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Prokaryotes have been cultured from a modern weathering profile developed on a approximately 365-million-year-old black shale that use macromolecular shale organic matter as their sole organic carbon source. Using natural-abundance carbon-14 analysis of membrane lipids, we show that 74 to 94% of lipid carbon in these cultures derives from assimilation of carbon-14-free organic carbon from the shale. These results reveal that microorganisms enriched from shale weathering profiles are able to use a macromolecular and putatively refractory pool of ancient organic matter. This activity may facilitate the oxidation of sedimentary organic matter to inorganic carbon when sedimentary rocks are exposed by erosion. Thus, microorganisms may play a more active role in the geochemical carbon cycle than previously recognized, with profound implications for controls on the abundance of oxygen and carbon dioxide in Earth's atmosphere over geologic time.

Models describing the evolution of the partial pressure of atmospheric oxygen over Phanerozoic time are constrained by the mass balances required between the inputs and outputs of carbon and sulfur to the oceans. This constraint has limited the applicability of proposed negative feedback mechanisms for maintaining levels of atmospheric O(2) at biologically permissable levels. Here we describe a modeling approach that incorporates O(2)-dependent carbon and sulfur isotope fractionation using data obtained from laboratory experiments on carbon-13 discrimination by vascular land plants and marine plankton. The model allows us to calculate a Phanerozoic O(2) history that agrees with independent models and with biological and physical constraints and supports the hypothesis of a high atmospheric O(2) content during the Carboniferous (300 million years ago), a time when insect gigantism was widespread.

Recent studies indicate that highly aged material is a major component of organic matter transported by most rivers. However, few studies have used natural 14C to trace the potential entry of this aged material into modern river food webs. Here we use natural abundance 14C, 13C, and deuterium (2H) to trace the contribution of aged and contemporary organic matter to an important group of consumers, crustacean zooplankton, in a large temperate river (the Hudson River, New York, USA). Zooplankton were highly 14C depleted (mean delta14C = -240 per thousand) compared to modern primary production in the river or its watershed (delta14C = -60 per thousand to +50 per thousand). In order to account for the observed 14C depletion, zooplankton must be subsidized by highly aged particulate organic carbon. IsoSource modeling suggests that the range of the aged dietary subsidy is between approximately 57%, if the aged organic matter source was produced 3400 years ago, and approximately 21%, if the organic carbon used is > or = 50 000 years in age, including fossil material that is millions of years in age. The magnitude of this aged carbon subsidy to river zooplankton suggests that modern river food webs may in some cases be buffered from the limitations set by present-day primary production.

To investigate the weathering of sedimentary organic matter and its role in regulating atmospheric oxygen, a theoretical modeling study is presented that addresses the fundamental controls on atmospheric oxygen uptake: erosion rate, organic matter content, and reaction rate. We compare model results with the previous part of this study that analyzed a drill core of black shale from the New Albany formation (Upper Devonian, Clay City, KY) for total and organic carbon, pyrite sulfur, porosity, permeability and specific surface area. As was observed in the field study, the model predicts that the loss of organic matter by oxidative weathering takes place across a reaction "front" where organic carbon content decreases sharply toward the land surface along with pyrite loss.The model is based on kinetic control of reaction of organic matter and pyrite with O 2 dissolved in soil water. The downward diffusion of gaseous O 2 partitions with dissolved O 2 in water films on sediment grains via Henry's law. Once a weathering profile is developed, the downward migrating O 2 reacts with shale organic matter and pyrite. Pyrite reacts faster with O 2 than does organic matter (for a given local concentration of oxygen) making the pyrite front generally deeper than the organic matter front. We explore the influence of differing erosion rates, atmospheric O 2 concentrations, organic matter contents, porosities, tortuosities, and rates of reaction (that could include possible acceleration due to microbes) on the oxygen consumption.We conclude, based on our modeling, that the erosion rate and the concentration of buried reduced matter, as opposed to the level of atmospheric O 2 , normally limits the rate of drawdown of atmospheric oxygen. For the vast majority of erosion rates and Phanerozoic oxygen levels, essentially all ancient reduced material is oxidized before reaching the surface. Only in regions of unusually rapid erosion or during very low atmospheric oxygen levels can rates of diffusion of O 2 in soils and rates of reaction control O 2 drawdown, leading to weathering that is O 2 -dependent. In this case erosion and rapid reburial of unoxidized organic matter would occur.

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