COMBINED CR-CD ISOTOPES REVEAL EDIACARAN-CAMBRIAN NUTRIENT AND REDOX DYNAMICS IN WESTERN GONDWANA

Introduction

The Ediacaran-Cambrian transition was a crucial time frame in Earth’s story, with important unidirectional changes taking place in the biosphere (first appearance and rapid diversification of complex organisms), lithosphere (advent of modern, cold-subduction plate tectonics), atmosphere (renewed oxygenation), climate (Snowball Earth and related ultra-greenhouse conditions) and overall biogeochemistry of the planet. In order to understand how changes in one of those spheres interacted with the others and vice-versa, proxies that can track crucial factors such as the relative levels of atmospheric oxygen and the degree of primary productivity in ancient oceans are necessary. In this study, we use two relatively novel proxies, Cr isotopes to track paleoredox conditions and Cd isotopes to track ancient productivity levels, on basins that record the Ediacaran-Cambrian transition in western Gondwana.

Cr from hydrothermal vent fluids is characterized by isotopically unfractionated δ⁵³Cr = 0.12 ± 0.11 ‰; Schoenberg et al., 2008), while continentally derived Cr, released by oxidative weathering, may be positively fractionated. Oxygen in the atmosphere causes the oxidation of immobile Cr(III) in weathered rocks to soluble Cr(VI) in the presence of Mn-oxides, which act as reaction catalysts (Oze et al., 2007). The positively fractionated dissolved Cr(VI) compounds reach the oceans via continental run-off where they either are sorbed onto particles or are reduced back to insoluble Cr(III) bearing species, either biologically or abiollogically (Janssen et al., 2020) or abiologically (Frei et al., 2009; Janssen et al., 2022). The overall dissolved Cr isotope signal may be further altered by biogenic redox processes in the water column. Such processes involve reduction by organic matter (Janssen et al., 2020), and incorporation into carbonates (Frei et al., 2011) and Fe–Mn crusts (Wei et al., 2018). Thus, the Cr records in chemical sediments are linked to atmospheric oxygenation, intensity of release of chromium from the continents, and levels of primary productivity.

Oceans receive Cd via the upper continental crust (δ¹¹⁴Cd = 0.01 ± 0.04 ‰; Rehkamper et al., 2012), rivers (δ¹¹⁴Cd = 0.1 to 0.3‰; Lambelet et al., 2013) and aerosols/dust (δ¹¹⁴Cd = 0.19 to 0.19‰; Bridgestock et al., 2017). The Cd composition of deep ocean seawater, however, is isotopically heavier than those inputs, with δ¹¹⁴Cd of 0.35 ± 0.12‰ (Frei et al., 2024 and references therein). This difference is balanced by light cadmium isotope signatures found in continental margin sediments, acquired through burial of organically-bound Cd (Janssen et al., 2019) and/or by burial of cadmium sulfide (CdS) if there is free aqueous hydrogen sulfide (Bryan et al., 2021). Therefore, the Cd isotope signatures can be used as a proxy to paleoproductivity levels in ancient waters.

Experimental

We studied sections from the Bambuí Group, covering the São Francisco craton in eastern Brazil, and from the Taguatyia Guazú Formation covering the Rio Apa craton in western Brazil. Both sections contain carbonate-rich units that record the latest Ediacaran/Cambrian transition in western Gondwana.

Powdered samples (4 × 2 g) were spiked with adequate amounts of a ^106-108Cd double spike, whereby a 2 g aliquot was additionally spiked with an appropriate amount of ^50-54Cr double spike. In order to ensure that only the carbonate portion of the samples was dissolved (i.e. to avoid detrital signatures), the sub-samples were attacked with a weak acid, 45 mL of 0.5 N HNO₃. After dissolution and recovery of the supernatant, Cr and Cd were separated through ion chromatography and analyzed on a IsotopX PHOENIX TIMS operating in static multi-collection mode at the Dept. of Geosciences and Natural Resource Management, University of Copenhagen, following the procedures described in Frei et al. (2024).

Results and Discussion (ARIAL 11 BOLD)

Carbonate samples from two drill cores of the Bambuí Group (Caxito et al., 2021 and references therein) indicate a complex evolving biogeochemical landscape, and can be subdivided in four major chemostratigraphic intervals: I-1) the post-glacial cap carbonate at the base of the Sete Lagoas Formation, which preserves a short-lived negative δ¹³C excursion and a sharp increase of ⁸⁷Sr/⁸⁶Sr from ca. 0.7075 to >0.7080. Short-lived oxic conditions in the wake of the Marinoan glaciation (ca. 635-600 Ma) are marked by a peak of δ⁵³Cr to +0.5‰. A quick recovery of primary productivity is indicated by δ¹⁴⁴Cd rising to +0.4‰; I-2) middle to upper Sete Lagoas Fm., with δ¹³C around 0‰ and slowly increasing. Anoxic conditions are indicated by unfractionated δ⁵³Cr, lower primary bioproductivity by lower δ¹⁴⁴Cd that persists upwards, and basin restriction by lower ⁸⁷Sr/⁸⁶Sr of ca. 0.7075, with fleeting and short-lived colonization of shallow-water oxic oases by biomineralizing organisms (600-540 Ma); I-3) upper Sete Lagoas Fm., where a transgression is marked by a sharp increase in δ¹³C to >+15‰ (the Middle Bambuí Excursion – MIBE). The overlying Serra de Santa Helena Fm. likely straddles the Ediacaran/Cambrian transition and marks a return to oxic conditions, with increasing δ⁵³Cr up to +1‰; I-4) the Lagoa do Jacaré Fm., which marks a sea-level fall and the return to anoxic conditions indicated by concomitant decreases in δ⁵³Cr, accompanied by typical early Cambrian ⁸⁷Sr/⁸⁶Sr of 0.7085, indicating renewed communication with the ocean.

We also studied carbonate levels related to phosphorites of the Sete Lagoas Formation in the Campos Belos/Arraias mining region of central Brazil. On these samples, δ¹¹⁴Cd hovers around 0‰ but reach values as high as 0.25‰ and as low as -0.5‰, indicating variable uptake of Cd, among other nutrients, into phototrophic plankton; while fractionated δ⁵³Cr values of 0.41 to 0.75‰ indicate the input of Cr(VI) generated in association with manganese oxides through continental oxidative weathering. The higher fractionated δ⁵³Cr occur along with the lower δ¹¹⁴Cd, suggesting that biological fractionation of Cr was not important.

The Ediacaran mixed siliciclastic carbonate Tagatiya Guazú Formation, Itapucumi Group, was deposited in a shallow rimmed ramp bordering the Rio Apa craton in northeastern Paraguay (Antunes et al., 2023). Cr isotopes are fractionated throughout the two studied sections, with authigenic δ⁵³Cr values ca. 0 to 0.5‰. Levels containing trace fossils are especially enriched, as high as ca. 1‰, indicating important oxidative weathering in the source areas. Cd isotopes show variable fractionation, reaching δ¹¹⁴Cd values as high as 0.5‰. These values indicate increased uptake of Cd, among other nutrients, into phototrophic plankton, supporting high levels of primary productivity.

Conclusions

Our integrated results shed light into the biogeochemical cycles during the rise of metazoans and the redox-nutrient controls on the distribution and development of the first complex organisms in western Gondwana. These organisms thrived in shallow oxygenated seas under favorable nutrient conditions that fueled trophic chains during the final amalgamation of the paleocontinent. Besides, they present direct evidence for the importance of oxidative weathering in the source areas of post-Marinoan shallow water phosphorite deposits, and link the marine phosphorus and oxygen cycles in the aftermath of Earth’s major Snowball glaciations.

Acknowledgements

This work is supported by CNPq, Brazil (Instituto GeoAtlântico, 405653/2022-0; Universal 408815/2021-3; Productivity in Science 304509/2021-3) and by Instituto Serrapilheira (Project MOBILE, Serra-1912-31510).