Effects of clay and organic matter on calcareous nannofossil ooze erodibility

Abstract

Bottom current activity has been responsible for the formation of a multitude of erosional and depositional features recorded in chalk. Advanced knowledge on the mobility and transport of unlithified calcareous nannofossil ooze by bottom currents is increasingly important not only for understanding the deposition of ancient chalk, but also for modelling the behaviour of modern pelagic carbonate sediments. Whereas the erosional behaviour of very pure calcareous nannofossil ooze has recently been investigated, the effect of organic matter (OM) and clay minerals on the erosional behaviour of calcareous nannofossil ooze is as yet unquantified. The results of the present study are based on laboratory flume studies utilizing chalk ooze with varying concentrations of smectite clay (1– 30 wt %), two types of OM, and bed porosity. Phytoplankton (Pavlova lutheri) was used as a proxy for particulate marine organic matter (MOM), and xanthan gum as a proxy for extracellular polymeric substances (EPS). The results show a significant decrease in nannofossil ooze mobility with increasing content of clay or MOM. Organic matter is found to reduce erodibility at much lower concentrations than clay minerals at porosities equivalent to those of the sea floor. At lower porosities, corresponding to some depth below the sea floor, organic matter and clay minerals are less effective in bed stabilisation. This suggests that clay minerals and especially organic matter will affect the likelihood of initiation of severe erosion on the sea floor, whereas their inhibiting effect will decrease as erosion scours progressively deeper into the sediment column. The effect of EPS is more complex than MOM, probably due to detachment of large aggregates from the bed and resulting increase in bed roughness. The choice of organic matter in sedimentological experiments may lead to significant differences in sediment behaviour and should therefore involve careful consideration.