A number that runs the wrong way
Methane in meltwater draining the North Greenland Ice Sheet was measured at between 12 and 20 nanomolar. That is far below what has been reported from the southwest of the same ice sheet: 270 nanomolar for Leverett Glacier, 1,575 for Isunnguata Sermia. The surprise is that the northern margin sits over an actively degassing petroleum system. A low concentration stands above a rich source.[1]
Being low does not mean being zero. All analysed waters are supersaturated by up to twenty times the atmospheric equilibrium value of 4.9 nanomolar, which makes these waters a net source of methane to the atmosphere. Sampling took place at the end of the melt season, which in this region typically occurs in September. The authors recall that late-season measurements in southwestern Greenland come in about tenfold below peak-season values. This figure does not count as an annual mean; it is a slice taken from the season's weakest end.[1]
Which account the isotopes change
The evidence sits not in the concentration but in the carbon itself. Meltwaters exhibit carbon isotope values from minus 41.9 to minus 35.1 per mil. Elsewhere across the ice sheet that value typically stays below minus 50 per mil and points to microbial production. Read together with the regional geological context, the heavier composition here makes the source predominantly thermogenic: a gas largely independent of subglacial microbial activity, formed far deeper and far earlier.[1]
The practical consequence is a correction to the account, not a prophecy. Models that estimate methane released as ice sheets retreat have mostly carried microbial production, which depends on organic matter and temperature beneath the ice and therefore has a ceiling. A geological reservoir behaves differently: where ice acts as a seal, lifting the seal is limited by the stock, and not by a production rate. The authors likewise highlight the need to account for geological methane reservoirs when assessing cryosphere-carbon feedbacks. The observation that would separate the two mechanisms at the northern margin is clear: how both concentration and isotope composition shift within the year, measured in the same catchments at the height of the melt season.[1]