Organic Carbon Dynamics in Fjords: A Case Study of Fiordland, New Zealand

Fjords have been identified as sites of enhanced organic carbon (OC) burial and may play an important role in regulating climate change on glacial-interglacial timescales. In this study, we use sediments from Fiordland, New Zealand to examine the relative importance of different terrestrially derived OC in fjord sediments and assess the role of hydrodynamic sorting in distributing sediments and OC along fjords. Sediments and terrestrial end-members were collected for organic geochemistry analysis to examine the efficiency of terrestrial OC burial, both in this system and in fjords globally. In addition, selective surface sediment samples were used for density fractionation separation, bulk OC and compound specific radiocarbon analysis (CSRA). End-member models indicated that 69% and 61% of OC in Fiordland and global fjord sediments were terrestrially derived, respectively. There was a decreasing trend of terrestrially derived OC down fjords, which was contributed by hydrodynamic sorting. Comparatively, less hydrodynamic sorting in fjords than large river estuary –coastal systems contributed to high burial rates of OC in fjords. However, our bulk OC radiocarbon and CSRA results suggested minimal contribution of petrogenic OC by bedrock erosion. Here we suggest that global fjords may play an important role in regulating atmospheric CO₂ levels by burying purely modern OC, in contrast to other systems. Globally, fjords bury ~11.3 ± 4.4 × 10¹² g OC_terr, which accounts for ~18% of all the OC_terr buried in all marine sediments annually. This new information has now increased the total global OC_terr pool buried in marine sediments from 58 to 63 (× 10¹² g), which better constrains the "missing sink" in OCterr burial in the global ocean. Our results indicate that watershed soil residence times, fjord water depth, and dissolved oxygen levels were more important than mass accumulation rates (MAR) in controlling OC burial.