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Glacial runoff contains relatively low levels of dissolved organic matter1. However, this material punches well

above its weight, stimulating biological productivity in recipient rivers and coastal waters2. Glacier-dissolved organic matter is old (radiocarbon measurements reveal ages of 2,800–3,900 years), making its high reactivity something of a surprise; ancient dissolved organic matter in other settings is biologically unreactive3. Writing in Nature Geoscience, Stubbins et al.4 show that anthropogenic aerosols, rich in fossil carbon, account for much of the ancient organic matter found in glacier runoff.

Glaciers in Alaska and Greenland, as well as in Svalbard and Iceland, blanket large areas of land. During spring and summer, when the snow and ice start to melt, glacial waters drain into nearby rivers and coastal waters. Dissolved organic carbon carried in these waters is thought to influence coastal marine ecosystems; for example, by stimulating heterotrophic activity2. Heterotrophs cannot produce their own organic matter by photosynthesis, and are therefore dependant on external sources.

Stubbins and colleagues4 examined the composition of dissolved organic matter in outflows, surface water, ice and snow associated with two glaciers in southeast Alaska to determine the source of this ecologically important material. The glacial organic matter contained large quantities of biologically reactive protein-like compounds, which could arise from biological activity on the glacier surface. However, the presence of fatty acids with long chain lengths, and sulphur- and nitrogen-containing organics, point to anthropogenic aerosols as a source5. Also present was an abundant supply of condensed aromatic compounds, which are common in soot aerosols and can only be formed by the thermal degradation of organic matter6. The age of the organic matter — which ranged from 2,640–7,800 years old— matched that of a depositional source heavily influenced by

the combustion of fossil fuels. Using these findings, Stubbins et al. suggest that much of the ancient organic matter in glacier outflow is sourced from the deposition of anthropogenic aerosols (Fig. 1).

Given the predominance of anthropogenic organic matter in glacier waters, Stubbins et al. suggest that the influence of glacier runoff on coastal ecosystems2 is an industrial-age phenomenon, fuelled by anthropogenic aerosols. As such, the structure of coastal food webs fed by glacial waters may have changed over the past century as a consequence of rising levels of anthropogenic aerosols. However, the implications of their findings are not confined to glacially fed coastal waters. The remote surface waters of the open ocean, for example, receive rain that also contains the same subsidy of ancient biologically

reactive organic matter. These surface waters are far from rivers and are starved of digestible inputs of organic matter from anything other than rain and dust7.

Microbiologists have a rule of thumb: wherever there is water, there is life8. Glacier surfaces are no exception9. A host of microorganisms thrive in the cold, watery environments that form on glaciers during spring and summer — a phenomenon known as glacier greening. Habitats range from pools on glacier surfaces to the wet, rotten ice that typifies many flatter glacier surfaces during summer10. Inhabitants include photosynthetic cyanobacteria and algae, which fix organic matter, and heterotrophic microorganisms that degrade organic matter. One would imagine that heterotrophic microorganisms would readily make use of the biologically active organic components in snow and ice melt,

BIOGEOCHEMISTRY

Ancient organics reign on glaciersGlaciers supply downstream ecosystems with reactive dissolved organic carbon during periods of ice and snow melt. An analysis of glacier meltwaters in Alaska shows that anthropogenic aerosols fertilize these waters, raising questions about glacier greening.

Martyn Tranter

=Anthropogenicorganic matter

Figure 1 | Deposition of anthropogenic aerosols on glacier surfaces. Stubbins et al.4 show that anthropogenic aerosols deliver biologically available organic matter (blue dots) to the surface of glaciers in Alaska . During periods of ice melt (right), this material is transported downstream to rivers and coastal waters, where it can stimulate the growth and activity of aquatic organisms. b, The influence of anthropogenic aerosols is unlikely to be confined to glacier-fed coastal waters alone — deposition on the surface of the remote open ocean (left) should also stimulate marine productivity.

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and release them only reluctantly, perhaps as a consequence of viral infection11. But the findings of Stubbins et al. — which testify to the presence of bioavailable organic matter in glacier runoff — could be evidence for a more biologically passive or neutral role for glacier surfaces. In this case, the study will stimulate great debate among cryospheric microbiologists, who will no doubt want to show that microbes really do consume the old biologically reactive organic matter in glacier waters. The stage is set for more exciting and provocative advances in the field.

Stubbins and colleagues4 show that anthropogenic aerosols could serve as a significant source of carbon for glacially

influenced ecosystems. But, like all good scientific advances, more riddles are thrown up than are answered. Often, they extend beyond the confines of the original subject, rather like old organic matter falling on the remote ocean surface. Here the characterization of aquatic dissolved organic matter in glacial environments has raised questions about the anthropogenic perturbation of the carbon cycle in coastal waters and the remote open ocean, as well as the nature of carbon uptake and release by a variety of marine and glacial microorganisms. Stubbins et al. show that ancient organics reign on glaciers, but their effects may be spread into much wider kingdoms. ❐

References1. Hood, E. & Scott, D. Nature Geosci. 1, 583–587 (2008).2. Fellman, J. et al. Mar. Chem. 121, 112–122 (2010).3. Hood, E. et al. Nature 462, 1044–1048 (2009).4. Stubbins, A. et al. Nature Geosci. 5, XXX–YYY (2012).5. Grannas, A. M., Hockaday, W. C., Hatcher, P. G., Thompson, L. G.

& Mosley-Thompson, E. J. Geophys. Res. Lett. 111, D04304 (2006).6. Koch, B. P. & Dittmar, T. Rapid Commun. Mass Sp.

20, 926–932 (2006).7. Chester, R. Marine Geochemistry 1st edn (Allen & Unwin, 1990).8. Priscu, J. C. et al. Science 280, 2095–2098 (1998).9. Anesio, A. M., Hodson, A. J., Fritz, A., Psenner, R. & Sattler, B.

Glob. Change Biol. 15, 955–960 (2009).10. Hodson, A. et al. Ecol. Monogr. 78, 41–67 (2008).11. Anesio, A. M. & Bellas, C. M. Trends Microbiol. 19, 52–57 (2011).

Martyn Tranter is at the Bristol Glaciology Centre, University of Bristol, University Road, Clifton, Bristol, BS8 1SS, UK. e-mail: m.tranter@bristol.ac.uk