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42672-B8
Holocene Coral-Reef Development of the SW Dominican Republic: Views Forward and Back in Time
This project continued to examine unique exposures of Holocene reefs in the western Dominican Republic. The three primary goals this year were to 1) strengthen the data to test patterns recognized in the first year, 2) characterize depth-related patterns of reef development and post-mortem coral alteration, and 3) examine new outcrops to broaden our understanding of spatial relationships between reef character and sediment stress.
The first goal was addressed primarily through vertical transects along the main Cañada Honda outcrop. Close-up photographs and detailed measurements were used to create “virtual cores” (one of the main objectives in the proposal) that will be compared to the more-detailed measurements of species abundance and diversity collected in Year I. Selected samples from each transect will be sent off for radiocarbon analysis to better constrain reef accretion both within and across reef facies. The end result will be a detailed characterization of reef development in response to temporal and spatial changes in water depth and sedimentation. Comparison of the “virtual cores” with the more complete picture provided by outcrop-scale measurements will allow us to assess the way we evaluate cores in more traditional Holocene reef studies. We also hope to address important questions about quantitatively comparing abundance and diversity of live corals measured on the reef surface to fossil communities preserved within reef interiors.
Quantitative measurement of coral shape has verified that a shift in coral morphology from hemispherical to conical to columnar colonies reflects increasing sediment stress. At Cañada Honda, conical colonies are dominantly the sediment resistant genus Siderastrea, while columnar colonies are more often the less tolerant Montastrea faveolata. The reefs to the east near Las Clavelinas are more heavily stressed. In response, Siderastrea spp colonies shift to columnar morphologies (i.e., a morphological shift to mitigate sedimentation), and Montastrea is largely absent (i.e., there is no more sediment-tolerant morphology available). Colony shape also impacts the intensity of post-mortem alteration. Columnar colonies in close proximity have no space between them to provide shelter for bioeroding species. In contrast, conical colonies still have small crypts between them, and bioerosion is higher, at statistically significant levels (a = 0.05), regardless of colony species. These data provide the first direct evidence to support the higher sedimentation levels that have been presumed to drive reef type and community structure within Bahia Enriquillo.
The identification of a storm layer in the Cañada Honda outcrop has provided an unusual opportunity to characterize reef-community patterns. In year I, we focused on coral growth. In Year II, we switched to bioerosion, with an intention of comparing post-mortem alteration in the outcrop to patterns found in modern reefs. Samples of M. faveolata and Siderastrea sp. were collected at regular intervals from the massive-coral facies just below the storm event bed. Detailed survey data and radiocarbon analyses will hopefully allow us to compute paleo-water depth for the samples. Comparative samples were collected along two transects across a modern reef on St. Croix, US Virgin Islands. Differences in the major bioeroders at the two sites support the presumption of higher-than normal sedimentation within Bahia Enriquillo throughout the Holocene. Analyses of depth-related patterns in the fossil and modern samples are in their early stages.
Our geographic range of outcrop measurements was enhanced this summer. Two new exposures to the east of where we worked in Year I will allow us to consider the effects of location and sedimentation on reef type. Only preliminary observations and measurements were taken this summer. A follow-up visit in 2008 will be used to collect more detailed data on coral abundance and diversity at these new outcrops.
PRF funds supported two undergraduate students in the field during Summer 2007, and will support them this Fall. Supplemental funds allowed the participation of Halard Lescisnsky and a student from Otterbein College. Lescinsky works on coral taphonomy; his investigations will strengthen our studies of post-mortem coral history. Funds from the Keck Geology Consortium and Oberlin College supported the two PRF students to collect comparative samples from a modern reef in the U.S. Virgin Islands. All of these will be used for Senior Research Projects at Oberlin. The students will present their data at the International Coral Reef Symposium sponsored by the International Society for Reef Studies in the summer of 2008. A post-meeting trip to the Enriquillo Valley will highlight the results of our studies. Advertisement of the trip has already resulted in inquiries about possible collaborative studies using these outcrops.
The PI presented project results at the Annual Meeting of the GSA and an SEPM short course on Quaternary reef development. The GSA presentation was expanded into a manuscript (Depth and Species-Related Patterns of Holocene Reef Accretion: A Critical Assessment of Existing Models), which is scheduled to appear this year as a special issue of Sedimentology dedicated to the achievements of Robert Ginsburg.
