"GICE GICE BABY" - BUT WAS THERE ICE? (Invited Presentation)

Many positive carbon isotope excursions identified in the early Paleozoic geologic record have been used to infer changes in the ocean–atmosphere-biosphere carbon system. A common interpretation is to link positive excursions to increased organic carbon burial, drawdown of atmospheric CO₂, global cooling, and biotic turnover events. We present results from a high-resolution, multi-proxy study of the GICE (Guttenberg Carbon Isotope Excursion), a prominent Late Ordovician positive carbon isotope excursion. In contrast to the general interpretation of cooling, our data show the GICE is coincident to warming. Our data are from multiple sections spanning one basin with detailed temporal and spatial comparisons of different proxies correlated using high-resolution event stratigraphy.

We measured carbonate and organic carbon isotopes in bulk samples and oxygen isotopes of conodonts from sites across the former extent of the epicontinental sea that covered Laurentia. A primary goal was to determine if the GICE reflects changes in the climate system and if those changes can account for lithologic and paleontologic changes that indicate cooling during this time period. We also measured neodymium isotopes preserved in conodonts to track changes in continental weathering and ocean circulation. Lastly, we used K-bentonite beds to constrain the relative timing of different geochemical and lithological events relative to each other across the basin.

The oxygen isotopes show a ~1.5‰ decrease that apparently is diachronous across the epicontinental sea. In sections along the southern margin, the decrease occurs a short stratigraphic distance below the GICE, whereas in interior settings, values decrease later, during the GICE interval. These results indicate that the GICE interval coincided with a transient warming event. This conclusion is supported by neodymium isotopic variations that suggest increased weathering rates correlated with the warming. Our results highlight the importance of high-resolution records from multiple sections for identifying underlying causes of other Paleozoic carbon isotope excursions.