We have developed a detailed time scale for the late Miocene
and later middle Miocene by correlating lithologic cyclicity
observed at Site 926 to the astronomically determined variations
in northern hemisphere summer insolation. Most of the equivalent
section at Site 925 can be correlated in the same manner. The
validity of the phase relationship that we have assumed, whereby
maximum terrigenous content is correlated with peak insolation,
is supported by examining the benthic 
data from the same site. The time scale
is probably accurate to the level of the individual precession
cycle back to about 7 Ma and in many intervals back to 13 Ma;
elsewhere, there are intervals where further work may require
adjustments of one or more pre-cession cycles.
At least in the upper part of the section investigated, we
conclude that the lithologic signal is present because the input
of terrigenous material to the Ceara Rise varied inversely with
northern hemisphere summer insolation. At the same time,
dissolution of foraminifers in the sediment as well as 
varied, chiefly in
response to obliquity, with a significant phase lag that probably
reflects the long residence time of carbon and nutrients in the
ocean.
1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2University of Cambridge, Godwin Laboratory, Free School Lane, Cambridge, CB2 3RS, United Kingdom. NJS5@cam.ac.uk
 |
 |