In their recent paper, Rea et al. (2006) claim to have discovered an area covering ~2 × 106 km2 located between ~28°S and 42.5°S in the central South Pacific that has been devoid of sediment since the Late Cretaceous (or covered by less than 7 m of sediment—the maximum thickness of sediment not detectable by the seismic-reflection profiling system used by Rea et al.). They named this area the South Pacific bare zone. This conclusion is based mainly on the interpretation of one N-S seismic-reflection profile, plus three gravity cores within this area. However, if we define a bare zone as being totally devoid of sediment, then the seismic-reflection profiling system deployed by these authors is clearly inadequate to test this hypothesis. Instead, there is abundant evidence to support the occurrence of an appreciable sediment cover within this supposedly bare zone.
The area studied by Rea et al. lies beneath the subtropical anticyclonic gyre, is characterized by very low productivity of the surface waters (Glasby, 1976a), and is one of the most remote areas in the world ocean (Stoffers et al., 1985). As such, it is characterized by low sedimentation rates and high abundances of manganese nodules. Previous studies have shown that the sediments in this region are dominantly pelagic clays, with sedimentation rates <1 mm/103 yr (Piper et al., 1985, 1987), and that almost the whole area south of 36°S is covered by manganese nodules with surface densities of up to 75%–100%, with some massive manganese crusts observed locally (Glasby, 1976b; Plüger et al., 1985; Plüger, 1989, personal commun.; Skornyakova et al., 1990). Berger et al. (1976) have estimated the Carbonate Compensation Depth (CCD) in this region to be ~4,200 m.
The age of the seafloor on which the nodules form in this region lies between ~25–70 Ma (Müller et al, 1997). Assuming that the sediment thickness in this area is 7 m (the maximum thickness of sediment not detectable by Rea et al.), then it can be calculated that the average rate of sedimentation in this region is in the range of ~0.1–0.3 mm/103 yr. This rate is much lower than the sedimentation rate previously estimated for this region (0.4 mm/103 yr; Glasby, 1991) and is about an order of magnitude lower than the actual sedimentation rates determined by 230Th/231Pa dating on sediment cores taken on a W-E transect across this region at 42°S during cruise SO-14 of R/V Sonne in 1980 (Schmitz et al., 1986). These low sedimentation rates reflect the extremely low input of aeolian dust from the deserts of Australia in this remote area (Prospero et al., 1989; Rea, 1994).
If we now take the thickest layers of manganese oxides in the manganese nodules collected during the Sonne cruise to be ~15–30 mm (based on photographs of nodule sections) and assume that these hydrogenous nodules have grown at rates of ~3.5–3.8 mm/106 yr since their formation (Plüger, 1989, personal commun.), then it can be calculated that these nodules formed over the period from ~3.9–8.6 Ma to the present. Nodule formation in this region is therefore quite recent. However, these age data are not sufficiently well constrained to permit paleoceanographic interpretation. In some nodules, the outer layer of the nodules consists of two discrete horizons of manganese oxides, each ~15 mm thick, implying an abrupt increase in ocean bottom current veloci ties at some stage (Glasby, 2006).
The area investigated by Rea et al. is one of five areas in the Pacific with the most abundant coverage of manganese nodules (Piper et al., 1987). The results presented here show unambiguously that this area is not a bare zone but is most likely characterized by sediment thicknesses somewhat less than 7 m. In order to validate this conclusion, it is suggested that one or two latitudinal transects be undertaken across the basin using systematic box coring, bottom photography, piston coring, and a deep-towed high-frequency seismic system. Provisional calculations suggest that more than a billion tonnes of deep-sea nodules occur within this poorly surveyed region, and that more detailed studies of the distribution and mode of formation of the nodules and their relation to the sedimentation regime are in order.