EARLY PERMIAN ABRUPT CLIMATE CHANGE INDUCED BY GLOBAL SHIFT IN THERMOHALINE CIRCULATION: DRIVERS AND TIPPING POINT

Earth scientists have traditionally interpreted evidence of rapid environmental change in the distant past as the result of equally abrupt cataclysms (bolide impacts, volcanoes, etc.). Evidence suggests, however, that slowly-changing earth systems drivers have worked in concert to create inherent instability, up to a point when such systems could no longer maintain their integrity and rapidly and irreversibly tipped into alternate states, hence recording geological events that mimic those brought by cataclysms.

Global distribution of late Paleozoic warm- and cold-water carbonates provides evidence of two diametrically opposed paleoceanographic-paleoclimatic states. State One lasted for more than 45 MY (Late Carboniferous to Early Permian) and was characterized by cold conditions in the southern hemisphere with many advances and retreats of land glaciers in Gondwana. Meanwhile, warm tropical-like conditions prevailed along the NW margin of Pangea, as shown by prolific carbonate factories and widespread biogenic mounds. State Two lasted for 33 MY (Early to latest Permian) and was marked by warm conditions in the southern hemisphere, with widespread development of coal measures, while cold-water carbonate and chert factories developed all along NW Pangea. The transition from one state to another occurred within less than one million years, some 280 Ma ago, mid way through Sakmarian time.

It is postulated that a rapid and irreversible shift in the oceanic thermohaline circulation (THC) caused the transition from Sate One to State Two against a backdrop of rising global temperatures and atmospheric CO₂ concentration, a slow northward drift of Pangea, and closure of the Uralian Trough. Prior to the mid-Sakmarian event, convergence of warm and cold water masses along NW Pangea was the main engine of THC, leading to net transport of heat from the southern to the northern hemisphere. It would seem that a net reduction in the influx of warm water via the Uralian Trough, combined with the northern edge of Pangea progressively encroaching upon polar regions, and warmer global temperatures, shifted the global THC to an alternate state. These findings imply that slowly changing earth processes, when pushed irreversibly in one direction, can create temporally abrupt, catastrophic, global-scale, life-stressing events.