Upload
tillie
View
48
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Evidence of coral reef accretion under unique environmental conditions: The Mid-Holocene fossil reefs of the Enriquillo Valley of Southwestern Dominican Republic. David N. Cuevas Miranda, Ph.D. US EPA Caribbean Environmental Protection Division, Guaynabo, PR Clark E. Sherman, Ph.D. - PowerPoint PPT Presentation
Citation preview
Evidence of coral reef accretion under Evidence of coral reef accretion under unique environmental conditions: The Mid-unique environmental conditions: The Mid-
Holocene fossil reefs of the Enriquillo Valley Holocene fossil reefs of the Enriquillo Valley of Southwestern Dominican Republicof Southwestern Dominican Republic
David N. Cuevas Miranda, Ph.D.US EPA Caribbean Environmental Protection Division, Guaynabo, PR
Clark E. Sherman, Ph.D.Dept. Marine Sciences, UPR-Mayagüez, PR
Wilson Ramírez, Ph.D.Dept. of Geology, UPR-Mayagüez, PR
Dennis K. Hubbard, Ph.D.Dept. of Geology, Oberlin College, Oberlin, OH
Geol. Soc. of America Southeastern Section MeetingMarch 20-22, 2013, San Juan, Puerto Rico
Section T12: Quaternary Caribbean Reef Systems
Statement of the Problem• High sedimentation
rates/resuspension is one of the primary causes for coral reef decline in many areas of the world.
• Yet, there is evidence of coral reefs that thrive in environments of very high sedimentation (e.g. inner reefs within the GBR in Australia)
• Natural vs. Anthropogenic induced sedimentation:– Is there really a difference?– If there is a difference, are the
effects on coral reefs different also?
Ikonos Image Mayagüez Bay, Western PR. (Courtesy of GERS Lab, Dept. Of Geology, UPRM)
Inner reefs in Mayagüez Bay with no coral and abundant macroalgae (Photo by J. Morelock)
Natural vs. anthropogenic
• Some limitations:– Earliest coral reef studies date back only to
the 1950’s– Possible changes within the coral reef require
long time
• Ecologists are looking at the fossil record.– If fine-scale changes documented over the
past decades are shown to be unique, then human impact comes as a reasonable explanation
Fossil Approach
• Study records of well-preserved fossil reefs in detail
• Compare the documented changes with those occurring in modern reef.
• Already been implemented in several studies (e.g. Pandolfi and Jackson, 2001; Aronson et al. 1998; 2002)
Previous Studies (Cont.)
Drill Cores (by C. Sherman)
Pleistocene Fossil Coral Reef, Curaçao, Netherlands Antilles
Cañada Honda Site
From Cuevas et al. (2009)
Study site
Modified from Mann et al. (2002)
Regional cross section across central Hispaniola (from Draper et al. 1994)
Landsat Image of SW Dominican Republic(Front cover of the 9th Caribbean Geological Conference)
Cordillera Central
Cuenca de San Juan
Sierra de Neiba
Cuenca de Enriquillo
Sierra de Bahoruco
General Geology
Haiti
Haiti
Bahia Enriquillo
Lago Enriquillo
Modified from Hubbard et al. 2008
10,000 CalBP
ca. 3-4 kybp - Closure
Modified fromBudd et al. 2006
Why these reefs?• Well-preserved subaerial
exposures (~10,000-4,000 yrs. ago, Mann et al, 1984; Taylor et al. 1985)
• The record allows for close examination of reef changes
• Influenced by high terrigenous sedimentation, before humans inhabited Hispaniola
Upstream Las Clavellinas Site
Bed of Acropora cervicornis
Sand and silt
Vertical Transect
• Comparison with modern reef community structure data from Southwestern Puerto Rico
Data summarized in Morelock et al. (2001)
Results
Community Structure:
• At least 22 species of Scleractinian corals (in transects)
• Siderastrea siderea most abundant (47.4%) followed by Montastraea faveolata (8.73%)
• Two major coral zones:– Branching coral zone– Siderastrea siderea zone
• Mix-coral subzone• Massive coral subzone• Platy coral subzone
Coral Species Distribution
Stratigraphy of CH, transect and coral sample location, and stratigraphy
Coral Morphologies
Coral Morphology
• Most morphologies typical of high sedimentation settings
Hubbard, 2005
High sedimentation
Sediment bands
Comparison with SW Puerto Rico
Results (cont.)
Sediment composition:
• The Cañada Honda fossil reef sediment is characterized by the high percentage of carbonate material.
• Carbonate varied from 84% to 96 %.
• A significant portion of CaCO3 from allochtonous (terrigenous) sediment
Model for sediment mineralogy in CH
Abundance (%) of constituent grains in CH sediment
Echinoid3.3%
Foraminifera0.5%
Sponges0.7%
Barnacles3.5%
Serpulid worms1.3%
Quartz/Feldspar0.5%
Bivalve15.2%
Gastropods5.1%
Calcareos algae0.1%
Red algae/Bryozoa0.3%
Coral21.1%
Non-skeletal48.5%
Discussion
• Coral community structure in Cañada Honda has distinctive features characteristic of siltation stress:– Remarkable abundance of sediment tolerant
coral species (e.g. S. siderea, M. faveolata, Agaricia spp.)
– Coral cover dominated mostly by two species• Large colonies, few juveniles
– Sediment bands within coral skeleton– Growth morphology (i.e. encrusting domes,
plates, cones).
Discussion (cont.)
• A. cervicornis zone similar to sites in protected environments with limited wave action throughout the Caribbean
• Unique assemblage in CH– Overwhelming abundances of Siderastrea
spp. – Absence of important coral genera (Diploria,
Mycetophyllia, Meandrina)
Discussion (cont.)• Coral development at
depths of ~10-12 m.– In typical reefs under
siltation stress corals do not grow beyond depths of ~10 m).
• High nutrients?:– phytoplanktonturbidity?– High abundance of
macroborers (Lithophaga and barnacles)
• High carbonate (CaCO3) content of terrigenous and reef sediment
Unique conditions in Cañada Honda
• Other studies have documented high salinity fluctuations (Greer and Swart, 2006)– Could explain high abundance of Siderastrea spp.
and the absence of other important genera
• High content of CaCO3 in terrigenous sediment allowed better light penetration
• Sedimentation events probably occurred sporadic, allowing time for reef corals to “keep-up” with rising sea-level and high sediment input.
Lessons from Cañada Honda
• Yes, coral reefs thrive within natural conditions of high sedimentation– Substantial evidence for siltation stress
• Highly resilient communities over thousands of years– Storms events– Mid-Holocene Thermal Maximum
• Greer and Swart 2006; Greer et al. 2009
Conclusions• Although there is evidence for high sediment
influx, the Cañada Honda fossil reef thrived and accreted for at least 3,000 years.
• Unique conditions:Protected marine setting + semiarid
conditions + high salinity fluctuations + high terrigenous sediment with high CaCO3
• Development of the CH reef occurred under conditions of high sedimentation and depths close to 10-12m.
• The high content of CaCO3 of the sediment
allowed more light penetration.
• Sedimentation was probably sporadic, allowing time for reef corals to grow back and “keep-up” with sedimentation and rising sea-level.
Acknowledgments
• UPR-NASA Space Grant Program• AGEP program of UPRM• Southeastern section of the Geological Society of America• Center for Research and Development of UPRM• Sea Grant Program at UPRM.• NSF-EPSCoR program of the UPR. • Geology Department at UPRM• Marine Science Department at UPRM• The Faculty of Arts and Sciences –UPRM• Members of the thesis committee• The staff of Hospital San Antonio, Mayagüez, Puerto Rico• Vanessa Rosario, Viviana Díaz and Elisa Torres.• Dennis Hubbard, Karla Parsons-Hubbard and all the students from Oberlin
College that were involved.• Secretaría de Estado y Ambiente of the Dominican Republic.• The staff of Hotel Iguana in La Descubierta.
Thank you!