Scott Rutherford and Terri King (Both at: Graduate School of Oceanography, URI, Narragansett, RI 02882; 401-874-6662; e-mail email@example.com, firstname.lastname@example.org)
We used evolutive cross-spectral analyses of ODP Legs 138 and 154 carbonate, and d18O to analyze the phase evolution of equatorial Atlantic and Pacific carbonate sedimentation from 0-6 Ma. While the phase changes in some orbital bands correspond to major paleoceanographic events, the phase relations in other bands do not. The change from Atlantic-type to Pacific-type carbonate stratigraphy appears in the phase relationship between Leg 138 carbonate and d18O. Prior to 3 Ma, the two records are nearly 180 out of phase with greatest coherence occurring in the 400 ky eccentricity band. After 3.0 Ma carbonate and d18O are nearly in phase with greatest coherence shifting to the 100 ky eccentricity band. Although there is significant coherence at obliquity and precessional bands, these frequencies appear unaffected by the factors that drove the eccentricity phase and frequency shift. If the eccentricity phase and frequency shift was related to changing deep water circulation caused by closure of the Panamanian Isthmus, then a different dissolution mechanism, or surface water conditions (e.g. productivity) must be forcing the obliquity and precessional-band carbonate fluctuations. General circulation models (Maier-Reimer et al., 1990) and planktic foraminiferal data (Chaisson, 1995) indicate that eastern equatorial Pacific upwelling remained essentially unchanged before and after isthmus closure, supporting our surface water interpretation. A comparison of the two equatorial carbonate records reveals a phase shift in the obliquity band at ~4.1 Ma. Prior to 4.1 Ma carbonate sedimentation in the two oceans was out of phase. From 4.1 Ma to the present, however, there has been a steady phase shift resulting in an in-phase relationship by ~2.0 Ma. A similar phase relationship is observed in the 19 ky and 23 ky precessional bands. The steepest gradient in the phase shift seems to propagate through the orbital bands, occurring first in the 19 ky (~4.5 Ma), then the 23 ky (~3.5 Ma) and finally the 41 ky at ~3.25 Ma (and 100 ky at ~3.0 Ma?). Hodell et al. (1985) cite isotope and faunal evidence for a Circumpolar Deep Water event at ~4.1 Ma that marks the re-organization of deep water circulation during isthmus closure. Our results indicate that different frequencies of carbonate sedimentation responded in turn to the changing circulation patterns.
1996,EOS, Trans. AGU, (77)