Abstract:
Astronomical tuning plays a central role in developing the chronostratigraphy of marine sediment sections. For Pleistocene sections, tuning is typically accomplished by matching the 41 and 23 ka components of oxygen isotopes (delta (super 18) O-41, delta (super 18) O-23) to orbital obliquity and precession with constant phase lags of 69 degrees and 78 degrees , respectively (depleted delta (super 18) O-41 lags maximum obliquity by 7.8 ka, while depleted delta (super 18) O-23 lags minimum precession by 5 ka). This approach places all records on the same time-scale relative to one another and relative to insolation forcing because Pleistocene delta (super 18) O is globally correlative. Thus, lead and lag relationships among climate variables measured at globally distributed sites can be assessed in an effort to understand the underlying physics of climate change. For Pleistocene sections, variables such as dust content, magnetic susceptibility, colour reflectance, and gamma-ray attenuation porosity evaluator (GRAPE) are typically tuned directly to precession or to a precession-dominated insolation target. Unlike Pleistocene delta (super 18) O, these variables are not globally correlative and few have unambiguous models linking them to insolation forcing. Consequently, the capacity to compare Pliocene lead and lag relationships among climate variables from different sites is considerably diminished, as is the capacity to evaluate the climate response to orbital forcing. Here a Pliocene tuning strategy is presented which builds on the precession-based approach by incorporating a final tuning step involving delta (super 18) O-41. Results from application to Site 659 and the Mediterranean Rossello Composite Section suggest qualified success. Application to other Pliocene sections is required to verify the utility of this astronomical tuning approach.