Warming in the Arctic is expected to be roughly twice as high as the global mean. Sea ice extent is declining dramatically over the last years and favors accelerating coastal erosion. With erosion rates as high as 25 m·yr-1, the release of organic carbon and nutrients from permafrost coasts has dramatic impacts on the global carbon cycle, on nearshore food webs and the local communities which are still relying on the marine biological resources.
During the Holocene, the delivery of sediment, particulate organic carbon (POC) and dissolved organic carbon (DOC) varied in response to temperature and relative sea level changes. Such phases of increased or reduced material input could serve as an analogue for future erosion scenarios. In the past, changing inputs of sediments, carbon, and nutrients may have altered the biogeochemical setting on the upper arctic shelves and may have impacted the global carbon cycle.
Recent flux estimates of sediment and POC from coastal erosion into the Arctic Ocean are ~430 Tg yr-1 sediment and 4.9-14 Tg yr-1 POC, which is comparable to if not higher than riverine fluxes. However, the fate of sediments and organic carbon once eroded from the cliff remains largely unknown and the release of DOC from melting ground ice in permafrost cliffs has not been considered yet.
Material supply over the Holocene is difficult to quantify as it depends on erosion of a coastline whose original configuration is not known. For example, large parts of the circum-arctic shelves were subaerially exposed during the last glacial maximum (LGM) and became flooded rapidly. Thus, early Holocene erosion of coastal permafrost deposits was probably stronger than today and released more terrestrial material. With the retreat of the coastline, the depocenters moved further southward and thereby successively reducing accumulation rates in the distal shelf areas. In other parts of the Arctic, however, glacio-isostatic rebound was significant so that global transgression was outpaced and therefore reduced shore line retreat. Even after the modern sea-level highstand was approached around 5,000 cal BP, there is evidence that the depositional system on the shelves took time to stabilize.
Quantitative estimates of erosion rates along Arctic coasts throughout the Holocene are still sparse and need substantial improvement to clarify the fate of terrigenous material in the Arctic Ocean.
EPIC Alfred Wegener Institut