Pyrite-lined shells as indicators of inefficient bioirrigation in the Holocene–Anthropocene stratigraphic record

Tomašových, Adam; Berensmeier, Michaela; Gallmetzer, Ivo; Haselmair, Alexandra; Zuschin, Martin

doi:10.5194/bg-18-5929-2021

Abstract. Although the depth of bioturbation can be estimated on the basisof ichnofabric, the timescale of sediment mixing (reworking) and irrigation(ventilation) by burrowers that affects carbonate preservation andbiogeochemical cycles is difficult to estimate in the stratigraphic record.However, pyrite linings on the interior of shells can be a signature of slowand shallow irrigation. They indicate that shells of molluscs initiallyinhabiting oxic sediment pockets were immediately and permanentlysequestered in reduced, iron-rich microenvironments within the mixed layer.Molluscan biomass-stimulated sulfate reduction and pyrite precipitation wasconfined to the location of decay under such conditions. A high abundance ofpyrite-lined shells in the stratigraphic record can thus be diagnostic oflimited exposure of organic tissues to O2 even when the seafloor isinhabited by abundant infauna disrupting and age-homogenizing sedimentaryfabric as in the present-day northern Adriatic Sea. Here, we reconstructthis sequestration pathway characterized by slow irrigation (1) by assessingpreservation and postmortem ages of pyrite-lined shells of theshallow-infaunal and hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) byevaluating whether an independently documented decline in the depth ofmixing, driven by high frequency of seasonal hypoxia during the 20thcentury, affected the frequency of pyrite-lined shells in the stratigraphicrecord of the northern Adriatic Sea. First, at prodelta sites with a highsedimentation rate, linings of pyrite framboids form rapidly in the upper5–10 cm as they already appear in the interiors of shells younger than 10 yearsand occur preferentially in well-preserved and articulated shells withperiostracum. Second, increments deposited in the early 20th centurycontain < 20 % of shells lined with pyrite at the Po prodelta and30 %–40 % at the Isonzo prodelta, whereas the late 20th centuryincrements possess 50 %–80 % of shells lined with pyrite at both locations.At sites with slow sedimentation rate, the frequency of pyrite linings islow (< 10 %–20 %). Surface sediments remained well mixed by depositand detritus feeders even in the late 20th century, thus maintainingthe suboxic zone with dissolved iron. The upcore increase in the frequencyof pyrite-lined shells thus indicates that the oxycline depth was reducedand bioirrigation rates declined during the 20th century. Wehypothesize that the permanent preservation of pyrite linings within theshells of V. gibba in the subsurface stratigraphic record was enabled by slowrecovery of infaunal communities from seasonal hypoxic events, leading tothe dominance of surficial sediment modifiers with low irrigation potential.The presence of very young and well-preserved pyrite-lined valves in theuppermost zones of the mixed layer indicates that rapid obrution by episodicsediment deposition is not needed for preservation of pyrite linings whensediment irrigation is transient and background sedimentation rates arenot low (here, exceeding ∼ 0.1 cm yr−1) and infaunal organismsdie at their living position within the sediment. Abundance ofwell-preserved shells lined by pyrite exceeding ∼ 10 % perassemblage in apparently well-mixed sediments in the deep-time stratigraphicrecord can be an indicator of inefficient bioirrigation. Fine-grainedprodelta sediments in the northern Adriatic Sea deposited since themid-20th century, with high preservation potential of reducedmicroenvironments formed within a mixed layer, can represent taphonomic andearly diagenetic analogues of deep-time skeletal assemblages with pyritelinings.

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