aus dem MARUM und dem Fachbereich Geowissenschaften der ...

MARIA S. MERIAN-Berichte, Cruise 20, Leg 4, Bridgetown – Freeport, 15.3.2012 – 7.4.2012
astrophorid sponges (Fig. 6.28c) and several species of stalked crinoids. The low-relief ridged
and almost flattened off-mound seabed shows locally cemented crusts (Fig. 6.28d) which might
be outwashed, weak ripple areas of calcareous ooze and sand but also boulder fields. The latter
serve as substrate for purple octorals (Fig. 6.28e) and large antipatharian colonies, again each
one housing a pair of bellotid fishes. Interestingly, in this region the CWC showed a distinct,
though temporarily variable, depth zonation. During ROV dives GeoB 16373 ("Mount Gay")
and 16374 "flourishing" coral gardens (Fig. 6.28c) have been observed on the visited mound
summits in depths shallower than 610-620 m. In contrast, during ROV dive GeoB 16381 the
summit of the visited mound only reached up as high as 650 m and it was entirely covered by
huge amounts of dead coral framework. This observation clearly points to a temporal
development of the CWC distribution in this region.
Also the two ROV dives higher up on the slope (GeoB 16387, 16388) over the flanks of an
isolated mound in 586-547 m depth shows no coral mound but instead a displaced olistolith that
is draped locally by coral framework. On ROV dive GeoB 16388 several graveyards with masses
of sea urchin coronas and masses of bivalve shells were encountered at the base of the mounded
structure (Fig. 6.28f). Among the collected shells, lucinid bivalves have been identified which
may indicate seepage in this area. Other indications for seepage (of whatever origin) were noted
on dive GeoB 16374 at 626 m water depth, when we spotted flurry water passing by.
It is most likely the case that a two-fold history explains the presence of clustered and often
E-W aligned mounds: first the slope failure and mass transport of rock slabs from the platform
perpendicular to the slope (E-W), second, the post-failure colonisation and mound growth by
corals. This also would explain the highly variable mound morphology of individual mounds
which are not current controlled but primarily controlled in their shape by the random
accumulation of displaced rock slabs.
6.5.5 Great Bahama Bank Sediment Sampling
(Claudia Wienberg, Dierk Hebbeln, Nina Joseph, Lelia Matos, Hector Reyes, Marco
Taviani, Klaus Dehning, Marco Klann, Maik Wilsenack)
Three mound structures were successfully sampled in the Great Bahama Bank region, all
belonging to the deep mound area between 700-600 m water depth observed during various
ROV dives (GeoB 16366, 16373, 16374, 16381). The first southernmost sampling site of this
area comprised a rather small mound structure (situated slightly south of Mount Gay, see Fig.
6.25). Two box cores collected from 660 m (GeoB 16367) and from 667 m (GeoB 16368) water
depth revealed quite different recoveries. Whereas GeoB 16367 consisted of Enallopsammia
rubble within a light grey muddy foraminifera-pteropod matrix, GeoB 16368 was made up by a
pale brown foraminifera-pteropod ooze with very few coral fragments. Both cores contained
scarce to no living fauna.
Slightly to the north, we selected one larger mound structure for intense sampling (Mount
Gay, 79°21.25’W, 24°33.60’N; Fig. 6.25). This mound was sampled with a series of box cores
and gravity cores (see below) covering its base, lower and middle slopes, and its top. The mound
base (GeoB 16375) and the lower slope (GeoB 16376) consisted of a pale brown muddy
sediment, whereby only the box core from the mound base contained a larger amount of coral
rubble. No obvious living macrofauna was recognized. The mid-slope core (GeoB 16377) was
washed out, however, some pale brown muddy sediment remained in the box. This was
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