HERB, SPICE, Jose, and HICE: Stratigraphic Clues to the Role of Oceanographic Change within the Marine Carbon Cycle

Large (>3‰), short-lived shifts in the carbon isotopic ratio of marine carbonate (δ¹³C) are relatively uncommon features of the stratigraphic record, and are generally attributed to transient change in fluxes within the global carbon cycle. The magnitude of the excursions fosters the hypothesis that they were accompanied by significant change in atmospheric composition and global climate. Although a bevy of explanations have been proposed to explain the occurrence of large δ¹³C excursions and their coincidence with major paleobiological events and/or sea level change, the identification of a general causal mechanism or sequence of events has remained speculative.

It is instructive to compare the patterns of variation during large δ¹³C excursions for their commonalities and differences. Global correlation based on δ¹³C suggests a recurring series of events within each group, and identifies similarities between the two groups. In both the Late Cambrian SPICE and Late Ordovician HICE (positive) δ¹³C excursions, maximum δ¹³C are reached quickly and second-order δ¹³C features are absent on the rising limb, despite differences in the long-term δ¹³C trends upon which the events are superimposed. Furthermore, the SPICE excursion reaches its maximum during an episode of trilobite diversification, while the HICE event occurs during a major mass extinction event. Interestingly, neither event imposes a change in the longer term evolution of marine δ¹³C carbon values. The Late Cambrian HERB and Early Ordovician Jose (negative) excursions also have strong similarities, including distinctive sedimentological antecedents and staggered δ¹³C codas. Both events occur immediately prior to a significant change in the long-term pattern of δ¹³C variation.

These similarities and differences suggest that change in the vertical exchange of carbon between the surface and deep ocean, had a general, primary, and anticipatory role in large marine carbon cycle perturbations during the Early Paleozoic.