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

Abstract

On geological timescales there is a temperature dependent feedback that means that increased degassing of CO 2 into the atmosphere leads to increased CO 2 drawdown into rocks stabilizing Earth's climate. It is widely considered that this thermostat largely comes from continental chemical weathering. An alternative, or additional, feedback comes from dissolution of seafloor basalt in low-temperature (tens of • C), off-axis, hydrothermal systems. Carbonate minerals precipitated in these systems provide strong evidence that increased bottom water temperature (traced by their O-isotopic compositions) leads to increased basalt dissolution (traced by their Sr-isotopic compositions). Inversion of a simple probabilistic model of fluid-rock interaction allows us to determine the apparent activation energy of rock dissolution in these systems. The high value we find (92 ± 7 kJ mol −1 ) indicates a strong temperature dependence of rock dissolution. Because deep-ocean temperature is sensitive to global climate, and the fluid temperature in the upper oceanic crust is strongly influenced by bottom water temperature, increased global temperature must lead to increased basalt dissolution. In turn, through the generation of alkalinity by rock dissolution, this leads to a negative feedback on planetary warming; i.e. off-axis, hydrothermal systems play an important role in the planetary thermostat. Changes in the extent of rock dissolution, due to changes in bottom water temperature, also lead to changes in the flux of unradiogenic Sr into the ocean. The decreased flux of unradiogenic Sr into the ocean due to the cooling of ocean bottom water over the last 35 Myr is sufficient to explain most of the increase in seawater 87 Sr/ 86 Sr over this time.

Effect of paleoseawater composition on hydrothermal exchange in midocean ridges

“…Possibly counterbalancing this effect is the amount of global hydrothermal fluid flux, which some have inferred was higher than modern in the Cretaceous (53, 54) as a consequence of globally higher seafloor generation rates. Our results therefore suggest that either low-temperature exchange of Sr between basalt and seawater (including weathering of ocean-island basalts) was responsible for a large fraction of the overall basalt-seawater exchange (43,49,55), or conversely, that there was an exceptionally small continental weathering flux of radiogenic Sr during the Cretaceous. Supporting the prior interpretation are records of low-temperature hydrothermal crust alteration (49), which suggest that higher water temperatures increased the magnitude of low-temperature basalt-seawater exchange.…”

“…There is evidence for elevated amounts of carbonate precipitation in oceanic crust at certain times in the geologic past (49,50). In the modern ocean, the amount of dissolved carbonate in seawater (∼2 mmol/kg as opposed to 28 mmol/kg SO 4 ), which would precipitate in an analogous manner to sulfate as fluids are heated, serves as only a small sink for seawater Ca and Mg.…”

“…Although we have not attempted to create a global model for seawater [Ca] and [Sr] through time, differences in lowtemperature fluxes and retrograde outputs from altered oceanic crust (e.g., refs. 43,49), along with potential differences in reaction rates caused by paleoseawater variability, would be important to consider [a dual-porosity model is presented in the SI Appendix, section I to explore the effects of reaction rate differences on hydrothermal exchange (SI Appendix, section I and Fig. S5)].…”

Evolution of the Global Carbon Cycle and Climate Regulation on Earth

“…The basic idea of this feedback is that any increases in CO 2 lead to increased surface temperatures, acceleration of the hydrological cycle, and larger fluxes of silicate chemical weathering, leading to greater CO 2 removal from the ocean‐atmosphere system through continental weathering (Berner et al, ; e.g., Urey, ; Walker et al, ). Marine silicate weathering feedback . Recent work has highlighted that rates of silicate mineral dissolution within mafic oceanic crust are sensitive to bottom‐water temperatures, essentially establishing a negative feedback with CO 2 given that sea surface and bottom water temperatures are intimately coupled (Brady & Gíslason, ; e.g., Coogan & Dosso, ). Silicate dissolution within marine sediments has also been noted to contribute to global alkalinity fluxes (Solomon et al, ; Wallmann et al, ).…”

Validation of the BASALT model for simulating off‐axis hydrothermal circulation in oceanic crust

“…This replacement of phases in the rock, which can persist for tens of millions of years, is responsible for geochemical exchange of major (Ca, Mg, C, Fe, Na, K, Al, and Si) and trace elements between the ocean and the upper oceanic crust [ Elderfield and Schultz , ]. For example, carbon uptake by oceanic crust alteration is thought to be comparable to CO 2 outgassing rates from mid‐ocean ridges [ Alt and Teagle , ; Staudigel et al , ; Gerlach , ], indicating that basalt alteration is important to global cycling of carbon and potentially long‐term climate regulation [ Sleep and Zahnle , ; Coogan and Gillis , ; Mills et al , ; Coogan and Dosso , ].…”