|
Overview
Since the Chernobyl accident in June of 1986, the "Cafe Thorium"
lab has been actively pursuing studies of the Black Sea using
Chernobyl, weapons testing and naturally occurring radionuclides
as tracers of oceanographic processes. These studies have
focused on determining the fluxes and rates of sedimentation
and water mixing in this unique basin. To learn more about
the Black Sea and our work there, pick a topic below, or for
the serious reader, check out the CV and request a reprint
or two.
Need caption. |
Black Sea Introduction
As with any oceanic basin, a wide variety of natural and man-made
radionuclides can be detected in the waters, sediments and
biota of the Black Sea. In contrast to other oceanic settings
however, the Black Sea is unusual in that it is the world's
largest anoxic basin, i.e. its deep waters are devoid of oxygen
and most marine life. As such, it has been extensively studied
due to the unique biogeochemical processes that occur across
the oxic/anoxic interface and in the sulfide-rich deep waters.
Due to the stability of these anoxic conditions, the Black
Sea can also be used as an analog to ancient anoxic seas.
Anoxia is established due to restricted mixing between the
brackish surface layers (salinities less than or equal to
18 ppt in upper 100-150m) and the deeper waters (approx. 22
ppt, max. depth = 2200m). Riverine fluxes are dominated by
inflow from the Danube and the rivers along the N boundary,
and outflow is restricted to the narrow and shallow Bosporus
Straits in the SW Black Sea. Mediterranean waters flow into
the Black Sea as an undercurrent in the Bosporus Straits.
The general counter-clockwise, or cyclonic, circulation pattern
leads to a doming of the density, or isopycnal surfaces in
the central basin, and a deepening of the depth of sulfide
onset along the margins.
For radiochemists, the low redox conditions in the Black Sea
provide an interesting site to study redox transformations
of radionuclides with multiple oxidation states, such as uranium
and plutonium. In addition, soluble tracers, such as 137Cs,
90Sr and 3H introduced from weapons testing and the Chernobyl
accident to Black Sea surface waters, can be used to estimate
physical exchange rates between the surface and deeper layers.
Finally, radionuclides can be used to date the deposition
record that has been well preserved in the deep sediments.
back to top
Mixing Studies between Oxic Surface Waters
and Anoxic Deep Waters
The earliest man-made radionuclides were delivered to the Black
Sea as fallout from the atmospheric nuclear weapons testing
programs which peaked in the mid-1960's. We can estimate the
longer term rates of mixing between surface and deep waters
from the distribution of the soluble tracers 137Cs, 90Sr and
3H derived from weapons testing.
We have also followed time-series distribution of Chernobyl
derived radionuclides which were delivered to Black Sea surface
waters in early May of 1986. By following the progression
of the highly soluble cesium isotopes from 1986-1992, we can
evaluate physical mixing rates bewteen the oxic and anoxic
layers on annual time-scales. Presented
normalized to salinity, these data show the rapid mixing of
the Chernobyl signal throughout the surface mixed layer, through
the Cold Intermediate Layer (CIL) and across the strong pycnocline
into the upper anoxic water zone.
For details, search for keyword "Black Sea" in:
CV
back to top
Studies of Black Sea sedimentation
Deep Black Sea sediments are anoxic and hence devoid of the
worms, crabs and other critters that typically mix-up radionuclide
records in marine sediments. Hence deep basin sediment profiles
of man-made 137Cs and 239,240Pu show peaks from weapons testing
fallout, and for 137Cs the more recent Chernobyl accident. 210Pb
shows a logarithmic decrease which follows the its radioactive
decay half-life of 22.3 years. Accumulation rates are on the
order of 70 gm m-2 yr-1, or 0.01-0.02 mm yr-1. Coastal Black
Sea sediments are overlain by oxic waters, and hence the sedimentary
profiles for Cs and
210Pb are mixed, or bioturbated and do not retain the weapons
testing peak. In a core collected in 1988, the Chernobyl derived
134Cs could still be seen in the upper layer. Accumulation rates
here are on the order of 3500 gm m-2 yr-1, though this is highly
site specific.
For details, search for keyword "Benitez" in: CV
back to top
Has the Black Sea Really Changed
While there is ample evidence of recent biological and chemical
changes to the Black Sea due to pollution, eutrophication and
overfishing, separating climatic or natural changes from man-made
one's is not a simple task.. Recently, using historical temperature,
salinity,
oxygen and hydrogen sulfide data from the US and FSU, we were
able to show that the oxic/anoxic interface is stable on decadal
time scales, and not shoaling as others had postulated. Also,
the so-called "sub-oxic" zone (i.e. depths where oxygen
and sulfide concentrations are less than 5 mM) appears to have
been a common feature, though the
extent or thickness of the sub-oxic zone is likely to vary with
the intensity of winter mixing and regional variations in mixing
between the cyclonic and anticyclonic gyres.
For details, search for keyword "change" in: CV
back to top
A Comparison of the 137Cs in the Black Sea to Other Oceans
back to top
Related Links
» Black Sea Environmental Internet Node
|
