Biogenic Silicate Research Articles

Fixation of dissolved silicate and sedimentation of biogenic silicate in the lower river Rhine during diatom blooms

Concentrations of dissolved silicate and particulate biogenic silicate were measured in three branches of the lower river Rhine in The Netherlands in order to analyse the role of this element in the eutrophication of the river basin. Particulate silicate followed the seasonal development of the phytoplankton, which was dominated by diatoms. The concentration of dissolved silicate fell during blooms (< 0.1 mg.l−1 ), but the amounts of biogenic silicate measured (∼ 1 mg.l−1) were insufficient to explain the seasonal decrease in the dissolved fraction; this indicates retention of silicate upstream. Some particulate biogenic silicate in river water settled in man-made sedimentation areas in the Rhine delta. The observations suggest that changes in silicate fixation in the Rhine may have contributed to the incidence of non-diatom phytoplankton blooms in receiving waters.

Mixing of Biogenic Siliceous and Terrigenous Clastic Sediments: South Belridge Field and Beta Field, California: ABSTRACT

The intermixing and interbedding of biogenically derived siliceous sediment with terrigenous clastic sediment in reservoirs of upper Miocene age provides both reservoir rock and seal and influences productivity by affecting porosity and permeability. Miocene reservoirs commonly contain either biogenic-dominated cyclic diatomite, porcelanite, or chert (classic Monterey Formation) or clastic-dominated submarine fan sequences with interbedded or intermixed siliceous members of biogenic origin. Biogenic-clastic cycles, 30-180 ft thick, at South Belridge field were formed by episodic influx of clastic sediment from distant submarine fans mixing with slowly accumulating diatomaceous ooze. The cycles consist of basal silt and pelletized massive diatomaceous mudstone, overlain by burrowed, faintly bedded clayey diatomite and topped by laminated diatomite. Cycle tops have higher porosity and permeability, lower grain density, and higher oil saturation than clay and silt-rich portions of the cycles. Submarine fan sediments forming reservoirs at the Beta field are comprised of interbedded sands and silts deposited in a channelized middle fan to outer fan setting. Individual turbidites display fining-upward sequences, with oil-bearing sands capped by wet micaceous silts. Average sands are moderately to poorly sorted, fine- to medium-grained arkosic arenites. Sands contain pore-filling carbonate and porcelaneous cements. Porcelaneous cement consists of a mixture of opal-A, opal-CT,more » and chert with montmorillonite and minor zeolite. This cement is an authigenic material precipitated in intergranular pore space. The origin of the opal is biogenic, with recrystallization of diatom frustules (opal-A) into opal-CT lepispheres and quartz crystals. Porcelaneous cement comprises 4-21% of the bulk volume of the rock. Seventy percent of the bulk volume of the cement is micropore space.« less

Removal of trace metals from seawater during a phytoplankton bloom as studied with sediment traps in Funka Bay, Japan

To study biological effects on the particulate removal of chemical elements from seawater, sediment trap experiments were carried out successively ten times throughout the spring phytoplankton bloom in Funka Bay. Sediment traps were deployed every one to two weeks at 1, 40 and 80 m depths. The settling particles obtained were analyzed for trace metals, phosphate and silicate. The propagation of diatoms in spring results in larger particulate fluxes than that of dinoflagellates. The biogenic silicate concentration is higher in the earlier period, when diatoms are predominant, than in the subsequent period, when dinoflagellates are predominant. The concentrations of aluminum, iron, manganese and cobalt in the settling particles comprising largely biogenic particles are lower during phytoplankton bloom. The concentration of copper is not reduced by the addition of biogenic particles, and its vertical flux is approximately proportional to the total flux, indicating that its concentration in the biogenic particles is nearly equal to that in the non-biogenic particles. The results for nickel and lead show the same tendency as for copper. Cadmium is more concentrated in biogenic particles than in non-biogenic particles, and the concentration of cadmium in the settling particles decreases with depth, similarly to phosphate and organic matter. Thus, metals in seawater are segregated by biological affinities, and the degree of incorporation into biogenic particles is in the order Cd > Pb, Ni, Cu > Co > Mn, Fe, Al. Biogenic particles are the most important agent controlling the vertical distribution of metals in the ocean. They remove the metals from the surface water, transport them through the water column, and regenerate them in the deep.

Regeneration of chemical elements from settling particles collected by sediment trap in Funka Bay, Japan

Sediment trap experiments were carried out 39 times during the years from 1977 to 1981 in Funka Bay, Hokkaido, Japan. The observed total particulate flux varies seasonally, that is, the particulate fluxes in winter and spring are larger than those in summer. The fluxes in all seasons increased with depth. Major components of settling particles are aluminosilicate in winter, biogenic silicate in spring and organic matter and terrestrial material in summer, respectively. The fluxes of each chemical component observed with sediment traps are normalized to that of Al by assuming that the actual flux of Al is equal to the accumulation rate onto the sediment surface. Vertical changes of the normalized flux of each chemical component indicate the following: Fe was not regenerated from the settling particles in the water column. Mn was regenerated from the settling particles in the lower layer exclusively between 80 m depth and the sediment surface. Cd was actively regenerated in the upper layer above 80 m depth. Phosphate was regenerated in the upper layer, while biogenic silicate was in the lower layer. The silicate regeneration, therefore, occurs after phosphate regeneration. The material decomposing in the water column below 40 m has an atomic ratio of P ∶ Si ∶ C = 1 ∶ 52 ∶ 128.

326Ra in Bering Sea sediment and its application as a geochronometer.

232Th, 230Th and 226Ra were determined for three pelagic cores collected in the northern North Pacific and the western and eastern basins of the Bering Sea. The surface sediment collected in the Pacific contains much 232Th and 230Th decreasing in concentration with depth indicating a sedimentation rate of 2.2 × 10-3 cm/yr. The Bering Sea sediments do not show vertical profiles of decreasing concentration with depth. This is due to a fast sedimentation rate in the Bering Sea. The excess radioactivity of 226Ra relative to that of 230Th was observed in the surface sediment collected in the eastern basin of the Bering Sea where the sediment was of siliceous ooze, showing a vertical profile decreasing in concentration with depth. The excess 226Ra is caused by the biogenic silicate material and has been used as a geochronometer. The sedimentation rate in the Bering Sea basin turns out to be 5 × 10-2cm/yr, which shows good agreement with that obtained by the 14C method. Therefore, 226Ra chronology is applicable to pelagic or hemi-pelagic sediment of siliceous ooze.

Cristobalite: Its Relationship to Chert Formation in Selected Samples from the Deep Sea Drilling Project

ABSTRACT Mineralogy and relationships of the silica minerals were studied in 96 pelagic siliceous sediments from Legs I through VI of the Deep Sea Drilling Project. Heating cycles at 650°C and 1010°C were used to convert opaline silica to cristobalite, for purposes of estimating opal content. Most of the free silica in these sediments is biogenic (radiolarians and diatoms). Silica deposition was especially abundant during Eocene time. The experimental evidence confirms earlier hypotheses for the diagenetic sequence: solution of biogenic silica within the sediment, precipitation of poorly-ordered cristobalite, gradual inversion to microcrystalline quartz (chert). Cherts formed in clay, silt, and biogenic siliceous ooze all contain cristobalite, as do siliceous mudstones. Cristobalite is absent from the cherts in nannofossil chalk, and from soft biogenic siliceous oozes. New findings include: 1) silica in a calcite matrix may crystallize directly to quartz, without intermediate cristobalite; 2) certain biogenic amorphous siliceous oozes will yield synthetic cristobalite at the surprisingly low temperature of 650°C; 3) cristobalite formed by diagenetic processes is characterized by mixed-layer tridymitic structure, whereas cristobalite formed by heating truly amorphous silica lacks tridymite layers.

Size Modes in Biogenic Carbonate Sediment, Southeastern Alaska

ABSTRACT Living communities of intertidal encrusting barnacles and submarine infaunal pelecypods supply skeletons to presently accumulating carbonate sediment in Sitka Sound, Alaska. The barnacle community contains an average of 1.95 105 gms/m2 of shell. Waves erode shells from the rocks; and transport through boulder beds produces shell fragmentation. Clam-bombing of rocks by seagulls and boring sponges and algae (?) or fungi (?) contribute to shell disintegration. Chemical interaction between shells and beach porewater is not active. Size-frequency distribution and point count data show that shell fragmentation of these barnacles and mollusks produces a continuous spectrum of particles between 12 and 0.1 mm. Size modes are weak and diffuse in the zone of grinding, but size modes are sharp and well developed in accumulated carbonate sediment of inner bays and beaches. Matching size-frequency distributions for biogenic carbonate and detrital silicate fractions of the same sample suggest size modes are the result of selective sorting during transport and do not reflect the size-frequency distribution of the source. Beach and inner bay carbonate accumulations are sandy gravels to gravelly sands and contain an average of 66 wt. percent CaCO3. Average mineralogy of the carbonate is 15-30 wt. percent aragonite and 85-70 percent calcite with calcites containing less than 1 mol. % Mg. Average constituent gram composition for 24 samples from Pirate Cove is barnacles 74, mollusks 12, echinoids 10, forams 2, and bryozoans 2 percent by volume. Radiocarbon ages for shells (1783 ± 160, 1698 ± 162 years) from trenches dug in beach deposits show these carbonates to he of post-Pleistocene age.