Abstract:
The particle flux in the present and over the last 5,000 years was
investigated in the Black Sea in a comparative study with samples from
time-series sediment traps and laminated core sediments. The sediment
trap samples were collected in the southwestern Black Sea over 2 1/2
years at sampling intervals of about two weeks. Sediment core samples
were derived from the central part of the western Black Sea, deposited
throughout the last 5,000 years during which the Black Sea was anoxic.
Conclusions from this study shed important light on temporal and
regional variability of the particle flux in the Black Sea, dominant
particle sources, and particle transport mechanisms.
Dominant particle sources are biogenic matter (coccolithophorids of
the species Emiliania huxleyi, diatoms, and silicoflagellates) and
terrigenous matter from the Danube and nearby local rivers. The
relative importance in the supply of these particles varies annually and
can be grouped into three phases: Phase I (June-October)
coccolithophorid production, Phase II (November-January) - resuspension
of coccoliths and terrigenous matter, and Phase III (February-May) river
input of terrigenous matter and production of diatoms and
silicoflagellates. Once removed from the surface water, particles
settle rapidly at a rate of 115 ±70 m/day.
Regionally, the particle flux varies considerably. Throughout the
last 1,000 years (sediment unit I), the particle flux (paleoflux) was
more than 5 times larger in the central part of the western Black Sea
than at present in the southwestern Black Sea, mostly because of the 11
times larger supply of coccoliths. The coccoliths were probably largely
produced on or adjacent to the Danube shelf in the northwestern Black
Sea and subsequently resuspended and transported offshelf by the fall
storms. Terrigenous matter in the central part of the western Black Sea
is higher by a factor of 3 compared to the southwestern Black Sea. The
coccoliths are concentrated in the white laminae (>93 % CaCO3), and
if the seasonal dynamics in the particle supply at the sediment trap
site is taken as a standard, the white laminae would be deposited
between about June and January. The black laminae contain largely
terrigenous matter and form during the peak river discharge period
between about February and May.
Compared to the last 1,000 years (unit I), the particle flux in the
central part of the western Black Sea between 1,000 and 5,000 years B.P.
was smaller by a factor of three, because the salinity was still too low
during this time period for the coccolithophorid Emiliania huxleyi to
exist. The Black Sea was a fresh water environment before more than
5,000 years ago and gradually became brackish; Emiliania huxleyi became
established after the salinity exceeded 11%.
The terrigenous matter supply remained about constant over the last
5,000 years. The western Black Sea is dominated by terrigenous input
from the Danube as revealed by the illite/montmorillonite ratio.
Seasonally, the terrigenous matter from the Danube appears to be
traceable in the southwestern Black Sea, as seen by the Ti/Al and
illite/montmorillonite ratios in the sediment trap samples.
