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| Tim Gallagher stands amongst his completed pits at Stengl. |
In a clearing on the north side of Stengl Lost Pines Biological Field Station lie twelve wood frames. At first glance, these seem to be small garden plots where plugs of the Texas State Grass, sideoats gramma, are prospering. But beneath these frames are pits that form an experiment set up by Tim Gallagher, a post-doc working with Associate Professor Dan Breecker in the Jackson School of Geosciences.
These twelve pits allow Tim to study the processes that control soil-carbonate precipitation and dissolution. Soil carbonates are preserved in the geologic record, in paleosols. Using the geochemistry of carbonate nodules, one can make inferences about past climate, past elevation, past atmospheric carbon dioxide levels, as these are great archives of paleo-environmental information. In modern times, however, running experiments like Tim’s allow us to understand the seasonal processes that control soil carbonate formation.
“In certain areas, there is a really strong seasonal bias,” says Tim, “so carbonate formation would form during these hot dry periods. That had been the current working idea. But as more study has been done, it seems in certain areas it is affected by the seasonal bias, and in other places it reflects the mean growing season conditions.” Tim’s experiment will help understand the seasonal processes when the carbonate is forming, which will have implications on how we interpret geologic samples.
Initially, Tim had planned to create a sand box experiment on the roof of the Jackson Building, but realized that the weight of soil would be a problem. The Lady Bird Johnson Wildflower Center was also an option, until Tim discovered that soil was too shallow in that area. Stengl Lost Pines thus became the right location, as the station has deep sandy soils without any carbonate.
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| A surprise toad. |
Each framed pit was set up in a meticulous manner. The pits were dug to be a meter deep. Tim homogenized the soil by tilling the clay and the sandy soil together, then added some calcium carbonate in six of the holes, and none in the other six to serve as a control group. Tim lined the sides of the pits with foam insulation board as a physical barrier to prevent things like lateral water flow and outside root growth. Soil gas composition is monitored by sampling from wells installed at 15 cm, 30 cm, and 60 cm. Soil moisture and temperature is monitored at 5 cm, 30 cm, and 60 cm.
The soil carbon dioxide fluctuates with carbonate dissolution and formation. When carbonate dissolves, it consumes some carbon dioxide and when it forms it releases carbon dioxide. Monitoring carbon dioxide is easy, but it also changes with root respiration and microbes since they respire and produce carbon dioxide as well as consume oxygen. Within these pits, Tim can passively see when the carbonate is distilling, without destruction, and measure other factors. “After a rain storm, how quickly and often does the soil switch back and forth between precipitation and dissolution?” Tim states. “Does it happen multiple times a year? Is it just this once seasonal thing or is it more chaotic?” Tim admits that at this point it’s hard to tell. “There are a lot of unknowns that no one has tried to get at.”
These plots have been a challenge to set up and maintain. After his test pit proved useful in March of this year, Tim dug the other pits in late April, at a time when heat and humidity have already settled over Central Texas. To beat the oppressive afternoons, Tim would wake at 5 a.m. and leave the house at 5:30 a.m. for his hour-long drive. The animals and insects also made their presences known: a curious family of hogs, toads unwilling to leave the pits, and a mud wasp determined to make a nest in Tim’s car. (The hogs passed through, thankfully, and the mud wasp was unknowingly transported to Bastrop, hidden inside the side mirror of Tim’s trusty Saturn.)
Tim believes this will be a long-term experiment, with time allowing him to see how both drought and years of higher precipitation effect things. There are many fundamental questions that this experiment can answer. “People haven’t done work like this,” Tim states. “It takes a lot of effort to dig and instrument the holes, and create new ways to look at it. This is an experimental approach, when you know the starting conditions.”
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| Excavation of the pits. | Setting walls in the pits. |
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| Instrumentation of a pit. | Mixing carbonate with soil. |
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| A pit with a carbonate horizon. | Framed pits with grass. |