Antiquity

The antiquity or age of the hadal community has been a contentious subject and under discussion since the first hadal organisms were recovered (Belyaev, 1989). To understand the invasion of the trenches both the historical oceanography and geology of the deep sea and trenches must be considered. The history of the physical deep-sea environment is characterised by extreme variability in temperature, oxygen and circulation (McClain and Hardy, 2010). The deep sea used to be much warmer than it is today as it has cooled by approximately 14–15ºC since the Eocene/Palaeocene boundary (55 Ma), following minor warming in the Late Cretaceous and a similar cool period at the Eocene/Oligocene boundary (34 Ma; Waelbroeck et al., 2001). Furthermore, deep-ocean circulation has alternated between two types of ocean; one driven by high-latitude deep water formation (thermohaline, THC), and one driven by salinity-induced stratification at low latitudes (halothermal, HTC). The former resulted in cold, oxygenated deep water and the latter in warm, saline deep water which reduced global circulation (Rogers, 2000; McClain and Hardy, 2010). THC conditions have existed since the Eocene–Oligocene transition, and the HTC conditions occurred back to the Triassic. During this period, deep-water anoxic events were both frequent and extensive (Jacobs and Lindberg, 1998; Rogers, 2000; Waelbroeck et al., 2001; Takashima et al., 2006), with the most severe events associated with rapid THC–HTC transitions in the mid-Cretaceous, and at the Permian/Triassic and Ordovician/Silurian boundaries (Horne, 1999).