Decadal, Sediment-Driven Sulfur Isotope Evolution in Lake Cadagno: Microscale Insights from SIMS

In “Decadal, sediment-driven sulfur isotope evolution in Lake Cadagno,” J. Dupeyron et al. report new dissolved H₂S and SO₄²⁻ isotope data together with microscale sulfur isotope analyses of sedimentary pyrite from the redox-stratified Lake Cadagno, Switzerland. By comparing data collected in 2022 with previously published measurements from 1991 and 2006–2008, the study reconstructs the multi-decadal evolution of sulfur isotopes in the lake’s water column and sediments.

A central component of this work is the use of secondary ion mass spectrometry (SIMS) to measure sulfur isotope compositions at the microscale.

“SIMS provides a unique opportunity to assess isotopic heterogeneity among individual grains within rocks or sediments. By applying this technique to measure the sulfur isotope composition of micrometer-sized pyrite grains from Lake Cadagno, we were able to demonstrate unambiguously that pyrite preserved the isotopic evolution of the dissolved sulfide pool over the past decades.” — Juliette Dupeyron, PhD, biogeochemist, University of Lausanne and Institut de physique du Globe de Paris

Study Context and Objectives

Lake Cadagno is a meromictic alpine lake with persistent redox stratification and moderate sulfate concentrations in deep waters. Microbial sulfate reduction (MSR) drives sulfide production in the anoxic deep waters and sediments. Previous incubation experiments in Lake Cadagno sediment reported near-equilibrium microbial sulfur isotope fractionation of 71.8‰.

The authors sought to determine whether comparable isotope fractionation occurs under natural conditions and whether such fractionation is preserved in sedimentary pyrite. To address these questions, they analyzed:

• Microscale sulfur isotope compositions of pyrite grains from surface sediments (0–5 cm)
• Bulk sulfur isotope compositions of chemically extracted pyrite
• New (2022) isotope data for dissolved H₂S in the water column and porewaters
• Previously published isotope data from earlier sampling campaigns

Given a sedimentation rate of 2.4 mm per year, the uppermost 5 cm of sediment represents approximately the past 20 years.

Results: A Dynamic H₂S Reservoir

The study documents a 15 to 20 ‰ enrichment in the δ³⁴S values of water column H₂S over the past two decades. In contrast, sulfate δ³⁴S values in the water column remained relatively constant over the same period.

Mass balance calculations indicate that this enrichment is driven by diffusion of ³⁴S-enriched H₂S produced in sediment porewaters into the overlying water column. The benthic H₂S flux exceeds the integrated rate of water column sulfide production by three orders of magnitude, demonstrating the significant contribution of sedimentary processes to the water column H₂S pool.

In porewaters, δ³⁴S values of H₂S increased between 1991 and 2022. The authors propose that this trend may be linked to changes in sedimentation regime and upward diffusion of H₂S produced from ³⁴S-enriched residual sulfate at depth.

Microscale SIMS Analysis of Sedimentary Pyrite

Sedimentary pyrite in the uppermost 5 cm occurs primarily as framboidal grains (8–20 μm in diameter) and euhedral grains (20–30 μm). These grains were analyzed by secondary ion mass spectrometry (SIMS).

The SIMS measurements revealed a wide range of δ³⁴S values, from −54.9 ± 0.2 ‰ to +4.6 ± 0.2 ‰. The median SIMS value (−10.5 ‰) aligns well with the bulk chemically extracted pyrite value (−4.0 ± 1.2 ‰).

Among 37 grains analyzed, only one framboidal grain exhibited a Δₚyr value (~79.6 ‰) consistent with the near-equilibrium microbial isotope fractionation previously measured in incubation experiments. Most other microscale Δₚyr values were below 55 ‰.

Importantly, most microscale δ³⁴Sₚyr values fall within the δ³⁴S range of dissolved H₂S measured between 1991 and 2022. This indicates that pyrite in the uppermost 5 cm recorded the temporal δ³⁴S evolution of the pooled H₂S reservoir over the past decades.

The authors infer that framboids formed in sediment porewaters, recording the δ³⁴S evolution of the pooled porewater H₂S reservoir.

Isotopic Homogenization and Reservoir Effects

The poor preservation of near-equilibrium microbial isotope fractionation in most pyrite grains suggests that sulfide was isotopically homogenized before precipitation as pyrite.

Sulfide accumulates in surface porewaters, and mixing of H₂S produced at different depths—originating from residual sulfate with varying degrees of ³⁴S enrichment—dilutes locally produced ³⁴S-depleted sulfide. As a result, sedimentary pyrite δ³⁴S values correspond to the pooled H₂S reservoir, masking near-equilibrium microbial isotope fractionation signatures.

Implications for Interpreting Euxinic Systems

The study offers an alternative perspective to traditional models that attribute H₂S reservoir dynamics in euxinic environments primarily to in situ water column MSR.

In Lake Cadagno, sedimentary production and accumulation of H₂S drive the build-up and mixing of the sulfide reservoir. Excess sulfide can diffuse into the water column, where it may undergo oxidation.

The authors conclude that the sulfur isotope composition of sedimentary pyrite alone may not distinguish between euxinic conditions in the water column and those restricted to the sediment.

Conclusion

By combining microscale SIMS analyses with bulk sulfur measurements and long-term monitoring, Dupeyron et al. demonstrate that the H₂S pool in Lake Cadagno is highly dynamic. The δ³⁴S increase in both porewater and water column H₂S is best explained by sedimentary processes, and this increase is faithfully recorded in pyrite formed during this period.

This study underscores the importance of integrating microscale isotope analysis with broader geochemical datasets to better understand sulfur cycling and pyrite formation in stratified systems.