Abstract:
We compared early to middle Miocene Sr isotope chronostratigraphies for two sites on the Ceara Rise in the western, equatorial Atlantic (ODP Leg 154) with previously published data from sites 289 (Hodell and Woodruff, 1994) and 747 (Oslick et al., 1994) to try to resolve a small, but consistent, discrepancy between chronostratigraphies from the earlier data sets. The age model for site 289 is based on biostratigraphy, but rapid early Miocene sedimentation rates (27 m/Ma) facilitate high resolution analyses. Site 747 (Kerguelen Plateau) has excellent magnetostratigraphy, but a sedimentation rate of only 3 m/Ma. Stratigraphies for Leg 154 sites are based on biostratigraphy, and early Miocene sedimentation rates range from 27 m/Ma at site 926 down to 8 m/Ma at site 929. Leg 154 Sr isotopic measurements were collected from hand-picked foraminifera, with samples spaced at approximately 0.25-0.5 Ma intervals. Sr chronostratigraphies were calculated by comparison to ninth order polynomials fit through data from sites 289 and 747. The differences in ages calculated from the two sites were generally small (0.2-0.7 Ma), but ages derived from site 289 data were consistently younger. Shipboard biostratigraphic control points for sites 926 and 929 correlate more accurately to ages derived from site 289 data. The discrepancy between site 289 and 747 chronostratigraphies has been attributed to errors in age assignments, interlaboratory differences, or differential diagenesis. Because of its greater burial depth, site 289 is most likely to be affected by diagenesis; however, the correlation between site 289 and the Leg 154 data, representing two sites with very different burial histories, suggests diagenesis can be eliminated as a possible cause.