Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels

Stefan C. Löhr, Martin J. Kennedy, Simon C. George, Robyn J. Williamson, Huiyuan Xu

Research output: Contribution to journalArticleResearchpeer-review

Abstract

The sapropel record of the eastern Mediterranean provides unique insight into the primary climatic, oceanographic, and biological drivers of organic carbon enrichment in marine sediments. The dominant source of organic matter, timing of oxygen depletion at the sea floor, and extent of metazoan reworking of these deposits remain unclear. These questions are addressed by combining microbeam imaging with bulk and molecular geochemical characterization of several Pliocene sapropels, revealing four microfacies which record distinct palaeoceanographic conditions, phytoplankton assemblages, and degrees of postdepositional reworking. The most organic‐rich, carbonate‐lean sapropel intervals consist of alternating 10–60‐μm‐thick organic and detrital mineral laminae. Petrographical features consistent with a pelagic origin, δ15N <−1.8 and the presence of 2α‐methylhopanes indicate that organic laminae formed by mass sedimentation of N‐fixing colonial cyanobacteria (Trichodesmium). The association of framboidal pyrite with organic laminae suggests that mass sedimentation led to the abrupt development of sea floor anoxia. Unlike similar Quaternary sapropels, no petrographic or biomarker evidence is found for a mat‐forming diatom contribution to these organic‐rich, laminated intervals. However, episodic mass sedimentation was not found to be a universal ingredient of sapropel formation. Carbonate‐lean, weakly laminated intervals contain >50 μm organomineral aggregates, interpreted as marine snow, whereas carbonate microfossil‐rich intervals record periods of nitrogen fixation and moderately increased primary production by a diverse assemblage of calcareous, organic‐walled, and siliceous plankton. The results presented here further show that burrowing by microscopic meiofauna impacted most sapropels, extending into seemingly laminated intervals below obvious disruption from burrowing macrofauna, indicating that metazoans influence organic carbon burial in oxygen‐depleted settings even where physical displacement of sediment is not visible.
LanguageEnglish
Pages293-317
Number of pages25
JournalThe Depositional Record
Volume4
Issue number2
DOIs
Publication statusPublished - Dec 2018

Fingerprint

sapropel
metazoan
reworking
cyanobacterium
Pliocene
sedimentation
burrowing
sediment
organic carbon
marine snow
carbonate
oxygen
meiofauna
microfacies
nitrogen fixation
microfossil
marine sediment
primary production
plankton
seafloor

Bibliographical note

Copyright the Author(s) 2018. 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.

Keywords

  • cyanobacteria
  • mass sedimentation
  • organic matter source
  • Pliocene
  • sapropel
  • sea floor anoxia

Cite this

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title = "Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels",
abstract = "The sapropel record of the eastern Mediterranean provides unique insight into the primary climatic, oceanographic, and biological drivers of organic carbon enrichment in marine sediments. The dominant source of organic matter, timing of oxygen depletion at the sea floor, and extent of metazoan reworking of these deposits remain unclear. These questions are addressed by combining microbeam imaging with bulk and molecular geochemical characterization of several Pliocene sapropels, revealing four microfacies which record distinct palaeoceanographic conditions, phytoplankton assemblages, and degrees of postdepositional reworking. The most organic‐rich, carbonate‐lean sapropel intervals consist of alternating 10–60‐μm‐thick organic and detrital mineral laminae. Petrographical features consistent with a pelagic origin, δ15N <−1.8 and the presence of 2α‐methylhopanes indicate that organic laminae formed by mass sedimentation of N‐fixing colonial cyanobacteria (Trichodesmium). The association of framboidal pyrite with organic laminae suggests that mass sedimentation led to the abrupt development of sea floor anoxia. Unlike similar Quaternary sapropels, no petrographic or biomarker evidence is found for a mat‐forming diatom contribution to these organic‐rich, laminated intervals. However, episodic mass sedimentation was not found to be a universal ingredient of sapropel formation. Carbonate‐lean, weakly laminated intervals contain >50 μm organomineral aggregates, interpreted as marine snow, whereas carbonate microfossil‐rich intervals record periods of nitrogen fixation and moderately increased primary production by a diverse assemblage of calcareous, organic‐walled, and siliceous plankton. The results presented here further show that burrowing by microscopic meiofauna impacted most sapropels, extending into seemingly laminated intervals below obvious disruption from burrowing macrofauna, indicating that metazoans influence organic carbon burial in oxygen‐depleted settings even where physical displacement of sediment is not visible.",
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author = "L{\"o}hr, {Stefan C.} and Kennedy, {Martin J.} and George, {Simon C.} and Williamson, {Robyn J.} and Huiyuan Xu",
note = "Copyright the Author(s) 2018. 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.",
year = "2018",
month = "12",
doi = "10.1002/dep2.49",
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Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels. / Löhr, Stefan C.; Kennedy, Martin J.; George, Simon C.; Williamson, Robyn J.; Xu, Huiyuan.

In: The Depositional Record, Vol. 4, No. 2, 12.2018, p. 293-317.

Research output: Contribution to journalArticleResearchpeer-review

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T1 - Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels

AU - Löhr, Stefan C.

AU - Kennedy, Martin J.

AU - George, Simon C.

AU - Williamson, Robyn J.

AU - Xu, Huiyuan

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N2 - The sapropel record of the eastern Mediterranean provides unique insight into the primary climatic, oceanographic, and biological drivers of organic carbon enrichment in marine sediments. The dominant source of organic matter, timing of oxygen depletion at the sea floor, and extent of metazoan reworking of these deposits remain unclear. These questions are addressed by combining microbeam imaging with bulk and molecular geochemical characterization of several Pliocene sapropels, revealing four microfacies which record distinct palaeoceanographic conditions, phytoplankton assemblages, and degrees of postdepositional reworking. The most organic‐rich, carbonate‐lean sapropel intervals consist of alternating 10–60‐μm‐thick organic and detrital mineral laminae. Petrographical features consistent with a pelagic origin, δ15N <−1.8 and the presence of 2α‐methylhopanes indicate that organic laminae formed by mass sedimentation of N‐fixing colonial cyanobacteria (Trichodesmium). The association of framboidal pyrite with organic laminae suggests that mass sedimentation led to the abrupt development of sea floor anoxia. Unlike similar Quaternary sapropels, no petrographic or biomarker evidence is found for a mat‐forming diatom contribution to these organic‐rich, laminated intervals. However, episodic mass sedimentation was not found to be a universal ingredient of sapropel formation. Carbonate‐lean, weakly laminated intervals contain >50 μm organomineral aggregates, interpreted as marine snow, whereas carbonate microfossil‐rich intervals record periods of nitrogen fixation and moderately increased primary production by a diverse assemblage of calcareous, organic‐walled, and siliceous plankton. The results presented here further show that burrowing by microscopic meiofauna impacted most sapropels, extending into seemingly laminated intervals below obvious disruption from burrowing macrofauna, indicating that metazoans influence organic carbon burial in oxygen‐depleted settings even where physical displacement of sediment is not visible.

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