Triple oxygen isotope (Δ'¹⁷O) analysis has recently be shown to be a powerful tool for identifying metabolic oxygen signatures in mammalian tooth enamel (Pack et al., 2013; Feng et al., 2022). Atmospheric O₂ is consumed by mammals for metabolic oxidation. The low triple oxygen isotope (Δ'¹⁷O) composition of air O₂ serves as a natural tracer for identifying metabolic oxygen in body water. Bioapatite precipitates in isotopic equilibrium with its parental body water and consequently records information on the air O₂. The Δ'¹⁷O of atmospheric O₂ is directly linked to pCO₂ and gross primary production, hence fossil teeth can be used for paleo- pCO₂ reconstructions.

To provide a modern baseline for this approach, we measured 128 individual mammal teeth for their bioapatite Δ'¹⁷O by automatic BrF₅ laser fluorination. The sample set includes diverse body size with a body mass range from 2 g to 6000 kg and physiology from different habitats. Taxon-specific oxygen mass balance models are developed for resolving principal dependencies and relationships.

The mass balance modelled data for all species agree within uncertainty with the measured data. The results show that Δ'¹⁷O not only correlates with body mass, but also with initial oxygen anomalies of inhaled air O₂, which allows for pCO₂ reconstruction on terrestrial mammalian tooth enamel. This documents the potential of tooth enamel Δ'¹⁷O analysis for metabolic rates of extinct vertebrates and paleoclimate reconstructions, especially for small mammals (Mb < 1 kg).

Pack et al. (2013) GCA, 102, 306–317.

Feng et al. (2022) GCA, 328, 85-102.

Carbonate δ¹⁸O and Δ₄₇ are used to reconstruct paleotemperatures. Because biogenic carbonate does not form in full equilibrium with seawater, species-specific temperature calibration curves are required for accurate temperature estimates. Apparent growth temperatures derived from corals, however, are generally inaccurate due to large and variable kinetic isotope effects, often termed “vital effect”. Triple oxygen isotope systematics can help identify if an organism forms carbonate in equilibrium with ambient water or not and thus if the δ¹⁸O and Δ₄₇ values provide accurate paleotemperatures. In addition, the chemical nature of the “vital effect” can be identified, because individual kinetic effects fall on characteristic trajectories in triple oxygen isotope space.

To examine these concepts a series of cold water and warm water corals as well as brachiopods are analyzed for δ¹⁸O and δ¹⁷O (expressed as Δ’¹⁷O) using the CO₂ spectrometer (TILDAS; Aerodyne Research) installed in Göttingen. Samples formed in equilibrium are expected to fall on the equilibrium curve. Most samples fall below the curve providing evidence that their oxygen isotope composition is biased by kinetic effects. We suggest that the “vital effect” in corals is dominated by a CO₂ absorption effect. Similar conclusions are derived from dual clumped (Δ₄₇ and Δ₄₈) isotope analyses of the same samples [1]. These authors suggested to correct for kinetic effects by back extrapolation to the “dual clumped” equilibrium line. The same concept can be applied in triple oxygen isotope space by back-extrapolation to the “triple oxygen isotope” equilibrium line.

[1] Davies et al. (2022). GCA 338, 66–78.

The reconstruction of Mesozoic seawater temperature is valuable to further understand the link between atmospheric carbon dioxide concentration and earth surface temperature variation. Belemnites are an effective archive for this purpose, owing to their distribution over an extensive latitudinal range in Jurassic and Cretaceous seas. However, uncertainty remains as to whether belemnites precipitate rostrum calcite in oxygen and clumped isotope equilibrium with surrounding seawater. Furthermore, recent study indicates that belemnite calcite may be more susceptible to thermal resetting of ∆₄₇ values through oxygen isotope exchange with internal water than other calcites (Looser et al., 2023).

Here, we demonstrate identification of thermal resetting of ∆₄₇ values in belemnite calcite using a combination of measured δ¹⁸O of fluid inclusions, δ¹⁸O and ∆₄₇ values of belemnite calcite. We then apply dual carbonate clumped isotope thermometry (i.e. the simultaneous measurement of ∆₄₇ and ∆₄₈), to assess the potential importance of kinetic limitations during belemnite biomineralization (Bajnai et al., 2020; Guo, 2020). We demonstrate that Maastrichtian agebelemnites sampled at 4 sites with a range of paleolatitudes from 34 to 45 ˚N, yield ∆₄₇ and ∆₄₈ values that fall on the experimentally derived equilibrium calibration of Fiebig et al., (2021) indicating that rostrum calcite precipitated in clumped isotope equilibrium. ∆₄₇-derived temperatures are combined with other proxy-based reconstructions of Maastrichtian seawater temperature to examine its latitudinal variability.

Microbes, plants and other organisms actively shape the Earth surface by a variety of processes. Current research on modern systems suggests that biota has a significant impact on the mobility of trace elements and there is growing evidence that it may have done so since the dawn of life. The secretion of extracellular compounds that bind strongly to iron and other micronutrients are evolutionary traits that may, besides acquiring bio-essential trace metals, also have helped in tackling with the toxicity of certain heavy metals. An example of such compounds are siderophores, which are produced today by many different plants, microbes and fungi. Besides iron, they promote the (bio)availability of different highly-charged cations in the natural environment.

In paleoclimate studies, the redox-sensitive trace elements Ce and U are commonly used as geochemical proxies (e.g., Ce anomalies, Th-U ratios, stable U isotopes) for reconstructing atmospheric oxygen levels through time. We investigated the effect of siderophores on the mobilization of rare earth elements, Th and U (isotopes) from natural rocks under anoxic, hypoxic and oxic conditions. We show that experimental water–rock interaction with siderophores under strictly anoxic conditions produces positive Ce anomalies – a geochemical signal that is usually attributed to the presence of atmospheric oxygen. Siderophores also influence the Th-U signal, but do not induce a significant U stable isotope fractionation. Thus, oxygen-independent fractionation during geo–bio interaction may hold the potential to use certain trace elements as a bio-proxy in addition to their current use as a redox proxy.

Cherts are sturdy and ubiquitous throughout Earth history and therefore potentially ideal climatic archives, yet their diagenetic formation challenges straightforward interpretations. Previous work shows that oxygen isotope ratios (δ¹⁸O_chert) are not only controlled by seawater T and δ¹⁸O_seawater, but also by basal heat flow (Q) and burial rates, i.e. the diagenetic T-t path (Tatzel et al., 2022).

To strengthen the empirical evidence for the control of Q and burial rates on δ¹⁸O_chert we exploit the geological framework of the Rhenish Massif: During the Lower Carboniferous siliceous sediments were deposited onto stretched continental crust with variable Q across the basin. The opal mud then transformed during burial diagenesis into chert while tectonic nappes were stacked onto the autochthonous sediments during the Armorica- Laurussia collision.

To isolate the effects of the T-t-path on δ¹⁸O_chert we sampled isochronous cherts and siliceous shales from the Northern and Eastern margin of the Rhenish Massif. A decreasing trend in δ¹⁸O_chert from West-to-East (by >6 ‰) testifies to differences in burial rates and paleo-Q. We derive constraints on burial rates from peak diagenetic temperatures using Raman Spectrometry of carbonaceous matter and deconvolve for paleo-Q. Triple oxygen isotope compositions (Δ’¹⁷O) show increasing variations with increasing δ¹⁸O_chert – presumably a reflection of a wider range of diagenetic conditions in low-Q settings.

Tatzel, M., Frings, P.J., Oelze, M., Herwartz, D., Lünsdorf, N.K., Wiedenbeck, M., 2022. Chert oxygen isotope ratios are driven by Earth’s thermal evolution. Proc. Natl. Acad. Sci. 119.