Geology Reference
In-Depth Information
in this process (Emrich
et al.,
1970; Romanek
et al.,
1992), the residual bicarbonate becomes
isotopically enriched.
Blackwelder, 1992), or that they are stirred up
bottom sediments, ultimately originating from
the decomposition of calcareous algae (Broecker
& Takahashi, 1966; Stockman
et al.,
1967; Morse
et al.,
1984). This debate has been heated over
the years, but regardless of the ultimate origin of
the mud component, this fi ne-grained sediment
seems to be the building material for most of the
other sedimentary particles on GBB. Mud becomes
ingested by benthic organisms forming faecal pel-
lets which harden to form peloids, some of which
become coated to form ooids or clumped together
to form grapestones. This process is supported
by the fact that the fi nest sediments have the nar-
rowest range of C and O isotopic compositions
(Fig. 2a), gradually increasing in range as diage-
netic and cementation processes add carbonate
material of differing generations and isotopic
compositions to the grains.
Sediments
Perhaps the most startling results arising from
this study are (i) the absence of signifi cant patterns
in the
13
C across the GBB (Fig. 5) and (ii) the lack
of a relationship between facies type and
13
C
(Fig. 3). The only exception to the latter is that
very large grains (>1000 μm) are slightly less
positive (+4.2 vs. +4.9‰). The relatively depleted
values are a result of the fact that these grains are
more likely to be composed of skeletal material,
which generally have lower
13
C values (see previ-
ous discussion). The origin of relatively high
13
C
values of the sediments compared with pelagic
LMC arises from two phenomena. First, as men-
tioned above the
13
C of the DIC averages +1.5‰
ranging from +0.4 to +2.4‰. Second, enrichment
of
13
C in aragonite is about 2.7
13
C
0.6‰
relative to only about 1‰ for LMC (Romanek
et al.,
1992). Hence equilibrium values for arago-
nite precipitated directly from surface waters of
GBB should range from ~ +2.6 to ~ +5.8‰ (this
range is calculated using the error of 0.6‰ and the
RELATIONSHIPS BETWEEN
δ
18
O
AND
δ
18
O
in the bulk samples and the separated size frac-
tions are shown in Fig. 2a and b. In
13
C and
The relationships between the
18
O
space, the data defi ne a triangle with the samples
possessing the most positive
13
C and
13
C values measured on the surface water DIC
(Fig. 8). This corresponds to a measured range of
+3.6 to +5.5‰ (see Tables 1 and 2).
As regards the
18
O values also
containing the narrowest range and the most ele-
vated
18
O, the variation in water
temperatures on GBB probably exceeds 10
o
C, but
averages 25
o
C. Using the standard palaeotempera-
ture equation (Epstein
et al.,
1953) and the values
reported in Table 2, the sediments if formed at
equilibrium should lie between
13
C values. These samples tend to be mud-
rich wackestones. This pattern (Fig. 11) probably
refl ects the relationships between those areas
on GBB where the water consistently possesses
the highest
18
O values.
These areas are probably fairly restricted in
geographical extent. In contrast, the areas where
there are wider ranges in
13
C (DIC) and
1.2 and +0.8‰.
This is a little more negative than the range of
the sediments (
18
O are more
prevalent and account for the majority of the
samples. The largest sized samples (500-1000 μm
and >1000 μm) have a much shallower relation-
ship between
13
C and
0.8 to +1.8‰) but still within the
errors of the temperature estimate (Fig. 11). So if the
predominantly aragonite sediments on GBB are in
equilibrium with the ambient waters, then why
do all the sediments, from the mud-dominated
wackestones to the grainstones, have similar
18
O refl ecting a larger
proportion of skeletal material in the samples
(Fig. 11).
13
C and
13
C
18
O compositions? The simple answer is
that the majority of the sediments are genetically
related; the exception being the small amount of
skeletal material mentioned previously.
The origin of the fi nest carbonate sediment
on GBB has been the subject of speculation
for over 50 years (Black, 1933; Smith, 1940a,b;
Cloud Jr., 1962). Authors have proposed that the
material is either a result of direct or algal induced
precipitation from seawater (Cloud Jr., 1962;
Morse
et al.,
1984; Shinn
et al.,
1989; Robbins &
and
IMPLICATIONS FOR CARBON ISOTOPE
STRATIGRAPHY
18
O of surface sedi-
ments of GBB shows that in spite of a large range
in sediment sizes, there is a relatively small spread
in the
13
C and
This study on the
13
C
values are a result of a combination of isotopic
13
C and
18
O values. The elevated
