Biogeochemical investigation of caves within Bahamian carbonate platforms.
The Bahamas are thousands of islands stretching along the coast of Florida and south to
the tropic of cancer. An aerialv iew, spectaculaar s it is, shows only a small part of the
islands, a larger part is submerged and discernible only by the abundance of blue dots
onshorea nd in the shallow water around the islands. The blue holes are entrancesto an
underwater world wherein biology seems to have slowed down to offer a fascinating
view of species rarely seen elsewhere, and of rock formations that tell a story of the
subterranianw orld below the islands.
My studies were inspired by curiosity and the desire to make a contribution to
the preservationo f this unique and irreplaceablew indow into the pastf or geologistsa nd
Important questions as to how the caves formed and when and what the role of
cavel ife might havep layed in the excavationp rocessesa re addressedin this thesis.
As a result of these studies bacteria must be considered as possibly active
participantsi n the formation processesa fter the time when the rising watertablef looded
all the Bahamian caves. The layering, often in hypersharp zones of biological material,
matches or is perhaps caused by, equally dramatic changes in hydrogen ions and acetate
and sulphate concentrations. In both caves, pH profiles were observed that varied
sharply over very short distances.
In all three study sites it could be shown that, vertically and horizontally, the
geochemical perimeters varied dramatically. Organically mediated processes are a
dominant control on dissolution within the fresh, mixing, and saline zone. The
combined generationof C02 results, known from earlier studies, and 112S and other
bacterially measured activity, supports this finding. Retention of suspended organic
matter was directly proportional to the salinity gradient. Elevated levels of DOC, POM,
and acetate at the two major density interfaces confirm this finding. In the results,
generated from CHN&S methods, wall rock material was found to contain residual
carbon for potential microbial use, and large bacterial populations were identified
through the SEM method in excess of what was measured within the water column.
Significant lepidocrocite deposits (dimorphose iron oxihydroxide) were
identified as a fraction of the cave sediment, and material known as "mung7, based on
amino-acid analysis, was shown to be proteinaceous. This mung appears to be unique
to caves in the Bahamas.
In a sense the cave system is an underground geo-biosphere wherein the
peculiar water flow patterns foster a specific, sometimes rich and unusual, fauna and
flora that is a treasure for science and a sight for humans to enjoy.