It has been estimated that Δ¹⁴C values of marine dissolved inorganic radiocarbon (Δ¹⁴C_DIC) are primarily governed by transport and radioactive decay. This implies that Δ¹⁴C_DIC can be considered as a radioconservative tracer which can be implemented into Earth system models without a full marine carbon cycle model. Here we evaluate the accuracies of the radioconservative modelling approach and of a further modelling approach which considers a different simplified representation of the marine radiocarbon cycle, presenting simulation results obtained with the ocean general circulation model FESOM and the marine biogeochemistry model REcoM. The relative uncertainties between the two simplified and the comprehensive treatments of the marine radiocarbon cycle are less than 5%. Therefore, the simplified Δ¹⁴C_DIC modelling approaches should be sufficiently accurate for many marine radiocarbon studies. 

How to cite: Butzin, M., Köhler, P., Völker, C., Ye, Y., and Lohmann, G.: How accurate are marine Δ14CDIC modelling approaches?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1481, https://doi.org/10.5194/egusphere-egu23-1481, 2023.

Fjords are recognised as important hotspots for the burial and storage of organic carbon (OC) within their sediment, which potentially provides a long-term climate regulation service. Annually, it is estimated that 18 Mt of OC is buried within fjord sediments with between 55 – 62% of the OC originating from the terrestrial environment. The transfer of OC from the terrestrial environment to the fjord sediments is likely a significant pathway for aged OC to reach the coastal ocean. By estimating the quantity and mapping the spatial distribution of aged OC within the fjord sediments, we can develop a better understanding of the processes that govern the transfer of terrestrial OC from the catchment to the sediment of fjords, further constraining their role in long-term climate regulation.

Here we bring together radiocarbon analysis with isotopic and biomarker measurements to investigate the age of the surficial sediments within 46 fjords across the North Atlantic. The fjords in this study range from Scottish systems with catchments dominant with OC rich peat to the glaciated systems of Svalbard and Greenland.  The results from this analysis highlight that a multiple natural and anthropogenic processes govern the quantity and distribution of aged OC across North Atlantic fjords ranging between glacial input of fossil OC to the erosion of aged terrestrial material facilitated by deforestation.  This study highlights the fundamental need to understand the processes that govern the transfer of OC across the land-ocean interface to allow the role these marine sedimentary systems play in long-term climate regulation to be constrained.    

How to cite: Smeaton, C., Haghipour, N., Austin, W., and Eglington, T.: The Role of Aged Organic Carbon in North Atlantic Fjord Sediments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2836, https://doi.org/10.5194/egusphere-egu23-2836, 2023.

The so-called sea spray effect is known to considerably influence the stable isotope fingerprint of coastal samples (plants, soil, bones, teeth). However, the impact of sea spray on radiocarbon analyses in environmental samples from coastal sites has not been investigated, yet. Sea spray aerosols, containing, e.g., HCO₃^- or CO₃^2- of marine origin, enter the terrestrial environment, shifting stable isotope values of terrestrial samples towards a seemingly marine isotope signature. Moreover, the sea spray is always accompanied by physiological effects in the sprayed plants, e.g., due to the salinity of the incorporated water, also visible in, e.g., stable carbon isotope data. A terrestrial herbivore, never consuming any marine food, can show a marine isotope signal due to the sea spray effect. While the marine reservoir effect, resulting from the consumption of marine food sources, can be investigated by calculating isotopic mixing models based on the δ¹³C_collagen and δ¹⁵N_collagen values of archaeological animal and human bones, the sea spray effect remains undetected in the δ¹³C_collagen and δ¹⁵N_collagen values of individuals consuming terrestrial protein (i.e., plant sources) influenced by marine aerosols. Therefore, it is important to investigate the influence of the sea spray on the radiocarbon signature.

The impact of either the direct or the accompanied, indirect sea spray effect can be visualized by an artificial sea spray experiment which was performed in a greenhouse. European beach grass (Ammophila arenaria, L.) was sprayed with mineral salt solution of different ion concentration but only traces of NaCl, with salty water from the Schlei inlet, collected next to the archaeological site of Haithabu (Germany), and with seawater from the Baltic Sea, collected at the western coast of Fehmarn island (Germany), respectively. Plants of all treatment groups were irrigated with Munich tap water (mainly originating from Mangfall valley). Radiocarbon analyses (F¹⁴C), stable as well as radiogenic isotope analyses (δ¹³C_DIC, δ¹³C_bulk, δ¹³C_cellulose, δ¹⁸O_cellulose, δ¹⁸O_sulfate, δ³⁴S_sulfate, δ³⁴S_{total S}, ⁸⁷Sr/⁸⁶Sr), and (trace) elemental analyses were conducted on the plant, soil, and water samples (spray water, irrigation water) using mass spectrometry (AMS, IRMS, TIMS, ICP-MS, IC).

Radiocarbon analyses of the plants showed an impact due to the artificial sea spray. The indirect sea spray effect, resulting from either salinity (NaCl) or HCO₃^- stress, has an impact on plants’ F¹⁴C. The study demonstrates that a substantial proportion of ¹⁴C, which is taken up by the sprayed plants, originates from either irrigation or spray water. Stomatal conductance is markedly reduced due to both salinity and bicarbonate stress. Accordingly, less atmospheric ¹⁴CO₂ can enter the plants via their stomata, while H¹⁴CO₃^-/¹⁴CO₃^2-/¹⁴CO_{2 (aq.)} can still be incorporated via the roots.

A multi-dimensional approach with a combined analysis of stable (δ¹³C, δ¹⁸O, δ³⁴S), radiogenic (⁸⁷Sr/⁸⁶Sr), and radiocarbon isotopes in environmental samples allows to depict a detailed image of biochemical and physiological processes associated with the sea spray effect and will help to reveal new insights into the sea spray impact on the isotopic fingerprint of plants, animals, and humans, including potential caveats for radiocarbon analyses in coastal regions.

How to cite: Göhring, A., Hüls, C. M., Hölzl, S., Mayr, C., and Strauss, H.: Indirect sea spray effects on radiocarbon data of coastal plants – how physiological reactions in plants can be visualized by a combined investigation of stable, radiogenic, and radiocarbon isotopes in a greenhouse experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-128, https://doi.org/10.5194/egusphere-egu23-128, 2023.

Rivers are important agents in the lateral transfer of carbon from terrestrial to the marine realm, thus forming a key component of the global carbon cycle. Carbon sources and transformations along the land-ocean aquatic continuum are dynamic with a complex interplay between dissolved and particulate, and inorganic and organic carbon pools. Elucidating interrelationships between these pools is hindered by multiple sources and processes that influence the carbon signatures of these pools in a dissimilar fashion. Icelandic streams and rivers offer an opportunity to directly assess the fluvial carbon pool dynamics due to the virtual absence of sedimentary rocks (e.g., shales, carbonates with a radiocarbon dead signature) that otherwise “muddy the waters” with respect to apparent sources and turnover times.

In order to characterize carbon transport patterns of Icelandic rivers and streams, we collected water samples from 43 river systems with watersheds that cover a wide range of catchment properties such as size, water discharge, climate as well as landcover. Here we assess the concentrations as well as the isotopic composition (¹³C, ¹⁴C) of particulate and dissolved organic carbon (POC; DOC, respectively) as well as dissolved inorganic carbon (DIC) alongside stable water isotopes (δ²H, δ¹⁸O) and major ion geochemistry.
Radiocarbon content (reported as fraction modern; F¹⁴C) of POC, DOC and DIC show similar patterns: lower F¹⁴C values (i.e., highest radiocarbon ages) are mostly associated with glacial runoff while higher F¹⁴C values (younger carbon) correspond to higher soil organic carbon content within the respective catchment. This dataset is but a first glimpse at carbon transport patterns in Icelandic rivers. Biomarker concentrations and isotopic compositions such as leaf waxes (n-alkanes, n-alkanoic acids) and soil derived lipids (branched glycerol dialkyl glycerol tetraether) will be used to further investigate provenance, transport and storage mechanisms in the diverse suite of Icelandic catchments.

How to cite: Gallarotti, N., Bröder, L., Lattaud, J., Haghipour, N., and Eglinton, T.: 14C-based deconvolution of relationships between carbon pools in Icelandic rivers and streams, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11287, https://doi.org/10.5194/egusphere-egu23-11287, 2023.

Soils are the largest carbon (C) reservoir in terrestrial ecosystems. There are still numerous uncertainties concerning the fate of soil organic carbon and its feedback on climate change. Radiocarbon is a useful approach to better understanding the carbon cycle. The nuclear weapon testing in the 1960^s induced a peak in ¹⁴C atmospheric concentration – a signal that can be used to trace the incorporation and turnover of C in soil. By separating the soil in different fractions and measuring the ¹⁴C in them, we can quantify how much C and for how long is stored in soils, and where soil organic carbon is stabilised.

Our study aimed at identifying the impact of climate and mineralogy on soil organic matter turnover on a regional scale. We analysed C pools and ¹⁴C contents in the organic layer, mineral soil (0-20cm) and its fractions from 54 sites across Switzerland. These 54 sites are systematically spread across natural climatic and geological gradients and were repeatedly sampled in the 1990s and 2014. The mineral soil was incubated for 181 days and ¹⁴C was measured in the respired CO₂. The mineral soil was fractionated according to density into particulate organic matter (POM) and mineral-associated organic matter (MAOM). We then oxidised the mineral-associated organic matter with hydrogen peroxide to remove its labile fraction of carbon. Our ¹⁴C dataset was analysed together with ancillary data comprising soil properties and climatic variables from the studied sites.

Our radiocarbon dataset showed that the carbon that was respired from the mineral soil originated predominantly from particulate organic matter. The bomb spike signal was incorporated in the organic layer and in the particulate organic matter, while the mineral-associated organic matter had turnover times on centennial to millennial time scales (from 94 to 3060 years). Further chemical oxidation of MAOM using hydrogen peroxide revealed a stronger depletion in radiocarbon of the residual fraction with Δ¹⁴C values ranging between -173 ‰ and -47 ‰. This indicates that the MAOM is a mixture of ¹⁴C-enriched organic matter and very old material.

With respect to the controlling factors of soil organic matter turnover time, the radiocarbon signature of the POM is most strongly affected by climatic variables such as mean annual temperatures. In contrast to POM, the mineral-associated organic matter, comprising the greatest pool of soil organic carbon is driven by chemical soil properties. For instance, older ¹⁴C ages are found in acidic soils with low pH values ranging between 3 and 4. In these soils, Al and Fe oxides concentrations are high. We showed that the concentrations of pedogenic oxides in the soil correlate with soil organic carbon concentrations in the mineral-associated organic matter. In soils with higher pH (>7), we can also find old ¹⁴C ages. In these soils, C is stabilised by interactions with calcium ions and carbonates.

Overall, our regional scale dataset shows that the net accumulation of labile soil organic matter seems to be climate sensitive, while mineralogy and weathering contribute most significantly to the stabilisation of organic carbon in the soil.

How to cite: Moreno Duborgel, M., Minich, L. I., Haghipour, N., González-Domíngez, B., Eglinton, T., and Hagedorn, F.: Using radiocarbon to identify the impact of climate and mineralogy on soil organic matter turnover, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8323, https://doi.org/10.5194/egusphere-egu23-8323, 2023.

Glacial/interglacial transitions in mountainous areas are marked by significant glacier retreat from forelands to inner massifs, resulting in large-scale and ephemeral lake formation that are subsequently filled (or not) by sediment transfer during lateglacial to postglacial times. When valley paleo-infills are preserved, they form precious archives to investigate (1) Alpine erosion dynamics and paleo-environmental conditions during key transition periods from full glacial stages to interglacials, and (2) glacial erosion patterns during susbequent glaciation.

In this contribution, we investigate such sedimentary deposits (locally called as "banquettes") in the French western Alps, and more precisely along the Isère valley and Val du Bourget. Previous research have attributed these deposits to the Riss – Würm transition due to their position under a basal compact till and to the MIS 6/5 transition up to early MIS 4 from palynological constraints, although no absolute ages has been available so far. Based on existing mapping of their spatial distribution and stratigraphic reconstructions, we sampled coarse-sand and sandy-gravel layers within these deposits for constraining both sediment deposition time (OSL dating) and provenance (glacial/postglacial origin, using terrestrial cosmogenic nuclide TCN ¹⁰Be in quartz). In addition, their spatial distribution provides estimates of maximum glacial erosion during the last glacial cycle, which can be subsequently used as spatial constraints for ice model predictions.

Our results confirm deposition times of these sedimentary units at the MIS 6/5 transition, with dating constraints from the late MIS 6 (ca. 145 ka) to the early MIS 5 (Eemien, 115-130 ka) for sandy layers. Upper sandy-gravel layers have younger deposition ages of ca. 80 ka, illustrating sediment fluxes at the transition from late MIS 5 to early MIS 4. We compare this temporal sequence to more recent sediment infills of the Isère valley (¹⁴C and OSL dating) during the Lateglacial to Holocene (MIS 2/1) transition. TCN data from sand samples also illustrate the sharp transition from full glacial to interglacial conditions, with a significant increase in ¹⁰Be concentrations from Lateglacial to post-glacial sediments. We propose that the observed signal can reflect changes in erosion rates, but also in glacier expansion or in paleo-environmental conditions, with export of stored subglacial sediments as well as the re-establishment of sediment/soil production and transfer along the catchment routing system following glacier retreat.

How to cite: Valla, P., Rolland, Y., Delunel, R., Carcaillet, J., and Crouzet, C.: Lateglacial to interglacial sediment infills in Alpine valleys: timing, sediment provenance and paleo-environmental conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8195, https://doi.org/10.5194/egusphere-egu23-8195, 2023.

Paleosols (PSs) contain valuable information about the climatic conditions under which they formed and constitute an outstanding archive of past weathering processes. Nevertheless, paleosol dating over most of the Quaternary remains challenging. Volcanic environments are unique sites for such purposes, as precise radiometric age determination can be achieved on volcanic units ‘bracketing’ PSs. Here, we present a combined geochemical and geochronological study of PSs spanning the last Myr in the Central Azores archipelago (Pico, Faial and São Jorge Islands; central North Atlantic). Precise K-Ar dating of lava flows on groundmass separates (unspiked Cassignol-Gillot technique) yield ages with a typical uncertainty of a few kyr, allowing us to tightly constrain PS ages and weathering rates near key paleoclimatic transitions. PS geochemistry further allowed us to reconstruct weathering conditions and estimate Mean Annual Precipitation and Temperature (MAP & MAT) by two proxies previously validated for other volcanic terranes (CIA-K and Clayeyness).

Four periods of PSs formation are constrained at 870-845 ka, ~725 ka, 320-280 ka and 130-45 ka. Most PSs formed just after interglacial peaks, with a few exceptions. Our MAP reconstructions are variable (600-1,500 mm/yr), but generally lower than current annual precipitations (~1,000 mm/yr). MAT estimates (14-28°C) are higher than present-day annual temperatures (~17.5°C). MAP & MAT variations are in general agreement with global climatic curves; the highest values (28°C, 1,500 mm/yr) are reached at ~855 ka, coinciding with an interglacial peak. The younger PSs (130-45 ka) indicate more stable MAP & MAT in the ranges 650-1,000 mm/yr and 15-20°C, respectively and seem to show a temperature decrease after the MIS5e interglacial stage.

Most paleosols were formed in a few kyr under high MAT (>17°C) and moderate to high MAP (>700 mm/yr), supporting a major influence of temperature on weathering kinetics. Parental material texture also had an important role, as several PSs formed upon pyroclastic deposits over most of their depth, whereas those developed on lava flows were generally restricted to the highly fragmented upper brecciated parts. Minimum vertical soil formation rates are in the range of ~0.3-4.5 cm/kyr, with a mean of ~1.7 cm/kyr and an outlier around ~10 cm/kyr. Those generally high values can be explained by the highly vesicular parental material, and by an exceptionally feldspar-rich (easily weathered) parental rock for the outlier.

As current precipitation and temperatures are higher than the threshold values of ~700 mm/yr and ~17°C under which most PSs formed, enhanced soil formation is expected for the near future, especially in the context of global warming and particularly in volcanic contexts. This may have important and fast impacts on local human activities, but also regarding CO₂ consumption by rock weathering and geological hazards.

How to cite: Hevia-Cruz, F., Hildenbrand, A., Sheldon, N. D., Chabaux, F., Marques, F. O., Carlut, J., and Zanon, V.: Paleoclimate and weathering on volcanic islands: insights from well-dated paleosols spanning the last Myr in the Central Azores, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9064, https://doi.org/10.5194/egusphere-egu23-9064, 2023.

The surficial 10 cm of Scotland’s saltmarshes are estimated to store 1.35 ± 0.33 Mt CO₂ equivalent, approximately 3.37% of Scotland’s national greenhouse gas emissions in 2020^1,2. This is largely achieved through effective preservation of organic matter (OM) in low oxygen, sulphidic soils¹. Saltmarshes gain organic carbon (OC) through in-situ (autochthonous) production by vegetation and benthic microalgae, and the accumulation of marine and terrestrial material during tidal inundation (allochthonous)³.

A key blue carbon challenge is to empirically understand, under current and predicted warmer conditions, the sources of OC accreted into and respired from saltmarshes. This can determine the proportion of the total OC pool which is additional through in-situ sequestration or from increased preservation of allochthonous OM³. Alongside ¹³C and ¹⁵N isotopes, radiocarbon (¹⁴C) analysis/dating can be used to determine the sources of saltmarsh surficial soil OC⁴.

We hypothesise that at ambient temperatures the younger and more labile, and predominantly autochthonous OM, will be preferentially decomposed.  But at elevated temperatures the aged, and predominantly allochthonous, OM pool will increasingly contribute to the respired greenhouse gases.

To test this hypothesis, we collected soil cores and surficial sediment samples from three contrasting Scottish saltmarshes. We analysed them for ¹⁴C to gain an understanding of the age and sources of the autochthonous and allochthonous OM accumulating. We also aerobically incubated sub-samples of the soil in temperature-controlled experiments at 11.1 ± 1°C (ambient) and 20 ± 1°C (elevated). The evolved CO₂ was collected on molecular sieve traps and analysed for ¹⁴C content/age.

Our results will facilitate comparison of the age of the bulk OM and the respired CO₂ to the thermogravimetrically measured reactivity of the OM determined using the recently developed Carbon Reactivity Index⁵. We will present our findings and introduce our ongoing work on anaerobic incubations of the same soils, which includes the novel measurement of the evolved ¹⁴CH₄.

Our research contributes to a growing evidence base for emissions from saltmarshes, and the sources of OC accreting in their soils, which is vital for understanding how they cycle carbon and their ability to mitigate climate change. It will contribute to the creation of saltmarsh carbon cycle models and inform work to include saltmarshes in the UK’s Nationally Determined Contributions.

References

• Smeaton C, Burden A, Ruranska P, et al. Using citizen science to estimate surficial soil Blue Carbon stocks in Great British saltmarshes. Front Mar Sci. 2022;9. Accessed November 28, 2022. https://www.frontiersin.org/articles/10.3389/fmars.2022.959459
• Scottish Goverment. Scottish Greenhouse Gas Statistics 2020.; 2022. https://www.gov.scot/publications/scottish-greenhouse-gas-statistics-2020/
• McTigue ND, Walker QA, Currin CA. Refining Estimates of Greenhouse Gas Emissions From Salt Marsh “Blue Carbon” Erosion and Decomposition. Front Mar Sci. 2021;8. Accessed January 26, 2022. https://www.frontiersin.org/article/10.3389/fmars.2021.661442
• Hajdas I, Ascough P, Garnett MH, et al. Radiocarbon dating. Nat Rev Methods Primer. 2021;1(1):1-26. doi:10.1038/s43586-021-00058-7
• Smeaton C, Austin WEN. Quality Not Quantity: Prioritizing the Management of Sedimentary Organic Matter Across Continental Shelf Seas. Geophys Res Lett. 2022;49(5):e2021GL097481. doi:10.1029/2021GL097481

How to cite: Houston, A., Austin, W., and Garnett, M.: A Novel Method for Radiocarbon Dating Greenhouse Gas Emissions from Saltmarsh Soils to Address Key Blue Carbon Challenges, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8033, https://doi.org/10.5194/egusphere-egu23-8033, 2023.

Measurements of radiocarbon (¹⁴C) in atmospheric methane (CH₄) provide a powerful tool to distinguish fossil from biogenic methane emissions, because fossil methane is completely devoid of ¹⁴C. However, these measurements are particularly challenging as CH₄ is at low concentration in the atmosphere and large volumes of air must be sampled.

At the Laboratory of Ion Beam Physics (LIP), ETH, we developed a portable sampler based on the laboratory prototype in Zazzeri et al. 2021 [1]. The new system enables extraction of carbon from CH₄ while sampling in the field, reducing the sample processing in the laboratory and allowing collection of a hundred liters of air onto a 0.5 g zeolite trap.

Here we present an overview of the sampling system and the technical developments that have been implemented at LIP. The relatively small size of the sampler and its interface with the gas ion source of the AMS system make ¹⁴CH₄ measurements much easier to perform. Its portability will enable collection of CH₄ samples in any environment, with the potential of assessing the radiocarbon signature of methane emissions that have not been yet characterized.

[1] Zazzeri, G., Xu, X., & Graven, H. (2021). Environmental Science & Technology, 55(13), 8535-8541.

How to cite: Zazzeri, G., Wacker, L., Haghipour, N., Gautschi, P., and Graven, H.: 14C analysis of atmospheric methane: development of a portable sampling system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12735, https://doi.org/10.5194/egusphere-egu23-12735, 2023.