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