The role of decadal and sub-annual water-level and wave variability in coastal carbon budgets

Abstract:

Relationships between coastal erosion and soil carbon dynamics are poorly constrained along the Laurentian Great Lakes’ coastal margins, despite morphodynamics comparable to those along marine and estuarine coasts. Soils contain the largest terrestrial pool of carbon, and erosion of coastal habitats removes carbon from storage as organic-rich soils are swept into marine and lacustrine environments. We developed soil carbon budgets at three sites along the Illinois shore of Lake Michigan, U.S.A., covering two temporal scales: A decadal record, which was based on historical aerial imagery (1939-2020); and a sub-annual record based on data from unoccupied aerial systems (2018-2020).

Rates and magnitudes of coastal erosion, assessed from aerial imagery, were coupled with soil carbon data from sediment cores to parameterize a carbon budget box model. This model quantifies the spatial and temporal variability in carbon storage and export across various coastal habitat types. Model advances beyond previous iterations include an increased ability to examine spatial variability, a temporally dynamic carbon inventory term, and an updated erosional term which better captures export of soil carbon along the shoreface. The model reveals that all sites acted as carbon sources over the period of investigation (1939-2020). The initial carbon stock of all sites was reduced by half over 80 years, a loss rate which is greatly imbalanced with the time required to store carbon in these habitats (394-1934 cal yrs BP).

Historical wave and water-level records suggest that carbon export is accelerated during periods of rising and high water levels. Most carbon losses are linked to high-energy storm events with northeasterly waves during high water-level conditions. The rate of carbon export from 2012 to 2020 (the most recent episode of water-level rise) is 1.6 times greater than any previously measured rate, owing to a rapid rate and large magnitude of water-level rise. Scientists and coastal managers can apply the insights gained from this improved carbon budget model to identify hot spots for carbon loss and make informed management decisions in light of anticipated future climate and hydrologic changes.