Marine Silica Cycle Research Articles

Biogenic silica accumulation in picoeukaryotes: Novel players in the marine silica cycle.

It is well known that the biological control of oceanic silica cycling is dominated by diatoms, with sponges and radiolarians playing additional roles. Recent studies have revealed that some smaller marine organisms (e.g. the picocyanobacterium Synechococcus) also take up silicic acid (dissolved silica, dSi) and accumulate silica, despite not exhibiting silicon dependent cellular structures. Here, we show biogenic silica (bSi) accumulation in five strains of picoeukaryotes (<2-3μm), including three novel isolates from the Baltic Sea, and two marine species (Ostreococcus tauri and Micromonas commoda), in cultures grown with added dSi (100 μM). Average bSi accumulation in these novel biosilicifiers was between 30 and 92 amol Si cell-1 . Growth rate and cell size of the picoeukaryotes were not affected by dSi addition. Still, the purpose of bSi accumulation in these smaller eukaryotic organisms lacking silicon dependent structures remains unclear. In line with the increasing recognition of picoeukaryotes in biogeochemical cycling, our findings suggest that they can also play a significant role in silica cycling.

Nudibranch predation boosts sponge silicon cycling

Diatoms play a key role in the marine silica cycle, but recent studies have shown that sponges can also have an important effect on this dynamic. They accumulate large stocks of biogenic silica within their bodies over long periods, which are thought to vary little on an intra-annual scale. The observation of an abrupt decline in sponge biomass in parallel with large increases in abundance of a spongivorous nudibranch (Doris verrucosa) led us to conduct a year-long study on the effect of nudibranch predation on the silicon budget of a sponge (Hymeniacidon perlevis) population. After 5 months of predation, the abundance of sponge individuals did not change but their biomass decreased by 95%, of which 48% was explained by nudibranch predation. About 97% of sponge spicules ingested by nudibranchs while feeding was excreted, most of them unbroken, implying a high rate of sponge silica deposition in the surrounding sediments. After predation, sponges partially recovered their biomass stocks within 7 months. This involved a rapid growth rate and large assimilation of dissolved silicon. Surprisingly, the highest rates of silicon absorption occurred when dissolved silicon concentration in seawater was minimal (< 1.5 µM). These findings suggest that the annual sponge predation-recovery cycle triggers unprecedented intra-annual changes in sponge silicon stocks and boosts the cycling of this nutrient. They also highlight the need for intra-annual data collection to understand the dynamics and resilience of sponge ecosystem functioning.

Explicit silicate cycling in the Kiel Marine Biogeochemistry Model version 3 (KMBM3) embedded in the UVic ESCM version 2.9

Abstract. We describe and test a new model of biological marine silicate cycling, implemented in the Kiel Marine Biogeochemical Model version 3 (KMBM3), embedded in the University of Victoria Earth System Climate Model (UVic ESCM) version 2.9. This new model adds diatoms, which are a key component of the biological carbon pump, to an existing ecosystem model. This new model combines previously published parameterizations of a diatom functional type, opal production and export with a novel, temperature-dependent dissolution scheme. Modelled steady-state biogeochemical rates, carbon and nutrient distributions are similar to those found in previous model versions. The new model performs well against independent ocean biogeochemical indicators and captures the large-scale features of the marine silica cycle to a degree comparable to similar Earth system models. Furthermore, it is computationally efficient, allowing both fully coupled, long-timescale transient simulations and “offline” transport matrix spinups. We assess the fully coupled model against modern ocean observations, the historical record starting from 1960 and a business-as-usual atmospheric CO2 forcing to the year 2300. The model simulates a global decline in net primary production (NPP) of 1.4 % having occurred since the 1960s, with the strongest declines in the tropics, northern midlatitudes and Southern Ocean. The simulated global decline in NPP reverses after the year 2100 (forced by the extended RCP8.5 CO2 concentration scenario), and NPP returns to 98 % of the pre-industrial rate by 2300. This recovery is dominated by increasing primary production in the Southern Ocean, mostly by calcifying phytoplankton. Large increases in calcifying phytoplankton in the Southern Ocean offset a decline in the low latitudes, producing a global net calcite export in 2300 that varies only slightly from pre-industrial rates. Diatom distribution moves southward in our simulations, following the receding Antarctic ice front, but diatoms are outcompeted by calcifiers across most of their pre-industrial Southern Ocean habitat. Global opal export production thus drops to 75 % of its pre-industrial value by 2300. Model nutrients such as phosphate, silicate and nitrate build up along the Southern Ocean particle export pathway, but dissolved iron (for which ocean sources are held constant) increases in the upper ocean. This different behaviour of iron is attributed to a reduction of low-latitude NPP (and consequently, a reduction in both uptake and export and particle, including calcite scavenging), an increase in seawater temperatures (raising the solubility of particulate iron) and stratification that “traps” the iron near the surface. These results are meant to serve as a baseline for sensitivity assessments to be undertaken with this model in the future.

Revisiting the sedimentary record of the rise of diatoms

Diatoms are a major primary producer in the modern oceans and play a critical role in the marine silica cycle. Their rise to dominance is recognized as one of the largest shifts in Cenozoic marine ecosystems, but the timing of this transition is debated. Here, we use a diagenetic model to examine the effect of sedimentation rate and temperature on the burial efficiency of biogenic silica over the past 66 million years (i.e., the Cenozoic). We find that the changing preservation potential of siliceous microfossils during that time would have overprinted the primary signal of diatom and radiolarian abundance. We generate a taphonomic null hypothesis of the diatom fossil record by assuming a constant flux of diatoms to the sea floor and having diagenetic conditions driven by observed shifts in temperature and sedimentation rate. This null hypothesis produces a late Cenozoic (∼5 Ma to 20 Ma) increase in the relative abundance of fossilized diatoms that is comparable to current empirical records. This suggests that the observed increase in diatom abundance in the sedimentary record may be driven by changing preservation potential. A late Cenozoic rise in diatoms has been causally tied to the rise of grasslands and baleen whales and to declining atmospheric CO2 levels. Here we suggest that the similarity among these records primarily arises from a common driver-the cooling climate system-that drove enhanced diatom preservation as well as the rise of grasslands and whales, rather than a causal link among them.

Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica‐depleted Paleozoic oceans

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 (δ30 Si) 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 δ30 Si and δ18 O 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 ~150μM 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.

Constraints on Earth System Functioning at the Paleocene‐Eocene Thermal Maximum From the Marine Silicon Cycle

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM, ca. 56 Ma) is marked by a negative carbon isotope excursion (CIE) and increased global temperatures. The CIE is thought to result from the release of 13C‐depleted carbon, although the source(s) of carbon and triggers for its release, its rate of release, and the mechanisms by which the Earth system recovered are all debated. Many of the proposed mechanisms for the onset and recovery phases of the PETM make testable predictions about the marine silica cycle, making silicon isotope records a promising tool to address open questions about the PETM. We analyzed silicon isotope ratios (δ30Si) in radiolarian tests and sponge spicules from the Western North Atlantic (ODP Site 1051) across the PETM. Radiolarian δ30Si decreases by 0.6‰ from a background of 1‰ coeval with the CIE, while sponge δ30Si remains consistent at 0.2‰. Using a box model to test the Si cycle response to various scenarios, we find the data are best explained by a weak silicate weathering feedback, implying the recovery was mostly driven by nondiatom organic carbon burial, the other major long‐term carbon sink. We find no resolvable evidence for a volcanic trigger for carbon release, or for a change in regional oceanography. Better understanding of radiolarian Si isotope fractionation and more Si isotope records spanning the PETM are needed to confirm the global validity of these conclusions, but they highlight how the coupling between the silica and carbon cycles can be exploited to yield insight into the functioning of the Earth system.

The Significance of Giant Phaeodarians (Rhizaria) to Biogenic Silica Export in the California Current Ecosystem

AbstractIn marine ecosystems, many planktonic organisms precipitate biogenic silica (bSiO2) to build silicified skeletons. Among them, giant siliceous rhizarians (&gt;500 μm), including Radiolaria and Phaeodaria, are important contributors to oceanic carbon pools but little is known about their contribution to the marine silica cycle. We report the first analyses of giant phaeodarians to bSiO2 export in the California Current Ecosystem. We measured the silica content of single rhizarian cells ranging in size from 470 to 3,920 μm and developed allometric equations to predict silica content (0.37–43.42 μg Si/cell) from morphometric measurements. Using sediment traps to measure phaeodarian fluxes from the euphotic zone on four cruises, we calculated bSiO2 export produced by two families, the Aulosphaeridae and Castanellidae. Biogenic silica export ranged from &lt;0.01 to 0.63 mmol Si · m−2 · day−1. These two families alone contributed on average 10% (range 0–80%) of total bSiO2 export from the euphotic zone. Their proportional contributions increased substantially in more oligotrophic regions with lower bSiO2 fluxes. Using the in situ Underwater Vision Profiler 5, we characterized vertical distributions of the giant phaeodarian family Aulosphaeridae to a depth of 500 m and inferred their contribution to bSiO2 export in deeper waters. We found a significant increase of Aulosphaeridae export (&lt;0.01 to 2.82 mmol Si · m−2 · day−1) when extended to mesopelagic depths. Using a global data set of in situ profiles, we estimated the significance of Aulosphaeridae to bSiO2 export and revealed that they can act as major exporters of bSiO2 to the mesopelagic zone in various regions.

Silicate weathering and North Atlantic silica burial during the Paleocene-Eocene Thermal Maximum

Abstract During the Paleocene-Eocene Thermal Maximum (PETM, ca. 56 Ma), thousands of gigatons of carbon were released into the ocean and atmosphere over several thousand years, offering the opportunity to study the response of ocean biogeochemistry to a carbon cycle perturbation of a similar magnitude to projected anthropogenic CO2 release. PETM scenarios typically invoke accelerated chemical weathering of terrestrial silicate rocks as a significant negative feedback driving the recovery and termination of the event. However, the implications of this mechanism for the geochemical cycling of silica during the PETM have received little attention. I use “back-of-the-envelope” calculations and a simple two-box geochemical model of the marine silica cycle to demonstrate that the sequestration of thousands of gigatons of carbon by enhanced silicate weathering during the PETM would have dramatically increased the riverine supply of dissolved silica (H4SiO4) to the oceans. This would have elevated seawater [H4SiO4], encouraging both increased opal (SiO2) production by siliceous organisms and enhanced preservation of SiO2 in the water column and sediments. Both of these factors would have promoted a prompt (due to the relatively short oceanic residence time of silica) increase in sedimentary opal burial, thus balancing the marine silica budget. Several recently recovered deep-sea sedimentary records from the central North Atlantic demonstrate elevated SiO2 content across the Paleocene-Eocene boundary, which I argue is the result of enhanced production and/or preservation of SiO2 in response to elevated [H4SiO4] in the North Atlantic, representing the ultimate fate of excess Si weathered from the continents during the PETM.

Sedimentology and geochemistry of Archean silica granules

The production of biogenic silica has dominated the marine silica cycle since early Paleozoic time, drawing down the concentration of dissolved silica in modern seawater to a few parts per million (ppm). Prior to the biological innovation of the first silica biomineralizing organisms in late Proterozoic time, inputs of silica into Precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes, although the mechanics of this abiotic marine silica cycle remain poorly understood. Cherty sedimentary rocks are abundant in Archean sequences, and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in Archean seawater. The recent discovery that many pure chert layers in early Archean rocks formed as sedimentary beds of sand-sized, subspherical silica granules has provided direct evidence for primary silica deposition. Here, we provide further sedimentological and geochemical analyses of early Archean silica granules in order to gain a better understanding of the mechanisms of granule formation. Silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths. The abundance and widespread distribution of silica granules in Archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column. The regular occurrence of silica granules as centimeter-scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation. Contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis. Looking to modern siliceous sinters for insight into silica precipitation, we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres. Consistent with this hypothesis, Archean ocean chemistry would have favored particle aggregation over gelling. Granule formation would have been most favorable under conditions promoting rapid silica polymerization, including high salinity and/or high concentrations of dissolved silica. Our observations suggest that granule sedimentation was often episodic, suggesting that granule formation may have also been episodic, perhaps linked to variations in these key parameters.

MESMO 2: a mechanistic marine silica cycle and coupling to a simple terrestrial scheme

Abstract. Here we describe the second version of Minnesota Earth System Model for Ocean biogeochemistry (MESMO 2), an earth system model of intermediate complexity, which consists of a dynamical ocean, dynamic-thermodynamic sea ice, and energy moisture balanced atmosphere. The new version has more realistic land ice masks and is driven by seasonal winds. A major aim in version 2 is representing the marine silica cycle mechanistically in order to investigate climate-carbon feedbacks involving diatoms, a critically important class of phytoplankton in terms of carbon export production. This is achieved in part by including iron, on which phytoplankton uptake of silicic acid depends. Also, MESMO 2 is coupled to an existing terrestrial model, which allows for the exchange of carbon, water and energy between land and the atmosphere. The coupled model, called MESMO 2E, is appropriate for more complete earth system simulations. The new version was calibrated, with the goal of preserving reasonable interior ocean ventilation and various biological production rates in the ocean and land, while simulating key features of the marine silica cycle.

The Impact of Depositional Events and Burial Rate On Carbonate-Silica Diagenesis In A Middle Jurassic Stromatactis Carbonate Mud Mound, Sainte-Baume Massif, Se France

Abstract Understanding the processes and conditions of chertification in carbonates is a challenging problem to assess the marine silica cycle. This contribution highlights parameters of internal silica recycling processes from their biogenic source to quartz cementation by describing a unique case of carbonate–silica diagenesis in a Mesozoic mud mound. Stromatactis carbonate mud mounds exposed in the Sainte-Baume Massif (Basse Provence, France) developed in an outer-shelf environment during the late Aalenian concavum ammonite Zone (Middle Jurassic). They form part of a cherty succession punctuated by hardgrounds and stratigraphically condensed intervals, interpreted to record deepening episodes. These mud mounds, rich in siliceous sponge spicules and stromatactis, are similar to their widespread Paleozoic counterparts but are particular in being extensively silicified. Carbonate–silica paragenesis forms a polymud fabric including four microcrystalline carbonates (M1 to M4; all low-Mg calcite, LMC), five spar cements (C1 to C5; C1 with high-Mg calcite, HMC, precursor mineralogy), and four silica phases (S1 to S4; S1 to S3 replacive chalcedony after opal-CT, S4 euhedral quartz cement). M1, assumedly related to the degradative calcification of siliceous sponges, forms a labyrinthine network into which M2 to M4 were infiltrated. Spar cements C1 to C3 include a successive decrease of δ18 O at fairly invariable δ13 C, interpreted to represent marine to shallow burial conditions, suggesting burial and successive transformation of marine bottom waters. Temperature estimation, based on the average δ18O value of C4 (δ18O −7.9‰), indicates a temperature of around 50°C at a maximum burial depth of about 1000 meters. Replacive chalcedony (S1 to S3) occurs between calcite cementation C3 and C4 in combination with traces of corrosion on C3. Mg-calcite was preferably replaced, particularly the precursor of cement C1 that surrounds peloids of M2, and preserved some ghost structures. Corrosion and substrate selectivity suggest that acidification, Mg-hydroxyl complexes, and surface area are the triggers for flocculation of a silica gel. Because dissolution of opaline sponge spicules started almost contemporaneously with C1, the dissolved silica was retained in a connate fluid as phases C1 to C3 were precipitated, and stagnant conditions prevailed for around 1 Ma. Bulk δ18O/δD values (S1–S3) below the marine chert line, together with the range of S1–S3 δ18O (δ18O SMOW = 28.3‰ ± 1.0) suggest silica flocculation in a shallow burial environment at temperatures of 25–30°C. The establishment of acidic conditions might have been favored by sulfide-oxidizing micro-organisms. Numerical simulation of early diagenetic silica flux in such marine sediments is consistent with the interpretation that small grain-size changes and fluid barriers (that create diffusion-controlled conditions), such as those induced by stratigraphic condensation, are key parameters to retain pore-water dissolved silica over geologic time. Inversely, Paleozoic stromatactis carbonate mud mounds typically form part of large-scale coarsening- and shallowing-upward successions that correspond to rapid burial. These conditions support convergent pore-water flux and drainage of pore-water dissolved silica in the shallow burial realm. Hence, spicule-rich mounds should tend to silicify if they form part of a condensed section.

Silica Use Through Time: Macroevolutionary Change in the Morphology of the Diatom Fustule

Diatoms have evolved an obligate requirement for silica for their ornamented cell wall. Diatom productivity and subsequent burial of their siliceous remains on the ocean floor is a major control on the marine silica cycle. The ecological success of the diatoms over the Mesozoic and Cenozoic is associated with biogeographic shifts in siliceous sponges, decreasing silicification in the radiolarians, and a global decrease in oceanic silicic acid concentrations. We review what is known about the evolutionary advantages of the silica frustule and how selection pressures, including decreasing silicic acid concentrations, changes in growth limiting nutrients other than Si, CO2, irradiance, predation and viral pressures have shaped the morphology of the frustule over the Mesozoic and Cenozoic.

Groundfish overfishing, diatom decline, and the marine silica cycle: Lessons from Saanich Inlet, Canada, and the Baltic Sea cod crash

In this study, we link groundfish activity to the marine silica cycle and suggest that the drastic mid‐1980s crash of the Baltic Sea cod (Gadus morhua) population triggered a cascade of events leading to decrease in dissolved silica (DSi) and diatom abundance in the water. We suggest that this seemingly unrelated sequence of events was caused by a marked decline in sediment resuspension associated with reduced groundfish activity resulting from the cod crash. In a study in Saanich Inlet, British Columbia, Canada, we discovered that, by resuspending bottom sediments, groundfish triple DSi fluxes from the sediments and reduce silica accumulation therein. Using these findings and the available oceanographic and environmental data from the Baltic Sea, we estimate that overfishing and recruitment failure of Baltic cod reduced by 20% the DSi supply from bottom sediments to the surface water leading to a decline in the diatom population in the Baltic Sea. The major importance of the marginal ocean in the marine silica cycle and the associated high population density of groundfish suggest that groundfish play a major role in the silica cycle. We postulate that dwindling groundfish populations caused by anthropogenic perturbations, e.g., overfishing and bottom water anoxia, may cause shifts in marine phytoplankton communities.

Silicic acid leakage from the Southern Ocean: A possible explanation for glacial atmospheric pCO2

Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO2. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO3 production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO2 from the interglacial 277 to 230–242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO2 sensitivity of general circulation models to carbonate pump forcing is ∼3.5–fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO2. We discuss the importance of the initial rain ratio of CaCO3 to organic carbon on atmospheric pCO2 and relevant sedimentary records that support and constrain this “silicic acid leakage” scenario.

Secular Distribution of Biogenic Silica through the Phanerozoic: Comparison of Silica‐Replaced Fossils and Bedded Cherts at the Series Level

Abstract The modern marine silica cycle is dominated by silica‐secreting phytoplankton, principally diatoms, but this cycle has evolved considerably during the Phanerozoic. We analyzed the temporal distribution of silica‐replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies. Trends in silica replacement of fossils match faunal radiations during the Ordovician and Siluro‐Devonian, with peaks in silica replacement in the Late Ordovician and Middle Devonian corresponding to peaks in the abundance and diversity of siliceous sponges. Sharp drops in the abundance of silica‐replaced fossils and/or bedded cherts coincide with four of the five major mass extinctions. No discernible decrease marks the extinction at the end of the Triassic. We expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling. Orogenic episodes, which may trigger continental glaciati...