We present carbonate dissolution records (carbonate content, sand percentage, and foraminiferal fragmentation indices) for Sites 927 (3315 m water depth) and 929 (4356 m), which were recovered during Ocean Drilling Program Leg 154 at the northeast slope of the Ceara Rise, western equatorial Atlantic. These records comprise the time interval from 2.6 to 1.0 m.y. Dating is based on tuning variations of the magnetic susceptibility records to orbital parameters.
Neogene sediments on the Ceara Rise consist primarily of biogenous carbonate ooze that is diluted by varying input of terrigenous material delivered by the nearby Amazon River. Similar to oxygen isotopic variations, the carbonate and sand content records are dominated by strong 41-k.y. cycles during the late Pliocene and the early Pleistocene, except for the period between 2.0 and 1.6 m.y., which shows predominantly precession-driven fluctuations. Higher mean carbonate and sand contents in both sites indicate a deeper lysocline and hence a better deep-water ventilation during this intermediate period. The intersite comparison of the accumulation rates shows that the lower sedimentation rates in the deeper Site 929 result exclusively from the lower carbonate accumulation due to dissolution. The accumulation rates of the terrigenous material are nearly identical over most of the record, which confirms the validity of the assumption that the sediment input for the two sites is exactly the same. Cross-spectral analysis indicates that Pliocene–Pleistocene changes in carbonate dissolution and hence in deep-water production appear to lead ice volume at the obliquity band, but are in-phase at the precessional band. However, the similarity between the foraminiferal dissolution indices, which are independent of terrigenous dilution, and the carbonate and sand content records reveal that the decay of terrestrial organic matter, which reduces carbonate ion concentration in the pore water, is more important for carbonate dissolution at the shallower Site 927 than changes in deep-water chemistry. This observation supports the idea of the influence of even supralysoclinal dissolution to the ocean carbon budget.
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).
2Fachbereich Geowissenschaften, Universität Bremen, 28334 Bremen, Federal Republic of Germany. bickert@zfn.uni-bremen.de
|
|