Phosphorus burial in vivianite-type minerals in methane-rich coastal sediments

L Joëlle Kubeneck,Wytze K Lenstra,Sairah Y Malkin,Daniel J Conley,Caroline P Slomp

doi:10.1016/j.marchem.2021.103948

Abstract

Sediments are a key sink for phosphorus (P) in coastal systems. This allows coastal areas to act as a filter for P that is transported from land to sea. Recent work suggests that vivianite-type ferrous iron (Fe(II))-P minerals may be more important as a sink for P in coastal sediments than previously thought. Here, we investigate the occurrence of such vivianite-type minerals in sediments of three eutrophic coastal sites with contrasting dynamics with respect to iron (Fe) and sulfur (S), covering a salinity range of 0 to 7. We only find authigenic vivianite-type minerals at the low and intermediate salinity sites, where Fe is available in excess over sulfide production. Sequential extractions combined with SEM-EDS and μXRF analysis point towards substitution of Fe in vivianite-type minerals by other transition metal cations such as magnesium and manganese, suggesting potentially different formation pathways modulated by metal cation availability. Our results suggest that vivianite-type minerals may act as a key sink for P in sediments of many other brackish coastal systems. Climate change-driven modulations of coastal bottom water salinity, and hence, Fe versus S availability in the sediment, may alter the role of vivianite-type minerals as a P burial sink over the coming decades. Model projections for the Baltic Sea point towards increased river input and freshening of coastal waters, which could enhance P burial. In contrast, sea level rise in the Chesapeake Bay area is expected to lead to an increase in bottom water salinity and this could lower rates of P burial or even liberate currently buried P, thereby enhancing eutrophication.

Highlights

Anthropogenic activities have increased terrestrial inputs of phos phorus (P) to coastal systems worldwide (Conley, 1999; Kemp et al, 2009; Slomp, 2011)
The sediments were organic-rich at all sites, with organic carbon contents ranging from 3.3 wt% at CB2.2 to ~6 wt% at the other two sites (Table 1, Supplemental Fig. S7)
Phosphorus entering the coastal zone from land does not necessarily reach the open sea as reactive P can be permanently removed from the water column via burial in coastal sediments (Froelich et al, 1982; Ruttenberg, 2003; Bouwman et al, 2013)

Summary

Introduction

Anthropogenic activities have increased terrestrial inputs of phos phorus (P) to coastal systems worldwide (Conley, 1999; Kemp et al, 2009; Slomp, 2011). Phosphorus can be removed from coastal waters through burial in sediments (Rut tenberg, 2003). Vivianite forms in sediments where dissolved phosphate (HPO42− ) and ferrous Fe (Fe2+) in porewaters are elevated. High porewater Fe2+ is observed when free dissolved sulfide (here, denoted as HS− ) formed during sulfate (SO42− ) reduction is low compared to the input of Fe (oxyhydr)oxides. Under these conditions, not all Fe-oxides are scav enged by HS− for the formation of Fe sulfides (FeSx; Berner, 1984). Vivianite formation is mostly observed below the sulfate-methane transition zone (SMTZ; Marz et al, 2008; Egger et al, 2015a)

Methods

Results

Discussion

Conclusion