Mineralogical Magazine, February 2008, Vol. 72(1), pp. 363–366

Silicate in anoxic marine

1, 2 1 3 3 4 1 K. WALLMANN *, G. ALOISI , M. HAECKEL ,P.TISHCHENKO ,G.PAVLOVA ,J.GREINERT ,S.KUTTEROLF AND 1 A. EISENHAUER 1 IFM-GEOMAR Leibniz Institute of Marine Sciences, Wischhofstrasse 1-3, 24148 Kiel, Germany 2 Laboratoire de Pale´oenvironnements et Pale´obiosphe`re, Universite´ Claude Bernard À Lyon I, 2, rue Dubois, F- 69622 Villeurbanne cedex, France 3 Pacific Oceanological Institute (POI), 43, Baltiyskaya Street, 690041 Vladivostok, Russia 4 Renard Centre of Marine Geology Department of Geology and Soil Science, Ghent University, Krijgslaan 281 s.8 B-9000 Ghent, Belgium Introduction reverse weathering was the dominant process in TWO cores retrieved at the northern the overlying surface sediments. Depth-integrated slope of Sakhalin Island, Sea of Okhotsk, were rates of marine weathering in methano- analysed for biogenic opal, organic carbon, genic sediments derived from the model À2 À1 carbonate, sulphur, major element concentrations, (81.4À99.2 mmol CO2 m yr ) are smaller mineral contents and dissolved substances than the marine weathering rates calculated from including nutrients, sulphate, methane, major the solid phase data (198À245 mmol À2 À1 cations, humic substances and total alkalinity. CO2 m yr ) suggesting a decrease in marine Down-core trends in mineral abundance suggest weathering over time. The production of CO2 that plagioclase feldspars and other reactive through reverse weathering in surface sediments À2 À1 silicate phases (olivine, pyroxene, volcanic ash) (4.22À15.0 mmol CO2 m yr ) is approxi- are transformed into smectite in the methanogenic mately one order of magnitude smaller than the sediment sections. The element ratios Na/Al, weathering-induced CO2 consumption in the Mg/Al and Ca/Al in the solid phase decrease underlying sediments. The evaluation of pore with sediment depth indicating a loss of mobile water data from other continental margin sites cations with depth and producing a significant shows that silicate weathering is a common down-core increase in the chemical index of process in methanogenic sediments. The global alteration. Pore waters separated from the rate of CO2 consumption through marine silicate sediment cores are highly enriched in dissolved weathering estimated here as 5À20 Tmol À1 Mg, total alkalinity, humic substances and boron. CO2 yr is as large as the global rate of The large contents of dissolved organic carbon in continental silicate weathering. Details of the the deeper methanogenic sediment sections present work can be found in (Wallmann et al., (50À150 mg dmÀ3) may promote the dissolution 2008). of silicate phases through complexation of Al3+ and other structure-building cations. A non-steady state transport-reaction model was developed and Methods applied to evaluate the down-core trends observed Sediments were taken by piston coring at the in the solid and dissolved phases. Dissolved Mg northern continental slope of Sakhalin Island and total alkalinity were used to track the in-situ during cruise SO178 with RV SONNE in rates of marine silicate weathering since thermo- JulyÀSeptember 2004. Core KL-13-6 was dynamic equilibrium calculations showed that retrieved at a water depth of 713 m (position: these tracers are not affected by exchange 52º43.88’N; 144º42.65’E) while core KL-29-2 processes with sediment surfaces. The modelling was taken further north (53º50.00’N; showed that silicate weathering is limited to the 144º14.23’E) at 771 m water depth. Pore fluids deeper methanogenic sediment section whereas were processed and analysed as previously described (Wallmann et al., 2006). Solids were characterized by polarization microscopy, XRD * E-mail: [email protected] and XRF. A numerical transport-reaction model DOI: 10.1180/minmag.2008.072.1.363 was applied to the pore water data set to calculate

# 2008 The Mineralogical Society K. WALLMANN ET AL.

rates of organic matter degradation, sulphate greater than the rates of reverse weathering and reduction, methane production, anaerobic oxida- cation exchange in Sakhalin slope sediments and tion of methane, carbonate precipitation and significantly larger than the rates of continental dissolution, and silicate weathering. Within this silicate weathering in the hinterland drained by modelling framework, down-core changes in the the Amur River. rate of silicate weathering were traced and tracked Pore water data obtained at other productive applying the concentration profiles of dissolved continental margin sites demonstrate that silicate Mg and total alkalinity (Wallmann et al., 2008). weathering is a common process in methanogenic sediments with high metabolic activity. Most of the CO2 formed during the anaerobic breakdown Results and discussion of organic matter is converted into alkalinity and Rapidly accumulating sediments composed of authigenic carbonates (Wallmann et al., 2008). biogenic opal, organic matter, plagioclase feld- Thus, silicate weathering in methanogenic marine spars, olivine, pyroxene, volcanic glass, clay sediments constitutes a significant sink for CO2 minerals and other products of continental weath- and a source for carbonate alkalinity that needs to ering are deposited at the Sakhalin slope. Reverse be included in future budgets of the global and weathering occurring in the upper section of the marine carbon cycles. The global rate of marine studied sediment cores induces the fixation of silicate weathering estimated as 5À20 Tmol 2+ + + + À1 dissolved cations (Mg ,Na,K,Li)in CO2 yr may be as high as the global CO2 authigenic clays and the consumption of carbo- consumption through continental weathering. nate alkalinity. Ammonium released during the breakdown of organic matter is adsorbed at sediment surfaces replacing adsorbed Ca2+ and Conclusions and perspectives Na+ that are released into the pore water via Marine sediments have previously been regarded cation exchange. Reactive (plagioclase as important CO2 sinks since large amounts of feldspars, olivine, pyroxene, volcanic glass) are organic carbon formed though photosynthesis are partly dissolved and transformed into cation- ultimately buried at the seafloor. It has, however, depleted silicate phases in the deeper methano- been ignored that the degradation of organic genic sediment sections (Fig. 1). matter in anoxic sediments works in a fundamen- The marine weathering of these silicates tally different way than aerobic degradation. 2+ 2+ + induces the release of Mg ,Ca and Na as While aerobic respiration recycles CO2 into the well as the formation of authigenic carbonates and atmosphere, anaerobic degradation, (Fig. 2). The CO2 which formed during carbonate coupled to marine silicate weathering, produces precipitation and breakdown of organic matter in large amounts of carbonate alkalinity and methanogenic sediments is completely neutra- authigenic carbonates. Thus, CO2 fixed in lized during silicate weathering. Rates of marine organic matter during primary production is not silicate weathering are one order of magnitude recycled but transformed into other carbon

FIG. 1. Composition of surface sediments and core base sediments retrieved at the northern Sakhalin slope (KL-29-2).

364 SILICATE WEATHERING IN ANOXIC MARINE SEDIMENTS

FIG. 2. Composition of pore fluids in sediments retrieved at the northern Sakhalin slope (KL-29-2). Red squares indicate data and blue curves represent the model results. TA is total alkalinity and RWE gives the rate of CO2 consumption via silicate weathering.

À À compounds (HCO3 ,CaCO3) during anoxic HCO3 inputs and through carbonate burial at the diagenesis. The global rate of carbon transforma- seafloor. The alkalinity budget of the Holocene tion in anoxic sediments is estimated here to be seems to be out of balance since TA 7.6À28.8 Tmol C yrÀ1. This value is at least as removal through carbonate burial (48 Tmol yrÀ1; large as the global rate of organic carbon burial in Berner and Berner, 1996) is larger than the marine sediments (13 Tmol C yrÀ1; (Hedges and riverine TA flux (38 Tmol yrÀ1; Berner and Keil, 1995) and therefore this previously ignored Berner, 1996). Part of this imbalance is caused by CO2 sink has to be considered in future budgets of glacial to interglacial sea-level changes affecting global carbon cycling. carbonate burial on the continental shelves. The The total alkalinity (TA) inventory of the TA generated in anoxic marine sediments through oceans is believed to be controlled by riverine silicate weathering and sulphur burial

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À1 (4.3À15.5 Tmol yr ) has, however, never been down of organic matter. Small CO2 contents in considered and may in fact account for a major natural gas and gas hydrates may be caused by portion of the apparent TA imbalance. marine weathering processes. Hence, the CO2 Marginal seas are known to act as CO2 pumps consumption via marine silicate weathering taking up atmospheric CO2 and exporting should be considered in basin modelling to carbonate alkalinity into the open ocean better predict the CO2 contents of economically (Tsunogai et al., 1999). This continental-shelf important natural gas and gas hydrate reservoirs. pump operating in the Sea of Okhotsk (Otsuki et Moreover, marine sediments are increasingly used al., 2003) and in other marginal seas (Thomas et for the disposal of CO2 separated from natural gas al., 2004) may remove large quantities of and in coal power plants (House et al., 2006). The anthropogenic CO2 from the atmosphere. Anoxic results presented in this study imply that sediments that are preferentially deposited at terrigenous sediments with large contents of productive continental margins may contribute reactive silicate phases might be well-suited for to the CO2 uptake through marine silicate sites of CO2 disposal since CO2 may be rapidly weathering and sulphur burial. neutralized by marine silicate weathering. The global rate of authigenic carbonate burial in anoxic marine sediments estimated here as À1 References 3.3À13.3 Tmol CaCO3 yr approaches the modern rate of pelagic carbonate burial (10 Archer, D.E. (1996) An atlas of the distribution of À1 Tmol CaCO3 yr ; (Archer, 1996). Productive calcium carbonate in sediments of the deep sea. continental margins with high rates of terrigenous Global Biogeochemical Cycles, 10, 159À174. are thus important and previously Berner, R.A. (2004) The Phanerozoic Carbon Cycle: ignored sites of carbonate accumulation. CO2 and O2. Oxford University Press. Approximately 50% of the Ca buried in authi- Berner,E.K.andBerner,R.A.(1996)Global genic carbonates originates from seawater such Environment: Water, Air and Geochemical Cycles. that the Ca balance of the ocean is also affected Prentice Hall. by anoxic diagenesis. Hedges, J.I. and Keil, R.G. (1995) Sedimentary organic In contrast to continental weathering, marine matter preservation: an assessment and speculative weathering is not directly coupled to average synthesis. Marine Chemistry, 49,81À115. global surface temperature and atmospheric House, K.Z., Schrag, D.P., Harvey, C.F. and Lackner, pCO2. Carbon transformations in anoxic sedi- K.S. (2006) Permanent storage in ments are rather fueled by the deposition of deep-sea sediments. PNAS, 103, 12291À12295. particulate organic matter and reactive silicate Otsuki, A.S., Watanabe, S. and Tsunogai, S. (2003) phases. As a result the consumption of CO2 in Adsorption of atmospheric CO2 and its transport to these sediments is controlled by continental the intermediate layer in the Okhotsk Sea. Journal of erosion and marine productivity. The negative Oceanography, 59, 709À717. climate feedback established by the temperature- Thomas, H., Bozec, Y., Elkalay, K. and De Baar, H.J.W. and pCO2-dependent rate of continental weath- (2004) Enhanced open ocean storage of CO2 from ering (Berner, 2004) is weakened by marine shelf sea pumping. Science, 304, 1005À1008. weathering processes since reactive silicate Tsunogai, S., Watanabe, S. and Sato, T. (1999) Is there a phases which are not consumed on land may be "continental shelf pump" for the absorption of weathered in methanogenic sediments. Marine atmospheric CO2? Tellus, B51, 701À712. weathering might, thus, amplify climate change Wallmann, K., Aloisi, G., Haeckel, M., Obzhirov, A., on geological time scales and could, for example, Pavlova, G. and Tishchenko, P. (2006) Kinetics of contribute to the draw-down of atmospheric CO2 organic matter degradation, microbial methane observed during the late Cenozoic and glacial generation, and gas hydrate formation in anoxic periods of the Quaternary. marine sediments. Geochimica et Cosmochimica The results presented in this study also have Acta, 70, 3905À3927. major implications for the applied geosciences. Wallmann, K., Aloisi, G., Haeckel, M., Tishchenko, P., Natural gas hydrates and gas reservoirs formed in Pavlova,G.,Greinert,J.,Kutterolf,S.and marine sediments contain large amounts of CH4 Eisenhauer, A. (2008) Silicate weathering in anoxic but usually much less CO2 even though both gases marine sediments. Geochimica et Cosmochimica are produced at equi-molar rates during the break- Acta, 72, 3067À3090.

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