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

Abstract

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.