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This paper is a comprehensive review devoted to topical issues in the study of opoka – marine sedimentary siliceous rocks of the Cretaceous-Paleogene age. For almost two hundred years of studying these sedimentary formations, the term “opoka” itself has not received proper recognition globally due to the lack of a detailed mineralogical definition and the general imperfection of the systematics of siliceous rocks. A distinctive feature of the opoka is the presence of opal-CT lepispheres, which form a recognizable framework of siliceous rock or form an obscure lepispheric/ cryptocrystalline mass. The article discusses various aspects of opoka – history of study, nomenclature, mineralogical composition, microtexture, the distribution of Cretaceous- Paleogene opoka, etc. The etymology of the genesis of the opoka and models of its origin at the early and late stages of diagenesis are critically discussed. In this review, attention is focused on recent studies, that provide detailed information about silica polymorphic forms distributed in the opoka, as well as its typical microtexture types. A model of the genesis of opoka and the relationship with global biogeochemical cycle of silica is presented. Differences recorded in opoka of different ages, geological and tectonic positions, indicate both the predetermination of many parameters of the mineral composition from the very genesis, and the disproportionate contribution of individual factors to the evolution of opoka rocks, namely the source of silicic acid, the role of siliceous bios in the mobilization of dissolved silica, the nature and volume of terrigenous influx from the adjacent landmass, secondary changes and mineral formation, etc.

The paper aims to build seasonal silica budgets in two macrotidal estuaries, the Elorn and Aulne estuaries of the Bay of Brest (North-Western France), based on modeling and measurements, in order to increase our understanding of the silica (Si) cycle at land-sea interfaces. A diagenetic model was developed to quantify benthic Si fluxes, e.g. aSiO2 deposition fluxes that are difficult to assess through direct measurements. Sediment cores were also seasonally sampled at six stations to provide data essential to parametrize and validate the model. Vertical profiles of porosity, burrowing depth, biodiffusive coefficients, concentrations of amorphous silica (aSiO2) and silicic acid (Si(OH)4 and the proportion of reactive aSiO2 were measured. The results show that sites sampled along the Elorn and Aulne estuaries constitute significant net Si deposition areas (1-4.5 mmol Si m-2 d-1), particularly in the upstream during winter and in midstream and downstream during summer. Year round, reprecipitation is negligible (< 3%) while burial accounts for the retention of ~ 30-80% of deposited aSiO2. In winter, burial dominates the benthic Si budget. As surface-integrated benthic Si fluxes are low compared to riverine aSiO2 fluxes, the Si export to coastal waters is high (93%) during winter. In contrast, in summer, burial accounts for 38% of river Si fluxes, and Si(OH)4 flux from the sediment is high as a result of enhanced benthic recycling and bioirrigation. Internal estuarine processes, e.g., benthic and pelagic primary production, dissolution and benthic Si fluxes, surpass river fluxes in magnitude during summer. Overall, we conclude that the Elorn and Aulne macrotidal estuaries are efficient filters of Si, retaining about 4-38% of river Si fluxes, and even 6-67% when accounting for retention in intertidal marshes, but with massive exports occurring during winter floods. [ABSTRACT FROM AUTHOR]

The paper aims to build seasonal silica budgets in two macrotidal estuaries, the Elorn and Aulne estuaries of the Bay of Brest (North-Western France), based on modeling and measurements, in order to increase our understanding of the silica (Si) cycle at land-sea interfaces. A diagenetic model was developed to quantify benthic Si fluxes, e.g. aSiO2 deposition fluxes that are difficult to assess through direct measurements. Sediment cores were also seasonally sampled at six stations to provide data essential to parametrize and validate the model. Vertical profiles of porosity, burrowing depth, biodiffusive coefficients, concentrations of amorphous silica (aSiO2) and silicic acid (Si(OH)4 and the proportion of reactive aSiO2 were measured. The results show that sites sampled along the Elorn and Aulne estuaries constitute significant net Si deposition areas (1-4.5 mmol Si m-2 d-1), particularly in the upstream during winter and in midstream and downstream during summer. Year round, reprecipitation is negligible (< 3%) while burial accounts for the retention of ~ 30-80% of deposited aSiO2. In winter, burial dominates the benthic Si budget. As surface-integrated benthic Si fluxes are low compared to riverine aSiO2 fluxes, the Si export to coastal waters is high (93%) during winter. In contrast, in summer, burial accounts for 38% of river Si fluxes, and Si(OH)4 flux from the sediment is high as a result of enhanced benthic recycling and bioirrigation. Internal estuarine processes, e.g., benthic and pelagic primary production, dissolution and benthic Si fluxes, surpass river fluxes in magnitude during summer. Overall, we conclude that the Elorn and Aulne macrotidal estuaries are efficient filters of Si, retaining about 4-38% of river Si fluxes, and even 6-67% when accounting for retention in intertidal marshes, but with massive exports occurring during winter floods.

Marine diatoms play a very crucial role in carbon export, and current food-web and become an important factor in global silica cycle. This then has made the mechanism of their biosilicification interesting to be a research subject. The classical theory states that the silica metabolism has been originated from the absorption of silicate ions, which might not give a suitable explanation for the solid silica silicification. In this study, mono-disperse Si/SiO2 fluorescent submicron-spheres were synthesized in aqueous solution, and applied in monitoring the extracellular solid silica accumulation of Chaetoceros sp. diatom. The Si/SiO2 submicron particles emitted light-blue color with the spectrum centered at 440 nm under the excitation of 365 nm UV light, similar to the typical excitation/emission pair of the DAPI fluorophore (excitation/emission: 358 nm/461 nm). The fluorescence-microscopic investigation showed that the Si/SiO2 particles delocalized on the diatoms’ surface and increased a silicic-acid-level surrounding the microalgae. As a consequence, the growth rate of the diatoms increased as the concentration of the SiO2 particles was at 120 mg/L, and reached 1.5 times higher than the growth rate calculated from the F2 media. The study not only introduces a new aspect to the extracellular metabolism of microalgae biosilicification corresponding to the global silica cycle, but also presents a new-type of culturing media using SiO2 nanoparticles for diatom cultivation, which increases the growth rate of artificial diatom-culturing for further applications.

The hydrological network in the Lachuá Ecoregion (EL), Alta Verapaz, Guatemala, hosts high levels of biodiversity and supplies water to 44 Mayan communities. Despite this critical fact, this network has been threatened by scarcely monitored industrial activities including the rapidly expanding oil palm monoculture (Elaeis guineensis Jacq). Regardless of Lachua’s freshwaters importance, there is little information on how this monoculture impacts them. We compared water-quality properties from streams in oil palm plantations (P), paddock and milpa systems (M), and primary forests (F) in EL. During 2015-2016, 13 rivers were sampled (5 times) for water temperature, pH, dissolved oxygen (DO), conductivity, hardness, chemical and biochemical oxygen demand (COD and BOD), and concentration of silica, nitrates, phosphates, and ammonia. Several parameters showed significant differences. P were 2.7ºC and 1.8ºC warmer than M and F and carried 1.4mg/L more nitrates than F. F carried 10.8mg/L and 11.8mg/L more silica than M and P. M showed intermediate temperatures and silica concentrations, as well as 14.8µS/cm and 8.9µS/cm lower conductivities than P and F. Additionally, COD in M was 9.9mg/L and 4.6mg/L lower than P and F. We believe higher temperatures and lower silica in P are due to the loss of riparian forest and their role in buffering temperatures and recycling silicon. In addition, the existence of secondary forest (guamil) in M might explain the intermediate temperatures and silica concentrations. Our results highlight the contributions of forests to waterways and suggest potential water-quality depletion from the oil palm expansion in EL.

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The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic‐to‐biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ30Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ30Si and δ18O of sponge spicules and radiolarians from Ordovician–Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150 μM during early Paleozoic time—a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed. [ABSTRACT FROM AUTHOR]

The Permian Phosphoria Rock Complex of the western USA contains an enigmatic assemblage of bioelemental rocks (i.e. phosphorites and cherts) that accumulated in a depositional system with no modern analogue. This study utilizes detailed sedimentological, stratigraphic and petrographic examination to evaluate the genetic relations of phosphorites, spiculitic chert and carbonates of the Ervay cycle (depositional sequence) and propose a unified oceanographic model for their deposition. The Ervay cycle contains three marine and one terrestrial facies association, each of which composes the bulk of a single lithostratigraphic unit. The marine facies associations include: (i) granular phosphorites (Retort Member); (ii) spiculitic cherty dolostones (Tosi Member); and (iii) marine to peritidal carbonates (Ervay Member). Red beds and intercalated gypsum (Goose Egg Formation) accumulated in the vast desert adjacent to the sea. The three marine members are chronostratigraphically distinct, successive and conformably stacked. They are not coeval facies belts. They reflect the progressive evolution of the epicontinental sea from the location of: (i) authigenic phosphogenesis (lowstand to transgression); to (ii) a glass ramp with biosiliceous (sponge) deposition (transgression); to (iii) a carbonate ramp (regression). This succession of switching biochemical sediment factories records the evolution of sea‐level, nutrient supply, upwelling, oxygenation and dissolved Si. Intense upwelling, potentially coupled with aeolian input, led to sedimentary condensation and phosphogenesis. Decreased upwelling intensity during transgression increased oxygenation sufficiently for a siliceous sponge benthos. Sponges were favoured over biocalcifiers due to elevated dissolved silica and a low carbonate saturation state. The cessation of sponge dominance and transition to a carbonate ramp occurred due to decreasing upwelling intensity, Si drawdown and an increased carbonate saturation state. These results provide insight into the role of Si loading in faunal turnover on glass ramps and highlight how differences in dissolved Si utilizers in pre‐Cretaceous versus post‐Cretaceous upwelling systems influence the resultant deposits. [ABSTRACT FROM AUTHOR]

Siliceous polycystines and phaeodarians are open‐ocean planktonic protists found throughout the water column and characterized by complex siliceous skeletons that are formed, at least partly, through the uptake of silicic acid. These protists contribute to the marine organic carbon (C) and biogenic silica (bSi) pools, but little is known about their contribution to the silica (Si) biogeochemical cycle. Here we report the first measurements of the Si uptake rate of polycystine and phaeodarian cells from samples collected in the Mediterranean Sea using the 32Si‐based method. The elementary composition (bSi, particulate organic carbon and nitrogen) of these organisms was also measured. Combining our results with published data on the distribution and abundance of Polycystina and Phaeodaria in the global ocean, we conclude that these organisms could contribute from 0.2 to 2.2 mmol Si m−2 of the marine standing stock of bSi and from 2 to 58 Tmol Si yr−1 (1% to 19%) of the global oceanic biogenic silica production. The implications for the global marine Si cycle are discussed. Key Points: Polycystines and phaeodarians are highly silicified cells (up to 11.6 μmol bSi mm−3)Over a large size spectrum, Si content of these siliceous rhizarians is associated with their cell sizeThese protists are able to consume dissolved Si and could contribute from 1% to 19% of the biogenic silica production of the global ocean [ABSTRACT FROM AUTHOR]

Abstract Field investigations of the Yangtze Estuary and its adjacent sea were carried out from July to August 2011. The distribution, source, transportation and transformation of biogenic silica (BSi) in suspended particulate matter (SPM) and core sediments were comprehensively investigated; dissolved silica (DSi) in pore water was also analyzed in this work. The budgets of reactive silica (RSi) and BSi in the East China Sea (ECS) were initially constructed on the basis of the above survey. The results indicated that the BSi distribution in this area was mainly affected by the input of the Yangtze River and Taiwan Warm Current, which was significantly correlated with SPM. The RSi flux input by rivers accounts for 17.6% of the total source of RSi in the ECS. Thus, these findings combined with the horizontal distribution of BSi in the Yangtze Estuary and its adjacent sea indicate that riverine input has a profound influence on the primary production of diatoms in the euphotic zone. Submarine groundwater exchange accounts for 22.3% of the DSi input, especially in the upwelling region, which will directly affect the euphotic nutrient structure. The DSi benthic flux from pore water to upper water exceeds riverine input by 3-fold, accounting for 11.5% of primary production in the ECS, which can alleviate the Si limiting effect caused by the decrease in DSi flux from the Yangtze runoff in recent years. Approximately 75.5% of BSi is dissolved and re-engaged in the ECS silicon cycle in the settlement process. Graphical abstract Unlabelled Image Highlights • BSi distribution in the investigated area was mainly affected by the input of Yangtze and Taiwan Warm Current. • Most terrestrial BSi dissolved and buried in the Yangtze estuary rather than entering the ocean. • The silicate flux released from the ECS seafloor was exceeded riverine input by 3-fold. [ABSTRACT FROM AUTHOR]

Abstract The geological cycles of carbon and silicon are linked via the silicate weathering feedback, but growing understanding of reverse weathering—the formation of authigenic clay minerals via back reaction of silica with cations—indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed. Recent carbon cycle modeling has suggested that enhanced reverse weathering during Precambrian time could have played an important role in regulating climate. The topology of the Precambrian silica cycle, including the size of the authigenic clay sink and its influence on the carbon cycle, is challenging to reconstruct due to inherent preservation biases in the rock record. Here we used an alternative approach using Si isotope values to invert for the magnitudes of Precambrian silica fluxes, by assuming that the silica cycle must be in mass balance with the bulk silicate Earth over geological time scales with respect to Si isotopes. To estimate uncertainties associated with these flux reconstructions, we used Monte Carlo simulations based on the natural variation exhibited in Si isotope ratio data from the Precambrian sedimentary record (n = 2118, spanning ~3.8 Ga to ~525 Ma) and modern authigenic clays (n = 123). The perspective of Si isotope mass balance shows that the previously observed increase in chert δ30Si values across Precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature. Results yielded a secular increase in the relative contribution of the authigenic clay sink across most of Precambrian time—this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater. This result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust. This approach also highlights key areas where improving Si isotope records could further enhance our constraints on the Precambrian silica cycle, and its relationship with the carbon cycle and climate stability. [ABSTRACT FROM AUTHOR]

Summary: Anecdotal evidence indicating substantial silica accumulation in tissues of bryophytes suggests that silica (phytolith) deposition evolved early on in embryophytes. To test this hypothesis, we conducted the first survey of phytolith content representing the major liverwort, moss and hornwort clades. We also assessed the diagnostic value of bryophyte phytoliths.Silica extracted from bryophyte material through wet‐ashing was described, focusing on abundance, classifying taxa as nonproducers, light producers and higher producers; and phytolith morphotypes. Ancestral state reconstruction of these characters was performed for mosses and liverworts using published phylogenies.Phytoliths are present in multiple subclades within liverworts, mosses and hornworts, but these phyla were not ancestrally high silica‐producers. Higher deposition occurs in liverworts and mosses with specialized water‐conducting cells.We hypothesize that active, high silica accumulation was not ancestral for embryophytes, but became possible in clades with increased water conductance. Phytoliths of diagnostic structures (e.g. pegged rhizoids) could help track bryophyte clades or water conductance evolution in the fossil record. [ABSTRACT FROM AUTHOR]

Chert, porcelainite, and other siliceous phases are exceptionally common in Atlantic sedimentary records of the early Eocene, but the origins of these facies remain enigmatic. The early Eocene was also the warmest interval of the entire Cenozoic Era, punctuated by numerous discrete warming events termed "hyperthermals," the largest of which is termed the Paleocene‐Eocene Thermal Maximum (~56 Ma). Here we present new and published lithologic and carbon isotope records of silica‐bearing lower Eocene sediments and suggest a link between the ubiquitous Atlantic cherts of that time period and hyperthermal events. Our data demonstrate that many of these Atlantic siliceous horizons coincide with negative carbon isotope excursions (a hallmark of hyperthermal events), including a previously unrecognized record of the Paleocene‐Eocene Thermal Maximum in the South Atlantic. Hyperthermal‐associated silica burial appears to be focused in the western middle to high latitudes of both the North and South Atlantic, with no association between siliceous facies and hyperthermal events found in the Pacific. We also present a new model of the coupled carbon and silica cycles (LOSiCAR) to demonstrate that enhanced silicate weathering during these events would require a rapid increase in total marine silica burial. Model experiments that include previously suggested transient reversals in the pattern of deep‐ocean circulation during hyperthermals demonstrate that such a mechanism can explain the apparent focusing of elevated silica burial into the Atlantic. This combination—a silicate weathering feedback in response to global warming along with a circulation‐driven focusing of silica burial—represents a new mechanism for the formation of deep‐sea cherts in lower Eocene Atlantic sedimentary records and may be relevant to understanding chert formation in other intervals of Earth history. Key Points: Chert and porcelainite are exceptionally common in the Atlantic during the Eocene, a warm period punctuated by numerous discrete hyperthermal eventsNew stable isotope records demonstrate that many of these Eocene Atlantic siliceous horizons correlate to hyperthermal events, including a new record of the Paleocene‐Eocene Thermal MaximumGeochemical modeling demonstrates that both the global silicate weathering response to hyperthermals and deep‐ocean circulation change are important in the formation of Eocene Atlantic hyperthermal‐associated cherts [ABSTRACT FROM AUTHOR]

Hydrosphere interactions and alteration of the terrestrial crust likely played a critical role in shaping Earth's surface, and in promoting prebiotic reactions leading to life, before 4.03 Ga (the Hadean Eon). The identity of aqueously altered material strongly depends on lithospheric cycling of abundant and water-soluble elements such as Si and O. However, direct constraints that define the character of Hadean sedimentary material are absent because samples from this earliest eon are limited to detrital zircons (ZrSiO4). Here we show that concurrent measurements of Si and O isotope ratios in Phanerozoic and detrital pre-3.0 Ga zircon constrain the composition of aqueously altered precursors incorporated into their source melts. Phanerozoic zircon from (S)edimentary-type rocks contain heterogeneous δ18O and δ30Si values consistent with assimilation of metapelitic material, distinct from the isotopic character of zircon from (I)gneous- and (A)norogenic-type rocks. The δ18O values of detrital Archean zircons are heterogeneous, although yield Si isotope compositions like mantle-derived zircon. Hadean crystals yield elevated δ18O values (vs. mantle zircon) and δ30Si values span almost the entire range observed for Phanerozoic samples. Coupled Si and O isotope data represent a constraint on Hadean weathering and sedimentary input into felsic melts including remelting of amphibolites possibly of basaltic origin, and fractional addition of chemical sediments, such as cherts and/or banded iron formations (BIFs) into source melts. That such sedimentary deposits were extensive enough to change the chemical signature of intracrustal melts suggests they may have been a suitable niche for (pre)biotic chemistry as early as 4.1 Ga. [ABSTRACT FROM AUTHOR]

Diatomites constitute a widely represented lithology in the Messinian sections of the circum-mediterranean Neogene marginal basins. Although traditionally interpreted as genuine evidence of the gradually restricted conditions that characterized the Mediterranean just before the Messinian salinity crisis, their coeval occurrence with a global intensification of the opaline production in the world oceans (late Miocene-early Pliocene biogenic bloom) suggests that an integrative analysis of the origins of these sediments is necessary. A comprehensive analysis of the geological and paleontological records suggests that the synergistic intervention of abiotic (tectonic and climate reconfigurations) and biotic (expansion of grass-dominated, opal-rich biomes) controlling factors may have promoted a remarkable enhancement of silica flux from continents to oceans, which in turn can explain the opaline burst that occurred during the late Miocene, at both the global and Mediterranean scale. The finely laminated pattern and the rich fossil content of diatomaceous deposits, that are usually considered to be byproducts of anoxic conditions, are briefly discussed. Some studies seem to indicate that, instead of anoxia, the aggregation and sedimentation of diatom tests may play a critical role in these processes. The lower Messinian diatomites of the Mediterranean region are generally interbedded with organic-rich sediments (sapropels) clearly attesting prolonged, precessionally-controlled periods of basin stratification and bottom water anoxia or hypoxia. A causal relationship between sapropel and diatomite deposition in the Mediterranean is proposed, considering the possible interplay between stratification-adapted diatoms and anaerobic bacteria and their respective role in influencing the marine silica cycle. [ABSTRACT FROM AUTHOR]

Three comprehensive surveys were performed in the Changjiang (Yangtze River) Estuary (CJE) to understand the biogenic silica (BSi) composition, behavior and budget. It is indicated that the BSi is composed of phytoliths, phytoplankton and sponges; phytolith BSi has 16 forms and account for 23% to 83% of the bulk BSi in the maximum turbidity zone. The budget shows that the major exogenous BSi source in the water column of the CJE is the riverine input, accounting for 95% of the total BSi input. Dominant processes that maintain BSi levels in the water column are the primary production (55 Gmol/a) and the subsequent BSi sedimentation (46 Gmol/a); and the BSi pool produced by the primary production represents two point three times the BSi loading of the Changjiang River and 63% of the BSi output, respectively. The net export (26 Gmol/a) of BSi from the CJE to the East China Sea and Yellow Sea roughly equals the riverine BSi loading. The observed total accumulation of BSi is one point seven times larger than the loading of total BSi output, with 53% to 88% of phytolith BSi and their assemblage, indicating that there has already been a 'filter' of terrestrial BSi. The reverse weathering in sediments is an important process for the reactive silica removal in the CJE due to authigenic alterations. It is indicated that the phytolith fluxes in the suspended load represent a significant BSi source in the estuary, and the CJE would act as a net BSi sink. [ABSTRACT FROM AUTHOR]

The terrestrial cycling of Si is thought to have a large influence on the terrestrial and marine primary production, as well as the coupled biogeochemical cycles of Si and C. Biomineralization of silica is widespread among terrestrial eukaryotes such as plants, soil diatoms, freshwater sponges, silicifying flagellates and testate amoebae. Two major groups of testate (shelled) amoebae, arcellinids and euglyphids, produce their own silica particles to construct shells. The two are unrelated phylogenetically and acquired biomineralizing capabilities independently. Hyalosphenids, a group within arcellinids, are predators of euglyphids. We demonstrate that hyalosphenids can construct shells using silica scales mineralized by the euglyphids. Parsimony analyses of the current hyalosphenid phylogeny indicate that the ability to “steal” euglyphid scales is most likely ancestral in hyalosphenids, implying that euglyphids should be older than hyalosphenids. However, exactly when euglyphids arose is uncertain. Current fossil record contains unambiguous euglyphid fossils that are as old as 50 million years, but older fossils are scarce and difficult to interpret. Poor taxon sampling of euglyphids has also prevented the development of molecular clocks. Here, we present a novel molecular clock reconstruction for arcellinids and consider the uncertainties due to various previously used calibration points. The new molecular clock puts the origin of hyalosphenids in the early Carboniferous (∼370 mya). Notably, this estimate coincides with the widespread colonization of land by Si-accumulating plants, suggesting possible links between the evolution of Arcellinid testate amoebae and the expansion of terrestrial habitats rich in organic matter and bioavailable Si.

Analysis of 161 nodular and bedded cherts shows that shallow-water biosiliceous facies, which were common in the Lower Ordovician, had mostly retreated from peritidal–lagoonal facies to deeper settings by the Middle Ordovician. For decades, this retreat was thought to have spanned the Cambrian and Ordovician Periods. Our data suggest that the basinward shift may have taken approximately 2–14 m.y. in the late Lower to early Middle Ordovician. A likely cause for the departure of these often siliceous-sponge cherts from shallow waters was depletion of shallow-water dissolved silica (DSi). Increased use of DSi by radiolarians may have commenced in the Middle Ordovician when radiolarians increased just as a pulse of radiolarite deposition began. Siliceous sponges may have been forced to relocate to deeper waters because radiolarians depleted shallow-water DSi. Apparent low relative sea level in the Middle Ordovician may have hampered the formation and/or preservation of shallow-water cherts by reducing available shelf area to host such deposits. However, the resurgence of shelf chert in the Upper Ordovician while peritidal cherts remained virtually absent suggests that biology was critical to driving the basinward shift of cherty facies. Formation and preservation of abundant latest Darriwilian through Sandbian cherts may have been favored in deep-water settings during a possible warming interval marked by sea-level rise and extensive anoxia and euxinia that coincided with an ongoing decline in 87 Sr/ 86 Sr ratios and increased sea-floor volcanic activity that probably enriched the DSi content of deep-ocean waters. Ocean circulation associated with the onset of cooling in the Katian may have optimized chert accumulation by stimulating surface water biosiliceous productivity before DSi-rich chert-preserving anoxic basins were ventilated. As Late Ordovician glaciation intensified, deep-ocean ventilation by thermohaline sinking of oxygen-rich waters probably decreased preservation potential of biosiliceous deposits while increasing oceanic recycling of silica. [ABSTRACT FROM AUTHOR]

Diatom assemblages and abundance in marine sediment cores are utilised as proxies of paleoceanographic conditions. However, diatoms are altered by taphonomic processes, which cause diatom accumulation rates to vary while simultaneously altering assemblages by removing some species from the sedimentary record. Furthermore, these taphonomic processes vary with space and time. Additionally, significant insights into the marine carbon and silicon cycles can be gained by assessing variability in diatom dissolution. Sediment core NBP0101-JPC24 from Prydz Bay, East Antarctica, provides a detailed Holocene record of diatom community shifts in an ecologically significant section of the Antarctic coast. The sedimentary and paleontologic record in JPC24 records the inception of the Prydz Bay gyre during deglaciation, as well as an increase in sea ice and in wind-related turbulence over the last 7000 years. A record of diatom preservation for JPC24 has been established by examining dissolution-induced changes in the morphology of Fragilariopsis curta, a sea-ice-related diatom common in JPC24. Diatom dissolution in NBP0101-JPC24 does not significantly correlate with the relative abundance of any diatom species or ecological species group. However, dissolution is low during the deglacial, prior to the inception of the Prydz Bay gyre and resultant polynya, and increases as the gyre increases in size and strength to present conditions. Circulation in the upper water column associated with the modern persistent gyre causes diatoms and other silt-sized particles to remain in suspension. Prior studies have shown that diatoms exposed to caustic marine water continue to dissolve until they are sequestered in sediments. Once sequestered in the sediments, diatoms will stop dissolving when a chemical equilibrium has been achieved between silicic acid in solution and solid opal. Diatom dissolution, therefore, is seen to be controlled by physical oceanographic factors rather than diatom ecology. [ABSTRACT FROM AUTHOR]

Taphonomic processes alter diatom assemblages in sediments, thus potentially negatively impacting paleoclimate records at various rates across space, time, and taxa. However, quantitative taphonomic data is rarely included in diatom-based paleoenvironmental reconstructions and no objective standard exists for comparing diatom dissolution in sediments recovered from marine depositional settings, including the Southern Ocean’s opal belt. Furthermore, identifying changes to diatom dissolution through time can provide insight into the efficiency of both upper water column nutrient recycling and the biological pump. This is significant in that reactive metal proxies (e.g. Al, Ti) in the sediments only account for post-depositional dissolution, not the water column where the majority of dissolution occurs. In order to assess the range of variability of responses to dissolution in a typical Southern Ocean diatom community and provide a quantitative guideline for assessing taphonomic variability in diatoms recovered from core material, a sediment trap sample was subjected to controlled, serial dissolution. By evaluating dissolution-induced changes to diatom species’ relative abundance, three preservational categories of diatoms have been identified: gracile, intermediate, and robust. The relative abundances of these categories can be used to establish a preservation grade for diatom assemblages. However, changes to the relative abundances of diatom species in sediment samples may reflect taphonomic or ecological factors. In order to address this complication, relative abundance changes have been tied to dissolution-induced morphological change to the areolae of Fragilariopsis curta , a significant sea-ice indicator in Southern Ocean sediments. This correlation allows differentiation between gracile species loss to dissolution versus ecological factors or sediment winnowing. These results mirror a similar morphological dissolution index from a parallel study utilizing Fragilariopsis kerguelensis , suggesting that results are applicable to a broad spectrum of diatoms typically preserved in the sediments. [ABSTRACT FROM AUTHOR]

A study of the temperate rain forests of New Zealand and Tasmania showed that their soil testate amoebae communities are composed of five groups of taxa: (1) seven taxa characteristic of wet acidic soils and Sphagnum peatlands (i.e., Amphitemidae, Apodera, Alcodera, Certesella, Cyphoderia, Placocista); (2) a group of 16 species of predatory Nebelids and Heleopera spp., characteristic of Sphagnum and rainforests; (3) a group of 17 species of litter and soil Euglypha, excluding the smallest ones; (4) a diverse population of other morphotypes common in other biomes; and (5) a population of small euryoecious taxa - Cryptodifflugia and Pseudodifflugia spp., Euglypha rotunda, E. laevis, Corythion and Trinema spp. This fifth group, with other r-selected protists (e.g., colpodid ciliates) appears in all habitats. Soil testate communities of other rainforests are composed of the same five groups and are distinguished by the first three assemblages. The fourth and fifth groups, often supplemented with a few Euglypha species, comprise the soil testate amoebae of other biomes. Nebelids and Heleopera, incorporating prey idiosomes into their shells, add an additional link to the role of Euglyphids in the silica cycle. Three Gondwanan Nebelid genera, Apodera, Alcodera, and Certesella were frequently observed, and the discovery of Alcodera cockayni in Tasmania extends its recorded distribution in the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

Mechanisms controlling the transfer and retention of silicon (Si) along continental margins are poorly understood, but play a major role in the functioning of coastal ecosystems and the oceanic biological pump of carbon. Deep-sea fans are well recognized as carbon sink spots, but we lack knowledge about the importance of the fans in the global Si cycle. Here, we provide a first estimate of the role played by the Congo deep-sea fan, one of the biggest in the world, in the Si cycle. Sediment cores sampled in the deep-sea fan were analyzed to build a Si mass balance. An exceptionally high accumulation rate of amorphous silica aSiO 2 (2.29 ± 0.58 mol Si m − 2 y − 1 ) was found, due to a high sedimentation rate and the presence of aluminum in the sediments. Although favored by bioirrigation, recycling fluxes remained low (0.3 mol Si m − 2 y − 1 ) and reconstructed input fluxes could only be explained by lateral inputs coming from the canyon. Preliminary calculations show that the rapid transport of aSiO 2 through the canyon and the excellent preservation efficiency in the sediments imply that 50% of aSiO 2 river inputs from the Congo River accumulate annually in the deep-sea fan. Si:C ratios in deep-sea fan sediments were very low (0.2) and only three times as high as those measured in the river itself, which suggests that material from the river and the continental shelf was delivered directly through the canyon, with very little time for Si and C cycle decoupling to take place. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ongoing global changes, such as climate warming and increasing supply of reactive nitrogen (N), are expected to affect essential ecosystem processes such as the decomposition of plant litter. Determining the influence of environmental heterogeneity on the magnitude of these effects remains an important task, with silicon (Si) availability being a notable component of this heterogeneity, especially for grasses. We conducted an outdoor enclosure experiment to test if increased Si supply to a widespread foundation species (.Phragmites australis) alters the effect of climate warming and excess N supply on litter decomposition by curbing fungal decomposers. Consistent with expectations, Si supply during plant growth reduced fungal biomass in decomposing leaf blades by 50%, an effect that was doubled by excess external N supply. These strong impacts, however, did not directly translate to reduced litter decomposition or associated changes in nutrient dynamics. Instead, plant tissue-specific effects determined the influence of Si, N, and elevated temperature on litter mass loss. Specifically, Si accelerated the decomposition of leaf sheaths, warming enhanced leaf-sheath and leaf-blade decomposition, and N decreased the decomposition of culm litter, in line with expectations based on differences in litter chemistry. Thus, despite highly detrimental effects of Si and N on fungal decomposers, compensation by other members of the microbial community could dampen the realized impact of these global-change factors on the decomposition of plant litter in the future. [ABSTRACT FROM AUTHOR]

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 % SiO 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, 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. [ABSTRACT FROM AUTHOR]

Background and aims: Plants absorb and carry soluble silica from soils and then deposit SiO · nHO within themselves producing amorphous silica particles known as phytoliths. Trace amount of organic carbon is occluded during phytolith formation referred to as phytolith-occluded carbon (PhytOC). This carbon fraction has been recognized as an important way of carbon biosequestration. Previous studies have investigated the PhytOC contents of many crop plants and their contribution to global carbon sink. However, the PhytOC in soil is less focused. In this study, we investigated the distribution of soil PhytOC in the Chinese Loess Plateau (CLP). Methods: Twenty-six soil profiles were collected in the Chinese Loess Plateau. A wet oxidation method was used for phytolith extraction. Occluded carbon was determined by element analyzer. Results: Our results showed that the soil PhytOC density (SPCD) ranged from 0.757 to 23.110 g/m among different soil profiles. The SPCD of profiles in the Southern CLP was generally higher than that in the Northern CLP. It was estimated that 5.35 Mt of PhytOC was stored in the upper soil of the CLP. We also estimated the annual phytolith flux into the Yellow River from the CLP by soil erosion and about 2.5 Mt of phytoliths eroded and transported into rivers per year. Conclusions: Our study indicated that PhytOC was one of the potential biosequestration way and phytoliths had an important influence on biogeochemical cycle of silica. Our results suggested that the soil PhytOC was mainly influenced by different plant communities. [ABSTRACT FROM AUTHOR]

Silicon (Si) is a key element in global biogeochemical cycling and recent research has shown that changes in the biological component of the Si cycle may lead to more rapid variations in the land–ocean Si transfer than previously thought. The objective of this paper is to better understand the controls on temporal Si dynamics in terrestrial ecosystems, by studying Si fluxes from a small forested catchment in central Belgium. An intensive monitoring program (2008–2010) of dissolved and amorphous silica (DSi and ASi) concentrations and load patterns show that DSi concentrations are significantly lower during winter–spring periods than during summer–autumn periods. In contrast to what was found in other studies, seasonal dynamics in Meerdaal forest are not controlled by variations in biological uptake or temperature, but mainly by the more important supply of pore-water to the groundwater table in winter–spring periods. Analysis of seasonal and event fluctuations in stream water DSi concentrations showed that final stream water is a mixture of old, DSi rich water pushed out of the soil, and new, DSi poor water delivered by quick flow. The mixing of old and new water finally resulted in streamwater DSi concentrations responding only moderately to variations in discharge (near-chemostatic behaviour). We estimated the total DSi export from the system to be ca. 65.1 × 10 3 mol km −2 year −1. Because Si delivery is biologically regulated through an important Si cycle in the vegetation-soil continuum, an anthropogenic (e.g. agricultural expansion) or climatic disturbance of terrestrial ecosystems may alter both water residence times through shifts in hydrological regimes and the DSi chemical equilibrium concentration in soils. In turn, these perturbations will potentially alter long-term DSi and ASi inputs to aquatic systems. [ABSTRACT FROM AUTHOR]

Radiolarites, commonly comprising chert-shale couplets, feature prominently in the Mesozoic stratigraphy of the Neotethyan realm. The origin and significance of these deposits, especially with regard to understanding palaeoproductivity in that region, remains controversial. This study examines transport processes and accumulation rates of radiolarians/radiolarites, as well as the chronology of radiolarite sedimentation based on revised data from the Southern Alps and new data from the Balagne Nappes, Alpine Corsica. Results suggest that the spatio-temporal distribution of carbonate and silica on Tethyan margins was governed by extensive lateral transport of radiolarian tests from topographic highs to basins. Palaeoclimatic, rather than palaeotectonic, changes seem to have triggered changes in surface fertility which, in turn, brought about major facies changes observed in Tethyan marginal basins: Regional platform demise - possibly due to eutrophication and onset of radiolarite deposition in the Early Bajocian - can be correlated with a positive shift in the carbon isotope curve, whereas platform recovery and the end of radiolarite accumulation correlates with a gradual decline in δ13C-values. The existence of a vigorous trans-Pangaean, equatorial current system that could have produced regional upwelling is questioned because of evidence for Middle Jurassic to Early Cretaceous oligotrophic conditions in the Proto-Caribbean and the Central Atlantic. The 'Caribbean River Plume Model' is proposed as a possible alternative to upwelling in the Jurassic Western Tethys. Dissolved nutrients may have been dispersed in low salinity lids that originated from river plumes of rivers from tropical Africa and warm-temperate Eurasia. Vast areas of the pre-Late Cretaceous world ocean must have accumulated radiolarites at low rates (<4 g cm−2 10−3years) represented by the condensed, long-lived (>50 Myr) radiolarite sections observed in many Circumpacific terranes. Radiolarian chert and claystone could be the normal pelagic sediment during Ordovician to Early Cretaceous time in far-offshore, open oceanic settings, where dilution by terrigenous material and periplatform shallow-water carbonates was absent. [ABSTRACT FROM AUTHOR]

Abstract: Between 2 and 3km depth, North Pacific deep waters contain a plume of water with high silicic acid concentrations. The plume extends outward from Cascadia Basin (the Washington Margin), where waters can contain in excess of 200μM off the coast of Oregon and Washington. To identify the source of the high Si concentrations in Cascadia Basin, we measured silicic acid and germanium concentrations in deep waters, and their fluxes from sediments using incubated cores. The mean flux of silicic acid into bottom waters is 0.81±0.05mmol/m2-day, and the Ge/Si ratio of this flux is 0.7±0.1μmol/mol. A box model, incorporating these results with hydrographic data, indicates that (1) no more than 5% of the silicic acid added to Basin deep waters can have a hydrothermal source (either hot or warm seeps), and (2) the total input of silicic acid to Basin deep waters is 0.06±0.02Tmol/y. This input is nearly all from remineralized biogenic debris and should contribute about 0.5% of the 14Tmol/y that are estimated to be necessary to maintain the North Pacific plume. [Copyright &y& Elsevier]

Abstract: The evolution of large marine mammals and their invasion of the pelagic realm is tied to the availability of sufficient food, which in turn is linked to upwelling areas and other highly productive regions in the ocean, as has been recognized for some time. Here I propose that silicate-supported upwelling (which provides the shorter food chain and hence the higher yield for apex consumers) and deep mixing within the Southern Ocean are the crucial ingredients of the system providing the stage for the evolution of whales. The Circumpolar Current receives silicate from various sources but especially from the North Atlantic Deep Water introduced in the Atlantic sector. The silicate is largely trapped in the Ring, some in the water, and some in the sediment, and is made available to shallower waters by unusually deep mixing. From these shallower depths silica-rich waters can enter the thermocline all through the southern hemisphere, stimulating diatom growth in upwelling systems to the equator and beyond. From the link diatoms–krill–whales and diatoms–krill–small fishes, it is readily apparent that the link between the climate narrative and the narrative of whale evolution (both for mysticetes and odontocetes) is the silica cycle. Studying it will generate hypotheses about whale evolution that can be tested using fossils. [Copyright &y& Elsevier]

Abstract: The processes controlling preservation and recycling of particulate biogenic silica in sediments must be understood in order to calculate oceanic silica mass balances. The new contribution of this work is the coupled use of advanced models including reprecipitation and different phases of biogenic silica with different dissolution characteristics as well as new data sets concerning experimental dissolution rates of marine particles and sediment opal obtained in flow-through reactors. We have used three models representing early diagenesis of biogenic silica in sediments that calculate the vertical distributions of dissolved silicate and solid silica in sediments. Model 1 contains one type of biogenic silica and the dissolution rate is constant, whereas model 2 contains a variable dissolution rate constant with sediment depth (representing aging) and one type of biogenic silica. Model 3 incorporates aging by describing two types of biogenic silica that differ by their dissolution properties. An explicit term of reprecipitation is incorporated into the three models. The distributions of dissolved silicate and solid silica predicted by steady-state calculations are compared to 4 observed data sets, from the Southern Ocean, the Equatorial Pacific and the North Atlantic, covering a wide range of sediment compositions, from opal-rich to opal-poor sediments. After adjustment of the critical parameters (the apparent silica dissolution rate constants, the biogenic silica flux deposited at the sediment–water interface and the reprecipitation rate), the second and third models provide good agreements between predicted and measured dissolved Si and solid silica profiles for each data set, except for the second model in the Equatorial Pacific. However, a large discrepancy between the experimentally derived dissolution rate constants and those calculated by the models is observed at all sites at depth in the sediment, suggesting that either artifacts arise during dissolution experiments such as over-representation of rapidly dissolving silica or variation of dissolution properties during the experimental procedures or the model oversimplifies the processes associated to silica dissolution and alumino-silicate reprecipitation and their interaction. [Copyright &y& Elsevier]

Chert deposits preserve a record of secular change in the oceanic silica cycle. The evolutionary radiation of silica-secreting organisms resulted in a transition from abiological silica deposition, characteristic of the Archean and Proterozoic eons, to the predominantly biologically controlled silica deposition of the Phanerozoic. Comparative petrography of Phanerozoic and Precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the Paleoproterozoic era (ca. 1.8 Ga). In Neoproterozoic and Mesoproterozoic strata, early diagenetic chertitication is largely restricted to peritidal environments. These early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures. The cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation. Some of the Paleoproterozoic cherts associated with iron formations, however, are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed. These primary cherts lack ghosts or inclusions of carbonate precursors, have fine-scale grain fracturing (possibly from syneresis), exhibit low grain-packing densities, and are not associated with unsilicified carbonate deposits of similar depositional composition. Cherts in some Paleoproterozoic iron formations (e.g., the Gunflint Formation, northwestern Lake Superior region) are composed of silica types similar to those in Phanerozoic sinters (e.g., the Devonian Rhynie and Windyfield cherts, Scotland). Such cherts may provide evidence that basinal, and perhaps global, oceanic silica concentrations were higher during the Paleoproterozoic era than at later times. [ABSTRACT FROM AUTHOR]

Oolites that were replaced by early diagenetic silica and were not reworked from their original depositional setting can serve as a useful tool to evaluate a previously suggested retreat of biosiliceous facies from peritidal environments to deeper water settings in the Early Paleozoic. The silica-replaced oolites suggest that the retreat was largely over by the Middle Ordovician, which is consistent with emerging compilations of the spatial and temporal distribution Ordovician cherts. Whether the retreat was a function of declining concentration of dissolved silica in shallow marine waters or of radiation of faunas from nearshore into offshore settings or some combination of both is an issue that may be clarified with improved resolution of the timing of the retreat. A small Upper Carboniferous peak in silica-replaced oolite abundance coincides with a minor rebound in shelf chert accumulation and the common presence of coastal flint deposits in Upper Carboniferous cyclothems, suggesting that cherty facies returned to shallow water environments at this time. [Copyright &y& Elsevier]

• Neoarchean silicified volcanic rocks from the Abitibi greenstone belt that range in composition from rhyolitic to basaltic overlap in silicon and oxygen isotope values with similar Paleoarchean silicified basalts from the Barberton greenstone belt. • Silicon isotope values measured from Neoarchean iron formation samples in the Deloro assemblage change with stratigraphic position, possibly due to a shift in the silicon isotope value of the reservoir, or silicification, which is more prevalent at the top of the section. • The Temagami greenstone belt iron formation silicon isotope data overlap with values measured from the upper Deloro iron formation, and could reflect local, basinal changes in the isotopic composition of the silicon isotope reservoir on short time scales. Silica-rich Precambrian rocks often preserve geochemical information and microfossil remnants from the early biosphere and could play a critical role in the formation of early crust. Because these rocks are important geochemical and paleontological archives, we need to better constrain their geochemical and isotopic attributes and generate a refined picture of the evolving Archean silica cycle. Here, we investigate a series of sub- to greenschist facies Si-rich Archean rocks from the ~ 2.7 Ga Abitibi greenstone belt, Canada, that represent chemical sedimentary rocks and rocks formed via silica-addition through the process of silicification. We report data for major and trace element geochemistry, multi-crystal silicon and oxygen isotopes of quartz using isotope ratio mass spectrometry, and texture-specific silicon isotope values measured using secondary ion mass spectrometry on Neoarchean chemical sedimentary rocks, their silicified equivalents, and associated silicified volcanic rocks. We find that in such a well-preserved terrane we can utilize petrographic textures and geochemical attributes to establish rock origin, distinguishing siliceous rocks that form via chemical sedimentation from those that form via silicification. Chemical sedimentary rocks display a wide range of 30Si-depleted silicon isotopes values that vary with stratigraphy similar to other Archean iron formation. Silicified volcanic rocks possess 30Si-enriched values, similar to Archean silicified basalts. We conclude that because silicon isotope values of iron formation shift toward 30Si-enriched values up stratigraphy, basinal changes in the composition of the silicon isotope reservoir may be preserved. Silicon isotope values of silicified volcanic rocks by contrast, likely represent precipitation from an isotopically heavy silicon reservoir, influenced by downward percolating seawater and upward moving convecting fluids interacting with host volcanic rock (basalt or andesite). Overall, we confirm that Neoarchean silicified rocks are 30Si-enriched like their Paleoarchean counterparts. [ABSTRACT FROM AUTHOR]

The rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the Ediacaran-Cambrian (E-C) interval, even if the exact timing and biological triggers of this event are still debated. Although Ediacaran-Cambrian chert depositions are widely distributed, multiple evidences converge to support their hydrothermal origin. In this study, we provide petrographic study supplemented by elemental concentration, SEM and Raman spectral data on the siliceous rocks from the Yanjiahe Formation (the Three Gorges area, South China), in order to ascertain the origin of included silica and discuss the involved processes of silica deposition. Contrary to most previous studies, our analyses suggest that studied cherts mainly resulted from biogenic silica, and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input. The deposition model of silica includes direct deposition of siliceous bioclasts, secondary replacement of carbonate by dissolved biosilica, and microbial induced silicification from seawater. The sink of silica recorded in the Yanjiahe Formation was characterized of a transition from typical Precambrian to Phanerozoic process. In addition, micropalaeontological and taphonomical analyses of the Yanjiahe Formation revealed two pulses of microfossil abundance and diversity in the Fortunian and Stage 2, respectively. They can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux. • Silica source in chert of the Yanjiahe Formation is mainly biogenic. • Silicification process was discussed in the Yanjiahe Formation. • Two pulses of blooming of microfossils are recognized in the studied chert. • Varied sources of silica from different settings in the Yangtze Block are discussed. [ABSTRACT FROM AUTHOR]

The processes affecting diatom dissolution and upper water column silica recycling are understood in modern environments. However, few studies have been undertaken to understand which processes have the greatest impact on diatom preservation in sedimentary archives. Furthermore, variability in diatom dissolution in the upper water column of past oceans is rarely quantified. Here we present a record of diatom dissolution, a proxy for upper water column silica recycling, from the Pliocene of the Ross Sea. Dissolution was quantified utilizing diatoms from the AND-1B core. These data were related to previously published diatom proxies of sea surface conditions. Diatoms were found to be better preserved in intervals dominated by sea ice diatoms than intervals dominated by cosmopolitan open ocean diatoms. However, preservation was not constant in the interval dominated by cosmopolitan diatoms, implying another taphonomic control. An antiphase relationship was found between diatom-inferred water temperature and dissolution; dissolution is less during cold intervals. A weaker positive correlation was found between diatom accumulation and preservation; dissolution is lower during periods of high diatom accumulation. The relationship between diatom accumulation and preservation reflects pore water saturation; individual diatoms contributes less silica to pore water saturation. However, increased growth rate is expected to result in thinner frustules and increased dissolution. These two opposing factors explain the weakness of the observed correlation. Interestingly, these results are different from those found in other Southern Ocean sediment cores. Comparative analysis highlights the importance of taphofacies on diatom preservation in understanding diatom dissolution data. • Diatom dissolution in Pliocene-aged sediments is evaluated. • Changes in diatom dissolution are likely driven by changes in growth rate. • Results contrast with similar studies in from deeper water settings. • The importance of taphofacies in microfossil analysis is highlighted. [ABSTRACT FROM AUTHOR]