Cryptic Fe-P cycling in sediments of a seasonally euxinic coastal system

Anthropogenic phosphorus (P) loading is a key driver of eutrophication and deoxygenation of coastal 
marine ecosystems. Vivianite, an authigenic Fe(II)-P mineral, can act as a major sink for P in coastal 
sediments. In brackish and marine systems, vivianite formation is typically observed below the sulfate
methane transition zone (SMTZ), where Fe²⁺ is not scavenged to form Fe sulfides. However, vivianite 
has also been detected in euxinic coastal systems, highlighting the need to better understand P cycling 
under these conditions.  

Here, we investigated how seasonal variations in bottom water redox conditions impact P recycling and 
burial in a seasonally euxinic marine coastal basin (Scharendijke basin, Lake Grevelingen, the 
Netherlands) using water column chemistry, porewater geochemistry, solid-phase extractions and 
mineralogical analyses (light microscopy, micro-XRF and SEM-EDS).  

Our data reveals cryptic Fe-P cycling in the surface sediments, with transformation of Fe-oxide bound 
P to vivianite occurring above the SMTZ during spring and summer, when bottom waters shift from 
oxic to euxinic. We show that dissimilatory Fe reduction supplies porewater Fe²⁺ that reacts with 
phosphate to form vivianite, which we confirmed by a combination of light microscopy, micro-XRF 
and SEM-EDS. High sedimentation rates promote the rapid burial of these vivianite-rich layers below 
the SMTZ, preventing later exposure to sulfide and ensuring permanent P burial.

Our results indicate that vivianite formation and preservation in sediments overlain by seasonally 
euxinic bottom waters reflect an interplay between Fe availability, sulfide production, phosphate supply, 
and sedimentation rate. Porewater profiles observed in the Baltic Sea suggest that vivianite formation 
above the SMTZ may occur across a range of sedimentary settings. These findings highlight a 
previously unrecognized pathway of P transformation in surface sediments of brackish and marine 
coastal systems.