Model of progressive ventilation of the Late Precambrian-Early Paleozoic ocean.

Abstract

The development and evolution of redox conditions with depth in the open ocean is traced for the Late Precambrian-Early Paleozoic, using an oceanographic advection-diffusion model and Henry's Law to predict redox conditions in the upper mixed layer and the main pycnocline for atmospheric oxygen contents from 1 to 100 percent Present Atmospheric Level of oxygen. Three climatic conditions: glacial, pre- to post-glacial, and non-glacial define different pycnocline thicknesses at any given atmospheric oxygen content. The results indicate that the oceans remained anoxic and sulfidic, until the content of oxygen in the atmosphere reached about 3 to 5 percent PAL. With various ventilation strategies related to climate and water mass formation, anoxic waters could have been a significant intermediate water between oxic and sulfidic waters. At about 25 percent PAL, aquatic aerobic respiration could occur in the surface waters at high latitudes permitting the development of active predation. At this time, the anoxic but non sulfidic water could provide a haven for non-motile primitive animal life on the shelf and in the water column in advance of developing oxic conditions in the overlying water. Only at 45 percent PAL would the tropical surface waters be ventilated sufficiently to permit aerobic respiration. Even at 100 percent PAL, suggested for the Lower Paleozoic, global anoxic conditions on the shelf are predicted during warm climates due to the lack of oxygen ventilation in deep water. - from Author