Coupled oceanic oxygenation and metazoan diversification during the early–middle Cambrian?

The early–middle Cambrian (Fortunian to Age 4) is characterized by a significant increase in metazoan diversification. Furthermore, this interval is marked by a prominent environmental and ecological expansion of arthropodand echinoderm-rich biotas. Recent redox work has suggested that this shift occurred during stable or decreasing marine oxygen levels, suggesting that these paleobiological and paleoecological transformations were decoupled from a redox control. We tested this idea by conducting new paleoredox analyses on Age 2–Age 4 Cambrian outer shelf (Jiuqunao-Wangjiaping), slope (Wuhe-Geyi), and basinal (Zhalagou) sections of the South China Craton. Multiple sections indicate that mid-depth waters transitioned from anoxic conditions during Cambrian Age 2 to stable oxic conditions during Cambrian Age 4. These findings suggest a stepwise expansion of oxic waters from shallow to deep settings during the early–middle Cambrian, consistent with a redox control of metazoan diversification and ecological expansion. More broadly, despite the surge in redox work over the past decade, this study highlights the need for continued coupled redox and paleontological studies to directly test models about the links between the evolution of animals, ecosystems, and marine redox conditions. INTRODUCTION During the early–middle Cambrian Period (Fortunian to Age 4) there was an explosion of metazoan diversity, including the appearance of small shelly faunas and more complex biotas dominated by arthropods and echinoderms (e.g., the Chengjiang Biota; Zhu et al., 2006). Although the record of early metazoan evolution is increasingly well understood, the underlying cause of these biotic events continues to be debated. Global redox changes (i.e., rapid rise or fall in atmospheric and oceanic oxygen levels) have been widely proposed; for example, the rapid rise of metazoan morphologic and taxonomic diversity during the early–middle Cambrian has been attributed to increasing atmospheric and oceanic oxygen levels (Knoll and Carroll, 1999; Chen et al., 2015b; Jin et al., 2016). However, the links between metazoan evolution and environmental oxygenation may have been complex or weak. A recent study suggested a decline in atmospheric and oceanic oxygen levels during Cambrian Age 3–Age 4 linked to rising bioturbation intensity beginning in Cambrian Age 2 (Boyle et al., 2014). Another study suggested static marine oxygen levels and dominantly anoxic conditions in all Cambrian based on statistical analysis of global iron speciation data (Sperling et al., 2015). Given this debate, new studies of early–middle Cambrian shallow to deep ocean systems are needed to evaluate the relationships among metazoan evolution, ecosystem structure, and ocean oxygenation. We conducted a high-resolution Fe-trace element geochemical study of lower-middle Cambrian (Fortunian to Age 4) sections of the South China Craton representing intermediateto deep-marine settings, including outer shelf (Jiuqunao-Wangjiaping, JW; note: Jiuqunao data were compiled from Och et al. [2016]), slope (Wuhe-Geyi, WG), and basinal (Zhalagou, ZLG) sections of the Yangtze Block (Fig. DR1 in the GSA Data Repository1). Linking our redox proxy analysis with detailed paleontological data from the lower-middle Cambrian of South China affords the unique opportunity to investigate the association, if any, between ocean-redox evolution and metazoan diversification during the early to middle Cambrian in South China and possibly elsewhere. LITHOSTRATIGRAPHY AND STRATIGRAPHIC CORRELATION The lower-middle Cambrian succession at JW (30°53′0.93′′N, 110°52′47.25′′E for Jiuqunao, 30°48′41′′N, 111°11′12′′E for Wangjiaping) comprises, moving upsection, the Yanjiahe, Shuijingtuo, Shipai, and Tianheban Formations (Fig. 1A). The upper Yanjiahe, uppermost Shuijingtuo, and Tianheban Formations consist mainly of limestones, whereas the Shuijingtuo and Shipai Formations are mainly black shales, mudstones, and siltstones. The lower-middle Cambrian succession at WG (26°45′34′′N, 108°24′33′′E for Wuhe, 26°48′12′′N, 108°14′10′′E for Geyi) comprises the upper Liuchapo, Jiumenchong, Bianmachong, and Balang Formations (Fig. 1B). The section consists mainly of black shales, mudstones, and siltstones, although phosphatic chert is present in the upper Liuchapo Formation and muddy limestones are present in the upper Jiumenchong Formation. The lower-middle Cambrian succession at ZLG (25°59′6′′N, 107°53′32′′E) consists of the Laobao, Zhalagou, and Duliujiang Formations (Fig. 1C). The Zhalagou and Duliujiang Formations are black shales, and the upper Laobao Formation is phosphatic chert. Correlations among the study sections are based on radiometric ages and extensive biostratigraphic work. The key tiepoints among sections are shown in Figure 1. The lowermost Cambrian black shale layer, which is a correlation marker across the Yangtze Platform, has been dated to early Cambrian Age 2 based on zircon U-Pb ages: 526.4 ± 5.4 Ma at WuheAijiahe, ~20 km from the JW section (Okada et al., 2014), and 522.3 ± 3.7 Ma at Bahuang, ~80 km from the WG section (Chen et al., 2015a). The upper Shuijingtuo Formation at JW and the upper Jiumenchong Formation at WG are dated to early Cambrian Age 3 based on the trilobite Hupeidiscus orientalis (Yang et al., 2016). The basal Shipai Formation at JW and uppermost Jiumenchong Formation at WG are constrained to 1 GSA Data Repository item 2017245, detailed geological settings, analytical methods, and supplemental figures and data tables, is available online at http://www .geosociety.org/datarepository/2017/, or on request from [email protected]. *E-mail: [email protected] GEOLOGY, July 2017; v. 45; no. 7; p. 1–4 | Data Repository item 2017245 | doi:10.1130/G39208.1 | Published online XX Month 2017 © 2017 eological Society of A erica. For permission to copy, contact [email protected]. 2 www.gsapubs.org | Volume 45 | Number 7 | GEOLOGY early Age 4 of the middle Cambrian (ca. 514–509 Ma) based on the trilobites Redlichia meitanensis and Mayiella (Yang et al., 2016). This age framework is confirmed by the trilobites Breviredlichia liantuoensis in the basal Tianheban Formation at JW and Arthricocephalus chauveaui in the middle Balang Formation at WG, which belong to the Megapalaeolenus Zone of middle Age 4 (Yuan and Zhao, 1999; Na and Kiessling, 2015). The trilobite Kunmingaspis, found in the basal Duliujing Formation at ZLG, belongs to the latest Chittidilla plana–Paragraulos kunmingensis trilobite Zone, demonstrating an age of late Age 4 for the lower part of that unit (Yuan and Zhao, 1999; Na and Kiessling, 2015). GEOCHEMICAL PROXIES FOR MARINE REDOX CONDITIONS Iron speciation is a widely used paleoredox proxy that, based on analysis of reactive iron phases in siliciclastic rocks, can track local watercolumn redox conditions (cf. Poulton and Canfield, 2011). Iron speciation is an empirically calibrated proxy, and therefore care must be taken when selecting samples. However, Fe speciation is one of the most extensively explored proxies and is robust when examined rocks are lithologically similar to those in the calibration studies. In modern and ancient oxic marine sediments the ratio of highly reactive iron (FeHR) to total Fe (FeT), i.e., FeHR/FeT, is commonly <0.38. In contrast, FeHR/FeT values exceeding this threshold are indicative of sediments deposited under anoxic conditions. When anoxia is indicated, the ratio between pyrite iron (FePy) and FeHR provides evidence for ferruginous (FePy/FeHR < 0.7–0.8) versus euxinic conditions (FePy/FeHR > 0.7–0.8). In order to validate paleoredox interpretations based on Fe speciation data, enrichments of redox-sensitive trace elements (RSTEs) (expressed as enrichment factors, or EFs) can be used as an independent proxy. RSTEs tend to be less soluble and more particle reactive under reducing conditions than under oxidizing conditions, leading to authigenic enrichments in anoxic facies (Algeo and Maynard, 2004). Mo is present in oxic water masses mainly as the conservative molybdate anion (MoO4 2–), which is converted to particle-reactive thiomolybdates (MoO4–xSx 2–, x = 1–4) through reaction with H2S under sulfidic conditions. U is present in seawater mainly as soluble U(VI) under oxic conditions, but is reduced to less soluble U(IV) under anoxic conditions. Transfer of seawater V to the sediment is associated with a two-step reduction process in which V(V) is converted to V(IV) under mildly reducing conditions and further to V(III) under euxinic conditions. Detailed descriptions of the analytical methods for iron speciation, trace metals, other geochemical analyses, and statistical tests used in this study are provided in the Data Repository. EARLY–MIDDLE CAMBRIAN MARINE REDOX CONDITIONS In the outer shelf JW section (Fig. 1A), Age 2–Age 3 (uppermost Yanjiahe and Shuijingtuo Formations) samples are characterized by high FeHR/FeT (0.92–1.00) and moderate to high FePy/FeHR (0.56–0.75), indicating ferruginous to euxinic conditions. Reducing conditions are supported by enrichments factors of 45 (7–52) for Mo and 18 (7–23) for U (note that reported values are the median and 16th–84th percentile range). V exhibits lower enrichment factors (2.3; 1.3–3.7) similar to that reported for ferruginous and euxinic units of Ediacaran age (e.g., Sperling et al., 2016). All Age 4 (Shipai and Tianheban Formations) samples are characterized by low FeHR/FeT (0.08–0.44) and low FePy/FeHR (0–0.62), suggesting oxic depositional conditions. This inference is consistent with low MoEF (0.3 ± 0.2; mean ± SD), UEF (0.8 ± 0.2), and VEF (0.7 ± 0.3) in the Age 4 formations. In the slope WG section (Fig. 1B), available Age 2 to early Age 4 (Jiumenchong and Bianmachong Formations) samples have variable FeHR/ FeT (0.23–0.88) and variable FePy/FeHR (0.24–0.89), indicating dominantly ferruginous conditions punctuated by episodic euxinia. In contrast, all older Age 4 (Balang Formation) samples show low FeHR/FeT (0.08–0.35) and low FePy/FeHR (~0), suggesting oxic depositional conditions, consistent with low MoEF (0.5 ± 0.2), UEF (0.3 ± 0.1), and VEF (0.3 ± 0.1). This is also consistent with trace metal enrichment patterns: samples from the lower member (0–29 m) show greater enrichments [MoEF = 18 (5–50), UEF = 8 (4–14), VEF = 7 (4–15)] than samples from the upper member (29–92 m) [MoEF = 12 (7–18), UEF = 2.3 (1.0–6.8), VEF = 1.0 (0.7–1.6)]. These small enrichments are consistent with sulfide restricted to sediment pore waters (cf. Scott and Lyons, 2012). 52 6. 4± 5. 4 M a H . o ri en ta ils R . m ei ta ne ns is P. l. B . l .