In the past 20 years, the exploration of deep ocean trenches has led to
spectacular new insights. Even in the deepest canyons, an unusual
variety of life and unexpectedly high benthic oxygen consumption rates
have been detected while microbial processes below the surface of the
hadal seafloor remains largely unknown. The information that exist comes
from geophysical measurements, especially related to seismic research,
and specific component analyses to estimate the carbon export. In
contrast, no information is available on metabolic activities in deeper
buried sediments of hadal environment. Here we present the first pore
water profiles from 15 up to 11 m long sediment cores recovered during
three expeditions to two hadal zones, the Japan Trench and the Atacama
Trench. Despite low levels of organic debris, our data reveal that rates
of microbial carbon turnover along the trench axes can be similar to
those encountered in much shallower and more productive oceanic regions.
The extreme sedimentation dynamics, characterized by frequent mass
wasting of slope sediments into the trenches, result in effective burial
of reactive, microbially available, organic material. Our results
document the fueling of the deep hadal biosphere with bioavailable
material and thus provide important understanding on the function of
deep-sea trenches and the hadal carbon cycle.