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biditic origin for the origin inasmuch as these re- masses in the eastern Mediterranean is the result of
flectors are continuous over a highly irregular to- increased evaporation and less fresh water dis-
pography. One possible origin of continuous layers charge or rainfall during cold periods; thus, in
are surfaces of nondeposition, perhaps related to this case, water stratification would be produced by
criticai sea level stands. A marked acoustic reflector the influx of less saline and less dense water from
also may indicate a marked change in the physical the Atlantic Ocean. In any case it is apparent that
properties of the mud such as water content and repetitive phases of stratification and stagnation
porosity rather than a change in texture. Stili during the Quaternary were basin-wide phenom-
another possible explanation for regionally exten- ena. Other examples of recent sapropel and
sive subbottom horizons might be concentrations sapropel-like deposits are reported from the Cari-
of foraminiferal tests or other microfossils. An aco Trench (Heezen et al., 1961), the Gulf of
example of this, recorded in the Alboran Sea California (Byrne and Emery, 1960; van Andel
(Huang and Stanley, 1972), is attributed to a re- and Shor, 1964), and the Black Sea (Ross and
giona! "bloom" related to a basin-wide event. Degens, 1974).
In generai the sapropel depositional models are
characterized by: (a) layering of water masses;
IMPLICATIONS OF STRAIT SEDIMENTATION TO
(b) stagnation of the bottom water, with forma-
CuRRENT REVERSALs
tion of an H 2 S rich zone; (c) seasonal (winter-
It is noteworthy that sapropels and associateci summer) or periodica! (eustatic changes) upwell-
sediment types which are distributed throughout ing and vertical mixing of water. These factors
the eastern Mediterranean do not occur in the favor the development and preservation of varve-
Strait of Sicily proper. Sapropel is cored only to like bedding (Figure 28A).
the east on the slope trending into the Ionian Ba- Although the depth of the three deep Strait of
sin. Most workers are of the opinion that these Sicily basin plains (1300-1700 m) is well below
dark organic-rich units are associateci with water that at which sapropel Iayers are found elsewhere
mass stratification-anaerobic conditions; they ac- in the centrai (Adriatic) and eastern (Ionian,
cumulateci during the warming phase of the Qua- Levantine basins) Mediterranean, no sapropels or
ternary climatic cycles, and not during the glacial other distinct evidence of stagnation are noted in
maximum (Ryan, 1972; and others). We assume the basin cores. On the contrary, structures pro-
that the upper sapropel layer on the slope east of duced by benthic organisms are commonly ob-
the Strait (core LY II-3 on Figure 34) is equiva- served, indicating that these deep narrow basins
lent to the upper sapropel layer in the eastern remained sufficiently oxygenated to support ben-
Mediterranean, which has been dated at between thic populations throughout the late Quaternary.
7500 and 9000 years BP (Ryan, 1972; van Straaten, Thus, it appears that vertical mixing prevailed on
1972). an almost continuing basis as a result of water mass
We accept the hypothesis which relates the depo- movement across the Strait of Sicily at a time when
sition of these organic layers with phases of anaero- sapropels were accumulating under stagnant con-
bic conditions and water mass stratification. ditions in the adjacent eastern Mediterranean.
Whether this stratification is the result of increased In this respect, core L Y II-6A west of the Strait
outflow of low salinity waters from the Black Sea Narrows (Figure 34) is of interest. The rate of
into the eastern Mediterranean coupled with de- sedimentation here is higher than in many other
creased evaporation rates (Olausson, 1961; Ryan, sectors of the Strait. The mud at the top of core
1972; Cita and Ryan, 1973; and others), or surface LY II-6A is dated as early Holocene (about 11,000
water warming (van Straaten, 1972), or an excess to 10,000 years BP), or well after sea level had be-
inflow of fresh water from rivers and melting ice gun to rise. Inasmuch as this core lies at a depth
and associateci current reversals at the Strait of of 755 m, the eustatic oscillation alone is not be-
Gibraltar (Mars, 1963; Huang et al., 1972; Nester- lieved to be the primary factor for erosion or non-
off, 1973) is not determined. Miiller (1973) pro- deposition in this sector. The region just west of
poses an alternative hypothesis on the basis of the Strait N arrows may be criticai for interpreting
nannoplankton analysis, i.e., that layering of water Quaternary oceanographic fluctuations since it oc-