Effects of drill cuttings on biogeochemical fluxes and macrobenthos of marine sediments

Experimental work was performed on drill cuttings sampled from off-shore drilling operations. The cuttings contained remnants of two different types of drilling muds: a water-ilmenite based mud and an olefin-barite based mud. In a pilot experiment, a gradient of up to 65 mm thick layers of water-ilmenite based cuttings were added to 78 cm2 core samples. In another set-up, 3 mm layers of water-ilmenite and olefin-barite based cuttings were added to replicate 1000 cm2 box-core samples with natural benthic communities transferred from the Oslofjord, S.E. Norway. Boxes with no addition and addition of 3 mm “clean” sediment were used for control. Increased consumption of oxygen and nitrate indicated the presence of degradable organic phases in both mud systems. The release of silicate showed a general decrease with increasing thickness of the cuttings layer, but maximum rates in the 3.1 mm treatment indicated increased bioturbation at low and moderate doses. The initial (field) composition of the macrobenthic communities was maintained throughout the 3 months experimental period, and at community level, no significant difference was observed between the four treatments at the end of the exposure period. However, after grouping the treatments into “clean” (untreated control and 3 mm clean sediment) and “cuttings” (3 mm water-ilmenite and 3 mm olefin-barite based cuttings), multivariate statistical analyses revealed a significant difference in community composition between clean and cuttings treatments, and three taxa showed significantly reduced abundances in the cuttings treatments. Chemical toxicity of mud components is assumed to be small, and the observed effect was more likely a result of physical properties such as the size or shape of cuttings particles.