Mangrove Mystery – Where Did All the Pumice Come From?

Tracking volcanic rock to the shores of Belize

For years, scientists have found pumice, a porous volcanic rock, scattered on the beaches and mangroves of Belize, despite the lack of volcanoes in the immediate area. Thanks to the persistence of one curious scientist, a diverse group has teamed up to solve this great pumice mystery, ultimately producing a better understanding of how geological, oceanographic and biological forces are linked in the Caribbean.

An aerial photo of Carrie Bow Cay and the Smithsonian research station looking north.

Credit: Dr. Candy Feller, Smithsonian Environmental Research Center

As a geological oceanographer at the Smithsonian National Museum of Natural History, Dr. Maggie Toscano has made a career of documenting how coastal systems have changed over thousands of years in response to rising sea levels, studying ecosystems from the marshes and barrier islands of the eastern US to the coral reefs and mangroves of the Caribbean. In recent years, her work has taken her to the Mesoamerican Barrier Reef offshore of Belize, where the Smithsonian has maintained a field station on Carrie Bow Cay since 1974 (see picture above). While there, investigating the very thick peat (fossil plant matter) accumulations underlying the mangrove islands, she came across something unexpected – an abundance of lightweight porous rocks on the sandy carbonate cay and trapped in mangrove roots above the high tide level (see picture on left from Maggie).

Pumice is the air-filled rock ejected by volcanoes during eruptions. We use it today to scrub our heels, and make light-weight concrete. Marine animals and plants hitch-hike on pumice, which can travel long distances as it floats on the surface of the sea and on other water bodies. And for years, it turns out that scientists have been finding and picking up this curious flotsam on Carrie Bow Cay. The naturally-occurring pumice clearly originated elsewhere, unrelated to the locally-produced, biogenic (animal-secreted) carbonate sediments of the nearby reef, lagoon and the Cay. Pumice small and large can be seen all over the mangrove areas of nearby Twin Cays, on the surface of the peat, stuck in between or perched upon mangrove prop roots. “It is everywhere! We step on it and trip over it constantly while doing field work,” Maggie said.

Visiting scientists attributed its presence to eruptions of Guatemalan volcanoes that produced pumice, which was carried down rivers to the Caribbean, and transported along the mainland coast and to the islands along the reef by ocean currents. This explanation made geological and oceanographic sense, so was not debated. That is, until January of 2012, when Dr. Juan Gonzalez, from The University of Texas – Pan American, made a trip to the research station. The usual generic explanation would not satisfy Juan – he wanted the specifics – and so, Maggie began investigating. 

However, she didn’t investigate solely to satisfy Juan’s curiosity. Determining the source of this wayward pumice in Belize would help illuminate the connections among a variety of seemingly unrelated geologic and oceanographic processes in Central America. And delving into these connections between land and sea could help scientists better understand how inputs from land and the currents themselves impact coral reef health, including nutrient flow and how organisms travel between reefs.


CREDIT: 

Map compiled by Julie Herrick with GVP GoogleEarth data layers.

Some quick research using information from the Smithsonian’s Global Volcanism Program (GVP) indicated that Santa María Volcano, located along the volcanic chain of Guatemala (Figure 3), experienced a major three-week eruption in 1902, with a colossal volcanic explosivity index of 6/10! The eruption spread pumice and ash as far away as Acapulco and Merida, Mexico -- more than 900 km north (Conway, 1995). This seemed the most logical source, and any other scientist might have been satisfied with that. 

But not Juan Gonzalez. “If this pumice was produced in 1902, why is it still on the surface of the sandy cays and on the surface of the mangrove peat after 110 years?” he asked. “Why isn’t it incorporated into the sediment and peat deposits?”