PERSISTENCE OF WIDESPREAD MARINE ANOXIA AFTER NEOPROTEROZOIC OXYGENATION AND ITS EFFECT ON EARLY ANIMAL EVOLUTION AND FOSSIL PRESERVATION (Invited Presentation)

Oxygenation of Earth surface environment at the end of the Neoproterozoic Era has been traditionally viewed as unidirectional, heralding the well-oxygenated world of the Phanerozoic Eon. While Earth’s atmosphere and surface oceans certainly crossed and maintained a threshold of oxygenation needed to support complex, energetic animals by the late Neoproterozoic, a growing body of geochemical evidence supports the hypothesis that oxygen-deficient deep waters persisted or returned in Cambrian oceans.

The persistence of anoxia in deeper Cambrian oceans would have profoundly influenced early animal life. Recent investigations have linked geochemical evidence for widespread expansion of marine euxinia — anoxic, H₂S-containing waters — to intervals of mass extinction in the Cambrian: the Precambrian/Cambrian boundary, the end Botomian and the recurrent Cambrian trilobite biomere extinctions. These data suggest that shoaling of euxinic waters led to the extinction of shelf fauna during these intervals. Thus, the broad patterns seen in Cambrian animal evolution — high rates of biological turnover and repeated trilobite extinctions — may reflect persistent oxygen deficiency in subsurface waters of Cambrian oceans.

Anoxic waters and ocean chemistry also appear to have played a fundamental role in the preservation of the Cambrian fossil record. Copious pyrite burial associated with widespread anoxia kept marine sulfate concentrations low, effectively reducing the oxidative potential of seawater. It also promoted the generation of ferruginous conditions — anoxic, Fe⁺²-containing waters — that have been hypothesized to promote Burgess Shale-type fossil preservation. Anoxia, both directly and indirectly, was likely responsible for the unique fossil preservational windows through which we view Cambrian marine life.

The pervasiveness of oxygen-deficient deep waters during the early Phanerozoic Eon shows that the oxygenation initiated during the Neoproterozoic was protracted and perhaps reversible and that the Cambrian was a period of transition between the oxygen-deficient deep oceans of the mid-Proterozoic and the well-oxygenated waters of the later Phanerozoic. As such, refining the record of early Phanerozoic marine anoxia remains an important goal for Earth historians.