NEODYMIUM AND OXYGEN ISOTOPIC CONSTRAINTS ON UPPER ORDOVICIAN PALEOCEANOGRAPHIC EVOLUTION ACROSS THE DUBUQUE/MAQUOKETA CONTACT IN NE IOWA AND SE MINNESOTA

Oxygen and neodymium isotope measurements of conodonts, phosphatic brachiopods, and authigenic phosphate show an 2-3 unit e_Nd increase but no consistent d¹⁸O_phos trends across the Dubuque/Maquoketa contact in NE Iowa and SE Minnesota. The lithologic transition between the limestone-rich Dubuque Formation, and the shalely Maquoketa Formation was sampled on a north-south transect at three locations in IA and MN. The contact between the two formations is a phosphatic hardground and interpretations from previous models have suggested that the phosphate was deposited during an upwelling of cool, nutrient rich, oxygen poor water. We tested this hypothesis using d¹⁸O_phos of species-specific conodont separates where possible (to minimize potential artifacts introduced by varying species abundances in mixed conodont assemblages). In addition, mixed separates, inarticulate brachiopods and authigenic phosphate were all sampled to increase the number of temperature estimates and to assess diagenetic overprinting. We tested for water mass changes through e_Nd measurements on the brachiopod Leptobolus.

No apparent temperature trends were observed through the interval in any of the d¹⁸O_phos results, but an offset of ~1‰ between the conodonts Drepanoistodus suberectus and Panderodus gracilis was present. In addition, e_Nd values from the Dubuque range from -12.8 to -11.4 and increase to -10.9 to -9.5 in the overlying Maquoketa. Our results are consistent with a paleoceanographic model influenced by sea level rise. The Dubuque and Maquoketa represent a transgressive-regressive subcycle with highstand occurring at the contact between the two formations. During the transgression, fresh-water runoff from the Taconic highlands could have driven a quasi-estuarine gyre that resulted in surface currents flowing basinward and out of the epeiric sea while cool ocean water flowed into the epeiric sea through the Sebree Trough. The incursion of ocean water and runoff from the Taconic highlands is reflected as increasing e_Nd. This interpretation and the apparent lack of temperature change suggests that the Dubuque/Maquoketa transition is best interpreted as a change in circulation patterns rather than a climatic event.