Tracing particulate organo-mineral export by a large glacio-nival river and a small headwater stream in Zackenberg valley, Greenland

Abstract:

Organo-mineral interactions play an important role in organic matter (OM) availability in soils, rivers and marine sediments, thereby mediating the release of carbon to the atmosphere. Yet how these interactions evolve during transport along the fluvial continuum is still poorly understood. Here we investigate the fate of OM released from disturbed permafrost soils to the aquatic environment. In Northeastern Greenland, we collected soil samples in a thermo-erosion gully and a retrogressive thaw slump; suspended solids and stream sediments along a glacio-nival river (Zackenberg River), including its tributaries, and a small headwater stream (Grænselv) affected by thermokarst features, both during a prolonged dry period and after a rain event. To evaluate the mobilized organic and mineral material transported during these different hydrological settings, we analyzed total suspended solids (TSS) and particulate organic carbon (POC) concentrations in the water and total organic carbon concentrations in the sediments along with bulk carbon isotopes (¹³C and ¹⁴C).

Preliminary results show large contrasts in TSS and POC concentrations as well as POC-Δ¹⁴C between the glacio-nival river and the headwater stream. These respective waterways respond differently to the rain event with the suspended load increasing by two orders of magnitude in the headwater stream but slightly decreasing in the larger Zackenberg River. POC concentrations before the rain event were generally higher for Zackenberg River than for Grænselv, however, the ratio of POC to TSS was significantly higher for the small stream. POC-Δ¹⁴C values were substantially lower in the vicinity of the eroding stream bank for Grænselv, whereas the Zackenberg main stem displayed fairly constant Δ¹⁴C values, despite the retrogressive thaw slump providing old POC. Relatively organic-rich particles low in Δ¹⁴C were also delivered by Zackenberg tributaries draining organic-rich topsoils underlain by sedimentary bedrock with ice-rich permafrost.

Molecular biomarker analyses will provide additional information on specific OM sources and the degradation status. Together with X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) analyses on the suspended mineral load, we aim to improve our understanding of the mineral matrix and its associations with OM.