TEMPORAL DECOUPLING OF DEEP-OCEAN AND SURFACE WATER IN THE TETHYS: EVIDENCE FROM THE CARBON ISOTOPE CURVE

The Permian - Triassic boundary marks the severest mass extinction in earth history. More than 90% of all skeleton building species became extinct. The Lower Triassic exhibits a decimated bio-community with an anachronistic fauna in a world evidently mainly very warm and with a weak polar temperature gradient.

Carbon isotope analyses of marine carbonates produce a marine carbon isotope curve showing huge oscillations in the Lower Triassic, seemingly episodic to periodic. The positive peaks seem to correlate with substage boundaries, evidencing that the changes in environmental conditions had a massive impact on the biota.

It has been noticed that bathymetrically deeper deposited sediments show less pronounced peaks than shallow water sediments. Two processes can explain this phenomenon: I) biotic activity and II) ocean stratification. Biotic activity preferentially incorporates ¹²C from the ocean water into the biomass. Deposition on the seafloor or reoxidation in deeper water of the biomass removes it from the surface seawater layer, which gets in this way passively enriched in ¹³C. A change in biotic activity thus can create a stronger enrichment of ¹³C in the surface water. Ocean stratification describes the reduction of mixing of surface and deeper ocean water. This controls the reflux of ¹²C from reoxidized organic material to the surface water. An increase in stratification (=reduction of circulation) thus also increases enrichment of ¹³C in the surface water.

As the substage boundaries positive δ¹³C excursions (Dienerian-Smithian, Smithian-Spathian) seem to be accompanied by climatic shifts a change in ocean circulation appears to be the dominant process for the changes in the marine carbon system. Biotic activity is less likely the main component, as the isotope shifts are simply too pronounced to be likely produced by an impoverished biota shortly after the severest mass extinction event, however, an increase in biotic activity might have played a limited role. Also volcanic activity that has been proposed as direct cause of the isotopic change is not very likely due to mass balance calculations. However, volcanic activity might have indirectly been an important force due to its effect on the climate and might have been the trigger for the climatic changes and the resulting variations in ocean circulation.