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A re-evaluation of facies on Great Bahama Bank II: variations in
the
13 C,
18 O and mineralogy of surface sediments
δ
δ
PETER K. SWART * , JOHN J.G. REIJMER †‡1 and ROBERT OTTO*
* Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science,
4600 Rickenbacker Causeway, Miami, FL 33149, USA (E-mail: pswart@rsmas.miami.edu)
IFM-GEOMAR, Leibniz-Institut für Meereswissenschaften, Dienstgebäude Ostufer, Wischhofstr.
1-3, D-24148 Kiel, Germany
Centre de Sédimentologie-Paléontologie, FRE CNRS 2761 “Géologie des Systèmes Carbonatés”,
Université de Provence (Aix-Marseille I), 3, place Victor Hugo, Case 67, F13331 Marseille Cédex 3, France
ABSTRACT
18 O of modern carbonate
sediments on Great Bahama Bank, ~290 surface samples were collected from a
grid of stations approximately 10 km apart between 2001 and 2004. These samples
were classifi ed using a modifi ed Dunham scheme, physically separated into six size
fractions and subsequently analysed for their mineralogy (aragonite, low-Mg calcite
and high-Mg calcite) and
13 C and
In order to investigate the spatial distribution of
13 C and
18 O values. A striking feature of these data is
13 C values of all the samples. Based on measurements of
13 C
the relatively positive
18 O of the dissolved inorganic carbon and the water, most of the sediments can
be considered to be in C and O isotopic equilibrium with the ambient waters. The high
and
13 C values are suggested to arise from isotopic enrichment of the dissolved inorganic
carbon pool by photosynthesis of seagrasses, benthic algae and cyanobacteria on the
platform and through the fractionation of HCO 3 during the precipitation of calcium
carbonate. Sediments that are not in C and O isotopic equilibrium are dominated more
by skeletal material. The data showed an absence of signifi cant spatial variation in
13 C of the sediments on the Great Bahama Bank and no clear spatial patterns relative
to the margin of the platform. The
18 O of the sediment showed more variation, with
the interior sediments being isotopically enriched relative to the platform margin. The
absence of signifi cant variations in the
13 C in the modern surface sediments of Great
Bahama Bank irrespective of facies type suggests that in the case of Great Bahama
Bank, downcore variations in
13 C cannot be related to changes in facies.
Keywords Stable isotope, shallow-water carbonates, Bahamas, grain-size, carbonate
platforms, salinity.
INTRODUCTION
with a companion study (Reijmer et al ., 2009)
examines spatial variations in the facies and geo-
chemistry of surface sediments on GBB. Although
there have been several papers that provided
some general geochemical characteristics on the
sediments on GBB (Lowenstam & Epstein, 1957;
Shinn et al., 1989), there have been no studies
which have described spatial variation in the inor-
ganic and organic mineralogy of the sediments
on the same scale as the original sedimentary
facies were investigated. Such data are important
as changes in the
The descriptions of Great Bahama Bank (GBB)
and its surface sediments (Illing, 1954; Newell
et al., 1959; Purdy, 1963a, 1963b; Traverse &
Ginsburg, 1966; Enos, 1974) made between 1950
and 1970 are widely used for modern biological
and geological studies on GBB. In addition, GBB
is often applied as an analogue for the study of
ancient carbonate platforms. This paper together
13 C of platform and periplat-
form carbonates are increasingly being used
for stratigraphic purposes (Vahrenkamp, 1996;
Saltzman et al., 2004) and for providing informa-
tion on the global CO 2 cycle in periods prior to the
1 Present address: Vrije Universiteit, Faculty of Earth
and Life Sciences (FALW), De Boelelaan 1085, 1081 HV
Amsterdam, The Netherlands.
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