In modern oceans, the calcareous skeletons of plankton are characterized by positive δ¹³C values because the dissolved bicarbonate in surface seawater is relatively depleted in carbon-12, a consequence of the preferential utilization of the lighter isotope during photosynthesis. At the K/T boundary, the gradient collapsed to zero, or a reversed gradient was temporarily established. The breakdown of the gradient is a manifestation of greatly reduced biomass production in the strangelove ocean after the terminal Cretaceous catastrophe (Hsü and McKenzie, 1985). In addition, we propose that the reversed gradient is possibly characteristic of an ocean in which a bacterial respiration control on the surface-water δ¹³C dominated over photosynthesis (McKenzie and others, 1989). We further suggest that the very large negative δ¹³C values across the K/T, Permian/Triassic, and Precambrian/Cambrian boundaries are evidence of “respiring oceans” after global catastrophes at era boundaries.

The origins of strangelove (zero-gradient) and respiring (negative gradient) oceans are related to reduced biomass productions after global catastrophes. Either an impact by a very large bolide or by explosive volcanism could be the ultimate cause of such catastrophes. From what we know now, however, the latter happened too frequently in geologic history to account for the rare era-boundary events.