Biogenic Silica Accumulation Rates Research Articles

Increase in Paleoproductivity Driven by Strengthening of Indian Summer Monsoon During Past ~14kyrs: Evidenced by Biogenic Silica Accumulation Rates at Southeastern Arabian Sea

We analysed the BSi (Biogenic Silica) contents of the marine core SK-313/1 from the southeastern Arabian Sea which is dated up to ~14.5 kyr BP. Results show that the maximum and minimum BSi contents are 2.5 and 0.5%, respectively. This is because of the low supply of silica skeleton and the high dissolution and dilution of terrigenous materials. The BSi mass accumulation rates indicate a trend of reduced paleoproductivity during the late Glacial period and increase during the Holocene, with global climatic shifts. Insolation changes at low latitudes, which regulate temperatures, may affect nutrient availability and, consequently productivity variations on a glacial–interglacial timescale. Notably, millennial events like the Younger Dryas (YD) periods, exhibited lower paleoproductivity, in contrast to the Bølling/Allerød (B-A) period and the middle Holocene, which displayed elevated levels of paleoproductivity. These millennial-scale fluctuations in siliceous productivity may be due to the changes in nutrient supply caused by the Indian summer monsoon (ISM). The period of increased paleoproductivity could be influenced by intensified chemical weathering and a boosted nutrient supply, which are consequences of the strengthened ISM during periods of relatively higher temperature.

South Georgia marine productivity over the past 15 ka and implications for glacial evolution

Abstract. The subantarctic islands of South Georgia are located in the Southern Ocean, and they may be sensitive to future climate warming. However, due to a lack of well-dated subantarctic palaeoclimate archives, there is still uncertainty about South Georgia's response to past climate change. Here, we reconstruct primary productivity changes and infer Holocene glacial evolution by analysing two marine gravity cores: one near Cumberland Bay on the inner South Georgia shelf (GC673: ca. 9.5 to 0.3 cal. kyr BP) and one offshore of Royal Bay on the mid-shelf (GC666: ca. 15.2 cal. kyr BP to present). We identify three distinct benthic foraminiferal assemblages characterised by the dominance of Miliammina earlandi, Fursenkoina fusiformis, and Cassidulinoides parkerianus that are considered alongside foraminiferal stable isotopes and the organic carbon and biogenic silica accumulation rates of the host sediment. The M. earlandi assemblage is prevalent during intervals of dissolution in GC666 and reduced productivity in GC673. The F. fusiformis assemblage coincides with enhanced productivity in both cores. Our multiproxy analysis provides evidence that the latest Pleistocene to earliest Holocene (ca. 15.2 to 10.5 cal. kyr BP) was a period of high productivity associated with increased glacial meltwater discharge. The mid–late Holocene (ca. 8 to 1 cal. kyr BP), coinciding with a fall in sedimentation rates and lower productivity, was likely a period of reduced glacial extent but with several short-lived episodes of increased productivity from minor glacial readvances. The latest Holocene (from ca. 1 cal. kyr BP) saw an increase in productivity and glacial advance associated with cooling temperatures and increased precipitation which may have been influenced by changes in the southwesterly winds over South Georgia. We interpret the elevated relative abundance of F. fusiformis as a proxy for increased primary productivity which, at proximal site GC673, was forced by terrestrial runoff associated with the spring–summer melting of glaciers in Cumberland Bay. Our study refines the glacial history of South Georgia and provides a more complete record of mid–late Holocene glacial readvances with robust chronology. Our results suggest that South Georgia glaciers were sensitive to modest climate changes within the Holocene.

Landscape Controls on Nutrient Stoichiometry Regulate Lake Primary Production at the Margin of the Greenland Ice Sheet

Global change is reshaping the physical environment and altering nutrient dynamics across the Arctic. These changes can affect the structure and function of biological communities and influence important climate-related feedbacks (for example, carbon (C) sequestration) in biogeochemical processing hot spots such as lakes. To understand how these ecosystems will respond in the future, this study examined recent (< 10 y) and long-term (1000 y) shifts in autotrophic production across paraglacial environmental gradients in SW Greenland. Contemporary lake temperatures and light levels increased with distance from the ice sheet, along with dissolved organic C (DOC) concentrations and total nitrogen:total phosphorus (TN:TP) ratios. Diatom production measured as biogenic silica accumulation rates (BSiARs) and diatom contribution to microbial communities declined across these gradients, while total production estimated using C accumulation rates and δ13C increased, indicating that autochthonous production and C burial are controlled by microbial competition and competitive displacement across physiochemical gradients in the region. Diatom production was generally low across lakes prior to the 1800’s AD but has risen 1.5–3× above background levels starting between 1750 and 1880 AD. These increases predate contemporary regional warming by 115–250 years, and temperature stimulation of primary production was inconsistent with paleorecords for ~ 90% of the last millennium. Instead, primary production appeared to be more strongly related to N and P availability, which differs considerably across the region due to lake landscape position, glacial activity and degree of atmospheric nutrient deposition. These results suggest that biological responses to enhanced nutrient supply could serve as important negative feedbacks to global change.

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literature.

Productivity and sea-surface temperature changes recorded during the late Eocene–early Oligocene at DSDP Site 511 (South Atlantic)

This study investigates paleoenvironmental changes during the Eocene–Oligocene transition (EOT) at Deep Sea Drilling Project (DSDP) Site 511 (South Atlantic), as inferred from lipid biomarker (long-chain diols, alkenones) and calcareous nannofossil accumulation rates, as well as changes in sedimentation regime (i.e. relative contributions of total organic carbon-TOC-, calcium carbonate, and biogenic silica). Sea-surface temperatures (SSTs) reconstructed from the alkenone unsaturation index UK′37 indicate a progressive but significant cooling (~8°C) from 34.5Ma to 33.6Ma, consistent with estimates derived from other temperature proxies (TEX86; δ18O) at the same site and for the same time interval. This cooling is associated with a marked increase in primary productivity, as indicated by high accumulation rates of biogenic silica, TOC, alkenones, long-chain diols, and calcareous nannofossils. Together, these results are consistent with an enhancement of upwelling conditions favorable to the development of siliceous organisms at DSDP Site 511, possibly induced by the Oi-1 glaciation in Antarctica that occurred during this period.

Lake carbonate-δ 18O records from the Yukon Territory, Canada: Little Ice Age moisture variability and patterns

A 1000-yr history of climate change in the central Yukon Territory, Canada, is inferred from sediment composition and isotope geochemistry from small, groundwater fed, Seven Mile Lake. Recent observations of lake-water δ 18O, lake level, river discharge, and climate variations, suggest that changes in regional effective moisture (precipitation minus evaporation) are reflected by the lake’s hydrologic balance. The observations indicate that the lake is currently 18O-enriched by summer evaporation and that during years of increased precipitation, when groundwater inflow rates to the lake increase, lake-water δ 18O values decrease. Past lake-water δ 18O values are inferred from oxygen isotope ratios of fine-grained sedimentary endogenic carbonate. Variations in carbonate δ 18O, supplemented by those in carbonate and organic δ 13C, C/N ratios, and organic carbon, carbonate and biogenic silica accumulation rates, document changes in effective moisture at decadal time scales during the early Little Ice Age period to present. Results indicate that between ∼AD 1000 and 1600, effective moisture was higher than today. A shift to more arid climate conditions occurred after ∼AD 1650. The 19th and 20th centuries have been the driest of the past millennium. Temporal variations correspond with inferred shifts in summer evaporation from Marcella Lake δ 18O, a similarly small, stratified, alkaline lake located ∼250 km to the southwest, suggesting that the combined reconstructions accurately document the regional paleoclimate of the east-central interior. Comparison with regional glacial activity suggests differing regional moisture patterns during early and late Little Ice Age advances.

Climate change and human impact at Sacrower See (NE Germany) during the past 13,000 years: a geochemical record

Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the effects of climate change and human impact on both lake ecosystem and landscape evolution for this region. Sacrower See, a hardwater lake located in Brandenburg, provides a unique sediment record covering the past 13,000 years which was used to reconstruct climatic and anthropogenic forcing on lacustrine sedimentation. Time control is provided by 12 AMS 14C dates of terrestrial plant remains, the Laacher See Tephra, and the onset of varve formation in AD 1870 (80 cal. BP). Geochemical (including XRF logging of major elements, CNS analyses as well as δ13Corg and δ15N measurements) and pollen analyses allowed detecting detailed environmental changes in the sediment record. During the Younger Dryas cold phase increased soil erosion and hypolimnetic oxygen depletion enhanced the nutrient supply to the lake water causing eutrophic conditions. The beginning of the Holocene is characterized by large changes in C/N ratios, total sulphur, δ13C of bulk organic matter as well as in K, Si, and Ti, reflecting the response of the lake’s catchment to climatic warming. Reforestation reduced the influx of detrital particles and terrestrial organic matter. The first, rather weak evidence of human impact is documented only in the pollen record at 5,500 cal. BP. However, until 3,200 cal. BP sedimentological and geochemical parameters indicate relatively stable environmental conditions. During periods of intense human impact at around 3,200, 2,800, and 900 cal. BP peaks in Ti and K represent phases of increased soil erosion due to forest clearing during the Bronze Age, Iron Age, and Medieval Times, respectively. In general, greater variation is observed in most variables during these perturbations, indicating less stable environmental conditions. The steady rise of biogenic silica accumulation rates during the Holocene reflects an increasing productivity of Sacrower See until diatoms were outcompeted by other algae during the last centuries. The applied multi-proxy approach fosters the interpretation of the sediment record to reveal a consistent picture of environmental change including environmental factors controlling lake ontogeny and the effects of human impact.

Late Quaternary vegetation history of southeast Africa: The molecular isotopic record from Lake Malawi

Accurate reconstructions of past hydrological variability are essential for understanding the climate history of the tropics. In tropical Africa, the relative proportion of vegetation utilizing the C 3 vs. C 4 photosynthetic pathway is mainly controlled by precipitation and thus past hydrological changes can be inferred from the vegetation record. In this study, biomarkers of terrestrial plants (lignin phenols and plant leaf wax carbon isotopes) are examined from a well-dated sedimentary record from Lake Malawi to provide a vegetation (aridity) record of the past 23 cal ka from southeast Africa. We suggest that the ratio of cinammyl to vanillyl (C/V) lignin phenols in Lake Malawi sediments mainly reflects inputs of C 3 trees (woody tissue) vs. C 4 grasses (non-woody tissue) and find that changes in the C/V ratio generally support variability noted in the n-alkane carbon isotope record. Together, these records provide evidence for increased C 4 vegetation (grasses) surrounding Lake Malawi during Last Glacial Maximum (LGM), the Younger Dryas, in the early Holocene, and from ~ 2 cal ka to the present, suggesting drier conditions at these times. Elevated inputs of C 3 vegetation are noted in the intervals from ~ 17–13.6 cal ka and ~ 7.7–2 cal ka, indicating wet conditions in southeast Africa. A relationship is noted between the n-alkane average chain length (ACL) and temperature, with longer ACLs associated with higher temperatures. Higher n-alkane carbon preference index (CPI) values correlate with higher mass accumulation rates of biogenic silica and may result from periodic increased northerly winds over Lake Malawi, which enhance upwelling and diatom productivity while simultaneously increasing erosion and transport of plant leaf waxes to the lake. The molecular data produced in this study suggest that the carbon isotopic signature of bulk sediment ( δ 13C TOC) in Lake Malawi is primarily a reflection of terrestrial inputs (C 3 vs. C 4 vegetation) and may not mainly reflect changes in algal productivity, as previously thought.

Eocene biogenic silica accumulation rates at the Pacific equatorial divergence zone

A synthesis of Eocene biogenic silica accumulation rates in the equatorial zone of the Pacific shows several relatively broad maxima spaced a few million years apart and extending from the uppermost Eocene into the lower Eocene. There is a distinct, step‐like decrease in biogenic silica mass accumulation rate at 38.5 Ma (just below the top of C18n.1n),which appears to be related to changes in oceanographic conditions in the eastern equatorial region. Eocene biogenic silica accumulation rates in the equatorial Pacific are distinctly lower than those in the Neogene. A comparison of Eocene accumulation rates and known areas of open ocean biogenic silica deposition with those in the more modern world leads us to conclude that there is no need to call upon an increased supply of dissolved silica to the world ocean to account for the siliceous‐rich deposits of the Eocene.

Modern accumulation rates of biogenic silica in Baikal bottom sediments: significance of diatom species composition

The modern distribution of biogenic silica (SiO 2biog) in Baikal surface (0–0.2 and 0–0.5 cm) sediments, its mass accumulation rates (MAR), and the processes responsible for this distribution were studied. The contents of SiO 2biog were determined in samples from 87 stations evenly distributed throughout the lake water area. These data were used to calculate the MAR of SiO 2biog and compile SiO 2biog and MAR distribution maps. The maps showed that the MAR varies significantly throughout the study area. Its highest values have been established in North Baikal. In Central and South Baikal the MAR values are much lower, and the minimum values are observed in the Selenga shoal. This MAR distribution pattern is consistent with the distribution of frustules of Aulacoseira baicalensis endemic diatoms throughout the surface sediments. Compared with other modern diatom species, this one has the most massive and heaviest frustules, which undergo the least dissolution when settling. This suggests that the above species plays a leading role in the accumulation of biogenic silica in modern Baikal bottom sediments, whereas the role of thin-walled species is minor because of their small mass and partial or complete dissolution in water on settling. Dilution of the sediments with terrigenous material is less significant for the SiO 2biog accumulation.

Diatom Paleolimnological Record of Holocene Climatic and Environmental Change in the Altai Mountains, Siberia

The sedimentary diatom records of three shallow lakes in the Altai Mountains, southern Siberia, were examined to assess the nature and timing of Holocene environmental changes. Few paleoenvironmental records, especially reconstructions not based on pollen, have been reported from this region. The lakes differ in elevation, annual precipitation, and catchment vegetation. Diatom assemblages in all lakes were dominated for the entire period of record by small benthic species of Pseudostaurosira Williams & Round, Staurosira Ehrenberg, and Staurosirella Williams & Round. Planktonic taxa only occur in very low abundances (<5%). The most diverse diatom flora was found in Dzhangyskol, which is situated at the lowest elevation within a forested catchment. A lack of detailed information on the ecological preferences of the dominant taxa and the complexity of environmental drivers make direct interpretation of the diatom record difficult. However, other proxies suggest that dramatic shifts in dominance between Staurosira elliptica and Staurosirella pinnata in Grusha Ozero reflect millennial-scale variability in climate. Together, chironomids and diatoms provide evidence of a cooling possibly correlative to the Younger Dryas Stade and subsequent early-Holocene warming consistent with pollen evidence of afforestation, which also is likely linked to increased humidity. By ~6000 cal year BP, the transition to a cooler, more continental climate had begun. The diatom record of Akkol shows significantly less variation in diatom community composition, but biogenic silica accumulation rates, a proxy for diatom productivity, appear to reflect climatic variability driven by insolation trends over the past 8000 years. Long-term variability in Dzhangyskol is not clearly linked to climate.

Radiolarian faunal turnover across the Oligocene/Miocene boundary in the equatorial Pacific Ocean

The global warming trend of the latest Oligocene was interrupted by several cooling events associated with Antarctic glaciations. These cooling events affected surface water productivity and plankton assemblages. Well-preserved radiolarians were obtained from upper Oligocene to lower Miocene sediments at Ocean Drilling Program (ODP) Leg 199 Sites 1218 and 1219 in the equatorial Pacific, and 110 radiolarian species were identified. Four episodes of significant radiolarian faunal changes were identified: middle late Oligocene (27.5 to 27.3 Ma), latest Oligocene (24.4 Ma), earliest Miocene (23.3 Ma), and middle early Miocene (21.6 Ma). These four episodes approximately coincide with increases and decreases of biogenic silica accumulation rates and increases in δ 18O values coded as “Oi” and “Mi” events. These data indicate that Antarctic glaciations were associated with change of siliceous sedimentation patterns and faunal changes in the equatorial Pacific. Radiolarian fauna was divided into three assemblages based on variations in radiolarian productivity, species richness and the composition of dominant species: a late Oligocene assemblage (27.6 to 24.4 Ma), a transitional assemblage (24.4 to 23.3 Ma) and an early Miocene assemblage (23.3 to 21.2 Ma). The late Oligocene assemblage is characterized by relatively high productivity, low species richness and four dominant species of Tholospyris anthophora, Stichocorys subligata, Lophocyrtis nomas and Lithelius spp. The transitional assemblage represents relatively low values of productivity and species richness, and consists of three dominant species of T. anthophora, S. subligata and L. nomas. The characteristics of the early Miocene assemblage are relatively low productivity, but high species richness. The two dominant species present in this assemblage are T. anthophora and Cyrtocapsella tetrapera. The most significant faunal turnover of radiolarians is marked at the boundary between the transitional/early Miocene assemblages. We also reviewed changes in other microfossil assemblages in the low latitudes during the late Oligocene through early Miocene. The microfossil assemblages of major groups show sequential changes near the Oligocene/Miocene (O/M) boundary (23.8 Ma). Many extinction events and some first occurrences of calcareous nannofossils and many occurrences of radiolarians are found from about 24.8 to 23.3 Ma, and first occurrences of planktic foraminifers and diatoms followed from 23.2 through 22 Ma. Hence, the O/M boundary is identified as a significant level for microfossil evolutions.

Preservation efficiencies and accumulation rates for biogenic silica and organic C, N, and P in high‐latitude sediments: The Ross Sea

Rates of biogenic sediment accumulation (biogenic silica and organic C, N, and P) and pore water flux have been established for a variety of depositional environments in the Ross Sea. On the basis of 14C measurements in kasten cores, sediment accumulation rates ranged from 250 cm kyr−1 in the coastal basin of Granite Harbor to 1–2 cm kyr−1 in the shelf and slope environments of the northern and eastern Ross Sea. Burial of biogenic material was most rapid in the southwestern Ross Sea, where biogenic silica accumulation rates ranged from 2 to 31 g cm−2 kyr−1 and organic carbon accumulation rates ranged from 0.05 to 1.4 g cm−2 kyr−1. In the northern and eastern Ross Sea, biogenic silica and organic carbon accumulation rates typically equaled 0.02–0.03 g SiO2 cm−2 kyr−1 and 0.002–0.004 g C cm−2 kyr−1. Flux core measurements were used to estimate seabed regeneration rates for biogenic silica, organic carbon, and phosphorus. Pore water fluxes, in general, showed much less variability across the Ross Sea than did the biogenic accumulation rates. Pore water silicate fluxes in the study area ranged from 0.6 to 5.3 g SiO2 cm−2 kyr−1, whereas carbon fluxes ranged from 0.1 to 1.2 g C cm−2 kyr−1 and phosphate fluxes varied from −0.006 to 0.012 g P cm−2 kyr−1. Seabed preservation efficiencies were calculated for biogenic silica, organic carbon, and phosphorus by combining the solid‐phase and pore water data. The seabed preservation efficiencies for biogenic silica (1–86%) were greater than for organic carbon (1–71%) at all nine stations examined. The preferential preservation of biogenic silica relative to organic carbon also was apparent in Ross Sea sediments because the biogenic silica/organic carbon ratio in the material buried in the seabed generally was 2 times greater than the ratio in sediment particles arriving at the sediment‐water interface. Sediment accumulation rate correlated strongly with both the biogenic silica and the organic carbon preservation efficiency data. P preservation efficiencies remained relatively high (24–65%) even when the accumulation rates were low (1–2 cm kyr−1) because of the near‐zero phosphate fluxes out of the seabed. Of the total P in the seabed (0.04–0.09 wt. %), approximately 25% existed in the form of organic P. The total amount of biogenic silica accumulating on the Ross Sea shelf is ∼2.3 × 1012 g SiO2 yr−1, which is approximately an order of magnitude less than the Ledford‐Hoffman et al. [1986] estimate that was based on 210Pb chronologies. Biogenic silica accumulation rates in the southern, central, and western Ross Sea increased during the mid and late Holocene, reaching their maximum values during the past 500 to 1000 years.

Fossil indicators of nutrient levels. 1: Eutrophication and climate change

Abstract Both phosphorus and nitrogen are elements essential to life. Availability of these biolimiting nutrients therefore exerts a major control on the carbon cycle through ocean productivity, bringing about changes in ocean-atmosphere chemistry and climate. This review begins with a brief account of the biological importance of phosphorus and nitrate to the cell, and introduces current terminology relating to the interlinked carbon-nutrient cycles in the oceans. Nitrate-limited, eutrophic ecosystems are associated with modern upwelling zones, characterized by high rates of delivery of new phosphorus, high rates of new primary production and export production. Fossil indicators include high accumulation rates of biogenic silica, apatite, baryte and organic matter, plus non-spinose, smaller planktonic foraminifera. Examples from biomineral chemistry include indications of high Ba/Ca, high Cd/Ca and increased differences between δ 13 C in planktonic and benthic foraminiferal test calcite. Such eutrophic indicators are shown to have peaked during glacial phases in the Quaternary, supporting a model in which Milankovitch-related reductions in solar isolation influenced the availability of nutrients, increasing primary productivity and lowering the levels of atmospheric carbon dioxide.

Biogenic silica accumulation and paleoproductivity in the northern basin of Lake Baikal during the Holocene

The effects of the Holocene climatic amelioration, productivity, and preservation on biogenic silica accumulation in the northern basin of Lake Baikal are evaluated by analyzing biogenic silica, organic carbon, nitrogen, and the stable carbon isotopic composition of organic matter in a 4 m piston core and a companion box core. In the early Holocene (11,000-8500 yr B.P.), biogenic silica accumulation was low because of low productivity, strong oligotrophy, and the influx of nutrient-limited alpine glacial meltwater. In the middle Holocene, productivity increased significantly during 8500-7000 yr B.P., but little biogenic silica accumulated in sediments because the dominant primary producer was not diatom flora and because of the relatively higher productivity caused by enhanced terrestrial nutrient influx. The highest accumulation rates of biogenic silica were those between 7000 and 4500 yr B.P., when aquatic productivity increased, limnological stratification occurred, a mature diatom flora developed. In the late Holocene (the past 4500 yr), biogenic silica accumulation decreased because of enhanced recycling and regeneration of silica and a low sedimentation rate.

Biogenic silica accumulation rate during the Holocene in the southeastern Indian Ocean

Three box-core transects were selected in the southeastern Indian Ocean to establish the variability of biogenic silica accumulation rate in the main subsystems of the Southern Ocean. Cycladophora davisiana and δ 18O chronologies, previously calibrated by 14C dates, and biogenic silica contents determined by X-ray diffraction analysis were used to calculate accumulation rates. Concurrently, a transfer function was used to quantify the silica loss during the biogenic particulate accumulation in the deep-sea sediments. The average rate of biogenic silica rain on the sea-floor, calculated from the accumulation rate and the amount of dissolved biogenic silica, ranges from less than 0.1 to 16 g opal cm −2 ka −l. During the Holocene, biogenic silica has accumulated at the highest rates on the Southeast Ridge, south of the Polar Frontal Zone. To the north and south, the accumulation rate drops where summer sea surface temperatures are above 8°C or lower than 2°C. Biogenic silica dissolution is maximum in marginal sea ice zone. Accumulation rates of biogenic silica can be a useful index to estimate changes of palaeoproductivity in the southeastern Indian Ocean, although there is no strict proportionality between accumulation and silica rain rates.

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.

Global Patterns in Opal Deposition from Late Cretaceous Late Miocene

Biogenic silica accumulation rates were calculated at Deep Sea Drilling Project (DSDP) sites from Legs 1-44, and were plotted on global reconstructions at seven 10-m.y. intervals. These data were then contoured. Because of the correlation between opaline sediments and upwelling, these maps may be used to trace paleocirculation patterns through time. They demonstrate a gradual transition from the Cretaceous circum-global equatorial current to the modern circum-Antarctic divergence, as Tethys closed and Australia and South America separated from Antarctica. Modern bottom-water circulation tends to concentrate opaline sediments in the Pacific and carbonate sediments in the Atlantic. Basinal opal fluxes were measured for each interval and, in combination with carbonate compensation depth (CCD) histories, reveal four lagoonal versus estuarine exchange reversals from the Late Cretaceous to the present. These reversals can be directly related to the development of new bottom-water sources with changing climatic and tectonic conditions.

The supply and accumulation of silica in the marine environment

Rivers and submarine hydrothermal emanations supply 6.1 × 10 14 g SiO 2/yr to the marine environment. Approximately two-thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep-sea deposits. Siliceous deep-sea sediments located beneath the Antarctic Polar Front (Convergence) account for over a fourth (1.6 × 10 14 g SiO 2/yr) of the silica supplied to the oceans. Deep-sea sediment accumulation rates beneath the Polar Front are highest in the South Atlantic with values as large as 53cm/kyr during the last 18.000 yr. Siliceous sediments in the Bering Sea, Sea of Okhotsk, and Subarctic North Pacific accumulate 0.6 × 10 14 g SiO 2/yr or 10% of the dissolved silica input to the oceans. The accumulation of biogenic silica in estuarine deposits removes a maximum of 0.8 × 10 14 g SiO 2/yr. Although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters, the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters. Siliceous sediments from continental margin upwelling areas (e.g. Gulf of California, Walvis Bay, or Peru-Chile coast) have the highest biogenic silica accumulation rates in the marine environment (69 g SiO 2 cm 2/kyr). Despite the rapid accumulation of biogenic silica, upwelling areas account for less than 5% of the silica supplied to the marine environment because they are confined laterally to such small areas.