Geochemical Signatures of Milankovitch Periodicity Recorded in the Rare Earth Element Content of Fossil Vertebrates from the Eocene-Oligocene White River Group of Northwest Nebraska

Following the early Eocene thermal maximum, global climates cooled from the Greenhouse climate of the late Mesozoic and early Paleogene to the Icehouse climate of the late Paleogene and Recent. The Eocene-Oligocene White River Group in northwest Nebraska and southwest South Dakota is a succession of nonmarine strata that records the final phases of this transition. Although the overall trend during this transition is a change to progressively drier conditions in this region, the Eocene-Oligocene boundary is characterized by a substantial decrease in paleotemperatures, but a negligible increase in aridity. Rare earth elements (REE) in vertebrate fossils from upper Eocene and Oligocene strata of Toadstool Geologic Park, northwestern Nebraska, provide a previously unrecognized archive of this change. REE signatures indicate that heavy REE-enriched alkaline groundwater reacted with light REE- and middle REE-enriched sediments to produce three component mixtures. REE signatures become increasingly light REE- and middle REE enriched toward the top of the section as the paleoenvironment became cooler and drier, suggesting that REE signatures may be climate proxies. Periodograms generated by spectral analysis of the Nd_N/Yb_N ratio indicate significant periodicities at ca. 1050, 800, 570, 440, and 225 ka. Cycles calculated from the Gd_N/Yb_N and La_N/Yb_N ratios are comparable. These periods are similar to modulations of Milankovitch-band astronomical, climate fluctuation, and stratigraphic periodicities. Although similar, the periodicities calculated do not correspond exactly with the longerterm astronomical Milankovitch cycles or their modulations. It is possible that Paleogene climate cycles in the North American interior were modified by local or regional orographic effects. However, the discrepancy might also be due to errors in sedimentation rate, hiatuses, or aliasing due to inadequate sampling frequency. More extensive sampling, perhaps over longer stratigraphic ranges, with reconsideration of depositional rates and possible lateral variability, may be required.