The analysis of stable isotopes in fossil mammals is a powerful tool to reconstruct paleoenvironmental and paleoecological conditions. Nevertheless, there are few works of this type focused on the Neogene of South America, specifically, on the Argentine Pampas. In this context, we perform an integrative approach for the Late Miocene-Early Pliocene of this region combining new U-Pb zircon dating and carbon and oxygen stable isotope analysis, to contextualize the paleoenvironmental and paleoecological evolution of the region. The δ¹³C values are used to reconstruct the diets and preferred habitats of the taxa, while changes in the δ¹⁸O values of animals forced to drink reflect variations in δ¹⁸O of meteoric water, controlled by temperature and evaporation rate. We selected a total of 270 bioapatite samples of δ¹³C_CO3 and δ¹⁸O_CO3 from eight localities of La Pampa and Buenos Aires provinces. Radioisotopic ages for six of them allowed us to pin down maximum deposition ages and a time interval of ca. 5 million years for our study (from 9.7±0.3 Ma at Arroyo Chasicó to 4.5±0.2 Ma at Farola Monte Hermoso), including the Chasicoan, Huayquerian, and Montehermosan stages/ages. We studied genera of Litopterna, Notoungulata, Rodentia, Pilosa, and Cingulata orders. During the Chasicoan Stage/Age, herbivore δ¹³C values point to mixed C₃–C₄ diets, evidencing the existence of favorable habitats for C₄ plants before their great expansion. By contrast, during the Huayquerian Stage/Age, taxa show values indicative of feeding preferentially on C₃ plants, except for some rodents that continued including C₄ plants in their diets (possibly related to an early specialization of this group). In the latest Huayquerian-Montehermosan stages/ages, herbivorous taxa incorporated a higher percentage of C₄ plants in their diets, coinciding with the global expansion of this type of vegetation. This change in δ¹³C values also reflects an increase in aridity and/or temperature since the Late Miocene-Early Pliocene in the area, coincident with results of other proxies. The δ¹⁸O values of the notoungulates support these interpretations, evidenced by higher δ¹⁸O values during the Chasicoan and latest Huayquerian–Montehermosan stages/ages; on the contrary, there is no trend recorded in litopterns, while only a slight decrease in δ¹⁸O values was obtained in rodents. These differences are possibly linked to the fact that each order records conditions at different scales; litopterns might register global hydrological conditions, notoungulates regional conditions, and rodents more local conditions. In addition, the difference with rodents is probably due to the fact that they obtained part of the water to cover their physiological needs by eating, in contrast to the notoungulates which needed to drink to meet these requirements.

This study was financed by the projects: PGC2018–094955–A–I00 and PID2022-138275NB-I00 (Ministerio de Ciencia e Innovación, Spain), 13G and 21G (FCEyN, UNLPam, Argentina), PGI 24 H/154 (Secretaría de Ciencia y Tecnología, UNS, Argentina). DSP acknowledges a predoctoral grant PRE2019–089848 and AEM and RLT funding from LA. TE. Andes – CONICET (2020-2021). This is a contribution of the research group UCM 910607 on Evolution of Cenozoic Mammals and Continental Palaeoenvironments.

How to cite: Sanz Pérez, D., Montalvo, C. I., Mehl, A. E., Tomassini, R. L., Hernández Fernández, M., and Domingo, L.: Paleoenvironmental and paleoecological changes during the early GABI in the Argentine Pampas: a stable isotope approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-344, https://doi.org/10.5194/egusphere-egu24-344, 2024.

This study examines marine sediment samples from Ocean Drilling Program Site 1195, Hole B (Lat 20°24.28'S; Long 152°40.24'E; water depth 420 m), located beneath the present-day pathway of the East Australian Current (EAC) on the shelf of Northeast Australia. The EAC represents the western boundary current of the South Pacific subtropical gyre. It originates from the bifurcation of the southern arm of the South Equatorial Current between 15°S and 20°S. The East Australian Current's evolution is linked to the tectonic reorganization of the Indonesian Gateway and the expansion of the West Pacific Warm Pool over geological time. Our analysis of foraminiferal relative abundance data revealed the existence of high surface productivity and reduced subsurface productivity during the 7.5–6.4 Ma Our findings suggest that the northward movement of Papua New Guinea may have commenced after 6.4 Ma, inferred from the consistent thinning of the mixed layer and shallowing of the thermocline. This aligns with the hypothesis proposing the formation of the New Guinea Coastal Undercurrent, potentially caused by the entrapment of a significant portion of the South Equatorial Current against the Papua boundaries, directing it northward. Additionally, we observe a significant decline in the relative abundance of Globigerinoides ruber and a shoaling of the thermocline during the Mid-Pleistocene Transition, coinciding with high surface productivity.

How to cite: Palei, R. R. and Gupta, A.: Paleoceanographic reconstruction of the NE shelf of Australia: Insights from surface and subsurface dynamics of the East Australian Current, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-462, https://doi.org/10.5194/egusphere-egu24-462, 2024.

The investigated area is part of a plateau situated at around 350m elevation in the south-western part of the South Carpathians. The area is crossed from north-west to south-east by dry valleys, which cut Pleistocene and Quaternary clastic deposits. Water is flowing across these valleys temporarily, after strong storms or during rainy periods. A groundwater geochemistry study was carried out on water samples collected from different springs and wells. The study was motivated by the fact that springs have represented the only source of potable water in the region, only recently completed by a few wells.

Spring distributions and geological data reveal the presence of a multi-layered system situated in the Lower Quaternary deposits. Precipitations and meteorological parameters were monitored in the region for a period over 10 years. The mean δ¹⁸O and δD values of groundwater reflect the yearly weighted mean of the isotopic composition of precipitation, demonstrating locally derived recharge. The aquifers are situated at different depths in clastic deposits; the shallower aquifers are affected by evaporations during the drought periods of the summer.

The water samples have concentrations of 157 to 852 mg/l for anion and 55 to 308 mg/l for cations, with TDS between 212 and 1157 mg/l. The total dissolved salts limit proposed in the guideline of WHO is 1200 mg/l, above the limit water having a bad taste, all the measured samples are below this limit.

The Piper ternary diagrams for spring water indicate that the dominant hydrochemical types is HCO₃^--Ca⁺²-Mg⁺² with transition toward higher SO₄^-2 and Mg⁺²  contents to the deeper aquifer. The anions vary from HCO₃^- with transition to no dominant- and with Cl^- contents higher for the shallower aquifer. The sequence of abundance of cations is generally Ca²⁺>Mg⁺²>K⁺>Na⁺ and for anions: HCO₃^->SO₄^-2>Cl^->NO₃^->F^-. The Gibbs diagrams indicate rock weathering as a major driving force for driving the groundwater ionic chemistry in the study area. Radiocarbon dating of DIC (dissolved inorganic carbon) indicates a sub recent recharge of the aquifers.

The vertical and lateral variations in groundwater chemistry may vary, and are influenced by lateral lithologic variation of the Quaternary clastic deposits. This is an unpredictable quality factor when taking the decision for the drinking water drill locations.

How to cite: Bojar, A.-V., Chmiel, S., Bojar, H.-P., Varlam, C., Barbu, V., and Pelc, A.: Spring distribution in Quaternary deposits, South Carpathians, Romania: isotope composition, chemistry and radiocarbon dating, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7606, https://doi.org/10.5194/egusphere-egu24-7606, 2024.

The Chernobyl 1986 accident, considered one of the worst of its kind, occupies the highest 7^th position on the seven levels IAEA International Nuclear and Radiological Event Scale. Following the thermal explosions which took parts, an impressive amount of new and spent nuclear fuel, rich in fission and neutron activation products was dispersed into the atmosphere at an altitude up to a few km, but without reaching the stratosphere. In this way, about 20 to 40 % of the total radiocesium inventory estimated to be 280 PBq of ¹³⁷Cs was transported by the atmospheric circulation contaminating significant areas of Ukraine, Belarus, Russia, Scandinavian countries, Central and Eastern Europe. Its presence was signaled also in Japan, Canada, and the United States.

Due to the geographical position of Romania in the vicinity of Ukraine, the total ¹³⁷Cs contamination of Romanian territory was estimated immediately after the Chernobyl accident at 51 ± 2 TBq, an estimation based on more detailed measurements performed during the 1993 y on 62 locations.

Under these circumstances, and for a more accurate estimation of the ¹³⁷Cs contamination, 747 soil samples covering the entire Romanian territory were collected, and the radiocesium inventory was measured by gamma-ray high-resolution spectroscopy performed in the low backgrounder laboratory located in the Slanic-Prahova former salt mine. Soil sampling and radiometric measurements were performed between 2016 and 2018 years, all data being recalculated for May 2016, i.e. 30 years after the Chernobyl accident.

The results showed for the 2016 radiocesium distribution an irregular pattern containing four maxima of which positions were quite different from the 1993 ones. Concerning the ¹³⁷Cs inventory, its total value decreased from 43 ± 2 TBq in May 1993 to 14.1± 0.7 TBq in May 2016, i.e. by a factor of 3 ± 0.3, twice of natural disintegration. This finding could be explained by taking into account that a significant amount of radiocesium was washed out by precipitation and, in a lower measure, was incorporated into plants.

The same data permitted evaluation of the total contribution of ¹³⁷Cs to the population exposure. Accordingly, in 1993 and even more so in 2016, the average supplementary annual effective dose did not exceed 1 mSv, i.e. the maximum annual effective dose considered not harmful for the unexposed population.

How to cite: Duliu, O. G., Blebea-Apostu, A.-M., Margineanau, R. M., Persa, D., and Gomoiu, M.-C.: On the radiocesium distribution on the Romanian territory 30 years after the Chernobyl disasters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8232, https://doi.org/10.5194/egusphere-egu24-8232, 2024.

The Middle Miocene Climatic Optima (MMCO; 17–14.5 Ma) is warmest over the last 23 Myrs, with higher pCO₂ (400–500 ppmv; Foster et al., 2012, Super et al., 2018), ~6 °C warming in a mid-latitude compared to the present (Flower and Kennett, 1994), and a large reduction (30–36 m) in Antarctic ice volume (Gasson et al., 2016). The eruption of CO₂ from the Columbia River flood basalt has been suggested as a primary cause of the MMCO. However, the mechanisms of the progressive global cooling after the MMCO remain highly controversial. Here, we provide novel paleoceanographic information on the Antarctic Intermediate Water (AAIW) in the Pacific sector of the Southern Ocean to improve our understanding of climate–ocean conditions during the MMCO. In this study, we analyzed the middle Miocene biopelagic sediments (> 90% CaCO₃ contents) from ODP Site 1120 (50°3.8′S, 173°22.3′E), located on the central Campbell Plateau off the South Island of New Zealand (Ando et al., 2011). Because the middle Miocene paleo-water depth at Site 1120 is estimated to be similar to the present water depth (~600 m), analyses of oxygen and carbon isotopic compositions of benthic foraminifer and neodymium (Nd) isotopes of fossil fish teeth/debris allow us to characterize the AAIW during the deposition. We present newly measured 17.5–8.5 Myr records of Nd isotopes of fossil fish teeth/debris, planktonic foraminiferal δ¹⁸O, δ¹³C, Mg/Ca, and Ba/Ca from Site 1120. The ε_Nd values ranged from -7.5 to -3.2 at 17.5–8.5 Ma. In contrast, during the Middle Miocene Climate Transition (MMCT, 14.5–13.5 Ma), ε_Nd values shifted rapidly toward more radiogenic values (~-3.2) and then gradually returned to less radiogenic values (-7 to -6). Such a large long-term variation of ε_Nd values has not been reported in previous datasets, which is the first records to represent the characteristics of the AAIW in the Pacific sector of the Southern Ocean during the MMCT. We argue for the two possible causes of these ε_Nd changes in the AAIW: 1) there might have been an anomalous supply of radiogenic Nd due to the intense physical weathering in West Antarctica caused by the onset of glaciation and 2) the equatorial surface water, characterized by high ε_Nd values, might have expanded toward the high latitudes and a part of the water mass was incorporated into the intermediate layer in the Southern Hemisphere.

How to cite: Khim, B.-K., Horikawa, K., and Asahara, Y.: Anomalous eNd change of the Antarctic Intermediate Water in the Pacific sector of the Southern Ocean during the Middle Miocene Climate Transition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3364, https://doi.org/10.5194/egusphere-egu24-3364, 2024.

The chromium (Cr) isotope system (δ⁵³Cr) is a promising tool to reconstruct changes in marine redox conditions and biological productivity through geological time, but uncertainties remain regarding the pathway of Cr from the water column to the sediment record (Janssen, 2021; Huang et al., 2021; Bruggmann et al., 2023). While sediment Cr concentration and isotope data are available from continental margin sites and oxygen minimum zones (e.g., Gueguen et al., 2016; Bruggmann et al., 2019), modern sediments from open marine settings are understudied. Such sediments can provide key information to understand how Cr from the water column or across the sediment-water interface accumulates in the sediment.

We will investigate the response of Cr concentrations and isotope compositions (i) in the authigenic fraction of different types of pelagic sediments (carbonate-rich versus clay-rich) (ii) from the North Atlantic to the North Pacific to complement published seawater δ⁵³Cr values, and (iii) their changes during well-characterised climate changes in the Quaternary, specifically the Last Glacial Maximum (LGM). The database of Cr isotope compositions in the modern global oceans resulting from this study will significantly enhance the interpretation of Cr isotope changes in sediments from the geological record.

References

Bruggmann S., Severmann S. and McManus J. (2023) Geochemical conditions regulating chromium preservation in marine sediments. Geochimica et Cosmochimica Acta 348, 239–257.

Gueguen B., Reinhard C. T., Algeo T. J., Peterson L. C., Nielsen S. G., Wang X., Rowe H. and Planavsky N. J. (2016) The chromium isotope composition of reducing and oxic marine sediments. Geochimica et Cosmochimica Acta 184, 1–19.

Huang T., Moos S. B. and Boyle E. A. (2021) Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone. Proc. Natl. Acad. Sci. U.S.A. 118.

Janssen D. J. (2021) Release from biogenic particles, benthic fluxes, and deep water circulation control Cr and δ⁵³Cr distributions in the ocean interior. Earth and Planetary Science Letters 574,117163.

How to cite: Bruggmann, S., Blaser, P., and Jaccard, S.: Chromium isotope compositions of sediments from the global oceans, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18475, https://doi.org/10.5194/egusphere-egu24-18475, 2024.

Proxy observations are the fundament for many insights in geosciences, such as the reconstruction of past environmental conditions from sediment cores. However, the labor-intensive nature of producing proxy data often renders high-resolution records prohibitively expensive. In contrast, state-of-the-art XRF analyses offer an efficient means of generating high-quality and very high-resolution elemental concentration data. Although these data initially provide only qualitative information about the bulk sample composition, their high-resolution renders them invaluable as a foundation for subsequent studies.

Here we explore the potential utility of high-resolution XRF data as a foundation for interpolating more scarce sedimentological, geochemical, and environmental analyses to the same high-resolution. We base the investigation on a long sediment core obtained from the Crozet Plateau in the Indian sector of the Southern Ocean, where changes in surface productivity, dust influx, and delivery of detritus from the nearby island dominate the sedimentological and geochemical signals.

We test different statistical methods for sediment classification and the interpolation of proxy data and discuss their reliability and limitations. For instance, application of a random forest model for the interpolation of carbonate and opal concentration conspicuously reveals the presence of distinct detrital layers. These layers are not readily discernible in the original XRF data or the sparse original opal and carbonate measurements.

We propose these straight-forward statistical methods have potential to be used on many other sediment records for which high-quality and high-resolution XRF data and certain proxy observations are available. Such an approach can streamline the interpretation of XRF data and enhance the investigation of specific environmental proxy observations at a high-resolution.

How to cite: Blaser, P., Lippold, J., Bruggmann, S., Bollen, M., Crosta, X., and Jaccard, S.: Interpolating proxy observations to high-resolution XRF data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18743, https://doi.org/10.5194/egusphere-egu24-18743, 2024.

Following the announcement of the retirement of the JOIDES Resolution drilling platform, it has become even more important to efficiently utilise the finite resource of marine sediment stored in IODP repositories. Marine sediments processed for inorganic geochemical analysis are often separated into fine (<0.63 µm) and coarse (>0.63 µm) fractions to help isolate benthic and planktonic foraminifera. However, organic matter can be associated with different particle size fractions and may have experienced different transport and diagenetic processes. Previous studies have suggested that sieving sediments into different size fractions does not affect the distribution of isoprenoidal [1,2] and branched glycerol dialkyl glycerol tetraethers (GDGTs) [3]. However, this has never been systematically investigated across a wide range of sample types (e.g., age, depositional environment, thermal maturity). It is also unclear whether size processing affects other lipid biomarker proxies (e.g., leaf waxes, alkenones).

Here we test whether processing marine sediments into different size fractions influences lipid distributions by separating sediments into fine (<0.63 µm) and coarse (> 0.63 µm) fractions and comparing these to corresponding bulk un-sieved sediments. Temperature reconstruction using the marine sea surface temperature proxy TEX₈₆ shows relatively minimal deviation (average ±0.12 TEX₈₆ units, or ~2-3 °C) between the bulk un-sieved sediment (i.e,. control) and fine (<63 µm) fraction, suggesting isoGDGTs are well preserved in the fine fraction. In contrast, relatively more variation is seen in the coarse fraction (±0.25 TEX₈₆ units, or >10 °C). We also analysed leaf wax derived n-alkanes extracted from the marine sediment to evaluate the impact on terrestrial biomarkers. The average chain length shows similar deviation in both the fine (±0.21 units) and coarse (±0.21 units) fractions relative to the bulk sediment, suggesting that either fraction is suitable for interpreting first-order changes in vegetation type.  Moving forward, our results suggest that the fine fraction of grain size-sorted sediment yield similar lipid distributions compared to the bulk un-washed sediment. However, coarse fractions often show large deviations from the bulk sediment across different proxies, perhaps making these unsuitable for biomarker-based climate reconstruction.

References
[1] Zachos et al., Geology, 34, 9, pp 737-740 (2006)
[2] Xiao et al., Global Biogeochemical Cycles, 37, e2022GB007648 (2023)
[3] Peterse and Eglinton, Frontiers in Earth Science, 5, 49 (2017)

How to cite: Hingley, J., Bray, P. S., Foster, G., Whiteside, J., Wade, B., and Inglis, G.: Testing the efficacy of grain size-sorted sediment for biomarker analysis to reconstruct palaeoclimate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9896, https://doi.org/10.5194/egusphere-egu24-9896, 2024.

Dust plays a vital role in global climate and environmental change by influencing the Earth’s radiation budget and providing nutrients to marine and terrestrial ecosystems. Modern dust emissions are dominated by North Africa, the Middle East and West Asia, which together provide ~80% of total global atmospheric dust. Geological deposits of dust can also act as valuable archives to study hydroclimate variability across a range of global states, with marine sediments in particular able to preserve high resolution and continuous records of past dust emissions. For example, dust preserved in Arabian Sea sediments has been used to provide climatic context for hominin evolution in East Africa, however, there is little understanding of where the deposited sediments originate and hence whether they truly record an African signal. Tracing the provenance of the lithogenic fraction in marine sediments is made particularly challenging by the lack of geochemical data in key potential source areas such as Mesopotamia, one of the most active dust emission regions in the Middle East. Here we present new radiogenic isotope data (Sr and Nd) from surface sediment samples, integrated with the satellite-derived dust source activation frequency (DSAF) maps and other published radiogenic data to characterise the geochemical fingerprint of dust-producing regions in the circum Arabian Sea. Our results provide a framework to trace sources of dust in geological archives including marine sediments, speleothems and ice cores and to identify the provenance of archaeological artefacts. We exemplify the utility of our approach by comparing our data to strategically positioned marine cores in the Arabian Sea to shed new light on key regional palaeoclimate reconstructions.

How to cite: Zhang, K., Kunkelova, T., Crocker, A. J., Gale, A., Xuan, C., and Wilson, P. A.: A radiogenic isotope framework to study palaeoaridity in the Middle East, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12864, https://doi.org/10.5194/egusphere-egu24-12864, 2024.

A +1.6 km borehole, southeast of the Lubambe Copper Mine, Zambia has intersected a seemingly continuous sedimentary sequence spanning the Neoproterozoic Kantanga Supergroup sedimentary succession within the Central African Copperbelt. This extensive sequence encompasses the Mindola Clastics Formation of the Lower Roan Subgroup, through the Upper Roan Subgroup, and the lower Nguba Group (including the Grand Conglomérat of the Mwale Formation and overlying Kakontwe Limestone). Lithofacies and mineralogical studies of this drillcore have revealed numerous sedimentary cycles, with clear evidence for evaporite formation during sedimentation and diagenetic processes throughout (e.g., bedded/vein anhydrite, chicken-wire textures, relic cements & pseudomorphs). Siliciclastic red-bed lithologies at the base transition into repeated cycles of shales/siltstones-dolostone-evaporite, succeeded by deeper water carbonate and siltstone intervals. Diamictites of the Mwale Formation (Grand Conglomérat unit) cap these cycles at the base of Nguba Group, interbedded with several thick carbonate beds (peloidal grainstones, dolostones +/- bioherm textures).

To explore the Upper Roan to Nguba Group carbonate successions, detailed mineral chemistry mapping (Tescan TIGER MIRA3 FEG-SEM, µXRF, pXRF) and C-O isotopic analysis were undertaken, with a focus on understanding global trends associated with the onset of diamictite formation. Mineral chemistry analysis revealed the influence of post-depositional processes on the sequence, including dolomite and silica alteration. Silicification selectively preserved peloids above the first diamictite occurrence while the original carbonate matrix dissolved, with later cement formation (hydrothermal?) associated with elevated Mn-Fe relative to earlier carbonates. However, most of the carbonate units from the upper successions are composed of dolomite grains/cement, with original textures often preserved (e.g., stromatolites, disrupted reefs).

Isotope sampling targeted the ‘least-altered’ carbonates from the upper Roan to lower Nguba Group, to minimize post-depositional alternation effects. The initial analysis revealed variable δ¹³C isotopic values, with ~negative trending excursions below the main diamictite body. The lack of notable covariance with corresponding O isotope values suggests representation of the original seawater composition, not impacted by local diagenetic/hydrothermal alterations. Further, the largest negative excursion is associated with a lithofacies change from non-stromatolite- to stromatolite-bearing dolomitic siltstones, suggesting local lithofacies changes may influence the isotope profile. Specifically, δ¹³C isotopic values drop from ~+4.2 ‰ to +0.37 ‰, before returning to ~+4.7 ‰ below the diamictite contact. While these units do not record the more extreme negative excursion (<-5 ‰) typically associated with the Sturtian glaciation recorded elsewhere, the overall curve may represent the Islay anomaly recorded globally in Cryogenic sections beneath the Sturtian diamictite. This negative excursion, in line with global C isotope trends (<-5 ‰) has also been identified within Copperbelt on both the Zambian and Democratic Republic of Congo sides (ongoing work), but with a more significant decrease in C isotope values, in line with global trends. The lack of exact duplication may be the result of local sedimentological factors, as shown by the association in lithofacies variation, an incomplete sample record, or perhaps the influence of the Sturtian is not fully recorded in the stratigraphy underlying these diamictites.

How to cite: Doran, A., Stacey, J., Torremans, K., Hitzman, M., Vincent, V., Hepburn, L., and Boyce, A. J.: Deciphering the Central African Copperbelt sedimentary succession: Chemostratigraphy of the Neoproterozoic metasedimentary sequences at Lubambe, Zambia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19482, https://doi.org/10.5194/egusphere-egu24-19482, 2024.

During the late Pliocene-early Pleistocene Tunisian calcretes were formed in a soil environment. Fabrics at the macro and micro scale show that these deposits are similar to rhizogenic modern analogue systems forming in Spain. We infer that a similar climate prevailed in Tunisia in the past, specifically winter-season rainfall and a dry summer with annual rainfall close to 430 mm/year in the center of Tunisia. Here, we provide further constraints on the climate under which the Tunisian calcrete formed by looking at vegetation structure, water advection and amount of seasonal rainfall.

To investigate (1) the palaeo-vegetation type(s) if they are C3 or C4 plants and (2) the source(s) of meteoric water of Tunisian calcretes during that era, stable isotopes of C and O were analysed in 25 samples taken from 5 horizontal laminar calcretes cores, from 3 sites: North (N36º.43.713; E10º.06.681’), Center (N35º.07.077’; E10º.14.545’), South (N33º.28.898’; E10º.23.602’). Results are expressed relative to the VPDB reference.
In the North, the δ¹⁸O samples show values varying from – 4.78 ‰ to -6.91 ‰. Likewise, central site cores are characterised by δ¹⁸O values ranging from -5.32 ‰ to -6.97 ‰. In contrast, the δ¹⁸O values from the South are more depleted (-8.82 ‰ to -7.20 ‰). Concerning the carbon isotope results, both central and southern sites show similarly enriched δ¹³C values with an average of -6 ‰, while the North site shows more depleted values (-11.3 ‰ to -9.6 ‰). The δ¹⁸O values are similar to those determined in the last deglaciation/early Holocene speleothem carbonates from caves in the Tunisian Atlas Mountains, indicating a North Atlantic source. The north-to-south difference in both isotope systems reveals a decoupling between precipitation δ¹⁸O and vegetation effects. The southern site shows more depleted water isotopes (a consequence of internal water recycling effects) and more enriched δ¹³C, consistent with C4 vegetation and/or lower soil respiration. The central site shows low water recycling, but southern-like dryland vegetation. The northern site shows low water recycling and C3 vegetation and/ or higher soil respiration. Consequently, although there is enhanced humidity in all three sites, the sites do not record the same amount of rainfall and the same response of the landscape to form calcretes.

How to cite: Jarraya, F., Mauz, B., Rogerson, M., Kallel, N., Elmejdoub, N., and Sghari, A.: Stable isotopes and palaeo-hydrological implications of Tunisian laminar calcretes during the late Pliocene - early Pleistocene, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2807, https://doi.org/10.5194/egusphere-egu24-2807, 2024.

Over geological times, the evolution of carbon isotope composition of carbonates (δ¹³C_carb) in the sedimentary record is punctuated by numerous positive isotopic excursions (CIEs), which reflect significant perturbations of the carbon cycle on Earth surface environments. Such isotopic events are mainly interpreted as a consequence of an increase of organic carbon burial in sediments. However, the significant spatial and temporal variability observed in many Proterozoic sedimentary successions recording positive CIEs still challenge this postulate. Among others, the potential influence of methanogenesis has been raised to explain at least a part of the variability observed during CIEs.

The Dziani Dzaha is a shallow tropical volcanic crater lake located on the Petite Terre Island of Mayotte (Comoros Archipelago, Indian Ocean). Its water most likely originate from the nearby ocean through bedrock seepage thanks to the fracturation associated with the phreatomagmatic eruption at that formed the crater 7 to 4 Kyr ago. Based on numerous analogies, this atypical modern lacustrine system was considered as an analogue of Proterozoic environments, and more specifically of those having recorded strongly positive δ¹³C values, such as for example the Lomagundi-Jatuli event. A previous study demonstrated the significant role of methane in the Dziani Dzaha carbon cycle through an intense organic matter degradation by methanogenesis associated with methane degassing into the atmosphere. Here, in order to investigate the processes responsible for the onset of this methanogenic activity, we present coupled C and S isotope records in a sediment core of the lake.

Based on geochemical and sedimentological evidences, four different units have been identified in the sediment core. From the bottom to the top, carbon and sulfur isotopic signatures in the first unit are similar to modern oceanic values (δ¹³C_org ~ -25‰, δ¹³C_carb ~ 0‰ and δ³⁴S_py ~ -20‰), which is consistent with a marine origin of the lake water. In the second unit, the δ³⁴S_py increases progressively from -20‰ to -10‰ while the δ¹³C of organic and inorganic carbon remain constant, which is consistent with a progressive consumption of the sulfate pool through the degradation of organic matter by sulfatoreduction in a restricted environment. The δ³⁴S_py shifts sharply to 35‰ at the transition with the third unit where it remains constant up to the top, while both organic and inorganic carbon isotopic signatures increase progressively from -25‰ to -14‰ and from 0‰ to almost 20‰, respectively. This turning point most likely results from a depletion of the initial sulfate pool to a point allowing part of the organic matter to be remineralized through methanogenesis with a degassing of methane into the atmosphere. These results highlight the potential of C-S-isotope coupling to identify a potential impact of methanogenesis on the carbon-isotope signatures observed in the sedimentary record through geological times.

How to cite: Cadeau, P., Ader, M., Cartigny, P., Jovovic, I., Adam, P., and Grossi, V.: From an Ocean-like to Methanogenesis-dominated carbon cycle in the Dziani Dzaha Lake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20288, https://doi.org/10.5194/egusphere-egu24-20288, 2024.