Geology Reference
In-Depth Information
that there is a systematic offset between bank and
off-bank isotope data in a Carboniferous exam-
ple. A possible explanation is the depletion in
13
C of carbonate bank waters due to respiration
of organic matter, which was demonstrated on
the modern Great Bahama Bank by Patterson &
Walter (1994a). Elevated
13
C may be observed
during high-amplitude transgressions when iso-
topically depleted bank water mixes with open
marine water (Immenhauser
et al
., 2003). Swart &
Eberli (2005) were able to correlate shallow-water
bank, slope and basin sections using
50 m in the south (Fig. 1b). Surface sediments
include reefal coralgal grainstones and
Halimeda
-
rich wackestones and packstones in the back reef
areas. Mollusc marls and siliciclastic sands are
found on the southern shelf and nearshore areas,
whereas molluscs, foraminifera and micritized
grains predominate on the northern shelf (Purdy
& Gischler, 2003).
Rasdhoo and Ari atolls are located in the
western chain of the Maldives archipelago
(Fig. 1c). Rasdhoo is a comparably small (60 km
2
)
atoll with an almost continuous reef margin and
a 40 m deep lagoon. Ari covers 2300 km
2
, the
reef margin is rather discontinuous with numer-
ous tidal channels and the lagoon is as deep as
80 m. Both lagoons contain patch reefs. Marginal
and reefal sediments are coralgal grainstones.
Lagoonal sediments are mollusc and foramin-
iferal wackestones and packstones, as well as
mudstone (Gischler, 2006). Mudstone occurs
only in Rasdhoo lagoon and most common grains
>125 μm are cemented faecal pellets.
The ramp offshore Kuwait deepens more or
less continuously to some 30 m over a distance
of 20 km (Fig. 1d). Isolated patch reefs occur in
the inner and outer ramps. Facies belts run more
or less parallel to bathymetric contours. They
include nearshore ooid-skeletal grainstone and
quartz sand, mid-ramp mollusc grainstone and
packstone, outer ramp mollusc marl wackestone
and coralgal grainstone at reefs (Gischler &
Lomando, 2005).
In all samples used in this study, carbonate
mineralogy is predominated by aragonite, which
usually ranges from 70 to 90%. High magnesium
calcite normally reaches 10-20% and low-
magnesium calcite <10% (Gischler & Lomando,
1999, 2005; Gischler, 2006).
13
C in the
Miocene to Holocene of the Bahamas. However,
13
C values along their sediment cores did not
correlate with the global
13
C trends. Apparently,
there are problems with the applicability of carbon
isotopes as environmental and stratigraphic prox-
ies in shallow-water depositional environments. In
this study, considerable variability of carbon and
oxygen isotopes in Holocene shallow-water car-
bonate sediments has been documented, the sig-
nifi cance of which will be discussed in relation to
the use of
13
C as a palaeoceanographic and strati-
graphic proxies in fossil shallow-water settings.
STUDY AREAS
Three areas formed the focus of the present study
(Fig. 1). The Belize-Yucatan carbonate platforms
include Glovers Reef, Lighthouse Reef, Turneffe
Islands and Banco Chinchorro, which range in size
from 200 to 550 km
2
(Fig. 1b). Surface-breaking reefs
surround interior lagoons that are up to 18 m deep.
Lagoonal patch reefs are common with the excep-
tion of Turneffe Islands, where interior lagoonal cir-
culation is restricted as a result of dense mangrove
growth. Reefal sediments are largely coralgal grain-
stones whereas lagoonal sediments are dominated
by fragments of molluscs, foraminifera and non-
skeletal grains (Glovers, Lighthouse, Chinchorro) as
well as
Halimeda
(Turneffe) (Gischler & Lomando,
1999). Holocene platform lagoon cores usually
show an up-core succession of Late Pleistocene/
early Holocene soil, early Holocene mangrove
peat and marine carbonates. Marine carbonate
sediments often show indications of bioturbation
and are predominantly composed of fragments of
mollusc, foraminifera and
Halimeda
. The succes-
sion overlies Pleistocene reef limestone, which is
usually altered diagenetically (Gischler, 2003).
The Belize Barrier Reef is 250 km long and
forms the margin of the Belize shelf, which
deepens from about 5 m in the north to some
METHODS
18
O values were determined on a
total of 559 surface sediment samples from the
three modern carbonate areas. These include
326 samples from the Belize-Yucatan platforms,
102 samples from the Belize barrier reef and
shelf, 38 samples from two Maldivian atolls and
93 samples from the Kuwait ramp. In addi-
tion, 69 carbonate particle end-members (coral,
mollusc, algae and echinoderm fragments; ooids
and peloids) were analysed with regard to C and O
isotopes. From the Belize platforms, carbon and O
isotopes along six Holocene lagoon cores were also
13
C and
The
