Ke Feng, Rachel Wood, Fred Toby Bowyer

The Late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE) event occurring at approximately 497 Ma and lasting for about 2-4 Myr is emblematic of a global-scale oceanic anoxia, coinciding with the trilobite mass extinction. It is significant in the biotic evolution spanning the Late Cambrian to Ordovician. Nevertheless, the driving mechanism behind the SPICE event is contentious, and its relationship with the trilobite mass extinction remains open. Therefore, to address these controversies and uncertainties, this study combined carbon and oxygen isotope analyses of the SPICE event in the Durness Group, with a comprehensive array of redox proxies, including I/(Ca+Mg) ratios, Fe speciation, TOC, and redox-sensitive trace elements, to elucidate the process of Durness SPICE event and provide insights.

Based on the sedimentological and geochemical characteristics of the Durness SPICE event, it can be delineated into five distinct stages corresponding to the transgressive-regressive cycles within the Sauk Megasequence. Stage I represents the pre-SPICE period of the Lower Systems Tract (LST). The δ13C baseline hovers around -1‰, and Mo-U covariation signals a semi-restricted environment. Low Al (mean 0.16 wt%) and Ti (mean 205 ppm) contents suggest minimal terrestrial input. The I/(Ca+Mg) ratios (mean 0.23) signify suboxic conditions, with TOC (mean 0.016 wt%) maintaining baseline values; Phase II marks the onset of the SPICE event in transgressive system tract (TST) with rising sea level. δ13C (mean 0.71‰) initiates an upward trend and Mo-U covariation shifts from semi-restricted to open conditions. Elevated Al (mean 0.67 wt%) and Ti (mean 432 ppm) concentrations point to increased terrigenous input. The TOC (mean 0.022 wt%) and I/(Ca+Mg) ratios (mean 0.25) briefly increase, returning subsequently to baseline, signifying heightened marine primary productivity that led to transient oxygenation and then progressed towards oxygen depletion. The δ13C peaks (2.8‰) during the Maximum Flooding Surface (MSF), leading into Phase III, characterized by a gradual sea-level drop, representing the Highstand Systems Tract (HST) and subsequent Falling-Stage Systems Tract (FSST). δ13C (mean 0.09‰) gradually returns to the baseline. The I/(Ca+Mg) ratio (mean 0.22) continues to decrease, and TOC (mean 0.016 wt%) remains at the baseline, confirming the intensification of suboxic conditions; Phase IV signifies a rapid regression, where the Mo-U covariance indicates a more restricted environment, and Al (mean 0.96 wt%) and Ti (mean 495 ppm) concentrations further increase, signifying enhanced terrigenous input. Simultaneously, TOC (mean 0.03 wt%) and the I/(Ca+Mg) ratios (mean 0.22) rise, signalling transient oxygenation associated with a renewed increase in marine primary productivity, causing a slight rise in δ13C (-0.31‰). A significant increase in U enrichment factors (EF) (mean 14.8) indicates a shift from suboxic to anoxic conditions; Phase V, the post-SPICE phase, corresponds to the Transgressive System Tract (TST) with rising sea levels. The Mo-U covariance indicates an open marine environment, while δ13C (mean -0.97‰) and TOC (0.017 wt%) remain at baseline. The declining I/(Ca+Mg) ratios (mean 0.19) signify increased anoxia, with a single Mo EF (103) suggesting potential localized euxinia. Additionally, FeHR/FeT (mean 0.68) and Fepy/FeHR (0.03) ratios suggest that after oxidation in Stage II, the Durness SPICE event primarily occurred under ferruginous conditions.

Through analyses of carbon isotopes, TOC, and comprehensive redox proxies, it is demonstrated that multiple eustatic fluctuations initiated the SPICE event, supporting the theory of anoxia resulting from increased nutrient input-driven marine primary productivity, thereby transitioning shallow tidal flats from suboxic to anoxic environments. Furthermore, it further elucidates the collaborative controls of deep nutrient-rich upwelling and enhanced continental weathering on the SPICE event, as well as the diffusion of deep basin euxinia to shallow shelves and the influence of palaeolatitude on weathering patterns, resulting in distinct local expressions of the global SPICE event at varying palaeolatitudes. This provides novel insights into the SPICE event. Moreover, the oxidation during the initial stages of the SPICE event challenges the hypothesis of widespread deep anoxia-driven trilobite extinction, favouring a scenario where rising sea levels led to increased marine primary productivity, disrupting the food chain, and triggering the trilobite mass extinction. Additionally, it introduces a potential driving force behind the significant and sustained increase in atmospheric oxygen levels during the post-SPICE period, which is the widespread ferruginous environment during the SPICE event. This enhances our understanding of the relationship between the SPICE event and the Great Ordovician Biodiversification Event occurring approximately 30-40 Myr later.