Exceptional sulfur and iron isotope enrichment in millimetre-sized, early Palaeozoic animal burrows

Pyrite-δ³⁴S and -δ⁵⁶Fe isotopes represent highly sensitive diagnostic paleoenvironmental proxies that express high variability at the bed (< 10 mm) scale that has so far defied explanation by a single formative process. This study reveals for the first time the paleoenvironmental context of exceptionally enriched pyrite-δ³⁴S and -δ⁵⁶Fe in bioturbated, storm-reworked mudstones of an early Ordovician storm-dominated delta (Tremadocian Beach Formation, Bell Island Group, Newfoundland). Very few studies provide insight into the low-temperature sulfur and iron cycling from bioturbated muddy settings for time periods prior to the evolution of deep soil horizons on land. Secondary ion mass spectroscopy (SIMS) analyses performed on Beach Formation muddy storm event beds reveal spatially distinct δ³⁴S and δ⁵⁶Fe values in: (a) tubular biogenic structures and trails (δ³⁴S ~ +40‰; δ⁵⁶Fe ~ -0.5‰), (b) silt-filled Planolites burrows (δ³⁴S ~ +40‰; δ⁵⁶Fe ~ +0.5 to + 2.1‰), and (c) non-bioturbated mudstone (δ³⁴S ~ +35‰; δ⁵⁶Fe ~ +0.5‰). δ³⁴S values of well above + 40.0‰ indicate at least some pyrite precipitation in the presence of a ³⁴S-depleted pore water sulfide reservoir, via closed system (Raleigh-type) fractionation. The preferential enrichment of ⁵⁶Fe in Planolites burrows is best explained via microbially-driven liberation of Fe(II) from solid iron parent phases and precipitation from a depleted ⁵⁴Fe dissolved Fe(II) reservoir. Rigorous sedimentological analysis represents a gateway to critically test the paleoenvironmental models describing the formation of a wide range of mudstones and elucidates the origins of variability in the global stable S and Fe isotope record.