A COUPLED OXGYEN ISOTOPE AND TRACE METAL RECORD OF GLACIOEUSTATIC SEA LEVEL VARIATION AND ENHANCED WATER COLUMN PRODUCTIVITY

Oxygen isotope records of phosphatic microfossils have the potential to extend detailed paleoclimate and paleoceanographic records much deeper in Earth history than previously possible. This study provides a high resolution, stratigraphically controlled, oxygen isotope database of Late Carboniferous cyclic sedimentary deposits within the North American midcontinent. These data, recorded within conodont apatite, yield oxygen isotope excursions of 1.7‰, 0.6‰, and 0.8‰. Each of these excursions occurs between the deep and shallow water facies of three North American midcontinent cyclic deposits: the Upper Ft. Scott (Desmoisian) and the Swope and Dennis (Missourian), respectively. The observed shifts are of the magnitude recorded by Cenozoic marine microfossils on glacial/interglacial timescales. These trends occur generally in phase with lithologic change attributed independently to changing sea level; however, the majority of the isotopic shift is observed within the black shale (deep water) facies. This does not support a model of gradual isotope and sea level change completely in phase with lithofacies shifts. We interpret this more complex record as evidence of efficient nutrient cycling with the onset of anoxic bottom waters and black shale deposition, leading to enhanced primary productivity in the overlying water column. Initially, the increased organic flux would sustain bottom water anoxia and black shale deposition despite decreasing sea level and any weakening of water column stratification. Eventually, sea level decrease linked to Gondwanan glacial growth led to more frequent disruption of the chemocline, allowing mixing between anoxic bottom water and the oxygenated surface water. Ultimately this shallowing resulted in loss of a distinct chemocline and cessation of black shale deposition. This model is supported by decreasing trace metal and organic C concentrations within the Hushpuckney black shale member of the Swope formation seen in previous studies. As with oxygen isotope records from more recent samples, interpretations of deeper Earth history require high resolution stratigraphically controlled sampling and careful examination of each cyclic record.