Spatial and temporal changes of dissolved and particular organic carbon in Iceland glaciers and glacier-fed streams

Glaciers are unique ecosystems with the potential to affect the aquatic carbon cycle, accumulating and releasing organic carbon (OC). OC stored in glaciers may be released as dissolved and particulate OC (DOC and POC), primarily via meltwater at the glacier’s surface into glacier-fed streams. Current global projections indicate an export of 78 Tg POC by 2050, representing more than double the DOC export (32.4 Tg DOC) predicted for the same period. However, POC data for glacier runoff is very limited and existing predictions are primarily based on an integrated approach, using single ice sampling points and mass balances to calculate an average annual export of glacier derived DOC. But this mass balance approach does not account for potential seasonal changes in OC, and may therefore not accurately reflect glacial OC export rates. Additionally, Icelandic glaciers are not included in global predictions of OC export, which is surprising as the largest nonpolar ice cap of Europe (Vatnajökull) is located in Iceland.

Therefore, we analyzed the concentration of DOC and POC, as well as its optical properties (absorbance and fluorescence) in glaciers and glacier-fed streams of Iceland. Sampling points covered ice samples from several glaciers of the Icelandic ice caps Vatnajökull, Langjökull, Hofsjökull, Myrdalsjökull, and Snaefellsjökull (110 ice samples) and water samples of the corresponding glacier-fed rivers (300 water samples) at the glacier termini. The majority of these sampling points were sampled seasonally (winter, spring, summer, autumn) and two times per day to cover temporal changes.

First results show, that DOC and POC concentrations in glacier-fed streams were found to range from 0.03 mg l-1 to 20.1 mg l-1, and from 0.1 mg l-1 to 33.0 mg l-1, respectively, whereas DOC and POC concentrations in glacier ice were found to range from 0.09 mg l-1 to 2.24 mg l-1, and from 0.3 mg l-1 to 39.4 mg l-1, respectively. Based on optical properties, we found that DOM is more proteinaceous and of recent origin (fresher) in summer and autumn. In contrast, DOM is more refractory with a higher contribution of a humic-like component in winter and spring. Based on the concentrations in glacier-fed streams we estimate an annual release of 0.032 Tg C yr-1 (DOC) and 0.128 Tg C yr-1 (POC) from Icelandic glaciers, assuming a mean glacier runoff of 1,500 m³ s-1 from the glaciers, and using the mean concentration of DOC and POC from our seasonal sampling points directly at the glacier terminus. If the annual release of DOC is weighted by the glaciated area of Iceland (11,060 km²), the calculated value is 0.0029 Gg C yr-1 km^-², clearly exceeding the area-weighted estimations of the Greenland Ice Sheet and the European Alps (0.0002 Gg C yr-1 km-² each).

Here for the first time, we analyzed the concentration of DOC and POC as well as its optical properties in proglacial streams of Iceland, location of Europe’s largest nonpolar ice cap, and thus established a comprehensive basis for improved prediction of global export of OC from glaciers.