Dramatic attenuation of continental weathering during the Ediacaran-Cambrian transition: Implications for the climatic-oceanic-biological co-evolution

Abstract

The Ediacaran–Cambrian (E-C) transition witnessed dramatic tectonic, climatic, oceanic, and biological co-evolutions at the dawn of the ‘Cambrian explosion’. Although glaciations were reported in some mid-high-latitude continents during this interval, large uncertainties remain concerning timing and extent, as well as their effect on the entire Earth's surface system. We here report magnesium (Mg) isotopic compositions, along with chemical index of alteration (CIA), Al/Ti ratios from a time-dated, nonglacial E-C boundary section on the mid-low-latitude Yangtze Block, South China, to constrain chemical weathering intensity and associated climatic, oceanic and biological co-evolution during this critical interval. Our data demonstrate a rapid and significant negative δ26Mg excursion (from +1.28 to −0.34‰) along with similar CIA and Al/Ti decreases across the E-C boundary, reflecting an apparent attenuation in chemical weathering intensity likely in response to severe climate cooling. This finding further affirms the far-field glaciation reported in the mid-high-latitude continents during the E-C transition. Under this condition, the intense climate cooling could have enhanced oceanic circulation and/or upwelling, and simultaneously caused prominent variations of seawater geochemistry (including alkalinity elevation) and nutrient flux as well as ecological stresses on metazoans. All these could reasonably account for extensive deposition of phosphorites and organic-rich black shales, and faunal turnover (extinction of Ediacaran fauna and appearance of skeletonized small shelly fauna) across this interval. These novel datasets thus shed new insights into the climatic-oceanic-biological interactions and time-specific deposition around the E–C transition at the prelude of the Cambrian explosive bioradiation.