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

Abstract

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.

SIMS Iron Isotope Measurements of the Balmat Pyrite Reference Material: A Non‐Unique δ56Fe Signature

“…The number of microscale δ 56 Fe analyses in pyrite, measured either by SIMS and/or LA-MC-ICP-MS, has increased drastically over the last 10 years (Agangi et al 2015, Czaja et al 2018, Decraene et al 2021a, Decraene et al 2021b, Dupeyron et al 2023, Galić et al 2017, Harazim et al 2020, Liu et al 2022, Marin-Carbonne et al 2020, Marin-Carbonne et al 2014, Nishizawa et al 2010, Virtasalo et al 2015, Virtasalo et al 2013, Yoshiya et al 2012, Yoshiya et al 2015a, Yoshiya et al 2015b, Zhang et al 2022a, Zhang et al 2022b, Zheng et al 2018. The pioneering work of Whitehouse and Fedo (2007) developed and proposed to use natural pyrite originating from the Balmat deposit (Adirondack Mountains, New York) as a Fe reference material for SIMS measurements.…”

“…2021b, Harazim et al . 2020, Whitehouse and Fedo 2007) and/or internal grain scale (e.g., Marin‐Carbonne et al . 2014).…”

“…However, in situ δ 56 Fe measurements by secondary ion mass spectrometry (SIMS) and/or laser ablation coupled to a multi-collector inductively coupled plasma-mass spectrometer (LA-MC-ICP-MS) are becoming more common for samples displaying large isotopic fractionation such as observed in pyrite, where the typical SIMS analytical precision for pyrite is around 0.20‰ (2s, Decraene et al 2021a). Indeed, over Earth's history the overall pyrite bulk δ 56 Fe variability ranges between -4 and +1‰, allowing SIMS δ 56 Fe investigation at the pyrite population level (e.g., Decraene et al 2021b, Harazim et al 2020, Whitehouse and Fedo 2007 and/or internal grain scale (e.g., Marin-Carbonne et al 2014). Interestingly, Dupeyron et al (2023) compiled available microscale δ 56 Fe and found an overall wider isotopic variability, i.e., -4 to +4‰, preserved in Precambrian sedimentary rocks opening a new avenue for in situ Fe isotope investigation.…”

Procedures from samples to sulfur isotopic data: A review

“…Later, the powders are dissolved in 5 mL of aqua regia in a vial to oxidize sulfur as SO 4 2− and dried at 150°C. MQ is added to the vial to redissolve the residue and collected as BaSO 4 120,292 …”

The sequestration of trace metals preserved in pyritized burrows

“…The introduction of "super-reactive" organic carbon in the form of extracellular polysaccharides (EPS) bioturbation within the burrow system, through mucus lining of the burrower drives sulphate reduction and OM degradation (Harazim et al, 2020;Sturdivant and Shimizu, 2017;Sutherland, 2006).…”