During the JR19002 Icebergs 3 cruise, dissolved organic carbon samples from the surface seawater were collected from Zodiacs, and samples from the sediment-seawater boundary were collected from the CTD rosette. Samples were collected with the motivation of sampling glacially-derived material and determining the biogeochemical composition and environmental significance with respect to iron and carbon cycling. Three main sites were visited during JR19002 from south to north: Marian Cove, Maxwell Bay, King George Island; Borgen Bay, Anvers Island; and Sheldon Cove, Ryder Bay, Adelaide Island. Funding: NERC grants, NE/P017630/1 (Dr Amber Annett) and NE/P003060/1 (Professor David Barnes).

Glacier meltwater supplies a significant amount of silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these essential nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. The dataset is used to examine the benthic cycling of the two nutrients at Patagonian fjord heads through geochemical analyses of sediment pore waters and reaction-transport modeling for Si. The dataset contains: (i) pore water redox-sensitive nitrate (NO3-) and dissolved manganese (DMn) concentration data, nutrient dissolved silicon (DSi) and iron (DFe) concentration and isotope data (delta30 Si, delta56 Fe); (ii) mild alkaline leachable (Si-Alk) and acid leachable (Si-HCl) sediment silica content and isotope data; and (iii) reaction transport model output for the benthic cycling of Si. The pore water and sediment samples were collected from four sites: SJ (48.228o S, 73.502o W, 106 m depth), SH (47.679 S, 73.715 W, 203 m depth), SP (48.179 S, 73.347 W, 248 m depth) and SB (47.787 S, 73.610 W, 151 m depth) in the Baker-Martinez Fjord Complex on the research vessel Sur-Austral in February 2017. Funded by NERC-CONICYT grant NE/P003133/1-PII20150106.