Spatial heterogeneity of sedimentary organic carbon in fjords around Stavanger, Norway – implications for upscaling

Abstract. Fjords are steep sided glacially carved troughs that have been inundated by the sea. Several global assessments have aimed to establish the role of fjords in the carbon cycle. According to these studies, fjords bury 18 Tg of organic carbon per year, 55 % to 62 % of that organic carbon is terrestrially sourced and 61 ± 16 % of the organic carbon in fjord sediments is thermally labile. Such quantitative estimates, while important for understanding the role of fjords in the global carbon cycle, often rest on data compilations that might not be representative for fjord environments as a whole and assumptions that might not hold. To test such assumptions, we present a local case study from fjords around Stavanger (Norway). Based on detailed investigations, we show that the seabed is heterogeneous in terms of substrate types covering the full grain-size spectrum from mud to boulders. Seabed areas where fine-grained sediment, and hence organic carbon, accumulates account for 50 % of the area while the remainder is characterised by coarse-grained sediment indicating erosion and transport. In depositional areas, rates of organic carbon accumulation vary between 18.7 g m⁻² yr⁻¹ and 82.6 g m⁻² yr⁻¹ and stocks from 0.1 kg m⁻² and 1.37 kg m⁻². The fraction of labile organic matter varies between 19 % and 44 %, while δ¹³C-values of the organic carbon fraction range from −27.44 ‰ to −21.23 ‰, indicating a strong variability of the sources of organic carbon over a comparatively small area. Taken together, these results attest to high environmental variability and spatial heterogeneity in the study site, putting several assumptions used in global assessments into question. We suggest steps to achieve more realistic results when upscaling from local studies to a higher level. Using available data on organic carbon accumulation rates from Norwegian coastal areas, we demonstrate how local results could be upscaled in a more robust way. We arrive at a tentative estimate of 0.41 –3.68 Tg yr⁻¹ of organic carbon accumulating in surface sediments (upper 10 cm) of fjords in mainland Norway.