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Milankovitch-climate theory provides a fundamental framework for the study of ancient climates. Although the identification and quantification of orbital rhythms are commonplace in paleoclimate research, criticisms have been advanced that dispute the importance of an astronomical climate driver. If these criticisms are valid, major revisions in our understanding of the climate system and past climates are required. Resolution of this issue is hindered by numerous factors that challenge accurate quantification of orbital cyclicity in paleoclimate archives. In this study, we delineate sources of noise that distort the primary orbital signal in proxy climate records, and utilize this template in tandem with advanced spectral methods to quantify Milankovitch-forced/paced climate variability in a temperature proxy record from the Vostok ice core (Vimeux and others, 2002). Our analysis indicates that Vostok temperature variance is almost equally apportioned between three components: the precession and obliquity periods (28%), a periodic "100,000" year cycle (41%), and the background continuum (31%). A range of analyses accounting for various frequency bands of interest, and potential bias introduced by the "saw-tooth" shape of the glacial/interglacial cycle, establish that precession and obliquity periods account for between 25 percent to 41 percent of the variance in the 1/10 kyr - 1/100 kyr band, and between 39 percent to 66 percent of the variance in the 1/10 kyr - 1/64 kyr band. These results are approximately two to four times greater than those published by Wunsch (2004) for the same Vostok time series. In all cases, most of the remaining variance is accounted for by the "100,000" year cycle, which is distinct from a background continuum that resembles autoregressive "red noise." Our analysis high- lights the importance of a comprehensive assessment of the climate signals in geologic records, and underscores the significance of orbital forcing/pacing as a primary agent of Pleistocene climate change.

Quantification of Milankovitch orbital cyclicity within ancient strata has become a principal tool for refinement of the geologic time scale. However, accurate characterization of the orbital signal in deep time paleoclimate records is commonly challenged by inadequate radiometric time constraints for calibration of the spatial rhythms to temporal periods. This problem can potentially introduce large errors into derivative orbital timescales. In this study we develop a new method for the identification and calibration of orbital cyclicity in cyclostratigraphic records. The method (average spectral misfit, or ASM) yields an objective estimate of the optimal sedimentation rate for a stratigraphic interval that preserves a record of orbital forcing. The technique also provides a formal statistical test for rejecting the null hypothesis (no orbital signal). Application of the method to assess orbital cyclicity in the upper Bridge Creek Limestone Member (Turonian) of the Western Interior Basin highlights the utility of this new cyclostratigraphic tool, and provides a means to independently evaluate conflicting interpretations of the lithologic cycles. Importantly, ASM offers a new consistent standard by which orbital timescales may be compared. Hence, the quality of an orbital timescale can be formally qualified by reporting its average spectral misfit and null hypothesis significance level. This technique will permit improvement of Mesozoic/Cenozoic orbital timescales and extension of orbital time scale development into the Paleozoic, as the method is not dependent upon well-constrained radiometric age data.

An integrated lithologic, paleontologic, and multi-proxy geochemical study of the Middle Devonian Oatka Creek Formation, a black shale in the northern Appalachian Basin, indicates that a number of different factors contributed to organic carbon-rich black shale deposition. Conditions leading to this organic-rich sedimentary deposit were ultimately controlled by a relative sealevel rise, dominantly eustatic but with possible contributions from local tectonics, which cut off the supply of carbonate to the basin. Geochemical proxy evidence –such as Mo/Ti, Fe/Ti, Corg, Spy, and δ34Spy –suggests that as sealevel continued to rise after the carbonate supply was cut off, a threshold was crossed at which point conditions in the basin shifted from dominantly anoxic to dominantly euxinic (anoxic-sulfidic bottom waters). Concurrent with the shift to dominantly euxinic conditions, the supply of siliciclastic sediments was cut off, resulting in a condensed horizon, as evidenced by the elemental ratios of Si/Al and K/(Fe+Mg) and the relative concentration of eolian silt as determined petrographically and from scanning electron microscopy. Sediment starvation in the basin appears to have facilitated the biogeochemical (re)cycling of C, N, and P. Specifically, the elemental ratios of C, N, and P and the stable carbon isotope composition of organic matter suggest that the preferential regeneration of P under anoxic conditions (and of N during the oxic phase of oxic/anoxic oscillation) led to enhanced primary production in surface waters, thereby maintaining euxinic conditions in the bottom waters through respiration of settling organic matter. Finally, it is observed that, though conditions in the basin seem to have remained consistently anoxic-sulfidic for some time after the initial shift to euxinic conditions, a progressive increase in siliciclastic sedimentation led to a corresponding decrease in the enrichment of various redox-related elements, illustrating the overriding control that sedimentation can have on geochemical proxy records.