11th ICRS Abstract book - Nova Southeastern University

1.13
Depth-Related Patterns of Infaunal Bioerosion in Two Modern and Two Fossil
Caaribbean Reefs: US Virgin Islands and the Dominican Republic
Timmons ERICKSON* 1 , Andrew ESTEP 1 , Dennis HUBBARD 1
1 Geology, Oberlin College, Oberlin, OH
Reef-accretion studies often focus on coral growth and underestimate the importance of
bioerosion. Recent findings suggest that not only is bioerosion an important part of the
accretion equation, but that bioerosion and sediment export might strongly impact depthrelated
patterns of reef building. However, studies of depth-related bioerosion within
individual reefs are generally lacking. Boring was quantified along a depth gradient in
two modern reefs (St. Croix: d = 3-10m; 3-27m) and two Holocene reefs (Dominican
Republic: d = 5-22m). In over 40 slabbed samples, galleries were traced and categorized.
Total area for each major bioeroder group (molluscs, various sponges, worms) was
measured within the outer bioeroded rim of the corals using NIH ImageJ.
The St. Croix reefs are exposed to higher energy and lower sedimentation than those in
the DR. This is reflected in a difference in the infaunal community at the two sites
(Lithophaga sp. in the DR versus sponges on St. Croix). Worm burrows are of minor
importance. The intensity of infaunal bioerosion on St. Croix is largely bimodal, with
maximum rates occurring near 6m (20-30%) and dropping abruptly to 5-15% beyond.
However, boring generally increases within the offshore samples from ~5% (10m) to
~15% (27m). In the fossil DR reefs, total bioerosion is likewise bimodal, with rates of 40-
60% in samples from paleodepths of 5-8m. Between 11 and 22m, total bioerosion ranged
from 20 to 30 percent. If sponges are considered alone, a pattern similar to what was seen
on St. Croix emerged.
This study provides only a snapshot of the final product. Similar studies addressing rates
of bioerosion by both grazers and infaunal borers will be needed to complete the picture.
Nevertheless, this study does show that higher rates of bioerosion correlate with
increased levels of nutrient availability either between sites or along a depth gradient.
1.14
The Role Of Constituent Durability On The Sediment Budget Of A Fringing Reef,
Lizard Island, Great Barrier Reef
Murray FORD* 1
1 School of Geography, Geology and Environmental Science, The University of
Auckland, Auckland, New Zealand
Tropical landforms associated with coral reefs such as reef islands and coastal plains are
often made entirely of carbonate sediments derived from surrounding reefs.
Consequently, sediment supply has a direct control on the development of such
landforms. Sediment is produced through the mechanical and biological erosion of the
reef framework and reef dwellers. The exchange between gross reef productivity and the
production of sediment is poorly understood. This is considered a ‘black box’ where the
inputs and outputs are well understood, but little is known of the internal workings of the
system. Furthermore, there is a dearth of knowledge regarding the fate of sediments
produced on reefs. This has significant implications for landforms adjacent to reefs. The
objective of this study is to enhance the understanding of the role durability plays in the
determining the composition of reef flat sediments and the implications of this on
sediment budgets and the availability of sediment for land-building processes. This
objective is addressed utilising both a detailed field investigation and through rigorous
laboratory experiments. Tumbling barrels were used to determine the durability of six
common constituents of sediment from a fringing reef on Lizard Island, Great Barrier
Reef. Each constituent was tested in tumbling barrels and the coefficient of durability
determined. The results of these experiments show the durability of common sediment
constituents vary on several orders of magnitude. The implications of this investigation
are discussed within the context of the sediment budget and the availability of sediment
for landform building processes.
Poster Mini-Symposium 1: Lessons from the Past
1.15
Response of soft-bottom macrobenthic coral reef communities to hurricane disturbance
(Reunion Island, Western Indian Ocean)
Dorothée TADDEI* 1 , Lionel BIGOT 1 , Heloïse ROUZE 2 , Patrick FROUIN 1
1 Biology, ECOMAR Lab, University of Reunion Island, Saint Denis cedex 9, Reunion,
2 University of Bretagne, Plouzané, France, Metropolitan
The impact of hurricane GAMEDE on soft-bottom macrofauna was evaluated in the three main
fringing reefs (St Gilles / La Saline, Etang Salé and St Pierre) of Reunion Island (Western
Indian Ocean). This baseline study of the macrobenthos will be used as part of a global survey
of the main soft-bottom areas recorded at Reunion Island. Sediment (grain-size, total organic
load and chlorophyll load) and community (diversity and density) variables were studied during
a warm season before and after the passage of the hurricane GAMEDE in 2006. Communities
of each fringing reef are poorly diversified (H’ = 2.23) characterized by abundant Actiniarian
(Edwardsia spp.) with more than 60 % of total abundance for some stations. Macrobenthic
abundance did not vary significantly after the hurricane disturbance despite slight changes
occurred in the most hydrodynamically exposed stations. Inversely community composition
was affected by a decreasing of diversity. The environmental factors could not explain the
macrofauna spatial variability as they showed different patterns of variability. The study finally
suggests that: 1) the fringing reef ecosystem is resistant despite subject to disturbance
(hurricane); 2) this stability is related to low diversity with species well adapted to strong
hydrodynamism; 3) a slight impact of the hurricane can be show on the most exposed
environments.
1.16
Late Quaternary Deposits On The Southern Belize Shelf: How Does The Mixed
Carbonate-Siliciclastic Succession Record Pleistocene Sea-Level Variation?
Eberhard GISCHLER* 1 , Robert GINSBURG 2 , Sachandra PRASAD 2
1 Geosciences/Geography, Johann Wolfgang Goethe University, Frankfurt am Main, Germany,
2 RSMAS-MGG, University of Miami, Miami, FL
The shelf of Belize (Central America) is a classic example of mixed carbonate-siliciclastic
system, which receives carbonate from the reefs and siliciclastics from the hinterland. Six cores
from the shelf of Belize are composed of coral grainstone and boundstone, molluskforaminiferal
wackestones and packstones, a variety of marls and clays, dolomite, and rare
quartz and shell-rich layers. The cores are between 67.7-105.8 m long and have a total thickness
of 529.5 m. In contrast to cores taken in the fore reef of the Belize Barrier Reef close-by, which
apparently recorded glacio-eustatic sea-level variation since marine isotope stage 11 by more or
less regular fluctuations in carbonate content and composition (Droxler et al. 2003), our Belize
shelf core exhibits no clear successional patterns or rhythms. Limestone sections, interpreted as
being deposited during highstands, and marls, clays and sands, considered as lowstand deposits,
alternate in a very irregular way. Thick limestone deposits predominate the upper core sections
down to 40-50 m below present sea-level. This section can be subdivided in a Holocene and late
Pleistocene part, based on preservation. Interestingly, no siliciclastics occur between Holocene
and Pleistocene limestones. Below these carbonate sections, brownish, reddish, gray and black
marls and clays occur, which reach thicknesses of 20-40 m. Underlying the siliciclastic
deposits, mollusk-foram packstones and wackestones, and fine-grained dolomite marls
predominate. No traces of subaerial exposure were found in the cores. The interpretation of the
shelf successions in the light of late Quaternary climate and sea-level fluctuations will strongly
depend on radiometric and biostratigraphic dating, which is currently underway. A limited
number of aragonitic corals are being dated by the U/Th-method and calcareous nanoplankton
in fine-grained limestone, marls and clays is investigated for biostratigraphy. Results will be
available by the time of the ICRS 2008 and will be presented together with the sedimentological
data.
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