Long-term evolution of terrestrial inputs from the Ediacaran to early Cambrian: Clues from Nd isotopes in shallow-marine carbonates, South China

Abstract

The emergence and radiation of metazoans have been widely attributed to a progressively more oxidizing surface environment through the Ediacaran-Cambrian transition interval. However, the root causes for atmospheric and oceanic oxygenation are still disputed. Long-term tectonic changes could possibly have led to atmospheric oxygenation but geochemical evidence for this linkage remains elusive. In this study, we analyzed the radiogenic Nd isotopic compositions (εNd) of shallow-marine carbonates from South China in order to track secular variations in terrestrial inputs from Ediacaran to early Cambrian time. Compared with most other geochemical indices, the Nd isotope system in carbonates is less susceptible to early diagenetic exchange and can thus act as a robust proxy for continental materials undergoing weathering. We interpret an abrupt excursion to lower εNd values during the middle Ediacaran as due to rapid exchange of different water masses. However, the more gradual trend towards lower εNd values from the Ediacaran to 2 early Cambrian, accompanied by increasing 87 Sr/ Sr ratios of the studied carbonates, likely indicates the enhanced weathering of old continental rock following the assembly of Gondwana. Increased net accumulation of atmospheric oxygen as a result of enhanced organic carbon burial may have benefited from intense continental denudation.

C, Sr, Nd isotope chemostratigraphy and zircon provenance of the Witvlei Group (Namibia): Neoproterozoic glaciations and seawater evolution

“…13B) with those recorded in the Doushantuo Formation (Yangtze Platform, S. China) and the Polanco Formation (Arroyo del Soldado Group) (Frei et al, 2011;Cui et al, 2015;Wei et al, 2019;Hohl et al, 2015b;Hu et al, 2016) may argue that the associated negative C isotope shift (Fig. 10C) is time-equivalent to the putative Shuram negative CIE (Sawaki et al, 2010;Xiao et al, 2012;Zhu et al, 2013;Wei et al, 2019).…”

“…In contrast to the global Sr trends, the short residence time of Nd (200-1000 yrs; Tachikawa et al, 1999) in seawater highlights the Nd isotope system as an ideal proxy for short-term and local water mass variations. For comprehensive background information on the long-term changes in Neoproterozoic carbonates, which is beyond the scope of this manuscript, the reader is referred to Shields andVeizer (2002), Halverson et al (2007; and Wei et al (2019).…”

“…A contemporaneous decrease in bio-essential Ni, Co and Cd over Sc ratios drop from values similar to Phanerozoic carbonate standard JDo-1 to concentrations close or below the UCC, arguing for a potential biological crisis, such as reduced nutrient availability in the water column. Wei et al (2019) speculated that the observed drastic changes in Nd isotopic compositions during the Shuram excursion could not be explained solely by increased weathering fluxes due to the amalgamation of Gondwana in the terminal Proterozoic-early Cambrian, as these changes would have also affected Phanerozoic seawater isotopic compositions. However, the Shuram anomaly and the presumably associated Sr and Nd isotope excursions are exclusively short-lived over a period of approximately 12 Ma (Rooney et al, 2020).…”

Primary or secondary? A dichotomy of the strontium isotope anomalies in the Ediacaran carbonates of Saudi Arabia

“…(Cui et al, 2016b;Cui et al, 2019); (E) Nama Group in southern Namibia (Ries et al, 2009). phosphorites from the Doushantuo and Dengying formations in South China (Yang et al, 1997); mudstones and fine-grained siliciclastics in Canada, Australia, Svalbard, and China (Cox et al, 2016); carbonates from the Doushantuo and Dengying formations in South China (Wei et al, 2019b). (Wei et al, 2019b); 3-Phosphorites from the Ediacaran Doushantuo and Dengying formations, South China (Yang et al, 1997); 4-Carbonates and shales from the Ediacaran Doushantuo Formation, South China (Hu et al, 2016); 5-Ediacaran Dhaiqa and Naghr formations, Saudi Arabia (this study).…”

Exploring Icehouse Cyclicity Pattern and Seawater Dynamics on an Ancient Carbonate Platform With Nd Isotopes (Carboniferous, Southern Kazakhstan)

“…In marine environments, the conservative behavior of Nd and other REEs is particularly observed in pure authigenic carbonates, which mostly preserve their primary REE signatures (F. Li et al., 2019; Luo et al., 2021; Webb et al., 2009). On the other hand, however, REEs in carbonates are susceptible to detrital contamination due to significantly higher REE contents in silicate particles than in authigenic carbonate phases (Wei et al., 2019). During marine silicate weathering, Nd is released from siliciclastic detritus into pore water, where it can be subsequently incorporated into authigenic minerals (e.g., Du et al., 2016).…”