Phanerozoic seawater Mg/Ca variations driven by supercontinent cycles.

Area of Science:

• Geochemistry
• Paleoceanography
• Geodynamics

Background:

• Seawater Mg/Ca ratios have fluctuated significantly throughout the Phanerozoic Eon.
• These oscillations correlate with changes in sedimentation, climate (icehouse-greenhouse cycles), and the evolution of marine calcifiers.
• The primary drivers behind these long-term seawater Mg/Ca variations have remained largely undetermined.

Purpose of the Study:

• To develop an inverse model linking seawater magnesium (Mg) concentration and Mg isotope composition.
• To quantify the major marine magnesium sinks by analyzing Mg isotopic fractionation during Mg-silicate formation and dolomitization.
• To identify the primary regulators of Phanerozoic seawater Mg/Ca oscillations.

Main Methods:

• Development of an inverse numerical model.
• Coupling of seawater Mg concentration and Mg isotope composition variations.
• Quantification of marine Mg sinks by exploiting contrasting Mg isotopic fractionation during silicate alteration and dolomitization.

Main Results:

• Increased seawater Mg/Ca during supercontinent assembly is attributed to reduced silicate alteration and dolomitization rates.
• Decreased seawater Mg/Ca during supercontinent stasis and breakup is linked to intensified Mg-silicate formation and dolomitization.
• Seafloor spreading, continental configuration, and climate modulate Mg-silicate and dolomite formation rates throughout the supercontinent cycle.

Conclusions:

• The supercontinent cycle is identified as a primary control on Phanerozoic seawater Mg/Ca.
• This control is exerted through the regulation of marine magnesium-silicate and dolomite fluxes.
• The study resolves the long-standing question regarding the drivers of long-term seawater Mg/Ca oscillations.