User:Beitz 48/Steptoean Positive Carbon Isotope Excursion (SPICE)

The Steptoean Positive Carbon Isotope Excursion (SPICE) was a geological event which occurred about 495 million years ago near the beginning of the International Furongian Epoch of the Cambrian Period. The SPICE represents an increase in the atmospheric ¹³C to ¹²C ratio, known as the δ¹³C, and lasted for around 2 to 4 million years.

A rise in the atmospheric δ¹³C ratio doesn't necessarily infer a rise in abolute atmospheric carbon concentration. An increase of δ¹³C such as during SPICE is rather interpreted as the result of increased carbon fixation primarily by plants, which fix ¹²C more readily than ¹³C, and the subsequent burial of this organic carbon in sedimentary rocks. This removal of atmospheric ¹²C in higher proportion than ¹³C signifies increased proliferation of life and shifts the ratio towards ¹³C. This shift during SPICE is interpreted to be a global disturbance in the carbon cycle, affecting the atmosphere and the oceans in equal proportion as described by Henry's law. Regional sea level changes, a rise in sea water temperatures, ocean anoxia, and trilobite and brachiopod extinctions are associated with the SPICE event, although the exact mechanisms causing these events are still unconfirmed.

One proposed cause of the SPICE is an increase in the burial of organic carbon, perhaps caused by increased primary productivity (e.g. photosynthesis) or enhanced organic matter preservation due to ocean deoxygenation (i.e. anoxia or euxinia). The spread of seafloor anoxia, facilitated by higher ocean temperatures, has also been proposed as the kill mechanism for the extinctions of marine organisms.