1 Introduction

Hadal trenches (water depth > 6,000 m) are mainly characterized by high hydrostatic pressure, low temperatures, oligotrophy, frequent geological activity, and isolation from the anthrosphere (Bao et al., 2018; Jamieson et al., 2010; Somero, 1992). However, organisms there are more vibrant than expected despite the limited input of organic matter (OM) from primary production in the surface ocean (Glud et al., 2013; Jamieson et al., 2010). The OM supply to hadal fauna is important because it sheds light on the carbon flow in hadal trenches and the biological adaptation to this extreme environment (Havermans & Smetacek, 2018; Jamieson et al., 2010). Many presumptions emerged, but the answer is still unclear. Since Ohara et al. (2012) first reported a flourishing serpentinite‐hosted ecosystem in the Shinkai Seep Field (~5,800 m deep; Figure 1), more similar communities have been found on the inner trench slope of the Mariana Trench (MT; Okumura et al., 2016). Thus, the potential release from the Earth's interior and/or the carrion transported from these ecosystems by ocean currents may provide food to hadal fauna. Moreover, earthquakes and the steep slope enhance the deposition rate by importing the surrounding sediment into trenches (Bao et al., 2018; Glud et al., 2013). Amphipods living on the refractory OM in these redeposited sediments seems feasible thanks to the revelation of a special enzyme within them (Kobayashi et al., 2012). A single source from the surface water was always thought to be impossible due to the low primary production rate, which was not fully discussed in previous studies. However, the high concentrations of anthropogenic pollution in hadal fauna hint to a primary OM supply from the surface water (Jamieson et al., 2017).

Figure 1 Open in figure viewer PowerPoint Sampling locations of amphipods from the Mariana Trench, Mussau Trench (MST), central New Britain Trench (NBT‐C), and eastern New Britain Trench (NBT‐E).

Radiocarbon (14C) is useful for tracing the cycling of OM in the ocean for three reasons. First, the Δ14C value represents the original source of OM without the effects of the trophic level, fractionation, or other biological processes (Eisenmann et al., 2017; Stuiver & Polach, 1977). Second, thermonuclear tests during the 1950s and 1960s doubled the amount of 14C in the atmosphere. This “bomb 14C” mixed quickly into the surface ocean and terrestrial carbon pools, enabling 14C to be traced within the carbon cycle on a short time scale, from years to decades (Hua & Barbetti, 2004). Third, Δ14C values of proposed sources of OM to hadal fauna differ substantially from one another. Carbon released from the Earth's interior is 14C free (Δ14C = −1000‰; Pohlman et al., 2009). Refractory OM has a low Δ14C because of an isotopic decay during the long residence time (Druffel et al., 1992; Williams & Druffel, 1987). In surface water, dissolved inorganic carbon (DIC) is in equilibrium with the atmospheric CO 2 and later is photosynthesized into OM, resulting in a high content of 14C in surface OM (Druffel et al., 2003).

In this study, we are first to trace the food source using Δ14C measurements of OM from hadal amphipods from three trenches, including the deepest trench, Mariana (Figure 1). The Δ14C results, along with the compiled database of Δ14C in different carbon pools in the West Pacific, demonstrate the food supply of these amphipods. Moreover, the importance of the Δ14C results are discussed, and specifically how they provide insights into the lifetimes of these hadal organisms.