Biogenic Silica Production Research Articles

Paleoproductivity, nutrient burial, climate change and the carbon cycle in the western equatorial Atlantic across the Eocene/Oligocene boundary

Paleoproductivity, nutrient burial, and carbon cycling were investigated across the Eocene/Oligocene (E/O) boundary (begin to end; 36.9–32.7 Ma at ∼40 kyr resolution, timescale of Shackleton et al. [1999]) at Ocean Drilling Program Site 925 on the Ceara Rise in the western equatorial Atlantic (3040 m present water depth; 748.26–850.70 mbsf). Downcore bulk sediment records of biogenic barium, total reactive phosphorus, biogenic silica, and calcium carbonate are interpreted to represent export production, net nutrient burial, biogenic opal production, and inorganic carbon burial, respectively. The global positive excursion in δ13C subsequent to the E/O boundary is recorded at Site 925. Export production appears to have been externally forced by orbital parameters at eccentricity frequencies during the study interval, based on spectral analysis of the biogenic barium and reactive phosphorus records. Biogenic silica production or preservation increased after the Eocene/Oligocene boundary to a higher baseline, although overall productivity and nutrient burial did not increase, based on barium and reactive phosphorus records. Thus, although absolute production did not increase at this site, a shift in relative abundance of siliceous versus carbonate productivity may have resulted in a change in relative organic carbon burial. This may have contributed to the positive excursion in global oceanic δ13C subsequent to the Eocene/Oligocene boundary, although the silica maximum persists after the carbon isotope excursion ends.

Magnetite dissolution in siliceous sediments

Magnetite dissolution, and consequent loss of magnetization, is widely observed in reducing sedimentary environments, where the decrease in Eh‐pH values with depth is driven by bacterially mediated degradation of organic carbon. We have observed low magnetizations in sediments with elevated pore water silica concentrations that arise from diagenesis of biogenic silica and/or silicic volcanic ash. These depletions in magnetization are greater than can be accounted for by dilution with magnetite‐poor sediments and suggest that postdepositional destruction of magnetite has occurred. Biosiliceous sediments usually also contain elevated concentrations of organic carbon, which makes it difficult to separate organic‐carbon‐related magnetite dissolution from other possible mechanisms for magnetite dissolution. However, the extent of magnetite dissolution in the sedimentary sequences that we have studied is not obviously related to the redox‐state of the environment. This suggests that other mechanisms might have given rise to magnetite dissolution in these siliceous sediments. Thermodynamic calculations indicate that magnetite is unstable under conditions of elevated dissolved silica concentrations (and appropriate Eh‐pH conditions) and predict that magnetite will break down to produce iron‐bearing smectite. A survey of magnetic susceptibility and pore water geochemical data from widely distributed Ocean Drilling Program sites supports this observed link between high dissolved silica concentrations and low magnetic susceptibilities. This observed link also holds for environments with low biogenic silica productivity (and low organic carbon content) but with high interstitial silica concentrations due to dissolution of silicic volcanic ashes. Dissolution of magnetite is therefore predicted to be a common feature of siliceous sedimentary environments.

Paleoenvironmental changes across the Cretaceous/Tertiary boundary at Flaxbourne River and Woodside Creek, eastern Marlborough, New Zealand

An integrated study of variation in siliceous microfossils, lithofacies, and other geochemical guides to environmental conditions through the Cretaceous/Tertiary (K/T) boundary transition at Flaxbourne River and Woodside Creek, coastal eastern Marlborough, indicates that the K/T impact disrupted oceanic conditions along the continental margin of eastern New Zealand for c. 1 m.y. Initial effects of the K/T event were a major reduction in carbonate production, associated with calcareous plankton extinctions, and significant increases in terrigenous clay and biogenic silica content. An absence of radiolarian extinctions or significant negative excursions in paleo‐productivity indicators (Ba, d13C) at the boundary, followed by rapid increases in the abundance of diatoms and spumellarian radiolarians, indicate that biogenic silica production partly compensated for the collapse in calcareous plankton. The earliest Paleocene recovery of calcareous plankton was short‐lived, giving way to a progressive increase in siliceous plankton abundance over c. 500 000 yr, which culminated in a c. 400 000 yr episode of peak biogenic silica production. The dominance of siliceous facies, coupled with the abundance of diatoms and spumellarian radiolarians, indicates climatic or oceanic conditions were significantly cooler than in the Late Cretaceous. Stepped increases in biogenic silica production show c. 100 000 yr periodicity, suggesting that early Paleocene lithofacies changes were influenced by climate forcing agents at the eccentricity bandwidth.

Spatial and temporal variability in phytoplankton iron limitation along the California coast and consequences for Si, N, and C biogeochemistry

Iron limitation was investigated along the Northern California coast in the summer of 1999. Small‐volume (1 l) shipboard iron addition bottle experiments were performed at 44 stations to gain the greatest possible temporal and spatial coverage of the area. Parameters measured in these 4‐day incubations included size‐fractionated chlorophyll a and particulate nutrients (C, N, and Si). Degrees of community iron limitation were quantified and compared using various iron limitation indexes, calculated as the ratio of chlorophyll a, particulate organic nitrogen, or particulate organic carbon produced in Fe‐amended bottles to the amounts produced in controls. Iron limitation occurred most frequently and was most severe on stations off the continental shelf, away from sedimentary sources of iron, as well as during relaxation events in aged upwelled water. Size‐fractionated chlorophyll a data did not suggest large Fe‐mediated changes in phytoplankton community composition. Fe limitation reduced phytoplankton production of particulate organic nitrogen and carbon, but had much less effect on biogenic silica production. The result is an increase in particulate Si:N and Si:C ratios of control samples, which were frequently double those of Fe‐amended samples. Particulate C:N ratios also decreased under Fe limitation, indicating that iron availability exerts a strong control on C, N, and Si elemental composition of phytoplankton, and thus on the biogeochemical cycling of these nutrients in the California upwelling region.

Growth and demise of Permian biogenic chert along northwest Pangea: evidence for end-Permian collapse of thermohaline circulation

The Permian Chert Event (PCE) was a 30 Ma long episode of unusual chert accumulation along the northwest margin of Pangea, and possibly worldwide. The onset of the PCE occurred at about the Sakmarian–Artinskian boundary in the Sverdrup Basin, Canadian Arctic, where it coincides with a maximum flooding event, the ending of high-frequency/high-amplitude shelf cyclicity, the onset of massive biogenic chert deposition in deep-water distal areas, and a long-term shift from warm- to cool-water carbonate sedimentation in shallow-water proximal areas. A similar and coeval shift is observed from the Barents Sea to the northwestern USA. A landward and southward expansion of silica factories occurred during the Middle and Late Permian at which time warm-water carbonate producers disappeared completely from the northwest margin of Pangea. Biotically impoverished and increasingly narrow cold-water carbonate factories (characterised by non-cemented bioclasts of sponges, bryozoans, echinoderms and brachiopods) were then progressively replaced by silica factories. By Late Permian time, little carbonate sediments accumulated in the Barents Sea and in the Sverdrup Basin, where the deep- to shallow-water sedimentary spectrum was occupied by siliceous sponge spicules. By that time, biogenic silica sedimentation was common throughout the world. Silica factories collapsed in the Late Permian, abruptly bringing the PCE to an end. In northwest Pangea, the end-Permian collapse of the PCE was associated with a major transgression and with a return to much warmer oceanic and continental climatic conditions. Chert deposition resumed in the distal oceanic areas during the early Middle Triassic (Anisian) after a 8–10 Ma interruption (Early Triassic Chert Gap). The conditions necessary for the onset, expansion and zenith of the PCE were provided by the thermohaline circulation of nutrient-rich cold waters along the northwestern and western margin of Pangea, and possibly throughout the world oceans. These conditions provided an efficient transportation mechanism that constantly replenished the supply of silica in the area, created a nutrient- and oxygen-rich environment favouring siliceous biogenic productivity, established cold sea-floor conditions, hindering silica dissolution, while increasing calcium carbonate solubility, and provided conditions adverse to organic and inorganic carbonate production. The northwest margin of Pangea was, for nearly 30 Ma, bathed by cold waters presumably derived from the seasonal melting of northern sea ice, the assumed engine for thermohaline circulation. This process started near the Sakmarian–Artinskian boundary, intensified throughout Middle and Late Permian time and ceased suddenly in latest Permian time. It led to oceanic conditions much colder than normally expected from the palaeolatitudes, and the influence of cold northerly-derived water was felt as far south southern Nevada. The demise of silica factories was caused by the rapid breakdown of these conditions and the establishment of a much warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean. Complete thawing of northern sea ice would have ended thermohaline circulation and led to warm and sluggish oceanic conditions inimical to the production, accumulation and preservation of biogenic silica.

Dissolution kinetics of biogenic silica from the water column to the sediments

Stirred flow-through experiments were conducted for the first time with planktonic biogenic silica (BSi). We investigated the dissolution kinetics of uncleaned and chemically cleaned BSi collected in ocean surface water, sediment traps, and sediments from the Norwegian Sea, the Southern Ocean, and the Arabian Sea. The solubility at 2°C is rather constant (1000 to 1200 μM). The dissolution rates are, however, highly variable, declining with water depth, and phytoplankton reactivity is two to three orders of magnitude higher than pure siliceous oozes. The reactivity decrease correlates well with an increase in the integrated peak intensity ratios of Si-O-Si/Si-OH measured by Fourier transform infrared (FTIR) spectroscopy. The removal of organic or inorganic coatings enhance the reactivity by at least an order of magnitude. Atomic Al/Si ratios of 0.03 to 0.08 in sedimentary diatom frustules decrease significantly to 0.02 as a result of removal of inorganic coatings and detritals present. Near equilibrium, the dissolution rates exhibit a linear dependence on the degree of undersaturation. At higher degrees of undersaturation—that is, at low concentrations of dissolved silica—the dissolution rates of uncleaned samples define a nonlinear trend.The nonlinear kinetics imply that the dissolution of natural BSi is strongly accelerated in silica-depleted surface waters. The FTIR results suggest that internal condensation reactions reduce the amount of surface reaction sites and are partly responsible for the reactivity decrease with depth. The high content of Al in sedimentary BSi is likely caused by precipitation of dissolved silica with Al dissolved from minerals in sediment. Nonbiogenic silica as coatings or detritals are partly responsible for the solubility and reactivity decrease of BSi in sediments. One order of magnitude different rate constants measured in Norwegian Sea and Southern Ocean sediment trap material support the so-called opal paradox—that is, high BSi accumulation rates in sediments in spite of low BSi production rates in surface waters of the Southern Ocean.

Resource limitation of phytoplankton growth in the Crozet Basin, Subantarctic Southern Ocean

In January–February 1999, we performed shipboard iron- and macronutrient-addition experiments in the Crozet Basin, Indian sector of the Subantarctic Southern Ocean, to evaluate the sufficiency of ambient iron and macronutrient concentrations for algal growth. Experiments were conducted with near-surface seawater collected from three locations in a narrow latitudinal band characterized by relatively low algal biomass (o0.7m gl � 1 chlorophyll a), low dissolved iron concentrations (o0.33 nM), and strongmeridional g radients in temperature, salinity and macronutrient concentrations: (1) the Polar Frontal Zone (PFZ) near 461S, 651 E( B19mM nitrate and 1.2mM silicic acid); (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF) near 44112 0 S, 63123 0 E( B5.4mM nitrate and 0.5mM silicic acid); and (3) the southern Subtropical Zone (STZ) near 43118 0 S, 62131 0 E( o0.1mM nitrate and B1.4mM silicic acid). Our experimental results reveal three distinct regimes of resource limitation of phytoplankton growth. In the PFZ, iron availability exerted the primary limitation on nitrate drawdown and biomass accumulation, thus community growth, with silicic acid availability exerting a secondary limitation on diatom growth and biogenic silica production. Within the SAF/STF, iron deficiency was also the primary limitation on algal community growth; however, here we observed evidence of secondary limitation of nitrate drawdown and biomass accumulation by silicic acid deficiency, via control of algal community structure—such that iron addition preferentially stimulated the growth of non-diatom nanoplankton—suggesting that the algal community was poised close to co-limitation by iron and silicic acid. As expected, our experimental results indicate that macronutrients (nitrate/phosphate) were the primary limitation on community growth in the STZ waters; however, our results also suggest that iron deficiency imposed a significant secondary limitation on community growth, particularly diatom growth, such that the algal community was poised close to co-limitation by macronutrients and iron. We conclude that these same regimes of resource limitation are likely to regulate phytoplankton growth and export production over much of the open-ocean Subantarctic region during the

Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996–1998

Abstract During the 1996–1998 Antarctic Environment and Southern Ocean Process Study (AESOPS), a component of US JGOFS, we obtained seasonal or longer-term data sets on the rates of production, vertical transport, remineralization and burial of particulate organic carbon (POC) and biogenic silica (BSiO2) in the Southern Ocean at 170°W between 55°S and 68°S. The AESOPS data records enable us to construct vertical C and Si budgets for the water column and upper sediments, with all estimates derived from direct measurement of the relevant fluxes. We constructed annual C and Si budgets for each of four ecologically distinct zonal bands within the system. For both POC and BSiO2 the greatest annual delivery to the sea floor (∼200 and 1400 mmol m−2 yr−1, respectively) and burial (∼6 and 160 mmol m−2 yr−1, respectively) were observed in the southern Antarctic Circumpolar Current (ACC) between 61.5°S and 65.5°S. That pattern is consistent with our observation that a diatom bloom propagated southward through the southern ACC during the spring and summer of 1997–1998, following the receding ice edge, and that this bloom was the main source of both POC and BSiO2 in the system on an annual basis. In the other zones the annual fluxes of POC and BSiO2 to the sea floor ranged from 19% to 67% of those in the zone traversed by the summer diatom bloom. The higher benthic fluxes of both POC and BSiO2 in the southern ACC imply that blooms similar to the one we observed in 1997–1998 occur commonly in the southern ACC, and that their high-productivity signature is transmitted to the sea floor. The data show preferential preservation of BSiO2 over POC throughout the water column and upper seabed. In the four zonal bands we consider, BSiO2 and POC are produced in mole ratios of 0.1–0.4, exported from the upper 100 m in ratios of 0.2–0.6, arrive at 1000 m in ratios of 1.5–4.5, reach the sea floor in ratios of 2.2–7.6, and are buried in ratios of 11.6–28. Despite the preferential preservation of BSiO2, accumulation of opal-rich sediments beneath the ACC does not result from unusually efficient preservation of siliceous material. The estimated BSiO2 preservation efficiency (burial-production) ranges from 1.2% to 5.5%, indistinguishable from the global average of 3%. Instead, opal-rich sediment accumulation in this region reflects very high annual rates of BSiO2 production in surface waters, along with very low accumulation rates of other sedimentary components (e.g., CaCO3 and detrital material). The observed high ratios of BSiO2 production to POC production in surface waters are consistent with the known tendency for the Si/C ratio of diatoms to increase when [Fe] is low. If greater Fe availability during the last glacial maximum permitted diatoms in the Southern Ocean to grow with lower, more normal Si/C ratios, export of diatom-produced POC could have occurred at 2–3 times its present rate. A corresponding increase in opal export or opal sediment accumulation is unlikely because silicic acid is almost totally depleted north of 65°S under present conditions. Thus, even large increases in POC production and export during glacial periods would not be reflected in the opal accumulation record.

Biogenic silica production rates and particulate organic matter distribution in the Atlantic sector of the Southern Ocean during austral spring 1992

Several of the components of the silicon cycleForthosilicic acid (Si(OH)4), biogenic silica (BSi), and biogenic silica production rates (rSi)Fhave been investigated, together with the distribution of particulate organic carbon (POC), particulate organic nitrogen (PON) and carbon primary production (rC), on a series of transects across three subsystems in the Atlantic sector of the Southern Ocean (61W): the seasonal ice zone (SIZ), the permanently open ocean zone (POOZ), and the southern boundary of the polar frontal zone (PFZ). The study was conducted in Spring 1992 as part of the European SO-JGOFS cruise aboard the R.V. Polarstern. High BSi concentrations (maximum: 11.7mmol Si l � 1 ) were recorded in late November at the southern border of the PFZ. In contrast, no large BSi biomass was found in the other subsystems studied. In the SIZ, no diatom bloom was observed, despite a sea-ice retreat of 200 km during the study period, and BSi biomass never exceeded 0.6mmol Si l � 1 . The POOZ also showed very low BSi biomass (o0.5mmol Si l � 1 ), and low BSi/POC molar ratios from the surface to 200 m (0.04–0.06 at 531S) suggest that this was an area where phytoplankton were not dominated by siliceous organisms. At the southern border of the PFZ, BSi/POC molar ratios were among the highest ever recorded in the surface waters of the Southern Ocean (maximum: 1.33). This could be a result of the presence of heavily silicified diatoms or also could reflect a more rapid recycling of POC as compared to BSi. High concentrations of BSi (>1.5mmol Si l � 1 ) extended well below the euphotic zone to 200 m depth between 491S and 511S, suggesting significant sedimentation of siliceous particles in that area. High values of rSi also were observed in the PFZ (29.6–60.7 mmol Si m � 2 d � 1 , during the production maximum) indicating that this subsystem is important in the biogeochemical budget of the Southern Ocean. High depth-integrated rSi/rC (0.25–0.46) and BSi/ POC (0.53–0.85) in the PFZ imply the production of diatoms rich in silica compared to organic matter. The high rates of silica production observed in the PFZ support the recent hypothesis that the formation of the abyssal siliceous oozes that encircle much of Antartica form primarily as the result of high levels of silica production in surface waters rather than as a result of high rates of opal preservation as has been suggested in the past. r 2002 Elsevier Science Ltd. All rights reserved. "

Shallow remineralization in the Weddell Gyre

The region influenced by the Polar Front in the Southern Ocean is characterized by relatively high productivity, which is mirrored in strong depletions of 234Th in the surface water, a good tracer of export production, and by high accumulation rates on the underlying seabed. Farther south, the Weddell Sea is generally considered a low productivity region with very low export fluxes. This finding is based on satellite observations, sediment accumulation rates, trap deployments, and phytoplankton distribution. If this would be true, 234Th should be close to equilibrium with its parent. However, in a series of high‐resolution transects of 234Th/238U across the Antarctic Circumpolar Current (ACC), 234Th was found to be depleted by 10–15% throughout the clear Weddell Gyre, only to reach equilibrium in sea‐ice covered regions of the coastal zone. Vertical profiles showed that the depletion was limited to the upper mixed layer and was balanced by an enrichment of similar magnitude at 100–250m depth. This implies that the export of particles below 250 m is negligible. Such shallow remineralization is in line with the discrepancies between biogenic silica production rates and sediment trap data observed in the Weddell and Ross Seas. These observations in the Weddell Sea are fully consistent with our inverse modeling results for both organic carbon and opal, and they are not inconsistent with TCO2 and oxygen sections that show a TCO2 enriched, oxygen reduced shallow subsurface layer. This blue ocean, characterized by upwelling of CO2 ‐enriched deep waters, supports sufficient productivity to be a net sink for CO2 to abyssal depths [Hoppema et al., 1999]. No record of this productivity and export is stored in the underlying sediment, which has important palaeoceanographic consequences.

Biogenic silica production in the Australian sector of the Subantarctic Zone of the Southern Ocean in late summer 1998

The distribution of orthosilicic acid (Si(OH)4), lithogenic silica (LSi), biogenic silica (BSi), and biogenic silica production rates (ρSi) have been investigated on a transect encompassing the Subantarctic Zone (SAZ) from the Subtropical Zone (STZ) to the northern Polar Frontal Zone (PFZ) in the Australian sector of the Southern Ocean at the end of summer 1998. Si(OH)4 distribution was characterized by a north‐south gradient (surface values: <0.1 μM in the STZ to >2 μM in the northern PFZ). The SAZ and STZ exhibited very low BSi values, while a deep (75–150 m) BSi maximum (up to 2.34 μmol Si L−1) characterized the PFZ. BSi growth rates were higher in the SAZ and STZ (up to 0.46 doubling d−1) as compared to the PFZ (0.01–0.10 doubling d−1) where the deep maximum probably resulted from phytodetritus accumulation at the end of the productive season. A very high KS value (30.7 μM Si(OH)4) was estimated in the PFZ surface waters, and it is concluded that multiple limitations including light, iron, and silicic acid availability act on the different subsystems. The study period is characteristic of the last stage of the seasonal cycle especially in the PFZ where ρSi values were very low (mean value: 2.58 mmol Si m−2 d−1) as compared to previous studies. In the SAZ and the STZ respective ρSi mean values were 1.07 and 0.60 mmol Si m−2 d−1, and the siliceous component does not appear to dominate the phytoplankton community.

A time series study of silica production and flux in an eastern boundary region: Santa Barbara Basin, California

A high‐resolution data set describing the annual silica cycle between January 1994 and August 1999 in the Santa Barbara Basin (SBB) captures processes and events occurring in both surface and deep basin waters. Increases in biogenic silica (bSi) concentrations in the surface waters indicated strong bloom events in the early spring of each year and sporadic blooms during the summer and fall. Lithogenic silica (lSi) was present at elevated concentrations in surface waters in the winter and spring. At times, the siliceous biomass in the SBB is comparable to that in major global upwelling regions. However, anomalously low annual depth‐integrated bSi concentrations were observed during the E1 Niño conditions of 1997–1998. A 1.5‐year time series of biogenic silica production rate measurements was used to calculate the highest resolution estimate of annual silica production to date, an estimated 5.5 mol Si m −2 yr−1. This silica production rate approaches that estimated for the major global upwelling zones (8.3 mol Si m−2 yr−1). Eastem boundary regions outside of the major coastal upwelling zones would account for 30–40% of global silica production if the Santa Barbara Basin is representative of these coastal regions. Since the sediments of the SBB contain a high‐resolution climate record, an understanding of the coupling between surface processes and deep basin sedimentation events is of value. Fluxes of bSi and lSi to 470 m were not strongly coupled to the seasonal cycle of particulate concentrations or production rates in surface waters. Continuous sediment trap collections of sedimenting particles indicate that both lSi and bSi tend to be exported at the highest rates during the spring and summer. As particles sink, lSi becomes enriched relative to bSi and consistently dominates the mass flux of silica at 470 m. Brief increases in the percent contribution of bSi to Si flux may create microlaminae within larger‐scale laminations which are thought to be annual varves. A comparison of annual bSi sedimentation rates with annual silica production estimates reveal that ∼30% of the annual silica production is exported to a depth of 470 m. This export rate is the highest observed to date and suggests the potential for large quantities of silica burial in coastal sediments.

Biogenic Silica Production in Selected Alpine Plant Species and Plant Communities

The biogenic silica extracted from samples of 28 alpine plant species belonging to 23 genera and nine families collected in the Swiss Alps (Valais) accounted for between 0.01 and 5.9% of the dry biomass of leaves and wood. Silica content, and plant contribution to the soil biogenic silica pool, varied widely among taxa. Plant net productivity and biogenic silica production from this study and from the literature have been used to predict the input made by different subalpine and alpine plant communities to soil-borne phytolith assemblages, and their contribution to the silicon biocycle.

Biogenic Silica Record of the Lake Baikal Response to Climatic Forcing during the Brunhes

This work presents a detailed, orbitally tuned biogenic silica record of continental paleoclimate change during the Brunhes chron. The Brunhes/Matuyama boundary lies within the warm isotopic stage 19 in Baikal, and the boundaries between eight lithological cycles correspond to terminations in the marine oxygen isotope record. The high amplitude and resolution of climatically driven changes in BioSi content in Lake Baikal sediments permits tuning of almost every precessional cycle during the Brunhes and reveals the structure of interglacial stages. For example, the last three interglacial stages (MIS 5, 7, and 9) clearly consist of five substages (a, b, c, d, e) corresponding to precessional insolation peaks. Abrupt and intense regional glaciations in Siberia during substages 5d and 7d were driven by extreme insolation minima. During substage 9d cooling was more gradual in response to more moderate forcing. The impact of strong glaciation is also observed in the middle of stage 15, where full glacial conditions appear to have lasted for over 30,000 yr during substages 15d, 15c, and 15b. Marine oxygen isotopic stage 11 appears to be the warmest period during the Brunhes in the Lake Baikal record, with at least three substages.A new hypothesis is presented regarding the response of the Lake Baikal BioSi record to insolation forcing. Based on the mechanism controlling modern diatom blooms, biogenic silica production is hypothesized to be dependent on changes in the heat balance of the lake and consequently on changes in the thermal structure of the water column. This mechanism is also sensitive to short-term sub-Milankovich cooling events, such as the mid-Eemian cooling, the Montaigu event during substage 5c, and a cooling which appears to be analogous to the Montaigu event during substage 9c. The continuity of the Lake Baikal paleoclimate record, its sensitivity to orbital forcing, and its high resolution make it an excellent candidate for a new “paleoclimatic stratotype” section for continental Asia.

Iron cycling and nutrient-limitation patterns in surface waters of the World Ocean

Abstract A global marine ecosystem mixed-layer model is used to study iron cycling and nutrient-limitation patterns in surface waters of the world ocean. The ecosystem model has a small phytoplankton size class whose growth can be limited by N, P, Fe, and/or light, a diatom class which can also be Si-limited, and a diazotroph phytoplankton class whose growth rates can be limited by P, Fe, and/or light levels. The model also includes a parameterization of calcification by phytoplankton and is described in detail by Moore et al. (Deep-Sea Res. II, 2002). The model reproduces the observed high nitrate, low chlorophyll (HNLC) conditions in the Southern Ocean, subarctic Northeast Pacific, and equatorial Pacific, and realistic global patterns of primary production, biogenic silica production, nitrogen fixation, particulate organic carbon export, calcium carbonate export, and surface chlorophyll concentrations. Phytoplankton cellular Fe/C ratios and surface layer dissolved iron concentrations are also in general agreement with the limited field data. Primary production, community structure, and the sinking carbon flux are quite sensitive to large variations in the atmospheric iron source, particularly in the HNLC regions, supporting the Iron Hypothesis of Martin (Paleoceanography 5 (1990) 1–13). Nitrogen fixation is also strongly influenced by atmospheric iron deposition. Nitrogen limits phytoplankton growth rates over less than half of the world ocean during summer months. Export of biogenic carbon is dominated by the sinking particulate flux, but detrainment and turbulent mixing account for 30% of global carbon export. Our results, in conjunction with other recent studies, suggest the familiar paradigm that nitrate inputs to the surface layer can be equated with particulate carbon export needs to be expanded to include multiple limiting nutrients and modes of export.

Phytoplankton processes. Part 2: Rates of primary production and factors controlling algal growth during the Southern Ocean Iron RElease Experiment (SOIREE)

Abstract The Southern Ocean Iron RElease Experiment (SOIREE) fertilised 200 km2 of polar waters (mixed-layer depth, 65 m) south of Australia in February 1999. During the 13-d SOIREE, iron enrichment stimulated primary production, algal growth and biogenic silica production rates. A floristic shift from small to large cells resulted in a greater contribution to community production by diatoms, which contributed >60% to community production towards the end of SOIREE. Column-integrated rates of primary production increased by more than 10-fold, and community production rates (normalised to chlorophyll) had doubled by day 4, but then declined after day 6 to initial rates. Iron enrichment increased the growth rates of the algal community from 0.02 to 0.15 d−1 (based on changes in in-situ chlorophyll concentrations), from 0.02 to 0.2 d−1 (based on algal carbon), and more than doubled initial rates (based on 14C) to 0.2 d−1 during SOIREE. However, these iron-elevated rates were less than the maximum rates (0.69 d−1) predicted for waters at 2.5°C. Community biogenic silica production rates increased by 6-fold, i.e. to a lesser extent than primary production rates, resulting in a 2-fold reduction in the silicic-acid : carbon uptake ratio. Shipboard perturbation experiments indicated that iron, rather than both iron and light, limited algal growth in these waters. However, a significant increase in light attenuation in situ due to algal self-shading, suggested that light limitation of algal growth might become significant after day 13 at this locale. Limitation of algal growth by both iron (and silicic acid and/or zinc) within iron-fertilised waters after day 8 of SOIREE was not evident from the results of a shipboard experiment. A simple mathematical model of particulate organic carbon (POC) potential export during SOIREE predicted large increases in export, due to both elevated production rates and a floristic shift to large cells. However, no significant increase in POC export was observed during SOIREE.

Silicon limitation of biogenic silica production in the Equatorial Pacific

During the EBENE cruise (November 1996), distributions of biogenic silica concentration and production rates were investigated in the surface waters of the equatorial Pacific (180°W, from 8°S to 8°N), with particular emphasis on the limitation of the biogenic silica production by ambient silicic acid concentrations. Integrated over the depth of the euphotic layer, concentrations of biogenic silica and production rates were maximum at the Equator (8.0 and 2.6 mmol m −2 d −1) and decreased more or less symmetrically polewards. Contribution of diatoms to the new production was estimated indirectly, comparing biogenic silica production rates and available data of new and export production in the same area. This comparison shows that new production in the equatorial area could mostly be sustained by diatoms, accounting for the major part of the exported flux of organic carbon. Kinetics experiments of silicic acid enrichment were performed. Half saturation constants were 1.57 μM at 3°S and 2.42 μm at the Equator close to the ambient concentrations. The corresponding V max values for Si uptake were 0.028 h −1 at 3°S and 0.052 h −1 at the equator. Experiments also show that in situ rates were restricted to 13–78% of V max, depending on ambient silicic acid concentrations. This work provides the first direct evidence that the rate of Si uptake by diatom populations of the equatorial Pacific is limited by the ambient concentration of silicic acid. However, such Si limitation might not be sufficient in itself to explain the low diatom growth rates observed, and additional limitation is suggested. One hypothesis that is consistent with the results of Fe limitation studies is that Fe and Si limitations may interact, rather than just being a mutually exclusive explanation for the HNLC character of the system.

Glacial to interglacial changes in non-carbonate and carbonate accumulation in the SW Pacific Ocean, New Zealand

Fluxes of carbonate and non-carbonate (terrigenous+biogenic siliceous) sediment, back to a maximum of marine isotope stage (MIS) 6, were determined for 54 cores from off eastern New Zealand. Mass accumulation rates (MARs) were derived using dry bulk densities measured directly from fresh cores or, in the case of dry archived samples, on estimates from a density/carbonate relationship.MARs exhibit wide spatial and temporal variability in response to (i) the location of cores relative to the terrigenous source of New Zealand, current pathways and local oceanographic features, and (ii) palaeoclimatic change. For MIS 1–2, non-carbonate accumulated at a high rate between 27 and 18 ka (calendar years), due to an influx of waterborne and aeolian terrigenous detritus together with localised production of biogenic silica under a northward intrusion of subantarctic waters, and an increase in wind-induced upwelling. By 18–12 ka, non-carbonate MARs reduced as winds ameliorated and sea level rose to confine some of the terrigenous load to the continental shelf. The Holocene transgression was accompanied by warmer sea surface temperatures, the formation of coastal and lacustrine sediment traps, and more widespread terrestrial vegetation. These conditions further reduced the deep oceanic non-carbonate flux to levels about half those of glacial periods.In contrast, carbonate MARs were lower in cold periods, reflecting lower productivity, increased dissolution and, in the case of the generally productive waters of Chatham Rise, to redistribution by bottom currents. However, off central New Zealand, carbonate production was higher on the continental margin, possibly because of enhanced upwelling under the strong wind regime of MIS 2. Such localised effects produced glacial carbonate MARs higher than interglacial MIS 1 values. For the rest of the region, carbonate fluxes increased to an early to middle MIS 1 maximum before generally declining in recent times.

Resolving the 'opal paradox' in the Southern Ocean

In the Southern Ocean, high accumulation rates of opal--which forms by precipitation from silica-bearing solutions--have been found in the sediment in spite of low production rates of biogenic silica and carbon in the overlying surface waters. This so-called 'opal paradox' is generally attributed to a higher efficiency of opal preservation in the Southern Ocean than elsewhere. Here we report biogenic silica production rates, opal rain rates in the water column and opal sediment burial rates for the Indian Ocean sector of the Southern Ocean, which show that the assumed opal paradox is a result of underestimated opal production rates and overestimated opal accumulation rates. Our data thus demonstrate that the overall preservation efficiency of biogenic opal in this region is substantially lower than previously thought, and that it lies within a factor of two of the global mean. The comparison of our revised opal preservation efficiencies for the Southern Ocean with existing values from the equatorial Pacific Ocean and the North Atlantic Ocean shows that spatial differences in preservation efficiencies are not the primary reason for the differences in sedimentary opal accumulation. The reconciliation of surface production rates and sedimentary accumulation rates may enable the use of biogenic opal in the reconstruction of palaeo-productivity when the factors that affect the Si/C ratio are better understood.

Production of biogenic silica by spring diatoms

The radioactive isotope 32Si was used to measure Si uptake during a spring bloom dominated by the diatom Thalassiosira nordenskioeldii. The diatoms converted virtually all dissolved silicic acid in the surface layer into biogenic silica. The rate of Si uptake was unsaturated in all the experiments conducted during the bloom. The half‐saturation constant for Si uptake declined from 6–12 mmol m‐3 early in the bloom to 2 mmol m‐3 when dissolved silicic acid concentration was low. The highest specific Si‐uptake rates measured (ca. 0.3 d‐1) were comparable to average net growth rates calculated from cell abundance measurements.