Abstract:
Based on benthic foraminiferal d18O from ODP Site 1143, a 5-Myr astronomical
timescale for the West Pacific Plio Pleistocene was established using an
automatic orbital tuning method. The tuned Brunhes/Matuyama paleomagnetic
polarity reversal age agrees well with the previously published age of 0.78 Ma.
The tuned ages for several planktonic foraminifer bio-events also agree well
with published
... dates, and new ages for some other bio-events in the
South China Sea were also estimated. The benthic d18O from Site 1143 is
highly coherent with the Earth's orbit (ETP) both at the obliquity and
precession bands for the last 5 Myr, and at the eccentricity band for the
last 2 Myr. In general, the 41-kyr cycle was dominant through the
Plio Pleistocene although the 23-kyr cycle was also very strong. The 100-kyr
cycle became dominant only during the last 1 Myr. A comparison of the benthic
d18O between the Atlantic (ODP 659) and the East and West Pacific (846 and 1143)
reveals that the Atlantic Pacific benthic oxygen isotope difference ratio (d18OAtl-Pac)
displays an increasing trend in three time intervals: 3.6 2.7 Ma, 2.7 2.1 Ma and
1.5 0.25 Ma. Each of the intervals begins with a rapid negative shift in d18OAtl-Pac,
followed by a long period with an increasing trend, corresponding to the growth of
the Northern Hemisphere ice sheet. This means that all three intervals of ice sheet
growth in the Northern Hemisphere were accompanied at the beginning by a rapid
relative warming of deep water in the Atlantic as compared to that of the Pacific,
followed by its gradual relative cooling. This general trend, superimposed on the
frequent fluctuations with glacial cycles, should yield insights into the processes
leading to the boreal glaciation. Cross-spectral analyses of the d18OAtl-Pac with
the Earth's orbit suggests that after the initiation of Northern Hemisphere glaciation
at about 2.5 Ma, obliquity rather than precession had become the dominant force
controlling the vertical structure or thermohaline circulation in the paleo-ocean. 
