Diatom Dissolution Research Articles

The effects of source water masses and internal recycling on concentrations of dissolved inorganic nutrients in the Gulf of Maine

It has been historically documented that deep, nutrient-rich, offshore slope waters that enter the Gulf of Maine through the Northeast Channel are the principal source of nutrients to the region. Silicate (Si(OH)4) and nitrate (NO3) in particular are potentially limiting nutrients for phytoplankton production. We examine here the extent to which nutrient variability in the region may be caused by internal recycling of organic material (i.e., chemical breakdown and dissolution of diatoms) versus variability in water mass sources, which can be identified by temperature and salinity properties. We present here the results from a gulf-wide survey conducted in October 2016. Nutrient samples were collected at 44 stations and compared to temperature, salinity, and beam attenuation profiles. Additionally, suspended particulate material in near-bottom waters was collected on filters at all stations and analyzed for biogenic silica. The results show that after being supplied via a slope water source, nitrate is likely to become depleted and silicate is likely to become enriched at any location in the gulf. We suspect that most of the nutrient variability is due to internal recycling, but there is evidence for an input of Scotian Shelf Water to the Jordan Basin region contributing nutrients at mid-depths and mixing with the deeper slope waters.

A record of diatom preservation from the AND-1B drillcore: The significance of taphofacies on diatom preservation

Abstract 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.

Lake Kivu Water Chemistry Variation with Depth Over Time, Northwestern Rwanda

The water of East African rift lakes contains large amounts of dissolved chemicals such as carbon dioxide, methane greatly and others like phosphate, silicate, Sulfate, Sulfide, Iron, Ammonia, Alkalinity etc. Lake Kivu is a large, deep rift basin lake located in the western branch of the East African rift zone that contains a methane gas deposit of great economic interest with two main sources: Inorganic carbon dioxide CO2 + 4H2 = CH4 + 2H2O and Organic methanogenesis CH3COOH =CH4 + CO2. Lake Kivu is a stratified, meromictic lake bordering Rwanda and the Democratic Republic of the Congo (DRC). The lake has a surface area of 2,370 Km2, a volume of 580 Km3 and a maximum water depth of 485 m. To characterize the vertical variation of Lake Kivu water chemistry, 8 samples of water were collected using Niskin bottles in Lake Kivu near Gisenyi town. Water samples were therefore collected at different depths: 0 m, 40 m, 90 m, 240 m, 290 m, 340 m, 340 m, and 390 m. Hatch kits were used to analyze water chemistry of samples taken of Sulfate, Sulfide, Iron, Ammonia, Alkalinity, Silica, PO4,andphosphorus.The results revealed that alkalinity increases in the monimolimnion part due to the precipitation of calcium carbonate in the upper levels of the water column and dissolution in the monimolimnion. The conductivity, dissolved oxygen, temperature and pH weremeasured by CTD Sonde. Water column data from these studies showed increasing concentrations with depth. The divide between the mixolimnion and monomolimnion is estimated at a depth of around 40 m. Higher amounts of silica observed closer to the shoreline is likely a result of an influx of siliciclastic sediment and increased silica with depth is likely a result of the dissolution of diatoms below the photic zone.
 Keywords: Monimolimnion, mixolimnion, water stratification, chemicals agents.

The Record of Environmental and Microbial Signatures in Ancient Microbialites: The Terminal Carbonate Complex from the Neogene Basins of Southeastern Spain

The Messinian microbialites of the Terminal Carbonate Complex (TCC) from the Neogene basins of southeastern Spain show both diversified morphologies and an excellent preservation of primary microbial microstructures. Their stratigraphic architecture, fabric (micro-, meso-, and macro-fabric), and mineralogical composition were investigated in eight localities from three sedimentary basins of southeastern Spain: The Sorbas and Bajo Segura basins and the Agua Amarga depression. Two recurrent microbialite associations were distinguished. Laterally linked low relief stromatolites predominated in Microbialite Association 1 (MA1), which probably formed in low energy lagoons or lakes with fluctuating normal marine to hypersaline water. The microfabrics of MA1 reflected the predominance of microbially induced/influenced precipitation of carbonates and locally (Ca)-Mg-Al silicates. Microbialite Association 2 (MA2) developed in high energy wave and tidal influenced foreshore to shoreface, in normal marine to hypersaline water. High-relief buildups surrounded by mobile sediment (e.g., ooids or pellets) dominated in this environment. MA2 microbialites showed a significant proportion of thrombolitic mesofabric. Grain-rich microfabrics indicated that trapping and binding played a significant role in their accretion, together with microbially induced/influenced carbonate precipitation. The stratigraphic distribution of MA1 and MA2 was strongly influenced by water level changes, the morphology and nature of the substratum, and exposure to waves. MA1 favorably developed in protected areas during third to fourth order early transgression and regression phases. MA2 mostly formed during the late transgressions and early regressions in high energy coastal areas, often corresponding to fossil coral reefs. Platform scale syn-sedimentary gypsum deformation and dissolution enhanced microbial carbonate production, microbialites being thicker and more extended in zones of maximum deformation/dissolution. Microbial microstructures (e.g., microbial peloids) and microfossils were preserved in the microbialites. Dolomite microspheres and filaments showed many morphological similarities with some of the cyanobacteria observed in modern open marine and hypersaline microbialites. Dolomite potentially replaced a metastable carbonate phase during early diagenesis, possibly in close relationship with extracellular polymeric substances (EPS) degradation. Double-layered microspheres locally showed an inner coating made of (Ca)-Mg-Al silicates and carbonates. This mineral coating could have formed around coccoid cyanobacteria and indicated an elevated pH in the upper part of the microbial mats and a potential dissolution of diatoms as a source of silica. Massive primary dolomite production in TCC microbialites may have resulted from enhanced sulfate reduction possibly linked to the dissolving gypsum that would have provided large amounts of sulfate-rich brines to microbial mats. Our results open new perspectives for the interpretation of ancient microbialites associated with major evaporite deposits, from microbe to carbonate platform scales.

Assessing the Influence of Saline and Alkaline Environments in the Preservation of Pampean Diatoms: An Experimental Approach

Diatom dissolution is controlled by environmental conditions prevailing during fossilization. In Pampean shallow lakes, diatom dissolution showed important correlations with gradients of salinity, pH, carbonate, and bicarbonate, being these the most probable causes of the observed shifts in Holocene assemblages preservation. In the present contribution, a series of experiments were conducted in order to demonstrate the effect that physical-chemical lake characteristics exert on Pampean diatom assemblages preservation. Three experimental assemblages were subjected to the effect of three concentrations of two salts, NaCl (0.6, 1.2 and 3M) and NaHCo3 (0.6, 0.9, and 1.2M), and two pH values (7 and 10). Aliquots of the experimental solutions were removed once each five days for 20 days, and analyzed for changes in dissolved silica concentration (SiDi), relative and absolute abundances of diatoms, and dissolution indices (DDI) based on the target taxon Cyclotella meneghiniana. All the experimental solutions increased the SiDi significantly, particularly since day 10. These increased SiDi values were accompanied by significant changes in the DDI, which reached maximum values at pH 10, and by evidence of dissolution observed in SEM images, whereas no significant changes in relative or absolute abundances of diatoms were registered. These experimental results demonstrated the impact that water chemistry can exert on diatom dissolution in Pampean shallow lakes, even during short-term exposures. Given the naturally high pH, NaCl and NaHCo3 concentrations characteristic of many of these lakes, these experimental findings can be confidently extrapolated to the interpretation of the dissolution trends found in modern and fossil sedimentary assemblages.

The role of oxygen conditions in the microbial dissolution of biogenic silica under brackish conditions

Regeneration of biogenic silica (BSi) is essential for the constant supply of dissolved silica (DSi) to diatoms in aquatic ecosystems, where these primary producers play an important role in carbon (C) drawdown. Increased DSi concentrations have been detected in the oxygen (O2) depleted bottom waters of e.g. the brackish Baltic Sea, which is suffering from one of the largest anoxic or hypoxic marine areas in the world. As hypoxia is likely to become more frequent also in coastal areas, the effects of suboxic conditions on the silicon (Si) cycle should be explored. To examine the effects of hypoxia on microbially mediated DSi release from diatom detritus and sediment to brackish seawater, a batch laboratory experiment was conducted. Hypoxic conditions facilitated more complete dissolution of diatoms than oxic conditions. However, the age, origin and physiological condition of the BSi significantly affected the dissolution rate, and the presence of sedimentary material decreased BSi dissolution. Inhibition of bacterial activity reduced the dissolution of diatoms under both oxic and hypoxic conditions, and T-RFLP of 16S rRNA genes confirmed that O2 conditions affected the composition of the bacterial community. Taken together, these results demonstrate that oxygen conditions and microbes play a fundamental role in regeneration of diatom BSi. Moreover, the results may explain the elevated DSi concentrations associated with O2 depletion in the bottom waters of the Baltic Sea.

Transformation of silicon in a sandy beach ecosystem: Insights from stable silicon isotopes from fresh and saline groundwaters

Dissolved silicon isotope compositions (δ30Si) have been analysed for the first time in groundwaters of beach sediments, which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater. The fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736μM, but homogeneous δ30Si of +1.0±0.15‰. By contrast, the seawater is strongly depleted in dissolved silica with concentrations of 3μM, and consequently characterised by high δ30Si of +3.0‰. The beach groundwaters are variably enriched in dissolved silica compared to seawater (23–192μM), and concentrations increase with depth at all sampling sites. The corresponding δ30Si values are highly variable (+0.3‰ to +2.2‰) and decrease with depth at each site. All groundwater δ30Si values are lower than seawater and most values are lower than dissolved δ30Si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments. In contrast to open North Sea sediments, diatom dissolution or formation of authigenic silica in beach sediments is very low (ca. 5μmolSig−1). Silica discharge from the beach to the coastal ocean is estimated as approximately 210molSiyr−1 per meter shoreline. Considering the extent of coastline this is, at least for the study area, a significant amount of the total Si budget and amounts to ca. 1% of river and 3.5% of backbarrier tidal flat area Si input.

Increased diatom dissolution in Prydz Bay, East Antarctica linked to inception of the Prydz Bay gyre

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.

Insights into the transfer of silicon isotopes into the sediment record

Abstract. The first δ30Sidiatom data from lacustrine sediment traps are presented from Lake Baikal, Siberia. Data are compared with March surface water (upper 180 m) δ30SiDSi compositions for which a mean value of +2.28‰ ± 0.09 (95 % confidence) is derived. This value acts as the pre-diatom bloom baseline silicic acid isotopic composition of waters (δ30SiDSi initial). Open traps were deployed along the depth of the Lake Baikal south basin water column between 2012 and 2013. Diatom assemblages display a dominance ( > 85 %) of the spring/summer bloom species Synedra acus var radians, so that δ30Sidiatom compositions reflect predominantly spring/summer bloom utilisation. Diatoms were isolated from open traps and, in addition, from 3-monthly (sequencing) traps (May, July and August 2012) for δ30Sidiatom analyses. Mean δ30Sidiatom values for open traps are +1.23‰ ± 0.06 (at 95 % confidence and MSWD of 2.9, n = 10). Total dry mass sediment fluxes are highest in June 2012, which we attribute to the initial export of the dominant spring diatom bloom. We therefore argue that May δ30Sidiatom signatures (+0.67‰ ± 0.06, 2σ) when compared with mean upper water δ30SiDSi initial (e.g. pre-bloom) signatures can be used to provide a snapshot estimation of diatom uptake fractionation factors (ϵuptake) in Lake Baikal. A ϵuptake estimation of −1.61 ‰ is therefore derived, although we emphasise that synchronous monthly δ30SiDSi and δ30Sidiatom data would be needed to provide more robust estimations and therefore more rigorously test this, particularly when taking into consideration any progressive enrichment of the DSi pool as blooms persist. The near-constant δ30Sidiatom composition in open traps demonstrates the full preservation of the signal through the water column and thereby justifies the use and application of the technique in biogeochemical and palaeoenvironmental research. Data are finally compared with lake sediment core samples, collected from the south basin. Values of +1.30‰ ± 0.08 (2σ) and +1.43‰ ± 0.13 (2σ) were derived for cores BAIK13-1C (0.6–0.8 cm core depth) and at BAIK13-4F (0.2–0.4 cm core depth) respectively. Trap data highlight the absence of a fractionation factor associated with diatom dissolution (ϵdissolution) (particularly as Synedra acus var radians, the dominant taxa in the traps, is very susceptible to dissolution) down the water column and in the lake surface sediments, thus validating the application of δ30Sidiatom analyses in Lake Baikal and other freshwater systems, in palaeoreconstructions.

An experiment to assess the effects of diatom dissolution on oxygen isotope ratios.

Current studies which use the oxygen isotope composition from diatom silica (δ(18) Odiatom ) as a palaeoclimate proxy assume that the δ(18) Odiatom value reflects the isotopic composition of the water in which the diatom formed. However, diatoms dissolve post mortem, preferentially losing less silicified structures in the water column and during/after burial into sediments. The impact of dissolution on δ(18) Odiatom values and potential misinterpretation of the palaeoclimate record are evaluated. Diatom frustules covering a range of ages (6 samples from the Miocene to the Holocene), environments and species were exposed to a weak alkaline solution for 48 days at two temperatures (20 °C and 4 °C), mimicking natural dissolution post mucilage removal. Following treatment, dissolution was assessed using scanning electron microscope images and a qualitative diatom dissolution index. The diatoms were subsequently analysed for their δ(18) O values using step-wise fluorination and isotope ratio mass spectrometry. Variable levels of diatom dissolution were observed between the six samples; in all cases higher temperatures resulted in more frustule degradation. Dissolution was most evident in younger samples, probably as a result of the more porous nature of the silica. The degree of diatom dissolution does not directly equate to changes in the isotope ratios; the δ(18) Odiatom value was, however, lower after dissolution, but in only half the samples was this reduction outside the analytical error (2σ analytical error = 0.46‰). We have shown that dissolution can have a small negative impact on δ(18) Odiatom values, causing reductions of up to 0.59‰ beyond analytical error (0.46‰) at natural environmental temperatures. These findings need to be considered in palaeoenvironmental reconstructions using δ(18) Odiatom values, especially when interpreting variations in these values of <1‰.

The silicon isotope record of early silica diagenesis

The heavy isotopes of silicon are strongly enriched in some of the youngest, early diagenetically formed porcellanite layers from the Southwest Indian Ridge (Pleistocene) and the Maud Rise (Pliocene). These porcellanite layers are composed of opal-CT and were formed by the conversion of amorphous silica (opal-A) from siliceous sediment via dissolution–reprecipitation. Their bulk δSi30 values range between 1.7 and 2.3‰. Detritus-poor siliceous sediment surrounding these layers is significantly lower at −0.3 to 1.5‰. Sequential chemical extractions of bulk siliceous sediment show (i) preferential dissolution of diatoms featuring higher δSi30 than radiolaria and Al–Si components. The detailed investigation of porcellanite layers by micro-scale Si isotope and Al/Si analyses using UV femtosecond laser ablation ICP mass spectrometry show that (ii) precipitation of authigenic aluminum silicates enriched in light Si isotopes drives pore waters to even higher δSi30. We suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert. We infer that past environmental conditions can be reconstructed with high fidelity from the stable Si isotope composition of chert when initial seawater Si concentrations were high (such as in the Precambrian). Exchange of Si between layers during phase transformation (from opal-A to opal-CT and from opal-CT to quartz) is impeded when variable amounts of detrital minerals are present, because they control rates of silica phase transformation and hence the timing of dissolution–reprecipitation during burial.

A record of Pleistocene diatom preservation from the Amundsen Sea, West Antarctica with possible implications on silica leakage

article i nfo Accumulation of fossil diatoms in marine sediments is variably affected by the production and dissolution of diatom frustules across space and time. No quantitative proxy exists for estimating the amount of silica lost to dissolution in Southern Ocean sediment cores. To address this gap, cultured valves of Fragilariopsis kerguelensis, the dominant planktonic diatom in the Pleistocene to recent Southern Ocean, were dissolved progressively under controlled laboratory conditions. Expansion, and eventual conjoining, of the areolae of F. kerguelensis advances through several phases, which are presented herein as a measure of diatom dissolution. In order to evaluate the validity of laboratory methods, this proxy is applied to sediments from piston core PS58/254 from the Amundsen Sea. Dissolution-induced degradation morphologies from laboratory experiments are seen to be identical to those preserved naturally in sediments, thus suggesting the laboratory conditions accurately reflect the natural process of dissolution. These resultsrepresentthe firstapplication of afullyquantitativeproxyfordiatomdissolution toSouthernOcean sediments, revealing that meaningful paleoceanographic data can be generated via taphonomic analysis of diatom valves. Intensity of diatom dissolution in core PS58/254 does not correspond to the relative abundance of any diatom species, clay mineralogy, or linear sedimentation rates. However, a correlation does exist between glacial cyclicity and diatom dissolution, indicating that variation in average wall thickness and susceptibility to dissolution reflects primary conditions during formation of the silica wall, transit through the water column, and eventual deposition. Higher overall silica dissolution is observed during each glacial period and enhanced silica preservation during interglacials. This result is consistent with the predictions of the silicic acid leakage hypothesis (SALH), whereby reduced rates of Si usage in the Southern Ocean lead to shifts in marine primary productivity in the tropics from coccolithophoriddominance duringinterglacialstodiatomdominanceduringglacials.Thiswouldfuela decrease in atmospheric pCO2duringglacials. Silica acidleakage isdrivenbya decreaseinthe Si:Nratioofdiatoms,caused by increased iron input to glacial-aged Southern Ocean waters. This resulted in more lightly silicified diatom valves in the Southern Ocean, which dissolved more quickly in the upper water column, thereby maintaining the high silica content of glacial-aged SO waters, and allowing for export of Si to upwelling zones in the lower latitudes.

Diatom species abundance and morphologically-based dissolution proxies in coastal Southern Ocean assemblages

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.

Dissolved silicon and its isotopes in the water column of the Bay of Bengal: Internal cycling versus lateral transport

The concentration of dissolved Si and its isotope composition are measured in the Bay of Bengal (BoB) region of the northern Indian Ocean; the isotope data are the first data set from the northern Indian Ocean. The measurements are made in eight depth profiles closely along the 87°E transect (GIO1 section of the international GEOTRACES program) and in a few samples from the northern shelf of the bay. Dissolved Si in the water column varies from ∼0.6 to ∼152.5μmol/kg, whereas the δ30Si data cover a range +1.2‰ to +3.6‰. The depth profiles of dissolved Si show generally lower values in the surface increasing with depth, whereas the pattern reverses in the case of δ30Si. These vertical distribution patterns of Si and δ30Si are similar to those reported in other oceanic regions and suggestive of the significant role of biological processes in governing Si biogeochemistry in the upper layers (top ∼1500m). In contrast, dissolved Si in near surface waters of the northern shelf and the southernmost station is exceptionally high. These results indicate a continental supply of dissolved Si from the Ganga–Brahmaputra river system (G–B) and submarine groundwater discharge (SGD) to the shelf region, and an intrusion of high salinity waters from the Arabian Sea in the southern bay. The δ30Si values of ∼1.34±0.10‰ for deep/bottom waters of the BoB (depth >1500m) are similar to those reported for the deep Southern Ocean and indicate the dominant control of water mass mixing. The dissolved Si concentrations in the bottom waters of the BoB are generally higher than those of the water mass endmembers, which suggest the need for an additional source of Si; in situ particle dissolution and/or benthic release in the central bay seem to be the potential candidate.The annual Si budget in the top ∼100m of the BoB seems to suggest that meso-scale eddies frequently occurring during non-monsoon periods can supply at the most ∼2.6gSi/m2/year, which is about 33% of the Si requirement to support new production in the bay. The supply of dissolved Si (∼1.3±0.5×1011mol/year) from the G–B river system and SGD has been calculated based on the distributions of dissolved Si concentration and δ30Si in the northern shelf waters. A comparison of this supply with the reported Si flux upstream of the estuarine zone indicates about 40% removal of dissolved Si in the G–B estuary. The mass balance of Si isotopes in the deep waters indicates that the dissolution of diatoms is the main cause of excess Si in the bay.

Silica cycling in the ultra-oligotrophic eastern Mediterranean Sea

Abstract. Although silica is a key plant nutrient, there have been few studies aimed at understanding the Si cycle in the eastern Mediterranean Sea (EMS). Here we use a combination of new measurements and literature values to explain the silicic acid distribution across the basin and to calculate a silica budget to identify the key controlling processes. The surface water concentration of ∼1 μM, which is unchanging seasonally across the basin, was due to the inflow of western Mediterranean Sea (WMS) water at the Straits of Sicily. It does not change seasonally because there is only a sparse population of diatoms due to the low nutrient (N and P) supply to the photic zone in the EMS. The concentration of silicic acid in the deep water of the western Ionian Sea (6.3 μM) close to the S Adriatic are an of formation was due to the preformed silicic acid (3 μM) plus biogenic silica (BSi) from the dissolution of diatoms from the winter phytoplankton bloom (3.2 μM). The increase of 4.4 μM across the deep water of the EMS was due to silicic acid formed from in situ diagenetic weathering of aluminosilicate minerals fluxing out of the sediment. The major inputs to the EMS are silicic acid and BSi inflowing from the western Mediterranean (121 × 109 mol Si yr−1 silicic acid and 16 × 109 mol Si yr−1 BSi), silicic acid fluxing from the sediment (54 × 109 mol Si yr−1) and riverine (27 × 109 mol Si yr−1) and subterranean groundwater (9.7 × 109 mol Si yr−1) inputs, with only a minor direct input from dissolution of dust in the water column (1 × 109 mol Si yr−1). This budget shows the importance of rapidly dissolving BSi and in situ weathering of aluminosilicate minerals as sources of silica to balance the net export of silicic acid at the Straits of Sicily. Future measurements to improve the accuracy of this preliminary budget have been identified.

Miocene diatom biostratigraphy of Gharandal Group, west-central Sinai, Egypt

Detailed diatom analysis has been carried out on one hundred forty-two samples collected from the Lower to Middle Miocene stratigraphic succession located in the Abu Qada basin, west-central Sinai. The succession is composed only of two lithostratigraphic units belonging to Gharandal Group; they are Rudies Formation followed directly by Kareem Formation. The diatom analysis shows interesting diverse and abundant different diatom assemblages of both marine and non-marine origin.Atotal of 84 diatom species and varieties belonging to 32 genera were identified. The preservation of the recorded taxa varies from poorly preserved in some samples to generally moderately to well preserve in other samples. Several intervals of pronounced diatom dissolution detected throughout the studied section. A biostratigraphic zonal framework consisting of three planktonic diatom zones is proposed and the Early /Middle Miocene boundary is determined accurately by means of diatom taxa.

Quantifying the impact of freshwater diatom productivity on silicon isotopes and silicon fluxes: Lake Myvatn, Iceland

Diatom productivity in the oceans plays a crucial role in the carbon cycle, but is strongly dependent upon the continental silicon supply. However, the relative influence of weathering and biological processes on continental Si fluxes remains poorly constrained. This study aims to quantify the impact of terrestrial diatom productivity on Si fluxes to the ocean. Lake Myvatn in North Iceland is one of the most productive lakes in the Northern Hemisphere, with nutrient-rich waters almost uniquely sourced by groundwater. The primary production is mainly controlled by diatom growth but also by cyanobacteria, and the lake output is via a single river, thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and Si budget of lake waters. Silicon stable isotopes (δ 30Si) provide a tracer of this biocycling, and have been measured in groundwater inputs to the lake, and in time-series monitoring of waters at the lake outlet. The δ 30Si values at the outlet range from + 0.70 ± 0.08 to + 1.42 ± 0.06‰, which is significantly heavier than the groundwater input (average cold and hot springs: + 0.50 ± 0.17‰, 2SD) and consistent with the preferential uptake of light Si isotopes by diatoms. The δ 30Si value at the outlet increases by up to 0.9‰ in spring and autumn relative to the Si isotope composition of the inflow. These seasonal diatom blooms can be modeled by an open system of Si uptake and affect Si fluxes at the outlet of the lake by up to 79%, or 53% integrated over the year. In the summer a shift to lighter δ 30Si values is correlated with a higher pH, which results in dissolution of diatoms releasing light Si isotopes. From mass balance, this seasonal diatom dissolution affects Si fluxes by up to 33%, but is limited to 3.7% integrated over the year. These results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived Si. They also demonstrate the pH dependency of diatom dissolution and/or preservation, which is likely to affect not only the continental Si fluxes to the ocean but also the Si recycling in the oceans themselves.

Algal-silica cycling and pigment diagenesis in recent alpine lake sediments: mechanisms and paleoecological implications

The quality and interpretability of the paleobiological record depends on the preservation of morphological and geochemical fossils. Siliceous microfossils and sedimentary pigments are often cornerstones in paleoecology, although the microbial and geochemical processes conducive to their preservation remain poorly constrained. We examined sediments from an alpine lake in Banff National Park (Alberta, Canada) where diatom frustules are completely dissolved within 50 years of deposition. Diatom dissolution, silica recycling, and diagenetic alteration of algal pigments were investigated, in conjunction with porewater geochemistry and microelectrode profiling of the sediment–water interface. Analysis of sediment trap material showed ~90% of biogenic silica (BSi) production is lost prior to burial. Silica flux calculations, based on dissolved silica (as H4SiO4) in pore-waters, show a further ~6% of total BSi is returned to the water column from the upper 4 cm of sediments, implying that only ~4% of total BSi is permanently archived in sediments. In situ sediment pH and O2 profiles reveal that aerobic respiration by bacteria fully consumes oxygen by a depth of 4 mm into the sediment, with associated strong pH and redox gradients. During sedimentation and early diagenesis, diatoms undergo loss of extracellular polymeric substances that coat their frustules, promoting silica dissolution and leading to the loss of the microfossil record by a depth of 3.25 cm. Sedimentary pigments similarly undergo rapid degradation, but diatom-related carotenoids persist below the depth of silica dissolution. This work provides new insights on diagenetic processes in lakes, with broad implications for the interpretation of sedimentary proxies for algal production.

Changes in lake stratification and oxygen distribution inferred from two contrasting records of magnetotactic bacteria and diatoms

Magnetotactic bacteria (MTB) and diatoms produce hard parts on seasonal timescales that continuously becomes embedded in various soft sediment archives. Consequently, they can, and are, frequently used to reconstruct past environmental conditions in oceans, fjords, and freshwater basins. In this study, we observe how these two cosmopolitan creatures can be used to track vertical shifts in the distribution of oxygen in a small mountain lake situated in western Norway. Several short and long sediment cores have been retrieved from the lake, allowing for a detailed reconstruction that spans the last 2500 years. During periods of enhanced runoff from the catchment the Oxic‐Anoxic Transition Zone (OATZ) is lowered down to the lake‐sediment interface, due to increased convection. We posit that this condition reduces the concentration of microaerophilic magnetite‐producing MTB, an interpretation partly supported by rock magnetic analyzes. Preservation of diatom frustules is, however, improved whenever this oxygen‐rich scenario prevails. Similarly, when runoff is low, and the OATZ is lifted above the lake‐sediment interface, MTB concentration seems to increase, whereas dissolution of diatoms is more common. This consistent antiphase relationship between MTB concentration and diatom preservation, occurring on a multidecadal timescale during lowest reconstructed lake‐water pH in the lake's history, suggest a coupled response to subtle changes in external environmental variables that affect catchment and lake hydrology.

Multi-proxy record of postglacial environmental change, south-central Melville Island, Northwest Territories, Canada

A sediment core from Lake BC01 (75"10.945?N, 111"55.181?W, 225"m asl) on south-central Melville Island, NWT, Canada, provides the first continuous postglacial environmental record for the region. Fossil pollen results indicate that the postglacial landscape was dominated by Poaceae andSalix, typical of a High Arctic plant community, whereas the Arctic herbOxyriaunderwent a gradual increase during the late Holocene. Pollen-based climate reconstructions suggests the presence of a cold and dry period ~12,000"cal yr BP, possibly representing the Younger Dryas, followed by warmer and wetter conditions from 11,000 to 5000"cal yr BP, likely reflective of the Holocene Thermal Maximum. The climate then underwent a gradual cooling and drying from 5000"cal yr BP to the present, suggesting a late Holocene neoglacial cooling. Diatom preservation was poor prior to 5000"cal yr BP, when conditions were warmest, suggesting that diatom dissolution may in part be climatically controlled. Diatom concentrations were highest ~4500"cal yr BP but then decreased substantially by 3500"cal yr BP and remained low before recovering slightly in the 20th century. An abrupt warming occurred during the past 70 yr at the site, although the magnitude of this warming did not exceed that of the early Holocene.