Post-Mesozoic Rapid Increase of Seawater Mg/Ca due to Enhanced Mantle-Seawater Interaction

Abstract

The seawater Mg/Ca ratio increased significantly from ~ 80 Ma to present, as suggested by studies of carbonate veins in oceanic basalts and of fluid inclusions in halite. We show here that reactions of mantle-derived peridotites with seawater along slow spreading mid-ocean ridges contributed to the post-Cretaceous Mg/Ca increase. These reactions can release to modern seawater up to 20% of the yearly Mg river input. However, no significant peridotite-seawater interaction and Mg-release to the ocean occur in fast spreading, East Pacific Rise-type ridges. The Mesozoic Pangean superocean implies a hot fast spreading ridge system. This prevented peridotite-seawater interaction and Mg release to the Mesozoic ocean, but favored hydrothermal Mg capture and Ca release by the basaltic crust, resulting in a low seawater Mg/Ca ratio. Continent dispersal and development of slow spreading ridges allowed Mg release to the ocean by peridotite-seawater reactions, contributing to the increase of the Mg/Ca ratio of post-Mesozoic seawater.

Direct measurement of the boron isotope fractionation factor: Reducing the uncertainty in reconstructing ocean paleo-pH

“…However, this constant cannot be applied reliably to different aqueous phase compositions, such as the aquatic chemistry assumed to compose the ancient ocean and/or other environmental settings. For example, fluid inclusions in marine halite crystals combined with geochemical modeling for ancient ocean composition have suggested that during the Cretaceous period (∼130 Ma), the Ca 2+ and Mg 2+ concentrations were respectively 20-29 mmol/kg higher and 5-15 mmol/kg lower relative to modern seawater (Ligi et al, 2013;Holt et al, 2014). Variations in Ca 2+ and Mg 2+ proportions in the ancient seawater would have affected specific ion interactions (e.g., ion-pairs formation), and thereby the apparent K * b value, rendering the binary approach inaccurate for determining paleo-pH by the Fig.…”

Controls on the formation of stratabound dolostone bodies, Hammam Faraun Fault block, Gulf of Suez

“…Mid Ocean Ridge basalts have differentially been reported as both a source and a sink for Mg in seawater (Ligi et al ., ). Hence, it is possible that Mg‐enrichment of Oligo—Miocene seawater occurred during fluid interaction with basaltic dykes and sills emplaced during rift initiation in proximity to the HFF, and in the Northern Gulf Basaltic Province (22 to 26 Ma; Montenat et al., 1986; Moustafa & Abdeen, ; Patton et al ., ; Bosworth et al ., ).…”

The Mg isotopic composition of Cenozoic seawater – evidence for a link between Mg-clays, seawater Mg/Ca, and climate

“…One intriguing possibility is that modern rivers underestimate the Mg/Ca of silicate weathering due to underappreciated contributions from subaerial weathering of basalt or submarine weathering of peridotite (Dessert et al, 2005;Ligi et al, 2013;Snow and Dick, 1995). There are a number of potential mechanisms -for example an increase in peridotite weathering associated with an increase in the fraction of global seafloor spreading occurring at slow spreading rates or an increase in weathering of flood basalts (Kent and Muttoni, 2008) and/or island arcs.…”