A review of the Si cycle in the modem ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy

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 reconstructio ns. However, because of spatial variations in the biogenic silica preservat ion, and in the degree of coupling between the marine Si and C biogeochemic al cycles, paleoreconstructions are not straitghtforward. A better calibrat ion of this proxy in the modem ocean is required, which needs a good unders tanding of the mechanisms that control the Si cycle, in close relation to t he 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 subs equent silicification processes, the regulatory mechanisms of which are unc oupled. This has strong implications for the direct measurement in the fiel d of the kinetics of Si(OH)(4) uptake and diatom growth. It also strongly i nfluences the Si:C ratio within diatoms, clearly linked to environmental co nditions. 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 d ominance and on the rain ratio between siliceous and calcareous material, w hich severely impacts surface waters pCO(2). A compilation of biogenic flux es collected at about 40 sites by means of sediment traps also shows a rema rkable pattern of increasing BSi:C-org ratio along the path of the "conveyo r belt", accompanying the relative enrichment of waters in Si compared to N and P. This observation suggests an extension of the Si pump model describ ed by Dugdale and Wilkerson (Dugdale, R.C., Wilkerson, F.P., 1998. Understa nding the eastern equatorial Pacific as a continuous new production system regulating on silicate. Nature 391, 270-273.), giving to Si(OH)(4) a major rule 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 relati on to C-org in terms of both dissolution and preservation mechanisms. Diffi culties in quantifying the dissolution of biogenic silica in the water colu mn as well as the sinking rates and forms of BSi to the deep, provide evide nce for a major gap in our understanding of the mechanisms controlling the competition between retention in and export from surface waters. The relati ve 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 compa rison 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 rep resent a major gap in our understanding of the processes governing the earl y diagenesis of BSI in sediments. The sediment composition (special emphasi s on Al availability), the sedimentation rate or bioturbation are shown to exert an important control on the competition between dissolution and prese rvation of BSI in sediments. It is suggested that a primary control on the kinetic and thermodynamic properties of BSi dissolution, both in coastal an d abyssal sediments, is exerted by water column processes, either occuring in surface waters during the formation of the frustules, or linked to the t ransfer of the particles through the water column, which duration may influ ence the quality of the biogenic rain. This highlights the importance of st udying the factors controlling the degree of coupling between pelagic and b enthic processes in various regions of the world ocean, and its consequence s, not only in terms of benthic biology but also for the constitution of th e sediment archive. The last section, first calls for the end of the ''NPZD" models, and for th e 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 diagnosti c model of the Si:C coupling, for a better understanding of the interaction s 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 m odels and OGCMs, with important implications for climate change studies and paleoreconstructions at regional and global scale. (C) 2000 Elsevier Scien ce B.V. All rights reserved.