18. UPPER QUATERNARY WESTERN ATLANTIC PALEOCEANOGRAPHY AND TERRIGENOUS SEDIMENTATION ON THE AMAZON FAN: A VIEW FROM STABLE ISOTOPES OF PLANKTONIC FORAMINIFERS AND BULK ORGANIC MATTER¹

R.R. Schneideltaer,² P.J. Müller,² B. Schlünz,² M. Segl,² W.J. Showers,³ and G. Wefer²

ABSTRACT

Three holes of 200 to 300 m in length (Holes 938A, 940A, and 942A) drilled during Ocean Drilling Program Leg 155 on the Amazon Fan have provided very expanded hemipelagic sediment records from crests of abandoned channel-levee systems. These sediments were analyzed for total organic and inorganic carbon contents, the delta¹³C ratios of total organic carbon, as well as delta¹⁸O and delta¹³C ratios of planktonic foraminifers, to evaluate western Atlantic paleocirculation and fan sedimentation processes with respect to upper Quaternary climate and sea-level change.

The foraminifer isotope records match the well-known pattern observed in cores from the open ocean and thus allow confirmation of preliminary shipboard bio- and magneto-stratigraphic chronologies. Isotopic excursions detected in the planktonic records in relatively undisturbed sediment sections can be related to similar events observed in the Southern Ocean, off southwest Africa, and in the Caribbean Sea. This indicates that changes in North Brazil Current hydrography off the Amazon River are closely connected to fluctuations in oceanwide surface circulation. There is no clear evidence for freshwater discharge events in the isotope records.

Organic and inorganic chemistry confirm the previous models of fan sedimentation for the last 140 k.y. Only during the maximum sea-level highstands was fan sedimentation dominated by fluxes of marine calcium carbonate and organic matter. During lowstands, below a threshold sea-level fall of 40 to 50 m, terrigenous input dominated the sedimentation via distributary channel-levee systems, with fan growth occurring at very high rates (~5 to 20 m/k.y.). Consistently low delta¹³C values of ~1‰– 27‰ for the organic matter in all glacial deposits suggest that the Amazon River load was transported directly to the upper and middle fan during sea-level lowstands, and not partially westward with the North Brazil Current as is the case of the modern ocean.

¹Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997. Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program).
²Fachbereich Geowissenschaften, Universitaet Bremen, 28334 Bremen, Federal Republic of Germany. rschneid@zfn.uni-bremen.de
³Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, U.S.A.