A meta-analysis of the Steptoean Positive Carbon Isotope Excursion: The SPICEraq database

Highlights

• Local conditions impact the stratigraphic expression of the SPICE event.

• Sections from 30 to 60°S paleolatitude have lower peak δ¹³C values than tropical paleolatitude sections.

• Shallow water sections tend to have lower δ¹³C values than deeper water sections.

• Most sections record a small negative δ¹³C excursion prior to the SPICE.

• Host-rock carbonate mineralogy appears to have an appreciable effect on δ¹³C values.

Abstract

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent chemostratigraphic feature in the Lower Paleozoic. It has been used to correlate Upper Cambrian carbonate strata globally, and is cited as intimately linked to the Crepicephalus-Aphelaspis trilobite extinction event and the Sauk II-Sauk III megasequence transition. Despite the global nature of the SPICE event, regional/local conditions serve as a control on the expression of the SPICE event in the rock record. In light of this, and to better understand how reliable the SPICE event is as a chemostratigraphic tool for correlation, we have created the “SPICEraq,” a database comprising 78 SPICE-bearing sections containing 6669 individual δ¹³C analyses. In this study, we quantitatively evaluate the variability in SPICE records, and document that, while the excursion is a global signature, its stratigraphic expression is influenced by such conditions as paleolatitude, paleocontinent, water depth, and facies. While the magnitude of the SPICE excursion is generally consistent (an ~4‰ increase), the peak δ¹³C values are quite variable (ranging from +0.35 to +5.87‰). Specifically, sections located between 30 and 60°S paleolatitude ca. 500 Ma record δ¹³C values ~1 to 2‰ lower than those from lower paleolatitudes. Sections deposited in shallow water depths and facies also record lower δ¹³C values than intermediate and deep-water facies; the deep-water facies exhibit the most ¹³C-enriched carbonates at the peak of the SPICE and post-excursion. The stratigraphic thickness of the excursion varies widely, ranging from <3 to ~884 m, and is significantly impacted by all categorical variables investigated in this study. This study documents that the rising limb of the SPICE is immediately preceded by a small negative δ¹³C excursion in 75% of sections with data collected pre-SPICE. While 32% of sections record a δ¹³C plateau during peak SPICE, its presence in the δ¹³C record does not appear to be influenced by any environmental conditions investigated herein. Altogether, these analyses indicate that regional/local conditions impact the stratigraphic expression of δ¹³C records, and thus care should be taken to use robust, quantitative measures to compare and correlate excursions.

Introduction

The carbon isotope record of the Cambrian Period (~540–485 Ma) shows considerable variation. Through the first two Cambrian Series (~540–497 Ma), fluctuating positive and negative excursions are common and characteristic (Maloof et al., 2005; Dilliard et al., 2007; Saltzman and Thomas, 2012). Following the end of Series 2, the δ¹³C_carb record stabilizes around 0‰ V-PDB (Vienna Pee Dee Belemnite standard; hereafter, δ¹³C refers to carbonate carbon), and is punctuated, mostly, by only lower magnitude excursions. The exception is the prominent Steptoean Positive Isotopic Carbon Excursion (SPICE) event near the Guzhangian–Paibian Stage boundary (which also corresponds to the North American Marjuman–Steptoean Stage boundary and the Miaolingian–Furongian Series boundary; Fig. 1) (Saltzman et al., 2000; Saltzman and Thomas, 2012; Zhao et al., 2019).

First named by Saltzman et al. (1998), the SPICE is a ~4 to 5‰ increase in δ¹³C values that has been used in global chemostratigraphic correlation. The SPICE has been documented in carbonate stratigraphic sections around the world, including North America (e.g., Saltzman et al., 1998; Hurtgen et al., 2009), South America (e.g., Sial et al., 2008), Europe (e.g., Pruss et al., 2019; Álvaro et al., 2008), Asia (e.g., Kouchinsky et al., 2008; Ng et al., 2014; Lim et al., 2015; Wotte and Strauss, 2015), and Australia (e.g., Schmid et al., 2018). With such a broad geographic occurrence of this pronounced excursion, the inference commonly adopted is that the SPICE represents a global perturbation of the carbon cycle (Saltzman et al., 1998). Further, not only has the excursion been identified in both shallow and deep water carbonates (Saltzman et al., 2000; Glumac and Mutti, 2007; Zuo et al., 2018), but also in a smaller magnitude excursion (+1 to +2‰) of organic carbon isotope values (δ¹³C_org) from organic-rich shales (Álvaro et al., 2008; Ahlberg et al., 2009; Ahlberg et al., 2019; Baker, 2010; Saltzman et al., 2011; Woods et al., 2011).

Other geochemical analyses have been utilized to investigate the SPICE event, including oxygen, sulfur, and uranium isotopes, as well as other elemental proxies (e.g., Elrick et al., 2011; Gill et al., 2011; Dahl et al., 2014; Wotte and Strauss, 2015; LeRoy and Gill, 2019). Unlike δ¹³C values, δ¹⁸O values do not show a consistent global pattern. In some sections, δ¹⁸O has an inverse relationship with δ¹³C, with the nadir of δ¹⁸O values corresponding to the peak δ¹³C values (Elrick et al., 2011). In other cases, δ¹⁸O values covary with δ¹³C and record a positive δ¹⁸O excursion, such as in the Kyrshabakty section in Kazakhstan (+1‰ V-PDB) (Wotte and Strauss, 2015) and the Kulyumbe section in Siberia (+4‰ V-PDB) (Kouchinsky et al., 2008). In many instances, however, δ¹⁸O does not covary with δ¹³C and may be virtually invariant through the SPICE, such as in multiple boreholes from Australia (Schmid, 2017), the Tangwangzhai section in China (Zhu et al., 2004), and the Shingle Pass section in Nevada, United States (Baker, 2010). A positive δ³⁴S excursion of varying amplitudes has also been shown to correlate with the SPICE in multiple sections around the globe (Hurtgen et al., 2009; Gill et al., 2011; Saltzman et al., 2011; Wotte and Strauss, 2015; LeRoy and Gill, 2019). Dahl et al. (2014) measured uranium isotopes in the Mt. Whelan core from Australia to further investigate the relationship between the SPICE event and possible euxinia. Iron, mercury, and molybdenum concentrations have been analyzed to evaluate the paleoredox conditions of SPICE occurrences in North America (LeRoy and Gill, 2019), Sweden (Gill et al., 2011), and Scotland (Pruss et al., 2019). Results from these studies suggest that the timing of anoxic and/or euxinic conditions in different sections is variable with respect to trilobite extinctions and peak δ¹³C values of the SPICE (Gill et al., 2011; Dahl et al., 2014; LeRoy and Gill, 2019; Pruss et al., 2019). Additionally, phosphatic brachiopods from Laurentia display δ¹³C values preserving the SPICE event in a manner similar to that of corresponding bulk carbonate analyses (Cowan et al., 2005; Auerbach, 2004; Elrick et al., 2011).

The SPICE event may have also been biologically important. It occurred coincidentally with a trilobite biomere turnover, but whether it was causally or only temporally linked remains a question (Saltzman et al., 1998, Saltzman et al., 2000; Gill et al., 2011; Gerhardt, 2014; Gerhardt and Gill, 2016; Schiffbauer et al., 2017). The onset of the rising limb is coincident with the first appearance datum (FAD) of the trilobite Glyptagnostus reticulatus and the base of the Pterocephaliid biomere (Fig. 1; Saltzman et al., 2000; Glumac, 2011). The FAD of G. reticulatus is also correlative with the base of the Aphelaspis zone following the two-phase extinction of Crepicephalus-zone and Coosella perplexa-subzone trilobites (Glumac, 2011; Schiffbauer et al., 2017). δ¹³C values rise toward a maximum of approximately +4 to +5‰ in the late Dunderbergia or early Elvinia zones, with the peak preceding the FAD of Irvingella major and coinciding with the Sauk II-Sauk III megasequence boundary (Sloss, 1963; Saltzman et al., 2000, Saltzman et al., 2004; Glumac, 2011; Wotte and Strauss, 2015). The return to background δ¹³C values occurs within the Elvinia zone and the latest Pterocephaliid biomere, preceding the Marjumiid-Pterocephaliid boundary and extinction event (Glumac, 2011; Wotte and Strauss, 2015; Saltzman et al., 2004). Much more broadly, the SPICE event occurred between the Cambrian radiation and the Great Ordovician Biodiversification Event (GOBE) and has been posited to reflect changes in paleoenvironmental conditions leading to either the GOBE or the Ordovician Plankton Revolution (Servais et al., 2016). Although no direct link has been established between the SPICE and these biotic events, the SPICE may record conditions that set the stage for development of the appropriate oxygenation and/or nutrient conditions for later biodiversification (Servais et al., 2008, Servais et al., 2016; Saltzman et al., 2011).

Several authors have documented the peak of the SPICE being coincident with the Sauk II–Sauk III megasequence boundary in Laurentian sections (Saltzman et al., 1998, Saltzman et al., 2004; Glumac and Mutti, 2007; Glumac, 2011). This necessitates deposition of strata recording the rising limb of the SPICE during the tail end of a regressive event, and subsequent deposition of strata capturing the falling limb during the beginning of a transgression. While the Sauk sequences specifically are directly applicable to Laurentian strata, some authors have suggested that sections elsewhere in the world may also record a shallowing event coincident with the SPICE (e.g., Chen et al., 2011; Wotte and Strauss, 2015; Wang et al., 2020), although this is not ubiquitous (e.g., Schiffbauer et al., 2017; LeRoy and Gill, 2019; Labotka and Freiburg, 2020). The SPICE is not the only carbon isotope excursion in the Paleozoic that is associated with eustatic sea-level changes. For instance, the positive Upper Ordovician Hirnantian isotopic carbon excursion (HICE) has been attributed to glacioeustacy (Melchin et al., 2013; Jones et al., 2020); and comparably, the positive δ¹³C excursion of the Silurian Ireviken event is suggested to have been linked to climate change (Rose et al., 2019). This latter excursion is sometimes recorded in regressive facies packages (Baltica and Canada), and sometimes in transgressive facies packages (USA, Great Britain, and Tunisia) (Rose et al., 2019), similar to what is observed for the SPICE.

Despite its reputed global nature, some workers have suggested that the SPICE event may be more strongly affected by regional/local depositional conditions than previously thought—and thus its utility for correlation may be imprecise. Some workers have cited regional tectonics, heterogeneity in the chemical gradients of seawater in the Furongian, and paleo-water depth as potential driving forces for this variability (Wotte and Strauss, 2015; Schiffbauer et al., 2017; Barili et al., 2018). Furthermore, the SPICE interval varies widely in stratigraphic thickness, from <3 m in the Wanliangyu section, China (Rising limb: ~1 m; Chen et al., 2011) to >800 m in the Kulyumbe section, Siberia (Rising limb: ~380 m; Kouchinsky et al., 2008). In addition to demonstrable local variability, some sections that have been previously identified as Steptoean in age do not capture an easily identifiable SPICE signal in their δ¹³C records, despite clear documentation of the event in nearby time-equivalent units (e.g., Pruss et al., 2016). The fossil assemblages in some of these sections indicate a likely Steptoean age (e.g., Huang et al., 2019), whereas in other sections, fossil data are sparse, causing ambiguity about the age of the units (e.g., Glumac and Mutti, 2007).

These disparities, among others later discussed, in published SPICE data make its use as a tool for global correlation more complicated than previously assumed. To date, no robust statistical analyses have been employed to quantitatively compare and contrast the stratigraphic expressions of the SPICE. Identification of the excursion has been done predominantly through C-isotope pattern-matching, for lack of a better term, and biostratigraphic and/or lithologic correlation. Here, we present a compilation of data from the published literature and the results of a meta-analysis to test for emergent patterns in the δ¹³C records of the SPICE and explore a variety of regional/local conditions during deposition and diagenesis that may have impacted its stratigraphic expression.