Conference Agenda

Archean seawater-precipitated cherts are among the best archives of Earth’s earliest inhabited oceans. However, using the triple oxygen isotope composition of chert to reconstruct ancient seawater composition and temperature requires understanding how post-depositional processes may have altered the original signal. In the Barberton Greenstone Belt, intensely hydrothermally silicified basaltic and clastic rocks commonly underlie Archean seawater-precipitated cherts, yet the effect of hydrothermal silicification on chert triple oxygen isotopes remains poorly constrained. To address this issue, we measured the triple oxygen isotope compositions of (1) seawater-precipitated cherts and (2) silicified volcanic and clastic sedimentary rocks from the belt. In a δ¹⁸O versus Δ’¹⁷O diagram, the seawater-precipitated cherts (δ¹⁸O = 16.4 to 20.3 ‰ and Δ’¹⁷O = −0.12 to −0.07 ‰) trend toward the field defined by completely silicified volcanic and clastic samples (δ¹⁸O=11.3 to 14.9 ‰ and Δ’¹⁷O values = −0.05 to −0.07 ‰). This pattern likely reflects post-depositional replacement or mixing of seawater-derived and hydrothermal silica, which may have obscured the preservation of original seawater temperature and compositional signals in the cherts. Mixing models indicate that quartz precipitated from seawater with δ¹⁸O values of 0‰ and lower than −5‰ at temperatures between 0 and 55°C could both explain the observed isotopic trend. Assuming precipitation of hydrothermal silica at 150°C and silicon dissolution during seawater–basalt interaction at approximately 200°C, we calculate negative δ¹⁸O values for the Archean hydrothermal fluids, supporting the existence of low-δ¹⁸O Archean oceans. Nevertheless, determining the original composition of Archean seawater-derived silica remains challenging.

The Shuram Excursion (ca. 574–563 Ma) represents the largest negative carbon isotope anomaly in Earth's history, with δ¹³C values reaching as low as −12‰, far below the mantle-derived baseline. Unlike earlier Neoproterozoic excursions linked to snowball Earth glaciations, the Shuram event lacks sedimentological evidence for global glaciation and occurs concurently to the rise of macroscopic fauna, making its origin particularly enigmatic. Proposed mechanisms include i) large-scale oxidation of marine organic matter followed by authigenic carbonate precipitation, ii) restricted-basin DIC effects, and iii) global diagenetic overprint. However, conventional δ¹³C–δ¹⁸O systematics cannot discriminate among these scenarios. Triple-oxygen-isotopes analysis, integrating δ¹⁸O and Δ'¹⁷O, offer a way to address this challenge by constraining carbonate formation pathways, thereby enabling insights into deep-time conditions even from non-pristine material.

Here, we investigate the triple-oxygen-isotopes signature of carbonate rocks from three Shuram Excursion localities: the Shuram Formation (Oman) and two sections of the Doushantuo Formation (Zhongling and Jiulongwan, South China). Shuram samples systematically deviate from Δ'¹⁷O-δ¹⁸O equilibrium, showing correlated depletions in δ¹⁸O and Δ'¹⁷O, indicating these carbonates did not precipitate in equilibrium with seawater, precluding direct use of oxygen isotopes as palaeotemperature proxies. Strikingly, the large δ¹³C variability is not mirrored in Δ'¹⁷O, which remains invariant across the full δ¹³C range, suggesting at least two distinct processes govern carbonate formation. Each locality defines a distinct Δ'¹⁷O-δ¹⁸O array, pointing to different formation histories. These observations provide key constraints on Shuram carbonate formation mechanisms, in light of which competing hypotheses for the origin of this anomaly will be discussed.

Oxygen isotope analyses of phosphates are widely used to investigate paleo-temperatures from conodonts or fish teeth. Beyond conventional δ¹⁸O_PO4, triple oxygen isotope (Δ'¹⁷O_PO4) analyses now provide an additional dimension that permits solving more complex flux models. In a first application the Δ'¹⁷O composition of ancient air O₂ is reconstructed from dinosaur tooth enamel (i.e. the oxygen inhaled by the dinosaur), which can be used to estimate paleo-pCO₂ and paleo gross primary productivity [1]. Such applications heavily rely on model parameters such as the equilibrium triple oxygen isotope curve for the phosphate-water system and on accurate normalization of measured Δ'¹⁷O_PO4 to the VSMOW-SLAP scale.

We have conducted enzymatic assay experiments with pyrophosphatase to attain an empirical equilibrium curve. The respective Ag₃PO₄ is analyzed using two independent methods. One is based on the combination of i) high temperature combustion in a TC/EA, ii) conversion of CO to CO₂ using high-voltage glow discharge and iii) triple oxygen isotope analysis via the Aerodyne Research Inc. TILDAS CO₂ spectrometer [2,3]. The second method is based on the laser fluorination approach commonly used for silicates and a range of other materials, including bioapatite [4]. Results from both techniques will be discussed in comparison to recent literature data. The final equilibrium curve is subsequently used for interpreting the first Δ'¹⁷O_PO4 conodont element data (Bagherpour et al. unpublished).

[1] Feng et al. (2025), PNAS; [2] Bajnai et al. (2023), G3; [3] Zahnow et al. (2026) Anal. Chem., [4] Feng et al. (2022), GCA.

The shells of the giant clam Tridacna are important archives for paleoenvironmental reconstruction. They can grow up to 1 m in size and record changing environmental conditions in their geochemical composition on timescales ranging from tens of years to sub-daily. Elemental ratio data at such high temporal resolution allows the reconstruction of paleo-seasonality and extreme weather events. However, the mechanisms behind the formation of daily elemental cycles remain uncertain.

To better understand the cyclic patterns and elemental uptake on daily scales, we cultured Tridacna under controlled conditions. We labelled shell growth during day and night by introducing a ¹³⁵Ba-isotope tracer in alternating 12-hour intervals. The calcification rates of the cultured Tridacna were five times higher during the day than at night. Most measured elemental ratios (B/Ca, Mg/Ca, Sr/Ca, Ba/Ca) decreased throughout the day but increased during the night, with Na/Ca showing the opposite pattern.

We deduce that daily elemental cycles are likely driven by the active transport of Ca²⁺ and HCO₃⁻ to the calcification site. This active transport is influenced by light, energy availability from photosymbionts, filter feeding, and possibly circadian rhythm.