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Disentangling the record of diagenesis, local redox conditions, and global seawater chemistry during the latest Ordovician glaciation

Ahm, Anne Sofie C; Bjerrum, Christian J and Hammarlund, Emma U. LU (2017) In Earth and Planetary Science Letters 459. p.145-156
Abstract

The Late Ordovician stratigraphic record integrates glacio-eustatic processes, water-column redox conditions and carbon cycle dynamics. This complex stratigraphic record, however, is dominated by deposits from epeiric seas that are susceptible to local physical and chemical processes decoupled from the open ocean. This study contributes a unique deep water basinal perspective to the Late Ordovician (Hirnantian) glacial record and the perturbations in seawater chemistry that may have contributed to the Hirnantian mass extinction event. We analyze recently drilled cores and outcrop samples from the upper Vinini Formation in central Nevada and report combined trace- and major element geochemistry, Fe speciation... (More)

The Late Ordovician stratigraphic record integrates glacio-eustatic processes, water-column redox conditions and carbon cycle dynamics. This complex stratigraphic record, however, is dominated by deposits from epeiric seas that are susceptible to local physical and chemical processes decoupled from the open ocean. This study contributes a unique deep water basinal perspective to the Late Ordovician (Hirnantian) glacial record and the perturbations in seawater chemistry that may have contributed to the Hirnantian mass extinction event. We analyze recently drilled cores and outcrop samples from the upper Vinini Formation in central Nevada and report combined trace- and major element geochemistry, Fe speciation (FePy/FeHR and FeHR/FeT), and stable isotope chemostratigraphy (δ13COrg and δ34SPy). Measurements of paired samples from outcrop and core reveal that reactive Fe is preserved mainly as pyrite in core samples, while outcrop samples have been significantly altered as pyrite has been oxidized and remobilized by modern weathering processes. Fe speciation in the more pristine core samples indicates persistent deep water anoxia, at least locally through the Late Ordovician, in contrast to the prevailing interpretation of increased Hirnantian water column oxygenation in shallower environments. Deep water redox conditions were likely decoupled from shallower environments by a basinal shift in organic matter export driven by decreasing rates of organic matter degradation and decreasing shelf areas. The variable magnitude in the record of the Hirnantian carbon isotope excursion may be explained by this increased storage of isotopically light carbon in the deep ocean which, in combination with increased glacio-eustatic restriction, would strengthen lateral- and vertical gradients in seawater chemistry. We adopt multivariate statistical methods to deconstruct the spatial and temporal re-organization of seawater chemistry during the Hirnantian glaciation and attempt to isolate the latent magnitude and global perturbation in the carbon cycle. We speculate, using a two component mixing model and residual estimates from principal component analysis, that the secular open ocean Hirnantian C isotope excursion possibly amounts to only ∼ +1.5‰. Such an increase could be mechanistically driven by the combination of sea-level fall, persistent deep water anoxia, and cooler glacial temperatures that increased the organic carbon burial efficiency in the deeper basins.

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author
publishing date
type
Contribution to journal
publication status
published
keywords
deep water anoxia, glacioeustasy, Hirnantian extinction, iron speciation, multivariate statistics, sulfur isotopes
in
Earth and Planetary Science Letters
volume
459
pages
12 pages
publisher
Elsevier
external identifiers
  • scopus:85006409353
ISSN
0012-821X
DOI
10.1016/j.epsl.2016.09.049
language
English
LU publication?
no
id
897cea99-6f7d-436f-992f-fe9a48fc614a
date added to LUP
2017-05-17 11:22:57
date last changed
2017-05-17 14:52:33
@article{897cea99-6f7d-436f-992f-fe9a48fc614a,
  abstract     = {<p>The Late Ordovician stratigraphic record integrates glacio-eustatic processes, water-column redox conditions and carbon cycle dynamics. This complex stratigraphic record, however, is dominated by deposits from epeiric seas that are susceptible to local physical and chemical processes decoupled from the open ocean. This study contributes a unique deep water basinal perspective to the Late Ordovician (Hirnantian) glacial record and the perturbations in seawater chemistry that may have contributed to the Hirnantian mass extinction event. We analyze recently drilled cores and outcrop samples from the upper Vinini Formation in central Nevada and report combined trace- and major element geochemistry, Fe speciation (Fe<sub>Py</sub>/Fe<sub>HR</sub> and Fe<sub>HR</sub>/Fe<sub>T</sub>), and stable isotope chemostratigraphy (δ<sup>13</sup>C<sub>Org</sub> and δ<sup>34</sup>S<sub>Py</sub>). Measurements of paired samples from outcrop and core reveal that reactive Fe is preserved mainly as pyrite in core samples, while outcrop samples have been significantly altered as pyrite has been oxidized and remobilized by modern weathering processes. Fe speciation in the more pristine core samples indicates persistent deep water anoxia, at least locally through the Late Ordovician, in contrast to the prevailing interpretation of increased Hirnantian water column oxygenation in shallower environments. Deep water redox conditions were likely decoupled from shallower environments by a basinal shift in organic matter export driven by decreasing rates of organic matter degradation and decreasing shelf areas. The variable magnitude in the record of the Hirnantian carbon isotope excursion may be explained by this increased storage of isotopically light carbon in the deep ocean which, in combination with increased glacio-eustatic restriction, would strengthen lateral- and vertical gradients in seawater chemistry. We adopt multivariate statistical methods to deconstruct the spatial and temporal re-organization of seawater chemistry during the Hirnantian glaciation and attempt to isolate the latent magnitude and global perturbation in the carbon cycle. We speculate, using a two component mixing model and residual estimates from principal component analysis, that the secular open ocean Hirnantian C isotope excursion possibly amounts to only ∼ +1.5‰. Such an increase could be mechanistically driven by the combination of sea-level fall, persistent deep water anoxia, and cooler glacial temperatures that increased the organic carbon burial efficiency in the deeper basins.</p>},
  author       = {Ahm, Anne Sofie C and Bjerrum, Christian J and Hammarlund, Emma U.},
  issn         = {0012-821X},
  keyword      = {deep water anoxia,glacioeustasy,Hirnantian extinction,iron speciation,multivariate statistics,sulfur isotopes},
  language     = {eng},
  month        = {02},
  pages        = {145--156},
  publisher    = {Elsevier},
  series       = {Earth and Planetary Science Letters},
  title        = {Disentangling the record of diagenesis, local redox conditions, and global seawater chemistry during the latest Ordovician glaciation},
  url          = {http://dx.doi.org/10.1016/j.epsl.2016.09.049},
  volume       = {459},
  year         = {2017},
}