Abstract. Although the depth of bioturbation can be estimated on the basis
of ichnofabric, the timescale of sediment mixing (reworking) and irrigation
(ventilation) by burrowers that affects carbonate preservation and
biogeochemical cycles is difficult to estimate in the stratigraphic record.
However, pyrite linings on the interior of shells can be a signature of slow
and shallow irrigation. They indicate that shells of molluscs initially
inhabiting oxic sediment pockets were immediately and permanently
sequestered in reduced, iron-rich microenvironments within the mixed layer.
Molluscan biomass-stimulated sulfate reduction and pyrite precipitation was
confined to the location of decay under such conditions. A high abundance of
pyrite-lined shells in the stratigraphic record can thus be diagnostic of
limited exposure of organic tissues to O2 even when the seafloor is
inhabited by abundant infauna disrupting and age-homogenizing sedimentary
fabric as in the present-day northern Adriatic Sea. Here, we reconstruct
this sequestration pathway characterized by slow irrigation (1) by assessing
preservation and postmortem ages of pyrite-lined shells of the
shallow-infaunal and hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) by
evaluating whether an independently documented decline in the depth of
mixing, driven by high frequency of seasonal hypoxia during the 20th
century, affected the frequency of pyrite-lined shells in the stratigraphic
record of the northern Adriatic Sea. First, at prodelta sites with a high
sedimentation rate, linings of pyrite framboids form rapidly in the upper
5–10 cm as they already appear in the interiors of shells younger than 10 years
and occur preferentially in well-preserved and articulated shells with
periostracum. Second, increments deposited in the early 20th century
contain < 20 % of shells lined with pyrite at the Po prodelta and
30 %–40 % at the Isonzo prodelta, whereas the late 20th century
increments possess 50 %–80 % of shells lined with pyrite at both locations.
At sites with slow sedimentation rate, the frequency of pyrite linings is
low (< 10 %–20 %). Surface sediments remained well mixed by deposit
and detritus feeders even in the late 20th century, thus maintaining
the suboxic zone with dissolved iron. The upcore increase in the frequency
of pyrite-lined shells thus indicates that the oxycline depth was reduced
and bioirrigation rates declined during the 20th century. We
hypothesize that the permanent preservation of pyrite linings within the
shells of V. gibba in the subsurface stratigraphic record was enabled by slow
recovery of infaunal communities from seasonal hypoxic events, leading to
the dominance of surficial sediment modifiers with low irrigation potential.
The presence of very young and well-preserved pyrite-lined valves in the
uppermost zones of the mixed layer indicates that rapid obrution by episodic
sediment deposition is not needed for preservation of pyrite linings when
sediment irrigation is transient and background sedimentation rates are
not low (here, exceeding ∼ 0.1 cm yr−1) and infaunal organisms
die at their living position within the sediment. Abundance of
well-preserved shells lined by pyrite exceeding ∼ 10 % per
assemblage in apparently well-mixed sediments in the deep-time stratigraphic
record can be an indicator of inefficient bioirrigation. Fine-grained
prodelta sediments in the northern Adriatic Sea deposited since the
mid-20th century, with high preservation potential of reduced
microenvironments formed within a mixed layer, can represent taphonomic and
early diagenetic analogues of deep-time skeletal assemblages with pyrite
linings.
Behaviour of Metal(loid)s at the Sediment-Water Interface in an Aquaculture Lagoon Environment (Grado Lagoon, Northern Adriatic Sea, Italy)