A Paleoecological Record of Tortola, British Virgin Islands: reconstructing a disturbed ecosystem through lake sediment and pollen analysis

Mitchell PowerCameron McIntireCameron D. McIntire and Mitchell J. Power

Department of Geography, University of Utah

Carbon Dating: C-14 radiocarbon dating was carried out via accelerator mass spectrometry (AMS) at the University of Georgia’s Center for Ap-plied Isotope Studies. Sediment was sampled near the bottom of Core 3 at a depth of 80.5cm. Radiocarbon chronology was calibrated using CALIB 6.0 calibration program.

Charcoal and Macro Fossil Analysis: Wood and grass charcoal was counted for each core. Samples were prepared in 1.0cm increments (Core 1 and 2) and a combination of 1.0cm and 0.5cm increments for Core 3. Most samples were weighed in 1.0g portions except in samples of high water concentration; in this case sample sizes ranged from 2.0-6.0g. Sam-ples were then diluted with 10% KOH, heated for 20 minutes, and stirred once. All samples were then passed through a 250μm screen. Seed and shell macrofossils were counted in addition to charcoal.

Loss on Ignition Analysis (LOI): Loss on ignition analysis was carried out with all three sediment cores. Lake sediment was sampled in 1.0cm (Core 1 and 2) and 0.5cm (Core 3) increments. 1.0g samples were dried at a minimum of 24hr at 70°C and measured for dry weight and calculated for moisture content. Samples were then combusted at 550°C for quantifying organic content, and again at 1000°C for carbonate (1).

Pollen Analysis: Pollen was prepared in 12 samples in 7cm incraments throughout the length of the largest and most complete sediment core (Core 3). Samples were processed using standard preparation techniques with a Lycopodium tracer as a control (2). Pollen samples were mounted on slides and identified at 400x magnification with a minimum count of 500 grains/spores per slide(3,4).

Background

Methods Results

I would like to thank my advisor Mitch Power for giving me the opportunity, resources, and his time to make this research possible. I also owe thanks to graduate students Yoshi and Macharia for all their help in teaching me labora-tory techniques and in answering my many questions. Stacy Morris for teaching me the finer points of pollen processing. Finally Jill Baeder and members of the Undergraduate Research Opportunities Program for their support and funding this project for the past two semesters.

Acknowledgements

Figure 5 (above):The bottom section of Core 3, the largest and most complete core in this study. Collected from the center of Belmont Pond in January of 2011. The section shown is 44cm in length and covers about 400 years of ecological information spanning from 1190AD to the mid 1600’s.

References

Figure 3: • Fire increased on the is-land during the intensifica-tion of corn agriculture by Ar-awak natives that had settled on Tortola by 100 BC.

• Fire activity decreased from 1500 AD to present, both in magnitude and fre-quency of fire events.

• LOI data show three abrupt shifts in organic and calcium content of the sediments during the last 800 year, po-tentially an indicator of past hurricanes

Ruppia maritima

From left to right: Plaque commemorat-ing the cholera epidemic that decimated 15% of Tortola’s population in the mid 1800’s (6). The car that escorted Queen Elizabeth II on her visit to the island in 1976, this was a period of deforestation due to the construction of new roads (7). Hurricane Earl, one of the most recent major climatic disturbances to the region (8).

1. Urquhart, G.R. 2009. Paleoecological record of hurricane disturbance and forest regeneration in Nicaragua. Quater nary International 195: 88–97.2. Faegri, K., Iverson, J., 1989. Textbook of Pollen Analysis, 4th Edition. Wiley, New York.3. Roubik, D. W. and J. E. Moreno. 1991. Pollen and Spores of Barro Colorado Island. St. Louis, MO, United States: Missouri Botanical Garden. 4. Colinvaux, P., P.E. De Oliveira and J.E.M. Patino. 1999. Amazon Pollen Manual and Atlas. Amsterdam, Netherlands: Harwood Acedemic Publishers. 5. Kimber, T.K. 1988. Martinique Revisited: The Changing Geographies of a West Indian Island. Texas: Texas A&M University Press. 6. Cholera memorial. Photograph. February 2010. Road Town, BVI. History of the British Virgin Islands. March 2012.7. Queen Elizabeth’s Lincoln. Photograph. Smugglers Cove, BVI. BVI Heritage Conservation Group. March 2012.8. Hurricane Earl. Satellite image. August 2010. NOAA Environmental Visualization Laboratory. March 2012.

Tortola is the largest island of the British Virgin Island archipelago as well as the most heavily populated. Tortola has a small coastal saltwater pond, Belmont Pond, that has been the focus of this research. Vegetation around Belmont Pond is highly diverse, ranging from dry scrub forest of cacti to wet palm swamps and mangrove trees. Sediment cores were analyzed for the charcoal content which were be used to infer significant fire events in addition to intact seeds and pollen preserved within pond sediment. Loss on ignition analysis was used to measure organic and calcium carbonate flux through time and allows us to make observations correlating with climatic disturbances such as hurricanes. This region of the British Virgin Islands also has a rich history of native as well as colonial colonization, resulting in a variety of land use changes such as large-scale agriculture (5). A combination of these techniques and research were used to create an ecological history for the area and prove to be useful tools for describing the past and present community structure around this site.

Figure 2: Map of Tortola and Great Thatch on the Virgin Island chain. A total of three gravity cores were collected from the study areas, indicated by arrows. Contour lines on ponds represent depth.

Figure 1: Pollen counts from Belmont Pond showing four select taxa (3). A total of 20 taxa were identified during pollen counting, but not shown in figure.