ND ISOTOPES REFLECT EUSTATIC AND CLIMATIC CHANGE DURING LATE PALEOZOIC ICE AGE: A RECORD FROM THE BIRD SPRING PLATFORM, WESTERN US

The late Paleozoic ice age (LPIA) consisted of numerous episodes of southern hemisphere glacial advance and retreat resulting in global sea level change. To better understand the far-field effects of these glaciations, we generated a record of seawater Nd isotopes using conodonts from the Bird Spring platform, western U.S.. In the late Paleozoic the Bird Spring platform formed near the equator along the western edge of North America in the southern Antler foreland basin and likely faced the eastern Panthalassic Ocean. It accumulated thick sequences of carbonate-rich marine sediments during the Carboniferous and Permian. Arrow Canyon carbonates reflect a primarily shallow water high-energy wave or tidally-dominated setting, while Marble Canyon deposits formed in low energy environments or as turbidites indicating sedimentation at the platform’s edge or on the upper slope.

Late Paleozoic ¹⁴³Nd/¹⁴⁴Nd values (expressed in epsilon notation and corrected for age, ε_Nd(t)) from Bird Spring platform conodonts exhibit a wide range, -12.0 to -5.7 and vary with sea level. The largest fluctuations of 3 to 5 epsilon units occur during the Late Mississippian and Early Pennsylvanian, coincident with the onset of major southern hemisphere glaciation, global regression and large eustatic swings. In coastal waters, the isotopic composition of dissolved Nd reflects changes in oceanic circulation and the interconnectedness of epicontinental seas with open marine waters, or variations in continental weathering patterns that alter the dissolved Nd inventory delivered to the shelf by rivers. We measured the lowest values in rocks deposited during relative sea level lowstands and/or progradational periods when material delivered via rivers was building out across the platform. Similar decreases are recorded at both Arrow Canyon and Marble Canyon implying enhanced contributions of unradiogenic Nd weathered from land caused the negative ε_Nd(t) excursions, rather than restricted circulation. More radiogenic values correspond to periods of sea level transgression with the highest ε_Nd(t) measured in conodonts extracted from rocks forming retrogradational or aggradational parasequences. We interpret the relatively radiogenic values to reflect encroachment of Panthalassic waters on the edge of the North American craton.