References

Biogeosciences

1726-4189

Copernicus Publications

Göttingen, Germany

10.5194/bg-11-5155-2014

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

Pogge von Strandmann

P. A. E.

https://orcid.org/0000-0001-7181-4165

¹ ² Forshaw

¹ ³ Schmidt

D. N.

Bristol Isotope Group, School of Earth Sciences, Bristol, UK

Institute of Earth and Planetary Sciences, University College London and Birkbeck University of London, Gower Street, London, WC1E 6BT, UK

now at: Department of Earth Sciences, Kingston University London, London, UK

25 09 2014

11 18 5155 5168

2014

This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/

This article is available from https://bg.copernicus.org/articles/11/5155/2014/bg-11-5155-2014.html

The full text article is available as a PDF file from https://bg.copernicus.org/articles/11/5155/2014/bg-11-5155-2014.pdf

Magnesium is an element critically involved in the carbon cycle, because weathering of Ca-Mg silicates removes atmospheric CO2 into rivers, and formation of Ca-Mg carbonates in the oceans removes carbon from the ocean-atmosphere system. Hence the Mg cycle holds the potential to provide valuable insights into Cenozoic climate-system history, and the shift during this time from a greenhouse to icehouse state. We present Mg isotope ratios for the past 40 Myr using planktic foraminifers as an archive. Modern foraminifera, which discriminate against elemental and isotopically heavy Mg during calcification, show no correlation between the Mg isotope composition (δ26Mg, relative to DSM-3) and temperature, Mg / Ca or other parameters such as carbonate saturation (ΔCO3). However, inter-species isotopic differences imply that only well-calibrated single species should be used for reconstruction of past seawater. Seawater δ26Mg inferred from the foraminiferal record decreased from ~0&permil; at 15 Ma, to −0.83&permil; at the present day, which coincides with increases in seawater lithium and oxygen isotope ratios. It strongly suggests that neither Mg concentrations nor isotope ratios are at steady state in modern oceans, given its ~10 Myr residence time. From these data, we have developed a dynamic box model to understand and constrain changes in Mg sources to the oceans (rivers) and Mg sinks (dolomitisation and hydrothermal alteration). Our estimates of seawater Mg concentrations through time are similar to those independently determined by pore waters and fluid inclusions. Modelling suggests that dolomite formation and the riverine Mg flux are the primary controls on the δ26Mg of seawater, while hydrothermal Mg removal and the δ26Mg of rivers are more minor controls. Using Mg riverine flux and isotope ratios inferred from the 87Sr / 86Sr record, the modelled Mg removal by dolomite formation shows minima in the Oligocene and at the present day (with decreasing trends from 15 Ma), both coinciding with rapid decreases in global temperatures.

Natural Environment Research Council

NE/I020571/1

References 1

Anand, P., Elderfield, H., and Conte, M. H.: Calibration of Mg / Ca thermometry in planktonic foraminifera from a sediment trap time series, Paleoceanography, 18, <a href="http://dx.doi.org/10.1029/2002PA000846">https://doi.org/10.1029/2002PA000846</a>, 2003.

Bentov, S. and Erez, J.: Impact of biomineralization processes on the Mg content of foraminiferal shells: A biological perspective, Geochem. Geophys. Geosyst., 7, <a href="http://dx.doi.org/10.1029/2005gc001015">https://doi.org/10.1029/2005gc001015</a>, 2006.

Berner, E. K. and Berner, R. A.: Global Environment: Water, Air and Geochemical Cycles, Prentice Hall, 1996.

Berner, R. A.: GEOCARB II: A revised model of atmospheric CO2 over Phanerozoic time, Am. J. Sci., 294, 56–91, 1994.

Berner, R. A., Lasaga, A. C., and Garrels, R. M.: The Carbonate-Silicate Geochemical Cycle and Its Effect on Atmospheric Carbon-Dioxide over the Past 100 Million Years, Am. J. Sci., 283, 641–683, 1983.

Black, J. R., Yin, Q. Z., and Casey, W. H.: An experimental study of magnesium-isotope fractionation in chlorophyll-a photosynthesis, Geochim. Cosmochim. Acta, 70, 4072–4079, 2006.

Bolou-Bi, E. B., Vigier, N., Poszwa, A., and Brenot, A.: Compared Mg isotope compositions of plants, rocks and waters, Geochim. Cosmochim. Acta, 71, A106, 2007.

Bolou-Bi, E. B., Vigier, N., Poszwa, A., Boudot, J.-P., and Dambrine, E.: Effects of biogeochemical processes on magnesium isotope variations in a forested catchment in the Vosges Mountains (France), Geochim. Cosmochim. Acta, 87, 341–355, 2012.

Busenberg, E. and Plummer, L. N.: Thermodynamics of magnesian calcite solid-solutions at 25 °C and 1 atm total pressure, Geochim. Cosmochim. Acta, 53, 1189-1208, <a href="http://dx.doi.org/10.1016/0016-7037(89)90056-2">https://doi.org/10.1016/0016-7037(89)90056-2</a>, 1989.

Chang, V. T. C., Williams, R. J. P., Makishima, A., Belshawl, N. S., and O'Nions, R. K.: Mg and Ca isotope fractionation during CaCO3 biomineralisation, Biochem. Bioph. Res. Co., 323, 79–85, 2004.

Coggon, R. M., Teagle, D. A. H., Smith-Duque, C. E., Alt, J. C., and Cooper, M. J.: Reconstructing Past Seawater Mg / Ca and Sr / Ca from Mid-Ocean Ridge Flank Calcium Carbonate Veins, Science, 327, 1114–1117, <a href="http://dx.doi.org/10.1126/science.1182252">https://doi.org/10.1126/science.1182252</a>, 2010.

Coltice, N., Seton, M., Rolf, T., Mueller, R. D., and Tackley, P. J.: Convergence of tectonic reconstructions and mantle convection models for significant fluctuations in seafloor spreading, Earth Planet. Sci. Lett., 383, 92–100, <a href="http://dx.doi.org/10.1016/j.epsl.2013.09.032">https://doi.org/10.1016/j.epsl.2013.09.032</a>, 2013.

Conrad, C. P. and Lithgow-Bertelloni, C.: Faster seafloor spreading and lithosphere production during the mid-Cenozoic, Geology, 35, 29–32, <a href="http://dx.doi.org/10.1130/g22759a.1">https://doi.org/10.1130/g22759a.1</a>, 2007.

de Nooijer, L. J., Spero, H. J., Erez, J., Bijma, J., and Reichart, G. J.: Biomineralization in perforate Foraminifera, Earth-Sci. Rev., 135, 48–58, 2014.

de Villiers, S., Dickson, J. A. D., and Ellam, R. M.: The composition of the continental river weathering flux deduced from seawater Mg isotopes, Chem. Geol., 216, 133–142, 2005.

Elderfield, H. and Ganssen, G.: Past temperature and δ18O of surface ocean waters inferred from foraminiferal Mg / Ca ratios, Nature, 405, 442–445, 2000.

Elderfield, H. and Schultz, A.: Mid-ocean ridge hydrothermal fluxes and the chemical composition of the ocean, Annu. Rev. Earth Pl. Sc., 24, 191-224, 1996.

Elderfield, H., Vautravers, M., and Cooper, M.: The relationship between shell size and Mg / Ca, Sr / Ca, δ18, and δ13C of species of planktonic foraminifera, Geochem. Geophys. Geosyst., 3, <a href="http://dx.doi.org/10.1029/2001gc000194">https://doi.org/10.1029/2001gc000194</a>, 2002.

Elderfield, H., Yu, J., Anand, P., Kiefer, T., and Nyland, B.: Calibrations for benthic foraminiferal Mg / Ca paleothermometry and the carbonate ion hypothesis, Earth Planet. Sci. Lett., 250, 633–649, <a href="http://dx.doi.org/10.1016/j.epsl.2006.07.041">https://doi.org/10.1016/j.epsl.2006.07.041</a>, 2006.

Erez, J.: The source of ions for biomineralization in foraminifera and their implications for paleoceanographic proxies, Rev. Mineral. Geochem., 54, 115–149, 2003.

Fantle, M. S. and DePaolo, D. J.: Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: Calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years, Geochim. Cosmochim. Acta, 70, 3883–3904, 2006.

Ferguson, J. E., Henderson, G. M., Kucera, M., and Rickaby, R. E. M.: Systematic change of foraminiferal Mg / Ca ratios across a strong salinity gradient, Earth Planet. Sci. Lett., 265, 153–166, <a href="http://dx.doi.org/10.1016/j.epsl.2007.10.011">https://doi.org/10.1016/j.epsl.2007.10.011</a>, 2008.

Foster, G. L., Pogge von Strandmann, P. A. E., and Rae, J. W. B.: The boron and magnesium isotopic composition of seawater, Geochem. Geophys. Geosyst., 11, Q08015, <a href="http://dx.doi.org/10.1029/2010GC003201">https://doi.org/10.1029/2010GC003201</a>, 2010.

Gaillardet, J., Dupre, B., Louvat, P., and Allegre, C. J.: Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers, Chem. Geol., 159, 3–30, 1999.

Galy, A., France-Lanord, C., and Derry, L. A.: The strontium isotopic budget of Himalayan Rivers in Nepal and Bangladesh, Geochim. Cosmochim. Acta, 63, 1905–1925, 1999.

Galy, A., Bar-Matthews, M., Halicz, L., and O'Nions, R. K.: Mg isotopic composition of carbonate: insight from speleothem formation, Earth Planet. Sci. Lett., 201, 105–115, 2002.

Georg, R. B., West, A. J., Vance, D., Newman, K., and Halliday, A. N.: Is the marine osmium isotope record a probe for CO2 release from sedimentary rocks?, Earth Planet. Sci. Lett., 367, 28–38, 2013.

Geske, A., Zorlu, J., Richter, D. K., Buhl, D., Niedermayr, A., and Immenhauser, A.: Impact of diagenesis and low grade metamorphosis on isotope (δ26Mg, δ13C, δ18O and 87Sr / 86Sr) and elemental (Ca, Mg, Mn, Fe and Sr) signatures of Triassic sabkha dolomites, Chem. Geol., 332–333, 45–64, 2012.

Hay, W. W.: The Pleistocene-Holocene fluxes are not the Earth's norm, in: Material fluxes on the surface of the Earth, National Academy Press, Washington D.C., 1994.

Higgins, J. A. and Schrag, D. P.: Constraining magnesium cycling in marine sediments using magnesium isotopes, Geochim. Cosmochim. Ac., 74, 5039–5053, 2010a.

Higgins, J. A. and Schrag, D. P.: Constraining magnesium cycling in marine sediments using magnesium isotopes, Geochim. Cosmochim. Ac., 74, 2039–5053, 2010b.

Higgins, J. A. and Schrag, D. P.: Records of Neogene seawater chemistry and diagenesis in deep-sea carbonate sediments and pore fluids, Earth Planet. Sci. Lett., 357–358, 386–396, 2012.

Hippler, D., Buhl, D., Witbaard, R., Richter, D. K., and Immenhauser, A.: Towards a better understanding of magnesium-isotope ratios from marine skeletal carbonates, Geochim. Cosmochim. Ac., 73, 6134–6146, <a href="http://dx.doi.org/10.1016/j.gca.2009.07.031">https://doi.org/10.1016/j.gca.2009.07.031</a>, 2009.

Holland, H. D.: Sea level, sediments and the composition of seawater, Am. J. Sci., 305, 220–239, 2005.

Holland, H. D. and Zimmermann, H.: The Dolomite Problem Revisited, Int. Geol. Rev., 42, 481–490, 2000.

Horita, J., Zimmermann, H., and Holland, H. D.: Chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites, Geochim. Cosmochim. Ac., 66, 3733–3756, 2002.

Huang, K. J., Teng, F. Z., Wei, G. J., Ma, J. L., and Bao, Z. Y.: Adsorption- and desorption-controlled magnesium isotope fractionation during extreme weathering of basalt in Hainan Island, China, Earth Planet. Sci. Lett., 359–360, 73–83, 2012.

Immenhauser, A., Buhl, D., Richter, D., Niedermayr, A., Riechelmann, D., Dietzel, M., and Schulte, U.: Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave – Field study and experiments, Geochim. Cosmochim. Ac., 74, 4346–4364, <a href="http://dx.doi.org/10.1016/j.gca.2010.05.006">https://doi.org/10.1016/j.gca.2010.05.006</a>, 2010.

John, C. M., Karner, G. D., Browning, E., Mark Leckie, R., and Mateo, Z.: Timing and magnitude of Miocene eustasy derived from the mixed siliciclastic-carbonate stratigraphic record of the northeastern Australian margin, Earth Planet. Sci. Lett., 304, 455–467, 2011.

Kisakürek, B., James, R. H., and Harris, N. B. W.: Li and δ7Li in Himalayan rivers: Proxies for silicate weathering?, Earth Planet. Sci. Lett., 237, 387–401, 2005.

Kozdon, R., Kelly, D. C., Kitajima, K., Strickland, A., Fournelle, J. H., and Valley, J. W.: In situ δ18O and Mg / Ca analyses of diagenetic and planktic foraminiferal calcite preserved in a deep-sea record of the Paleocene-Eocene thermal maximum, Paleoceanography, 28, 517–528, <a href="http://dx.doi.org/10.1002/palo.20048">https://doi.org/10.1002/palo.20048</a>, 2013.

Kump, L. R., Brantley, S. L., and Arthur, M. A.: Chemical, weathering, atmospheric CO2, and climate, Ann. Rev. Earth Pl. Sc., 28, 611–667, 2000.

Lea, D. W., Mashiotta, T. A., and Spero, H. J.: Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing, Geochim. Cosmochim. Ac., 63, 2369–2379, 1999.

Lear, C. H., Elderfield, H., and Wilson, P. A.: Cenozoic deep-sea temperatures and global ice volumes from Mg / Ca in benthic foraminiferal calcite, Science, 287, 269–272, <a href="http://dx.doi.org/10.1126/science.287.5451.269">https://doi.org/10.1126/science.287.5451.269</a>, 2000.

Lear, C. H., Elderfield, H., and Wilson, P. A.: A Cenozoic seawater Sr / Ca record from benthic foraminiferal calcite and its application in determining global weathering fuxes, Earth Planet. Sci. Lett., 208, 69–84, 2003.

Levasseur, S., Birck, J. L., and Allègre, C. J.: The osmium riverine flux and the oceanic mass balance of osmium, Earth Planet. Sci. Lett., 174, 7–23, 1999.

Li, G. and West, A. J.: Evolution of Cenozoic seawater lithium isotopes: Coupling of global denudation regime and shifting seawater sinks, Earth Planet. Sci. Lett., 401, 284–293, 2014.

Li, G., Ji, J., Chen, J., and Kemp, D. B.: Evolution of the Cenozoic carbon cycle: The roles of tectonics and CO2 fertilization, Global Biogeochem. Cy., 23, GB1009, <a href="http://dx.doi.org/10.1029/2008GB003220">https://doi.org/10.1029/2008GB003220</a>, 2009a.

Li, G., Ji, J., and Kemp, D. B.: Evolution of the Cenozoic carbon cycle: The roles of tectonics and CO2 fertilization, Global Biogeochem. Cy., 23, GB1009, <a href="http://dx.doi.org/10.1029/2008GB003220">https://doi.org/10.1029/2008GB003220</a>, 2009b.

Li, W. Q., Chakraborty, S., Beard, B. L., Romanek, C. S., and Johnson, C. M.: Magnesium isotope fractionation during precipitation of inorganic calcite under laboratory conditions, Earth Planet. Sci. Lett., 333, 304–316, <a href="http://dx.doi.org/10.1016/j.epsl.2012.04.010">https://doi.org/10.1016/j.epsl.2012.04.010</a>, 2012.

Li, W.-Y., Teng, F. Z., Ke, S., Rudnick, R. L., Gao, S., Wu, F.-Y., and Chappell, B. W.: Heterogeneous magnesium isotopic composition of the upper continental crust, Geochim. Cosmochim. Ac., 74, 6867–6884, 2010.

Martinez-Boti, M. A., Mortyn, P. G., Schmidt, D. N., Vance, D., and Field, D. B.: Mg / Ca in foraminifera from plankton tows: Evaluation of proxy controls and comparison with core tops, Earth Planet. Sci. Lett., 307, 113–125, <a href="http://dx.doi.org/10.1016/j.epsl.2011.04.019">https://doi.org/10.1016/j.epsl.2011.04.019</a>, 2011.

McArthur, J. M., Howarth, R. J., and Bailey, T. R.: Strontium isotope stratigraphy: LOWESS version 3: Best fit to the marine Sr-isotope curve for 0-509 Ma and accompanying look-up table for deriving numerical age, J. Geol., 109, 155–170, 2001.

Millot, R., Vigier, N., and Gaillardet, J.: Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada, Geochim. Cosmochim. Ac., 74, 3897–3912, 2010.

Misra, S. and Froelich, P. N.: Lithium Isotope History of Cenozoic Seawater: Changes in Silicate Weathering and Reverse Weathering, Science, 335, 818–823, 2012.

Mottl, M. J. and Wheat, C. G.: Hydrothermal circulation through mid-ocean ridge flanks: Fluxes of heat and magnesium, Geochim. Cosmochim. Ac., 58, 2225–2237, 1994.

Mucci, A.: Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater, Geochim. Cosmochim. Ac., 51, 1977–1984, <a href="http://dx.doi.org/10.1016/0016-7037(87)90186-4">https://doi.org/10.1016/0016-7037(87)90186-4</a>, 1987.

Muller, R. D., Sdrolias, M., Gaina, C., Steinberger, B., and Heine, C.: Long-term sea-level fluctuations driven by ocean basin dynamics, Science, 319, 1357–1362, <a href="http://dx.doi.org/10.1126/science.1151540">https://doi.org/10.1126/science.1151540</a>, 2008.

Nehrke, G., Keul, N., Langer, G., de Nooijer, L. J., Bijma, J., and Meibom, A.: A new model for biomineralization and trace-element signatures of Foraminifera tests, Biogeosciences, 10, 6759–6767, <a href="http://dx.doi.org/10.5194/bg-10-6759-2013">https://doi.org/10.5194/bg-10-6759-2013</a>, 2013.

Ni, Y., Foster, G. L., Bailey, T., Elliott, T., Schmidt, D. N., Pearson, P. N., Haley, B., and Coath, C. D.: A core top assessment of proxies for the ocean carbonate system in surface-dwelling foraminifers, Paleoceanography, 22, PA3212, 2007.

Nürnberg, D., Bijma, J., and Hemleben, C.: Assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperatures, Geochim. Cosmochim. Ac., 60, 2483–2483, 1996.

Oliver, L., Harris, N., Bickle, M., Chapman, H., Dise, N., and Horstwood, M.: Silicate weathering rates decoupled from the 87Sr / 86Sr ratio of the dissolved load during Himalayan erosion, Chem. Geol., 201, 119–139, 2003.

Opdyke, B. N. and Wilkinson, B. H.: Surface area control of shallow cratonic to deep marine carbonate accumulation, Paleoceanography, 3, 685–703, 1988.

Palmer, M. R. and Edmond, J. M.: Controls over the strontium isotope composition of river water, Geochim. Cosmochim. Ac., 56, 2099–2111, 1992.

Pogge von Strandmann, P. A. E.: Precise magnesium isotope measurements in core top planktic and benthic foraminifera, Geochem. Geophys. Geosyst., 9, Q12015, <a href="http://dx.doi.org/10.1029/2008GC002209">https://doi.org/10.1029/2008GC002209</a>, 2008.

Pogge von Strandmann, P. A. E., Burton, K. W., James, R. H., van Calsteren, P., and Gislason, S. R.: The influence of weathering processes on riverine magnesium isotopes in a basaltic terrain, Earth Planet. Sci. Lett., 276, 187–197, 2008.

Pogge von Strandmann, P. A. E., Elliott, T., Marschall, H. R., Coath, C., Lai, Y. J., Jeffcoate, A. B., and Ionov, D. A.: Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths, Geochim. Cosmochim. Ac., 75, 5247–5268, 2011.

Pogge von Strandmann, P. A. E., Opfergelt, S., Lai, Y. J., Sigfusson, B., Gislason, S. R., and Burton, K. W.: Lithium, magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in Iceland, Earth Planet. Sci. Lett., 339–340, 11–23, 2012.

Ra, K., Kitagawa, H., and Shiraiwa, Y.: Mg isotopes in chlorophyll-a and coccoliths of cultured coccolithophores (Emiliania huxleyi) by MC-ICP-MS, Mar. Chem., 122, 130–137, 2010.

Raitzsch, M., Dueñas-Bohórquez, A., Reichart, G.-J., de Nooijer, L. J., and Bickert, T.: Incorporation of Mg and Sr in calcite of cultured benthic foraminifera: impact of calcium concentration and associated calcite saturation state, Biogeosciences, 7, 869–881, <a href="http://dx.doi.org/10.5194/bg-7-869-2010">https://doi.org/10.5194/bg-7-869-2010</a>, 2010.

Richter, F. M., Mendybaev, R. A., Christensen, J. N., Hutcheon, I. D., Williams, R. W., Sturchio, N. C., and Belsos Jr., A. D.: Kinetic isotopic fractionation during diffusion of ionic species in water, Geochim. Cosmochim. Ac., 70, 277–289, 2006.

Saenger, C. and Wang, Z.: Magnesium isotope fractionation in biogenic and abiogenic carbonates: implications for paleoenvironmental proxies, Quaternary Sci. Rev., 90, 1–21, 2014.

Saulnier, S., Rollion-Bard, C., Vigier, N., and Chaussidon, M.: Mg isotope fractionation during calcite precipitation: An experimental study, Geochim. Cosmochim. Ac., 91, 75–91, <a href="http://dx.doi.org/10.1016/j.gca.2012.05.024">https://doi.org/10.1016/j.gca.2012.05.024</a>, 2012.

Shackleton, N. J. and Kennett, J. P.: Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: oxygen and carbon isotope analysis is DSDP Sites 277, 279, and 281, in: Initial Reports of the Deep Sea Drilling Project, US Government Printing Office, 1975.

Teng, F. Z., Li, W. Y., Ke, S., Marty, B., Dauphas, N., Huang, S., Wu, F.-Y., and Pourmand, A.: Magnesium isotopic composition of the Earth and chondrites, Geochim. Cosmochim. Ac., 74, 4150–4166, 2010a.

Teng, F. Z., Li, W. Y., Rudnick, R. L., and Gardner, L. R.: Contrasting lithium and magnesium isotope fractionation during continental weathering, Earth Planet. Sci. Lett., 300, 63–71, 2010b.

ter Kuile, B.: Mechanisms for calcification and carbon cycling in algal symbiont-bearing foraminifera, in: Biology of Foraminifera, edited by: Lee, J. J. and Anderson, O. R., Academic Press Limited, 73–92, 1991.

Tipper, E. T., Galy, A., and Bickle, M. J.: Riverine evidence for a fractionated reservoir of Ca and Mg on the continents: Implications for the oceanic Ca cycle, Earth Planet. Sci. Lett., 247, 267–279, 2006a.

Tipper, E. T., Galy, A., Gaillardet, J., Bickle, M. J., Elderfield, H., and Carder, E. A.: The magnesium isotope budget of the modern ocean: Constraints from riverine magnesium isotope ratios, Earth Planet. Sci. Lett., 250, 241–253, 2006b.

Tipper, E. T., Galy, A., and Bickle, M.: Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?, Geochim. Cosmochim. Ac., 72, 1057–1075, 2008.

Tipper, E. T., Gaillardet, J., Louvat, P., Capmas, F., and White, A. F.: Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California, Geochim. Cosmochim. Ac., 74, 3883–3896, 2010.

Tipper, E. T., Calmels, D., Gaillardet, J., Louvat, P., Capmas, F., and Dubacq, B.: Positive correlation between Li and Mg isotope ratios in the river waters of the Mackenzie Basin challenges the interpretation of apparent isotopic fractionation during weathering, Earth Planet. Sci. Lett., 333, 35–45, <a href="http://dx.doi.org/10.1016/j.epsl.2012.04.023">https://doi.org/10.1016/j.epsl.2012.04.023</a>, 2012.

Vance, D., Teagle, D. A. H., and Foster, G. L.: Variable Quaternary chemical weathering fluxes and imbalances in marine geochemical budgets, Nature, 458, 493–496, 2009.

Walker, J. C. G., Hays, P. B., and Kasting, J. F.: A Negative Feedback Mechanism for the Long-Term Stabilization of Earths Surface-Temperature, J. Geophys. Res.-Oc. Atm., 86, 9776–9782, 1981.

Wanner, C., Sonnenthal, E. L., and Liu, X.-M.: Seawater δ7Li: A direct proxy for global CO2 consumption by continental silicate weathering?, Chem. Geol., 381, 154–167, 2014.

Wimpenny, J., Burton, K. W., James, R. H., Gannoun, A., Mokadem, F., and Gislason, S. R.: The behaviour of magnesium and its isotopes during glacial weathering in an ancient shield terrain in West Greenland, Earth Planet. Sci. Lett., 304, 260–269, <a href="http://dx.doi.org/10.1016/j.epsl.2011.02.008">https://doi.org/10.1016/j.epsl.2011.02.008</a>, 2011.

Wombacher, F., Eisenhauer, A., Bohm, F., Gussone, N., Regenberg, M., Dullo, W. C., and Ruggeberg, A.: Magnesium stable isotope fractionation in marine biogenic calcite and aragonite, Geochim. Cosmochim. Ac., 75, 5797–5818, <a href="http://dx.doi.org/10.1016/j.gca.2011.07.017">https://doi.org/10.1016/j.gca.2011.07.017</a>, 2011.

Yoshimura, T., Taniguchi, M., Inoue, M., Suzuki, A., Iwasaki, N., and Kawahata, H.: Mg isotope fractionation in biogenic carbonates of deep-sea coral, benthic foraminifera, and hermatypic coral, Anal. Bioanal. Chem., 401, 2755–2769, 2011.

Young, E. D. and Galy, A.: The isotope geochemistry and cosmochemistry of magnesium, in: Geochemistry of non-traditional stable isotopes, edited by: Johnson, C. M., Beard, B. L., and Albarede, F., Mineralogical Society of America, Geochemical Society, Washington D.C., 197–230, 2004.

Zachos, J. C., Pagani, M., Sloan, L., Thomas, E., and Billups, K.: Trends, rhythms, and aberrations in global climate 65 Ma to present, Science, 292, 686–693, 2001.

Zachos, J. C., Kroon, D., Blum, P., Bowles, J., Gaillot, P., Hasegawa, T., Hathorne, E. C., Hodell, D. A., Kelly, D. C., Jung, J.-H., Keller, S. M., Lee, Y. S., Leuschner, D. C., Liu, Z., Lohmann, K. C., Lourens, L., Monechi, S., Nicolo, M., Raffi, I., Riesselman, C., Röhl, U., Schellenberg, S. A., Schmidt, D., Sluijs, A., Thomas, D., Thomas, E., and Vallius, H.: Proceedings of the Ocean Drilling Program, Initial Reports Volume 208, College Station TX (Ocean Drilling Program), 1–112, 2004.

Zimmermann, H.: Tertiary seawater chemistry – Implications from primary fluid inclusions in marine halite, Am. J. Sci., 300, 723–767, 2000.