Chemical Weathering and Organic Carbon Export from Arctic Watersheds, North Slope, Alaska

Recent climate change has dramatically impacted the Arctic, and models suggest that accelerated effects will occur in the future as CO2 continues to accumulate in the atmosphere. Understanding this problem is important because arctic environmental change could have planetary-scale repercussions within human timescales. In particular, oxidation of organic carbon stored in permafrost might create a positive feedback to global warming. While numerous studies have focused on the relationship between warming and carbon cycling in the Arctic, specific effects of rising atmospheric CO2 levels on chemical weathering processes and coupled changes in organic carbon dynamics have received relatively little attention. This project has two integrated goals: to identify mineral weathering reactions and hydrologic processes that control how and at what rate the inorganic solute geochemistry of water evolves during transport within soils and streams and to establish linkages between chemical weathering phenomena and organic carbon export. The work will focus on the major ion and isotope geochemistry of headwater streams and creeks draining the North Slope of Alaska. Based on preliminary data, the guiding hypothesis of the work is that the ratio of carbonate to silicate weathering fluctuates in response to seasonal changes in discharge and permafrost active layer depth, with the highest ratios observed during base flow conditions and maximum active layer depth and the lowest ratios observed during peak flow conditions and minimum active layer depth. To test this hypothesis and its implications for understanding organic carbon export, the team will: 1) Quantify rates of major cation release, CO2 consumption, and dissolved organic carbon (DIC) production by carbonate and silicate weathering; 2) Evaluate controls on the Ca, Sr, and C (of DIC) isotope composition of rivers and soils; and 3) Establish relationships between these tracers and other river water constituents, including H and O isotopes of water, organic matter concentrations, and the isotope composition of organic matter (both C and N). They will collect water, sediment, bedrock, and soil samples. Water sampling will occur from spring thaw through fall freeze-up. In the laboratory, they will conduct leaching and digestion experiments to quantify mineral weathering end members. They will synthesize data using mass-balance modeling, carbonate equilibria calculations, hydrograph separations, and elemental and isotope mixing equations. This study represents one of the first integrative efforts to elucidate fundamental linkages between the isotope and organic geochemistry of Arctic Alaskan rivers. Because warming will likely alter the ratio of carbonate to silicate weathering via several feedback mechanisms, this study will establish a novel method for monitoring Arctic environmental change at the watershed scale.

近年来，气候变化已对北极地区造成显著冲击，模型预测随着大气中二氧化碳（CO₂）持续累积，未来其影响还将进一步加剧。对此问题的研究具有重要意义，因为北极环境变化可能在人类时间尺度内引发全球性的连锁反应。具体而言，永久冻土（permafrost）中储存的有机碳的氧化作用，可能会对全球变暖形成正反馈效应。尽管已有大量研究聚焦北极地区变暖与碳循环之间的关联，但大气CO₂浓度升高对化学风化过程以及有机碳动态耦合变化的具体影响，却相对缺乏关注。本项目包含两项整合性研究目标：一是明确控制土壤与溪流中水体无机溶质地球化学演化过程及其速率的矿物风化反应与水文过程；二是厘清化学风化现象与有机碳输出之间的关联机制。本研究将聚焦阿拉斯加北坡汇水区源头溪流的主要离子与同位素地球化学特征。基于前期观测数据，本研究的核心假说为：碳酸盐风化与硅酸盐风化的比值会随径流量与冻土活动层厚度的季节变化发生波动——基流状态与活动层厚度最大时，该比值达到峰值；而洪峰流量状态与活动层厚度最小时，该比值处于最低水平。为验证该假说及其对理解有机碳输出的启示意义，研究团队将开展以下工作：1）量化碳酸盐与硅酸盐风化过程中主要阳离子释放量、CO₂消耗量，以及溶解有机碳（Dissolved Organic Carbon, DIC）的生成速率；2）解析河流与土壤中钙、锶以及溶解无机碳中碳同位素组成的控制因素；3）明确上述示踪剂与其他河流水体组分之间的关联，包括水体氢、氧同位素组成、有机质浓度，以及有机质的碳、氮同位素组成。研究团队将采集水体、沉积物、基岩与土壤样品。水体采样将覆盖从春季融冻至秋季封冻的完整周期。实验室阶段将开展浸提与消解实验，以量化矿物风化端元的相关参数。研究将通过质量平衡模型、碳酸盐平衡计算、水文过程线分割，以及元素与同位素混合方程对数据进行整合分析。本研究是首批系统性阐明阿拉斯加北极地区河流同位素与有机地球化学之间内在关联的研究之一。鉴于气候变暖可能通过多种反馈机制改变碳酸盐与硅酸盐风化的比值，本研究将建立一种全新的流域尺度北极环境变化监测方法。