Abstract:
Seismic stratigraphic evidence from the western and northern North
Atlantic indicates that a major change in abyssal circulation occurred in
the latest Eocene to earliest Oligocene. In the northern North Atlantic,
the widely-distributed reflector R4 correlates with an unconformity that
can be traced to its correlative conformity near the top of the Eocene.
This horizon reflects a change from weakly circulating (Eocene) to
vigorously circulating bottom water (early Oligocene). Sediment
distribution patterns provide evidence for strong contour-following
bottom water flow beginning at reflector R4 time; this suggests a
northern source for this bottom water, probably from the Arctic via the
Norwegian-Greenland Sea and Faeroe-Shetland Channel. Erosion and
current-controlled sedimentation continued through the Oligocene;
however, above reflector R3 (middle to upper Oligocene), the intensity of
abyssal currents decreased. Above reflector R2 (upper lower Miocene)
current-controlled sedimentation became more coherent and a major phase
of sedimentary drift development began. This resulted from further
reduction in speeds and stabilization of abyssal currents.
Paleontological and stable isotopic data support these interpretations.
In the Bay of Biscay/Goban Spur regions, a major δ180 increase
began at -38 Ma (late Eocene), culminating in a rapid (<0.5 my)
increase in δ180 just above the Eocene/Oligocene boundary (-36.5 Ma).
A rapid δ13C increase also occurs at -36.5 Ma in these sites. Major
changes in benthic foraminiferal assemblages also occurred between the
middle Eocene and the earliest Oligocene: 1) In the Labrador Sea, a
predominantly agglutinated assemblage was replaced by a calcareous
assemblage between the middle Eocene and early Oligocene; 2) In the
abyssal (> 3km) Bay of Biscay, an indigenous Eocene calcareous fauna
including Nuttallides truempyi, Clinapertina spp., Abyssammina spp.,
Aragonia spp., and Alabamina dissonata became extinct between the middle
Eocene and earliest Oligocene; 3) In shallower sites (< 3km paleodepth)
throughout the Atlantic, a Nuttallides truempyi-dominated assemblage was
replaced by a Globocassidulina subglobosa--Gyroidinoides-Cibicidoides
ungerianus-Oridorsalis assemblage in the early late Eocene (-40-38.5
Ma). These faunal and isotopic changes represent the transition from
warm, old, corrosive Eocene bottom waters to colder, younger (lower C02
and higher pH, hence less corrosive) early Oligocene bottom waters.
