The Yucátan Peninsula's mangrove sinkholes are known sources of soil carbon, and researchers just found the most carbon-rich one yet, marking an important step in assessing the economic value of these ecologically vital areas — and highlighting the need for their conservation.

Around 2,000 of these sinkholes, known as cenotes in Spanish, dot the Yucátan Peninsula, and indigenous Mayan communities own most of them. These people, as well as the unique fauna that inhabit these sinkholes, stand to directly benefit from preservation, as growing tourism and increased agricultural burdens threaten their integrity, according to a paper published Wednesday in Biology Letters.

"Our main interest is to conserve these habitats, and to know and to give a value to these habitats, so they can be conserved in a more urgent way than currently," said second author Nadia S. Santini, a research fellow working in ecosystem ecology at the National Autonomous University of Mexico. "I don't know if lots of people still see them just as pools to swim in, without understanding how much more is over there."

The Yucátan Peninsula comprises much of Mexico's southeastern states, as well as most of Belize and parts of Guatemala. On its eastern coast lies the Caribbean Sea, and its western coast curls upward, forming what looks a continental bowl under the Gulf of Mexico. 

Where some cenotes were blasted into existence by the Chicxulub impact that doomed the dinosaurs, the cenotes studied by Santini and her colleagues developed from rain and seawater that gradually eroded the Yucátan's limestone deposits, which, over time, collapsed and formed sinkholes. The sinkholes, along with fluctuating sea levels, eventually gave mangroves a chance to display their knack for growing vertically.

Mangroves are trees or shrubs growing along the coastal tropics and subtropics, and because they live in flooding conditions, the soil in which they root contains little oxygen. 

According to Santini, scientific evidence shows that sea level rise decelerated in the mid-late Holocene. But as sea levels rose over time and filled the cenotes with seawater, the mangroves surrounding them grew upward, leaving thick deposits of peat, or preserved roots, in the water underneath. These deposits can be up to 6 meters deep, and have remained largely untouched for at least the last 3,000 years, sealed by their underwater environment, keeping their massive carbon stores untouched.

To determine the soil carbon content of these deposits, two of Santini's co-authors donned scuba gear and retrieved soil samples from three cenotes in the Yucátan. One of the sampling sites at casa cenote revealed a carbon stock of 2,792 megagrams of carbon per hectare, the most-ever described in mangroves.

The carbon-dense soil poses a potential climate threat. If the soil is ever exposed to air, whether via deforestation or groundwater extraction, it will react with oxygen and emit carbon dioxide, according to Santini. While cutting down mangroves is illegal in Mexico, deforestation remains an issue, as even Pemex, Mexico's state-owned petroleum company, cut down mangroves — against a government order — to build an oil refinery in the Yucátan.

Yucátan tourism also applies pressure to the cenotes. Prior to COVID-19, a record number of tourists visited in 2019: more than 3.2 million. These tourists help foster pollution, and the tourism industry in Yucátan relies on groundwater for washing and bathing, because there is nowhere else to get fresh water but the ground. 

"Unfortunately, there is not a lot of planning in the tourism industry, so there has been a lot of overexploitation of the groundwater," said Santini. 

Aside from the tourism industry, pressure to industrialize once-traditional agricultural practices of the area have polluted the groundwater system with fertilizer and pesticides, according to Santini. These threats can harm the wide range of unique organisms found only in the area, such as the Yucátan jay, the Yucátan amazon and the Mexican blind brotula, a cenote cavefish that is the only member of its genus Typhliasina.

The findings of this study can be a first step in protecting cenotes, the authors say. By converting the carbon content of a cenote's reservoir into equivalent carbon dioxide emissions, landowners can figure out how much money they could make by protecting their cenotes.

"We give them value because of the carbon market, but we also need to think about them as an important habitat," Santini said in an interview with The Academic Times. "It's unfortunate that we need to give them a monetary value, but they have a value in terms of biodiversity too, and I think that's very important."

The money matters because a number of green companies and projects in Mexico are investing in carbon offsets. One is MexiCO2, a company run by a Mexican financial institution, which works closely with the country's Secretariat of Natural Resources, Natural Forestry Commission and The National Institute of Ecology and Climate Change.

Scientifically, Santini is also interested in studying the cenotes more to better understand their carbon history and biodiversity. But, just as important is getting this information to the appropriate government agencies, as well as the Mayan people who live among the cenotes, for whom the choice of conservation, according to Santini, is a common one in rural Mexico: create farmland or preserve forest. 

"If you instead give them money for conservation, then they're going to conserve," said Santini. "The government needs to give them alternatives and, really, programs that value our environment, because our environment is what is going to keep us going."

The study, "Mangrove sinkholes (cenotes) of the Yucatan Peninsula, a global hotspot of carbon sequestration," published May 5 in Biology Letters, was authored by M. F. Adame, Griffith University; N.S. Santini, Universidad Nacional Autónoma de México; O. Torres-Talamante, Colectividad Razonatura; and K. Rogers, University of Wollongong.