Geology Reference
In-Depth Information
26
o
N
18
O (‰)
V-PDB
δ
25
o
N
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
−
0.2
−
0.4
24
o
N
−
0.6
−
0.8
23
o
N
79
o
W
78
o
W
77
o
W
Fig. 6.
Spatial composition map of
km
18
O of bulk carbonate sediments
from GBB.
0
0
50
50
100
100
13
C
compared with portions of the interior (Fig. 9a).
These changes are strongly correlated with the
percentage of aragonite and arise because carbon-
ate materials produced from the decomposition of
algae, such as the various calcareous green algae
commonly found on coral reefs (
Halimeda
sp
.,
Penicillus
sp
., Acetabularia
sp
.
and others),
have very positive
signifi cantly more negative (up to 2‰) in
DISCUSSION
13
C
Previous work on the variation of
δ
on modern carbonate platforms
There have been relatively few previous studies
on the distribution of
13
C of sediments on mod-
ern carbonate platforms (Lowenstam & Epstein,
1957; Weber, 1967; Weber & Schmalz, 1968;
Weber & Woodhead, 1969; Shinn
et al.,
1989; Dix
et al.,
2005). Studies such as the one carried out
at Heron Island (Weber & Woodhead, 1969) exam-
ined variations over a relatively small area, a coral
atoll approximately 5
13
C values (typically > +4‰).
13
C of most LMC and HMC
components is close to or below 0‰ (Keith &
Weber, 1965; Weber, 1965; Milliman, 1974; Land,
1989). Hence, a mixing line between aragonite
and LMC or HMC will produce a covarying trend.
However, a close inspection of the data (Fig. 9)
shows two apparent trends, one at concentrations of
In contrast, the
20 km. The Heron Island
study showed substantial variation in
13
C pro-
gressing across the reef, with the margins being
