Post-depositional formation of vivianite-type minerals alters sediment phosphorus records

Phosphorus (P) concentrations in sediments are
frequently used to reconstruct past environmental conditions
in freshwater and marine systems, with high values thought
to be indicative of a high biological productivity. Recent
studies suggest that the post-depositional formation of vivianite,
an iron(II)-phosphate mineral, might significantly
alter trends in P with sediment depth. To assess its importance,
we investigate a sediment record from the Bornholm
Basin that was retrieved during the Integrated Ocean
Drilling Program (IODP) Baltic Sea Paleoenvironment Expedition
347 in 2013, consisting of lake sediments overlain
by brackish–marine deposits. Combining bulk sediment geochemistry
with microanalysis using scanning electron microscope
energy dispersive spectroscopy (SEM-EDS) and
synchrotron-based X-ray absorption spectroscopy (XAS), we
demonstrate that vivianite-type minerals rich in manganese
and magnesium are present in the lake deposits just below
the transition to the brackish–marine sediments (at 11.5 to
12 m sediment depth). In this depth interval, phosphate that
diffuses down from the organic-rich, brackish–marine sediments
meets porewaters rich in dissolved iron in the lake sediments,
resulting in the precipitation of iron(II) phosphate.
Results from a reactive transport model suggest that the peak
in iron(II) phosphate originally occurred at the lake–marine
transition (9 to 10 m) and moved downwards due to changes
in the depth of a sulfidization front. However, its current position
relative to the lake–marine transition is stable as the
vivianite-type minerals and active sulfidization fronts have
been spatially separated over time. Experiments in which
vivianite was subjected to sulfidic conditions demonstrate
that incorporation of manganese or magnesium in vivianite
does not affect its susceptibility to sulfide-induced dissolution.
Our work highlights that post-depositional formation
of iron(II) phosphates such as vivianite has the potential
to strongly alter sedimentary P records particularly in systems
that are subject to environmental perturbation, such as a
change in primary productivity, which can be associated with
a lake–marine transition.