Carbon burial in two Greenland fjords shows no direct link to glacier type

Fjord systems play a crucial role in the burial and long-term storage of organic carbon (OC). Despite their importance, Greenland's fjords remain underrepresented in global carbon budgets, even though accelerated melt of the Ice Sheet alters these ecosystems through increased freshwater discharge and iceberg calving, ultimately driving glacier retreat inland. This study compares sediment TOC, TN, and Chl-a content as well as δ13C, δ15N and organic carbon burial rates (OCBRs) in two neighbouring Greenland fjords: Nuup Kangerlua, dominated by marine-terminating glaciers (MTGs), and Ameralik, influenced by a land-terminating glacier (LTG). Although subglacial upwelling enhances primary productivity in Nuup Kangerlua, this does not translate into correspondingly higher surface sediment organic matter content or significantly higher OCBRs compared to Ameralik, where no such upwelling occurs. Instead, the average OCBRs were similar between the two fjords with 18.0 ± 1.6 g C m−2 yr−1 in Nuup Kangerlua and 16.2 ± 1.7 g C m−2 yr−1 in Ameralik. In Nuup Kangerlua, sediment Chl-a content in the upper 10 cm ranged from 0.08 to 9.8 µg g−1 and TOC from 0.05 \% to 1.32 \%, whereas in Ameralik they ranged from 0.35 to 20.1 µg g−1 and 0.13 \% to 2.43 \%, respectively. The elevated values in Ameralik are linked to a deep depositional basin that promotes OC accumulation and strongly contributes to the relatively high average OCBR. Furthermore, between 8 \% and 28 \% of the annual surface production in Nuup Kangerlua is ultimately buried in the sediments, whereas this proportion is substantially higher in Ameralik: 25 \% to 62 \%. The weaker coupling between surface production and sedimentary OC burial in Nuup Kangerlua versus Ameralik underscores the need for further research to disentangle the interactions driving primary production, carbon transfer in the food web, and the lateral and vertical transport, degradation and preservation of OC in fjord sediments.