Glacier Melting Sets Free Organic Carbon

Posted by on Feb 12, 2015

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Research has shown that glaciers have a greater role than was previously known in the movement of organic carbon into and through aquatic ecosystems, including the oceans. Organic Carbon (OC) refers to carbon contained in organic compounds that is originally derived from decaying vegetation, bacterial growth, and metabolic activities of living organisms. It serves as a primary food source for marine organisms, particularly microbes. In addition, it contributes to the acidification of water. Particularly in freshwater ecosystems, excessive OC can result in a brownish coloration. In fact, the amount of OC is often used as an indicator of overall water quality.

Figure 1. Location of glacier DOC samples classified by type. a–d, Samples were collected from a wide variety of glacial environments including: Alaska (a), Tibet (b), Dry Valley glaciers in Antarctica (c), and the Greenland Ice Sheet (d). (Source: Hood et al.)

Figure 1. Location of glacier DOC samples classified by type. a–d, Samples were collected from a wide variety of glacial environments including: Alaska (a), Tibet (b), Dry Valley glaciers in Antarctica (c), and the Greenland Ice Sheet (d). (Source: Hood et al.)

A recent research shows that the increase in glacier runoff through melting and iceberg calving has led to a rise of OC flux entering marine and lacustrine ecosystems, and this flux is expected to grow in the coming decades. According to the article, glacier ecosystems accumulate OC from primary production on the glacier surface, particularly in cryoconite deposits, and also from the deposition of carbonaceous material derived from terrestrial and anthropogenic sources.

To quantify the total storage of OC in terrestrial ice reservoirs, the study integrates measurements of organic carbon from mountain glaciers, ice sheets in Greenland, and Antarctica Ice Sheet, with data from locations that span five continents (see Figure 1). It turns out that that largest amount of OC is located in Antarctica, followed by Greenland and mountain glaciers. However, it is found in the study that a large portion of the OC released from melting glaciers is from mountain glaciers and peripheral glaciers which exit from the Greenland ice sheets (see Figure 2). The surprisingly disproportionately high DOC export from mountain glaciers and Greenland is associated with their glacier mass turnover rate, which is higher than in Antarctica. Even as glaciers are losing ice through melting and caving at their lower ends, they continue to receive new snow at the top, which converts to ice—a process of flow, which contributes to the movement of OC through the glaciers.

Figure 2. Storage and flux of glacier DOC. Total glacier storage of DOC (a) and annual DOC export in glacier runoff (b) for MGL, GIS, and AIS.

Figure 2. Storage and flux of glacier DOC. Total glacier storage of DOC (a) and annual DOC export in glacier runoff (b) for AIS (Antarctic Icesheet), GIS (Greenland Icesheet) and MGL (mountain glaciers). (Source: Hood et al.)

Dissolved organic carbon (DOC) and particulate organic carbon (POC), two major components of the OC, are both significant components in the carbon cycle, because they are primary food sources in aquatic food webs. In particular, DOC forms complexes with trace metals, which can be transported and consumed by organisms. This may have drastic affects on marine life, “because this material is readily consumed by microbes at the bottom of the food chain,” said U.S. Geological Survey research glaciologist and co-author of the research Shad O’Neel. The microbes are an important source of food for plankton and for larger organisms in the seas, including crustaceans and fish.

 

Iceberg Calving (Source: Flickr/Indistinct)

Iceberg Calving (Source: indistinct/Flickr)

The study raises questions of the implications of OC input for carbon dioxide concentration in atmosphere. The authors suggest that glacier-derived OC shows a high degree of biological availability, when compared to other terrestrial sources. Hence, it is more likely to result in more rapid decomposition of dead marine organisms, which otherwise would fall from upper zones of the oceans to deeper sections, where they would remain for long periods. This decomposition, in turn, contributes to carbon dioxide outgassing from the oceans to the atmosphere.

For another story about the effects of glaciers on ocean chemistry and ecology, look here.

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