biogenic silica accumulation in the ross sea and the importance of antarctic continental shelf deposits in the marine silica budget abstract 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 cm2 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 1014 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 1014 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

biogenic silica dissolution in sediments of the southern ocean i solubility abstract a stirred flow through reactor technique was used to determine silica solubilities in sediments collected with a multicorer in the indian sector of the southern ocean antares i cruise the results show that the apparent silica solubility in the cores may decrease increase or remain constant with depth the silica solubility profiles are best explained by the early diagenetic interactions between biogenic silica and soluble aluminum derived from detrital material by combining the solubility data with measured dissolved silica profiles it is shown that the variable asymptotic pore water silica levels in the cores cannot be explained solely by differences in silica solubility in sediments that experience a significant detrital input the simultaneous reprecipitation of dissolved aluminum and dissolved silica prevents water silicic acid from reaching saturation with the dissolving biogenic silica the principal oceanographic control on pore water silica build up in the cores studied is the ratio of the deposition fluxes of biogenic silica and detrital material solubility differences inherited from the biomineralization process in the surface waters do not appear to have a significant effect on the observed pore water silica levels

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

biological uptake and accumulation of silica on the amazon continental shelf abstract water column and seabed samples were obtained from 92 stations on the amazon continental shelf during october of 1979 uptake of silica near and southeast of the river mouth began at a salinity of 8 and accounted for 17 of the riverine silica flux to this region uptake northwest of the river mouth began at a salinity of 20 and resulted in 33 removal of the riverine silica flux examination of filtered suspended solids revealed abundant diatoms in the surface waters including coscinodiscus skeletonema synedra and thalassiosira the biological uptake of silica appears to be dependent on three factors turbidity turbulence and nutrient availability there was no evidence of abiological removal of silica in the amazon estuary 75 to 88 of the silica removed from surface waters by diatoms dissolves prior to accumulation in the seabed based on the mean biogenic silica content of shelf sediment 0 25 and estimates of rates of sediment accumulation the biogenic silica accumulation rate on the shelf is 2 1012 g yr which represents only 4 of the dissolved silica supplied by the amazon river biological uptake of silica in estuarine surface waters may not accurately reflect permanent removal of biogenic silica to the seabed because of dissolution which occurs in bottom waters and near the sediment water interface

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 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 1014g sio2 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 1014g sio2 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 1014g sio2 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 sio2 cm2 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

the geochemistry of inorganic germanium in natural waters we present the initial results from a survey of the distribution of dissolved inorganic germanium in rivers in the oceans and in hydrothermal vents and plumes in the ocean germanium cycling mimics that of silica since siliceous organisms incorporate ge as a trace constituent in biogenic opal the vertical and horizontal distributions of ge concentrations in seawater are thus identical to those of silica ge si 0 7 10 6 and reflect uptake into and dissolution from the frustules of siliceous organisms germanium enters the ocean via dissolution of continental and seafloor silicates the naturally weathered flux from continents to oceans carries a dissolved ge si atom ratio in rivers of about 0 7 10 6 which reflects the ratio in average continental granites rivers draining industrial regions with significant coal combustion are contaminated with ge leached from fly ash particles resulting in ge si ratios up to tenfold above the naturally weathered background the flux to the oceans via high temperature alteration of seafloor basalts hot springs produces a ge anomaly in hydrothermal plumes over the east pacific rise crest 20 s combining the observed ge and 3he anomalies in plumes with 3he si observations in hot springs suggests that the ge si hydrothermal flux to the oceans is about 4 10 6 near the ratio for average oceanic basalts 2 6 10 6 vent effluents from the galapagos hot springs measured directly display ge si ratios of 16 10 6 suggesting that the ratio is variable between vent fields metalliferous sediments accumulating on the epr crest 17 s are enriched in germanium suggesting that a fraction of the hydrothermal germanium source on the rise crest is scavenged into these rapidly accumulating iron and manganese hydroxyoxide phases

seismic moment ratio of aftershocks with respect to main shocks the empirical bath s law indicates that the earthquake process is self similar and provides an opportunity to estimate the magnitude of the largest aftershock subsequent to a main shock however the analysis of this relation is limited to a small magnitude range and also depends on the aftershock selection rules as an alternative we analyze in this paper the cumulative seismic moment of aftershocks relative to the main shock moment because i it is a physical quantity that does not only take the largest aftershock into account ii background activity can be considered and as a result estimations are less affected by selection rules and iii the effects of the catalog cut off magnitude can be corrected what leads to larger magnitude range for the analysis we analyze the global preliminary determination of epicenters u s geological society catalog combined with centroid moment tensor focal mechanisms and find that the seismic moment release of aftershocks is on average approximately 5 of the main shock seismic moment we show that the results can be well fitted by simulations of the epidemic type aftershock sequence model in particular we test whether simulations constrained by predictions of the static stress triggering model proposing a break of self similarity due to the finite seismogenic width are in agreement with observations our analysis shows that the observed dependency on the main shock magnitude as well as systematic variations with the main shock fault plane solution can be both explained by the constraints based on the static stress triggering

sediment accumulation rates and carbon fluxes to bottom sediments at the western bransfield strait antarctica abstract mean sediment and carbon accumulation rates in the western bransfield strait during the last ca 100 years were determined at three sites by using 210 pb as a radiotracer of sedimentation processes rates 210 pb profiles showed moderate mixing in the upper 8 5 12 cm which was attributed to bioturbation sediment accumulation rates calculated assuming no mixing below the surface mixed layer sml were found to be relatively high between 0 03 and 0 09 g cm 2 yr 1 in good agreement with previously reported data based on both 210 pb and 14 c dating in surrounding areas these results together with the calculated excess 210 pb fluxes to the bottom sediments and the 210 pb and 210 po distributions in the water column indicated the presence of a large net advective flux of material in the area highlighting the importance of glacial related sedimentation processes in this semi enclosed sea carbon in bottom sediments was mainly organic and its content was moderately low 0 65 1 25 being slightly higher within the sml reflecting the presence of inhabiting organisms however organic carbon oc accumulation fluxes 3 1 6 7 g c m 2 yr 1 were considerable due to the high sediment accumulation rates 210 pb dated profiles allowed us to estimate the amount of carbon exported from the water column and buried in the bottom sediment which represents about 3 7 of the mean annual primary production in the euphotic layer of the bransfield strait the burial efficiency of oc in the sediment was estimated to be approximately 60 80

modern sediment dispersal and accumulation on the outer poverty continental margin abstract the poverty margin was sampled in 2005 and 2006 as part of an international initiative to examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal systems at mud dominated coasts the nsf margins source to sink initiative 210pb accumulation rates generally decrease from 1 g cm2 y in the mid shelf depocenter on the outer shelf to 0 1 g cm2 y on the mid slope plateau range 0 04 2 53 g cm2 y higher accumulation rates are observed all along the outer poverty shelf extending over the shelf break onto the upper poverty slope in canyons and gullies rates are fast in gullies that incise into the shelf edge 0 75 1 g cm2 y particularly in the area between waipaoa canyon and poverty canyon and in the axis of poverty canyon 1 29 1 89 g cm2 y below 1200 m water depth rates in the axis of poverty canyon are no more rapid than those found at open slope cores in similar water depths 0 12 0 15 g cm2 y excess 210pb profiles generally exhibit steady state characteristics except in the axis of poverty canyon where non steady state 210pb profiles are observed 137cs was not found above minimum detection limits or above statistical background levels in any cores from the poverty margin 7be was found distributed widely along and across the margin during both summer and winter periods observed to depths of up to 4 cm widespread presence of 7be on the margin in both winter and summer suggests that terrestrially derived sediment is reaching the outer shelf and upper slope throughout the year a sediment budget based on 210pb accumulation rate data shows that approximately 13 18 of the waipaoa annual discharge is accumulating on the outer poverty margin as a whole and approximately 11 15 of the annual discharge is accumulating on the continental slope alone notably approximately 28 of the sediment annually accumulating in the outer margin is being sequestered in slope gullies and canyons although they represent only 6 of the area of the outer margin

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

productivity of the glacial ocean discussion of the iron hypothesis an increase in the productivity of the glacial age southern ocean has been postulated to explain the decrease in pc0 of the atmosphere observed in ice cores a plausible mechanism has been proposed elsewhere that productivity is limited by the availability of fe in this region and that the greater supply of eolian dust during glacial time removed this limit recently published evidence from cores from the southern ocean suggests that in fact there was no change in productivity in the assumed manner glacial age productivity was indeed greatly increased in the equatorial pacific and in the eastern boundary upwelling systems the cause presumably was the mechanical action of glacial age winds rather than a greater supply of fe however a role of increased supply of micronutrients from the continents in the increase of equatorial productivity during glacial time cannot be excluded such enhancement from increased supply of dust would have the interesting corollary of more efficient export transfer to depth possibly contributing to nutrient depletion in glacial age deep intermediate waters there is some indication as well of a general decrease in nutrient content in the tropical thermocline in the western pacific during the last several million years a depletion that may have been fostered by increasing supply of dust from emerging asian highlands alternative explanations are available the case for fe as a major modifier of productivity and biogenic sedimentation on geological time scales cannot be made in the absence of criteria diagnostic for fe supply as opposed to stirring and upwelling

biogenic silica recycling in surficial sediments across the polar front of the southern ocean indian sector abstract the processes controlling preservation and recycling of particulate biogenic silica in superficial sediments must be understood before one uses biogenic silica as a proxy in paleooceanographic studies and in order to compute oceanic mass balances for silica in this respect the antarctic ocean is certainly a key region due to its high productivity and export of biogenic silica in order to quantify sedimentary fluxes and identify crucial processes that allow the preservation of biogenic silica pore water and solid phase silica profiles were performed on sediment cores from the southern ocean indian sector during the antares 1 cruise in combination with solubility data reported by van cappellen and qiu 1997a a process model representing the early diagenesis of silica was developed in this model a dependence with depth of the kinetic constant was introduced to allow the preservation of biogenic silica in sediment porewater undersaturated with respect to that phase using this steady state model it is proposed that a proportionality of the reactivity of the biogenic silica with its settling flux is necessary to explain the observed profiles it is then shown using this model that the preservation of biogenic silica is not a linear function of the deposited flux using a modified version of this model containing an explicit term of reprecipitation we hypothesize that reprecipitation alone cannot counterbalance dissolution and that its effect is certainly related to a decrease in either surface solubility or kinetics of dissolution

asymptotic forms and solutions of the model for silica opal diagenesis in bioturbated sediments early diagenesis of the coupled dissolved silica opal system in bioturbated sediments may be explained by one of three possible models of increasing mathematical complexity i e the simple but unused constant opal model abbreviated c o the schink et al 1975 model abbreviated sg f for which wong and grosch 1978 have supplied an analytical solution designated as w g and the improved but much more sophisticated model proposed by schink and guinasso 1980 that must be solved numerically s g scaling analysis and computational comparisons show that the c o model and the sg f model as calculated via the w g solution are asymptotically valid forms of more complete s g model for the limits of large and small opal concentrations respectively specifically the c o model is found to provide an excellent approximation to the vastly more complicated s g model if the amount of opal preserved at depth in the sediment b satisfies the inequality b 0 1 1 db ds cs cw 1 b 1 1 where is the porosity db is the mixing coefficient ds is the tortuosity corrected molecular diffusivity of silica cs is the solubility of opal cw is the silica concentration in the overlying waters b is the intrinsic density of opal and is a unit conversion constant if b and c are in different units schink and guinasso 1980 have criticized wong and grosch 1978 for utilizing their solution of the earlier sg f model to describe opal accumulation a situation for which they believed the sg f model was invalid this study has found however that for conditions characteristic of the deep sea the sg f model and so the w g solution remain reasonably accurate even if small amounts of opal escape dissolution and collect but that radical divergence from the s g model can be expected if the flux of silica is sufficient to create a siliceous ooze this reflects the minor role played by the divergence of the advective flux in the balance of terms in the diagenetic conservation equations when little opal is preserved these findings should resolve any uncertainty and controversy over the use of the w g solution opal diagenesis in bioturbated shelf like sediments appears to be adequately described by the c o model alone the c o model coupled where necessary to the w g solution constitutes an attractive alternative to the s g model because relatively simple analytical methods of solution may be employed rather than advanced numerical techniques

sedimentation on the ross sea continental shelf antarctica abstract glacial marine sedimentation has prevailed throughout most of the early glacial history of the ross sea region however since late miocene to early pliocene time the west antarctic ice sheet has advanced onto the continental shelf perhaps several times glacial troughs of the western ross sea may also have formed at this time these glacial troughs border outlet glaciers of the east antarctic ice sheet a widespread lodgement till facies was deposited during this glacial advance holocene retreat of this expanded ice sheet left no conspicuous meltwater record so decoupling of the ice sheet from the seafloor rather than rapid melting is implied modern glacial sediment is far less prevalent than biogenic sediment and reworked glacial sediment except in coastal areas adjacent to small ice shelves and ice tongues little or no debris is being transported beyond the calving line of the ross ice shelf siliceous mud and ooze occurs in the western ross sea where surface circulation is most vigorous and warm deep waters up well this siliceous sediment along with terrigenous silt and clay blankets the seafloor below approximately 300 m water depth most of the ross sea continental shelf is deeper than this surface sediments on shallow banks and coastal platforms have clearly been reworked by marine currents the most vigorous bottom current activity occurs in those areas where geostrophic currents impinge onto the shelf debris flows and turbidity currents are also important in eroding and redistributing glacial sediments on the shelf terrigenous silt and clay are more abundant in surface sediment than basal till the source of this silt and clay is problematic the most plausible sources being onshore transport of suspended sediment by impinging currents and subglacial meltwater input

the effect of sediment mixing on pb 210 accumulation rates for the washington continental shelf abstract nine cores from the washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from pb 210 profiles th 234 profiles indicate mixing coefficients of 140 cm 2 yr 1 for the seabed offshore from the columbia river and 47 cm 2 yr 1 for the seabed of the mid shelf silt deposit 75 km north of the columbia river these large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer 10 cm within one year after emplacement at the seabed surface observed depths of cs 137 penetration within the seabed are compared with depths predicted from the surface mixed layer thickness and the pb 210 accumulation rate the observed and predicted depths agree well at all but one station at this station a combination of th 234 co 60 pb 210 and cs 137 profiles suggests that active mixing occurs below the intensely mixed surface layer the agreement of pb 210 and cs 137 data at the other stations however indicates a general absence of deep mixing on the washington shelf thus the apparent accumulation rates calculated from pb 210 profiles below the intensely mixed surface layer on the washington shelf generally reflect the true rates of sediment accumulation

geochemical characteristics of western ross sea antarctica sediments abstract a geochemical study of bottom sediments from 50 stations in various ross sea sedimentary environments ranging from the coastal strip to the external platform situated off southern victoria land between the david and mariner glaciers has demonstrated the silicate nature of these sediments and significant variations in the major element concentrations the presence and concentrations of the trace elements studied cr ni co v cu pb zn and cd differ from one geographic area to another and within a given basin a statistical analysis of the geochemical variables using the cluster method shows that si is independent of the other elements which group into associations a multivariate statistical analysis based on geochemical granulometric and environmental parameters identifies five distinct sediment groups located in different areas of sedimentation and with peculiar geochemical characteristics group 1 on the external platform which comprises very fine sediments is characterised by the highest contents of mg loi cr ni and v and of cu and zn group 2 from the banks formed by pelitic sands is distinguished on the basis of lower cu and zn and higher cr and si group 3 on the internal platform consisting of sandy sediments comprises high values of na al ca and pb and low values of zn and v group 4 in the coarse coastal sands is enriched in co and ca na k and al group 5 formed by materials with different grain size distribution located south of coulman island is distinguished by a very high content of zn fe ti and na with cr co pb and si being found at lower levels the materials are glacial detrital are derived from the rocky hinterland formations and are reworked by transport and deposition processes depending on the energy level of the marine sedimentary environments given the scarcity of comparison data from the hinterland rocks it is difficult to establish the parent rocks of these glaciomarine sediments on the basis of geochemical data alone only in the case of the sediments south of coulman island can one be certain of a contribution from the mcmurdo volcanics for the external platform sediments the highest contents of si and l o i confirm the input of material from biogenic neoformation

antares i a biogeochemical study of the indian sector of the southern ocean abstract the first expedition of the antares program the french contribution to southern jgofs investigated the biology and the chemistry of the benthic environment and the water column in a region located between 60 s 72 e and 39 s 55 e in this special issue we present a detailed description of the transports of water masses across the polar front zone which are corroborated by chemical and microbial tracer studies a description of the sediment water interface bridging biological chemical and microbial studies and new insights into the diagenesis of biogenic silica

cycling of organic carbon and biogenic silica in the southern ocean estimates of water column and sedimentary fluxes on the ross sea continental shelf we examined the cycling of organic carbon and biogenic silica in the water column and upper sediments of the ross sea seeking to understand the processes leading to the formation of opal rich organic poor sediments over much of the southern ocean between january 1990 and december 1994 we conducted three cruises performing tracer incubation studies 14c 15n 30si 32si to measure rates of primary production nitrate based new production biogenic silica production and biogenic silica dissolution in the upper 50 m over most of the ross sea shelf in spring mid summer and late summer we deployed sediment traps from january 1990 to early march 1992 to measure the mid water 250 m and near bottom gravitational fluxes of particulate organic carbon nitrogen and biogenic silica year round at three sites and obtained sediment cores at 15 sites to assess the accumulation rates of organic carbon and biogenic silica in all known sediment regimes on the shelf at 9 of those sites we also measured nutrient efflux from the sediments enabling us to calculate benthic recycling fluxes of organic matter and opal these data permit estimates of the annual production near surface recycling vertical sinking flux delivery to the seabed benthic regeneration and long term burial of both organic and siliceous material integrated over a 3 3 105 km2 area that covers 75 80 of the ross sea shelf the resulting annual budgets for carbon and silica indicate highly selective preservation of biogenic silica over organic carbon between 50 and 250 m in the water column as well as in the upper seabed selective preservation of silica within the upper 50 m is not indicated and both organic matter and silica are transported from 250 m to the sea floor with virtually 100 efficiency the sio2 c mass ratios for surface layer production 250 m sinking flux delivery to the seabed and long term burial are approximately 0 85 6 1 6 2 and 27 respectively this progressive enrichment in silica results in long term burial of 5 8 of the biogenic silica and 0 17 of the organic carbon produced by phytoplankton in the surface layer a factor of 30 greater preservation efficiency for silica than for carbon nevertheless the ratio of opal burial to opal production in the ross sea is only about twice the apparent global average of 3 and 1 3 of the estimated burial production ratio for the southern ocean as a whole it thus appears that both silica preservation and the decoupling between the cycles of silica and carbon must be even more effective in the waters overlying abyssal southern ocean sediments than they are over the ross sea shelf