FIRST PAIRED CARBONATE-CARBON AND ORGANIC CARBON ISOTOPE RECORDS ACROSS THE LUDFORDIAN (LATE SILURIAN) SUGGEST ELEVATED PCO2 BEFORE THE LARGEST PHANEROZOIC CARBON ISOTOPE ANOMALY

The Ludfordian (late Silurian) time interval records one of the largest perturbations of the carbon cycle during the Phanerozoic Eon, which was predated by distinct mass extinction of marine organisms. This interval is characterized by a large positive carbonate-carbon isotope excursion (carbonate CIE), enriched to more than +8‰,which has been globally recognized. Even though the mid-Ludfordian carbonate CIE has been studied for more than 20 years, controversies exit regarding its origin and nature. To better constrain the carbon cycle we analyzed organic carbon isotopic variation and for the first time report a paired record of carbonate-carbon and organic carbon isotope variation across the mid-Ludfordian anomaly. Our samples were obtained from carbonate-marl successions of the Perunica microplate (mid palaeo-latitudes of northern peri–Gondwana). The observed carbonate-carbon and organic carbon isotope values closely covary during most of the CIE and therefore indicate a photosynthetic origin of organic CIE. The only exception is a short interval at the onset of the carbonate CIE, where distinct decoupling was observed. Carbonate-organic carbon isotope differences (~ 31‰) significantly increase from the onset of the carbonate CIE to about ~ 33‰ suggesting elevated pCO₂ in the first half of the rising limb of the mid-Ludfordian carbonate CIE and before onset of the organic CIE. However, in the second half of the rising limb of the carbonate CIE, the carbonate-organic carbon isotope differences quickly drop to pre-excursion levels. The latter stratigraphic level coincides with a globally recognized sea-level drop. Our data reveal a significant delay in the reaction of the organic carbon isotope reservoir during an anomalous increase of carbonate-carbon isotope values. Therefore, the cause of the largest Phanerozoic perturbation of carbon cycle seems to be connected with processes influencing the carbonate-carbon isotope reservoir.