A highly redox-heterogeneous ocean in South China during the early Cambrian (∼529–514 Ma): Implications for biota-environment co-evolution

Abstract

The-Cambrian Explosion‖ is known for rapid increases in the morphological disparity and taxonomic diversity of metazoans. It has been widely proposed that this biological event was a consequence of oxygenation of the global ocean, but this hypothesis is still under debate. Here, we present high-resolution Fe-S-C-Al-trace element geochemical records from the Jinsha (outer shelf) and Weng'an (outer shelf) sections of the early Cambrian Yangtze Platform, integrating these results with previously published data from six correlative sections representing a range of water depths (Xiaotan, Shatan, Dingtai, Yangjiaping, Songtao, and Longbizui). The integrated iron chemistry and redox-sensitive trace element data suggest that euxinic mid-depth waters dynamically coexisted with oxic surface waters and ferruginous deep waters during the earliest Cambrian, but that stepwise expansion of oxic waters commenced during Cambrian Stage 3 (~521-514 Ma). Combined with data from lower Cambrian sections elsewhere, including Oman, Iran and Canada, we infer that the global ocean exhibited a high degree of redox heterogeneity during the early Cambrian, consistent with low atmospheric oxygen levels (~10-40% of present atmospheric level, or PAL). A large spatial gradient in pyrite sulfur isotopic compositions (δ 34 S py), which vary from a mean of-12.0‰ in nearshore areas to +22.5‰ in distal deepwater sections in lower Cambrian marine units of South China imply low concentrations and spatial heterogeneity of seawater sulfate, which is consistent with a limited oceanic sulfate reservoir globally. By comparing our reconstructed redox chemistry with fossil records from the lower Cambrian of South China, we infer that a stepwise oxygenation of shelf and slope environments occurred concurrently with a gradual increase in ecosystem complexity. However, deep waters remained anoxic and ferruginous even as macrozooplankton and suspension-feeding mesozooplankton appeared during 3 Cambrian Stage 3. These findings suggest that the-Cambrian Explosion‖ in South China may have been primarily a consequence of locally improved oxygenation of the ocean-surface layer rather than of the full global ocean. Our observations are inconsistent with predicted changes in ocean chemistry driven by early Cambrian animals, suggesting that the influence of early Cambrian animals on contemporaneous ocean chemistry, as proposed in previous studies, may be overly exaggerated.

Reorganisation of Earth’s biogeochemical cycles briefly oxygenated the oceans 520 Myr ago

“…This implies that earlier positive δ 98 Mo excursions (~2 ‰) in Terreneuvian phosphorite deposits (Wen et al, 2011), and perhaps in the latest Ediacaran (Kendall et al, 2015), represent episodic events rather than persistent ocean oxygenation. Similarly, detailed studies of the bottom water redox conditions in the Nanhua Basin, South China, suggest that oxygenated waters also invaded shallower part of the basin later in the Stage 3 (Jin et al 2016). As such, the oxygenation history of the early Cambrian ocean appears more dynamic than previously thought (e.g., Dahl et al, 2010;Sperling et al, 2013;Chen et al, 2015).…”

“…Specifically, the molybdenum isotope (δ 98 Mo) record from shales and phosphorites show several fluctuations recorded in ~560 to 520 Myr old stratigraphic sections in China (Wille et al, 2008;Wen et al, 2011;Xu et al, 2012;Chen et al, 2015;Kendall et al, 2015;Wen et al, 2015). The last positive δ 98 Mo excursion is broadly correlated to the first appearance of trilobites in China, strong Mo enrichments and with the oxygenation event reported here (Dahl et al, 2010;Xu et al, 2012;Chen et al, 2015;Wen et al, 2015;Jin et al, 2016). The coincident positive δ 98 Mo and δ 238 U excursions point to a widespread oxygenation episode in the earliest Cambrian Stage 3 oceans where O 2 -rich waters expanded and affected the Mo and U isotope composition of seawater as the overall burial fluxes of Mo and U into anoxic and euxinic settings decreased.…”

Preface: Atmospheric and oceanic oxygenation and evolution of early life on Earth: New contributions from China

“…This oceanic structure likely did not change in nature until the Cambrian Age 3 (ca. 520 Ma) (Jin et al, 2016).…”

“…However, recent studies have yielded evidence of a highly redoxstratified structure for the early-Earth oceans (>520Ma), in which a mid-depth euxinic water mass was maintained dynamically on continental shelves between oxic proximal-shelf waters and ferruginous deep-ocean waters (i.e., the "euxinic wedge" model; Fig. 1d; Jin et al, 2016;Guilbaud et al, 2015;Poulton and Canfield, 2011;Li et al, 2010;Poulton et al, 2010). Notably, this highly redox-stratified structure of ocean chemistry has been proposed to help the buildup of a large dissolved organic carbon reservoir in the Ediacaran deep oceans (Wang et al, 2015;Li et al, 2010).…”

Oxygen minimum zones in the early Cambrian ocean

“…Most current models linking early animal evolution to redox chemistry assume a horizontally stratified ocean (Wood et al, 2015;Jin et al, 2016). However, modern oceans exhibit a marked lateral redox variability, most notably where combinations of high primary productivity and metazoan activity (Bianchi et al, 2013) give rise to wedge-shaped oxygen minimum zones (Sperling et al, 2015a).…”