datasheet3_Geochemistry of Coastal Permafrost and Erosion-Driven Organic Matter Fluxes to the Beaufort Sea Near Drew Point, Alaska.csv

Accelerating erosion of the Alaska Beaufort Sea coast is increasing inputs of organic matter from land to the Arctic Ocean, and improved estimates of organic matter stocks in eroding coastal permafrost are needed to assess their mobilization rates under contemporary conditions. We collected three permafrost cores (4.5–7.5 m long) along a geomorphic gradient near Drew Point, Alaska, where recent erosion rates average 17.2 m year−1. Down-core patterns indicate that organic-rich soils and lacustrine sediments (12–45% total organic carbon; TOC) in the active layer and upper permafrost accumulated during the Holocene. Deeper permafrost (below 3 m elevation) mainly consists of Late Pleistocene marine sediments with lower organic matter content (∼1% TOC), lower C:N ratios, and higher δ13C values. Radiocarbon-based estimates of organic carbon accumulation rates were 11.3 ± 3.6 g TOC m−2 year−1 during the Holocene and 0.5 ± 0.1 g TOC m−2 year−1 during the Late Pleistocene (12–38 kyr BP). Within relict marine sediments, porewater salinities increased with depth. Elevated salinity near sea level (∼20–37 in thawed samples) inhibited freezing despite year-round temperatures below 0°C. We used organic matter stock estimates from the cores in combination with remote sensing time-series data to estimate carbon fluxes for a 9 km stretch of coastline near Drew Point. Erosional fluxes of TOC averaged 1,369 kg C m−1 year−1 during the 21st century (2002–2018), nearly doubling the average flux of the previous half-century (1955–2002). Our estimate of the 21st century erosional TOC flux year−1 from this 9 km coastline (12,318 metric tons C year−1) is similar to the annual TOC flux from the Kuparuk River, which drains a 8,107 km2 area east of Drew Point and ranks as the third largest river on the North Slope of Alaska. Total nitrogen fluxes via coastal erosion at Drew Point were also quantified, and were similar to those from the Kuparuk River. This study emphasizes that coastal erosion represents a significant pathway for carbon and nitrogen trapped in permafrost to enter modern biogeochemical cycles, where it may fuel food webs and greenhouse gas emissions in the marine environment.

阿拉斯加博福特海海岸的侵蚀速率持续加快，正加剧陆地来源有机质向北冰洋的输入通量；当前亟需对受侵蚀海岸多年冻土（permafrost）中的有机质储量开展更精准的估算，以评估当代环境下有机质的活化速率。我们在阿拉斯加德鲁角（Drew Point）附近的地貌梯度带上，采集了3根长度为4.5~7.5米的多年冻土岩芯，该区域近年的平均海岸侵蚀速率为17.2米/年。岩芯垂向序列特征显示，活动层与浅层多年冻土中的富有机质土壤及湖相沉积物（lacustrine sediments）的总有机碳（Total Organic Carbon, TOC）占比为12%~45%，这些沉积物形成于全新世时期。埋藏深度超过3米的深层多年冻土主要由晚更新世海相沉积物构成，其有机质含量较低（总有机碳占比约1%）、碳氮比更低且δ¹³C值更高。基于放射性碳定年的有机碳累积速率估算结果显示：全新世时期的有机碳累积速率为11.3±3.6克总有机碳·平方米⁻¹·年⁻¹，晚更新世（距今12~38千年）时期则为0.5±0.1克总有机碳·平方米⁻¹·年⁻¹。在残留海相沉积物中，孔隙水盐度随埋藏深度增加而升高；尽管全年气温低于0℃，但近海平面处的高盐度（解冻样品盐度约为20~37）抑制了沉积物冻结。我们结合冻土岩芯的有机质储量估算结果与遥感时序数据，对德鲁角附近9千米海岸段的碳通量进行了估算。21世纪（2002~2018年）期间，该区域总有机碳的海岸侵蚀通量平均值为1369千克碳·米⁻¹·年⁻¹，几乎是此前半个世纪（1955~2002年）平均通量的两倍。我们估算的该9千米海岸段21世纪总有机碳年侵蚀通量为12318公吨碳·年⁻¹，这一数值与库帕鲁克河（Kuparuk River）的年总有机碳通量相近；库帕鲁克河流域覆盖德鲁角以东8107平方千米的区域，是阿拉斯加北坡第三大河流。德鲁角海岸侵蚀携带的总氮通量也得到了量化，其数值同样与库帕鲁克河的总氮通量相近。本研究表明，海岸侵蚀是封存于多年冻土中的碳与氮进入现代生物地球化学循环的重要途径，这些物质可在海洋环境中促进食物网运作并加剧温室气体排放。