Synchronous positive δ13Ccarb and δ13Corg excursions during 497–494 Ma: From a CO2 concentrating mechanism dominated photosynthesis?

Abstract

Steptoean Positive Carbon Isotope Excursion (SPICE) event is featured particularly as the synchronous positive δ13Ccarb and δ13Corg excursions, which are recorded worldwide in the Late Cambrian (ca. 497–494 Ma). However, the cause of this event remains elusive. To better understand the underlying mechanism of the contemporaneous positive isotope excursions, here we present a series of geochemical data from two drilling wells of the Tarim Basin, China and some relevant data reported from worldwide sections. CO2 concentrating mechanisms (CCMs, e.g., utilized by cyanobacteria) developed in response to decrease in pCO2 and increase in pO2 during the SPICE event, in which the HCO3− enriched in 13C but depleted in 2H is utilized for photosynthesis, is proposed to result in the synchronous positive excursions of δ13Ccarb and δ13Corg coupled by a negative shift of δDorg as exemplified by the samples from the Tarim Basin. The SPICE event was followed by an increase in plankton, which could relate to triggering CCMs and maintaining the biodiversity of marine organisms during the event.