Life and the Oceans
Published:January 01, 1983
The sialic plutonism that transformed prevailing petrographic, sedimentary, and paleogeographic styles from Archean to Proterozoic aspects lasted for 600 million years. It created the first extensive cratonal surfaces, with all their potential for interaction among a diversity of microbiotas and their physical surroundings. Earth, in fact, was essentially completed during the Proterozoic. Continental and oceanic crusts and waters achieved their approximate present dimensions and chemistries. O2-releasing photosynthesis became established. Life at the cellular level differentiated, culminating in eucaryotic heredity and leading to Metazoa at the Proterozoic-Phanerozoic transition. A kinetic lag between sources and sinks of O2 eventually created an oxygenous atmosphere, its development being reflected by such sedimentary peculiarities as detrital uraninite, banded-iron-formation, distinctive soil profiles, red beds, and perhaps sedimentary phosphorites and metal sulfides. Combined with biological evidence, the temporal distribution of such materials suggests an O2 growth curve connecting these provisional points: (1) near zero O2 until ~2.3 × 109 years ago, (2) 1 percent present atmospheric level (PAL) O2 2-2.3 × 109 years ago, (3) ~7 percent PAL 670 × 106 years ago, (4) ~ 10 percent PAL 550 × 106 years ago, and (5) present atmospheric levels beginning ~ 400 × 106 years ago.