The expansion of phytoplankton utilizing CO2 concentrating mechanisms during the late Cambrian SPICE event

Abstract

The Steptoean positive carbon isotope excursion (SPICE) event represents one of the most pronounced perturbations of the global carbon cycle during the Early Paleozoic, postdating the end-Marjuman extinction. Profound oceanographic and biological changes occurred during the SPICE event, yet the underlying mechanisms driving this isotopic excursion and its precise linkages to concurrent environmental and biotic changes remain unresolved. This study presents high-resolution geochemical data from the Wangcun section in South China, revealing coupled positive δ¹³C_carb and δ¹³C_org excursions and a negative δD_org shift. Multiple geochemical proxies confirm that these signatures preserve primary environmental signals rather than diagenetic artifacts. The SPICE event exhibits diachronous onset and variable magnitude across different depositional settings, challenging the notion of a globally synchronous anoxic event. These findings support the hypothesis that enhanced organic carbon burial and isotopic fractionation were potentially linked to the expansion of phytoplankton utilizing CO₂ concentrating mechanisms. The development of CO₂ concentrating mechanisms in phytoplankton may have played a pivotal role in modulating isotopic responses across depositional facies. This research provides new insights into the biogeochemical dynamics and evolutionary implications of the SPICE event, bridging the Cambrian Explosion and the Great Ordovician Biodiversification Event.