High Carbon Mineralization Rates in Subseafloor Hadal Sediments—Result of Frequent Mass Wasting

Abstract

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.

Sediment Accumulation and Carbon Burial in Four Hadal Trench Systems

“…These assessments demonstrate that despite highly elevated microbial mineralization, hadal regions accumulate relatively large amounts of organic material. The current findings align with other recent investigations that demonstrating the importance of tectonic induced mass wasting for translocating organic carbon to the hadal environment (Bao et al., 2018; Kioka et al., 2019b; Zabel et al., 2022) and previous indirect evidence arguing for high carbon accumulation in hadal sediments (Luo et al., 2017; Xu et al., 2021), as well as recent findings of highly elevated sequestration of black carbon in some hadal settings (Zhang et al., 2022).…”

“…Additionally, our assessments also suggest that trenches are characterized by continuous accumulation of material focusing, even during periods without mass wasting (Figure 4b). Both the continuous and the erratic enhancement of material deposition might have important implications for the balance between microbial mineralization and burial of organic material and thereby long‐term carbon sequestration in trench regions and the deep sea in general (Zabel et al., 2022).…”

“…Generally, the vertical transport of organic material from the surface ocean attenuates strongly with ocean depth (Berger et al., 1987), but during the past two decades it has been realized that hadal trenches can act as depocenters for organic material (Danovaro et al., 2003; Ichino et al., 2015; Xu et al., 2021). In addition, some trench sediments hold surprisingly high levels of microbial and meiofaunal biomass (e.g., Brandt et al., 2020; Itoh et al., 2011; Schauberger et al., 2021; Zabel et al., 2022) and hadal sediments appear to express elevated O 2 consumption rates as compared to adjacent abyssal sites with water depths of 5,000–6,000 m (Glud et al., 2013, 2021; Luo et al., 2019; Wenzhöfer et al., 2016). These observations imply that deposited organic materials in trenches do not only consist of resilient pre‐aged organic material but also include relatively labile and nutritious organics that can sustain highly active microbial communities thriving at extreme hydrostatic pressure.…”

Organic matter degradation causes enrichment of organic pollutants in hadal sediments

“…It is also likely that the bulk organic carbon quality as sorbent for PCBs changes as it degrades in the trench. While a fraction of the labile organic carbon is preserved along the trench axis 15 , 40 , the inert fraction of the surface sediment organic matter generally increases with the water depth at the targeted sites. There was a marked difference in PCB concentrations of the hadal and non-hadal sites.…”

Disentangling Effects of Sea Surface Temperature and Water Depth on Hydroxylated Isoprenoid GDGTs: Insights From the Hadal Zone and Global Sediments

“…Input of materials through lateral advection is significant in the hadal zone due to the unique V‐shaped topographical feature and frequent mass‐wasting events (Glud et al., 2021; M. Liu et al., 2021; Xiao et al., 2020b; Zabel et al., 2022). In the Atacama Trench, all the trench axis sites have lower OH‐GDGT‐0%, higher RI‐OH′, and higher $${\mathrm{R}\mathrm{I}\mbox{-}\mathrm{O}{\mathrm{H}}^{\prime }}_{\text{TEM}}$$ SST estimates compared to the adjacent oceanward abyssal plain site (Figure S1 in Supporting Information ).…”