『Abstract
The results of a theoretical isotope mass balance model are presented
for the time dependence of burial and weathering-plus-degassing
fluxes within the combined long-term carbon and sulfur cycles.
Averaged data for oceanic δ13C and δ34S
were entered for every million years from 270 to 240 Ma (middle
Permian to middle Triassic) to study general trends across the
Permian-Triassic boundary. Results show a drop in the rate of
global organic matter burial during the late Permian and a predominance
of low values during the early-to-middle Triassic. This overall
decrease with time is ascribed mainly to epochs of conversion
of high biomass forests to low biomass herbaceous vegetation resulting
in a decrease in the production of terrestrially derived organic
debris. Additional contributions to lessened terrestrial carbon
burial were increased aridity and a drop in sea level during the
late Permian which led to smaller areas of low-lying coastal wetlands
suitable for coal and peat deposition.
Mirroring the drop in organic matter deposition was an increase
in the burial of sedimentary pyrite, and a dramatic increase in
the calculated global mean ratio of pyrite-S to organic-C. High
S/C values resulted from an increase of deposition in marine euxinic
basins combined with a decrease in the burial of low-pyrite associated
terrestrial organic matter. The prediction of increased oceanic
anoxia during the late Permian and early Triassic agrees with
independent studies of the composition of sedimentary rocks.
Weathering plus burial fluxes for organic carbon and pyrite sulfur
were used to calculate changes in atmospheric oxygen. The striking
result is a continuous drop in O2 concentration
from 〜30% to 〜13% over a twenty million year period. This drop
was brought about mainly by a decrease in the burial of terrestrially
derived organic matter, but with a possible contribution from
the weathering of older organic matter on land. It must have exerted
a considerable influence on animal evolution because of the role
of O2 in respiration. Some examples are the
extinction of many vertebrates, loss of giant insects and amphibians,
and the restriction of animals to low elevations. It is concluded
that the extinction of plants may have contributed to the extinction
of animals.』
1. Introduction
2. Method of calculation
3. Results and discussion
3.1. Organic matter burial
3.2. Oceanic anoxia
3.3. Atmospheric oxygen
4. Conclusions
Acknowledgments
References