The Rock! by Al Curran

*Note to readers: this blog is a bit more involved and in-depth than our normal reports, but given that this is our “Science Research Week” on Ambergris, it seems appropriate.  Please bear with me….


Just one very special rock sample can tell us much about the geological history of Ambergris Caye, including stages of major global climate change that occurred in the not too distant geological past. The island is very low, mostly less than 2 meters above present sea level, and long north to south, but narrow east to west, very much like the barrier islands of the U.S. mid-Atlantic coast. Also like the barriers islands, Ambergris is young geologically. Relatively hard limestone of Late Pleistocene age (~120,000 yrs. before present = ybp) lies just below present-day sand, soil, and vegetation, and this limestone unit runs essentially north-south to form the “spine” of the island.


Image #1 shows the stratigraphic column (1.5 meters) exposed in a recent cut along the entry road to a new and large development on North Ambergris Caye called Grand Belizean Estates (GBE). This is a huge development of about 1,200 small lots on ~190 acres of mostly low, mangroves-covered land. Apparently most of the lots have been sold, but only a few houses have been built to date. GBE is a story unto itself and for another time.


The stratigraphic column and our rock sample (Image #2) reveal a thick, fossiliferous bed (Unit 1) at the base of the section and an overlying brown, hard, thinly laminated layer of material called caliche (Unit 2). Unit 1 contains a diverse fauna of marine mollusks (Image #3 – mostly fossil bivalves, with scattered gastropods), indicating conditions of shallow-marine sediment deposition during the Last Interglacial (~130-115,000 ybp), when sea level was up to +6-9 meters higher than today and the climate was warm.


The arrow in Image #2 marks a profound change in depositional conditions and also in global climate. Sea level dropped abruptly beginning about 115,000 ybp with the onset of the Last Glacial, when large and thick continental ice sheets formed on the northern parts of the Northern Hemisphere continents and spread south.  At peak ice advance (~20,000 ybp), sea level had dropped by about 130 meters below present level. The land that today is Ambergris was literally high, fully exposed, and connected to the mainland, so not an island at that time!


During this time of glacial conditions, which lasted for about 100,000 years, the limestone of Unit 1 was exposed at the surface, and subject to dissolution by naturally weakly acidic rainwater. As the limestone was slowly dissolved away, a thin, laminated rind of iron-rich microcrystalline calcite formed, followed by additional laminae comprising Unit 2. This is the caliche layer, and the brown color reflects its iron content. Source of the iron also is a story for another time. Interestingly, the slowly-forming, relatively thin caliche layer represents about 100,000 years of global glaciation, whereas the rapidly deposited, much thicker limestone layer represents only about 10 to 15 thousand years of interglacial time.

At about 11,000 ybp, the continental ice sheets began to melt and rapidly recede; concurrently, sea level began to rise. This change also marks the beginning of the Holocene Epoch, our present interglacial time of warm climate. As sea level reached the edge of the Belizean continental shelf, the Mesoamerican barrier coral reef began to develop. With more flooding of the land, the island of Ambergris Caye took form. Sea level continues to rise today, and this presents a major challenge for the developed parts of Ambergris. Our Coral Ed seawalls and mangroves projects are both addressing aspects of this challenge.



Image captions:

Image #1: The stratigraphic section on the entry road to Grand Belizean Estates (scale = 1.5 m; yellow box indicates collection point for the rock sample of Image #2).


Image #2: The rock sample with units numbered; arrow marks the important stratigraphic contact between Last Interglacial and Last Glacial depositional conditions.

Image #3: Close-up of Unit 1 fossil-rich beds; bivalve (clam) shell fossils are dominant.


Al Curran & The Coral Ed Team




1 thought on “The Rock! by Al Curran

  1. You have to explain about the iron content–don’t leave me in suspense!

    I’m so happy to see that the program has become an integral part of the San Pedro summer. Smithies have so much to give to the island and the Belizean children have a lot to give back to their teachers.

    Grand Belizean Estates? Glad the building is slow, but hope they build before the sites are under water. So sad that development is altering this beautiful place.

    Hope all is well with you.

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