The demise of a diverse magnetofossil assemblage across the Eocene-Oligocene Transition in a Northwest Atlantic sediment drift.
Abstract
The Eocene-Oligocene Transition (EOT, 34 Ma) marks a step change in the Earth climate system. How marine productivity and benthic ecosystems transitioned to the presence of permanent ice on Antarctica and changes in ocean and atmospheric circulation is informed primarily by observations from equatorial and high southerly latitudes. Few of these studies have examined how magnetotactic bacteria (MTB) communities— prokaryotic biomarkers for suboxia and, potentially, marine productivity, nutrient availability, and Corg supply— responded to the EOT. To fill this gap, IODP Expedition 342 (Newfoundland Sediment Drifts) Site U1411 recovered an exceptional record of the EOT from the northwest Atlantic Ocean: the interval between the C12r/C13n and C13r/C15n chron boundaries spans >60m of nannofossil-ooze-rich pelagic clay; linear sedimentation rates exceed 5cm/kyr during the onset of the Oi-1 positive oxygen isotope excursion; carbonate microfossil preservation is near pristine; and the sediment retains a detailed record of the Earth's magnetic field behavior. At Site U1411, the EOT, which is nearly coeval with the C13n/C13r chron boundary, is magnetically characterized by a 2-order of magnitude decrease in the intensity of the natural remanent magnetization and anhysteretic remanent magnetization from the Eocene to the lower Oligocene that is not accompanied by a dramatic change in the lithostratigraphy. Distinct central ridges in high-resolution FORC diagrams and double Verwey transitions in low-temperature remanence measurements demonstrate the biogenic origin of abundant single-domain magnetite in upper Eocene sediments and their absence in lower Oligocene sediments. These observations, and the morphology of the magnetofossil assemblage, are confirmed with scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron diffraction. The diversity and preservation of the MTB assemblage is interpreted within the context of continuous 2cm resolution CaCO3, Mn, S, and Fe X-ray fluorescence records. Our results hint at dramatic changes in sediment redox conditions in the North Atlantic during the lead up to, onset of, and acme of Oi-1 that controlled the diversity and preservation of MTB during the transition from a greenhouse to icehouse.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMGP43D..02L
- Keywords:
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- 1505 Biogenic magnetic minerals;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1512 Environmental magnetism;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1518 Magnetic fabrics and anisotropy;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM