Weddell Sea: dissolved organic matter stays unchanged during dense-water formation, 178 samples

Researchers analysed 178 water samples from 38 stations at the Weddell Sea Deep Water (WSDW) formation sites on the southern Weddell Sea shelf, according to a study published on nature.com. The samples were collected during two expeditions of the research icebreaker Polarstern, in 2016 and 2021. The authors report that the molecular and structural properties of solid-phase extractable dissolved organic matter (SPE-DOM) remained unchanged during dense-water formation nature.comnature.comnature.comOpen source ↗. They estimate that 0.155 ± 0.03 PgC are transferred annually from the Weddell Sea to Antarctic Bottom Water (AABW).

Why the Weddell Sea shelf matters

AABW is the coldest, densest and most voluminous water mass in the global ocean. It forms from dense shelf waters produced along the Antarctic continental shelf. The Weddell Sea is where half of AABW forms.

AABW is decreasing in volume due to climate change, with potential consequences for abyssal carbon storage, where carbon is kept away from the atmosphere for centuries. Dense water masses on the Weddell Sea shelf have an average residence time of 6 to 12 years, longer than on the Ross Sea shelf (below 3.6 years). Together with intense seasonal primary production on the shelf, this suggested that newly produced organic matter might be transformed before export to depth.

What the measurements show

Of the 178 samples, 116 were assigned to seven water masses involved in WSDW formation, from the surface down to 4433 m. The team combined physicochemical and quantitative analyses of dissolved organic carbon with molecular characterisation of SPE-DOM by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) nature.comnature.comnature.comOpen source ↗.

The composition did not change along the transformation pathway. The authors conclude that shelf processes minimally affect the organic matter exported to the deep sea, likely because of remineralisation to inorganic carbon, enhanced by the long residence times on the shelf. As a result, the dominant SPE-DOM signature in WSDW is established before dense shelf waters mix with warmer off-shelf waters.

The authors expect the molecular composition of the matter transferred to AABW to remain conserved despite the ongoing climate-driven weakening of AABW. The molecular analysis covers the solid-phase extractable fraction, which recovers only part of the total dissolved organic carbon.