Anthropogenic aerosols as a source of ancient dissolved organic matter in glaciers

Glacier-derived dissolved organic matter represents a quantitatively significant source of ancient, yet highly bioavailable carbon to downstream ecosystems1. This finding runs counter to logical perceptions of age–reactivity relationships, in which the least reactive material withstands degradation the longest and is therefore the oldest2. The remnants of ancient peatlands and forests overrun by glaciers have been invoked as the source of this organic matter1,3,4. Here, we examine the radiocarbon age and chemical composition of dissolved organic matter in snow, glacier surface water, ice and glacier outflow samples from Alaska to determine the origin of the organic matter. Low levels of compounds derived from vascular plants indicate that the organic matter does not originate from forests or peatlands. Instead, we show that the organic matter on the surface of the glaciers is radiocarbon depleted, consistent with an anthropogenic aerosol source. Fluorescence spectrophotometry measurements reveal the presence of protein-like compounds of microbial or aerosol origin. In addition, ultrahigh-resolution mass spectrometry measurements document the presence of combustion products found in anthropogenic aerosols. Based on the presence of these compounds, we suggest that aerosols derived from fossil fuel burning are a source of pre-aged organic matter to glacier surfaces. Furthermore, we show that the molecular signature of the organic matter is conserved in snow, glacier water and outflow, suggesting that the anthropogenic carbon is exported relatively unchanged in glacier outflows. Glaciers and ice sheets combined represent the second largest reservoir of water in the global hydrologic system and glacier ecosystems cover 10% of the Earth, yet the carbon dynamics underpinning these ecosystems remain poorly understood5. Increased understanding of glacier biogeochemistry is a priority, as glacier environments are among the most sensitive to climate warming and industrial forcing. Anthropogenic combustion products are the main sources of the aerosol organic carbon deposited on glacier surfaces and a driver of glacier recession6. However, the biogeochemical role and fate of this aerosol carbon once deposited to glacier ecosystems has not been addressed. Here we demonstrate that anthropogenic aerosol organic matter (OM) is the main source of fossil dissolved organic matter (DOM) in glacier waters and