Comparative organic geochemistries of soils and marine sediments

Abstract

Striking similarities and sharp contrasts exist between the geochemistries of organic matter in surface soils and marine sediments. The contrasts result in part from physical differences in the two environments and their indigenous biota. Vascular plants predominate on land, where soils are deeply leached by percolating water and receive organic matter from falling debris and penetrating roots. The large size of vascular plants, and their high concentrations of carbon-rich biomacromolecules such as cellulose, lignin and tannin, necessitate recycling by aggressive consortia of microorganisms, including fungi armed with O 2-requiring oxidative enzymes. In the ocean, nitrogen-rich microorganisms produce and recycle most organic matter in the water column, from which degraded particles rain onto the underlying sea floor. Water saturation restricts O 2 penetration into sediments accumulating along most continental margins to less than several centimeters, below which biomacromolecules must be broken down hydrolytically with nitrate and sulfate as the primary electron acceptors. In both soils and sediments, plant products are degraded extensively by microorganisms, leaving small organic remnants which are soluble in base and depleted in conventionally measurable biochemicals. Much of the surviving organic matter is intimately associated with mineral surfaces and enclosed within particle aggregates, and thus may be physically protected from microbial attack. Degradation under oxic conditions is severe both on land and within surface ocean deposits. As a result, even physically protected organic matter can slowly be mineralized, along with intrinsically resistant substrates such as lignin, pollen, kerogen and coal. The only long-term shelter from mineralization is within anoxic marine sediments which accumulate one mole of organic carbon for every 500–1000 fixed by photosynthetic organisms. The buried organic matter joins the geological cycle, surfacing again millions of years later as kerogen uplifted in continental rocks. Chemists investigating organic matter in soils or sediments employ distinct strategies and experimental methods for disparate purposes. Soil studies focus primarily on bulk properties linked to complex system functions such as fertility and erosion. Investigations of sedimentary organic matter are more molecularly-based and directed toward interpretations of water column processes and paleorecords. With the pressing need for more efficient large-scale research, the time is ripe for increased interchange between chemists studying subaerial and subaqueous systems. Combination of methods (such as preparative particle sorting and solid-state NMR) for determining the forms and physical distributions of organic matter in soils, with highly sensitive tracer techniques being developed in the aquatic field is a particularly promising crossover area. The aim of this review is to facilitate such interactions between soil and sedimentary organic geochemists by a comparative evaluation of conditions, concepts and opportunities in both fields.