| Organic matter decomposition is a ubiquitous process at Earth's surface and is the driving force for many mineral dissolution-precipitation reactions. These carbon transformations link the atmospheric and hydrospheric carbon reservoirs, and regulate global climate and elemental cycles on short and long timescales. The impact of the rapid rise in atmospheric CO2 from fossil fuel burning is one of the most important areas of environmental research. It is anticipated that an amount up to 10 times that currently stored in the atmosphere will be released over the next several centuries. Near surface terrestrial and coastal environments are the most likely sites for short-term carbon storage. This dissertation focuses on the biogeochemical pathways and consequences of organic carbon - mineral interactions in a terrestrial ecosystem and in the shallow marine environment.; In shallow marine sediments, decomposition of organic matter largely determines the amount of calcium carbonate that is removed from the oceans and is preserved in the fossil record. To examine carbonate preservation in modern marine sediments, pore water and sediment geochemistries were analyzed from the San Blas Archipelago, Panama. Here, iron reduction of Fe-Mg aluminosilicate minerals is an important diagenetic pathway that limits the extent of carbonate dissolution by inhibiting acid generation. In addition, the formation of Fe-Mg aluminosilicate minerals may regulate the cycle of Fe, Mg, Si, and Al in the oceans, albeit on much longer time scales than the terrestrial carbon fluxes.; To better understand terrestrial carbon cycling, the carbon transformations in a temperate forested watershed in Northern Michigan were investigated in detail. The flux of dissolved organic and inorganic carbon from soil horizons into groundwater aquifers was tracked by measuring the concentration and isotopic composition of carbon in soils, gases and waters. Carbon storage in groundwaters, and the flux of carbon from the watershed is maximized where carbonate-bearing soils are in contact with CO2-rich soil gases. Importantly, most dissolved carbon in groundwater that is exported out of the watershed by rivers is not affected by carbonate precipitation. Thus, these freshwater systems provide a rapid carbon “shuttle” between terrestrial and shallow marine reservoirs, operating on relatively short human timescales. Taken together, these results establish that reactions involving geological materials may not only be important for the long-term carbon cycle, but also for shorter human timescales, with implications for past and future climate changes. |
