many factors. Considering that phytoplankton in the Paleo-Yangtze Grand Underwater Delta is mainly diatom (80% on average), about 0.21–0.88 billion tons of biogenic silica source sediment is produced in the area annually. With the reduction of land source sediment occurring in recent years in the area, biogenic silica sediment may be one of the dominant sediment sources for maintaining the future stability of the Paleo-Yangtze Grand Underwater Delta.

Distribution of Biogenic Silica in Sediments of the Yellow River (Upper and Middle Reaches)

An experiment is held to extract the biogenic silica from Yellow River sediments to explore how such four influencing factors as extraction time, the type and concentration of extracting agent, and solid-liquid ratio affect the content of biogenic silicon in the samples. Besides, the distribution characteristics of biogenic silica in sediments of the Yellow River (upper and middle reaches) are discussed based on the measured results. The experiment results demonstrate the optimal conditions for the extraction of biogenic silica in sediments of the Yellow River: weight 0.050g of sediment sample at 90°C and extract biogenic silica for 7 hours with 40mL 1.0mol/L Na2CO3 solution as the extracting agent. The biogenic silica content of the surface sediments in the upper and middle reaches of the Yellow River varies greatly from 0.07-3.06mg/g. In descending order, the samples are arranged as: MD1>XH2>BT9>WLT8>SSG7>SZS5>XG4>GD3>WH6. The highest content appears in Maduo segment at the source of the Yellow River, and the lowest content appears in Wuhai segment.

Silicon isotope and silicic acid uptake in surface waters of Marguerite Bay, West Antarctic Peninsula

The silicon isotope composition (δ30Si) of dissolved silicon (DSi) and biogenic silica (BSi) provides information about the silicon cycle and its role in oceanic carbon uptake in the modern ocean and in the past. However, there are still questions outstanding regarding the impact of processes such as oceanic mixing, export and dissolution on the isotopic signature of seawater, and the impacts on sedimentary BSi. This study reports the δ30Si of DSi from surface waters at the Rothera Time Series (RaTS) site, Ryder Bay, in a coastal region of the West Antarctic Peninsula (WAP). The samples were collected at the end of austral spring through the end of austral summer/beginning of autumn over two field seasons, 2004/5 and 2005/6. Broadly, for both field seasons, DSi diminished and δ30Si of DSi increased through the summer, but this was accomplished during only a few short periods of net nutrient drawdown. During these periods, the δ30Si of DSi was negatively correlated with DSi concentrations. The Si isotope fractionation factor determined for the net nutrient drawdown periods, ɛuptake, was in the range of −2.26 to −1.80‰ when calculated using an open system model and −1.93 to −1.33‰ when using a closed system model. These estimates of ɛ are somewhat higher than previous studies that relied on snapshots in time rather than following changes in δ30Si and DSi over time, which therefore were more likely to include the effects of mixing of dissolved silicon up into the mixed layer. Results highlight also that, even at the same station and within a single growing season, the apparent fractionation factor may exhibit significant temporal variability because of changes in the extent of biological removal of DSi, nutrient source, siliceous species, and mixing events.Paleoceanographic studies using silicon isotopes need careful consideration in the light of our new results.

Use of biogenic silica in sediment to record diatom density and their responses to environmental change in Sanggou Bay maricultural area over the past 200 years

Two sediment cores (S1 and S2 from shellfish and kelp cultivation areas, respectively) were collected from the Sanggou Bay maricultural area for the analysis of biogenic silica (BSi), an ideal proxy for the reconstruction of diatom density. The results show that prior to the advent of mariculture, the maximum diatom density in the two cores were observed in the 1880s before the first industrial revolution, and the maximum densities were 6500 × 104 ind m−3 and 5300 × 104 ind m−3 for S1 and S2, respectively. The diatom density was primarily affected by natural factors, such as the sea surface temperature (SST), the East Asian Winter Monsoon Index (EAWM), and the wild shellfish feeding pressure. The diatom density was affected by different cultivation methods and maricultural density during the maricultivation period. The diatom density decreased to 2800 × 104 ind m−3 in the shellfish cultivation area (S1) in 1996, and the diatom density was only 3600 × 104 ind m−3 in the kelp cultivation area (S2). Subsequently, a shellfish-kelp polyculture approach was implemented and diatom density started to continuously increase. The total diatom densities in S1 and S2 were restored to 3700 × 104 ind m−3 and 4200 × 104 ind m−3 in 2006, respectively. Nonetheless, the total diatom density in core S1 was only 55% of the density in the 1880s. This observation indicates that additional efforts are required to promote modifications to improve the cultivation environment and establish an environmentally-friendly cultivation model.

Composition and variability in the export of biogenic silica in the Changjiang River and the effect of Three Gorges Reservoir

Silicon (Si) plays an essential role in biogeochemical processes, but is still poorly characterized in the river system. This study addressed the biogenic silica (BSi) composition, origin and variation in the Changjiang River, and estimated the impacts of natural processes and human activities on the river Si cycling. Our results indicate that phytoliths comprised 14%–64% of BSi, while diatoms accounted for 34%–85% of BSi. The Changjiang River transported 620Ggyr−1 of BSi and 2100Ggyr−1 of dissolved silicate (DSi) loadings, respectively; 55% of the BSi and 51% of the DSi fluxes are transported during the high discharge period from June to September. The Changjiang River carried phytolith BSi mostly comes from the middle and lower reaches area. The ratio of BSi/(BSi+DSi) has decreased from 0.47 before 1980 to 0.19 in 2013–2014 due to the direct retention of BSi. The BSi sedimentation in the Three Gorges Reservoir would cause a decrease of total reactive silica, but contribute to approximately 4%–16% of the DSi loading at the Jiangyin station due to its dissolution. This study demonstrates that phytoliths represent a significant contribution to the biogeochemical cycle of silica in coastal waters, and in-stream process exerts a great influence on the river Si loading and cycling.

Deep ocean mass fluxes in the coastal upwelling off Mauritania from 1988 to 2012: variability on seasonal to decadal timescales

Abstract. A more than two-decadal sediment trap record from the Eastern Boundary Upwelling Ecosystem (EBUE) off Cape Blanc, Mauritania, is analysed with respect to deep ocean mass fluxes, flux components and their variability on seasonal to decadal timescales. The total mass flux revealed interannual fluctuations which were superimposed by fluctuations on decadal timescales. High winter fluxes of biogenic silica (BSi), used as a measure of marine production (mostly by diatoms) largely correspond to a positive North Atlantic Oscillation (NAO) index (December–March). However, this relationship is weak. The highest positive BSi anomaly was in winter 2004–2005 when the NAO was in a neutral state. More episodic BSi sedimentation events occurred in several summer seasons between 2001 and 2005, when the previous winter NAO was neutral or even negative. We suggest that distinct dust outbreaks and deposition in the surface ocean in winter and occasionally in summer/autumn enhanced particle sedimentation and carbon export on short timescales via the ballasting effect. Episodic perturbations of the marine carbon cycle by dust outbreaks (e.g. in 2005) might have weakened the relationships between fluxes and large-scale climatic oscillations. As phytoplankton biomass is high throughout the year, any dry (in winter) or wet (in summer) deposition of fine-grained dust particles is assumed to enhance the efficiency of the biological pump by incorporating dust into dense and fast settling organic-rich aggregates. A good correspondence between BSi and dust fluxes was observed for the dusty year 2005, following a period of rather dry conditions in the Sahara/Sahel region. Large changes of all bulk fluxes occurred during the strongest El Niño-Southern Oscillation (ENSO) in 1997–1999 where low fluxes were obtained for almost 1 year during the warm El Niño and high fluxes in the following cold La Niña phase. For decadal timescales, Bakun (1990) suggested an intensification of coastal upwelling due to increased winds (''Bakun upwelling intensification hypothesis''; Cropper et al., 2014) and global climate change. We did not observe an increase of any flux component off Cape Blanc during the past 2 and a half decades which might support this. Furthermore, fluxes of mineral dust did not show any positive or negative trends over time which might suggest enhanced desertification or ''Saharan greening'' during the last few decades.

Simulating the modern δ30Si distribution in the oceans and in marine sediments

AbstractThe δ30Si of biogenic silica ( ) in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past. The application of this proxy, however, requires an understanding of the modern δ30Si distributions and their controlling mechanisms. Here we present results from a modern climate simulation with a coupled ocean‐sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation. In agreement with previous studies, biological fractionation combined with physical transport and mixing determines the oceanic distribution of simulated δ30Si. A new finding is a distinct seasonal cycle of δ30Si in the surface ocean, which is inversely related to that of silicic acid concentration and mixed layer depth. We also provide the first simulation results of sedimentary δ30Si, which reveal that (1) the distribution in the surface sediment reflects the exported signal from the euphotic zone and (2) the dissolution of biogenic silica in the sediment acts as a source of relatively light δ30Si into the bottom waters of the polar oceans, while it is a source of heavier δ30Si to the subtropical South Atlantic and South Pacific.

Response of Lacustrine Biota to Late Holocene Climate and Environmental Conditions in Northernmost Ungava (Canada)

Sediment cores from three lakes located in the northernmost region of Ungava, Quebec (Canada) were examined to define aquatic community and ecosystem variability during the Late Holocene period. A chironomid-based transfer function was used to reconstruct August air temperature trends, and lacustrine primary production was inferred from sedimentary biogenic silica content and siliceous microfossil abundances. Trends in primary production, sediment organic matter content (estimated through loss on ignition), and chironomid-inferred temperature were compared to explore potential effects of environmental change on biotic assemblage composition at centennial to millennial time scales. Although no direct correlation between chironomid-inferred August air temperature and primary production was observed, we found indications that both chironomid and diatom communities were responding to the same overarching regional climatic and environmental processes. Over the last decade, northern Quebec has been undergoing notable, rapid warming that contrasts with the relative inertia of the past few millennia. This study provides a baseline against which recent and future environmental changes in this region can be compared.

Early diagenetic alterations of biogenic and reactive silica in the surface sediment of the Yangtze Estuary

Sedimentary biogenic silica (BSi) is an important parameter for understanding biogeochemical processes in estuarine ecosystems. In this study, a two-step mild acid–mild alkaline extraction procedure was used to leach BSi and its early diagenetic products from the sediments of the Yangtze Estuary. A Si/Al correction of the mild alkaline leachable silica (Si-Alk) was applied to estimate the contents of BSi in the sediments. The BSi contents varied from 18.90 to 120.10μmolSi/g in the sediments, whereas mild acid leachable silica (Si-HCl) and Si-Alk levels ranged from 17.43 to 73.56 and from 19.56 to 185.63μmolSi/g, respectively. Furthermore, the degrees of diagenetic alteration of biogenic and reactive silica were also calculated and discussed. The diagenetic alteration ratios of biogenic and reactive silica increased seaward during May, August and November 2012, whereas an opposite trend was observed in March 2013. The diagenetic alteration of the biogenic and reactive silica was mainly controlled by the redox conditions in benthic sediments. Additionally, the deposition of fresh diatoms and authigenic products could temporarily affect the distribution of silica pools in the sediments and ultimately affect the diagenetic alteration ratios of biogenic and reactive silica. Detailed investigations are still necessary to understand the early diagenetic processes of biogenic and reactive silica in this warm temperate area.

Changes in the micro- and nanostructure of siliceous valves in the diatom Synedra acus under the effect of colchicine treatment at different stages of the cell cycle.

The important role of the cytoskeleton in the morphogenesis of siliceous frustule components, which are synthesized within the diatom cells, has been revealed due to experiments with microtubule inhibitors. It has been shown that colchicine entering the diatom cell inhibits polymerization of tubulin, the main protein of microtubules, thereby disrupting the normal processes of biogenic silica deposition and daughter valve morphogenesis. In this study, experiments with a synchronized culture of the pennate diatom Synedra acus have been performed to determine the timing and duration of the formation of various valve components and analyze the effect of colchicine at a subtoxic concentration on the structure of daughter valves at different stages of their morphogenesis. Electron microscopic analysis has revealed several types of micro- and nanoscale anomalies in daughter valve morphology, with their frequency varying depending on the time of colchicine treatment. Laser scanning microscopy of preparations vitally stained with Tubulin Tracker Green has shown that polymerized tubulin at early stages of valve morphogenesis is localized along the periphery of the developing valve. This is evidence for an important role of microtubules in the horizontal growth of the valve at the stage when its general structural pattern is established, including its shape and arrangement of basic micro- and nanostructures. Treatment with a microtubule inhibitor at a certain stage of valve morphogenesis makes it possible to obtain new forms with a specific structure of siliceous components that hold promise for use in nanotechnologies.

The Southern Ocean silica cycle

The Southern Ocean is a major opal sink and plays a key role in the silica cycle of the world ocean. So far however, a complete cycle of silicon in the Southern Ocean has not been published. On one hand, Southern Ocean surface waters receive considerable amounts of silicic acid (dissolved silica, DSi) from the rest of the world ocean through the upwelling of the Circumpolar Deep Water, fed by contributions of deep waters of the Atlantic, Indian, and Pacific Oceans. On the other hand, the Southern Ocean exports a considerable flux of the silicic acid that is not used by diatoms in surface waters through the northward pathways of the Sub-Antarctic Mode Water, of the Antarctic Intermediate Water, and of the Antarctic Bottom Water. Thus the Southern Ocean is a source of DSi for the rest of the world ocean. Here we show that the Southern Ocean is a net importer of DSi: because there is no significant external input of DSi, the flux of DSi imported through the Circumpolar Deep Water pathway compensates the sink flux of biogenic silica in sediments.

Independent measurement of biogenic silica in sediments by FTIR spectroscopy and PLS regression

We present an independent calibration model for the determination of biogenic silica (BSi) in sediments, developed from analysis of synthetic sediment mixtures and application of Fourier transform infrared spectroscopy (FTIRS) and partial least squares regression (PLSR) modeling. In contrast to current FTIRS applications for quantifying BSi, this new calibration is independent from conventional wet-chemical techniques and their associated measurement uncertainties. This approach also removes the need for developing internal calibrations between the two methods for individual sediments records. For the independent calibration, we produced six series of different synthetic sediment mixtures using two purified diatom extracts, with one extract mixed with quartz sand, calcite, 60/40 quartz/calcite and two different natural sediments, and a second extract mixed with one of the natural sediments. A total of 306 samples—51 samples per series—yielded BSi contents ranging from 0 to 100 %. The resulting PLSR calibration model between the FTIR spectral information and the defined BSi concentration of the synthetic sediment mixtures exhibits a strong cross-validated correlation ( $$ {\text{R}}^{ 2}_{\text{cv}} $$ = 0.97) and a low root-mean square error of cross-validation (RMSECV = 4.7 %). Application of the independent calibration to natural lacustrine and marine sediments yields robust BSi reconstructions. At present, the synthetic mixtures do not include the variation in organic matter that occurs in natural samples, which may explain the somewhat lower prediction accuracy of the calibration model for organic-rich samples.

Combining limnology and palaeolimnology to investigate recent regime shifts in a shallow, eutrophic lake

In this study, we demonstrate that an integrated approach, combining palaeolimnological records and limnological monitoring data, can increase our understanding of changing ecological patterns and processes in shallow lakes. We focused on recent regime shifts in shallow Lake Krankesjön, southern Sweden, including the collapse of the clear-water state in 1975 and its subsequent recovery in the late 1980s. We used diatom, hydrocarbon and biogenic silica sediment records, in concert with limnological data sets on nutrient concentrations, water clarity, chlorophyll-a and water depth, to investigate the shifts. The shift from clear to turbid conditions was abrupt and occurred over 1 to 2 years, whereas recovery of the clear-water state was more gradual, taking 4–5 years. In 1978, shortly after the first regime shift in water clarity, the diatom community underwent a significant shift. It became less diverse, with decreased abundance of epiphytic and planktonic taxa. Despite rising phosphorus concentrations and lower abundance of submerged macrophytes, Lake Krankesjön has remained in the clear-water state over the past 20 years, although this state seems to be increasingly unstable and susceptible to collapse. The complex reactions of the entire lake ecosystem to major changes in lake-water clarity, as shown by the palaeolimnological variables investigated in this study, emphasize the importance of careful lake and catchment management if a stable, clear-water state is desired.

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

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 steady-state. 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 long-term 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 % SiO2 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.

Surface sediment dinoflagellate cysts from the Hudson Bay system and their relation to freshwater and nutrient cycling

Abstract Surface sediment samples from the Hudson Bay system were analysed in order to examine the role of key regulators of arctic marine productivity — light and nutrients as affected by freshwater stratification and sea-ice cover — on the spatial distribution and production of dinoflagellate cysts. Total cyst fluxes vary from 0.2 × 10 6 to 30.6 × 10 6 cysts m − 2 a − 1 , with the highest values observed in eastern Hudson Bay. A total of 24 cyst taxa, representing 11 genera of five orders, were identified and distribution maps of the most common taxa have been produced. This is the first record of Echinidinium aculeatum , Echinidinium karaense , cf. Echinidinium delicatum , Islandinium brevispinosum , Selenopemphix quanta , cysts of Protoperidinium americanum , cysts of cf. Biecheleria sp. and Polarella glacialis in the Hudson Bay system. Dinoflagellate cyst assemblages show distinct spatial patterns revealing three compositional domains: eastern Hudson Bay, western-central Hudson Bay and Hudson Strait. The eastern domain is characterised by a dominance of autotrophic cysts of Pentapharsodinium dalei whereas the western-central domain is characterised by autotrophic Operculodinium centrocarpum with some contribution by heterotrophic Polykrikos sp. var. arctic morphotype and Polykrikos spp. Sites from Hudson Strait are distinguished by an overwhelming prevalence of heterotrophic Protoperidiniaceae cysts, mainly Islandinium minutum , and have the highest values of sedimentary biogenic silica, used as a proxy for diatom productivity. Sediment geochemical tracers are used as proxies for freshwater inputs (lignin and its biomarkers) and nitrate availability (nitrogen isotopes), and sea-ice concentrations derived from passive microwave data as a proxy for light availability. Sea-ice regulated length of the dark season has a negligible influence on the proportion and production of heterotrophic (dark-adapted) versus autotrophic (light-dependent) dinoflagellate cysts, perhaps due to the location of our study area on the southern fringe of the Arctic. Instead, cyst populations in Hudson Bay are primarily regulated by vertical stratification and nitrate availability, while in Hudson Strait the pivotal mechanism constitutes food availability.

Depthwise variation of carbon, nitrogen and biogenic silica in sediment from lakes of Pokhara Valley, Nepal

Phewa and Begnas lakes in Pokhara Valley are playing important roles in ecological, economic, cultural and aesthetic aspects. However, due to increase in anthropogenic pressure and activities in the lake catchment, the nutrient load in the lakes has been substantially increased. The growth of algae and phytoplankton is largely dictated by carbon (C), nitrogen (N), phosphorus (P), and silica (Si) availability. Moreover, the increase in P and N loading leads to greater diatom production resulting in the accumulation of biogenic silica (BSi) in sediments. Thus, variation in C, N and BSi in sediment reflects the nutrient dynamics in the lake. In this connection, C, N and Si concentration serves as an indicator for growth of algae, including diatom, phytoplankton productivity, and overall phytoplankton efficiency. The preceding researches have mostly explained limnological characteristics and plankton diversity studies. Hence, information about sediment characteristics associated with nutrient loading and eutrophication seems yet to be wanting. The present study has been carried out to evaluate vertical variation of C, N and BSi in core sediments of the Phewa and Begnas lakes. The C and N was determined by CHNS analyzer and BSi was extracted by wet-alkaline digestion method and determined by molybdosilicate method. The result revealed increase in C, N and BSi contents toward the surface sediments compared to underlying layers suggesting increased rate of accumulation in recent deposits. The magnitude of C, N and BSi was higher in city-ward core of Phewa Lake explaining the influence of the urban inputs ...

Species-specific responses of late Miocene Discoaster spp. to enhanced biosilica productivity conditions in the equatorial Pacific and the Mediterranean

Census data of a major Cenozoic calcareous nannofossil genus (Discoaster) have been acquired from Site U1338, located near the Equator in the eastern Pacific Ocean and drilled in 2009 during Integrated Ocean Drilling Program (IODP) Expedition 321. The investigated 147.53 m thick upper Miocene sediment sequence is primarily composed of biogenic carbonate and biogenic silica. Diatom biostratigraphic data were used to develop a revised biomagnetostratigraphic age model, resulting in more variable late Miocene sedimentation rates. Carbonate content variations mainly reflect dilution by biogenic silica production, although intense carbonate dissolution affects a few shorter intervals. Abundance variations of discoasters show no distinct correlation with either carbonate or biosilica contents. The two dominant Discoaster taxa are D. brouweri and D. variabilis, except for a 12 m thick interval where D. bellus outnumbers the sum of all other discoasters by a factor of 4.6. Data presented indicate that first D. hamatus and then D. berggrenii both evolved from D. bellus. Three unusual morphotypes, here referred to as Discoaster A, B and C, increase in relative abundance during episodes of enhanced biosilica production in the upper half of the investigated sequence (Messinian). Strikingly similar morphotypes have been observed previously in Messinian age sediments from the Mediterranean, characterized by alternating deposition of biogenic carbonate and biosilica. This suggests a species-specific response among some of the late Miocene discoasters to broader oceanographic and climatic forcing that promoted episodes of enhanced deposition of biogenic silica.

Extent of diagenetic transformations in severely altered biogenic silica deposits from Tunisia: new insights from mineralogy and geochemistry

Sedimentary biogenic silica from Redeyef in Gafsa basin (southern Tunisia) was analysed for its 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra and complemented by X-ray diffraction and SEM observations. The 29Si MAS NMR spectrum is characterized by the abundance of hydroxylated silicon, displayed in resonance intensities and reflects a clear tendency towards dissolution of diatomaceous amorphous silica and the occurrence of the hydrated silica, which is the main component that ensures the diagenetic transition via the mechanism of dissolution–precipitation to other more crystalline silica phases, after the lost of its hydroxyls groups (water) by heating (burial). 27Al MAS NMR reveals two coordinations of Al; the octahedrally coordinated Al suggests the presence of clay relics trapped during crystal growth or a microcrystalline zeolite (clinoptilolite detected by SEM observations), while the tetrahedrally coordinated Al suggests the presence of minor quantities of minerals with tetrahedral Al, such as an Al-rich fluid and/or minerals such as feldspars.

Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography

Abstract. Southern Ocean biogeochemical processes have an impact on global marine primary production and global elemental cycling, e.g. by likely controlling glacial-interglacial pCO2 variation. In this context, the natural silicon isotopic composition (δ30Si) of sedimentary biogenic silica has been used to reconstruct past Si-consumption:supply ratios in the surface waters. We present a new dataset in the Southern Ocean from a IPY-GEOTRACES transect (Bonus-GoodHope) which includes for the first time summer δ30Si signatures of suspended biogenic silica (i) for the whole water column at three stations and (ii) in the mixed layer at seven stations from the subtropical zone up to the Weddell Gyre. In general, the isotopic composition of biogenic opal exported to depth was comparable to the opal leaving the mixed layer and did not seem to be affected by any diagenetic processes during settling, even if an effect of biogenic silica dissolution cannot be ruled out in the northern part of the Weddell Gyre. We develop a mechanistic understanding of the processes involved in the modern Si-isotopic balance, by implementing a mixed layer model. We observe that the accumulated biogenic silica (sensu Rayleigh distillation) should satisfactorily describe the δ30Si composition of biogenic silica exported out of the mixed layer, within the limit of the current analytical precision on the δ30Si. The failures of previous models (Rayleigh and steady state) become apparent especially at the end of the productive period in the mixed layer, when biogenic silica production and export are low. This results from (1) a higher biogenic silica dissolution:production ratio imposing a lower net fractionation factor and (2) a higher Si-supply:Si-uptake ratio supplying light Si-isotopes into the mixed layer. The latter effect is especially expressed when the summer mixed layer becomes strongly Si-depleted, together with a large vertical silicic acid gradient, e.g. in the Polar Front Zone and at the Polar Front.

Evolution of silica accumulation in the earth’s history and its relation to biota development

Two branches of silica rockevolution related to oceans and continental blocks have been outlined. The boundaries of changes in types of silica rocks, their formation, and development of the organic world were established: (i) the beginning of the Phanerozoic was related to transformation from microbial to biogenic silica deposition (or to a change in the influence of microbial activity on silica accumulation); (ii) cessation of the formation of cherts enriched in organic matter, which was synchronized with land colonization by organisms in the Devonian, and (iii) an explosive style of global silica accumulation related to the appearance and development of diatoms, which correlates with appearance of angiosperm plants in tropical forests.