Neogene Low-Latitude Magnetostratigraphy from Site 710 and Revised Age Estimates of Miocene Nannofossil Datum Events

Abstract

Ocean Drilling Program Leg 115 recovered, for the first time, a largely continuous Neogene magnetostratigraphic sequence from an equatorial environment. This sequence (Site 710) has been correlated to the geomagnetic polarity time scale by means of nannofossil biostratigraphy. When the Miocene nannofossil magnetobiochronology from Site 710 is compared with other predominantly mid-latitude magnetobiochronologies, an evaluation of consistency can be made. The results require substantial revision of the age estimates of Miocene nannofossil zonal markers, relative to the values presented in the currently accepted Neogene nannofossil chronology.

Biostratigraphic Summary for Leg 138

“…Our estimate for the upper limit of Cyclicargolithus floridanus agrees well with that given by Backman et al (1990), based on an interpolation at central Pacific DSDP Site 574. Olafsson (1991) showed that this species survived much longer at mid-latitude Site 608 (see also Raffi et al, this volume).…”

“…However, Backman et al (1990) based their estimate on an interpolation between the base of Discoaster hamatus (for which they used an age of 10.5 Ma) and the upper limit of Sphenolithus heteromorphus at 13.5 Ma; their estimate would be closer to ours using our age (discussed above) for the base of D. hamatus, and we conclude that our estimate is reliable.…”

“…Their age is based on an inteΦolation between the ages for the base of Catinaster coalitus and the upper limit of Sphenolithus hetero-morphus, using the ages obtained by Backman et al (1990). Using our ages for these two events, the discrepancy is significantly reduced.…”

“…First, our age for the base of Discoaster hamatus, when converted back to BKF85, is 9.87 Ma, whereas Backman et al (1990) derived an age of 10.50 Ma at tropical Indian Ocean DSDP Site 710 with primary magnetostratigraphic control. However, Olafsson (1991) obtained an age of 9.9 Ma in Site 608 in the North Atlantic, also with primary magnetostratigraphy.…”

Explanatory Notes

“…Zonal codes are those of Martini (1971). Age column references represent: (1) Thierstein et al, 1977;(2) Sato et al, in press; (3) Backman and Shackleton, 1983; (4) Rio et al, in press; (5) Backman and Pestiaux, 1986;(6a) Berggren et al, 1985a;(6b) Berggren et al, 1985b; (7) Backman et al, 1990;(8) Zijderveld et al, 1986;(9) Rio et al, 1990; (10) Fornaciari et al, 1990. specimens cannot be identified at the species and/or generic level).…”