Geology Reference
In-Depth Information
c
. 10% in the other platform locations. In the
Maldive examples, Rasdhoo Atoll sediments con-
tain peloids in contrast to Ari Atoll. The Kuwait
ramp is rich in non-skeletal grains such as ooids,
peloids and aggregate grains, just like the above-
mentioned example of Great Bahama Bank (Purdy,
1963; Swart & Eberli, 2005; Swart
et al
., 2009).
back-reef or lagoonal settings, largely a result of
the lowering of pH during organic matter oxida-
tion. Sediment in
Callianassa
-rich settings is espe-
cially prone to dissolution (Sanders, 2001). There
is also geochemical evidence of dissolution and
reprecipitation of calcium carbonate in Holocene
and modern platform sediments of Florida Bay
(Burns & Swart, 1992; Patterson & Walter, 1994b).
No major changes in mineralogy were observed,
however, shifts in trace element (strontium, mag-
nesium) and stable isotopes of carbon and oxy-
gen were signifi cant. Recrystallization of modern
shallow-water skeletal grains from calcareous
algae and foraminifera is widespread (Reid &
Macintyre, 1998) and was demonstrated in the
fi ne grain-size fraction of sediments of the Belize
platforms (Gischler & Zingeler, 2002). Few geo-
chemical studies indicate a lowering of minor
element content, e.g., strontium (Reid & Macintyre,
1998; Gischler & Zingeler, 2002), however, it is
largely unknown as yet to which degree early
recrystallization processes also modify the isoto-
pic composition of these carbonate grains.
For pre-Holocene settings, late diagenetic effects
in the meteoric and burial environment have to be
considered as well. Processes described include
dissolution and recrystallization, neomorphism,
LMC precipitation and dolomitization. In addition,
lagoonal carbonate sediment can possibly become
compacted and eventually develop features indic-
ative of pressure dissolution. These processes
are discussed at length in textbooks, and, because
this study focuses on the Holocene, they will not
be treated in detail. Melim
et al
. (2001) described
a Neogene example based on the investigation of
diagenesis in two cores in Great Bahama Bank.
Taphonomic and diagenetic fi lters
Subsurface results from the Belize platforms
show that major fl uctuations of several per
mille in
13
C along individual Holocene sections
are associated with meteoric alteration near the
Pleistocene/Holocene hiatus. However, isotopic
variation along individual cores above the meteor-
ically infl uenced horizon were rather low (<1.2%).
Possible explanations for the latter observation
include taphonomic processes such as bioturba-
tion and early diagenetic effects.
Bioturbation by the burrowing shrimp
Calli-
anassa
characterizes large parts of the lagoonal
areas of the Belize platforms (Gischler & Lomando,
1999). The construction of large and complex bur-
rows in unconsolidated sediment,
Callianassa
is able to rework and mix sediment to depths of
almost 1 m (Scoffi n, 1992). We speculate that dur-
ing this process, isotopic variability is apparently
reduced, as seen in the bulk of modern sediments.
The fossil record of callianassids (Thalassinidean
shrimps) extends to the Jurassic (Myrow, 1995)
and
Callianassa
-type bioturbation is known from
carbonate platforms as old as the Cretaceous
(Sanders, 2001). The trace fossil
Thalassinoides
is reported from sedimentary rocks as old as
the Cambrian (Myrow, 1995). Other organisms
observed in bioturbation in the modern environ-
ments studied here include mollusc bivalves,
echinoderms, polychaetes and fi sh. The fossil
record of bioturbation for some of these organism
groups goes as far back as the late Proterozoic,
however, in general the Palaeozoic record is much
more fragmentary as compared with the Meso-
Cenozoic record (Bromley, 1990).
Early diagenetic effects also impact on the iso-
tope composition of Holocene lagoonal carbon-
ate sediments in this study, which are to a large
part made up of the metastable minerals aragonite
and HMC. A number of previous studies have
shown scanning electron microscopy (SEM) evi-
dence of partial dissolution of carbonate grains
such as
Halimeda
, foraminifera and coralline
algae in modern Caribbean reef settings (Perry,
2000). Partial dissolution occurs preferentially in
Signifi cance for carbon isotope stratigraphy
13
C for stratigraphic cor-
relation purposes has been applied for about a
decade to ancient shallow-water carbonate set-
tings of ramps and platforms spanning the time
from the Precambrian to the Tertiary (Joachimski &
Buggisch, 1993; Jenkyns, 1995; Vahrenkamp,
1996; Ferreri
et al
., 1997; Mutti
et al
., 1997;
Grötsch
et al
., 1998; Wissler
et al
., 2003; Saltzman
et al
., 2004). In these studies,
The application of
13
C ranges from
about
1 to +5‰ and individual stratigraphic
sections show an absolute range from approxi-
mately 1‰ to 4‰. Only Valladares
et al
. (1996)
reported a range of up to 8‰ for
13
C along inves-
tigated Cretaceous sections. Likewise, Cozzi
et al
.
(2004) presented large
13
C ranges from
8 to +3‰
