Sr Record of Seawater : New Constraints on 1 Past Changes in Oceanic Carbonate Fluxes 2 3

16 The isotopic composition of Phanerozoic marine sediments provides important 17 information about changes in seawater chemistry. In particular, the radiogenic 18 strontium isotope (Sr/Sr) system is a powerful tool for constraining plate tectonic 19 processes and their influence on atmospheric CO2 concentrations. However, the 20 Sr/Sr isotope ratio of seawater is not sensitive to temporal changes in the marine 21 strontium (Sr) output flux, which is primarily controlled by the burial of calcium 22 carbonate (CaCO3) at the ocean floor. The Sr budget of the Phanerozoic ocean, 23 including the associated changes in the amount of CaCO3 burial, is therefore only 24 poorly constrained. Here, we present the first stable isotope record of Sr for 25 Phanerozoic skeletal carbonates, and by inference for Phanerozoic seawater 26 (δSrsw), which we find to be sensitive to imbalances in the Sr input and output 27 fluxes. This δSrsw record varies from ~0.25‰ to ~0.60‰ (vs. SRM987) with a 28 mean of ~0.37‰. The fractionation factor between modern seawater and skeletal 29 1 Present address: Center for Space and Habitability + Institut für Geologie, Universität Bern, Baltzerstrasse 1 + 3, 3012 Bern, Switzerland 2 Present address: Leyegasse 4, 69117 Heidelberg, Germany * Corresponding author. Phone: +41 31 631 8533, fax: +41 31 631 4843 E-mail addresses: hauke.vollstaedt@csh.unibe. ch (H. Vollstaedt), aeisenhauer@geomar.de (A. Eisenhauer), kwallmann@geomar.de (K. Wallmann), fboehm@geomar.de (F. Böhm), jfietzke@geomar.de (J. Fietzke), vliebetrau@geomar.de (V. Liebetrau), Andre.Krabbenhoeft@tesa.com (A. Krabbenhöft), juraj.farkas@gelogy.cz (J. Farkaš), Adam.Tomasovych@savba.sk (A. Tomašových), jraddatz@geomar.de (J. Raddatz), jveizer@uottawa.ca (J. Veizer) s u r c e : h t t p s : / / d o i . o r g / 1 0 . 7 8 9 2 / b o r i s . 4 3 7 1 1 | d o w n l o a d e d : 1 8 . 1 1 . 2 0 1 9 calcite ∆Srcc-sw, based on the analysis of 13 modern brachiopods (mean δSr 30 of 0.176±0.016‰, 2 standard deviations (s.d.)), is -0.21‰ and was found to be 31 independent of species, water temperature, and habitat location. Overall, the 32 Phanerozoic δSrsw record is positively correlated with the Ca isotope record 33 (δCasw), but not with the radiogenic Sr isotope record ((Sr/Sr)sw). A new 34 numerical modeling approach, which considers both δSrsw and (Sr/Sr)sw, 35 yields improved estimates for Phanerozoic fluxes and concentrations for seawater Sr. 36 The oceanic net carbonate flux of Sr (F(Sr)carb) varied between an output 37 of -4.7x10mol/Myr and an input of +2.3x10mol/Myr with a mean 38 of -1.6x10mol/Myr. On time scales in excess of 100Myrs the F(Sr)carb is proposed to 39 have been controlled by the relative importance of calcium carbonate precipitates 40 during the “aragonite” and “calcite” sea episodes. On time scales less than 20Myrs 41 the F(Sr)carb seems to be controlled by variable combinations of carbonate burial rate, 42 shelf carbonate weathering and recrystallization, ocean acidification, and ocean 43 anoxia. In particular, the Permian/Triassic transition is marked by a prominent 44 positive δSrsw-peak that reflects a significantly enhanced burial flux of Sr and 45 carbonate, likely driven by bacterial sulfate reduction (BSR) and the related alkalinity 46 production in deeper anoxic waters. We also argue that the residence time of Sr in 47 the Phanerozoic ocean ranged from ~1Myrs to ~20Myrs. 48