Buried shells in Houston are no treasure

Zeng noticed the shells in the roadbeds of Texas. Builders put them there as far back as the mid-19th century because the materials were plentiful and cheap. Her studies of the shells have added a small piece to the global puzzle of how human enterprise has altered the natural cycle of carbon dioxide, a greenhouse gas that plays a major role in global warming. Her results were reported recently in the online journal Biogeochemistry. Zeng and her mentor and co-author Carrie Masiello, a Rice assistant professor of Earth science, analyzed Spring Creek and Buffalo Bayou to quantify how much carbon dioxide these waterways release in a natural process that, under ideal conditions, keeps the atmosphere in balance. Spring Creek, which runs primarily through rural areas north and west of metro Houston, produced numbers in line with what Masiello had anticipated from a 2005 study she and others had done on the Amazon River. The Amazon releases roughly as much CO2 to the air and the ocean as the rainforest absorbs through plant growth every year. The same happens in Texas: Carbon moves through the forest to the soil, into waterways and back to the atmosphere (a cycle called ecosystem residence time) in as little as a few years. Buffalo Bayou, in the heart of Houston, is similar to Spring Creek in the amount of CO2 released. However, in Buffalo Bayou, radiocarbon dating of CO2 in water samples from various locations and times showed some carbon was almost 5,000 years old. "We knew from the isotope data that there was carbonate input to Buffalo Bayou, but we were thinking, 'There's no limestone in this region,'" Zeng said. (Limestone, a sedimentary rock composed of shells and other organic material compacted over millennia, would have accounted for the bizarre readings.) Then she and Masiello looked down. "It took us almost six months to figure out what was going on," Masiello said. "When you cut your grass in Houston, the blades don't stay on the surface of your soil for 5,000 years. We thought there was just no way our radiocarbon numbers were right. We walked around for a long time and finally looked at the ground. That's when we saw the shells and thought, 'Where did those come from?'" The simple answer is Galveston Bay, the main source of hundreds of millions of cubic yards of oyster shells from eons-old beds. Contractors dredged the bay, crushing shells and mixing them with concrete or using them as is for roadbeds until Texas outlawed their operations in the '70s. "The shell roads built in the early 20th century are buried under the surface, and they're slowly decomposing," Masiello said. "Urban acid rain falls on the shells and dissolves them, releasing a pool of CO2 that moves into the groundwater. On a rainy day, that CO2 gets swept out of the soil and pushed into the river. So when we date CO2 in Buffalo Bayou, it's extremely old because it's carrying the age of these fossil shells."