ARCTIC

"Sources, transport, and degradation of permafrost-derived organic carbon in a warming Arctic: the Siberian Shelf"

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 Obiettivo del progetto (Objective)

'Among the mechanisms that could move significant quantities of green house gases into the atmosphere within this century, climate scientists are particularly concerned about the carbon (C) locked in permanently frozen Arctic ground that is now being released because of warming temperatures. Given the extent of this mega-pool of C (~1,400 Gt of C) susceptible to climate-induce changes compared to the atmospheric reservoir (~750 Gt of C), the potential release of this stock into the atmosphere due to thawing is considered a serious risk for the future climate. Recent studies suggest that a significant fraction of previously frozen soil will be re-located along the Arctic shelf. Indeed there are ample of evidence indicating that land-to-ocean fluxes of organic carbon (OC) along the Arctic coasts are changing because of thermal collapse of coastal permafrost and increase of the river runoff. However, the fate of this material once re-introduced in the marine carbon cycle is poorly constrained. With this project we propose to address this critical knowledge gap by analyzing the composition and physical properties of surface sediments from the Siberian Shelf. Our overarching objective is to develop and test a sensitive proxy of land-derived OC degradation in a system that will experience a massive supply of terrigenous material in the near future. The extent of degradation will be assessed using mineral surface-normalized concentrations of terrigenous OC. This latter will be characterized at molecular level using a suite of terrigenous biomarkers including lignin phenols, cutin-derived products, and high molecular weight compounds (n-alkanols, n-alkanoic acids and n-alkanes). The degree of soil-OC degradation will be assesses as a function of the depletion of the terrigenous biomarkers loadings relative to the original concentrations in river and permafrost samples. The study will be carried out on different sediment size fractions to avoid sorting bias.'

Introduzione (Teaser)

Climate warming is expected to result in the transport of organic carbon (OC) from the land to the Arctic Ocean, thereby affecting marine geochemistry at high latitudes. An EU-funded project investigated the amount of OC transported and the climate response to this phenomenon.

Descrizione progetto (Article)

In the Arctic, huge quantities of carbon are locked in permanently frozen ground. Its potential release into the atmosphere due to thawing of the permafrost could represent a serious risk for the future climate. Recent studies indicate that a significant fraction of previously frozen soil will be transported across the East Siberian Arctic Shelf (ESAS) because of increased river runoff. However, the fate of this material once it enters the Arctic Ocean is not well understood.

The ARCTIC project addressed this critical knowledge gap. Project partners investigated the composition and physical properties of surface sediments collected from across the ESAS. The consortium also characterised marine and land-derived carbon using a large number of molecular biomarkers.

Biomarkers used included lignin, phenols, cutin-derived products and high-molecular-weight compounds (n-alkanols, n-alkanoic acids and n-alkanes). The analyses focused on differences in density, size and settling fractions to overcome the potential bias due to sorting during transport of the sediment over the ESAS.

Results showed that land-derived OC, referred to as terrigenous organic carbon (TerrOC), can vary greatly across the ESAS. In the inner shelf, a significant fraction of OC is associated with plant debris. However, in the outer-shelf, most of the OC is bound to the mineral matrix, mainly as fine sediment.

The plant debris is retained in the inner shelf because, despite its light density, it is relatively large in size, resulting in a high settling velocity. Therefore, as the sediments move across the ESAS the relative concentration of the different terrigenous biomarkers changes.

Furthermore, by focusing on particular size and density fractions the ARCTIC consortium could measure TerrOC degradation at the macromolecular level along the sediment transport. This showed that both degradation and winnowing of TerrOC significantly affects the composition of the permafrost once it enters the Arctic Ocean.

The ARCTIC project provided valuable data on the sources, transport and degradation of permafrost-derived OC resulting from the warming of the Arctic. This will help scientists to understand how a thawing of the permafrost may contribute to climate change.