Calcium carbonate veins in ocean crust record a threefold increase of seawater Mg/Ca in the past 30 million years

Abstract

Chemical (Sr, Mg) and isotopic (δ18O, 87Sr/86Sr) compositions of calcium carbonate veins (CCV) in the oceanic basement were determined to reconstruct changes in Sr/Ca and Mg/Ca of seawater in the Cenozoic. We examined CCV from ten basement drill sites in the Atlantic and Pacific, ranging in age between 165 and 2.3 Ma. Six of these sites are from cold ridge flanks in basement <46 Ma, which provide direct information about seawater composition. CCV of these young sites were dated, using the Sr isotopic evolution of seawater. For the other sites, temperature-corrections were applied to correct for seawater-basement exchange processes. The combined data show that a period of constant/low Sr/Ca (4.46 -6.22 mmol/mol) and Mg/Ca (1.12 -2.03 mol/mol) between 165 and 30 Ma was followed by a steady increase in Mg/Ca ratios by a factor of three to modern ocean composition. Mg/Ca -Sr/Ca relations suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes. However, additional forcing may be involved in explaining the timing and magnitude of changes. A plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering, which in turn is a result of subduction-zone recycling of CO2 from pelagic carbonate formed after the Cretaceous slow-down in ocean crust production rate. veins (CCV) in the oceanic basement were determined to reconstruct changes in Sr/Ca 20 and Mg/Ca of seawater in the Cenozoic. We examined CCV from ten basement drill 21 sites in the Atlantic and Pacific, ranging in age between 165 and 2.3 Ma. Six of these 22 sites are from cold ridge flanks in basement <46 Ma, which provide direct information 23 about seawater composition. CCV of these young sites were dated, using the Sr 24 isotopic evolution of seawater. For the other sites, temperature-corrections were 25 applied to correct for seawater-basement exchange processes. The combined data 26 Calcium carbonate veins in ocean crustshow that a period of constant/low Sr/Ca (4.46 -6.22 mmol/mol) and Mg/Ca (1.12 -27 2.03 mol/mol) between 165 and 30 Ma was followed by a steady increase in Mg/Ca 28 ratios by a factor of three to modern ocean composition. Mg/Ca -Sr/Ca relations 29 suggest that variations in hydrothermal fluxes and riverine input are likely causes 30 driving the seawater compositional changes. However, additional forcing may be 31 involved in explaining the timing and magnitude of changes. A plausible scenario is 32 intensified carbonate production due to increased alkalinity input to the oceans from 33 silicate weathering, which in turn is a result of subduction-zone recycling of CO 2 from 34 pelagic carbonate formed after the Cretaceous slow-down in ocean crust production 35 rate. 36 37 KEYWORDS 38 seawater evolution; mid-ocean ridges; hydrothermal circulation; carbonate veins 39 40 INTRODUCTION 41Circulation of seawater through the flanks of mid-ocean ridges has profound 42 effects on the chemistry of both seawater a...

Alteration of ocean crust provides a strong temperature dependent feedback on the geological carbon cycle and is a primary driver of the Sr-isotopic composition of seawater

“…Carbonate minerals are found in the lava section of the crust where they occur in veins, filling pore spaces and replacing igneous phases (e.g., Staudigel et al, 1981;Alt and Teagle, 1999;Coggon et al, 2004;Gillis and Coogan, 2011;Rausch et al, 2013). Bulk-rock CO 2 contents are controlled by the abundance of secondary carbonates.…”

Hydrogeologic Properties, Processes, and Alteration in the Igneous Ocean Crust

“…Vein carbonates recovered from core samples of the oceanic crust are valuable recorders of fluid chemical evolution as the seafloor ages, indicating variations in both crustal and seawater composition and temperature (e.g., Expedition 301 carbonates, Coggon et al, 2004Coggon et al, , 2010Gillis & Coogan, 2011;Rausch, Böhm, Bach, Klügel, & Eisenhauer, 2013).…”

Composition and timing of carbonate vein precipitation within the igneous basement of the Early Cretaceous Shatsky Rise, NW Pacific

“…As shown in several recent studies (Coggon et al, 2010;Rausch et al, 2013;Rausch et al, in prep. ), the elemental and isotopic composition of calcite veins can be used to reconstruct past seawater compositions, providing that the veins have formed at ambient seawater temperatures or if the geochemical evolution of the fluid can be reconstructed (Coggon et al, 2010;Coggon and Teagle, 2011).…”

“…Recently, however, it was shown that important seawater parameters (Sr isotopic composition, Mg/Ca, Sr/Ca ratios) are reliably recorded in calcium carbonate veins, if precipitated at near ambient seawater temperature (Coggon et al, 2004;Coggon et al, 2010, Rausch et al, 2013 or can be reconstructed if the precipitation temperature is known (Coggon and Teagle, 2011). In this paper we report Mg/Ca and seawater Sr-isotope stratigraphy will be used to discuss possible vein precipitation ages, which in turn can be compared with measured or assumed basement formation ages.…”