At Sites 925-929, an orbitally tuned time scale was generated for the Pliocene from 5 to 2.5 Ma by correlating precessional magnetic susceptibility cycles to the 65°N summer insolation record that was based on the astronomical solution of Laskar et al. (1993).
Two Pliocene time intervals were investigated. The time interval from 3.3 to 2.6 Ma includes the intensification of northern hemisphere glaciation. The second interval from 5 to 4.5 Ma monitors the warmer Pliocene and is believed to reflect a major event in the closure history of the Panama Isthmus. A bathymetric comparison between sand content and carbonate accumulation records suggests that the early Pliocene was marked by stronger carbonate dissolution and a shallower lysocline than the middle Pliocene. A drastic increase in carbonate preservation occurred at about 4.6 Ma that was probably associated with the emergence of the Panama Isthmus. Variations in the lysocline depth were dominated by the 41-k.y. tilt cycle, indicating a strong coupling to high-latitude climate forcing. In contrast, the dominance of precession cycles in less undersaturated water masses well above the lysocline may indicate changes in carbonate production and carbonate flux. Middle Pliocene maxima in carbonate dissolution at 4356 m water depth lag minima of ice volume by 12 k.y. at the obliquity band, which is typical for Pleistocene Pacific records. This phase lag decreased toward shallower water depths returning to the Atlantic type of carbonate preservation.
Cyclic fluctuations in the supply of Amazon sediments responded with nearly equal concentration of variance to the precession and obliquity periods. The 41-k.y. fluctuations in the supply of Amazon siliciclastics may result from sea-level changes as inferred from an in-phase relationship with benthic . Benthic oxygen isotopes show no response to orbital precession. This suggests that climatological changes in South America and/or changes in the intensity of the North Brazilian Coastal Current are strong candidates in controlling the precessional flux of terrigenous material to Ceara Rise.
1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2GEOMAR Forschungszentrum für marine Geowissenschaften,Wischhofstraße 1-3, D-24148 Kiel, Federal Republic of Germany. firstname.lastname@example.org