Coupled carbon‑iron‑phosphorus cycling in the Rainbow hydrothermal vent field

Hydrothermal venting has been shown to play a role in the global biogeochemical cycles of three bio-essential elements: iron (Fe), carbon (C) and phosphorus (P). However, our insight into the coupled cycling of Fe and associated C and P in hydrothermal plumes as well as their long-term fate in the underlying sediments remains limited. We present a detailed study, tracing the biogeochemical pathways of hydrothermally sourced Fe and the associated C and P from a buoyant to a neutrally buoyant plume and to the underlying sediments. Combining chemical and micro(spectro)scopic methods, we characterize particulate and dissolved phases recovered from the water column and sediment at two sites: one located directly in the active Rainbow hydrothermal vent field at 36°N on the mid-Atlantic ridge (MAR) and one located 3 km NE of the active vents. Our results show that the precipitates in one of the largest hydrothermal plumes on the MAR consist of aggregates of Fe nanoparticles, comprising poorly-ordered Fe oxyhydroxides and polycrystalline Fe sulfides, coated with carbon that is likely of organic origin. The sediments underlying the hydrothermal plume show enrichments in organic C, Fe, P and vent-derived trace metals such as Cu that decrease with distance from active vents. The enrichment in organic C and persistence of apparently highly-reactive Fe phases after sediment burial may reflect enhanced preservation potential of both phases as a result of the formation of organic-mineral complexes. We further demonstrate that the scavenging of dissolved seawater phosphate (PO₄³⁻) by Fe nanoparticles is constrained to early stages of particle formation and sorption reactions in the buoyant hydrothermal plume and that P burial close to the vent field is driven by the deposition of the Fe nanoparticles. In the sediment, P is then efficiently retained through sink-switching from Fe-bound to more stable, authigenic apatite phases. As such, the sediments underlying the Rainbow vent seem to faithfully record coupled emission, scavenging and burial of essential elements and therefore offer potential for reconstruction of past venting activity. The deposition and burial of partly reduced plume material (e.g., Fe sulfides) in an oxic deep-sea sediment results in sediment chemistry and diagenesis that is very specific to the hydrothermal environment, while the redox signature of the plume is gradually lost. Beyond its role as a potential source of bioavailable Fe, we highlight how hydrothermal venting represents an efficient sink for organic C and bioavailable P. The findings contribute to understanding the profound impact of geological episodes of increased hydrothermal activity on ocean biogeochemistry and the coupled ocean-climate system.