PEDOGENIC CARBONATE CONCRETIONS IN IOWA’S LOESS SOILS: A MODERN CARBON SINK?

Glaciers have left their mark on almost all of Iowa, including the Loess Hills of western Iowa. This unique feature was formed when receding melt waters left fine-grained sediment exposed in the Missouri River Valley. Westerly winds transported this sediment all throughout Iowa, leaving the larges accumulation adjacent to the Missouri River Valley, creating the Loess Hills. Within the loess soils, carbonate concretions are found in the top few meters and despite their ubiquity, little is known about the origins of these concretions.

Our main goals are to determine what these concretions are, how formed and if they are still forming, and to examine their role as a carbon sink. X-ray diffraction concluded that the concretions are composed of predominantly calcite and dolomite, with detrital class consisting of quartz, and feldspars. δ13C and δ18O measurements indicate a marine rather than a pedogenic origin for the carbonate. Paleozoic limestone formations are a possible source material that was transported and redistributed as loess, and could have also supplied the carbonate. We hypothesize that the carbonate concretions were formed after carbonate grains dissolved due to weathering and soil respiration, and then were re-precipitated as a secondary carbonate concretion surrounding roots due to photosynthesis. Radiocarbon dating of organic carbon in the concretions indicated they are around 2,500 years old. This places their timeline of formation well after the last glacial retreat. Through optical microscopy, specifically fluorescence, we are working to determine if these concretions are laminated, suggesting successive growth events. If they are, microscale isotopic measurements might indicate if the carbon source has changed through time, and if concretion formation is a sink for modern atmospheric carbon dioxide.