Geology Reference
In-Depth Information
measured, totalling 139 samples. The extensive
raw data are deposited in electronic form.
Samples were reduced with a sample split-
ter and ground with a mortar and pestle. Ground
samples were analysed at the University of
Frankfurt with a ThermoFinnigan MAT 253 mass
spectrometer using the Gas Bench II (Finnigan
Inc.) and at the University of Miami using an
automated carousel attached to a Finnigan-MAT
251 mass spectrometer. Isotopic data are corrected
using the procedures of Craig (1957) modifi ed for a
triple collector mass spectrometer. Data are quoted
relative to Vienna Pee Dee Belemnite (V-PDB)
standard according to the conventional notation.
The analytical precision at both laboratories is
approximately 0.1‰. Isotope data were compared
with mineralogical composition, i.e., percent-
ages of aragonite, high-magnesium calcite (HMC),
low-magnesium calcite (LMC) determined using
X-ray diffraction and to facies, i.e., amounts of
constituent particles as quantifi ed by point count-
ing in thin-section.
2
Coral-algal facies
Halimeda facies
Mollusc-foram facies
Non-skeletal facies
−
2
2
6
13
C(
‰
)
0
−
2
−
4
Fig. 2.
C-O-isotope plot of sediments from Belize-Yucatan
platforms. Note the overlap of isotopic composition of
different facies.
RESULTS
0
Belize-Yucatan carbonate platforms,
Caribbean Sea
−
1
Surface sediments
13
C) range from +1.0 to
+5.5‰ and oxygen isotopes (
Carbon isotope values (
−
2
18
O) from
2.3 to
0‰ (Fig. 2). There are no isotopic differences
between carbonate facies, i.e., it is not possible
to delineate facies based on their
−
3
Echinoderm
Peloids
Penicillus
Halimeda
Molluscs
Red algae
Coral
Foraminifera
18
O
values. End-members of constituent particles
exhibit even wider ranges (Fig. 3). As regards
13
C and
−
4
13
C, corals usually have the lowest values,
whereas non-skeletal grains such as peloids and
green calcareous algae such as
Penicillus
show
the highest values.
There are only a few cases of statistically signifi c-
ant correlations between isotopes and abundance
of carbonate grains (Tables 1-4). In the shallow
and open platforms of Lighthouse and Chinchorro
there are negative correlations between
−
5
−
2
0
2
4
6
13
C (‰)
Fig. 3.
C-O isotopes of end-members, Belize-Yucatan
platforms. Isotopic variation is larger as compared with
variation in bulk samples.
13
C,
18
O and abundance of coral and coralline algal
fragments. In the Glovers and Turneffe areas, no
signifi cant correlations between isotopes and
constituent grains exist. Signifi cant correlations
between
Correlations are positive. There are no signifi cant
correlations between isotopes and water depth.
18
O and the fi ne grain-size fraction
(<125 μm) exist only in Lighthouse. Statistically
signifi cant correlations between
13
C,
Subsurface sediments
18
O and
aragonite content occur only at Chinchorro.
13
C,
13
C ranges from
In six cores,
0.2 to +5.2‰
(Fig. 4a). All cores show an up-core trend towards
