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Clues on Middle Caicos

low-level-wet-passageWhat can the Conch Bar cave system tell us about sea levels in the past?

By Gina E. Mosely M.Sc., School of Geographical Sciences, University of Bristol, UK with Professor Peter L. Smart and Dr. David A. Richards

Reports on climate change and its effects are becoming a regular occurrence within the media as researchers try to establish the impacts of global warming. At present, the polar ice caps and mountain glaciers are being significantly reduced due to surface warming and over the past several decades sea levels have risen as a result. However, in more recent years oceanic rise has been documented as occurring at an accelerated rate (IPCC, 2007). How high will sea levels rise in the future? How do we go about finding the answer? To assist in answering such questions we look to the past.

The last time that sea levels were above present-day levels was approximately 125,000 years ago during the last warm interglacial period. Sea levels at that time are estimated to have been up to 6 metres above the present level. Researchers, however, are particularly interested in an interglacial period beginning around 423,000 years ago known as Marine Isotope Stage 11 (MIS 11). The Earth’s orbit around the sun, which is one of the key drivers for cold glacial/warm interglacial periods, shows similarities between MIS 11 and the present day, allowing it to be used as an indicator of future conditions.

Beaches form at sea level as sand is deposited by the incoming waves and can therefore be used to establish former sea levels. Some beaches on Eleuthera in the Bahamas were formed during MIS 11 at a height of +22 metres above today’s sea level (Hearty, 1998), although there is debate about the evidence. It is thought that for sea levels to have risen so dramatically in the past, the entire Greenland and West Antarctic ice sheet must have collapsed. Such a rise in the future would have very serious implications globally. Understanding the heights that sea levels have reached in the past and the rates at which they rose and fell is therefore important to assist climate modellers with predicting future changes.

Sea level records are produced across the globe using a variety of proxies including coral reefs, beaches, sediment cores from the bottom of the ocean and cave deposits. The most complete record comes from coral reefs in Papua New Guinea, but the reefs there have been uplifted over the years creating uncertainty in the sea level record produced from them (Richards et al, 1994). The Turks & Caicos Islands are generally believed to be stable, making them an ideal study site for sea level research.

The porous limestone Islands are saturated through and through with saltwater at depth. And on top of the saltwater sits a layer of less-dense freshwater known as the freshwater lens. The freshwater/saltwater boundary is chemically aggressive and called the “mixing zone” or halocline. It is here that caves, called flank margin caves, form (Mylroie and Carew, 1990). Sea levels fall and rise with the expansion and contraction of ice sheets during cold glacial and warm interglacial periods, so changes in sea level result in changes in cave development, as new passages form at the elevation of the new mixing zone.

During September 2007, I visited the Conch Bar Cave system, Middle Caicos, with Professor Peter Smart of the University of Bristol, UK. This cave is the longest surveyed sub-aerial (above water) system in the Bahamas and Turks & Caicos Islands with over 5 kilometres of passages. (Some of the original survey information from 1994 is currently on display in the TCI National Museum on Grand Turk and the National Environmental Centre in Providenciales.) A particular reason for visiting the cave is that the highest passages are well above sea level and could provide evidence for the MIS 11 high stand.

We worked in the cave for four days from dawn until dusk, measuring the elevation of passages and cave deposits throughout the whole system. In some places this involved wading in deep water, in others balancing precariously on ledges to reach the ceiling of the large chambers and squeezing through the narrow corkscrew near the Indian Entrance. In total, 84 cross sections of passage were sketched, taking account of main ceiling height, former floors and low notches. Bells and tubes extending up into the roof were also recorded. Using surveying equipment comprising a level and staff, 689 detailed measurements of ceiling height were taken relative to water level within the cave. The water level, however, changes with the tide, making it necessary for a monitoring station to be set up to correct all the measurements to one fixed point.

Preliminary analysis of the results shows cave passages above sea level at +17 to +20.5 metres, +6.5 to +10 metres, +2.5 to +6 metres, +2 to +3 metres and -0.5 to + 1.5 metres. John Garvin, Mark Parrish and James Hurley of the Caicos Caves Project have also explored a lower level of passage at -15 to -12.5 metres and have provided data for our study. There are two possible explanations for the high passages — either Middle Caicos is subject to earth movements as are other islands near the Caribbean/North American tectonic plate boundary, or the highest levels confirm high sea stands associated with complete melting of the ice sheets.

We are now trying to find the age of the various levels in the caves by dating of the cave deposits such as stalagmites, stalagtites and flowstone. These contain an isotope of uranium called 234U which decays over a long period of time to an isotope of thorium, 230Th. The testing process works in a similar way to radiocarbon dating (14C) but with a longer range of application (400,000 years into the past compared to 50,000 years). A stalagmite will always be younger than the formation of the passage, therefore dating the deposit provides a minimum age for passage formation. Additionally, cave deposits do not form underwater so they therefore stop growing when flooded by rising sea levels. Through dating the point at which the deposit stopped growing and knowing the elevation that it was growing at, it is possible to establish how high sea levels were at a particular time.

A number of stalagmite samples were collected with permission from the TCI Department of Environment & Coastal Resources (DECR) for use in this study and will be dated using the state of the art, modern mass spectrometric facilities at the University of Bristol. Efforts at present are being concentrated into obtaining an age for the formation of the passage in Upper Entrance that is 17 metres above sea level.

upper-entranceIn addition to the work on Middle Caicos, one day was spent looking at the caves of East Caicos to assess sites for further study. Members of the Caicos Caves Project, TCI National Trust and DECR guided our visit.

We visited four caves on East Caicos which had been previously located. Stubb’s Guano Cave and East Caicos Cave #2 were both found to be dry caves which had previously been mined for guano. Both caves contained old wooden artefacts that had been used in the mining industry and inside the former, the date “1883” was inscribed in the wall. These two caves also appeared to have two passage elevations at approximately +1 to +2.5 metres and +3 to +7 metres above sea level. East Caicos #2 also contained a large pool of water at one end of the cave though no underwater sections were found.

The impressive Edison’s Cathedral Cave was found to be largely flooded and contained some underwater passages. The ceiling of the cave is extremely high and may be greater than +9 metres. Wooden structures and several pieces of old machinery were found within the cave, though it was difficult to decide whether this cave had been used in the guano mining industry or if the machinery served another purpose. Large rocks were stacked on shelves high above the water which indicates that at some point in the past the floor was much higher.

The final cave we investigated was a sinkhole with a large collapsed entrance. The cave, which was mainly flooded, had ceilings up to approximately 3 metres high and appeared to have only one phase of sub-aerial passages. Divers John Garvin and Mark Parrish explored considerable sections of underwater passages which most probably relate to another phase of passage development.

Further survey work is now needed on East Caicos. Other high level caves may be present in Flamingo Hill, the highest point of the Turks & Caicos Islands and so a return trip is planned to locate and survey these. Meanwhile, dating work continues at Bristol and we are also investigating other high level caves in the Bahamas, such as Hatchet Bay Cave on Eleuthera.

This trip was funded partly through the UK Natural Environment Research Council (NERC), Studentship number NER/S/A/2005/13256. The authors would especially like to thank John Garvin, Mark Parrish and James Hurley of the Caicos Cave Project and Kim Mortimer for diving and logistical support on East Caicos, Middle Caicos and Providenciales. Thanks are extended to Brian Manco and Judnel “Flash” Blaise of the TCI National Trust and Brian Riggs of the Department of Environment & Coastal Resources for guidance and expertise during the visit to East Caicos. Thanks also go to Capt. Cardinal Arthur of Middle Caicos for his excellent seamanship. We would also like to thank the TCI Government for their permission to undertake this sea level research.

References

Hearty, P.J., 1998. The geology of Eleuthera Island, Bahamas: A rosetta stone of Quaternary stratigraphy and sea-level history, Quaternary Science Reviews, 17, pp. 333–355.

Lemke, P., J. Ren, R.B. Alley, I. Allison, J. Carrasco, G. Flato, Y. Fujii, G. Kaser, P. Mote, R.H. Thomas and T. Zhang, 2007. Observations: Changes in Snow, Ice and Frozen Ground. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Mylroie, J.E., and Carew, J.L., 1990. The flank margin model for dissolution cave development in carbonate platforms, Earth Surface Processes and Landforms, 15, pp. 413–424.

Richards, D.A., Smart, P.L. and Lawrence-Edwards, R., 1994. Maximum sea levels for the last interglacial period from U-series ages of submerged speleothems, Nature, 367, pp. 357–360.



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Kevin
Nov 22, 2016 15:38

Having recently explored a lot of the Hatchet Bay Cave in Eleuthera Bahamas with my wife (we were both formally wild cave tour guides for the U.S. Forest service at Blanchard Springs Caverns in Arkansas) we have had a hard time finding detailed information about the cave. Maps and info about the upper section and the lowest section in particular. We would like to return with more time to more thoroughly explore and craw through the smaller cracks. I found a zoomed out map that showed the upper section extends much further that we thought and didn’t have the extra time to explore and the lower level was at high tide. We walked a short distance into the lower level but will need to return with better equipment to stay wet. One report mentioned the lower level extended @1200 meters. Any help would be greatly appreciated.

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Agile LeVin—photographer, explorer and chronicler of everything TCI on his website www.visittci.com—took this drone photo of the multi-textured wetlands of West Caicos. He was part of the expedition that investigated the site of the historic pirate attack in the area. For more information and photos, go to page 48.

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