FROM SURFACE TO SEDIMENT: INVESTIGATING THE SPATIAL VARIABILITY OF CARBON STABILIZATION IN ARCTIC LAKE SEDIMENTS

Lakes play a pivotal role in global carbon (C) cycling and are critical zones for C transformations as lake sediments store large amounts of organic carbon (OC), acting as long-term geologic sinks. Understanding processes controlling lake C cycling is of importance in the Arctic due to changes driven by rapidly rising temperatures. It remains uncertain whether Arctic lakes will continue acting as C sinks or sources in the face of global climate change. The amount of C stored in lakebed sediments is controlled by 1) how much C reaches the lakebed and 2) how that C is altered both biologically and chemically at the lakebed. Terrestrial runoff from permafrost thaw, groundwater, and phytoplankton productivity control the amount of OC delivered to the lakebed. Lakebed C can be lost from sediments through microbial respiration, of which, aerobic respiration is thought to be the most efficient mode of OC remineralization however, oxygen-rich conditions may foster OC stabilization with metal oxides, like iron (Fe) and manganese (Mn), protecting labile OC. Large deposits of fresh OC to sediments can drive anoxic conditions in which Fe and Mn reduction are favorable pathways for OC remineralization, potentially transforming these systems into C sources through the production and release of greenhouse gases. To investigate the vulnerability and fate of OC-metal complexes, we took sediment cores and porewater samples in Toolik Lake, an Arctic lake in Northern Alaska. Preliminary results from porewater samples suggests that portions of Toolik Lake are dominated by high concentrations of dissolved Fe while others are dominated by Mn. Differences in sediment metal concentrations could alter how much C is sequestered in the lakebed suggesting that portions of the lake may function as better C sinks than others. Dissolved inorganic carbon isotope porewater profiles also indicate that some parts of Toolik Lake are hotspots for organic matter degradation. Future work will identify spatial differences in the abundance and nature of OC-metal complexes in sediments while also characterizing OC sources delivered to the lakebed and their impact on C, Fe, and Mn cycling to assess the complexity of both C delivery and sedimentary sequestration which is crucial for informing the global C budget.