High mercury accumulation in deep-ocean hadal sediments.

Area of Science:

• Marine Geochemistry
• Oceanography
• Environmental Science

Background:

• Ocean sediments are the primary global sink for mercury (Hg) sequestration, playing a crucial role in the mercury cycle.
• Existing global mercury flux data for deep-ocean sediments are inferred, not directly measured, and lack data from the deepest hadal zone (>6 km).

Purpose of the Study:

• To report the first direct measurements of mercury fluxes in hadal trenches and adjacent abyssal areas.
• To investigate mercury concentrations and accumulation rates in these previously unstudied deep-ocean environments.

Main Methods:

• Direct measurement of mercury fluxes in sediments from the Atacama and Kermadec hadal trenches and nearby abyssal zones (2-6 km depth).
• Analysis of mercury concentrations in marine sediments.
• Estimation of hadal mercury accumulation based on sediment mercury and particulate organic carbon (POC) fluxes.

Main Results:

• Mercury concentrations up to 400 ng g⁻¹ were recorded, the highest in marine sediments far from known pollution sources.
• Hadal and abyssal sediments within trench systems showed similar mercury concentrations and fluxes, but the systems differed significantly.
• Measured mercury fluxes were 6-15 times higher than inferred deep-ocean averages, with median fluxes 22-56 times higher.

Conclusions:

• Certain hadal and abyssal sediments act as significant mercury accumulation hotspots.
• Despite comprising only ~1% of the deep-ocean area, hadal zones may contribute 12-30% of total deep-ocean mercury accumulation.
• Further research on deep-ocean mercury fluxes is essential for accurate global mercury cycle modeling.