Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

Abstract

Cryogenian (∼720-635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth's history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ²⁶Mg values (+0.56 to +0.95)-indicative of an episode of intense chemical weathering-occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ²⁶Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO₂ concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO₂ and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition.

Keywords: South China; cap carbonate; chemical weathering; magnesium isotopes; snowball Earth.

Fig. 1.

Compilation of Mg isotope data. (A) Boxplots showing the Mg isotopic compositions of major Mg reservoirs. The box encompasses the 25 and 75 percentiles, the vertical line inside the box represents the median value, the whiskers represent the 2.5 and 97.5 percentiles, and the gray dots are outliers. The cyan and orange vertical dashed lines represent average δ²⁶Mg of seawater (SW, –0.83) (47) and UCC (–0.22) (32), respectively. See SI Text and Dataset S1 for references. (B) Cross-plot of δ²⁶Mg versus CIA for weathered residues in basalt weathering profiles. Green bar, fresh basalt (21, 22); cyan squares, saprolites developed from basaltic diabase dike, South Carolina (22); purple triangles, soils above basalts, Iceland (48); red circles, saprolites developed from tholeiitic basalts South China (21); orange diamonds, bauxites developed from Columbia River basalts, United States (19). (Inset) In detail, the relationship between CIA and δ²⁶Mg values of saprolites from South China and Columbia River.

Fig. 2.

Stratigraphic profiles of δ²⁶Mg and δ¹³C of the upper Nantuo Formation and the basal Doushantuo Formation cap carbonate in the 14ZK core and the Bahuang section. Units I through VI refer to six lithostratigraphic units in the upper Nantuo Formation. Pink dashed line represents the stratigraphic correlation line. For different grain sizes, c/s is clay/silt, f is fine sand, m is medium sand, c is coarse sand, and g is granules.

Fig. S1.

(A) Late Neoproterozoic paleogeographic map of South China Block (modified from ref. 23). Sample localities are marked by circled numbers. (Inset) Map shows the location of the Yangtze Platform (highlighted in blue) and the Cathaysia Block. (B) Geological map of studied area, showing the location of the 14ZK core and the outcrop section at Bahuang (modified from ref. 60).

Fig. S2.

Field, drill core, and petrographic photographs. (A) Photograph of drill core showing sandstone/siltstone with parallel bedding in unit II of 14ZK core. (B) Photograph of drill core indicating diamictite in unit I of 14ZK. (C) Petrographic photomicrograph showing petrography of siltstone in unit IV at 14ZK. (D) Thin section photomicrograph of sandstone in unit II at 14ZK. (E) Thin section photomicrography showing poorly sorted matrix of diamictite in unit I at 14ZK under perpendicular polarized light. (F) Field photograph showing the contact between the Doushantuo cap carbonate and the underlying pebbly siltstone in the topmost part of the Nantuo Formation at the Bahuang section. (G) Massive diamictite in the Nantuo Formation at Bahuang section. (H) Thin section photomicrograph showing calcite cements infilling sheet-crack structures in the cap carbonate at Bahuang section. (I) Thin section photomicrograph of siltstone in the topmost part of the Nantuo Formation at Bahuang section. (J) Thin section photomicrograph of pebbly sandstone in the topmost part of the Nantuo Formation at Bahuang section. Arrows highlight lonestones in the Nantuo Formation.

Fig. S3.

Cross plots of the molar ratios of (A) K/Ti, (B) Mg/Ti, (C) Ca/Ti, and (D) Fe/Ti vs. δ²⁶Mg values of fine-grained siliciclastic components in Nantuo (NT) and Doushantuo (DST) formations.

Fig. S4.

XRD chromatograms of (Upper) the representative Nantuo Formation sample (14ZK-83) and (Lower) the representative Doushantuo cap carbonate (14ZK-C5). The Nantuo samples are dominated by feldspar (albite), quartz, smectite, illite, and chlorite, and Doushantuo cap carbonate is dominated by dolomite, but also composed of quartz, chlorite, illite, and albite.

Fig. 3.

Modeling results showing the relationships between the partial pressure of atmospheric CO₂ (pCO₂), (A) seawater pH, and (B) carbonate saturation state (Ω) in an equilibrated atmosphere−ocean system, assuming a sea surface temperature of 10 °C and salinity of 35‰. Contour lines indicate different concentrations of total DIC in seawater. Orange shade refers to the estimated seawater pH and Ω at the onset of Snowball deglaciation (8, 37, 38). Blue line and green shade represent the range of seawater pH and Ω during cap carbonate deposition based on triple-oxygen isotopes (42) and boron isotopes (40, 41), respectively.

Fig. S5.

Three-isotope plot illustrating Mg isotopic composition of samples from the Nantuo (NT) Formation and Doushantuo (DST) cap carbonate, and in-house standards (San Carlos olivine, Hawaii seawater). All data fall along the terrestrial equilibrium mass-dependent fractionation curve (solid line) with a slope of 0.5193.