Geology and Paleoceanography
Studies of ostracodes in surface sediments
from piston core and boxcore tops, col-
lected mainly from the Eurasia Basin of
the Arctic Ocean and analyzed using
multivariate quantitative and clustering
techniques, have shown that many ostra-
code species that form assemblages in the
Arctic Ocean have distinct bathymetric
ranges that are related to the depth-
stratified water masses of the present-day
water column. Thus, we are able to recog-
nize assemblages of ostracodes that are diagnostic of:
Liz Osborne pro-
Polar surface water (0100 m), below which lies a 100-m-thick halocline layer;
cores in the Polar
Arctic intermediate water (Atlantic water) (200800 m); and
Sea core laboratory.
Arctic Ocean deep water (to 4000 m or more).
These water masses are in themselves easily differentiated from one another
because each has a characteristic temperaturesalinity signature, which is related
to water density, which in turn, determines their position in the stratified water
column. We, therefore, are able to recognize and "calibrate" these modern analog
ostracode assemblages with what is known of the physicochemical properties of
each water mass or subdivisions of the water mass into upper and lower parts.
Thus, "depth-related" ostracode assemblages in modern-day Arctic seafloor sed-
iments are proxy indicators of water mass type. Moreover, downcore changes in
ostracode assemblages, when compared to the modern analog coretop assem-
blages by a quantitative method called the modern analog technique (MAT),
will result in a proxy record of the water-mass history over a basin, abyssal
plain, ridge, rise or plateau in the deeper (≥4 km) to shallower (~1 km) parts of
the Arctic Ocean during late Quaternary time.
We have found:
Well-preserved ostracodes and good biostratigraphies downcore in box-
cores from the Mendeleyev and Lomonosov Ridges and slopes;
Ostracodes in 18 boxcore tops; and
Well-preserved adult specimens of the deep-sea ostracode genus Krithe to
be used in shell-chemistry studies.
The longest of the 18 boxcores is 46 cm long. On average, sedimentation rates
are on the order of 12 cm per 1000 radiocarbon years (1 kyr) in the CAO for
the Holocene (the last 10,000 kyr). Sedimentation rates during the last glacia-
tion have been variously estimated at 0.10.2 cm/kyr up to 0.5 cm/kyr, based
on accelerator mass spectrometry (AMS) 14C dating. Thus, the ages of the pre-
Holocene sections of the boxcores in all likelihood span all of the last glacia-
tion, which dates from approximately 7010 kyr ago, and may be older. All of
the boxcores, then, must record the time of the last glacial maximum (approx-
imately 18 kyr ago) through to the present.