Description: We report results from time-series decay and sequential leaching experiments of laboratory cultured and coastal plankton to elucidate the mechanisms controlling barite formation in seawater. Batch-cultured diatoms (Stephanopyxis palmerina) and coccolithophorids (Emiliania huxleyi) were let to decay in the dark for 8–10 weeks, suspended in aerated seawater. The development of barite crystals was monitored by Scanning Electron Microscopy (SEM). A similar experiment was conducted with plankton collected during the spring-bloom in Vineyard Sound (MA). In addition to SEM, suspended particles were sequentially leached for Ba (distilled water rinse; 10% (v/v) HNO3 rinse at room temperature; 30% (v/v) HCl at 80°C overnight; 50% (v/v) HNO3 at 80°C overnight) immediately after collection, and after 10-week decay in seawater, in seawater poisoned with HgCl2, and in seawater spiked with 135Ba. Both experiments showed an increase in the number of barite crystals during decay. The spring-bloom plankton had initially a large pool of labile Ba, soluble in distilled water and cold dilute HNO3 that was lost from the plankton after 10-week decay in both axenic and nonaxenic conditions. In contrast, Ba in the decayed plankton samples was predominantly in forms extracted by hot HCl and hot HNO3 acids, which were attributed to presence of barite Ba and refractory organic Ba respectively. The increase in barite crystal counts under a Scanning Electron Microscope (SEM), the increase in HCl extractable Ba relative to organic carbon, and the loss of a large fraction of Ba during plankton decay suggest that living plankton consists of a relatively large pool of labile Ba, which is rapidly released during plankton decomposition and acts as the main source of Ba for barite formation in supersaturated microenvironments. Since mass balance indicates that only a small proportion (2 to 4%) of the labile-Ba pool is converted to barite, the availability of microenvironments that could locally concentrate Ba released by plankton decay seems to be the main limiting factor in barite precipitation.

Description: Due to the major role played by diatoms in the biological pump of CO2, and to the presence of silica-rich sediments in areas that play a major role in air-sea CO2 exchange (e.g. the Southern Ocean and the Equatorial Pacific), opal has a strong potential as a proxy for paleoproductivity reconstructions. However, because of spatial variations in the biogenic silica preservation, and in the degree of coupling between the marine Si and C biogeochemical cycles, paleoreconstructions are not straitghtforward. A better calibration of this proxy in the modern ocean is required, which needs a good understanding of the mechanisms that control the Si cycle, in close relation to the carbon cycle. This review of the Si cycle in the modern ocean starts with the mechanisms that control the uptake of silicic acid (Si(OH)4) by diatoms and the subsequent silicification processes, the regulatory mechanisms of which are uncoupled. This has strong implications for the direct measurement in the field of the kinetics of Si(OH)4 uptake and diatom growth. It also strongly influences the Si:C ratio within diatoms, clearly linked to environmental conditions. Diatoms tend to dominate new production at marine ergoclines. At depth, they also succeed to form mats, which sedimentation is at the origin of laminated sediments and marine sapropels. The concentration of Si(OH)4 with respect to other macronutrients exerts a major influence on diatom dominance and on the rain ratio between siliceous and calcareous material, which severely impacts surface waters pCO2. A compilation of biogenic fluxes collected at about 40 sites by means of sediment traps also shows a remarkable pattern of increasing BSi:Corg ratio along the path of the "conveyor belt", accompanying the relative enrichment of waters in Si compared to N and P. This observation suggests an extension of the Si pump model described by Dugdale and Wilkerson (1989, doi:10.1038/34630), giving to Si(OH)4 a major role in the control of the rain ratio, which is of major importance in the global carbon cycle. The fate of the BSi produced in surface waters is then described, in relation to Corg, in terms of both dissolution and preservation mechanisms. Difficulties in quantifying the dissolution of biogenic silica in the water column as well as the sinking rates and forms of BSi to the deep, provide evidence for a major gap in our understanding of the mechanisms controlling the competition between retention in and export from surface waters. The relative influences of environmental conditions, seasonality, food web structure or aggregation are however explored. Quantitatively, assuming steady state, the measurements of the opal rain rate by means of sediment traps matches reasonably well those obtained by adding the recycling and burial fluxes in the underlying abyssal sediments, for most of the sites where such a comparison is possible. The major exception is the Southern Ocean where sediment focusing precludes the closing of mass balances. Focusing in fact is also an important aspect of the downward revision of the importance of Southern Ocean sediments in the global biogenic silica accumulation. Qualitatively, little is known about the duration of the transfer through the deep and the quality of the material that reaches the seabed, which is suggested to represent a major gap in our understanding of the processes governing the early diagenesis of BSi in sediments. The sediment composition (special emphasis on Al availability), the sedimentation rate or bioturbation are shown to exert an important control on the competition between dissolution and preservation of BSi in sediments. It is suggested that a primary control on the kinetic and thermodynamic properties of BSi dissolution, both in coastal and abyssal sediments, is exerted by water column processes, either occuring in surface waters during the formation of the frustules, or linked to the transfer of the particles through the water column, which duration may influence the quality of the biogenic rain. This highlights the importance of studying the factors controlling the degree of coupling between pelagic and benthic processes in various regions of the world ocean, and its consequences, not only in terms of benthic biology but also for the constitution of the sediment archive. The last section, first calls for the end of the "NPZD" models, and for the introduction of processes linked to the Si cycle, into models describing the phytoplankton cycles in surface waters and the early diagenesis of BSi in sediments. It also calls for the creation of an integrated 1-D diagnostic model of the Si:C coupling, for a better understanding of the interactions between surface waters, deep waters and the upper sedimentary column. The importance of Si(OH)4 in the control of the rain ratio and the improved parametrization of the Si cycle in the 1-D diagnostic models should lead to a reasonable incorporation of the Si cycle into 3-D regional circulation models and OGCMs, with important implications for climate change studies and paleoreconstructions at regional and global scale.

Description: An array of five bottom-tethered moorings with 19 PARFLUX time-series sediment trap at three depths (1 and 2 km below the surface, and 0.7 km above the sea-floor) was deployed in the western Pacific sector of the Southern Ocean, along 170°W. The five stations were selected to sample settling particles in the main hydrological zones of the Southern Ocean. The sampling period spanned 425 days (November 28, 1996-January 23, 1998) and was divided into 13 or 21 synchronized time intervals. A total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass (TMF), organic carbon, carbonate, biogenic silica, and lithogenic particles. The fluxes of biogenic material were higher than anticipated, challenging the notion that the Southern Ocean is a low-productivity region. Organic carbon fluxes at 1 km depth within the Polar Frontal Zone and the Antarctic Zone were relatively uniform (1.7-2.3 g/m**2/yr), and about twice the estimated ocean-wide average (ca. 1 g/m**2/yr). Carbonate fluxes were also high and uniform between the Subantarctic Front and ca. 64°S (11-13 g/m**/yr). A large fraction of the carbonate flux in the Antarctic Zone was due to the presence of pteropod shells. Coccoliths were found only to the north of the Polar Front, and calcium carbonate became the dominant phase in the Subantarctic Zone. In contrast, carbonate particles were nearly absent near 64°S. Latitudinal variations in biogenic silica fluxes were substantial. The large opal flux (57 g/m**2/yr) measured in the Antarctic Zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the Polar Front. Unlike biogenic material, fluxes of lithogenic particles were among the lowest measured in the open-ocean (0.12-0.05 g/m**2/yr), reflecting a very low dust input.