Direct evidence for organic carbon preservation as clay-organic nanocomposites in a Devonian black shale; from deposition to diagenesis

Martin John Kennedy, Stefan Carlos Löhr, Samuel Alex Fraser, Elizabeth Teresa Baruch

Research output: Contribution to journalArticleResearchpeer-review

Abstract

The burial of marine sourced organic carbon (OC) in continental margin sediments is most commonly linked to oceanographic regulation of bottom-water oxygenation (anoxia) and/or biological productivity. Here we show an additional influence in the Devonian Woodford Shale, in which OC occurs as nanometer intercalations with specific phyllosilicate minerals (mixed-layer illite/smectite) that we term organo-mineral nanocomposites. High resolution transmission electron microscopic (HRTEM) images provide direct evidence of this nano-scale relationship. While discrete micron-scale organic particles, such as Tasmanites algal cysts, are present in some lamina, a strong relation between total organic carbon (TOC) and mineral surface area (MSA) over a range of 15% TOC indicate that the dominant association of organic carbon is with mineral surfaces and not as discrete pelagic grains, consistent with HRTEM images of nanocomposites. Where periods of oxygenation are indicated by bioturbation, this relationship is modified by a shift to lower OC loading on mineral surfaces and reduced MSA variability likely resulting from biological mixing and homogenization of the sediment, oxidative burn down of OC and/or stripping of OC from minerals in animal guts. The TOC-MSA relationship extends across a range of burial depths and thermal maturities into the oil window and persists through partial illitization. Where illitization occurs, the loss of mineral surface area associated with the collapse of smectite interlayer space results in a systematic increase in TOC:MSA and reorganization of organic carbon and clays into nano-scale aggregates. While the Woodford Shale is representative of black shale deposits commonly thought to record heightened marine productivity and/or anoxia, our results point to the importance of high surface area clay minerals for OC enrichment. Given that the vast majority of these clay minerals are formed in soils before being transported to continental margin settings, their mineralogy and attendant preservative potential is primarily a function of continental climate and provenance making these deposits a sensitive recorder of land as well as oceanographic change.

LanguageEnglish
Pages59-70
Number of pages12
JournalEarth and Planetary Science Letters
Volume388
DOIs
Publication statusPublished - 15 Feb 2014
Externally publishedYes

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black shale
Shale
Organic carbon
clays
diagenesis
Nanocomposites
nanocomposites
organic carbon
minerals
Minerals
clay
carbon
mineral
surface area
total organic carbon
illitization
anoxia
oxygenation
Oxygenation
smectite

Bibliographical note

Copyright the Author(s) 2013. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

Cite this

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title = "Direct evidence for organic carbon preservation as clay-organic nanocomposites in a Devonian black shale; from deposition to diagenesis",
abstract = "The burial of marine sourced organic carbon (OC) in continental margin sediments is most commonly linked to oceanographic regulation of bottom-water oxygenation (anoxia) and/or biological productivity. Here we show an additional influence in the Devonian Woodford Shale, in which OC occurs as nanometer intercalations with specific phyllosilicate minerals (mixed-layer illite/smectite) that we term organo-mineral nanocomposites. High resolution transmission electron microscopic (HRTEM) images provide direct evidence of this nano-scale relationship. While discrete micron-scale organic particles, such as Tasmanites algal cysts, are present in some lamina, a strong relation between total organic carbon (TOC) and mineral surface area (MSA) over a range of 15{\%} TOC indicate that the dominant association of organic carbon is with mineral surfaces and not as discrete pelagic grains, consistent with HRTEM images of nanocomposites. Where periods of oxygenation are indicated by bioturbation, this relationship is modified by a shift to lower OC loading on mineral surfaces and reduced MSA variability likely resulting from biological mixing and homogenization of the sediment, oxidative burn down of OC and/or stripping of OC from minerals in animal guts. The TOC-MSA relationship extends across a range of burial depths and thermal maturities into the oil window and persists through partial illitization. Where illitization occurs, the loss of mineral surface area associated with the collapse of smectite interlayer space results in a systematic increase in TOC:MSA and reorganization of organic carbon and clays into nano-scale aggregates. While the Woodford Shale is representative of black shale deposits commonly thought to record heightened marine productivity and/or anoxia, our results point to the importance of high surface area clay minerals for OC enrichment. Given that the vast majority of these clay minerals are formed in soils before being transported to continental margin settings, their mineralogy and attendant preservative potential is primarily a function of continental climate and provenance making these deposits a sensitive recorder of land as well as oceanographic change.",
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Direct evidence for organic carbon preservation as clay-organic nanocomposites in a Devonian black shale; from deposition to diagenesis. / Kennedy, Martin John; Löhr, Stefan Carlos; Fraser, Samuel Alex; Baruch, Elizabeth Teresa.

In: Earth and Planetary Science Letters, Vol. 388, 15.02.2014, p. 59-70.

Research output: Contribution to journalArticleResearchpeer-review

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