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this trend because other specific depositional mech- slope environments in ancient sediments.
anisms particular to this environment are involved The cumulative percent of the different sediment
(discussed in a later section). types in each core also has been calculated (Table
The distribution of turbiditic sand and sand-silt 4). This was accomplished as follows. First, the
sediments appears independent of depth. Other t o tal percent in each core of four ma jor sediment
environmental factors related to the boundary con- groups (legend a, h, c, d in Figure 36B) were cal-
ditions of the environment (such as distribution of culated. Then the cumulative percent of each
channels, natural levees, small basins, etc.) may be group of sediment was calculated in the following
of primary importance in their distribution. sequence: a, a + h, a + h + c, and a + h + c +
Hemipelagic mud is more closely related to d. The results are shown schematically in the inter-
depth, showing a decrease with an increase in pretative diagram in Figure 36n, which displays
depth. The turbiditic mud type also shows a cor- the variance of sediment types in cores as a func-
relation with depth, but in contrast to the hemi- tion of depth. For each sediment type two graphic
pelagic mud, its importance increases with depth. limits are depicted: one corresponds to the upper
The correlation coefficients between depth and the cumulative limit of probability of a given sediment
percent of sediment type in the cores and the re- group (thin line); the other represents the lower
gression lines have been calculated for both types cumulative limit of probability for this sediment
of mud (Figure 35A). The value for hemipelagic group (heavy line). It is apparent from this graph
mud is r = -0.69, y = 106.7- 0.018x; for turbiditic that hemipelagic mud is the most important sedi-
mud it is r = 0.70, y = -5.8 + 0.014x. Both cor- ment type in the neritic-bathyal environment. The
relations are statistically significant at the a = 0.01 importance of the turbiditic mud type increases
leve l. with depth; below 2500 m the percent of turbiditic
The correlation is not very strong in either case, mud increases sharply and becomes as important as
inasmuch as only about half of the variance (47% hemipelagic mud. The sand-silt sediment type also
for the hemipelagic and 49% for the turbiditic mud) increases in cores paralleling an increase in depth.
in the percent of sediment type in cores can be ~x­ Coarse calcareous sand and shallow water mud are
plained by a change in depth. However, it is ubiquitous in the shallow water environment and
interesting that depth, which is only one of severa! their importance decreases sharply below the 200
possible environmental factors, apparently con- to 500 m zone. The distribution of sapropel, or-
trols about half of the variance in the distribution ganic ooze, and related deposits which do not occur
of these sediment types. on the Strait proper is not depicted on the graph.
Another significant aspect of the relationship
between depth and sediment distribution can be
BIOTURBATION AS AN ENVIRONMENTAL INDICATOR
inferred from the graphic representation. Core data
from the Balearic Basin plain (Rupke and Stan- Bioturbation, an indicator of biomass and ben-
ley, 1974) have also been plotted. A sharp change thic activity, is considered in this discussion of
in trend of the regression line is clearly evident Strait sedimentation. The preservation of primary
when the data from the Balearic plain are com- structures in marine sediments is the result of a
pared to the data from the Strait of Sicily cores. delicate balance between rate of sedimentation and
The intersection of the regression lines from both rate of benthic activity on, and just below, the
sets of data occurs just beyond 2500 m, which cor- water-sediment interface (Moore and Scruton,
responds well with the depth of the basin plain- 1957). The degree of bioturbation in cores against
base of slope break. depth is depicted graphically in Figure 37. The
This type of correlation between sediment se- correlation coefficient is r = -0.87 (significant at
quences and depth may be applicable in other the a = 0.01 level) and the regression line is y =
parts of the Mediterranean, but further testing is 169.6-0.101x. Data from core KS 110, a deep basin
needed. This model, taking into account the sta- core, was not used in the calculation; it is too short
tistica! limitations of the technique, also may be of to provide reliable data. The correlation between
considerable importance for the interpretation of depth and degree of bioturbation (about 75% of the
depth of paleoslope and recognition of base-of- variance) is higher than the correlation between