Vivianite is a key sink for phosphorus in sediments of the Landsort Deep, an intermittently anoxic deep basin in the Baltic Sea

Abstract

a b s t r a c t Phosphorus (P) is an essential nutrient for marine organisms. Its burial in hypoxic and anoxic marine basins is still incompletely understood. Recent studies suggest that P can be sequestered in sediments of such basins as reduced iron (Fe)-P but the exact phase and the underlying mechanisms that lead to its formation are unknown. In this study, we investigated sediments from the deepest basin in the Baltic Sea, the Landsort Deep (site M0063), that were retrieved during the Integrated Ocean Drilling Project (IODP) Baltic Sea Paleoenvironment Expedition 347. The record comprises the whole brackish/marine Littorina Sea stage including past intervals of extensive hypoxia in the Baltic Sea that occurred during the Holocene Thermal Maximum (HTM HI ) and the Medieval Climate Anomaly (MCA1 HI and MCA2 HI ). Various redox proxies (e.g. the presence of laminations and high Mo contents) suggest almost permanent bottom water hypoxia during the Littorina Sea stage in the Landsort Deep. The bottom waters were likely even seasonally anoxic or sulfidic during the MCA1 HI and MCA2 HI , and permanently sulfidic during the HTM HI . With the use of micro-analysis of sieved minerals (SEM-EDS, XRD and synchrotron-based XAS), we show that Mn-and Mgrich vivianite crystals are present at various depths in the Littorina Sea sediments. We also have indications for vivianite in the MCA1 HI , MCA2 HI and HTM HI deposits. The formation of vivianite thus likely explains the high Febound P fraction throughout the whole Littorina Sea stage. Shuttling of Fe and Mn from the shelves into the basin and high inputs of P in settling organic matter are likely key drivers for vivianite formation. Our study shows that vivianite can likely form in near-surface sediments under a broad range of bottom water redox conditions, varying from hypoxic and anoxic to sulfidic.

Holocene Refreshening and Reoxygenation of a Bothnian Sea Estuary Led to Enhanced Phosphorus Burial

“…Major shifts in water column conditions can also affect P diagenesis in the subsurface by creating distinct geochemical gradients. For instance, vivianite crystals can form at lake/marine transitions in sediments, where upward diffusing Fe 2+ from the lacustrine deposits meets PO 4 from the overlying organic-rich marine deposits, as demonstrated recently for the Baltic Sea (Dijkstra et al 2016). Moreover, enhanced input of organic matter may result in an upward shift of the SMTZ within the sediment (Egger et al 2015b).…”

“…The exchangeable P fraction was targeted by an extraction with a magnesium chloride solution (1 M MgCl 2 brought to pH 8 with sodium hydroxide) for 0.5 h. Afterward, Fe-bound P was targeted with a CDB solution (0.3 M trisodium citrate and 25 g/L sodium dithionite buffered to pH ∼ 7.6 in 1 M sodium bicarbonate) for 8 h, followed by a wash step with 1 M MgCl 2 for 0.5 h. In addition to Fe-oxidebound P, reduced Fe-phosphates such as vivianite also dissolve in the CDB step of the SEDEX extraction (Dijkstra et al 2016). The authigenic Ca-P fraction was targeted with a sodium acetate solution (1 M) buffered with acetic acid (pH 4) for 6 h, and again followed by a short 1 M MgCl 2 wash step (0.5 h).…”

“…In many lake sediments, in contrast, bottom water SO 4 2− concentrations are low and may allow for accumulation of Fe 2+ and PO 4 in the porewater and subsequent authigenic vivianite formation (Nriagu and Dell 1974;Rothe et al 2014;O'Connell et al 2015). In sediments of brackish/marine basins, an excess supply of reactive Fe-oxides may also act as a key driver of vivianite formation (Jilbert and Slomp 2013;Dijkstra et al 2016;Reed et al 2016). In addition to vivianite formed near the sediment-water interface, vivianite can also form diagenetically in brackish/marine basins below the sedimentary sulfate/methane transition zone (SMTZ) when porewater Fe 2+ accumulates in the absence of HS − and meets porewater PO 4 (März et al 2008;Hsu et al 2014;Egger et al 2015a).…”

Phosphorus Cycling and Burial in Sediments of a Seasonally Hypoxic Marine Basin

“…This has been attributed to binding of P to Fe-(oxyhydr)oxides, which is supported by the presence of a large stock of Fe-bound P in the surface sediment (Sulu-Gambari et al 2016a). In this latter study, however, the stock of Fe-bound P was determined with an operational SEDEX extraction (Ruttenberg 1992) that can also extract vivianite (Dijkstra et al 2014;Dijkstra et al 2016;Egger et al 2015). Thus, a role for vivianite besides P bound to Fe-(oxyhydr)oxides cannot, at present, be excluded.…”

“…Merging and normalisation of spectra was performed with the Athena data analysis package (Ravel and Newville 2005). For comparison, reference spectra for a suite of P minerals were obtained from Dijkstra et al (2016) and Egger et al (2015) and supplemented with adsorbed-P spectra, taken from the ID21beamline P-XANES spectra database at the ESRF (ID21 P-XANES database).…”