High mercury accumulation in deep-ocean hadal sediments

Abstract

Ocean sediments are the largest sink for mercury (Hg) sequestration and hence an important part of the global Hg cycle1. Yet accepted global average Hg flux data for deep-ocean sediments (> 200 m depth) are not based on measurements on sediments but are inferred from sinking particulates2. Mercury fluxes have never been reported from the deepest zone, the hadal (> 6 km depth). Here we report the first measurements of Hg fluxes from two hadal trenches (Atacama and Kermadec) and adjacent abyssal areas (2–6 km). Mercury concentrations of up to 400 ng g−1 were the highest recorded in marine sediments remote from anthropogenic or hydrothermal sources. The two trench systems differed significantly in Hg concentrations and fluxes, but hadal and abyssal areas within each system did not. The relatively low recent mean flux at Kermadec was 6–15 times higher than the inferred deep-ocean average1,3, while the median flux across all cores was 22–56 times higher. Thus, some hadal and abyssal sediments are Hg accumulation hot-spots. The hadal zone comprises only ~ 1% of the deep-ocean area, yet a preliminary estimate based on sediment Hg and particulate organic carbon (POC) fluxes suggests total hadal Hg accumulation may be 12–30% of the estimate for the entire deep-ocean. The few abyssal data show equally high Hg fluxes near trench systems. These results highlight a need for further research into deep-ocean Hg fluxes to better constrain global Hg models.

Mercury Accumulation in Marine Sediments – A Comparison of an Upwelling Area and Two Large River Mouths

“…However, a recent study of isotope mass balance suggests that evasion flux estimations from models are likely overestimated (Jiskra et al, 2020). The current estimate of deep-ocean Hg accumulation also shows that Hg flux in the deep ocean is higher than the currently accepted deep-ocean average Hg flux (Sanei et al, 2021).…”

The hadal zone is an important and heterogeneous sink of black carbon in the ocean

“…1730-1810 AD (8-6 cm) for the BC/TOC ratio (Fig. 7b) 16 . These abrupt changes may be caused by mass-wasting events, such as submarine landslides, because of the high seismicity 17,50 .…”

“…The BC preserved in marine sediments represents a long-term carbon sink for atmospheric CO 2 . The sedimentation rate obtained from regression of 210 Pb ex (excess 210 Pb activity) versus core depth ranges from 0.026 to 0.20 cm yr -1 (Supplementary Table 5) 16 . Based on the data of sedimentation rate, BC content and dry sediment bulk density, we calculated the accumulation rate of BC in five trench regions, i.e., MT, NBT, BT, KT, and AT.…”

“…The hadal zone is mainly composed of trenches formed at the subduction zones. Despite representing only ~1% of total seafloor area, the hadal zone accounts for 45% of the ocean's depth range 15,16 . Hadal trench ecosystems are one of the least explored habitats on Earth due to extreme challenges of sampling 15 .…”

Mercury evidence for combustion of organic-rich sediments during the end-Triassic crisis

“…However, recent studies have painted a more nuanced picture, revealing that terrestrial and nearshore sections received Hg from multiple sources, including seawater, terrestrial materials (e.g., vegetation, soil), and the atmosphere [18][19][20][21][22] . Although processes of removal of Hg from pelagic seawater are complex 23,24 , analysis of a pelagic open-ocean section far from continental influences is needed to gain a better understanding of atmospheric Hg fluxes during the T-J transition 15 .…”