EVIDENCE FOR SEAFLOOR SPREADING RATE CHANGES FROM SEAWATER CHEMISTRY, UPPER EOCENE TO PRESENT

Recent oceanic crust generation models for the time since the Mesozoic have come to very different conclusions about the possibility of changes in the rate of seafloor spreading. An inconstant spreading rate would alter the major-ion composition of the oceans, by altering the mid-ocean ridge brine flux. These brines are created by cycling seawater through the hot basaltic rocks; this interaction creates a brine with high concentrations of Ca²⁺, and devoid of Mg²⁺ and SO₄^2-. Therefore, one effect of a decrease in the seafloor spreading rate is a correlative decrease in the production of mid-ocean ridge brines, which lead to a decrease in oceanic Ca²⁺ concentrations with a corresponding increase in Mg²⁺ and SO₄^2- concentrations. Published models of secular changes in the major ion composition of seawater indicate that equilibrium would be achieved within 20 Ma at a constant seafloor spreading rate. Therefore, any sustained change in seawater composition would indicate sustained change in the flux of mid-ocean ridge brine, and by proxy, a sustained change in the rate of seafloor spreading and oceanic crust production.

A dataset of fluid inclusion analyses (574 total analyses) compiled from Eocene through Miocene halites was used to quantify the major-ion evolution of seawater over the past 35 Ma. Of these analyses, 122 were completed as part of this study, and the remaining 454 were previously reported. The parent waters for each halite were determined by using available sedimentological and geochemical criteria coupled with the fluid inclusion data; non-marine and ambiguous-source halites, and their corresponding fluid inclusion analyses, were identified and separated prior to quantifying seawater compositions. The resulting dataset (304 total analyses: 91 from this study and 213 previously reported) was used to determine the major-ion composition of seawater over the past 35 Ma.

Fluid inclusions from these marine halites indicate that the concentration of Ca²⁺ has decreased by ~40% and Mg²⁺ and SO₄^2- have increased by ~40% in seawater over the past ~35 Ma. These changes, which occur over 35 Ma, indicate a slow decrease in seafloor spreading during Cenozoic. Therefore, the models which indicate an inconstant rate of oceanic crust production are consistent with the seawater composition since the Upper Eocene.