Cenozoic global cooling and increased seawater Mg/Ca via reduced reverse weathering

Abstract

Authigenic clay minerals formed on or in the seafloor occur in every type of marine sediment. They are recognized to be a major sink of many elements in the ocean but are difficult to study directly due to dilution by detrital clay minerals. The extremely low dust fluxes and marine sedimentation rates in the South Pacific Gyre (SPG) provide a unique opportunity to examine relatively undiluted authigenic clay. Here, using Mg isotopes and element concentrations combined with multivariate statistical modeling, we fingerprint and quantify the abundance of authigenic clay within SPG sediment. Key reactants include volcanic ash (source of reactive aluminium) and reactive biogenic silica on or shallowly buried within the seafloor. Our results, together with previous studies, suggest that global reorganizations of biogenic silica burial over the Cenozoic reduced marine authigenic clay formation, contributing to the rise in seawater Mg/Ca and decline in atmospheric CO₂ over the past 50 million years.Reverse weathering reactions on or in the seafloor are a major sink of many elements and alkalinity in seawater. Here, the authors show how reduced rates of reverse weathering may be responsible for global cooling and increased seawater Mg/Ca over the past 50 million years.

Fig. 1

End-member model and δ²⁶Mg of bulk sediment at Site U1366. a Mass fractions of the six end-members that comprise the deep-sea pelagic sediment at Site U1366, as modeled with constrained least squares multiple linear regression. b Mass accumulation rates of each end-member calculated from the modeled mass fractions and total mass accumulation rates. c Measured δ²⁶Mg (‰) values for the same bulk sediment samples. Green vertical shading highlights the samples with δ²⁶Mg values within the range of average upper continental composition (0−14 mbsf) and helps identify the samples with heavier δ²⁶Mg (14−20 mbsf, to the right of the green vertical shading) caused by the incorporation of seawater Mg into authigenic phases

Fig. 2

Comparison of two techniques that identify authigenic clay at Site U1366. Measured δ²⁶Mg (‰, all y-axes) values are plotted against the modeled mass fraction (x-axes) for each of the six components of the bulk sediment (a–f). a The significant correlation (r ² = 0.51; P < 0.01) between δ²⁶Mg and the mass fraction of Mg-enriched altered ash is consistent with heavier Mg isotopes being incorporated from seawater into an authigenic clay component. c Samples with a high mass fraction of dust plot near the δ²⁶Mg values typical of bulk-silicate earth (green shaded region), further indicating consistency between the two techniques

Fig. 3

Mechanism for reverse weathering affecting seawater Mg/Ca and climate. The proposed mechanism by which geographic reorganization of biogenic Si deposition and a reduction of global authigenic clay formation could cause an increase in seawater Mg/Ca and global cooling, as is observed over Cenozoic. The decreased sink of alkalinity in the ocean would be balanced by a reduction in global silicate weathering until the charge-weighted masses of sources are balanced by the sinks of alkalinity