Erosion of Organic Carbon from Permafrost Zones in the Arctic as a Geological Carbon Dioxide Sink

Abstract:

Soils of the northern high latitudes store carbon over millennial timescales and contain almost double the carbon stock of the atmosphere. The exposure and decomposition of aged organic matter in these soils is a carbon dioxide (CO₂) source to the atmosphere. Permafrost thaw over the coming century may result in a significant CO₂ release. However, some of this soil organic carbon in permafrost zones can be eroded and input to rivers. If it escapes degradation during river transport and is buried in ocean sediments, it instead contributes to a longer-term (>10⁴ yr), geological CO₂sink.

Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers draining permafrost zones remains poorly constrained. We quantify POC source, flux and fate in the Mackenzie River Basin, the main sediment supplier to the Arctic Ocean, using radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC. The eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5800±800 yr. Rivers eroding continuous permafrost zones contribute the oldest biospheric POC. Based on the measured biospheric POC content and annual sediment flux, we calculate a biospheric POC flux of 2.2 (+1.3/-0.9) TgC yr^-1 from the Mackenzie River to the Arctic Ocean, three times the CO₂ drawdown by silicate weathering. Offshore we find evidence for efficient terrestrial carbon burial over the Holocene period. Our findings demonstrate how erosion of organic carbon-rich, high latitude soils can result in a significant geological CO₂sink.

We postulate that this geological CO₂ sink is sensitive to climate conditions in the Arctic. The transfer can operate when high latitudes host carbon stocks in soil, and while rivers can erode and transfer sediments to the Arctic Ocean. Over the last 1Ma, the erosional transfer was likely to have been enhanced during interglacials. We propose that erosion of biospheric carbon by large rivers in the Arctic could play an important role in long-term CO₂ drawdown, coupling the carbon cycle to climatic conditions at high latitudes.