An Experimental Perspective on Co-Deposition of Marine Organic Matter With Clays and Silt

Abstract

Muds are common on modern shelves and in deep sea environments. Their consolidated counterparts (shales & mudstones) dominate the sedimentary rock record. Experimental work with clay and silt was extended by adding an approximation of marine snow (degraded finely ground kelp) to moving clay suspensions. Suspensions that consisted largely of organic matter (OM), up to mm-sized aggregates (floccules) formed that travelled mostly in suspension at velocities that would transport and deposit flocculated clays in bedload (10-25 cm/sec). OM floccules only accumulate in bedload at low flow velocities (~5 cm/sec). Incremental addition of clays to this organic suspension resulted in a gradual increase of the critical velocity of sedimentation, due to increasing mineral “ballast” in organic-mineral hybrid floccules. The OM component is largely dispersed plant debris and microbial EPS (extracellular polysaccharide) mucus and highly hydrated. 1 liter of organic sludge contain a mere 30 g OM (dry weight). Due to this low density of the OM component, a 1:1 ratio (dry weight basis) of OM/clay means that 1 kg of clay (0.36 l) needs to be combined with 31 l of organic sludge, a sludge to clay volume ratio of 86. In a homogeneous mixture the clay therefore comprises a little over 1% of the total volume, too little to effectively affect settling characteristics and critical velocity of sedimentation of floccules. Once the OM loading of the mixture has been reduced to what would amount to 10% TOC in a consolidated (water-free) deposit, the OM to clay weight ratio is approximately 1:4 (instead of 86:1). At that point the “ballast” (clays) dominates floccule behavior and critical velocities of sedimentation are similar to that of flocculated clays (~25 cm/sec) and we observe ripple formation and generation of laminated muds. Other sediment aspects affected by OM addition are change of erosion behavior with temperature and oxygen availability. At low temperatures erosion resistance increases because of a “stiffening” effect by the EPS matrix (the “jello” hardens), and reducing oxygen supply appears to slow down microbial degradation of the EPS matrix. Our experimental sediments suggest that kerogen streaks in black shales may reflect situations when the surface sediment is overwhelmed by marine snow/detritus, whereas organo-mineralic aggregates form as marine snow is mixed with an abundance of mineral detritus during bedload transport of OM and mud components.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018