Iron-rich I-type cosmic spherules — micrometeorites formed by the complete melting and oxidation of extraterrestrial Fe, Ni metal particles — incorporate oxygen from the Earth’s atmosphere. As a result, they can be used to assess the triple oxygen isotope composition of atmospheric O₂, offering insights into CO₂ levels and global primary production [1,2]. When recovered from sedimentary rocks, these spherules can preserve a record of atmospheric conditions dating back billions of years. To date, no published triple oxygen (and triple iron) isotope data exist for fossil I-type cosmic spherules. This study aims to address this gap as we establish using fossil I-type cosmic spherules as an archive of Earth's atmospheric oxygen isotope composition and by quantifying associated CO₂ levels during key geologic periods. We analyzed a collection of fossil I-type cosmic spherules extracted from Phanerozoic sedimentary rocks, focusing on both triple oxygen and triple iron isotope compositions. This approach allowed us to reconstruct the triple oxygen isotope anomalies in past atmospheric O₂ and to assess the potential terrestrial alteration of these spherules. Our data reveal moderate ancient CO₂ levels during the Miocene (~8.5 Ma) and late Cretaceous (~87 Ma). We also demonstrate the competitive precision of using I-type cosmic spherules for paleo-CO₂ determination. Additionally, our work indicates that morphologically intact spherules can be isotopically altered by terrestrial processes, underscoring the need for rigorous sample screening.

[1] Pack et al. (2017), Nat. Commun. 8, 15702.

[2] Fischer et al. (2021), Paleoceanogr. Paleoclimatol. 36, e2020PA004159.

In light of anthropogenic climate change, the use of palaeo-CO₂ proxies to reconstruct past atmospheric pCO₂ is essential for understanding the impact of CO₂ variations on the Earth's climate. The mechanistic leaf gas exchange model of Franks et al. (2014) utilises the relationship between atmospheric pCO₂ and chemical and morphological traits of leaf fossils to reconstruct pCO₂. While the model has shown promising results, its broader applicability remains unclear, particularly whether there are consistent off-sets in pCO₂ estimates among major plant clades. Here, we evaluate the performance of the model across different phylogenetic groups (five ferns, six gymnosperms, five angiosperms), with generic input values and recommended adjustments to isotopic values, assimilation rates and the scaling of stomatal conductance. Results show species-specific variations with generic values; a clade-level bias is therefore unlikely. As a consequence, the correction factors to remove a phylogenetic effect from isotopic values do not show improvements across plant groups. Adjustments to the assimilation rate based on phylogeny, habit and habitat result in less species-specific variation and in an improvement in pCO₂ estimates. While the minor change in the scaling value of stomatal conductance for woody angiosperms results in inconsiderable differences in accuracy, precision decreases due to higher error rates. Overall, the results of this study show that the Franks model has a strong potential for reconstructing past atmospheric pCO₂ when applied in a multitaxon framework, especially with recommended assimilation rate values.

Speleothem carbonates are key archives for the reconstruction of terrestrial paleo-environmental conditions, but it is challenging to resolve between the temperature, hydrological and kinetic information that is recorded in their stable isotopic composition. Recently, it has been demonstrated that dual clumped isotope thermometry, i.e., analysis of Δ₄₈ alongside Δ₄₇ in CO₂ evolved from phosphoric acid digestion of carbonate [1], holds the potential to detect and quantify kinetic departures from isotopic equilibrium without having to know the oxygen isotope composition of the parent water, and, moreover, to correct for kinetic biases in carbonate formation temperatures [2].

We performed paired Δ₄₇-Δ₄₈ measurements on speleothems to investigate to which extent isotopic disequilibrium is recorded in these archives. No significant disequilibrium bias is recorded in slowly grown pool carbonates and cryogenic carbonates, and Δ₄₇ derived temperatures agree with expected formation temperatures. Most stalagmites, sampled closest to their growth axis where disequilibrium should be least pronounced, record a slight, but significant -Δ₄₇ disequilibrium bias. While the bulk set of investigated stalagmites exhibits a significant +Δ₄₈ offset from equilibrium, the extent of Δ₄₈ disequilibrium in individual samples remains below analytical resolution. Disequilibrium biases originate from rapid dehydration/dehydroxylation of bicarbonate occurring dissolved in the drip water. These results demonstrate that stalagmite Δ₄₇ values, without any additional evidence, should only be considered upper estimates of cave temperature, even if corresponding dual clumped data plots indistinguishably from the Δ₄₇-Δ₄₈ equilibrium line.

[1[ Fiebig et al. (2019), Chem. Geol. 522, 186-191

[2] Bajnai et al. (2020) Nat. Commun. 11:4005

Various proxy records have suggested widespread permafrost degradation in northern high latitudes during interglacial warm climates, including the mid Holocene (MH, 6000 years before present) and the last interglacial (LIG, 127 ka BP), and linked this to substantially warmer high-latitude climates compared to the pre-industrial period (PI). However, most Earth system models suggest only modest warming or even slight cooling in terms of annual mean surface temperatures during these interglacials. Here, we combine paleo permafrost reconstructions derived from speleothem and pollen records with paleoclimate simulations of the AWI-ESM-2.5 climate model and the CryoGridLite permafrost model to investigate the ground thermal regime and freeze-thaw dynamics in northern high-latitude land areas during the MH and the LIG. The simulated paleo permafrost extents are broadly in agreement with proxy contrainsts from speleothems. The simulations further revealed that not only changes in mean temperatures, but also the enhanced seasonal temperature amplitude due to a different orbital forcing have driven permafrost and ground ice dynamics during past interglacial climates. Our results provide an additional explanation of reconstructed periods of marked permafrost degradation in the past, which was driven by deep surficial thaw during summer, while colder winters allowed for permafrost persistence in greater depths. Our findings suggest that past interglacial climates have limited suitability as analogues for future permafrost thaw trajectories, as rising mean temperatures paralleled by decreasing seasonal amplitudes expose the northern permafrost region to magnitudes of thaw that are likely unprecedented since at least Marine Isotope Stage 11c (about 400 ka BP).

Triple metal(oid) isotope systems can provide valuable insights into underlying fractionation mechanisms. However, the limited isotopic variability and the precision constraints of MC-ICP-MS analyses often hinder the resolution of differences in three-isotope space. As a result, correlated isotope ratios are typically used only as quality indicators for δ-value measurements.

In this study, I introduce a straightforward method to reduce uncertainty in triple isotope slope measurements using MC-ICP-MS. This approach is applied to Mg isotopes in carbonates, demonstrating improved precision. Additionally, two case studies of Si isotopes show that differences between equilibrium and kinetic fractionation slopes -on the order of ~10 ppm per ‰- can be resolved by enhancing the spatial resolution or simply by high-precision solution analyses for fractionations of approximately 3 ‰.

These advances in resolving triple isotope variations open new avenues for the application of metal(oid) isotopes as proxies in paleoenvironmental reconstructions.

Phosphoric acid digestion of carbonates is associated with fractionations of both bulk oxygen and clumped isotopes. Accurate knowledge of the effect of cation substitution on the degree of isotopic clumping in the carbonate phase (∆₆₃, ∆₆₄) and on acid fractionation factors (∆^*₄₇, ∆^*₄₈) is crucial for accurate temperature reconstructions based on clumped isotope measurements (∆₄₇, ∆₄₈) of the extracted CO₂. Previous studies have yielded contradicting results whether a universal ∆^*₄₇ acid fractionation factor and ∆₄₇-T relationship is valid for all carbonate mineralogies, and a systematic investigation of mineralogy-specific effects on ∆₄₈ and ∆^*₄₈ is still lacking.

We have determined ∆₄₇ and ∆₄₈ values of stochastic (i.e, ∆₆₃ = ∆₆₄ = 0) and non-stochastic calcites, aragonites, dolomites, witherites‚ and siderites with outstanding precision. We demonstrate that stochastic calcite, aragonite, dolomite, witherite and siderite exhibit statistically indistinguishable ∆_{47, CDES90} and ∆_{48, CDES90} values. In addition, ∆_{47, CDES90} and ∆_{48, CDES90} values of non-stochastic aragonites, (proto-)dolomites and witherite correspond to calcite equilibrium values^[1] predicted by their independently known formation temperatures. These results provide evidence that calcite, aragonite, dolomite and witherite share indistinguishable ∆^*₄₇, ∆^*₄₈ and equilibrium ∆₆₃-∆₆₄-T relationships. Consequently, the calcite-specific equilibrium ∆₆₃-∆₆₄-T relationships^[1] can be reliably applied to aragonite, dolomite, and witherite. More investigations are necessary to clarify its validity for siderite.

^[1]Fiebig, J. et al. Chem. Geol. 670, 122382 (2024)