Tight benthic-pelagic coupling drives seasonal and interannual changes in iron‑sulfur cycling in Arctic fjord sediments (Kongsfjorden, Svalbard)

• Published in: Journal of Marine Systems
• Main Authors: Herbert, Lisa C., Michaud, Alexander B., Laufer-Meiser, Katja, Hoppe, Clara J.M., Zhu, Qingzhi, Aller, Robert C., Jørgensen, Bo Barker, Wehrmann, Laura M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2022
• Subjects: Arctic; Svalbard; Climate change; glacier; Kongsfjord*; Kongsfjorden; Phytoplankton
• Online Access: https://oceanrep.geomar.de/id/eprint/54218/; https://oceanrep.geomar.de/id/eprint/54218/1/Herbert%20et%20al.pdf; https://doi.org/10.1016/j.jmarsys.2021.103645

Highlights • Investigation of seasonal Fe-S-C cycling in Arctic fjord sediments and water column. • Results show benthic respiration and increased benthic Fe2+ flux over winter. • Findings suggest that fjord sediments respond rapidly to water column changes. • With glacial retreat, fjords may produce less benthic Fe and sequester less carbon. Glaciated fjords are dynamic systems dominated by seasonal events such as spring phytoplankton blooms and pulses of glacial sediment-bearing meltwater delivery. These fjords are also characterized by strong spatial gradients in environmental factors such as sedimentation rate and primary productivity from the glacier-influenced head to the marine-influenced mouth. Such seasonal variations and spatial gradients, combined with the ongoing influence of climate change, generate non-steady state conditions, which have a strong impact on the mineralization of organic carbon in the fjord sediments and the flux of nutrients from the seabed. In order to investigate the role of fjord seasonal events and variability on diagenetic cycling of iron (Fe) and sulfur (S), we sampled Kongsfjorden (Svalbard, 79°N) in the spring, mid-summer, and late summer. We investigated sediment structure and biogeochemistry, conducted laboratory experiments to determine reaction rates, and compared these findings to water column productivity and turbidity. We found that rapid sedimentation near the glacial input buried algal matter-rich layers that fueled sub-surface peaks in mineralization rates over multi-year timescales. Sulfate reduction rates were limited by organic carbon availability and competition with Fe-reducers, while Fe reduction was controlled by the availability of reactive Fe(III) oxides. Pore water Fe2+ concentrations were influenced by sulfur cycling pathways and abiotic reactions such as carbonate precipitation and potentially reverse weathering. Seasonal changes in sedimentation and organic carbon supply caused lower sulfate reduction and sulfide production rates in spring, driving ...