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

hydrothermal germanium over the southern east pacific rise germanium enrichment in the oceanic water column above the southern axis of the east pacific rise results from hydrothermal solutions emanating from hot springs along the rise crest this plume signature provides a new oceanic tracer of reactions between seawater and sea floor basalts during hydrothermal alteration in contrast to the sharp plumes of 3he and manganese the germanium plume is broad and diffuse suggesting the existence of pervasive venting of low temperature solutions off the ridge axis

transfer of germanium to marine sediments insights from its accumulation in radiolarites and authigenic capture under reducing conditions some examples through geological ages in the geosphere germanium ge has a chemical behavior close to that of silicon si and ge commonly substitutes for si in small proportions in silicates studying the evolution of the respective proportions of ge and si through time allows us to better constrain the global si cycle the marine inventory of ge present as dissolved germanic acid is facing two main sinks known through the study of present sediments 1 incorporation into diatom frustules and transfer to sediments by these shuttles 2 capture of ge released to pore water through frustule dissolution by authigenic mineral phases forming within reducing sediments our goals are to determine whether such a bio induced transfer of ge is also achieved by radiolarian and whether ge could be trapped directly from seawater into authigenic phases with no intervention of opal secreting organisms shuttles to this end we studied two paleozoic radiolarite formations and geological formations dated of devonian jurassic and cretaceous deposited under more or less drastic redox conditions our results show that the ge si values observed for these radiolarites are close to slightly above those measured from modern diatoms and sponges in addition our results confirm what is observed with some present day reducing sediments the ancient sediments that underwent reducing depositional conditions are authigenically enriched in ge furthermore it is probable that at least a part of the authigenic ge came directly from seawater the recurrence and extent through time and space of anoxic conditions affecting sea bottoms have been quite important through the geological times consequently the capture of ge by reducing sediments must have impacted ge distribution and in turn the evolution of the seawater ge si ratio

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

variation of rb sr ratios in the loess paleosol sequences of central china during the last 130 000 years and their implications for monsoon paleoclimatology abstract rb sr and magnetic susceptibility have been measured in the last interglacial glacial loess profiles at luochuan and huanxian central china a high degree of similarity between the parameters in both profiles suggests that variations of rb sr ratios in the sequence can be regarded as an indicator of east asian summer monsoon strength matching the rb sr record with the specmap 18o curve suggests that the rb sr ratio responds sensitively to changes of the east asian monsoon induced by global ice volume variation

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

primary productivity and the silica cycle in the southern ocean atlantic sector abstract tentative data for the biogenic silica production in a section along 10 e are calculated from the dissolved silica distribution north of a sharp silica front found south of the antarctic polar front and near the maud rise production was relatively high these areas have also high bottom relief causing redistribution of sediments as is demonstrated from 210pb analyses this overestimates the importance of such areas in budget calculations of the accumulation of biogenic silica dissolved silica concentrations in interstitial waters are highly variable low values are explained by the incorporation of al in diatom frustules dissolution in the surface sediments is 3 times larger than burial this is more than previously assumed calculations of the flux out of the sediments from gradients in interstitial waters and in bottom waters using diffusion coefficients determined from 222rn profiles give reasonable agreement

mechanisms of blueschist creation and preservation in an a type subduction zone mount olympos region greece deformation and metamorphism in the mount olympos region greece are the result of subduction of continental crust a type subduction during the alpine orogeny new structural and petrologic data from this region provide insight into the creation of low geothermal gradients in a type subduction zones and the preservation of blueschist facies assemblages early convergent deformation was accompanied by greenschist amphibolite facies metamorphism blueschist facies mineral assemblages overprinted higher grade assemblages during large scale episodic shortening that occurred during and after continental collision prehnite pumpellyite facies assemblages are postkinematic with respect to contractional fabrics but predate a prolonged period of extension that was not accompanied by metamorphism under plating and extension permitted unroofing and preservation of the blueschists with no over printing by greenschist facies assemblages

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

diatom distribution and paleoceanographic reconstruction in the southern ocean present and last glacial maximum abstract diatom assemblage and preservational data are used to reconstruct paleoceanographic conditions at the last glacial maximum 18 000 yrs bp in the southern ocean from these data the following points can be made about the last glacial maximum in this region 1 contraction and slight northern shift of the belt of well preserved diatoms appears to be related to the northward shift of late spring early summer sea ice cover 2 presence of open ocean though poorly preserved diatom assemblages to the south of this belt strongly suggest that during many summers large areas of the southern ocean were ice free 3 the distribution of thenitzschia kerguelensis factor both in surface sediments and at the last glacial maximum indicates that the gyre systems particularly the weddell gyre were intensified during glacial times 4 although data are sparse in the higher latitude south atlantic there is an indication that the weddell polynya also existed during glacial times although it was shifted a few degrees northward

germanium silicon fractionation during biogenic opal formation model results and empirical data show that germanium is fractionated from silica during biogenic opal formation in a two box model of the ocean particulate ge si can vary between the average ocean ge si and the average ocean ratio times the ge fractionation coefficient the two box model requires a ge fractionation coefficient near 0 48 to reproduce si and ge concentrations similar to the ocean analysis of paired water and opal samples yields a ge fractionation coefficient of 0 51 as a consequence of this fractionation the ge si of opal can be an upwelling indicator small shifts of opal ge si in adjacent centimeter scale lighter and darker layers in the miocene monterey formation are consistent with other sedimentologic indicators of productivity and upwelling variations opal ge si are lower in lighter layers and are consistent with interpretations that the lighter layers were formed by increased productivity in overlying waters

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

pliocene variations in the position of the antarctic convergence in the southwest atlantic middle to late pliocene 4 1 1 9 ma variations in the positions of surface water masses and migrations of the polar front in the southwest atlantic are inferred from a factor analysis of radiolarian assemblages from dsdp site 514 faunal results suggest that surface water masses underwent progressive cooling as the polar front zone pfz advanced northward during the latest gilbert to late gauss chron the late gilbert chron 4 12 3 86 ma was the waning phase of a mid gilbert warm interval northern subantarctic surface water prevailed between 4 12 and 3 97 ma indicative of surface water temperatures warmer than those during the succeeding pliocene a long term sea surface cooling trend began between 3 97 and 3 86 ma which moved the pfz northward and closer to site 514 bringing cooler southern subantarctic surface waters to the site this movement northward of cooler surface waters is inferred to be related to the initial growth of sea ice and ice shelves throughout regions of west antarctica which were deglaciated or unglaciated during the prior warming interval of the gilbert chron a major late gilbert early gauss chron increase in the production of antarctic bottom water aabw resulted in an erosional or nondepositional hiatus at site 514 spanning 3 86 to 3 18 ma increased aabw activity at this time is linked to continued growth of expansive ice shelves and sea ice in the west antarctic region in the east antarctic margin and possibly in the marine embayments of east antarctica decreased aabw production led to a 3 2 ma resumption of sedimentation at site 514 and elsewhere in the southern ocean antarctic surface water assemblages above the hiatus 3 18 3 16 ma suggest that the pfz first passed over site 514 during the 3 86 3 18 ma interval of the hiatus following this initial invasion of antarctic surface waters between 3 16 and 2 94 ma the pfz retreated to a position near or south of the site this slight retreat of the pfz with a corresponding decrease in aabw activity is possibly related to a significant reduction in the area of shelf and sea ice as grounded ice expanded in the west antarctic region we interpret the widespread global enrichment of 18o at approximately 3 2 ma to be partially related to a cooling of surface waters the production of cooler bottom waters and an expansion of west antarctic ice volume associated with its grounding phase between 3 5 and 2 7 ma based on surface water mass migrations at site 514 the west antarctic ice volume increased significantly slightly prior to or at approximately 3 2 ma decreased between 3 14 and 2 7 ma and then increased by 2 67 ma to a more stable configuration with a volume at least as great as that at 3 2 ma during the late gauss 2 94 2 47 ma four northward advances of the pfz led to the migration of antarctic surface waters over the site by 2 67 ma and continued dominance of antarctic surface water through the remainder of the pliocene we suggest that this permanent change from subantarctic to antarctic surface water mass dominance was linked indirectly to the initial growth of northern hemisphere ice and a reduction of sea level we propose a model of simultaneous northern and southern hemisphere ice volume growth induced by interhemisphere positive feedback which is responsible for the major late gauss advance of antarctic surface waters and the global enrichment of 18o at 2 5 ma

planktonic and benthic foraminiferal extinction events during the last 100 m y abstract five major planktonic and benthic foraminiferal extinction events occurred during the past 100 m y at the cenomanian turonian c t boundary at the cretaceous tertiary k t boundary in the latest paleocene in the late eocene and in the early middle miocene to clarify the character of the five events i estimated 1 paleo waterdepths and paleoenvironments where foraminifera were affected 2 the duration of each extinction event and 3 evaluated possible causes for each extinction event on the basis of recent literature these extinction events can be classified into three fundamentally different types 1 the mass extinction event at the k t boundary was abrupt and concentrated among surface dwelling species 2 extinctions at the c t boundary and during the latest paleocene were abrupt and mainly affected intermediate and deep water taxa over short periods 3 the late eocene and early middle miocene events affected surface and deep water species and involved stepwise or gradual extinctions over a long period 4 m y i attributed the extinction events to 1 a bolide impact for the k t event 2 a decrease in dissolved oxygen in intermediate and deep waters due to climatic warming for the c t and latest paleocene events and 3 long term global cooling for the late eocene and early middle miocene events the following relationships exist between the causes and types of extinction events 1 large extraterrestrial impacts strongly affected surface dwelling organisms in short term extinction events 2 intermediate and deep water low oxygen conditions induced by climatic warming have severely affected intermediate and deep water organisms over medium and short terms

inorganic germanium and silica in the indian ocean biological fractionation during ge si opal formation hydrographic profiles of inorganic germanium and silica in the western indian ocean confirm the silicalike behavior of inorganic germanium in seawater combining all existing oceanic data indian atlantic pacific bering sea and antarctic these new data provide an improved oceanic ge to si relationship ge pm 0 699 0 004 si m 3 6 0 4 simple box models of the ocean demonstrate that the positive ge intercept in this relationship results from biological fractionation of the ge si ratio siliceous organisms discriminate against germanium during production of biogenic opal this result is confirmed by ge si opal analyses of diatoms and radiolaria separated from siliceous oozes of holocene core tops a rapid rise in diatom ge si opal recorded in an antarctic piston core over the interval from the last glacial 18 thousand years before present to the holocene present suggests that biosiliceous productivity was lower and preformed silica concentrations were higher in southern ocean surface waters during glacial times

punctuated evolution of global climate cooling during the late middle to early late miocene high resolution planktonic foraminiferal and oxygen isotope records from the mediterranean high resolution planktonic foraminiferal and oxygen isotope records are presented from a mediterranean deep marine succession dated astronomically between 12 12 and 9 78 ma planktonic and benthic oxygen isotope records are punctuated by two episodes of 18o increase which have astronomical ages of 11 4 and 10 4 ma and correspond to the mi5 and mi6 events of miller et al 1991a these ice growth events coincide with low amplitude variations in the 1 2 myr obliquity cycle and are accompanied by significant faunal changes in the mediterranean such as the arrival of neogloboquadrinids the increase in abundance of the g apertura g obliquus group and the areal differentiation between n atlantica and n acostaensis short term variations in the planktonic foraminiferal and oxygen isotope records correspond to dominantly precession controlled sedimentary cycles features of the sapropel grey marl layers indicate that the short term astronomically controlled circum mediterranean climate changes remained basically the same over the last 12 myr

the marine geochemistry of germanium ekasilicon the geochemical behavior of dissolved germanium in rivers estuaries and the oceans is similar to that of silicon it is chemically weathered from continental rocks and transported by rivers to the oceans where it is taken up by siliceous organisms and deposited in biogenic opal the ratio of germanium to silicon buried in siliceous oozes must reflect the oceanic input ratios from rivers crustal weathering and sea floor vents mid ocean ridge hydrothermal weathering the opal germanium silicon ratio may thus monitor relative variations in these weathering processes and provide information about the geochemical history of seawater

signal propagation related to the north atlantic overturning 1 changes of the meridional overturning circulation moc due to surface heat flux variability related to the north atlantic oscillation nao are analyzed in various ocean models i e eddying and non eddying cases a prime signature of the forcing is variability of the winter time convection in the labrador sea the associated changes in the strength of the moc near the subpolar front 45 n are closely related to the nao index leading moc anomalies by about 2 3 years in both the eddying and non eddying simulation further south the speed of the meridional signal propagation depends on model resolution with lower resolution non eddying case 4 3 resolution the moc signal propagates equatorward with a mean speed of about 0 6 cm s similar as spreading rates of passive tracer anomalies eddy permitting experiments 1 3 show a significantly faster propagation with speeds corresponding to boundary waves thus leading to an almost in phase variation of the moc transport over the subtropical to subpolar north atlantic

the geochemistry of germanium in deep sea cherts nineteen chert samples from a continuous core of the dsdp leg 17 hole 167 were analysed for ge in addition we analysed five samples from other cores the ages range between late jurassic and late eocene the concentration of ge changes with age from 0 87 ppm in the oldest samples to 0 23 ppm in the youngest equivalent to a gesi decrease from 0 72 10 6 to 0 19 10 6 the decrease in gesi is well correlated with the 87sr86sr ratio in sea water of the relevant age the interpretation of this trend may reflect a different levels of gesi in sea water as a result of a different ratio between hydrothermal and riverine input b a diagenetic trend in siliceous sediments c recording by radiolaria a transition between a radiolaria dominated ocean with relatively high gesi ratios in sea water and diatom domination or d a combination of the above

tertiary paleoceanic chemical variability unintended consequences of simple geochemical models five lines of concrete quantitative evidence about oceanic chemical composition during the past 65 m y can provide insights into the controlling processes and their temporal variations but the system constraints are not yet fully understood the calcium carbonate compensation depth mean carbon isotopic ratio and strontium isotopic ratio have all changed significantly during this time while changes in lithium to calcium and strontium to calcium ratios have been much smaller when these tracers are considered separately simple scenarios of changes in geochemical and geological factors can be used to explain the historical records these factors include 1 increases in continental chemical weathering rates and net continental fluxes to the oceans 2 changes in the isotopic composition of material weathered from continents to the oceans 3 decreases in the proportion of calcium carbonate deposited on continental shelves 4 changes in the deposition or weathering rates of certain types of sediments and 5 changes in hydrothermal circulation because these geochemical cycles are linked to each other and to others such as nutrients and global atmospheric carbon dioxide an explanation invoked for one tracer must satisfy the direction magnitude and timing of the patterns for all the tracers several simple scenarios for geochemical variations have been evaluated for their consequences both intended and unintended and compared to the historical records despite the attractiveness and possible justifications of some simple hypotheses for example decreases in organic carbon burial rates increases in carbonate weathering rates on the continents or increases in all continental weathering rates no one simple scenario appears to be consistent with both the timing and magnitude of the changes observed during the past 65 m y for all tracers considered the contradictions uncovered can be useful in defining what knowledge is crucial to further understanding of oceanic chemical history better knowledge is needed of absolute deep ocean sediment accumulation rates and their temporal variability of the extent and time scale of variability in the net carbon isotope input to the ocean from continental weathering of steady state fluxes for oceanic mass balances and the extent to which these mass balances approach steady state behavior and of the links and interactions between the geochemical cycles of various elements

rare earth major and trace elements in chert from the franciscan complex and monterey group california assessing ree sources to fine grained marine sediments abstract rare earth element ree major and trace element analyses of 77 samples from the mesozoic franciscan complex and miocene monterey group of california usa indicate that magnitudes of the ce anomaly ce ce and total ree abundances sree in these rocks are controlled dominantly by a the amount of included metalliferous material b the amount of direct terrigenous input and c the overall burial rate the relative importance of these individual processes varies across an ocean basin in such a way that ree relative fractionations and sree correspond with its depositional environment our conclusions are supported by an extensive review of published ree data from various river coastal open ocean and ridge influenced waters as well as terrigenous pelagic and metalliferous sediment the chert sequences record deposition in spreading ridge open ocean basin and continental margin environments the relative importance of metalliferous and terrigenous influences and the role of scavenging from seawater may be estimated from a sample s inferred depositional distance km from the spreading ridge the major element chemistry and the stratigraphie context the ce ce values are low 0 29 in the vicinity of spreading ridges and trend to successively higher 1 values with decreasing metalliferous and increasing terrigenous influences other ree indicators ree lu n la n eu eu also respond to metalliferous and terrigenous influences near the spreading ridge chert ree appears determined by burial rate while shale ree responds to metalliferous abundance in open ocean and continental margin environments ree is controlled by sediment exposure time and therefore is related inversely to burial rate ree data are consistent with the major elements fe2o3 mno and al2o3 in reflecting the respective metalliferous and terrigenous sources p2o5 and cao do not vary with ce ce in any environment but cao shows a weak negative correlation with ree trace element trends e g v cr cu ge si rb sr y th u in general can similarly be accounted for in the context of depositional position relative to metalliferous and terrigenous inputs rees in chert appear less affected by post depositional processes than either major or trace elements indicating that rees are better tools for paleoceanographic or tectonic reconstructions that are based on chert chemistry this work complements recent efforts to elucidate the factors determining large scale ree distributions in marine sediments and has direct bearing on previous attempts to use the ce anomaly in older rocks and sediments as a recorder of secular variations and anoxic events in the overlying water column

dissolution and preservation of antarctic diatoms and the effect on sediment thanatocoenoses abstract comparison of southern ocean diatom populations from i surface ocean production ii underlying antarctic sediments and iii laboratory dissolution experiments demonstrates that dissolution can account for the temporal and spatial variations in sedimentary diatom assemblages observed in southern ocean sediments increasing dissolution causes relative depletions in n kerguelensis k enrichments in t lentiginosa l and slight enrichments in e antarctica a this reflects the relative susceptibility to dissolution of the three species that dominate antarctic sediments we have devised a preservation index for the southern ocean based on the ratio k k l to estimate relative extents of dissolution and applied it to natural assemblages holocene southern ocean sediments display increasing opal preservation toward higher latitudes but south of the antarctic polar front preservation decreases in the order well preserved se indian s atlantic sw indian se pacific poorly preserved dissolution also accounts for the pattern of diatom assemblages in the last glacial maximum lgm sediments of the indian and pacific sectors but in the atlantic increased e antarctica abundances at lgm must have resulted from an increase in surface ocean production of this species holocene and lgm diatoms in atlantic and pacific sector sediments are equally well preserved but in the indian sectors holocene sediments are better preserved than those of lgm age paleoceanographic and paleoclimatic transfer functions derived from factor analyses of variations in the sedimentary abundances of these three diatoms have ignored the effects of differential dissolution on thanatocoenosis and thus should be interpreted with caution

late cenozoic ge si record of marine biogenic opal implications for variations of riverine fluxes to the ocean we have determined germanium silicon ratios in purified diatoms and radiolarians from siliceous sediments in holocene core tops one late pleistocene piston core and four high latitude southern ocean deep sea drilling project dsdp sites ranging in age from holocene to oligocene high latitude holocene diatoms in the 10 to 38 m size fraction ge si opal 0 693 0 039 10 6 faithfully record the present oceanic ratio ge si seawater 0 699 0 004 10 6 this confirms that diatoms from siliceous oozes are monitors of seawater ge si and may record past changes in rates of delivery of weathering products to the sea from continental and seafloor weathering large diatoms and radiolarians display ratios much lower than ge si seawater suggesting that ge si opal to some extent displays the effects of biological fractionation from 0 to 2 5 m y b p ge si opal in diatoms from dsdp sites ranges from about 0 54 to 0 70 10 6 similar to the range observed in one late pleistocene piston core over the last 25 kyr from 2 5 to 6 m y b p the ratio is more variable while prior to 10 m y b p ratios are clearly higher averaging about 0 9 10 6 in the early to middle miocene low values of the ratio are consistent with global weathering regimes dominated by river silica input to the sea while higher ratios suggest periods of enhanced hydrothermal input or reduced fluvial contribution nevertheless an oceanic one box model with variable river and hydrothermal inputs and biogenic opal output cannot adequately account for the magnitude or the timing of changes observed in the record suggesting the existence of other sources sinks for inorganic germanium and or silicon to or from the sea