11th ICRS Abstract book - Nova Southeastern University

1-13
Unusual Serpulid/Tufa Bioherms Cap Corals of a Mid-Holocene Fringing Reef,
Dominican Republic: Paleoenvironmental and Climatic Controls
H. Allen CURRAN* 1 , Kelsey WINSOR 1 , Lisa GREER 2 , Bosiljka GLUMAC 1
1 Geology, Smith College, Northampton, MA, 2 Geology, Washington & Lee University,
Lexington, VA
Meter-scale mid-Holocene serpulid/tufa bioherms in the form of mounds and terraces cap
corals of the exposed fringing reef bordering much of the Enriquillo Valley, Dominican
Republic. Composition of these unusual bioherms resembles that of two other localities
described in the literature: Miocene mounds of the Paratethys Basin of Ukraine/Poland
and Late Triassic deposits in the western Tethys of Italy. Rarity of such bioherms and
their similarities indicate that their formation was the result of convergence of specific
environmental conditions. In each case, restricted waters of less-than-normal marine
salinity reduced ecological competition and favored opportunistic serpulid aggregation,
as well as concentrated calcium carbonate for tufa precipitation. In the Enriquillo Valley,
limited hard substrate, wave action, calcium carbonate-rich spring waters entering at the
surface of ancient Lago Enriquillo, and periods of lake-level stability also contributed to
bioherm formation. These contributing factors were a result of larger-scale climatic
conditions characteristic of the mid-Holocene Caribbean, as well as local geological
constraints. Around 5 ka, slowed sea-level rise, increased precipitation and resultant
fluvial sedimentation owing to northward migration of the Intertropical Convergence
Zone (ITCZ), and possible regional tectonic uplift restricted the mouth of the ancient
Enriquillo seaway. This caused the demise of the Enriquillo fringing coral reef, and later
the dead coral colonies commonly served as substrate for development of serpulid/tufa
bioherms. Repeated changes in Lago Enriquillo water level and salinity created
conditions that supported not only bioherm formation, but also diverse molluscan and
ostracode faunas found within many bioherms. In late Holocene time, drier conditions
prevailed as the ITCZ moved southward and water levels of Lago Enriquillo receded,
exposing this spectacular fringing reef-serpulid/tufa mound and terrace complex.
1-14
Is Acropora Cervicornis A Canary in The Global Warming Coal Mine? Lessons
From The Mid-Holocene Dominican Republic
Lisa GREER* 1 , Julia JACKSON 2 , H. Allen CURRAN 3 , Tom GUILDERSON 4 , William
P. PATTERSON 5 , Lida TENEVA 6 , Elizabeth RHEA 7 , Jenny SHULTIS 8
1 Geology, Washington and Lee University, Lexington, VA, 2 University of Texas-Austin,
Austin, TX, 3 Smith College, Northampton, MA, 4 Lawrence Livermore National
Laboratory, Livermore, CA, 5 University of Saskatchewan, Saskatoon, SK, Canada,
6 Lamont-Doherty Earth Observatory, Palisades, NY, 7 Washington and Lee University,
Lexington, VA, 8 RCHS, Lexington, VA
The recent, well-documented decline of Acropora cervicornis throughout the wider
Caribbean region has been cause for considerable alarm, with the decline interpreted as a
warning of greater coral reef devastation in the future. Elevated sea-surface temperatures,
white band disease, anthropogenic stress, and storm activity have all been cited as
potential causes for the decline. In order to understand the optimal conditions and
threshold environmental range for A. cervicornis growth, an extensive mid-Holocene
fringing reef located in the Dominican Republic was examined for evidence of
paleoenvironmental variability during A. cervicornis dominance over a >3000 year
period. Over 50 radiocarbon and 234U/230Th dates from an 11 m vertical exposure of A.
cervicornis indicate continual accumulation between ~9.5 to ~7.3 ka with only minor
reversals (mixing events). The species continued to thrive at the site until at least ~5.8 ka.
This time range was marked by increasing regional sea-surface temperature to a mid-
Holocene thermal maximum (HTM), a time comparable to, if not warmer than, present.
δ13C and δ18O data from the fossil A. cervicornis specimens indicate high-magnitude
changes in precipitation during reef formation with no resulting break in coral
accumulation. Taphonomy and morphology data suggest the species thrived under high
sediment stress, variable bioerosion, and differential exposure to wave activity. Highresolution
δ13C and δ18O data from modern A. cervicornis growing at comparable
depths off Barbados (2007 collections) show stable isotope ‘signatures’ most similar to
corals that grew at the height of the HTM. A. cervicornis from the Dominican Holocene
can be characterized as a relatively hardy survivor during highly variable environmental
conditions. The data and inferences derived from this study indicate that the cause for
recent A. cervicornis decline is not strictly due to rising sea surface temperature.
Oral Mini-Symposium 1: Lessons From the Past
1-15
The Emperor Has No Coral—An Inconvenient Truth?
Eugene SHINN* 1 , Barbara LIDZ 2
1 College of Marine Science, University of South Florida, St. Petersburg, FL, 2 U. S. Geological
Survey, St. Petersburg, FL
The Emperor Has No Coral—An Inconvenient Truth?
High-resolution subbottom profiling, reef drilling, and mapping of benthic habitats along the
Florida Keys reef track demonstrate that moribund non-accreting coral reefs outnumber live
accreting reefs about 100 to 1, based on reef-tract area. Sub-circular patch reefs restricted
mainly to turbid nearshore areas in the lower Keys compose the majority of living/accreting
reefs. Linear offshore shelf-edge areas are not accreting, and Holocene coral accumulation
during the past 6 ka is generally less than 2 m thick. The thickest accretions consist mainly of
coral spurs rooted directly on a Pleistocene unconformity at the platform margin. Less than 2%
of Florida reefs have kept pace with the rise in Holocene sea level and are generally located
shoreward of the platform margin. Such coral growth, or lack thereof, presents a paradox.
Growth rates of all common coral reef species, especially the rapidly growing acroporids,
should have kept pace with the well-documented rise in sea level over the past 6 ka. Why did so
few reefs keep pace or accrete to present sea level? That Holocene coral growth has
experienced setbacks before has been confirmed by 14C ages of fossil Acropora cervicornis
fragments that are so common in backreef sands. Carbon-14 data reveal two 500-year periods of
non-growth centered on 4.5 ka and 3 ka. The present period of rapid coral demise has spanned
only about 30 years. Thus, past periods of non-growth indicate times of environmental crises
that predated modern human invasion of the Florida Keys. These observations challenge the
highly popular notion that the present declines in Florida, and elsewhere, are anthropogenic in
origin.
1-16
New Ideas About Caribbean Coral-Reef Development: A View From Ye Shoulders Of
Giants
Dennis HUBBARD* 1
1 Geology, Oberlin College, Oberlin, OH
Three cores from Lang Bank described at the 1977 ISRS Meeting set the direction of the coralreef
discussion for the next three decades. A perceived lag in reef initiation led to ideas about
"inimical bank waters" and their control on reef building. High accretion rates reported for
many Caribbean sites led us to ask why reefs capable of outpacing even the most rapid sea-level
rise could be left behind. A possible gap in accretion on Lang Bank at 8,000 CalBP raised
interesting possibilities. Dirty water flowing off the newly flooded bank or a sudden jump in sea
level provided solutions to Schlager's "paradox". Combined with assurances that reef accretion
should mirror coral growth, we developed our prevailing models. More recent studies suggest
that some of the foundational ideas upon which these paradigms are built may warrant
reexamination. Closer analyses of Caribbean cores reveal little variation in accretion rates down
the forereef. New Lang Bank cores show that A. palmata reefs not only continued to build, but
thrived across the "reef gap" that was attributed to "inimical bottom waters" in Miami and later
to a jump in sea level. This was verified at other Caribbean/Atlantic sites. Reef building can be
generally characterized as transgressive between 11,000 and 7,000 CalBP due to a steady but
rapid sea-level rise, and regressive thereafter, as it slowed below ca. 4m/ky. Superimposed on
this pattern are two enigmatic lapses in Acropora, starting at ca. 6,000 and 3,000 CalBP, well
after sea level rise had slowed relative to reef accretion. Paradoxically, these events created
gaps in the Caribbean Acropora assemblage that were not associated with earlier, documented
SL jumps. The presentations at the 1977 Miami meeting were a watershed for valuable thought,
and the return to Fort Lauderdale provides a fitting opportunity to take the next step.
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