Micro-scale geotechnical variability in continental slope and abyssal sediments influenced by the oxygen minimum zone in the Arabian Sea

We describe a new micro-scale method of measuring the penetration resistance of the surficial layers of slope and abyssal plain sediments within and below the OMZ on the Oman continental slope. The micro-scale load resistance penetrometer (MLRP) continuously measures the penetration resistance (q) of a cylindrical probe driven at constant velocity into cores of sediment recovered by sub-coring box core samples. Measurements are recorded at 0.8 mm depth intervals into the sediment by an automated data acquisition system. These provide continuous depth profiles of penetration resistance (q) and its variability (qr) in surficial sediments (0–100 mm). The results represent the first published application of this technique to slope and abyssal plain sediments. The q data divide the stations into shallow OMZ (SOMZ) stations (391–854 m), deep OMZ (DOMZ) stations (987–1285 m), and abyssal non-OMZ (ANOMZ) stations (3392–3396 m). One station (688 m) was anomalous. Penetration resistance (q) was positively related to shear strength and inversely related to water content, but not to bioturbation measured as burrow numbers. Relaxation and pull-out data were obtained from three stations. Three patterns of qr were observed, consisting of well-confined data, data containing large negative and positive spikes, and data showing a progressive down-core increase in spikes. Four additional measures of q variability were defined, which show statistically significant relationships with burrow numbers. An increase in bioturbation, estimated by burrow numbers, is associated with a decrease in micro-scale variability in q. This is related to the activity of burrowing macrofauna. Our results are discussed in the context of biogeotechnical modifications of sedimentary environments.