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

ABSTRACT: Pyrite-?³⁴S and -?⁵⁶Fe isotopes represent highly sensitive diagnostic paleoenvironmental proxies that express high variability at the bed (34S and -?⁵⁶Fe 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 ?³⁴S and ?⁵⁶Fe values in: (a) tubular biogenic structures and trails (?³⁴S?~?+40‰; ?⁵⁶Fe?~?-0.5‰), (b) silt-filled Planolites burrows (?³⁴S?~?+40‰; ?⁵⁶Fe?~?+0.5 to +?2.1‰), and (c) non-bioturbated mudstone (?³⁴S?~?+35‰; ?⁵⁶Fe?~?+0.5‰). ?³⁴S 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.

SUBMITTER: Harazim D

PROVIDER: S-EPMC7679392 | biostudies-literature | 2020 Nov

REPOSITORIES: biostudies-literature

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

Harazim Dario D Virtasalo Joonas J JJ Denommee Kathryn C KC Thiemeyer Nicolas N Lahaye Yann Y Whitehouse Martin J MJ

Scientific reports 20201120 1

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, Be ...[more]

PMID: 33219284

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Project description:The precise interpretation of environmental sulfur isotope records requires a quantitative understanding of the biochemical controls on sulfur isotope fractionation by the principle isotope-fractionating process within the S cycle, microbial sulfate reduction (MSR). Here we provide the only direct observation of the major ((34)S/(32)S) and minor ((33)S/(32)S, (36)S/(32)S) sulfur isotope fractionations imparted by a central enzyme in the energy metabolism of sulfate reducers, dissimilatory sulfite reductase (DsrAB). Results from in vitro sulfite reduction experiments allow us to calculate the in vitro DsrAB isotope effect in (34)S/(32)S (hereafter, [Formula: see text]) to be 15.3 ± 2‰, 2?. The accompanying minor isotope effect in (33)S, described as [Formula: see text], is calculated to be 0.5150 ± 0.0012, 2?. These observations facilitate a rigorous evaluation of the isotopic fractionation associated with the dissimilatory MSR pathway, as well as of the environmental variables that govern the overall magnitude of fractionation by natural communities of sulfate reducers. The isotope effect induced by DsrAB upon sulfite reduction is a factor of 0.3-0.6 times prior indirect estimates, which have ranged from 25 to 53‰ in (34)?DsrAB. The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of [Formula: see text] is similar to the median value of experimental observations compiled from all known published work, where (34)? r-p = 16.1‰ (r-p indicates reactant vs. product, n = 648). This value closely matches those of MSR operating at high sulfate reduction rates in both laboratory chemostat experiments ([Formula: see text] 17.3 ± 1.5‰, 2?) and in modern marine sediments ([Formula: see text] 17.3 ± 3.8‰). Targeting the direct isotopic consequences of a specific enzymatic processes is a fundamental step toward a biochemical foundation for reinterpreting the biogeochemical and geobiological sulfur isotope records in modern and ancient environments.

Dataset Information

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

Publications

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

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