The Role of Seafloor Hydrothermal Systems in the Evolution of Seawater Composition During the Phanerozoic

Seafloor hydrothermal systems clearly have played an important role in controlling the composition of seawater over geologic time. However, controversy exists concerning their role as drivers of secular variability in major element composition of seawater during the Phanerozoic. The history of Phanerozoic changes in calcium (Ca²⁺) and magnesium (Mg²⁺) concentration has been reconstructed through analysis of fluid inclusions in evaporites, and this history is inconsistent with global geochemical cycling models that link both volcanic CO₂ release and Mg²⁺ uptake / Ca²⁺ release to inferred rates of seafloor production. These models generate little variability in Mg²⁺ concentrations because of compensating effects. Some models that recreate the observed trends either do not conserve alkalinity or do not link seafloor spreading, inferred from past variations in continental flooding presumed to reflect changing mid-ocean-ridge volumes, to volcanic CO₂ production. The most comprehensive models reproduce the variations in seawater Mg²⁺/Ca²⁺ through time quite well, but not because this ratio is tied to variations in seafloor spreading rate, and despite considerable mismatches to the Mg²⁺ record. This cacophony arises in part from our lack of quantitative understanding of fundamental relationships between heat flow, seafloor production rates, sea level, hydrothermal circulation rates, volcanic CO₂ release rates, and ocean chemical changes. Nevertheless, the current fluid inclusion data seem to indicate that ocean composition has an "attractor" that drives Ca²⁺ and Mg²⁺ toward equilibration with seafloor hydrothermal mineral assemblages [20 mm (millimolal) Ca²⁺ and 0 mm Mg²⁺], thwarted by other processes (weathering and riverine inputs, biogenic mineral precipitation) that drive the system away from this attractor.