Comment on "Formation of Thetis Deep metal-rich sediments in the absence of brines, Red Sea" by Pierret et al. (2010)

The recent paper by Pierret et al. (2010) interestingly suggests that metal-rich hydrothermal sediments in Thetis Deep have been deposited in the absence of brines, in contrast to many of the other Red Sea deeps. We however were concerned by the claim that "...brine layers never occurred in the Thetis Deep." This claim was based on radiocarbon dates of samples from one piston core (334PC) reported by Scholten et al. (1991). The oldest C14 date reported by Scholten et al. was 18,000 y. Our study of the geology of Thetis Deep has only recently been published (Mitchell et al., 2010) and unfortunately would not have been available to Pierret et al. The magnetic anomaly data from this area clearly show at least one or two field reversals (Chu and Gordon, 1998 and Izzeldin, 1987) suggesting that sea-floor spreading has occurred here since at least the Brunhes-Matuyama boundary at 0.78 Ma and the central Red Sea evaporites have been dated as Late Miocene (Whitmarsh et al., 1974) or around 5 Ma. A sequence of core samples would need to be analysed including material extending to much older dates than currently available to demonstrate that brines have never existed here. We also question whether sedimentation rates at 334PC can be accurately applied to those of core 1016 analysed by Pierret et al. Fig. 1 shows the core locations against a background of our new multibeam data. Core 334PC is on the opposite side of the depression from 1016.