Orbital Forcing of Climate: Cenozoic and Mesozoic

We seek to understand how orbital forcing influences the land, ocean, and atmosphere in times of changing and minimal ice volume. We also seek to identify and understand the non-stationary responses of the Earth’s climate system (i.e., the change or decoupling of responses relative to each other and the orbital forcing over time). We do so by studying biotic, sedimentological, and geochemical variations at the scale of ancient orbital cycles, and by developing our understanding of which climate mechanisms could be responsible for producing cyclic sedimentation.

Our understanding of the role orbital forcing plays in Plio-Pleistocene climate change focuses on the roles of solar radiation and ice. Changes in ice volume provide most of the signal in d18O curves, provide one clear positive feedback loop (ice-albedo feedback), and explain repetitive stratigraphic packages of onlap and offlap. However, for much of the Cenozoic and Mesozoic, ice was confined to either the Antarctic region, or not present in any large amounts anywhere on the globe. Likewise, large changes in tropical climates reflect changes in temperature, precipitation, and winds rather than the direct effects of ice. Our department maintains a strong research interest in the history of orbital forcing of climate both before and after the development of large northern hemisphere glaciation (Professors Herbert, Matthews, and Prell). We seek to extend the orbital theory of climate to warmer worlds of the past, to study both stratigraphic and paleoclimatic applications of the orbital model. The implications of orbital forcing are integrated into many of our other research themes.

Stratigraphic applications of the orbital model include improving the Geomagnetic Polarity Timescale by tuning magnetozones and/or biozones with the orbital chronometer, and developing quantitative models of cyclic sedimentary fluxes that match geological observations. Much of our research effort requires that we develop techniques to rapidly acquire time series data, and statistical approaches to interpreting time series. Successful examples include the use of optical densitometry on color slides, of visible reflectance on core surfaces, and a new application of infrared spectroscopy to core surfaces.

Orbital forcing has played an important role in climate changes even during the relatively short time-period of the Holocene. For instance, precessional forcing of the African monsoon drove massive changes in continental hydrology, including the well-known "greening of the Sahara." This wet interval in northern and equatorial Africa during the early Holocene, known as the African Humid Period, varies in its amplitude, rates of change, and timing in different parts of the African continent due to regional land-surface and sea-surface temperature feedbacks. Professor Russell is working at several sites across the African continent to examine how feedbacks from vegetation and SSTs affected the rate and timing of the demise of the African Humid Period, and how these dynamics may differ from monsoonal systems in South America and Asia.