Biogenic Silica Measurements Research Articles

Biogeochemical behavior and distribution of biogenic silica in marine sediments from Izmir Bay, Aegean Sea (Turkey)

The distribution of BSi (biogenic silica) in surface sediment and pore water from Izmir Bay was investigated using Si-only and Si/Al methods for determining the appropriate methods. Surface sediment samples were collected using a box core from 15 stations in the inner Izmir Bay from March to May 2008. The relationships between biogenic silica, heavy metals and geochemical data were investigated. Si/Al ratios (weight/weight) ranged between 0.88–1.39 for positive BSi values and 1.21–12.409 for negative BSi values.The silica dissolution values showed a rapid increase in Al concentrations during the first 5h of the experiment when the silica dissolved, followed by constant silica level at all stations. The dissolution curves in all stations have been stable in a maximum DSi (dissolved silica) value after 5h. The linear curve of Al against the silica dissolution explained the biogenic silica content from intercept. The BSi measurements for this study give more accurate results according to the method of Si/Al. Factor analysis was used to determine the relationships between silica and geochemical data. BSi and Sipore have a strong association with manganese. The diatomeae productivity in the water column has a significant importance on the stability of Mn in the sediment. Carbonate content of sediments controls the Mn anomalies from terrestrial sources in the bay. Maximum values of BSi at inner parts of the bay are due to the freshwater source at the deep.

Amplified bioproductivity during Transition IV (332 000–342 000 yr ago): evidence from the geochemical record of Lake El'gygytgyn

Abstract. To date, terrestrial archives of long-term climatic change within the Arctic have widely been restricted to ice cores from Greenland and, more recently, sediments from Lake El'gygytgyn in northeast Arctic Russia. Sediments from this lake contain a paleoclimate record of glacial-interglacial cycles during the last three million years. Low-resolution studies at this lake have suggested that changes observed during Transition IV (the transition from marine isotope stage (MIS) 10 to MIS 9) are of greater amplitude than any observed since. In this study, geochemical parameters are used to infer past climatic conditions thus providing the first high-resolution analyses of Transition IV from a terrestrial Arctic setting. These results demonstrate that a significant shift in climate was subsequently followed by a rapid increase in biogenic silica (BSi) production. Following this sharp increase, bioproductivity remained high, but variable, for over a thousand years. This study reveals differences in the timing and magnitude of change within the ratio of silica to titanium (Si/Ti) and BSi records that would not be apparent in lower resolution studies. This has significant implications for the increasingly common use of Si/Ti data as an alternative to traditional BSi measurements.

Diatom-inferred lateglacial and Holocene climatic variability in the South Carpathian Mountains (Romania)

Climatic and environmental changes were studied using siliceous algae (diatoms and chrysophycean cysts) analyses in a mountain lake in the South Carpathian Mountains for the lateglacial and Holocene. Lake Brazi (1740 a.s.l.) is a small, shallow lake in the Retezat Mountains. According to the basal radiocarbon dating of the core, the lake was formed around 15,750 cal BP and sediment accumulation has been continuous ever since. High resolution diatom analyses were carried out together with loss-on-ignition and biogenic silica measurements. Total epilimnetic phosphorus (TP) concentrations and pH were reconstructed quantitatively. Ten statistically significant zones were distinguished, six of which appeared in the lateglacial and Early Holocene suggesting the instability of diatom assemblages after deglaciation. The uppermost zone covered six thousand years, which suggests that diatom assemblages remained quite stable during the second half of the Holocene. High lake level occurred between 9500 and 9000 cal BP. After 6300 cal BP the lake level gradually increased again until 5800 cal BP. A further increase was detected at 3000 cal BP, while after 1500 cal BP water level decreased. Although there is limited knowledge regarding the environmental constraints of several diatom taxa present in the sequence (e.g. Aulacoseira laevissima), which makes the precise environmental interpretation of the diatom record more difficult, the available data provide an important new insight into the environmental history of Lake Brazi and the Carpathian Mountains. The most remarkable changes in the aquatic ecosystem were observed at 6000 cal BP in the Holocene and 12,800 cal BP, at the onset of the Younger Dryas. Marked changes were also detected at 10,500–10,300; 9800; 9500–9000; 8900–8800; 8300; 6300–5800 and 3100–3000 cal BP.

Problems with biogenic silica measurement in marginal seas

Surface sediment samples from the Bohai, Yellow Sea, and the Pacific were used to assess biogenic silica (BSi) content and to study uncertainties in BSi measurements. The contents of BSi in the Bohai and Yellow Sea are all less than 1%. The dissolution of BSi in sediments from the Bohai and Yellow Sea is very important to maintain high levels of silicate in the water column. The non-biogenic silica from clay minerals has an obvious effect on BSi of sediment samples in the Bohai and Yellow Sea with low BSi and high clay minerals. The solid to solution ratio was found to have a great influence on BSi measurement, which can induce uncertainties up to 75%. The effect of loss by sorption and centrifugation is negligible. Interlaboratory comparison of techniques for BSi measurement by the wet alkaline extraction technique of Mortlock and Froelich [Deep-Sea Res. 36 (1989) 1415–1426] with clay correction was suggested to give no significant differences. However, differences in sediment compositions and reagent to sample ratio may limit the application of the wet alkaline extraction method.

Diatom succession, silicification and silicic acid availability in Belgian coastal waters (Southern North Sea)

The significance of silicon in the diatom bloom dynamics of eutrophicated Belgian coastal waters (Southern Bight of the North Sea) was assessed by investigating diatom species succession and silicification level (Si:C) in relation with environmental control during an annual cycle from early February to mid-December 1995. The Si:C of natural diatom communities was determined by 2 methods. The measurement of biogenic silica after alkaline digestion of particulate material and the estimate of diatom carbon biomass derived from microscope observations were performed on weekly samples from Stn 330 in Belgian coastal waters. Alternatively, parallel 24 h kinetics experiments of 32Si uptake and 14C incorporation into proteins, as an index of phytoplankton biomass production, into diatom-dominated natural communities were conducted in simulated natural conditions. The seasonal pattern of diatom growth was characterised by 3 diatom assemblages dominated by respectively small colonial species, Chaetoceros spp.-Schroederella sp. and Rhizosolenia spp. The colonial haptophyte Phaeocystis bloomed for 2 mo, co-occurring with the 2 latter diatom communities. The 3 diatom assemblages were characterised by distinct Si:C with values varying from 0.80 to 0.05 during the growing season. The Si:C of the blooming assemblages are discussed with respect to inter- and intra-specific variations based on changes in ambient controlling factors (nutrient, light and temperature) and compared with information available from the literature for individual diatom species. The results suggest that silicic acid availability controls the Si:C of the main diatom assemblages. This is supported by the positive relationship between the diatom Si:C and silicic acid concentrations. We conclude that silicic acid availability is an important factor in the selection of diatom species, shaping the diatom succession in temperate coastal waters.

Effect of pH on the measurement of biogenic silica

For investigations of the biogenic silica content of plankton, sediment trap, and sediment samples, alkaline extraction is the most common procedure. Different extraction techniques, with respect to the pH of the leaching agent and the method applied to correct for the release of SiO2 by non-biogenic silica phases as clays or feldspars, are applied. By sequential extraction of marine sediment samples and clay standards with solutions of pH 8 to 14, we investigated the sensitivity of these leaching techniques to the pH of the leaching solution. In addition to the dissolved silica concentration, the temporal increase of the dissolved aluminum concentration of the leaching agent was measured. For sediments of low biogenic silica content, the results are considerably affected by the pH of the leaching agent and relative deviations of more than 300% were observed. Correction by the calculated SiO2/Al2O3 ratios of non-biogenic silica phases had no significant effect on the observed pH dependence. Measurements of samples which exceed ∼10% biogenic silica are only negligibly affected by the pH of the leaching solution. Based on the pH dependence and investigations of a pure Miocene diatom sample, sequential alkaline extraction with a pH of 12.5 is recommended for the determination of biogenic silica.

An interlaboratory comparison for the measurement of biogenic silica in sediments

Abstract An interlaboratory comparison was made to assess the variability in the measurement of biogenic silica (BSi) in sediments among aquatic scientists. The purpose of this study was to (1) assess the present state of BSi analysis by aquatic scientists, (2) establish the range of variability in the measurement of BSi across the community of aquatic scientists (paleolimnologists, paleooceanographers, limnologists, estuarine scientists, and oceanographers), and (3) to determine if patterns in the measurement were related to specific methodologies used or to treatments. This study included 30 laboratories primarily using a variety of different wet chemical extraction techniques. The six samples used in the interlaboratory comparison were from modern freshwater and coastal marine depositional environments. The samples were chosen to cover a wide range of BSi concentrations, contain no sponge spicules or radiolarians, and were easy to digest relative to relict samples. Independently measured BSi concentrations of the same sediment ranged widely. BSi concentrations determined by X-ray diffraction were significantly higher than concentrations determined by wet chemical methods. The percent standard deviation of the mean in samples analyzed by wet chemical digestion techniques was highest in the samples with the lowest BSi concentration (Table 3) with 67.5% variation in sample 6 (1.31±0.88 wt.% as SiO2), and lowest at the highest BSi concentration with 21.2% variation in sample 2 (44.3±9.38 wt.% as SiO2). An Analysis of Variance (ANOVA) was used to test for significant differences between treatments for the following variables: (1) pH of digestion solution, (2) digestion solution (Na2CO3 or NaOH), and (3) whether a mineral correction was made. Although significant differences were found, inspection of the raw data suggests that several anomalous points may be responsible for the significant differences obtained. Therefore, the ANOVA was rerun following removal of 7 laboratories from the comparison. These laboratories were removed if they measured BSi concentrations more than ±1 standard deviation from the mean in at least three of the six samples used in the intercomparison. For this analysis no significant differences were found between laboratories. Finally, no significant differences were found between the two most widely used techniques to measure BSi in sediments (DeMaster, D.J., 1979. The marine budgets of silica and 32 Si . PhD Dissertation, Yale University, 308 pp.; Mortlock, R.A. and Froelich, P.N., 1989. A simple method for the rapid determination of biogenic opal in pelagic marine sediments. Deep-Sea Res., 36, 1415–1426.). The importance of optimizing the technique for the type of sample and of making a determination of slope for the mineral correction are explored.

Biogenic-silica accumulation in the Ross Sea and the importance of Antarctic continental-shelf deposits in the marine silica budget

Thirty-five box cores were collected from the continental shelf in the Ross Sea during cruises in January and February, 1983. Pb-210 and Pu-239, 240 geochronologies coupled with biogenic-silica measurements were used to calculate accumulation rates of biogenic silica. Sediment in the southern Ross Sea accumulates at rates ranging from ≤0.6 to 2.7 mm/y, with the highest values occurring in the southwestern Ross Sea. Biogenic-silica content in surface sediments ranges from 2% (by weight) in Sulzberger Bay and the eastern Ross Sea to 41% in the southwestern Ross Sea. Biogenic-silica accumulation in the southwestern Ross Sea averages 2.7 × 10 −2 g/ cm 2/ y and is comparable to accumulation rates in high-productivity, upwelling environments from low-latitude continental margins ( e.g., Gulf of California, coast of Peru). The total rate of biogenic-silica accumulation in the southern Ross Sea is approximately 0.2 × 10 14 g/ y, with most of the accumulation occurring in basins (500–1000 m water depth). If biogenic-silica accumulation in the southern Ross Sea continental shelf is typical of other basins on the Antarctic continental shelf, as much as 1.2 × 10 14 g/ y of silica could be accumulating in these deposits. Biogenic-silica accumulation on the Antarctic continental shelf may account for as much as a fourth of the dissolved silica supplied to the world ocean by rivers and hydrothermal vents.