Linking Biogeochemical Cycles of Nitrogen and Oxygen in Euxinic Devonian Basins
Abstract
Berner’s 2009 reconstruction of Phanerozoic atmospheric pO2 (1) features an approximately 30% decline in O2 spanning the Lower to Upper Devonian, a period characterized by the emergence and expansion of terrestrial forest ecosystems and episodic high organic matter deposition now found as epicontinental basinal black shales. There is an apparent contradiction between high rates of organic matter burial coupled with an increasing areal extent of photosynthetic O2 production and declining levels of atmospheric O2. This study sought to resolve that contradiction by examining the role of terrestrial nitrgen in the development of anoxygenic photosynthetic ecosystems in euxinic Devonian basins utilizing stable isotope and biomarker analyses. By fixing C into biomass without producing O2 as a byproduct, sulfide-driven anoxygenic photosynthesis can serve to moderate the flux of O2 to the atmosphere generated by oxygenic photosynthesis (2). Sulfur isotope values indicate temporally and spatially widespread euxinia in Middle and Upper Devonian epeiric seas and the presence of isorenieratane, a biomarker for green sulfur bacteria, indicating that anoxygenic photosynthesis was also widespread. High levels of marine primary productivity and the consequent development a sulfidic water column in Middle and Upper Devonian epeiric basins were likely the result of the emergence of a substantial new source of fixed N. Modern lowland tropical forests are net exporters of N to aquatic systems and it is likely that warm, moist Devonian lowland forests featured a similar N biogeochemistry. Riverine and atmospheric deposition of terrestrial fixed N facilitated an increase in the relative contribution of anoxygenic photosynthesis to total global C fixation, resulting in the 30% pO2 decline over a period of about 30 million years. The Upper Devonian expansion of forests into upland and temperate environments, where the N excess seen in modern forests is uncommon, reversed the O2 decline by increasing the ratio of terrestrial O2 produced to terrestrial fixed N exported and decreasing the relative contribution of anoxygenic photosynthesis. (1) Berner, R.A., 2009, Phanerozoic Atmospheric Oxygen: New Results Using The Geocarbsulf Model, American Journal of Science, 309(7), pp. 603-6. (2) Johnston, D.T., Wolfe-Simon, F., Pearson, A. & Knoll, A.H., 2009, Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age, Proceedings of the National Academy of Sciences of the United States of America, 106(40), pp. 16925-9.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMPP31A1619T
- Keywords:
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- 0404 BIOGEOSCIENCES / Anoxic and hypoxic environments;
- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 1055 GEOCHEMISTRY / Organic and biogenic geochemistry;
- 4900 PALEOCEANOGRAPHY