CO2 AS THE DRIVER OF CHANGES IN SILURIAN CLIMATE?

A major early Silurian positive carbon isotope excursion began at or near the top of the Pterospathodus amorphognathoides conodont biozone (herein referred to as the Ireviken Excursion), and is recorded in sections in Europe, Australia and North America. We have documented the shift in carbonate carbon in three successions in the mid-continent of North America, also conducting paired carbon isotope analyses of carbonate and organic carbon on the section of the Wayne Formation in central Tennessee. The results indicate that a positive shift in δ¹³C_carb of +3.9‰ coincided with a minimum in δ¹³C_org of -30.1‰. This data indicate that pCO₂ was increasing during the onset of the Ireviken Excursion. Subsequently, the δ¹³C_carb curve decreased to a stable baseline near +1‰ in the upper part of the Maddox, while the δ¹³C_org curve increased back to baseline. The rate and timing of changes in the two isotopic records reveal a complex, counter-intuitive relationship suggesting that high organic carbon burial initially coincided with increasing pCO₂ during the early Silurian Ireviken Excursion. This is proposed to reflect warming in polar regions that switched the site of deep water formation to low latitudes. As a result, organic carbon burial increased in the anoxic deep oceans, which provided a negative feedback on rising pCO₂ levels. The alteration of deep ocean circulation also lowered nutrient availability in shelf environments, which severely affected the biota and allowed a healthy carbonate factory to develop throughout the tropics.