Modes of Organic Matter Burial in Mudstones - Observations from Flume Experiments
Abstract
The burial of organic matter (OM) in marine sediments is one of the key geological pathways for global carbon sequestration and intimately linked to levels of atmospheric oxygen and global climate. Carbonaceous mudstones are the sedimentary rock type that records this process in the rock record, and it was long thought that these rocks bear witness to situations where marine snow (OM from surface waters) and mineral matter (mostly clays) settled through oxygen deficient stagnant bottom waters to the seafloor. An increasing number of black shale successions, however, shows evidence of deposition from bottom currents that transported flocculated muds in bedload. Such a mode of deposition seems initially incompatible with enhanced organic matter preservation, particularly in the case of many ancient black shales that accumulated in shallow shelf seas. Upon further consideration, however, accumulation of bedload transported flocculated clay-organic aggregates (dynamic mode) may actually enhance organic matter preservation when compared to gravity settled (passive mode) accumulations of marine snow and mineral grains. Based on the TEM study of experimental sediments where simulated marine-snow and clays were deposited from moving suspensions as well as via gravity settling, we find that flow-deposited OM-clay mixtures show indications of roll-aggregation by floccules and a less porous fabric, whereas still-water settled muds show a more "open" pore structure. In the latter case the microfabric is characterized by dispersed clumps of OM and clays, whereas the microfabrics of flow deposited OM-clay mixtures show small scale OM-bridges between clay particles and OM "coatings" on clays. We propose that the differences in OM-clay association and pore structure exert a significant difference on the way in which the OM in these sediments will be bacterially processed in surface sediments and on how much of the initial OM is likely to enter the rock record. The temperature of depositing flows may also affect how sediment is deposited and what microfabrics we might expect to see in the rock record. Experiments conducted with water cooled to deep ocean temperatures showed a significant change in depositional behavior due to the approximately 50% higher viscosity of cold (5 degrees Celsius) vs warm (23 degrees Celsius) water. The parallel increase in shear stress lowers the critical velocity of sedimentation for the sediments (from 20 cm/sec to 13.5 cm/sec). Further study may reveal significant differences in depositional fabrics between mudstones that were deposited in a warm water environment (tropical shelf) vs. a cold water environment (arctic shelf, deep sea).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS41D1763S
- Keywords:
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- 3000 MARINE GEOLOGY AND GEOPHYSICS;
- 4200 OCEANOGRAPHY: GENERAL;
- 4219 OCEANOGRAPHY: GENERAL / Continental shelf and slope processes;
- 4524 OCEANOGRAPHY: PHYSICAL / Fine structure and microstructure