Mangrove Mystery – Where Did All the Pumice Come From?

Mapping the Pumice Journey

Finding pumice on the shores of Belize is a common occurrence based on a lot of anecdotal evidence, and may have figured prominently in Mayan life over the centuries. Maggie’s Mayan friend Doroteo Cho, a Belize Fisheries Officer on Twin Cays, reports that the locals use ground pumice to make strong cinderblocks for buildings, and use pieces to clean their comal, or cast-iron tortilla pans. He reports that old Mayan pottery shards have a texture indicating the inclusion of ground pumice, whereas modern pottery is made of all clay. According to Doro, pumice is not in constant supply along the coast, which supports the idea of periodic erosion and transport of new loads.

This map shows the possible pathway of pumice transport to the shores of Belize (the green line) amidst major drainage basins and wind and current patterns. Click the photo to see the full version of this map.

Credit: Image adapted from Ezer et al., 2005; WRI, 2006; Burke and Sugg, 2006; and Chérubin, et al., 2008.

But what path, precisely, did this pumice take to reach Belize from the Guatemalan Highlands? Maps of drainage networks that reach the Gulf of Honduras and currents in the western Caribbean Sea are important for describing how the pumice could have been transported. The Río Motagua and Río Dulce rivers have watersheds that, combined, drain 8,270 sq mi (21,420 sq km) of the northeastern slopes of the Guatemalan Highlands (see above figure) (Burke and Sugg, 2006), contributing large loads of suspended sediments and organic matter to the Caribbean. Under normal conditions, sediment, seeds and pumice introduced to the gulf by rivers get picked up by nearshore currents, and are carried along a circuitous path to their eventual resting place on mainland shores, sandy reef cays and mangrove islands.  The northwestward flow of the main Caribbean Current is located offshore of the Belize reefs, but smaller currents are known to move along the shore and near the reef (Soto et al., 2009, and Burke and Sugg, 2006). These smaller currents could have transported the pumice from the Guatemalan volcanoes.

But even mapping the complex journey of the pumice wasn’t enough to quench Juan’s curiosity. Further laboratory analysis of the composition of the pumice will narrow down which volcano likely produced the material. Using a microscope (optical or scanning electron), the minerals within the pumice can be identified and more advanced techniques will further define the rock composition. This information can be compared to the chemical composition from the tephra marker units of each eruption to determine when and from which volcano the pumice was originally formed.

The precise identifications of volcanic origin, sample ages and watersheds will teach Maggie, Julie and Juan about land-to-ocean linkages and pathways for each new load of pumice introduced into this river-ocean conveyor. Furthermore, they could help ecologists better understand the sources and impacts of foreign materials on Caribbean ecosystems. Biologists, ecologists, and physical oceanographers investigating Caribbean reefs have found that biological and inorganic materials floating along those pathways have reached the reefs on a regular basis. Introduced nutrients delivered by the currents can be harmful to reef health; however, larval transport and replenishment of reef species down-current is potentially beneficial. Additionally, pumice may serve as a raft for attached marine life such as barnacles (See example of encrusted pumice from eruption at Home Reef Volcano on left, found at Marion Reef, Australia a year later. Photo: Scott Bryan) and encrusting organisms to move from one area of the Caribbean to the other.

Investigating the source of pumice found along the shore of the Caribbean offers an interdisciplinary opportunity to derive linkages and connectivity between seemingly unrelated realms of volcanoes, rivers, ocean currents, coral reefs, and mangroves. Maggie, Julie and Juan hope this work will highlight the widespread and dynamic connections between radically different natural environments, and finally satisfy Juan’s questions.

References

Burke, L. and Sugg, Z., 2006. Hydrologic Modeling of Watersheds Discharging Adjacent to the Mesoamerican Reef: Analysis Summary, World Resources Institute, 35 p.

Soto, I., Andrefouet, S., Hu, C., Muller-Karger, F.E., Wall, C.C., Sheng, J., and Hatcher, B.G., 2009. Physical connectivity inthe Mesoamerican Barrier Reef System inferred from 9 years of ocean color observations, Coral Reefs, 28:415–425.