EMPLOYING PASSIVE ACOUSTICS AS A TEMPORALLY PRECISE MONOLOGUE FOR CONSTRAINING EBULLITIVE METHANE FLUXES IN WARMING SUBARCTIC LAKES

Systematic difficulties in capturing the large spatial and temporal variability of ebullition (bubbling) has promoted a broad range of uncertainty in our understanding of the role of lakes as key emitters of atmospheric methane (CH₄). With the projected warming and ongoing thawing of high-latitude frozen peatlands abundant in small lakes and ponds, there is an increasing need for methods that provide high-temporal resolution delineating precisely when and under what circumstances ebullitive fluxes occur. Employing the well-established Minnaert resonance formula as a reliable proxy for bubble volume, we designed a system of passive acoustic hydrophone sensors calibrated to continuously record ebullition from lakes at 160 kbits/sec. We present here the results of three summer field seasons (2011-2013) of acoustic and manual bubble flux measurements from four subarctic lakes situated in discontinuous permafrost regions of northern Sweden and Alaska. Results show trends similar to prior lake measurements in the subarctic. We found wide variation in CH₄ concentrations, spanning between 0.10 to 95.16%. Fluxes ranged from 0-279.72 mg CH₄ m^-2 d^-1 and averaged 12.03 mg CH₄ m^-2 d^-1 (n = 401) over the three-year period. High-resolution time series analysis of our measurements will be compared alongside standard meteorological parameters such as atmospheric pressure, temperature, rainfall, water table, wind speed, and radiative inputs to infer dominant external forcings on ebullition. Radiocarbon and ¹³C/¹²C ratios of bubble samples collected from Swedish lakes in July 2013 are to be subsequently analyzed for age, transport and production mechanisms.