Neodymium Isotope Ratios and a Positive d13C Excursion: Connecting Oceanographic and Climate Changes Near the M4-M5 Sequence Boundary of the Late Ordovician

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A significant positive carbon isotope excursion first described from the Guttenberg Member of the Decorah Formation in Iowa called the GICE is a defining feature of Late Ordovician chemostratigraphy. It occurs near the M4-M5 sequence boundary of the Late Ordovician and has been correlated across the globe using prominent K-bentonite ash beds and conodont biomarkers. The cause of the GICE has been debated, with some arguing that it represents the initiation of a glacial period in a greenhouse climate 10 Ma before the end Ordovician glaciation and extinction. A variety of cooling signals have been identified across the M4-M5 sequence boundary support this hypothesis, but evidence contradicting this cooling hypothesis remains. Neodymium isotopes were collected from Rochester, MN, to determine if the movement of water masses over space and time played a role in the cooling signals witnessed across the M4-M5 and the GICE. Original δ13C data were collected from Dickeyville, WI, for comparison to existing carbon isotope curves showing the GICE. The GICE is well preserved at Dickeyville, WI, and plots similarly to previous studies from the area. It occurs after two significant and widespread K-bentonites (Deicke, Millbrig) and reaches excursion values characteristic of this location (~+2.0‰). εNd data from Rochester, MN, show values initially increasing, from -16.5 to -12.16 epsilon up to the Deicke K-bentonite. A shift in εNd values occurs sometime between the Millbrig and Deicke K-bentonites. After the Millbrig K-bentonite, εNd values are lower, ranging from -14.3 to -16.65. These results are indicative of high weathering rates from the nearby Precambrian shield and possibly a major regression occurring between the Millbrig and Deicke K-bentonites. However, a previously recorded δ18O decrease in the study area and lithostratigraphic evidence for a transgression across the M4-M5 conflicts with a glacially induced regression. Local tectonic quiescence during the GICE precludes a tectonically driven regression causing a low εNd weathering flux. Therefore the decrease in εNd values between the Millbrig and Deicke K-bentonites could be indicative of a climate induced spike in weathering rates. These findings imply that previous work suggesting εNd as a direct proxy for eustatic changes is complicated during periods of changing climate and weathering flux.