Why are hadal zones important?

Deep sea trenches of the hadal depth zone (6-11 km) are hotspots for high microbial activity because they receive an unusually high flux of organic matter, made up of animal carcasses, and sinking algae, originating from the surrounding shallower seabeds.

Technician Anni Glud from University of Southern Denmark is handling sediment samples retrieved from the bottom of Kermadec Trench. Photo: Ronnie N. Glud.

Some of this might get moved during earthquakes and sink further down into the deepest parts of the trenches.

- So even though the trenches only amount to about 2 pct. of the global ocean area, they have a relatively large impact on marine carbon balance and thus on the global carbon cycle, says project leader, professor Ronnie N. Glud, University of Southern Denmark.

On the bottom of the deep sea, microbial mineralization releases nutrients and inorganic carbon, ultimately sustaining continued primary production, while a fraction of the organic material is retained in the sediment record.

Regulation of O2 in the atmosphere and oceans

The balance between mineralization and burial of organic material is – on geological time scales – part of the most important feed-back mechanism for regulating the O2 availability in the atmosphere and oceans.

The efficiency of benthic mineralization, therefore has critical implications for ocean chemistry, nutrient availability, redox conditions and for life in the global ocean – and on Earth.

Professor Bo Thamdrup and technician Anni Glud from University of Southern Denmark working in the lab onboard the research vessel Tangaroa. Photo: Ronnie N. Glud.

More than half of global benthic carbon mineralization

The volume-specific benthic carbon mineralization rate decreases three orders of magnitude from the coastal ocean to the abyssal plains at 4-6 km water depth. However, given the vast extent of the deep sea, activity in abyssal and hadal sediment is estimated to contribute >50% of the global benthic carbon mineralization.

This value might even be underestimated as data from great water depth are poorly represented in the global database and there is evidence that the pelagic degradation efficiency of sinking aggregates declines significantly with increasing water depth leading to deposition of relatively fresh material at great depth.

Trench bottoms receive a lot of organic material

Early evidence for high organic carbon content and high abundance of meiofauna (animals in the size of .03-0.5 mm) and microbes in recovered sediment samples suggested that hadal trenches could act as depocenters for organic material but still little is known about the nature and amount of depositing organic material, or the extent to which this material is mineralized or retained.

Recently, however, we demonstrated a doubling of in situ benthic mineralization rates in Challenger Deep relative to adjacent abyssal sediments.

Read about it here. 

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