Radiocarbon signature of dissolved organic carbon (DOC) in arctic rivers - An evaluation of DOC extraction methods and a Lena Delta case study

• Main Author: Grotheer, Hendrik
• Format: Thesis
• Language: unknown
• Published: 2012
• Subjects: Arctic; Arctic Ocean; Climate change; Global warming; lena delta; permafrost; Siberia
• Online Access: https://epic.awi.de/id/eprint/37413/; https://epic.awi.de/id/eprint/37413/1/Grotheer_MasterThesis.pdf; https://hdl.handle.net/10013/epic.45062; https://hdl.handle.net/10013/epic.45062.d001

Riverine discharge of dissolved organic carbon (DOC) represents an important part of the global carbon cycle. Substantial effort is being invested to determine fluxes, composition and isotopic signature of terrestrial DOC to the oceans and its influence on climate change. However, all predictions of the climate change are depending on the quality of the data that’s being used. Unfortunately, there is a severe lack of knowledge about the DOC data quality, due to under-investigated methodological constrains during riverine DOC sample processing. The first part of this thesis compares three extraction methods commonly used to determine the stable and radiogenic carbon signature of DOC. Ultraviolet-oxidation, roto-evaporation and solid phase extraction were compared to unravel blank carbon incorporation, data reliability and methodological limitations. Ultraviolet-oxidation shows small blank incorporation but provides most reliable bulk DO14C data. The roto-evaporation results in smallest blank carbon incorporation but also the removal of non-polar low-molecular-weight components. However, this does not alter the bulk DO14C signature for samples from temperate and arctic regions. The roto-evaporation has been proven to be an inexpensive and reliable alternative to the ultraviolet-oxidation. Solid phase extraction showed highest blank carbon incorporation and additionally discrimination against the bioactive fraction of DOC. Uncertainties about the blank incorporation and the removal of the important bioactive DOC makes the solid phase extraction an inappropriate method to extract riverine DOC. The second party applies the roto-evaporation method to a set of Lena Delta (northeast Siberia) samples in preparation for DO14C measurements. Stable and radiogenic isotopy reveals that the DOC in the Delta originates from local litter leaching and near-surface chemical weathering of Holocene soils. Upstream DOC is been removed en route to the Delta, suggesting that the majority of export carbon to the Arctic Ocean originates from coastal-near regions. Progressive isotopic depletion of coastal near DO14C during the late season suggests increasing permafrost thawing in response to global warming. The increase in thawing depth is highest close to the Arctic Ocean and is weakened towards the Siberian hinterland. However, no such trend could be observed for North American permafrost suggesting that Siberian permafrost and climate response faster to and will be more influenced by ongoing climate changes.