Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation
Bulk-carbonate carbon isotope ratios are a widely applied proxy for investigating the ancient biogeochemical carbon cycle. Temporal carbon isotope trends serve as a prime stratigraphic tool, with the inherent assumption that bulk micritic carbonate rock is a faithful geochemical recorder of the...
Main Authors: | , , , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-11-01
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Series: | Climate of the Past |
Online Access: | https://www.clim-past.net/13/1635/2017/cp-13-1635-2017.pdf |
Summary: | Bulk-carbonate carbon isotope ratios are a widely applied proxy for
investigating the ancient biogeochemical carbon cycle. Temporal carbon
isotope trends serve as a prime stratigraphic tool, with the inherent
assumption that bulk micritic carbonate rock is a faithful geochemical
recorder of the isotopic composition of seawater dissolved inorganic carbon.
However, bulk-carbonate rock is also prone to incorporate diagenetic signals.
The aim of the present study is to disentangle primary trends from diagenetic
signals in carbon isotope records which traverse the Permian–Triassic
boundary in the marine carbonate-bearing sequences of Iran and South China. By
pooling newly produced and published carbon isotope data, we confirm that a
global first-order trend towards depleted values exists. However, a large
amount of scatter is superimposed on this geochemical record. In addition, we
observe a temporal trend in the amplitude of this residual
<i>δ</i><sup>13</sup>C variability, which is reproducible for the two studied
regions. We suggest that (sub-)sea-floor microbial communities and their
control on calcite nucleation and ambient porewater dissolved
inorganic carbon <i>δ</i><sup>13</sup>C pose a viable mechanism to induce
bulk-rock <i>δ</i><sup>13</sup>C variability. Numerical model calculations
highlight that early diagenetic carbonate rock stabilization and linked
carbon isotope alteration can be controlled by organic matter supply and
subsequent microbial remineralization. A major biotic decline among Late
Permian bottom-dwelling organisms facilitated a spatial increase in
heterogeneous organic carbon accumulation. Combined with low marine sulfate,
this resulted in varying degrees of carbon isotope overprinting. A simulated
time series suggests that a 50 % increase in the spatial scatter of organic
carbon relative to the average, in addition to an imposed increase in the
likelihood of sampling cements formed by microbial calcite nucleation to 1
out of 10 samples, is sufficient to induce the observed signal of carbon
isotope variability. These findings put constraints on the application of
Permian–Triassic carbon isotope chemostratigraphy based on whole-rock
samples, which appears less refined than classical biozonation dating
schemes. On the other hand, this signal of increased carbon isotope
variability concurrent with the largest mass extinction of the Phanerozoic
may provide information about local carbon cycling mediated by spatially heterogeneous
(sub-)sea-floor microbial communities under suppressed bioturbation. |
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ISSN: | 1814-9324 1814-9332 |