germanium silicon fractionation in the weathering environment abstract we present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the ge si system as a tracer of silicate weathering processes in both modern and ancient environments intensely weathered soils developed on hawaiian basalts have bulk soil ge si ratios 2 to 10 times higher than fresh basalt e g 10 to 25 mol mol vs 2 5 mol mol soil ge concentrations increase with si and decrease with fe suggesting that ge sequestration is related to accumulation of secondary soil silicates rather than retention in soil fe oxy hydroxides sequential extractions of these soils suggest that ge si fractionation takes place by ge sequestration during the initial precipitation of secondary soil aluminosilicates principally allophane further si loss and changes in mineralogy as these soils age result in little additional ge si fractionation ge si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals kaolinite has a ge si ratio 5 9 mol mol higher than the plagioclase from which it forms 3 1 mol mol whereas accumulation of primary quartz ge si 0 5 mol mol prevents granitic soils from attaining high ge si ratios laboratory synthesis of allophane confirms that ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution

iron and manganese diagenesis in deep sea volcanogenic sediments and the origins of pore water colloids abstract volcanogenic sediments are typically rich in fe and mn bearing minerals that undergo substantial alteration during early marine diagenesis however their impact on the global biogeochemical cycling of fe and mn has not been widely addressed this study compares the near surface 0 20 cm below sea floor cmbsf aqueous no 3 and solid phase reactive fe distributions between two volcanogenic sediment settings 1 a deep sea tephra rich deposit neighbouring the volcanically active island of montserrat and 2 mixed biosiliceous volcanogenic sediments from abyssal depths near the volcanically inactive crozet islands archipelago shallow penetration of o2 aq into montserrat sediments was observed 5 cmbsf and largely controlled by the oxidation of organic matter dissolved fe and mn distributions in montserrat pore waters were lowest in the surface oxic layer 0 3 m fe 32 m mn with maxima 20 m fe 200 m mn in the upper 1 15 cmbsf unlike montserrat fe and mn in crozet pore waters were ubiquitously partitioned between 0 2 m and 0 02 m filtrations indicating that the pore water distributions of fe and mn in the traditionally termed dissolved size fraction are dominated by colloids with respective mean abundances of 80 and 61 plausible mechanisms for the origin and composition of pore water colloids are discussed and include prolonged exposure of crozet surface sediments to early diagenesis compared to montserrat favouring nano particulate goethite formation and the elevated dissolved si concentrations which are shown to encourage fine grained smectite formation in addition organic matter may stabilise authigenic fe and mn in the crozet pore waters we conclude that volcanogenic sediment diagenesis leads to a flux of colloidal material to the overlying bottom water which may impact significantly on deep ocean biogeochemistry diffusive flux estimates from montserrat suggest that diagenesis within tephra deposits of active island volcanism may also be an important source of dissolved mn to the bottom waters and therefore a source for the widespread hydrogenous mnox deposits found in the caribbean region

authigenic uranium relationship to oxygen penetration depth and organic carbon rain we have measured u in benthic incubation chambers sediment pore waters and in sediments along the california continental margin sedimentary u uptake rates based on a combination of sediment pore water profiles and benthic incubation chambers generally agree with those predicted from sediment accumulation rate data this agreement supports the view that most of the continental margin sedimentary u is delivered by diffusion across the seawater sediment boundary the average rate of authigenic u accumulation for all the sites examined here is 0 2 nmol cm 2 y 1 which is consistent with published global estimates of sedimentary u uptake in addition the accumulation rate of u in sediments exhibits a nonlinear relationship with the oxygen penetration depth and a linear relationship with the organic carbon rain rate these relationships highlight the potential utility for the u accumulation rate as a proxy for these processes

the missing silica sink revisiting the marine sedimentary si cycle using cosmogenic 32si burial of biogenic silica bsitotal in high sedimentation rate continental margins remains highly uncertain cosmogenic 32si t1 2 140 years can be used to trace the fates of bsitotal postdeposition including as opal bsiopal and diagenetically altered opal bsialtered the latter dominantly authigenic clay bsiclay to determine the magnitude and form of bsitotal storage in coastal sediments conventional operational leaches targeting bsiopal and bsialtered including bsiclay were modified for large scale samples necessary for measurement of 32si the 32si activity was used to estimate total biogenic silica burial bsitotal bsiopal bsialtered in several depositional settings gulf of papua gulf of mexico long island sound and in the previously studied amazon guianas deltaic system in subtropical and temperate regions 32si was detected in both traditional biogenic silica leaches bsiopal and residual authigenic clays traditional bsiopal and modified operational leaches designed to target the most reactive authigenic silicates bsialtered consistently underestimate authigenic clay formation bsiclay and thus the magnitude of bsitotal burial in temperate coastal zones and subtropical deltas by 2 4 fold in tropical deltas 32si activities in the residual fraction after removal of bsiopal demonstrate rapid and almost complete alteration of initial bsiopal to new forms most likely bsiclay globally 4 5 4 9 tmol yr si may be trapped in marine nearshore deposits as rapidly formed clay bsiclay 100 of the missing silica sink in the marine silica budget

diagenetic fractionation of ge and si in reducing sediments the missing ge sink and a possible mechanism to cause glacial interglacial variations in oceanic ge si the average ge si ratio in the ocean is determined by the budgets for each of these elements previous budget formulations have assumed that the only important sink for both elements is burial as opal based on studies of the si cycle and the close oceanic coupling observed between inorganic ge and si distributions however these budgets implied two paradoxes 1 hydrothermal flow through ocean ridges is smaller than predicted by other tracers and 2 the lower ge si ratio of opal deposited during glacial times compared to that deposited during interglacial times required enhanced weathering during cooler drier climates both paradoxes could be resolved if a significant sedimentary sink for ge other than opal burial could be identified and the objective of this study was to search for one two pore water profiles collected in equatorial pacific sediments show that ge and si behave similarly in the upper 10 cm of sediment indicating no evidence for a significant non opal sink for ge in oxic sediments by contrast profiles in several cores from the california margin demonstrate that in reducing sediments ge diagenesis is poorly coupled to si diagenesis significant ge removal is evident both downcore and sometimes in the near surface benthic flux chamber measurements at three continental slope stations all with an oxic layer less than 1 cm thick and large iron gradients in near surface pore waters showed that 55 9 of the ge released by opal dissolution is sequestered however at two locations with anoxic sediments but little pore water fe 2 in the upper 2 cm flux measurements indicated little fractionation from the oceanic ratio during diagenesis implicating the importance of iron for fractionating ge from si during diagenesis if the ge sequestration observed in the iron rich ca margin sediments is typical of all slope sediments using a depth range of 200 1000 m then the ge sink is sufficient to bring the hydrothermal budget based on ge into concurrence with that based on other tracers the temporal variation in oceanic ge si could be explained if ge and si inputs remain constant and the effective diagenetic fractionation of ge increases by a factor of 2 3 during glacial times increased fractionation would require that glacial periods are characterized by increased opal dissolution in iron rich reducing sediments this could be caused by 1 thinning of the oxygenated sediment layer in response to decreased bottom water oxygen concentrations or increased rain of organics to the sea floor 2 increased rain of iron rich detrital sediments in areas receiving high opal rain 3 increased rain of opal to sediments in margin areas if the oceanic ge si ratio reflects increased rain of diatom opal or organic carbon in margin areas during glacial periods it may indicate an increase in the efficiency of the biological pump for co2 during glacial times

germanium geochemistry in the southern california borderlands abstract concentrations of ge oh 4 denoted as ge i or inorganic germanium and si oh 4 were measured in water column and pore water samples from san pedro and san nicolas basins in the southern california borderlands there is a characteristic linear relationship between ge i and si in the water column which implies that the distributions of germanium and silica are controlled by similar processes in contrast the pore water chemistry of germanium appears to differ from that of silica particularly in the reducing organic rich sediments of san pedro basin silica concentrations in san pedro basin pore water asymptotically increase to 480 m at 35 40 cm depth ge i concentrations in the topmost 2 cm of pore water from san pedro basin are an order of magnitude greater than bottom water concentrations below 2 cm ge i concentrations decrease exponentially with a half distance of about 8 cm cycling of ferric oxyhydroxides at a redox boundary is the most likely cause of the ge i enrichment in san pedro basin pore water while authigenic pyrite formation with the concomitant inclusion of germanium is a probable but undocumented cause of the downcore decrease in ge i pore water ge i and silica concentrations in sediments of san nicolas basin are generally greater than bottom water concentrations but vary irregularly with depth probably due to irrigation

effect of subduction of the mendocino fracture zone on tertiary sedimentation in southern california abstract the age and paleogeographic setting of middle to late tertiary fluvial deposits in southern california are consistent with the theory that subduction of the mendocino fracture zone produced a north facing north moving slope and zone of deformation in the north american plate the mendocino fracture zone was a 1 km high north facing scarp in the farallon plate that moved northward at about 3 cm yr 1 subduction of this scarp produced isostatic stress in the overlying north american plate if this stress were expressed in the crust as a north facing slope then a given area should have experienced the following sedimentary evolution 1 normal probably oceanward drainage when the slope was far to the south 2 reorientation to northward drainage as the slope moved to a position immediately south of the area 3 deposition of coarse sedimentary units possibly accompanied by down to the north faulting when the slope reached the area 4 uplift deformation and erosion of previously deposited sediments as the area was uplifted along the slope and 5 reestablishment of normal drainage compilation of data from sedimentary sequences in southern california supports this model in particular the time of fracture zone passage was marked by strong dominance of northward stream flow these relationships suggest that middle tertiary fluvial sedimentation in the southern california area was strongly influenced by passage of the fracture zone

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

the early diagenesis of iron in pelagic sediments a multidisciplinary approach abstract biogeochemical reactions of iron within pelagic sediments from the eastern and western equatorial atlantic are investigated by means of pore water chemistry chemical leaching experiments and total elemental determinations color reflectance spectroscopy rock magnetic measurements and tem observations of the magnetic fraction results indicate that in the presence of nitrate ascorbate a weak reducing agent leachable iron decreased with depth from the sediment surface within this upper sediment region iron assimilation by bacteria is indicated as magnetosomes were found by tem observations throughout the whole core extractions with a strong reducing agent dithionite representing iron bound to iron oxides correlated linearly with the concentration of highly coercive magnetic minerals and with the reflection intensity of red color within the same iron redox zone the abrupt decrease of red color reflection intensity relative to the amount of iron oxides below the iron redox boundary is the result of a decrease in the specific surface area of iron oxyhydroxide oxides magnetic parameters imply a smaller average grain size of the magnetic fraction below the iron redox boundary arising from an increase in biogenic magnetite formed by magnetotactic assimilatory bacteria dissolution of very fine grained magnetite and the gradual decrease of coarser grained terrigenous titano magnetite with depth the early diagenetically formed magnetic fraction withstands subsequent dissolution and gives pronounced peaks of magnetic parameters within sediments with high carbonate and low terrigenous matter content

germanium isotopic variations in igneous rocks and marine sediments a new technique for the precise and accurate determination of ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal the analyses were performed using a continuous flow hydride generation system coupled to a mc icpms samples have been purified through anion and cation exchange resins to separate ge from matrix elements and eliminate potential isobaric interferences variations of 74ge 70ge ratios are expressed as 74ge values relative to our internal standard and the long term external reproducibility of the data is better than 0 2 for sample size as low as 15 ng of ge data are presented for igneous and sedimentary rocks and the overall variation is 2 4 in 74ge representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of ge is not unique co variations of 74ge 70ge 73ge 70ge and 72ge 70ge ratios follow a mass dependent behaviour and imply natural isotopic fractionation of ge by physicochemical processes the range of 74ge in igneous rocks is only 0 25 without systematic differences among continental crust oceanic crust or mantle material on this basis a bulk silicate earth reservoir with a 74ge of 1 3 0 2 can be defined in contrast modern biogenic opal such as marine sponges and authigenic glauconite displayed higher 74ge values between 2 0 and 3 0 this suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the 74ge of the seawater to 3 0 this suggests that seawater is isotopically heavy relative to bulk silicate earth and that biogenic opal may be significantly fractionated with respect to seawater deep sea sediments are within the range of the bulk silicate earth while mesozoic deep sea cherts opal and quartz have 74ge values ranging from 0 7 to 2 0 the variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz further work is now required to determine ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification

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

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

a core top calibration of b ca in the benthic foraminifers nuttallides umbonifera and oridorsalis umbonatus a proxy for cenozoic bottom water carbonate saturation abstract we present modern b ca core top calibrations for the epifaunal benthic foraminifer nuttallides umbonifera and the infaunal oridorsalis umbonatu s to test whether b ca values in these species can be used for the reconstruction of paleo co 3 2 o umbonatus originated in the late cretaceous and remains extant whereas n umbonifera originated in the eocene and is the closest extant relative to nuttallides truempyi which ranges from the late cretaceous through the eocene we measured b ca in both species in 35 holocene sediment samples from the atlantic pacific and southern oceans b ca values in epifaunal n umbonifera 85 175 mol mol are consistently lower than values reported for epifaunal cibicidoides cibicides wuellerstorfi 130 250 mol mol though the sensitivity of co 3 2 on b ca in n umbonifera 1 23 0 15 is similar to that in c wuellerstorfi 1 14 0 048 in addition we show that b ca values of paired n umbonifera and its extinct ancestor n truempyi from eocene cores are indistinguishable within error in contrast both the b ca 35 85 mol mol and sensitivity to co 3 2 0 29 0 20 of core top o umbonatus are considerably lower as in other infaunal species and this offset extends into the paleocene thus the b ca of n umbonifera and its ancestor can be used to reconstruct bottom water co 3 2 whereas o umbonatus b ca appears to be buffered by porewater co 3 2 and suited for constraining long term drift in seawater b ca

environmental control on concretion forming processes examples from paleozoic terrigenous sediments of the north gondwana margin armorican massif middle ordovician and middle devonian and sw sardinia late ordovician abstract concretions of various compositions are common in the paleozoic terrigenous successions of the north gondwana margin this study focuses on phosphatic p and siliceous si concretions present in some successions of the armorican massif nw france and sw sardinia w italy it shows that they consist of mudstones fine to very fine grained sandstones or shellbeds with a more or less abundant p cement and form a continuum between a phosphatic end member and a siliceous biogenic end member the p 2 o 5 contents are ranging from 0 26 to 21 5 and are related to apatite the sio 2 contents vary from 25 to 82 and are linked both to a terrigenous phase and to a biogenic silica phase concretions showing the lower p contents p 2 o 5 2 al 2 o 3 5 comparison with the surrounding sediments shows that all the concretions are enriched in chlorite and in middle rare earth elements la s gd s 0 12 0 72 and some of them in y up to 974 ppm rare earth elements more than 300 ppm and sr 260 880 ppm the concretions with highest biogenic silica concentrations are contained in the outer shelf sediments whereas the other concretions are present from the proximal part of the inner shelf to the outer shelf a genetic model in two stages is proposed during early diagenesis the dissolution of shells and degradation of organic matter progressively enrich the pore water in dissolved si ca and p when the suboxic zone is reached p precipitation begins leading to the formation of protoconcretions in shallow environments the relative permeability of sediments and the winnowing or reworking of the upper few centimetres by bottom currents allow for suboxic conditions to be maintained leading to p rich concretion formation in deeper environments the anoxic zone is reached more rapidly thereby preventing extensive phosphogenesis nevertheless in the protoconcretions the early p cement preserves pore spaces from compaction in the presence of biogenic siliceous particles the fluids are enriched in dissolved silica and diffuse towards the protoconcretions silica precipitation can thus occur later in the intergranular spaces

conversion of diatoms to clays during early diagenesis in tropical continental shelf muds in coastal muds downdrift from the amazon river mouth marine diatom frustules are rapidly converted to various forms of authigenic aluminosilicate phases during burial the dominant neoformed crystalline phases have a composition similar to k smectite yield electron diffraction patterns characteristic of clay minerals and exhibit a range of crystal sizes and morphologies including euhedral pseudohexagonal crystals and anhedral flakes replacing biogenic silica a poorly crystalline or microcrystalline k rich and fe rich aluminosilicate material also replaces the siliceous frustules the conversion process is not always complete leaving relics of the original frustule in the resulting authigenic aluminosilicate phases laboratory incubation experiments with cultured diatoms demonstrate that the conversion process occurs in 20 23 months the conversion of biogenic silica to authigenic clays occurs throughout the amazon deltaic deposits and presumably takes place in other comparable depositional settings biogenic silica alteration provides proof for a direct link between the biogeochemical cycle of silica in nearshore environments and the neoformation of cation rich aluminosilicate phases and it may prove to be important for oceanic geochemical cycles as a sink for si k and other elements incorporated in the authigenic aluminosilicates rapid formation of authigenic k smectite may also represent a reaction stage leading to eventual formation of illitic clays during later diagenesis

a test of ge si opal as a paleorecorder of ge si seawater late pleistocene variations of germanium to silicon ratios in marine diatom shells from sediment cores ge si opal are coherent with the global isotope record of glacial to interglacial climate change these variations are thought to reflect changes in ge si seawater driven by climate modulated alterations in oceanic ge si sources and sinks however an important criterion for interpreting ge si opal as a monitor of whole ocean ge si seawater is that the opal burial ratio be insensitive both to local diatom production and surface ocean silica concentrations so called biological fractionation effects and to differential dissolution artifacts so called diagenesis offsets here we test these assumptions by comparing model ocean sediment ge si opal distributions with data from holocene and glacial sediments across the high latitude indian antarctic ocean siliceous ooze belt in contrast to the model the data show no gradients in either holocene or glacial ge si opal values across productivity zones displaying dramatic changes in biosiliceous production opal burial and dissolution this evidence supports the contention that fractionation effects are small and that observed down core variations in ge si opal faithfully record secular changes in ge si seawater

a search for an early holocene caco3 preservation event the co2 record for air bubbles from the byrd station ice core suggest a drawdown in the ocean atmosphere carbon reservoir during the early holocene such a drawdown would require a corresponding increase in the co3 ion concentration in the deep sea we report here the results of a search in atlantic sediments for evidence that the lysocline showed a corresponding deepening while both the pteropod and the calcite preservation records we have obtained are consistent with expectation they are not conclusive

redox zonation of elements at an oxic post oxic boundary in deep sea sediments abstract deep sea sediments generally provide limited evidence of redox control on the post depositional redistributions of elements this is because only a fraction of any element is reactive and observed elemental distributions are the result of a number of competing processes which are simultaneously active so that redox effects are not dominant a detailed investigation is reported of a core which is unusual in showing unequivocal evidence of direct redox control operating in a homogeneous environment that is relatively free from competing processes a progressive oxidation front has migrated slowly downwards into a uniform turbidite over a long period ca 250 ky the reactive fractions of several redox sensitive elements now show a remarkable degree of organisation around the redox boundary of this front with the different elements exhibiting individual responses solid phase enrichments are not coincident but are arranged in a clear succession of concentration peaks along the redox gradient from oxic to postoxic conditions the succession is mn i fe se and pt cu and pb v zn and sb and u the movement directions of most elements can be inferred from the peak shapes against the homogeneous turbidite background composition the system seems to have been essentially isolated within the sediment for most elements so that no loss or gain can be ascribed to redox processes exceptions are u and v which have been augmented by uptake from ocean bottom waters into the sediments by an efficient redox trap mechanism

the influence of organic matter diagenesis on caco3 dissolution at the deep sea floor abstract in situ benthic flux chamber measurements were performed at seafloor locations in the atlantic and pacific oceans where the bottomwaters were supersaturated approximately saturated and undersaturated with respect to calcite there was no evidence of significant dissolution at or above the saturation horizon below the saturation horizon however significant benthic fluxes of titration alkalinity and calcium out of the sediments were observed indicating appreciable dissolution where observed the rate of dissolution is consistent with a rate expression of reaction order 4 5 and a rate constant in the range of 0 05 to 0 5 per day a numerical simulation was constructed to represent organic matter and caco3 diagenesis at the seafloor at the supersaturated and near saturation sites the model over predicts the influence of metabolically produced co2 on the extent of caco3 dissolution it is possible that these results can be reconciled by a better understanding and parameterization of organic matter mineralization and porewater transport processes in surface sediments the primary factors to be considered include 1 the oxidation state of the carbon being oxidized 2 the vertical distribution of oxidation rates including the role of doc in sediment remineralization 3 the influence of nondiffusive transport of near surface porewaters and 4 the influence of episodic depositional events on organic and inorganic carbon cycling

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

glacial enrichments of authigenic cd and u in subantarctic sediments a climatic control on the elements oceanic budget we examine the possibility that glacial increase in the areal extent of reducing sediments might have changed the oceanic cd inventory thereby decoupling cd from po4 we suggest that the precipitation of cd sulfide in suboxic sediments is the single largest sink in the oceanic cd budget and that the accumulation of authigenic cd and u is tightly coupled to the organic carbon flux into the seafloor sediments from the subantarctic ocean and the cape basin south atlantic where oxic conditions currently prevail show high accumulation rates of authigenic cd and u during glacial intervals associated with increased accumulation of organic carbon these elemental enrichments attest to more reducing conditions in glacial sediments in response to an increased flux of organic carbon a third core overlain by circumpolar deep water cpdw as are the other two cores but located south of the antarctic polar front shows an approximately inverse pattern to the subantarctic record the contrasting patterns to the north and south of the antarctic polar front suggest that higher accumulation rates of cd and u in subantarctic sediments were driven primarily by increased productivity this proposal is consistent with the hypothesis of glacial stage northward migration of the antarctic polar front and its associated belt of high siliceous productivity however the increase in authigenic cd and u glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts suggesting greater partitioning of organic carbon into the sediments during glacial intervals lower oxygen content of cpdw and higher organic carbon to biogenic silica rain rate ratio during glacial stages are possible causes higher glacial productivity in the cape basin record very likely reflects enhanced coastal up welling in response to increased wind speeds we suggest that higher productivity might have doubled the areal extent of suboxic sediments during the last glacial maximum however our calculations suggest low sensitivity of seawater cd concentrations to glacial doubling of the extent of reducing sediments the model suggests that during the last 250 kyr seawater cd concentrations fluctuated only slightly between high levels about 0 66 nmol kg 1 on glacial initiations and reaching lowest values about 0 57 nmol kg 1 during glacial maxima the estimated 5 lower cd content at the last glacial maximum relative to modern levels 0 60 nmol kg 1 cannot explain the discordance between cd and 13c such as observed in the southern ocean this low sensitivity is consistent with foraminiferal data suggesting minimal change in the glacial cd mean oceanic content

germanium incorporation into sponge spicules development of a proxy for reconstructing inorganic germanium and silicon concentrations in seawater abstract measurements of germanium ge in deep sea sponge skeletons are presented for sponges collected by dredge and for spicules isolated from a range of deep sea sediment cores germanium to silicon si ratios ge si sp for sponge silica ranged between 0 075 and 0 380 mol mol which are much lower than the present day seawater ge si ratio of 0 7 mol mol a plot of ge si sp versus estimated seawater ge and si concentration produced a linear relationship with the ge content of spicules increasing with seawater ge concentration plots of ge si sp versus depth for both dredged sponges and sediment bound spicules produced oceanic profiles similar to those of dissolved ge and si concentration to explain the fractionation seen in the sponge ge si sp data two models are presented the first model used to interpret the data assumes that sponges only respond to the ge content of the ambient seawater implying that ge incorporation into sponge silica is independent of the seawater ge si ratio up to a si concentration of about 100 mol l this model is consistent with the 68 ge uptake results of davie et al e i davie t l simpson r garonne experimental germanium incorporation into siliceous sponge spicules biol cell 48 1983 191 202 for cultured spicules their results showed that the incorporation of ge in sponge silica is only dependent on the ge concentration of the water in which a sponge is growing the second model used to explain the data assumes that ge si sp fractionation results from subtle differences in the uptake kinetics of ge and si while the assumptions used by each model to describe the data are different it is possible to use sponge ge si sp data to reconstruct palaeo ge concentrations using model i and to reconstruct palaeo si concentrations using both models palaeo si concentrations estimated using both models are in good agreement

primary silica precipitate at the precambrian cambrian boundary in the south oman salt basin sultanate of oman abstract an organic rich laminated chert silicilyte consisting of up to 90 microcrystalline quartz that formed at the precambrian cambrian boundary acts as a light oil reservoir in the subsurface of the south oman salt basin sultanate of oman fully encased in salt domes it was first discovered during the 1990 s hydrocarbon exploration activities of petroleum development oman because of the economic significance and the unconventional reservoir characteristics there is great interest in understanding the origin of the laminated chert its source of silica and mode of precipitation in an anoxic sulphur rich stagnant and highly saline basin the homogeneous distribution and high values of stable si isotope composition avg 30 si 0 83 0 28 coupled with a low molar ge si ratio 6 of the microcrystalline matrix quartz clearly reveal dissolved silica in the seawater as the si source whereas hydrothermal or biogenic e g sponge derived silica can be excluded silica precipitation from seawater was likely the result of a dramatic increase in salinity in response to salt dissolution atop and adjacent to the edges of transtensional depressions on the deep basin floor thus markedly reducing the solubility of amorphous silica in these brine filled seawater depressions this saturation triggered the formation of silica gel which accumulated at the basin floor forming a soft silica rich layer on bacterial mats giving rise to a laminated sediment the mean number of laminae is ca 32 per year suggesting that layering is non annual and controlled by processes such as fluctuations in nutrient supply lunar driven re mixing or diagenetic segregation the transformation of the silica gel to microcrystalline quartz occurred below 45 c indicating a less than 4 5 18 o composition of the pore water during microcrystalline quartz formation

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

germanium silicon ratios in the copper river basin alaska weathering and partitioning in periglacial versus glacial environments 1 glacial to interglacial variation in the germanium to silicon ratio ge si of biogenic opal in oceanic sediment cores has been proposed to record terrestrial silicate weathering intensity however data connecting ge si ratios to weathering are sparse especially for cold regions to characterize ge si of the dissolved loads of rivers draining modern glacial and periglacial regions we measured ge and major element chemistry of more than 30 streams in the copper river basin of south alaska we measured significantly higher ge si in rivers with greater than 15 glacial cover in their basins than in rivers draining areas with less glacial cover which we attribute to preferencial subglacial weathering of biotite we also ran laboratory experiments to investigate the role of differential sorption of ge and si on hydrous iron oxides in moderating the ge si ratio of rivers in recently deglaciated basins these experiments indicate that ge is preferentially adsorbed over si by hydrous iron oxides over a wide range of ph values and support differential sorption as a potentially significant fractionation process our measured high ge si in glaciated basins is in striking contrast to the low ge si in the oceanic record during glacial periods the glacial to interglacial changes in marine ge si cannot simply reflect changes in glacier cover on land as the mineralogy of silicates weathered and processes partitioning ge in soils must also strongly influence the fluvial ge and si inputs to the ocean

germanium silicon fractionation in a river influenced continental margin the northern gulf of mexico abstract in this study we have sampled the water column and sediments of the gulf of mexico to investigate the effects of high riverine terrigenous load and sediment redox conditions on the cycling of ge and si water column ge si ratios across the gulf of mexico continental shelf range from 1 9 to 25 mol mol which is elevated compared to the global ocean value of 0 7 mol mol the ge enrichment in the gulf of mexico seawater is primarily due to anthropogenic contamination of the mississippi river which is the main ge and si source to the area and to a smaller extent due to discrimination against ge during biogenic silica bsi production ge si 1 2 1 8 mol mol especially by radiolarians and siliceous sponges ge si 0 6 1 1 mol mol most sediment pore waters ge si 0 3 4 5 mol mol and sediment incubation experiments benthic flux ge si 0 9 1 2 mol mol indicate precipitation of authigenic phases that sequester ge from pore waters non opal sink this process appears to be independent of oxidation reduction reactions and suggests that authigenic aluminosilicate formation reverse weathering may be the dominant ge sink in marine sediments compilation of previously published data shows that in continental margins non opal ge burial flux is controlled by bsi supply while in open ocean sediments it is 10 100 times lower and most likely limited by the supply of lithogenic material we provide a measurement based estimate of the global non opal ge burial flux as 4 32 mmol yr 1 encompassing the 2 16 mmol yr 1 needed to keep the global marine ge cycle at steady state

early diagenesis of germanium in sediments of the antarctic south atlantic in search of the missing ge sink pore water and solid phase geochemistry profiles were obtained from several cores between 41 degree s and 53 degree s in the atlantic sector of the southern ocean pore water nitrate manganese and iron profiles delineate standard redox zones in these sediments and help characterize those with classic vs burn down behaviors pore water si and ge profiles demonstrate that ge released during opal dissolution is removed pervasively throughout the uppermost interval of silicate release and also downwards into the suboxic zone by as yet unidentified precipitation mechanisms these results indicate that early diagenesis of ge is uncoupled from that of opal solid phase extractions fe mn u mo ge cu ni co v and cd in a few cores suggest that anthigenic ge removal in the suboxic zone is not associated with peaks in authigenic mn cycling mno sub 2 and related metals but rather with processes deeper in the sediments perhaps fe or u diagenesis below the interval of ge removal pre water ge increases linearly with depth by over two orders of magnitude indicating a deep below recovery source of large magnitude the fraction of opal derived ge precipitated authigenically in these sediments ranges from approximately 1 to 96 and correlates strongly with the detrital fraction asmore well as the detrital to opal ratio both of which generally decrease from north to south the ge sink observed in these sediments would need to be globally representative to account for the entire missing ge sink in today s oceanic ge balance which seems unlikely benthic fluxes of ge and si estimated from these pore water profiles and from measurements in three benthic flux chamber experiments at high carbon rain continental margin sites demonstrate that the ge si rate released from the seafloor in locations with high benthic silicate and carbon fluxes is congruent with holocene opal dissolution ge si approximately 0 7 x 10 sup minus 6 in contrast ge si flux ratios in areas with low silicate fluxes are ge depleted ge si 0 5 x 10 sup minus 6 the authors speculate that glacial interglacial changes in oceanic ge si as recorded in diatoms may be due in part to variations in this authigenic ge sink perhaps related to shifts in siliceous productivity from open ocean antarctic siliceous oozes during interglacials to areas of higher detrital input sub antarctic continental margins and possibly also to generally more reducing conditions in sediments during glacials less

seawater mediated interactions between diatomaceous silica and terrigenous sediments results from long term incubation experiments reactors containing frustules of the cultured diatom thalassiosira punctigera suspended in seawater were incubated with or without added sediment from the mississippi river delta or the congo river fan the diatom frustules were separated from the terrigenous sediments by a dialysis membrane thereby only allowing the exchange of dissolved species one series of incubations was carried out in the laboratory at room temperature 21 c and for a period of 10 months another series of reactors was deployed along a mooring in the mozambique channel at three water depths 500 1250 and 2000 m for a period of 22 months chemical analyses after total destruction of frustules collected at the end of the incubations showed elemental transfer from seawater mg and k and the sediments al fe mn p and ca to the frustules in the presence of the terrigenous sediments the dissolved silicate concentrations at the end of the incubations were systematically lower that those measured in the incubations without the sediments in addition electron microscopy revealed the formation of new mineral precipitates these included amorphous deposits on the frustules containing si fe al mg k and p as well as euhedral clay crystallites differences were observed between the incubations performed in the laboratory and those deployed at sea likely as a result of differences in redox conditions temperature and reaction time overall the interactions between biogenic silica seawater and lithogenic minerals reduce the regeneration of nutrient silicon fixed by siliceous organisms these interactions take place on relatively short time scales months to years and affect not only the marine cycle of silicon but also those of other major and minor elements such as al fe mn k and mg

evidence from authigenic uranium for increased productivity of the glacial subantarctic ocean authigenic uranium is precipitated in reducing sediments and therefore responds both to changes in particulate organic carbon flux to the sediment and to changes in the oxygen concentration of bottom waters by examining a large number of cores over a wide latitudinal and depth range in the atlantic sector of the southern ocean we hope to distinguish between a predominantly productivity driven authigenic u signal and a circulation or sediment focusing signal we find little to no authigenic u in holocene sediments throughout the south atlantic ocean glacial sediments north of 40 s lack authigenic u whereas sediments from the subantarctic zone show substantial enrichments in authigenic u relative to the holocene up to 5 3 ppm the widespread distribution of glacial u enrichment even in cores with no glacial interglacial change in mass accumulation rate implies that u deposition was not caused by an increased supply of organic carbon via sediment focusing authigenic u and organic carbon in a shallow core 1000 m from the subantarctic region show the same glacial increase found in the deep cores because this site was well ventilated during the last glacial maximum its record provides further evidence that increased glacial productivity of the subantarctic region contributed to the high concentrations of authigenic u found in the glacial sections of deep cores

redox conditions and trace metal cycling in coastal sediments from the maritime antarctic abstract redox sensitive trace metals mn fe u mo re nutrients and terminal metabolic products no 3 nh 4 po 4 3 total alkalinity were investigated for the first time in pore waters of antarctic coastal sediments the results of this study reveal a high spatial variability in redox conditions in surface sediments from potter cove king george island western antarctic peninsula particularly in the shallower areas of the bay the significant correlation between sulphate depletion and total alkalinity the inorganic product of terminal metabolism indicates sulphate reduction to be the major pathway of organic matter mineralisation in contrast dissimilatory metal oxide reduction seems to be prevailing in the newly ice free areas and the deeper troughs where concentrations of dissolved iron of up to 700 m were found we suggest a combination of several factors to be responsible for the domination of metal oxide reduction over sulphate reduction in these areas these include the increased accumulation of fine grained material with high amounts of reducible metal oxides a reduced availability of metabolisable organic matter and an enhanced physical and biological disturbance by bottom water currents ice scouring and burrowing organisms based on modelled iron fluxes we calculate the contribution of the antarctic shelf to the pool of potentially bioavailable iron feb to be 6 9 103 to 790 103 t yr 1 consequently these shelf sediments would provide an feb flux of 0 35 39 5 mg m 2 yr 1 median 3 8 mg m 2 yr 1 to the southern ocean this contribution is in the same order of magnitude as the flux provided by icebergs and significantly higher than the input by aeolian dust for this reason suboxic shelf sediments form a key source of iron for the high nutrient low chlorophyll hnlc areas of the southern ocean this source may become even more important in the future due to rising temperatures at the wap accompanied by enhanced glacier retreat and the accumulation of melt water derived iron rich material on the shelf

silicon isotopic fractionation in marine sponges a new model for understanding silicon isotopic variations in sponges abstract the silicon si isotope 30 si composition of deep sea sponges from near antarctica subantarctic waters tasmania seamounts and subtropical waters north of new zealand vary widely between 0 87 and 3 40 vs nbs28 depth profiles show that sponge 30 si compositions trend to lower values with increasing depth this is exemplified by sponges from the tasmania seamounts where 30 si varies from 0 87 to 3 13 over a depth range from 100 m to 1200 m these changes in 30 si of sponges are inconsistent with a rayleigh type isotope fractionation model requiring constant 30 si fractionation between sponge and seawater we conclude that overall si isotope fractionation 30 si 30 si sponge 30 si seawater is influenced by seawater si concentration with more fractionated lower isotope values being associated with sponges collected from waters high in si we invoke and fit a model whereby the 30 si fractionation varies as a function si influx and efflux using this model it appears that 30 si fractionation during transport into the sponge is constant at 1 34 the model also shows asymptotic behaviour with 30 si trending towards a maximum of 6 02 at very high si concentrations these results suggest that the 30 si composition of fossil spicules may be useful for reconstruction paleo si concentrations during the past

germanium silica ratios in diagenetic chert nodules from the ediacaran doushantuo formation south china abstract germanium silica ge si ratios of dolostone and mudstone hosted chert nodules from the ediacaran 635 542 ma doushantuo formation in the yangtze gorges area south china are reported these chert nodules typically have a calcite rim a pyrite rim and a silica core the latter sometimes containing disseminated pyrite the silica core was precipitated by early diagenetic replacement of carbonate and silty muddy sediments two types of chert nodules are identified based on their mineralogy and geochemistry type 1 chert nodules are poor in disseminated pyrite in the silica core they also have low al and show a strong positive correlation between al contents and ge si with a near zero or negative intercept in contrast type 2 chert nodules contain abundant disseminated pyrite in the silica core and show a weakly positive correlation between ge si ratios and al contents with a large positive intercept on the ge si axis the ge si of doushantuo nodules are greater than those of cretaceous deep sea cherts suggesting that the ge si ratio of ediacaran seawater porewater was greater than the cretaceous due to the more effective discrimination against ge by inorganic opal precipitation relative to biogenic opal precipitation the positive correlation between ge si and al can be interpreted using a mixing model with a pure chert characterized by a low ge si ratio and an al rich endmember characterized by a high ge si ratio the latter is most likely represented by a clay component but the model based estimate of the ge si ratio inferred for the al rich clay endmember is much higher than that of phanerozoic clay minerals these high ge si ratios for the clay endmember may be related to the generally high ge si ratio of ediacaran seawater but could also be related to clay organic matter interactions organic matter absorbed to clays could provide an additional source of ge because certain organic molecules are known to have a high affinity for ge due to their strong metal ion chelating properties the high ge si ratio of the al rich endmember in type 1 chert nodules suggests that ge in porewaters from which these cherts precipitated may have been dominated by ge organic complexes the low ge si ratio inferred for the al rich endmember in type 2 chert nodules is therefore taken to indicate that ge was released from organic matter perhaps due to anaerobic degradation of organic matter accompanied by the formation of pyrite and was redistributed between clay organic endmembers and pure chert resulting in a non zero intercept in the ge si vs al 2 o 3 plots these observations suggest that a strong terrestrial influence in a restricted sedimentary basin or a high content of dissolved organic carbon in ediacaran seawater and porewater may have contributed to the dominance of ge organic complexes in the doushantuo basin in the yangtze gorges area

enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands a sink for atmospheric co2 atmospheric co2 and global climate are closely coupled since 800 ka co2 concentrations have been up to 50 higher during interglacial compared to glacial periods because of its dependence on temperature humidity and erosion rates chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition cooler and drier conditions and lower non glacial erosion rates suppressed in situ chemical weathering rates during glacial periods however using systematic variations in major element geochemistry sr nd isotopes and clay mineral records from ocean drilling program sites 1143 and 1144 in the south china sea spanning the last 1 1 ma we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials we attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for 9 of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial interglacial variation of 80 ppmv as a result if similar magnitudes can be identified in other tropical shelf slope systems the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced the drawdown of atmospheric co2 during cold stages of the quaternary this in turn would have caused an intensification of glacial cycles

sensitivity of silicon isotopes to whole ocean changes in the silica cycle a 2 box model has been used to assess the impact of both long and short term budgetary imbalance in the silica cycle on the average silicon isotopic composition 30si of the ocean and marine sediments over a 100 ky time span such as a quaternary glacial cycle a sustained change in the riverine flux of silicon to the oceans could alter the average 30si of seawater and the average 30si of opal outputs by a few hundredths to a few tenths of permil this would be largely tied to a change in the 30si of silicon entering the ocean due to a shift in the proportion of riverine and non riverine sources of silicon a doubling of the riverine flux of silicon would have little impact on average marine 30si but a sustained halving of river inputs could interfere with use of 30si as a tracer of nutrient utilization studies on the longer term focussed on the transition from a high silicic acid to low silicic acid ocean associated with the rise of the diatoms this transition is marked by drop in the average 30si of seawater from greater than 1 9 down to about 0 8 the isotopic composition of diatom opal however has an isotopic composition that sticks close to the 0 8 of the inputs and is thus unlikely to provide information about the transition to the low silicic acid ocean of the modern day however the 30si of opal produced in the deep sea for example by sponges should document this transition

early diagenesis of biogenic silica in the amazon delta alteration authigenic clay formation and storage deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets a variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating amazon delta sediments indicates that both of these general assumptions are unjustified apparent lack of significant biogenic silica storage in deltaic environments particularly in the tropics may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica biogenic silica particles buried in suboxic amazon delta deposits can be unaltered partially dissolved covered with aluminosilicate or metal rich coatings or completely reconstituted into authigenic k fe rich aluminosilicate minerals pore water k mg f si and solid phase distributions direct observations of particles laboratory experiments and depositional context indicate that authigenic clays form rapidly 1 yr in the seasonally reworked surface layer 0 5 2 m of the delta topset and are disseminated during sediment remobilization fe al oxide rich debris derived from the tropical drainage basin is an abundant reactant and thus the supply of biogenic silica is a major control on the amount of clay formed the mild 1 na2co3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0 1n hcl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution well crystallized clays are not significantly affected by this modification nor is bulk amazon river bed sediment the two step procedure indicates that 90 of the biogenic silica originally present in deposits is converted to clay or otherwise altered raising the effective quantity of biogenic silica stored from 33 to 296 mol si g 1 1 8 sio2 biogenic si stored in the delta increases away from the river mouth across shelf and along the dispersal system where primary production is highest the k si ratio of labile authigenic material is 0 19 mol mol 1 far higher than amazon river suspended matter 0 07 mol mol 1 diagenetic models indicate formation rates in the mobile sediment layer of 2 8 mol k g 1 yr 1 16 mol si g 1 yr 1 inclusion of authigenic alteration products of biogenic silica in estimates of reactive si burial increases the deltaic storage of riverine si to 22 of the amazon river input the rapid formation of aluminosilicates from biogenic sio2 seawater solutes and remobilized fe al oxides represents a form of reverse weathering rapid reverse weathering reactions in tropical muds and deltaic deposits the largest sediment depocenters on earth confirms the general importance of these processes in oceanic elemental cycles

a global ge isotope budget we present measurements of ge isotope composition and ancillary data for samples of river water low and high temperature hydrothermal fluids and seawater the dissolved 74ge composition of analyzed rivers ranges from 2 0 to 5 6 which is significantly heavier than previously determined values for silicate rocks 74ge 0 4 0 7 escoube et al geostand geoanal res 36 2 2012 from which dissolved ge is primarily derived an observed negative correlation between riverine ge si and 74ge signatures suggests that the primary 74ge fractionation mechanism during rock weathering is the preferential incorporation of light isotopes into secondary weathering products high temperature 150 c hydrothermal fluids analyzed in this study have 74ge of 0 7 1 6 most likely fractionated during fluid equilibration with quartz in the reaction zone low temperature 25 63 c hydrothermal fluids are heavier 74ge between 2 9 and 4 1 and most likely fractionated during ge precipitation with hydrothermal clays seawater from the open ocean has a 74gesw value of 3 2 0 4 and is indistinguishable among the different ocean basins at the current level of precision this value should be regulated over time by the isotopic balance of ge sources and sinks and a new compilation of these fluxes is presented along with their estimated isotopic compositions assuming steady state non opal ge sequestration during sediment authigenesis likely involves isotopic fractionation 74gesolid solution that is 0 6 1 8

biogenic silica a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon on geological time scales 106 years the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric co2 through chemical weathering of ca and mg silicate minerals in continental rocks rivers transport the soluble products of weathering cations alkalinity and silicic acid to the oceans where they are utilized by marine ecosystems on decadal to glacial interglacial time scales however large biotic fluxes and storages of si within terrestrial and freshwater ecosystems need to be taken into account recent studies have emphasized the importance of si accumulating plants which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths these include grasses sedges palms some temperate deciduous trees and conifers and many tropical hardwoods landscapes dominated by accumulator plants such as tropical rainforests grasslands herbaceous wetlands and bamboo forests act as silica factories important silica bioengineers in freshwater ecosystems comprise diatoms sponges and chrysophytes this paper reviews the biological role of si in higher plants the impact of vegetation on rates of chemical weathering the fluxes of si through catchment ecosystems lakes and rivers and the potential contribution of new geochemical and isotopic tracers to si biogeochemistry multiproxy investigations of lake sediments will provide novel insights into past variations in si biocycling from terrestrial to aquatic realms on 10 106 year time scales copyright 2008 john wiley sons ltd

silica and germanium in pacific ocean hydrothermal vents and plumes dissolved silica si and inorganic germanium ge concentrations were measured in hydrothermal fluids from black smoker vents on the east pacific rise 21 n epr and the southern juan de fuca ridge 45 n sjdfr north and south cleft sites axial volcano these typically display end member concentrations ranging from 16 to 23 mm si and 150 to 280 nm ge and end member ge si ratios clustering between 8 and 14 10 6 more than 10 fold greater than the ratio entering the ocean via rivers 0 54 10 6 and being recycled in seawater 0 7 10 6 excess concentrations of dissolved si and ge above oceanic background are observed in mid water hydrothermal plumes over mid ocean ridge mor spreading centers on the southern epr sepr 10 20 s and the sjdfr the largest si and ge concentration anomalies occur over the north cleft segment of the sjdfr these are a factor of three greater than anomalies over the sepr 10 20 s excess ge correlates with excess3he in plumes at a ge 3he molar ratio of about 1 104 approximately the same ratio as in black smokers these observations combined with low abundances of ge in femn rich metalliferous sediments suggest that ge and si behave conservatively in mid ocean ridge hydrothermal plumes a simple ocean si and ge balance constrained by the global river silica flux and ge si ratios in hydrothermal vents rivers and biogenic silica suggests that the global hydrothermal silica flux is about 1 4 1011 mole yr 1 much lower than that estimated from3he either 1 70 80 of the ge flux to the ocean is removed in as yet undiscovered sinks not opal or 2 only 10 of the mantle to ocean3he and heat fluxes is associated with mor hydrothermal convection through the 350 c isotherm 90 is off ridge or 3 the oceanic ge si 3he and87sr86sr balances today are far from steady state

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

patagonian origin of glacial dust deposited in east antarctica vostok and dome c during glacial stages 2 4 and 6 abstract the source area of continental dust deposited at both vostok and dome c sites east antarctica during the glacial stages stage 2 18 ka last glacial maximum stage 4 60 ka and stage 6 160 ka of the last two climatic cycles has remained constant the isotopic composition 87 sr 86 sr and 143 nd 144 nd of the ice core dust has been compared with the isotopic composition of the potential source areas antarctica new zealand southern africa australia and south america this comparison reveals the southern south american provenance of the dust for all three glacial periods as has already been shown for the lgm 1 we show that the patagonian loess and the marine shelf sediments from the argentine continental shelf which was variably emerged during glacial periods display different isotopic compositions and that the composition of the patagonian loess best matches the signature of the ice core dust the identification of the patagonian region as the source of the windblown dust deposited over east antarctica during all the latest glacial periods permits a better understanding of paleo atmospheric circulations of the last climatic cycles and a better constraint on the parametrization of dust sources for paleoatmospheric general circulation models

biogenic opal in southern ocean sediments over the last 450 000 years implications for surface water chemistry and circulation we present records of biogenic opal percentage and burial rate in 12 piston cores from the atlantic and indian sectors of the southern ocean these records provide a detailed quantitative description of changing patterns of opal deposition over the last 450 kyr the striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the southern ocean under different global climate conditions three broad depositional patterns can be distinguished northernmost records 39 42 s latitude are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity records from cores just north of the present antarctic polar front 46 49 s show even larger increases in opal burial rate during glacial intervals but have variance concentrated in the 100 and 23 kyr bands southernmost records 51 55 s are completely out of phase with those to the north with greatly reduced opal burial rates during glacial periods taken as a whole the opal records show no evidence for the increased total antarctic productivity predicted by recent geochemical models of atmospheric co2 variability the areal expansion of southern ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial interglacial opal patterns the excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region however local circulation changes may act to modify this basic signal possibly accounting for the observed differences in the opal variance spectra

the isotope composition of inorganic germanium in seawater and deep sea sponges abstract although dissolved concentrations of germanium ge and silicon si in modern seawater are tightly correlated uncertainties still exist in the modern marine ge cycle germanium stable isotope systematics in marine systems should provide additional constraints on marine ge sources and sinks however the low concentration of ge in seawater presents an analytical challenge for isotopic measurement here we present a new method of pre concentration of inorganic ge from seawater which was applied to measure three ge isotope profiles in the southern ocean and deep seawater from the atlantic and pacific oceans germanium isotopic measurements were performed on ge amounts as low as 2 6 ng using a double spike approach and a hydride generation system coupled to a mc icp ms germanium was co precipitated with iron hydroxide and then purified through anion exchange chromatography results for the deep i e 1000 m depth pacific ocean off hawaii nearby loihi seamount and the deep atlantic off bermuda bats station showed nearly identical 74 70ge values at 3 19 0 31 2sd n 9 and 2 93 0 10 2sd n 2 respectively vertical distributions of ge concentration and isotope composition in the deep southern ocean for water depth 1300 m yielded an average 74 70ge 3 13 0 25 2sd n 14 and ge si 0 80 0 09 mol mol 2sd n 12 significant variations in 74 70ge from 2 62 to 3 71 were measured in the first 1000 m in one station of the southern ocean near sars seamount in the drake passage with the heaviest values measured in surface waters isotope fractionation by diatoms during opal biomineralization may explain the enrichment in heavy isotopes for both ge and si in surface seawater however examination of both oceanographic parameters and 74 70ge values suggest also that water mass mixing and potential contribution of shelf derived ge also could contribute to the variations combining these results with new ge isotope data for deep sea sponges sampled nearby allowed us to determine a ge isotope fractionation factor of 0 87 0 37 2sd n 12 during ge uptake by sponges although ge has long been considered as a geochemical twin of si this work underpins fundamental differences in their isotopic behaviors both during biomineralization processes and in their oceanic distributions this suggests that combined with si isotopes ge isotopes hold significant promise as a complementary proxy for delineating biological versus source effects in the evolution of the marine silicon cycle through time

zinc and germanium in the sedimentary rocks of gale crater on mars indicate hydrothermal enrichment followed by diagenetic fractionation zinc and germanium enrichments have been discovered in sedimentary rocks in gale crater mars by the alpha particle x ray spectrometer on the rover curiosity concentrations of zn 910 840 ppm and ge 65 58 ppm are tens to hundreds of times greater than in martian meteorites and estimates for average silicate mars enrichments occur in diverse rocks including minimally to extensively altered basaltic and alkalic sedimentary rocks the magnitude of the enrichments indicates hydrothermal fluids but curiosity has not discovered unambiguous hydrothermal mineral assemblages we propose that zn and ge rich hydrothermal deposits in the source region were dispersed in siliciclastic sediments during transport into the crater subsequent diagenetic mobilization and fractionation of zn and ge is evident in a zn rich sandstone windjana zn 4000 ppm ge 85 ppm and associated cl rich vein stephen zn 8000 ppm ge 60 ppm in ge rich veins garden city zn 2200 ppm ge 650 ppm and in silica rich alteration haloes leached of zn 30 200 ppm in moderately to highly altered silica rich rocks ge remained immobile relative to leached elements fe mn mg and ca consistent with fluid interaction at ph 7 in contrast crosscutting ge rich veins at garden city suggest aqueous mobilization as ge f complexes at ph 2 5 multiple jarosite detections by the chemin x ray diffractometer and variable zn concentrations indicate diagenesis of lower mount sharp bedrock under acidic conditions the enrichment and fractionation of zn and ge constrains fluid events affecting gale sediments and can aid in unraveling fluid histories as curiosity s traverse continues

river fluxes of dissolved silica to the ocean were higher during glacials ge si in diatoms rivers and oceans two centric marine diatom species thalassiosira oceanica and thalassiosira antarctica were grown in batch cultures to determine the incorporation of germanium ge and silicon si into siliceous shells opal the results were modeled as ge si isotope fractionation during exponential growth diatoms take up and incorporate ge si from solution without major discrimination against ge during stationary phase growth near silica limitation the antarctic species t antarctica discriminates slightly against ge but integrated ge si opal produced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio these results confirm experiments using radioactive 68ge that showed absence of fractionation during diatom silica uptake azam and volcani 1981 in contrast to two box ocean models that invoke 50 ge discrimination by diatoms to explain the observed excess surface ocean germanium concentration murnane and stallard 1988 froelich et al 1989 and late pleistocene ocean sediment ge si opal records mortlock et al 1991 runs of a 10 box ocean ge and si model pandora with 50 discrimination reproduce the excess surface ocean ge but introduces curvature into the deep ocean ge versus si relationship that is not observed in the oceans thus 50 fractionation is not supported by either cultures or models if diatoms do not fractionate ge si then late pleistocene ge si opal variations in piston cores are caused not by changes in local biosiliceous production and silica utilization mortlock et al 1991 but rather by whole ocean changes in ge si seawater the marine ge si opal record of the last 450 kyr can be modeled as transient oceanic responses to instantaneous continental climate transitions consistent with the chemical weathering model of murnane and stallard 1990 glacial periods are characterized by lower continental weathering intensity lower ge si riv and two fold higher dissolved silica river fluxes marine ge si opal records thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering

germanium silica ratio and rare earth element composition of silica filling in sheet cracks of the doushantuo cap carbonates south china constraining hydrothermal activity during the marinoan snowball earth glaciation abstract the rapid diversification of eukaryotes within 3 million years after the marinoan global glaciation suggests the potential linkage between the snowball earth event and biological evolution to understand how the global glaciation and the biological evolution were linked it is necessary to reconstruct the sequence of events with high temporal resolution that occurred during the deglaciation in this study we focus on silica filling within sheet cracks sheet cracks abundantly occur in the basal part of the marinoan cap carbonate 635 ma and are characterized by the interconnected horizontal and vertical cracks that are sequentially cemented by dolospars silica and calcite as the second stage of crack filling the origin of silica filling remains controversial if the seawater origin is confirmed the silica filling might archive the signal of ocean chemistry in the aftermath of snowball earth event in this study we measured the germanium silica ratios ge si and rare earth elements ree compositions of silica filling from the doushantuo cap carbonate in the yangtze platform south china the silica filling has low ge si ratios 1 mol mol and displays a light ree lree depleted ree pattern normalized to paas with positive eu anomalies eu eu these results indicate the predominant seawater origin of silica with some contribution from hydrothermal fluid suggesting the penecontemporaneous precipitation of silica filling with respect to sheet crack formation in order to quantify the contribution from hydrothermal fluid we developed a binary mixing model by using ge si ratio and eu eu as two independent proxies the modeling results indicate mixing of 5 vol hydrothermal fluid with contemporaneous seawater or 20 wt of si in silica filling from hydrothermal fluid the modeling results also show that the synglacial hydrothermal flux might be one order of magnitude lower than the modern level suggesting extremely weak hydrothermal activities during the marinoan snowball glaciation

oceanic germanium silicon ratios evaluation of the potential overprint of temperature on weathering signals 1 diatom remains indicate the oceanic ge si ratio mmol mol has varied temporally ranging from miocene values of 0 9 to last glacial maximum lgm values of 0 55 with the present value of 0 69 these ratios lie between those for the primary sources for these elements continental weathering ge si 0 5 and hydrothermal fluids ge si 11 previous explanations for temporal variation have focused on variations in the relative strength of the primary inputs alternatively the cause may be temporal variation in the relative strength of the two sinks for ge opal and a nonopal sink found in margin sediments the importance of the latter depends on the rate of opal rain to the seafloor in margin environments as temperature decreases opal dissolution rates in the water column decrease permitting a larger fraction of the opal rain to reach the seafloor before dissolving this increases the rate of ge uptake into nonopal phases a model calculation predicts that a decrease of 3 5 c in lgm ocean temperature should increase opal rain to the margin seafloor by a factor of 1 5 an increase of 3 5 ci n miocene temperature should decrease rain by a factor of 0 6 the predicted changes for ge uptake in the nonopal sink can account for essentially all of the temporal ge si variation recorded in diatoms the absence of larger changes constrains silicate weathering rates relative to the present as 106 16 during the lgm and 88 12 during the mid miocene 15 20 ma however additional uncertainty exists because some factors of potential importance remain unconstrained index terms 0330 atmospheric composition and structure geochemical cycles 1050 geochemistry marine geochemistry 4835 4850 1886 hydrology weathering 1625 4845 oceanography biological and chemical nutrients and nutrient cycling 4267 oceanography general paleoceanography keywords germanium silicon weathering

theoretical prediction for several important equilibrium ge isotope fractionation factors and geological implications article i nfo this study estimates equilibrium fractionation factors in the ge isotope system including the dominant aqueous ge oh 4 and geo oh 3 species in seawater ge bearing organic complexes e g ge catechol ge oxalic acid and ge citric acid and ge in quartz or opal albite k feldspar olivine and sphalerite like structures estimations are based on urey model or bigeleisen mayer equation and high level quantum

pore fluid fe isotopes reflect the extent of benthic fe redox recycling evidence from continental shelf and deep sea sediments pore fluid fe isotopes may be a unique tracer of sediment respiration by dissimilatory fe reducing bacteria but to date pore fluid fe isotope measurements have been restricted to continental shelf settings here we present 56fe values of pore fluids from two distinct sedimentary settings 1 a riverine dominated site on the northern california margin eel river shelf 120 m water depth and 2 biogenic opal rich volcaniclastic deep sea sediments from the southern ocean north and south of the crozet plateau 3000 4000 m water depth the fe isotope compositions of crozet region pore fluids are significantly less fractionated 56fe 0 12 to 0 01 than the eel river shelf 56fe 0 65 to 3 40 and previous studies of pore fluid fe isotopes relative to average igneous rocks our data represent the first measurements of fe isotope compositions in pore fluids from deep sea sediments a comparison of pore fluid 56fe with the relative abundance of highly labile fe in the reactive sedimentary fe pool demonstrates that the composition of fe isotopes in the pore fluids reflects the different extent of sedimentary fe redox recycling between these sites

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

coupled ge si and ge isotope ratios as geochemical tracers of seafloor hydrothermal systems case studies at loihi seamount and east pacific rise 9 50 n germanium ge and silicon si exhibit similar geochemical behavior in marine environments but are variably enriched in seafloor hydrothermal fluids relative to seawater in this study ge isotope and ge si ratio systematics were investigated in low temperature hydrothermal vents from loihi seamount pacific ocean 18 54 n 155 15 w and results were compared to high temperature vents from the east pacific rise epr at 9 50 n loihi offers the opportunity to understand contrasting ge and si behavior in low temperature seafloor hydrothermal systems characterized by abundant fe oxyhydroxide deposition at the seafloor the results show that both ge si and 74 70ge in hydrothermal fluids are fractionated relative to the basaltic host rocks the enrichment in ge vs si relative to fresh basalts together with ge isotope fractionation 74 70gefluid basalt up to 1 15 at epr 9 50 n and 1 64 at loihi are best explained by the precipitation of minerals e g quartz and fe sulfides during higher temperature seawater rock reactions in the subsurface the study of fe rich hydrothermal deposits at loihi largely composed of fe oxyhydroxides shows that ge isotopes are also fractionated upon mineral precipitation at the seafloor we obtained an average ge isotope fractionation factor between fe oxyhydroxide ferrihydrite and dissolved ge in the fluid of 2 0 0 6 2sd and a maximum value of 3 6 0 6 2sd which is consistent with recent theoretical and experimental studies the study of a hydrothermal chimney at bio 9 vent at epr 9 50 n also demonstrates that ge isotopes are fractionated by approximately 5 6 0 6 2sd during precipitation of metal sulfides under hydrothermal conditions using combined ge si and estimated ge isotope signatures of ge sinks and sources in seawater we propose a preliminary oceanic budget of ge which reveals that an important sink referred as the missing ge sink may correspond to ge sequestration into authigenic fe oxyhydroxides in marine sediments this study shows that combining ge si and 74 70ge systematics provides a useful tool to trace hydrothermal ge and si sources in marine environments and to understand formation processes of seafloor hydrothermal deposits

predicting benthic fluxes of silicic acid from deep sea sediments 1 benthic fluxes of silicic acid are driven by the dissolution of biogenic opal deposited at the seafloor the fluxes vary over several orders of magnitude and correlate inversely with the mass ratio of lithogenic matter to biogenic opal of the sediments the role of the lithogenic or detrital sediment fraction was studied in batch reactors by monitoring the release of silicic acid from mixtures of a well characterized biosiliceous ooze from the southern ocean and model lithogenic materials kaolinite or basalt the rate constant of silicic acid production and the saturation silicic acid concentration decreased systematically with increasing detrital to opal mass ratio of the mixtures both effects which are attributed to the release of soluble aluminum from lithogenic mineral phases were incorporated into an early diagenetic model to calculate diffusive benthic silicic acid fluxes from deep sea sediments the apparent silica dissolution rate constant the saturation or asymptotic pore water silicic acid concentration and the concentration of biogenic opal were the critical parameters determining the model derived benthic fluxes predicted benthic fluxes of silicic acid were compared to values reported for deep sea sediments with known detrital to opal ratio and asymptotic pore water concentration when taking into account the dependence of pore water silicic acid build up on the detrital sediment fraction good agreement was found between predicted and measured benthic fluxes over a wide range of oceanic areas and sediment composition from opal rich to opal poor sediments the model provides a means to estimate the benthic flux of silicic acid for deep sea sediments for which the asymptotic silicic acid concentration and the composition of the uppermost sediment are known

early diagenesis of organic matter in peru continental margin sediments phosphorite precipitation abstract pore water chemistry total dissolved co 2 nh 4 no 3 no 2 po 4 si oh 4 ca mg fe mn so 4 h 2 s and f and titration alkalinity solid phase chemistry c org p org c tot n tot f si opal and s ii and sediment characteristics porosity dry bulk density and formation factors were determined on a centimeter scale spacing in the upper 20 40 cm of sediments under intense upwelling areas on the peru continental shelf these data demonstrate that carbonate fluorapatite cfa is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content up to 20 c org very high porosity 0 96 ml cm 3 and very low dry bulk density 3 dissolved phosphate concentrations at the sediment water interface range from 20 to 100 m orders of magnitude higher than bottom water concentrations and much higher than predicted from regeneration of organic matter the mechanism of this interfacial phosphate release is unclear but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments fluoride is incorporated into cfa by diffusion from the overlying seawater and carbonate ions are provided from pore water alkalinity magnesium concentrations in this reaction zone are not significantly different from those of seawater suggesting that magnesium depletion is not a necessary prerequisite for cfa precipitation the environment of precipitation is interface linked rather than driven by organic diagenesis of phosphorus deeper in the sediment most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region but almost all show the precipitation signature near the interface this interface linked early diagenetic porewater environment for the precipitation of cfa explains many of the geochemical characteristics of phosphorites and provides a testable model to compare the modern phosphogenic analog with ancient phosphorite deposits two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites

determination of the mg mn ratio in foraminiferal coatings an approach to correct mg ca temperatures for mn rich contaminant phases the occurrence of manganese rich coatings on foraminifera can have a significant effect on their bulk mg ca ratios thereby biasing seawater temperature reconstructions the removal of this mn phase requires a reductive cleaning step but this has been suggested to preferentially dissolve mg rich biogenic carbonate potentially introducing an analytical bias in paleotemperature estimates in this study the geochemical composition of foraminifera tests from mn rich sediments from the antarctic southern ocean odp site 1094 was investigated using solution based and laser ablation icp ms in order to determine the amount of mg incorporated into the coatings the analysis of planktonic and benthic foraminifera revealed a nearly constant mg mn ratio in the mn coating of 0 2 mol mol consequently the coating mg mn ratio can be used to correct for the mg incorporated into the mn phase by using the down core mn ca values of samples that have not been reductively cleaned the consistency of the coating mg mn ratio obtained in this study as well as that found in samples from the panama basin suggests that spatial variation of mg mn in foraminiferal mn overgrowths may be smaller than expected from mn nodules and mn ca carbonates however a site specific assessment of the mg mn ratio in foraminiferal coatings is recommended to improve the accuracy of the correction