LARGE-SCALE GROUNDWATER FLOW AND SEDIMENTARY DIAGENESIS IN CONTINENTAL SHELVES ARE IMPORTANT CONTROLS ON THE MAJOR ION CHEMISTRY OF THE OCEAN

Marine chemical budgets for Ca, Mg, Na, and K have been assumed to be controlled by river input, hydrothermal circulation at mid-ocean ridges, carbonate precipitation, and low-temperature basalt alteration, but these budgets have been difficult to balance. A new generation of isotope budgets, especially δ²⁶Mg, compound the problem. We show that these budgets can be balanced by including the effects of long-term (100s of thousands to millions of years) saline groundwater flow and sedimentary diagenetic reactions in continental shelf basins. Water fluxes were based on 17 passive continental margin basins, roughly 20% of passive margins. We decompacted overpressured basins to estimate compaction-driven flow and estimated geothermal fluxes based on the sediment composition of the basin and generalized numerical models. Chemical fluxes were estimated based on the spectrum of observed compositions for deep basin fluids, including fluids from clastic basins, carbonate basins, and CaCl₂ brines. Global fluid and chemical fluxes were comparable to mid-ocean ridges. The new chemical budgets balanced with inputs from low-temperature basalt alteration representative of low temperature sites (10-20°C, Dorado outcrop), with less than 3% inputs from higher temperature sites. Fluxes from active margins were not included, because fluxes from accretionary prisms in active margins are small compared to passive margin fluxes, but other fluxes (e.g. forearc basins) could contribute. Major shifts in ocean chemistry over the Phanerozoic (the last 540 My) have been explained by changes in weathering and circulation at the mid-ocean ridge, but these shifts are also consistent with changes in continental shelf volume and groundwater flux over time.