DECLINING PCO2 AND THE PHYTOPLANKTON OF TWO PHANÆROZOIC OCEANS

Phanærozoic oceans are thought to have alternated between "calcite" and "aragonite" oceans, as evidenced by differences in carbonate precipitation, evaporite chemistry and the chemistry of fluid inclusions in brine salts. These changes in ocean composition can be viewed as cyclic, following the rise and fall of MOR hydrothermal brine activity. The standing taxon diversity of organic walled microplankton (acritarchs + dinoflagellates) follows this alternation tightly, flourishing during an Early Palæozoic first "calcite" sea (CI) and a second Mid-Triassic – Palæogene (CII) one. The phytoplankton composition of these two "calcite" oceans was completely different, however, since all three major extant phytoplankton groups (diatoms, dinoflagellates and coccoliths) evolved contemporaneously with CII, and none is known with certainty to have been present during CI.

We examine the hypothesis that the biotic difference between CI and CII phytoplankton was caused by a secular trend in declining pCO₂, superimposed upon the cyclic trend in sea water composition. Even though the ancestors of all three modern phytoplankton groups existed during the Palæozoic, higher CO₂ levels in the CI ocean appear to have supressed the extracellular secretion of either silica or calcite as a selective advantage in phytoplankton evolution. Higher CO₂ (aq) during the Palæozoic obviated the need for carbon concentration mechanisms (CCMs) in the contemporaneous phytoplankton and CI phytoplankton were probably lacking both CCMs and efficient RubisCO. The consequent Devonian decline in pCO₂ during the initial rise of vascular plants on land, caused severe extinction in the phytoplankton who did not recover until the CII began in the Mid-Triassic. During the intervening "aragonite" sea, the rise of vascular plants together with the burial of terrestrial carbon provided the mechanism for maintaining low pCO₂. During CII, lower pCO₂ caused a rise in the oceanic pH, ultimately favoring HCO₃^- uptake (the evolution of CCM’s) and extracellular mineralization.