This study presents carbonate concentration, accumulation, and
preservation data for the middle to late Miocene (~5-10 Ma) to
examine changes in carbonate sedimentation associated with
surface productivity and dissolution at the Ceara Rise in the
western equatorial Atlantic. Time series of magnetic
susceptibility-based estimates of calcium carbonate concentration
for Ceara Rise Sites 925, 928, and 929, which together form a
transect that spans approximately 1500 m of water depth
(3041-4356 m), show substantial changes in the depth gradient of
calcium carbonate concentration between 5 and 10 Ma. The
estimated carbonate concentrations at the shallow Site 925 were
relatively high (
= 77.2%)
throughout this whole time interval. Prior to about 5.5 Ma, only
trace amounts of carbonate are preserved at the deep Site 929,
indicating that the carbonate compensation depth (CCD) was close
to or shallower than the depth of this site during most of the
middle to late Miocene. To further examine these gradients, we
generated carbonate concentration, accumulation, and
foraminiferal preservation data for four representative time
slices from Sites 925, 928, and 929.
The results show that, during the late Miocene, the quasi-cyclic changes in carbonate concentration at shallow Site 925 can be attributed to either production or dilution by terrigenous material, depending on the interval. Calcium carbonate sedimentation at Site 925 averaged >20 g/m2/yr, more than twice the Holocene rates. The foraminiferal assemblage was extremely well preserved, and the sediment fluxes reflect carbonate rain rates to the seafloor. These high accumulation rates are comparable to the highest sediment trap-measured fluxes of carbonate from the equatorial Pacific. In contrast, calcium carbonate at the deep sites is heavily dissolved and dissolution was the dominant control of concentration changes. This decrease reflects a shoaling of the CCD, which coincides with a period of sea level rise. The results are not consistent with an increase of North Atlantic Deep Water (NADW) during the late Miocene, although further work is necessary to evaluate the relative contributions of NADW and southern component water at this site.
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).
2Department of Geological Sciences, Brown University, Providence, RI, 02912 U.S.A. dmurray@brown.edu
3Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, U.S.A.
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