aus dem MARUM und dem Fachbereich Geowissenschaften der ...

MARIA S. MERIAN-Berichte, Cruise 20, Leg 4, Bridgetown – Freeport, 15.3.2012 – 7.4.2012
6.4.2. The Bimini Slope: The Water Column Structure
(Christian Dullo, Silke Glogowski, Thorsten Garlichs)
Off Bimini only one CTD station was performed (GeoB 16356). The upper 70 m of the water
column are obtained by Florida Current Surface Water (FCSW) with the lowest salinities around
36.14 in water depth of 70 m (see Fig. 6.26). Here, the FCSW is typically influenced by the
Antilles Current. Below 152 m follows the Subtropical Underwater (SUW; salinity: 36.90).
Decreasing salinities towards the deep (minimum salinity in 748 m water depth: 34.94) mark the
lower portion of the SUW that is already influenced by the Antarctic Intermediate Water
(AAIW), although this water mass was not clearly identified in this station. For a comparison
with the following CTD station in the next working area slightly further south, the Great Bahama
Bank area, see chapter 6.5.2.
6.4.3 The Bimini Slope: Bathymetry and Sub-Seafloor Structures
(Gregor Eberli, Paul Wintersteller, Dierk Hebbeln)
The modern slope of the western Great Bahama Bank (incl. the northern Bimini Slope) is the last
one of a series of prograding clinoforms that advanced the platform margin more than 25 km
towards the west during the last 12 Myr (Eberli & Ginsburg, 1987; Anselmetti et al., 2000). The
modern slope receives large amounts of fine-grained sediment with up to 90 m accumulation
during the Holocene (Wilber et al., 1990) but the older strata also contain abundant calcareous
turbidites and slump units (Betzler et al., 1999). The recently obtained MBES bathymetry map of
Mulder et al. (in press) documents that large-scale slope failures and mass transport complexes
occur along the modern carbonate slope. During cruise MSM20-4 it could be demonstrated that
these erosional products are the core of the numerous mound structures observed in the MBES
data and formerly be interpreted as CWC mounds. Two ROV dives along the Bimini Slope
revealed that these mounds are sparsely covered by CWC or even nearly barren of any CWC
colonisation and that these mounds (at least most of them) are most probably not formed by coral
fragments and hemipelagic sediments (see chapter 6.5).
North of Bimini the slope is dissected by a series of shallow canyons that originate in a 30km-long
array of scars at the upper slope in a water depth of ~450 m (Fig 6.22). The canyons
run west down slope and bend slightly to the south. Numerous mound structures with heights
of >100 m were found at the canyons' mouth and along the canyons' flanks. Three different sites
were defined for detailed studies within this 30x30-km-sized Bimini Slope region (Fig. 6.22).
The first site is located close the toe-of-slope including a 100-m-high mound named
“Wienberg mound”. The second area is characterised by a cluster of mounds at the confluence
of three canyons and is situated in the middle of the entire canyon system. However, this site
was not investigated by sampling as ROV dives in sites 1 and 3 revealed that the mounds as
large boulders with scarce or no coral coverage. The third site comprises the upper end of the
canyons below the erosional scar.
The MBES bathymetry map acquired on board R/V MARIA S. MERIAN illustrates the slope
with its large anastomosing canyon system in great detail (Fig. 6.21). The canyons are bifurcating
and at the lower slope they bend to the south. Large mounded features of the "Mulder
map" (Fig. 6.22) are here recognizable as blocks and boulders. The entire system seems to have
very little fine-grained off-bank transported sediment that is smothering mounded features
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