GSA Connects 2021 in Portland, Oregon

Paper No. 21-1
Presentation Time: 9:00 AM-1:00 PM


ZAMBITO IV, James1, MCLAUGHLIN, Patrick I.2, HOLICKY, Madeline G.3, FARBARIK, Olivia3, CLINKSCALES, Emily N.3 and FRIES, Morgan A.3, (1)Department of Geology, Beloit College, 700 College St, Beloit, WI 53511-5509; Department of Geology, Beloit College, Beloit, WI 53511, (2)Indiana Geological and Water Survey, Indiana University, 611 N. Walnut Grove, Bloomington, IN 47405, (3)Department of Geology, Beloit College, 700 College St, Beloit, WI 53511-5509

The Mid-Late Givetian (Middle Devonian) was a volatile time in Earth’s history. Marine faunal migration and extinction, forest evolution and increased atmospheric oxygenation, eustatic fluctuation, carbon-cycle perturbation, warming/cooling, aridity variation, and decreased oxygen in epeiric seas occurred over a period of <2Myr during and after the complex Global Taghanic Biocrisis (GTB).

In the Northern Appalachian Basin Lansing Core, an expanded stable isotope profile of inorganic carbon (δ13CCARB) through the GTB spans an incredible range of ~8‰ (~+3 to near -5). Notably, there is a -1.5‰ excursion at the basal Tully Formation (GTB-onset; baseline values ~+1‰) coincident with warming and influx of paleoequatorial taxa. A subsequent positive peak in values (~+3‰) within the Tully (mid-GTB) corresponds with cooling and endemic taxa return. Onset of a negative shift in values begins in the uppermost Tully, reaching a plateau of almost -2‰ in the lower Geneseo Formation (end-GTB) during an additional warming pulse. In the overlying Sherburne Formation this plateau in values drops to near -5‰ associated with evidence of another warming pulse. This overall positive-to-negative shift in values during and after the GTB is documented globally as the lightest values in the Devonian, but the magnitude of negative shift discovered here is more extreme than previously reported.

In the studied section, stable isotopes of total organic carbon (δ13CTOC) show a complex relationship to δ13CCARB. In the Geneseo, δ13CTOC follows a similar trend to δ13CCARB with a shift to lighter values. However, δ13CTOC and δ13CCARB patterns are decoupled within the overlying Sherburne, with δ13CTOC exhibiting a positive shift of ~+1.5‰ and large degree of scatter. In more basinal successions this decoupling is not present. Thus, the positive δ13CTOC shift may record influx of terrestrial organics with prograding siliciclastics. Ultimately, the light δ13CCARB values in the Tully, Geneseo, and Sherburne, coincident with evidence for warming, suggest pulsed release of sequestered methane to the global ocean-atmosphere system. Future work will include documentation of carbon isotope patterns in Mid-Late Givetian age rocks along transects in the Appalachian Basin and globally to differentiate regional from global signatures.