Sensitivity of the Marine Strontium Budget and of the Long Term Carbon Cycle to the Composition of Young Igneous Rocks

The variations in the strontium isotope ratio (87Sr/86Sr) in seawater are interpreted as a balance between fluxes of radiogenic strontium (Sr) from continental surfaces and unradiogenic Sr fluxes from mid-oceanic ridge and island arc basalts. We revisit this interpretative framework by showing that the evolution of the 87Sr/86Sr ratio of young silicic igneous rocks is a critical variable in the Sr budget. We use the secular variations in hafnium isotopes (ɛHf) from detrital zircons of sediments to reconstruct the changes in the 87Sr/86Sr ratio of young silicic igneous rocks through time. We show that the 87Sr/86Sr ratio in seawater correlates with the initial 87Sr/86Sr ratio of young silicic igneous rocks at millions of years timescale. The correlation is weak during ocean opening stages or late stages of supercontinent amalgamation. In these periods, the Sr flux from mid-oceanic ridge basalts or from recycled silicates uplifted in collisional orogens probably dominate the marine Sr budget. The correlation is strong during periods of supercontinent dispersal and assembly when volcanic arcs are widespread. In these periods, the average 87Sr/86Sr ratio of young silicic igneous rocks and associated siliciclastic sediments is a first-order control of the marine Sr budget at million years timescale. This compositional variable could play a key role in regulating the seawater budget of other incompatible elements, particularly Calcium and Magnesium, and by extension in controlling the long term carbon cycle. We suggest that long-term climate sensitivity varies with the secular evolution of the composition of young igneous silicates forming on the Earth's surface through time.