11th ICRS Abstract book - Nova Southeastern University

1.20
Grain Size And Sedimentary Constituents As Indicators Of Changing
Environments And Energy Levels in Smugglers' Cove, St. Croix, U.s.v.i.
Matthew KLINMAN* 1 , Sarah CHAMLEE 2 , Selina TIRTAJANA 3 , Donald BARBER 1 ,
Donald POTTER 2 , Karla HUBBARD 4 , Dennis HUBBARD 4
1 Geology, Bryn Mawr College, Bryn Mawr, PA, 2 Geology, University of the South,
Sewanee, TN, 3 Earth & Environmental Science, Wesleyan University, Middletown, CT,
4 Geology, Oberlin College, Oberlin, OH
Twelve 1.5-3.0m vibrocores (diameter = 7.6 cm) were taken along two shoreperpendicular
transects in Smuggler’s Cove, St. Croix (US Virgin Islands) to understand
the development of the lagoon and possible changes in major environments (primarily
open and bioturbated sand versus grassbeds with little or no burrowing) over time.
Surface samples were also taken along a third transect to provide a more systematic view
of onshore-offshore patterns of grain size, organic content and sedimentary constituents
as a result of both position in the lagoon and lagoon-bottom character. Grain-size
analyses were completed on the surface samples and at 20-cm intervals throughout all the
cores. Thin sections of the sand fraction were used to characterize sedimentary
constituents. Terrigenous content in both the sand and mud fractions were determined
using a Magnetic Susceptibility Meter, a CO2 Coulometer combined with loss-on-ignition
analysis, and digestion using HCl and Hydrogen Peroxide. Additional work was done
analyzing diatom species composition down-core. The depth of a hard antecedent surface
in the deepest cores indicates that deposition in the lagoon began between 7,000 and
6,000 years ago when rising sea-level initially flooded the surface. Assuming a constant
rate of sedimentation, the lagoon deposits accumulated at rates of 0.4 – 0.6 m/ky. In the
sand fraction, terrigenous input is typically confined to areas very close to the beach;
terrestrially derived mud has a broader impact. Antecedent topography plays an important
role in both flooding history and the relative impact of the developing reef. Changes in
both constituent types and grain size within the carbonate fraction appear to reflect highly
variable conditions, but show a general reduction in wave energy and some shifting of
environments as Tague reef developed. Elevated levels of total carbon in all the surface
samples may reflect increased runoff from recent upland development.
1.21
Preservation Potential Of Lagoon Faunas And The Role Of Deep Bioturbation in
Shell Bed Formation
Caitlin TEMS* 1 , Ashley BURKETT 2 , Karla HUBBARD 3
1 Geology, Colorado College, Colorado Springs, CO, 2 Geology, Muskingum College,
New Concord, OH, 3 Geology, Oberlin College, Oberlin, OH
Twelve 1.5-3.0m vibrocores (diameter = 7.6 cm) were taken along two shoreperpendicular
transects in Smuggler’s Cove, St. Croix (USVI) to address the preservation
potential of lagoon environments and to understand patterns and processes of shell bed
formation. Molluscs were sampled from three common lagoon environments: seagrass
beds, open carbonate sand, and areas affected by burrowing callianassid shrimp. Mollusc
samples near the surface were used to establish the community structure of each
environment, and all shells were further characterized by taphonomic condition. Previous
work has established that lagoon environments can be recognized by the taphonomic
condition of the molluscan community without regard to taxonomic composition.
Results from the cores indicate that shell beds were not as common a feature in the
subsurface of the lagoon as has been previously assumed. Most cores had very few shell
beds greater than the thickness of any single shell. This was true in both areas with and
without obvious burrowing activity. However, the bottom of nearly every core that
reached the hard antecedent subsurface (pre-Holocene?) exhibited a shelly lag generally
in the range of 20 cm thick. This lag was always associated with the hard pavement,
which varied in depth across the lagoon. The uppermost 70cm of core is similar to the
present surface environment both taxonomically and taphonomically; bivalves are better
preserved than gastropods. The faunal assemblage near the core bases did not reflect the
present surface assemblage nearly as well, and all shells were generally better preserved
regardless of type. The published depth estimates for callianassid burrowing may be low
if these lags are the result of shrimp activities. On the other hand, if lags are not the result
of bioturbation, they may reflect more rapid burial of shells during earlier phases of
lagoon development.
Poster Mini-Symposium 1: Lessons from the Past
1.22
Holocene Reef Accretion Along the North Side of Bahia Enriquillo (Western Dominican
Republic): Unique Insights Into Patterns of Reef Development in Response to
Paleoceanography and Sea-Level Rise
Dennis HUBBARD* 1 , Wilson RAMIREZ 2 , David CUEVAS 3
1 Geology, Oberlin College, Oberlin, OH, 2 Geology, University of Puerto Rico-Mayaguez,
Mayaguez, Puerto Rico, 3 Marine Science, University of Puerto Rico-Mayaguez, Mayaguez,
Puerto Rico
Well-exposed Holocene reef outcrops along the northern side of the Enriquillo Valley record
continuous reef accretion between 9,000 and 5-6,000 CalBP, when the bay closed. All the zones
on Caribbean reefs today are well represented. Despite high sedimentation (>2-3 mm/yr) that
nearly matched the reduced rates of coral growth (~1-3mm/yr), over 25 species of coral were
present on the reef, and coral abundance averaged 50-75%. The importance of sediment-tolerant
corals (especially Montastraea spp. and Siderastrea spp.) supports the premise of elevated
sediment stress, as does the importance of conical and columnar colonies resulting from the
inhibition of lateral growth. The high rates of bioerosion (especially in conical colonies that
provide cryptic space) and the dominance of Lithophaga and other molluscs likewise argue for
a significant input of mud and the nutrients that adsorb onto the surfaces of finer grains. A
storm-debris layer up to a meter thick, and dominated by what has been described as Madracis
myriaster was deposited across the massive-coral zone ca 8,000 years ago, and provides a
time-line that defines reef morphology. Preliminary taphonomic studies along this surface
indicate that as little as half of the coral along any time-contemporaneous surface was alive at
the same time. Thus, measurements of coral abundance in outcrop probably overestimate livecoral
cover when compared to modern monitoring studies. Direct observations within the
outcrops and radiometric dating along several vertical transects confirm that the most
continuous section located in Cañada Honda reflects transgression until ca. 7,000 CalBP when
sea level slowed, followed by regression as shallower portions of the reef dominated by
Acropora cervicornis began to build out over the older and deeper forereef. Continuous
outcrop exposure provides marked advantages over cores, and does not support a sudden and
rapid rise of sea level proposed for ca. 8,000 CalBP.
1.23
Historical And Present Status Of The Pearl Oyster, Pinctada Margaritifera, in The
Northwestern Hawaiian Islands
Elizabeth KEENAN* 1,2 , Russell BRAINARD 3 , Lawrence BASCH 4
1 NOAA, NOS, Papahanaumokuakea Marine National Monument, Honolulu, HI, 2 Research
Corperation of the University of Hawaii, Honolulu, 3 NOAA, PIFSC, Coral Reef Ecosystem
Division, Honolulu, HI, 4 University of Hawaii Manoa,Pacific Islands Cooperative Ecosystem
Studies Unit, Honolulu, HI
Populations of the black-lipped pearl oyster, Pinctada margaritifera, at Pearl and Hermes Atoll
in the Northwestern Hawaiian Islands (NWHI) were first reported in 1928 and heavily
harvested over the next 2 years. Approximately 150,000 pearl oysters were either exported or
killed during the exploitation. An expedition in 1930 to assess post-harvest population status
determined the population to be severely depleted. Limited surveys in 1994 and 2000 found
only a few pearl oysters and led to the conclusion that the population remained depleted. From
2003 to 2006 quantitative observations of Pinctada margaritifera were collected, including the
location, size, depth, habitat, and orientation of individual pearl oysters on the reef. The surveys
in 2003 discovered a higher abundance of pearl oysters in shallow waters than after the late
1920’s harvest, though few oysters were found at deeper depths. Further studies in 2006
included a depth-stratified distribution survey to determine whether the 2003 depth distribution
was accurate or if the results were an artifact of survey method. The depth-stratified study
confirmed the 2003 finding of a predominantly shallow water distribution for pearl oysters at
Pearl and Hermes Atoll. These results, along with density estimates, strongly suggest that there
has been no significant population increase or recovery of pearl oysters since harvest occurred
there over 75 years ago. This implies limitations on population growth due to Allee effects
operating on fertilization and early life history ecology. The research conducted in 2004 through
2006 also included surveys at four other reefs in the NWHI: Kure Atoll, Midway Atoll, Maro
Reef, and French Frigate Shoals. Pearl oysters were found to be rare at the other NWHI
locations surveyed; indicating an apparent absence of minimum viable populations outside of
Pearl and Hermes Atoll, within the northern Hawaiian Archipelago.
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