Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over phanerozoic time

Abstract

The biogechemical cycles of carbon and sulfur contain within them the principles processes that control the level of atmospheric oxygen over Phanerozoic time. These are the production, deposition, burial and eventual oxidative weathering of organic matter and the diagenetic formation, burial and eventual oxidative weathering of sedimentary pyrite. Observations of modern sedimentary environments indicate that: (1) most organic matter, on a worldwide basis, is deposited in marine shelf-deltaic sediments, rather than in high-productivity upwelling regions or anoxic basins; (2) burial rate and the degree of preservation of organic matter are positively correlated with total sedimentation rate; (3) the rate of decomposition of organic matter in natural waters and sediments is highly dependent on the source and degree of aging of the organic matter itself, and is largely unaffected by the concentration of the principal oxidants, O2 and SO42-; (4) pyrite sulfur concentration in normal marine environments correlates well with organic carbon concentration due to the limitation of bacterial sulfate reduction by reactive organic matter that escapes aerobic destruction during deposition and bioturbation; (5) depth-integrated sulfate reduction in normal marine sediments correlates positively with total sedimentation rate; (6) in euxinic sediments excess pyrite is formed from sulfidic bottomwaters leading to a lack of correlation between organic carbon and pyrite sulfur, a good correlation between pyrite sulfur and total iron, and high degrees of pyritization of iron; and (7) very low concentrations of pyrite are found in continental fresh water sediments, regardless of organic matter or iron content, due to limitation of pyrite formation by the low concentration of dissolved sulfate in fresh water. Through the use of carbon and sulfur isotope mass balance models, it is possible to calculate original rates of burial of organic carbon (C) and pyrite sulfur (S) over Phanerozoic time. Unusually high calculated rates of organic carbon burial during Permo-Carboniferous time are attributed to: (1) the rise of vascular plants on the continents; (2) the availability of vast flooded lowlands (swamps) on the emergent continent of Pangea where deposited organic matter could be preserved; and (3) the high C/P ratio of vascular plant material which enabled greater organic carbon burial, per unit of phosphorus burial, than was the case previously for marine organic matter. High C/S ratios for Permo-Carboniferous time are attributed to a large proportion of total worldwide organic matter burial occurring in fresh water swamps and lakes. Low C/S ratios for the early Paleozoic are attributed to a much greater abundance of euxinic basins than at present and to the lack of deposition of vascular plant-derived organic matter at that time. There is little doubt that atmospheric O2 has varied over Phanerozoic time. However, straightforward calculation of atmospheric O2 levels, from rates of organic matter burial etc. derived from the isotope mass balance models, leads to geologically unreasonable fluctuations, even when linear O2 negative feedback is introduced to the modelling. There is a need for additional quantifiable and geologically reasonable feedback mechanisms that constrain the modeling of atmospheric O2 variation. Three possible candidates discussed here are: (1) splitting of the reservoirs of materials undergoing weathering into old and young sub-reservoirs, which enables rapid isotope recycling via more rapid weathering of younger rocks; (2) changing rates of worldwide erosion and total sediments burial which simultaneously enables variations in O2 production, due to changes in burial rates of organic matter and pyrite, and in O2 consumption, due to changes in rates of exposure of organic matter and pyrite to oxidative weathering; and (3) changing the stoichiometry of O2 uptake during weathering by making it dependent on atmospheric O2 level.