Eroding Permafrost Coastlines Release Biodegradable Dissolved Organic Carbon to the Arctic Ocean

Coastal erosion mobilizes large quantities of organic matter (OM) to the Arctic Ocean where it may fuel greenhouse gas emissions and marine production. While the biodegradability of permafrost-derived dissolved organic carbon (DOC) has been extensively studied in inland soils and freshwaters, few studies have examined dissolved OM (DOM) leached from eroding coastal permafrost in seawater. To address this knowledge gap, we sampled three horizons from bluff exposures near Drew Point, Alaska: seasonally thawed active layer soils, permafrost containing Holocene terrestrial and/or lacustrine OM, and permafrost containing late-Pleistocene marine-derived OM. Samples were leached in seawater to compare DOC yields, DOM composition (chromophoric DOM, Fourier transform ion cyclotron resonance mass spectrometry), and biodegradable DOC (BDOC). Holocene terrestrial permafrost leached the most DOC compared to active layer soils and Pleistocene marine permafrost. However, DOC from Pleistocene marine permafrost was the most biodegradable (33 +/- 6% over 90 days), followed by DOC from active layer soils (23 +/- 5%) and Holocene terrestrial permafrost (14 +/- 3%). Permafrost leachates contained relatively more aliphatic and peptide-like formulae, whereas active layer leachates contained relatively more aromatic formulae. BDOC was positively correlated with nitrogen-containing and aliphatic formulae, and negatively correlated with polyphenolic and condensed aromatic formulae. Using estimates of eroding OM, we scale our results to estimate DOC and BDOC inputs to the Alaska Beaufort Sea. While DOC inputs from coastal erosion are relatively small compared to rivers, our results suggest that erosion may be an important source of BDOC to the Beaufort Sea when river inputs are low. Arctic coastlines are rapidly eroding into the ocean. Soils along these coastlines contain large quantities of organic matter (OM) that dissolves in seawater and may be consumed by microbes. If this dissolved organic matter (DOM) is biodegradable, it can be an important energy source to coastal food webs and/or quickly decomposed to greenhouse gases. We used laboratory experiments to examine the chemical composition of the DOM that is released from eroding soils into seawater and measured its biodegradability. We compared results for three different layers of soil at Drew Point, Alaska, including near-surface soils that thaw seasonally and deeper soils that are perennially frozen (permafrost). Our results show that different layers, which contain OM of different sources and ages, have distinct chemical characteristics that impact biodegradability. While rivers supply more OM to the Alaska Beaufort Sea than coastal erosion, our results show that DOM released from all soil layers is highly biodegradable, and that DOM from deep permafrost is the most biodegradable. We deomonstrate that coastal erosion can be an important source of OM to Arctic coastal ecosystems, particularly in locations and seasons (e.g., late summer) that receive fewer river inputs. Eroding soils and permafrost along the Alaska Beaufort Sea coast leach biodegradable dissolved organic carbon into seawater Biodegradability was higher in leachates from Late-Pleistocene relict marine permafrost than Holocene terrestrial soils and permafrost Permafrost leached aliphatic and peptide-like molecular formulae that were not present or less abundant in active layer soils