In the Pit, the contacts between glacial and interglacial strati-graphic levels are abrupt, with dramatically differing sediments on each side of the contact. Within each climatic interval, the sedimentary transitions across stratigraphic levels are much more subtle, or, in the case of arbitrary levels, lacking entirely (Barnosky and Rasmussen, 1988; Bell and Bar-nosky, 2000). This pattern suggests episodic deposition, with periods of nondeposition (unconformities or disconformities between climatic intervals) separating a sequence of discrete sediment packages, each of which samples some unknown slice of time within its respective climate interval. Which of the several middle Pleistocene glacial and interglacial stages are represented is unknown, although the dating considerations discussed earlier provide some constraints.
If the correlation expressed in figure 7.4 is correct, Pit levels 1-3 probably accumulated during interglacial stage 19 or 21, using the oxygen isotope chronology of Raymo et al. (1997) and Raymo (1998) (figure 7.5). Because the correlation of Pit to Velvet Room sediments suggests that levels 1-3 are older than 780 Ka, stage 21 is the most likely candidate. In that case, the climatic intervals that precede levels 1-3 may match sequentially with the glacial-interglacial cycles interpreted from the oxygen isotope curve (Raymo et al., 1997; Raymo, 1998), as indicated in figure 7.5 (climatic intervals 22, A, B, C). This would be consistent with the sedimentological and fau-nal evidence (chapter 23), which suggests that levels 1-3 represent the warmest and most arid of the warm periods, because the oxygen isotope excursion that precedes stage 22 (A in figure 7.5) was considerably less pronounced than that for stage 21. Alternatively, some other sequential combination of the glacials and interglacials noted earlier and those labeled D, F (glacials) and E (interglacial) in figure 7.5 could be represented. It does not seem likely that glacials or interglacials older or younger than these are represented, in view of the suspected age of the Pit and Velvet Room sequences.
This interval of time is of interest from a climatic point of view in that it spans at least part of the transition from a 41,000-year rhythm for glacial-interglacial cycles to a 100,000-year rhythm. Transition to the 100,000-year cycle began about 1 Ma ago. By about 800 Ka ago the 100,000-year cycle started to become dominant (Raymo, 1998), and it was firmly established by circa 640 Ka ago (Schmieder et al., 2000). Thus the upper cycle at Porcupine Cave (levels 1-5) very probably accumulated during the time of the 100,000-year periodicity, whereas the lower Porcupine Cave levels may sample the transitional climatic intervals as the 41,000-year rhythm switched over to 100,000-year periodicity.
FIGURE 7.5 Potential correlation of the Porcupine Cave stratigraphie sequences with global climate changes indicated by the oxygen isotope curve. The oxygen isotope data are adapted from Raymo et al. (1997).
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