Brief report on Belize carbonate environments field trip Feb. 2012

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1 Brief report on Belize carbonate environments field trip Feb In february 2012 I travelled to San Pedro (Ambergris Caye) Belize (Figure 1) to examine and study depositional environments of carbonate rocks on the Belize Shelf. Over the period from the 19 th to the 27 th of February I used boats, a kayak and a bicycle to explore various depositional environments on and around Ambergris Caye, including the main reef, the outer shelf, the island itself, restricted lagoons and the inner shelf (see Figure 2). In most of these environments I took photographs, described the conditions (e.g., water depth, ph and temperature, sediment type), and sampled and described the sediments. Figure 1 Location map While in San Pedro I also made contact with a number of people that may be able to assist me with planning for a follow-up trip to Belize with V.I.U. Earth Science students in Figure 2 Depositional environments on the northern Belize shelf (from Mazzullo, 2006) 1) the barrier reef 2) the outer shelf 3-4) lagoons 5-10) the inner shelf

2 The Barrier Reef The main Belize barrier reef lies about one km offshore from San Pedro, and is easily accessible by boat or kayak. The reef is a critically important habitat in the region, and is home to dozens of different types of corals, sponges and algae and many different types of fish ranging right up to sharks, rays and turtles. Water depth on the crest of the reef is less than 1 m. On the inner edge of the reef the depth approaches 2 m, while on the outer edge it drops to several hundred m. The barrier reef also serves to protect inshore areas from the strong Caribbean swells, and from a geological perspective, it provides an abundance of shell material that is incorporated into the sediments of the outer and inner shelves. Like other barrier reefs around the world, this one is vulnerable to the effects of tourism, development, fishing, storms and climate change. Belize has strong environmental protection systems in place to ensure that the reef is preserved (including a Marine Reserve system), but the authorities can do nothing to protect the reef from tropical storms and increasing water temperature and acidity, all of which are likely to become more extreme as a consequence of climate change.

3 The Outer Shelf In the Ambergris area the outer shelf ranges in width from 1 to several km, and is characterized by water depths ranging from 0.5 to 3 m. It has a sandytextured bottom comprised mostly of carbonate fragments eroded from the reef itself. One of the key features of the outer shelf is the presence of marine grasses, such as Thallasia testudinae (see photos to the right). Not only do these grasses provide habitat for numerous small animals, they play a critical role in trapping sediment, thus allowing the shelf area to be maintained. In some areas grasses have been destroyed by resorts, whose guests prefer bare sandy shores, and also by frequent passage of motor boats used by fishermen and to transport tourists and residents. Lagoons There are several large restricted-circulation marine lagoons on Ambergris, and they represent an important habitat and carbonate depositional environment. The lagoon areas are well populated with mangroves, which, like the marine grasses, serve an important role as animal habitat and stabilization for sediments. Indeed, without mangroves, it is unlikely that islands like Ambergris would exist at all.

4 Inner Shelf The inner shelf in northern Belize is 20 to km wide, but water depths are still generally in the 2 to 5 m range. The inner shelf is generally isolated from the reef, either by islands like Ambergris, or by distance, and thus the carbonate sediments are different than those on the outer shelf. In some areas the sediment is comprised of fine mud transported in suspension from farther offshore (top photo). In other areas of the inner shelf the carbonate sediment is comprised mostly of shells of molluscs or of foraminifers: mmsized zooplankton (protists) living suspended in the shallow water. In some parts of the inner shelf the sediments also include amorphous carbonate peloids, some of which originate as fecal pellets from fish and crustaceans. As in the lagoons of Ambergris Caye, mangroves are critical in stabilizing sediments in the inner shelf region. The shelf is dotted with small islands some inhabited most of which would not exist without the sediment trapping properties of the mangrove roots and stems.

5 I have learned a great deal about carbonates by travelling to the Ambergris Caye area of Belize to observe the depositional environments and processes with my own eyes. As a result, I am now in a much better position to teach my students about carbonate deposition and carbonate rocks than I was before. During the rest of this term I will be using samples, photographs and information from this trip in my GEOL-201 Sedimentary Geology course. I will also be using these materials in other courses in the future. I believe that, because of its accessibility and the amount of research that has been done in this area by others, the Ambergris area is an ideal place to take students for a GEOL-390 Special Field Studies course. I propose to take a group of students to Ambergris during Spring Break in I have now made contact with a number of services providers in the area (regarding accommodation, boat transportation and snorkelling tours) and with a scientist at the Hol Chan Marine Reserve office in San Pedro. Reference Mazzullo, S, 2006, Late Pliocene to Holocene platform evolution in northern Belize, and comparison with coeval deposits in southern Belize and the Bahamas, Sedimentology, V. 53, p Prepared by: Steven Earle, PhD Earth Science Department March 2012