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each other out in a mixture of grain types (see e.g.
Fig. 7). Corals usually show the most negative val-
ues, whereas non-skeletal grains such as peloids
and ooids and calcareous algae are isotopically
heaviest. Variation within skeletal grain groups
may be explained by taxonomic differences in iso-
topic fractionation and also by seasonal isotopic
variation within skeletons (Emiliani
et al
., 1978;
Wefer & Berger, 1991). As a consequence of the
latter observation, small fragments of skeletons
may not necessarily yield representative
2
1
0
Non-skeletal facies
Peloid
Ooid
−
1
13
C and
Molluscan facies
Coral algal facies
18
O signals.
Spatial trends in isotopic composition of bulk
samples are visible in the Kuwait ramp example
in that carbon isotopes decrease from nearshore
to offshore. In the atoll, barrier reef and platform
examples, however, no clear trends are discern-
ible. In Glovers, Lighthouse and Chinchorro,
Coral
Gastropod
−
2
Bivalve
Echinoderm
Foraminifera
−
3
−
2
0
2
4
6
13
C
decreases from the platform interiors to margins.
In the Belize Barrier Reef transects 1, 3, 5 and B
there is an offshore increase in
13
C (‰)
Fig. 7.
C-O-isotope plot of sediments and end-members
from Kuwait ramp. Note relatively clear separation of
different facies and larger isotopic range of constituent
particles as compared with bulk samples.
13
C, however, in
transects 4 and A, there is an offshore decrease.
Likewise, carbonate facies can be characterized
by their
18
O composition in the ramp
example, but not in the other locations. Statistical
analyses show that there are also more signifi cant
correlations between isotope composition and
carbonate mineralogy, abundance of grain types
or facies on the Kuwait ramp. Also, all measured
parameters including
13
C and
values (range
c
.
3 to +2.3‰) show consider-
able differences between individual sites. In the
Belize Barrier Reef and Maldive atoll samples,
values are in the same range. Belize-Yucatan
platform samples show slightly heavier values
and Kuwait ramp samples even more positive
values. One possible explanation for these differ-
ences might be salinity, which is normal marine
in the Maldives (around 35‰), normal marine to
hyposaline (30-35‰) on the Belize shelf (Purdy
et
al
., 1975), elevated (35-42‰) in Belize-Yucatan
platform lagoons (Gischler
et al
., 2003) and hyper-
saline (39-43‰) on the Kuwait ramp (Gischler &
Lomando, 2005).
For comparison, Weber (1967) and Weber &
Woodhead (1969) measured ranges in
18
O values cor-
relate signifi cantly with water depth, i.e., with
depositional energy on the ramp example. This
difference is probably a consequence of the fact
that the continuous depositional energy gradi-
ent on the Kuwait ramp largely controls carbon-
ate facies. In the atoll, barrier reef and platform
examples, depositional energy gradients are more
variable and morphological and environmental
variability is much greater.
Several workers have found signifi cant positive
correlations between
13
C and
13
C of bulk
carbonate sediments of +1.6 to +4.5‰ and +0.75 to
+3.2‰ in Bermuda and on Heron Reef off Australia,
respectively. The
13
C and the concentra-
tion of aragonite on various carbonate platforms
from numerous locations such as the Bahamas
(Weber, 1967; Swart & Eberli, 2005; Swart
et al
.,
2009), Australia (Weber & Woodhead, 1969) and
the NW Australian shelf (Dix
et al
., 2005). In
our study, such a correlation was only present
in the Chinchorro platform, Rasdhoo Atoll and
the Kuwait ramp samples, but not in the other
examples investigated. A potential reason for this
difference is the elevated content of isotopically
heavy non-skeletal grains at these three locations.
Among the Belize-Yucatan platforms, Chinchorro
has 30% peloids on average as compared with
18
O values from these settings
ranged from
0.6 to +0.4‰ in Bermuda and from
0.4‰ on Heron Reef. Land (1989) com-
piled isotope data of shallow-marine carbonate
sediments. These values range from about
2.1 to
3.5 to
13
C and from
18
O.
+5‰ for
3.5 to +2‰ for
18
O values of constituent car-
bonate sediment particles such as corals, algae,
molluscs, foraminifera and others from the Belize
and Kuwait locations exhibit even larger varia-
tions as compared with the bulk samples. Land
(1989) came to a similar conclusion, which can
be explained by extreme isotope values cancelling
13
C and
The
