21. EARLY PLIOCENE DEEP-WATER CIRCULATION: STABLE ISOTOPE
EVIDENCE FOR ENHANCED NORTHERN COMPONENT DEEP WATER1
Katharina Billups,2 A. Christina Ravelo,3
and James C. Zachos2
ABSTRACT
High resolution benthic (Cibicidoides spp.) stable isotope and
percent sand fraction records were constructed for the early
Pliocene (3.3-4.7 Ma) at Ocean Drilling Program (ODP) Sites 925
(3042 m water depth) and 929 (4361 m water depth). These sites
are located in the modern mixing zone of North Atlantic Deep
Water (NADW) and Antarctic Bottom Water (AABW). We evaluate
changes in deep-water circulation between the two sites using the
vertical stable isotope gradient. The oxygen isotope records of
both sites have well defined maxima that correspond to
established glacial isotope stages for this time interval.
Between 4.2 and 3.7 Ma, average 
values from the shallower Site 925 are higher than
at the deeper Site 929 by 0.2‰, indicating that NADW may
have been relatively warm and salty compared to today. Site 929 values correspond well to
previously published records, suggesting an average decrease in
the isotopic composition of ocean water of ~0.4‰ with
respect to the late Holocene. At the deeper Site 929, there is
considerable 
variability with extremely low minima. These results indicate
that Site 929 was highly sensitive to changes in the relative
flux of northern vs. southern component deep water. Site 929 minima coincide with
glacial stages suggesting a deep-water circulation link similar
to that observed during the late Pliocene/Pleistocene. At Site
925, values are
consistently high and minima can be entirely accounted for by
global variability.
These data indicate a relatively strong NADW flux and suggest
that the shallower Site 925 remained within the core of NADW
throughout the majority of the time interval. The prevailing
influence of northern component deep water evident at these sites
is consistent with climate models that attribute early Pliocene
warmth to increased northward heat transport.
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).
2Earth Sciences Department, University of
California, Santa Cruz, CA 95064, U.S.A. kbillups@earthsci.ucsc.edu
3Marine Sciences, University of California, Santa
Cruz, CA 95064, U.S.A.
