BG2.8

Pinna nobilis is a large bivalve endemic to the Mediterranean Sea that lives in shallow coastal areas. Due to its size and relatively fast shell growth rates, it is an interesting taxon for high resolution geochemical and sclerochronological research. Subsequently to previous analyses of δ¹⁸O and δ¹³C in P. nobilis shells, here, we investigate nitrogen isotopes in the carbonate-bound organic matrix (δ¹⁵N_CBOM) of this species. Our objectives were to test if P. nobilis shells (i) can be used as an indicator of the isotopic baseline of the system, and (ii) is a good candidate for obtaining high-resolution temporal data on environmental δ¹⁵N variability. Due to the multiple mass mortality events of P. nobilis spreading throughout the Mediterranean, including the Adriatic Sea, we also tested if (iii) P. nobilis geochemistry changes as a response to diseases.

Shells were opportunistically collected by skin diving from 4 shallow coastal localities in the eastern Adriatic, as a part of a project on mortality monitoring. Specimens from Lim channel (October 2019), Kaštela Bay (January 2020) and Mali Ston Bay (November 2019) were collected alive, while in Pag Bay, shells of three recently dead specimens were collected in September 2020. Tissue and epibionts were removed and shells carefully cleaned and air-dried. Shell powder was collected by milling sample swaths by hand using a DREMEL Fortiflex drill equipped with a 300 μm tungsten carbide drill bit. For δ¹⁵N_CBOM analysis, three shells from each locality were processed and three replicas were collected from each of these shells by milling shallow lines parallel to the growth axis from the internal shell surface. In addition, high-resolution δ¹⁵N_CBOM data were obtained for one shell from Kaštela by milling lines (N=40) perpendicular to the major growth axis from the external shell surface. From this shell we also collected shell powder for δ¹⁸O_shell and δ¹³C_shell analysis to enable placing δ¹⁵N_CBOM into temporal context. Isotope samples were analyzed Union College on an elemental analyzer - isotope ratio mass spectrometer.

Results indicate significant differences in δ¹⁵N_CBOM between sampling localities, with lowest values recorded for shells from Pag Bay (3.73±0.36‰), and highest for shells sampled in Lim channel (7.04±0.63‰). High-resolution δ¹⁵N_CBOM data obtained from the shell collected from Kaštela Bay corresponded to a time interval from spring 2018 to spring 2019. These data showed relatively small variations (5.02±0.33‰). However, δ¹⁵N_CBOM values increased to 8.65±1.61‰ closest to the shell margin, and were coupled with a decrease in δ¹³C_shell values, indicating that this animal was experiencing stressful conditions several months prior to its death. According to our findings, δ¹⁵N_CBOM values serve as an indicator of the isotopic baseline of the ecosystem as well as a potential powerful tool to study bivalve physiology.

Research was the supported by the Croatian Science Foundation, research project BivACME.

How to cite: Peharda, M., Gillikin, D., Schöne, B., Verheyden-Gillikin, A., Uvanović, H., Markulin, K., Šarić, T., and Župan, I.: Nitrogen isotope sclerochronology - insights into coastal environmental conditions and Pinna nobilis ecology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-454, https://doi.org/10.5194/egusphere-egu21-454, 2021.

The Smooth clam Callista chione is a commercially important venerid bivalve. It is widely distributed in the eastern Atlantic Ocean and the Mediterranean Sea and inhabits sandy sediments in coastal waters at depths down to 180 m. With specimens that can reach 10 cm of shell length and with a lifespan of more than four decades, C. chione represents an interesting archive for sclerochronological research. The aim of this study was to analyse possible variations in δ¹⁸O_shell and δ¹³C_shell values between C. chione specimens collected in different parts of the Mediterranean Sea.

Callista chione shells were collected alive from three localities: (1) Caleta de Vélez in the north region of the Alborán Sea, Spain; (2) Gulf of Venice, Italy, North Adriatic, and (3) west coast of the Istrian peninsula, Croatia, also in the North Adriatic. At the first two localities, specimens were obtained from catch of the commercial fishing vessels, while in Istria they were collected by SCUBA diving. Prior to analysis, the external shell surface was physically cleaned by grinding. Shell powder for δ¹⁸O_shell and δ¹³C_shell analysis was then collected by milling narrow sample swaths in the outer shell surface and processed at Mainz University using a GasBench II - IRMS.

Modelled daily temperature and salinity values were obtained for each locality and used for calculating the predicted δ¹⁸O_shell values. For Caleta de Vélez, daily temperature and salinity values were obtained by MEDSEA model; for the Venetian region by the AdriSC climate model, and for Istria by the 3D numerical model ROMS. Temporal alignment of measured δ¹⁸O_shell values was conducted manually in Excel by best-fitting measured isotope data to predicted δ¹⁸O_shell curves.

Seasonal δ¹⁸O_shell cycles were observed in all studied specimens. Temporal alignment of measured and modelled δ¹⁸O_shell values clearly showed that C. chione grew fast during the warm part of the year, while slower growth occurred during the winter months. Samples collected in Caleta de Vélez had the narrowest range of δ¹⁸O_shell values (-0.43 to +1.73 ‰), while δ¹⁸O_shell values in C. chione from Istria showed the largest amplitudes (-1.61 to +2.67 ‰). Growth patterns varied between sampling localities.

The δ¹³C_shell values varied strongly between localities and specimens. Highest δ¹³C_shell values were obtained for C. chione shells from Caleta de Vélez (0.51 ± 0.03 ‰, range -0.19 to +1.06 ‰). The broadest range of δ¹³C_shell values (-3.37 to -0.08 ‰) were measured in shells from the Venetian region. These samples also had the lowest mean (-1.42 ± 0.14 ‰). Shells from Istria had δ¹³C_shell values ranging from -1.57 to +0.38 ‰ (mean: -0.42 ± 0.28 ‰). Observed differences between localities are statistically significant (Kruskal Wallis H = 150.4, p < 0.001).  Isotope data obtained for this study were compared with data from a previous study on the same species in the Eastern Adriatic.

Research was supported by the Croatian Science Foundation, research project BivACME.

How to cite: Uvanović, H., Schöne, B. R., Janeković, I., Denamiel, C., Mazzoldi, C., Baro, J., Markulin, K., and Peharda, M.: Callista chione – geochemical archive of δ18O and δ13C data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7559, https://doi.org/10.5194/egusphere-egu21-7559, 2021.

The tribe Lithospermeae (fam. Boraginaceae) represents one of very few taxa vascular plants that accumulate appreciable amounts of calcium carbonate in their tissues. The CaCO₃ is localized in the pericarp sclerenchyma, which makes their small fruits (nutlets) mechanically durable and provides their good preservation in sediments and cultural layers. Fossil Lithospermeae fruits appear as whitish, slightly elongated entities, 3-5 mm in length.  At archaeological sites, the nutlets can be of diverse origin: in most contexts they represent carpological evidence for weed flora of the past, however, some findings suggest that they were used for decorative purposes (beads etc.).  

Here we overview the potential use of fruit carbonate of Lithospermeae in paleoecological research.   

¹⁴C-dating: Fruit carbonate of the taxon can be successfully dated with radiocarbon.  

The ¹⁴C concentration in the CaCO₃ fraction of modern nutlets is well-correlated to the recent atmospheric ¹⁴C levels. Radiocarbon ages of old nutlets are in good correspondence with the age ranges of archaeological contexts. Obviously, fruit carbonate can represent a geochemically closed system for millennia in sediment environments.               

δ¹⁸O values: Our data based on an array of herbarium exemplars of Lithospermeae, suggest that the δ¹⁸O of fruit carbonate is distinctively sensitive to the amount of atmospheric precipitation during the warm season. The degree of correlation between δ¹⁸O and local air temperatures is lower.

We further performed an experiment on gromwell (Buglossoides arvensis (L.) I.M.Johnst), irrigated by water with different oxygen isotope signatures. The δ¹⁸O values of fruit CaCO₃ showed correlation to the δ¹⁸O of irrigation water. The oxygen isotope fractionation in fruit carbonate turned out to be surprisingly low with 1000lnα = 4.72±3.49, which is relatively close to foraminiferal CaCO₃.

 δ¹³C values: In contrast to the oxygen isotope signature, we did not find a strong correlation of the δ¹³C values of fruit carbonate to precipitation and temperature.  However, the photosynthetic origin of carbon in fruit CaCO₃ admits a possibility of some links of δ¹³C to ambient factors.  

How to cite: Pustovoytov, K. and Riehl, S.: Fruit carbonate in Lithospermeae: 14C, stable isotope composition and potential as a paleoenvrionmental proxy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11778, https://doi.org/10.5194/egusphere-egu21-11778, 2021.

Benthic nepheloid layers (BNLs) are particle-rich layers that can extend over a thousand meter or more above the seafloor and are thought to be produced by the resuspension of fine sediments from strong bottom currents. They can often be subdivided into two sublayers: (i) a lower sublayer in contact with the seabed, where particle concentrations are the largest and which roughly coincides with the bottom mixed layer (BML); and (ii) an upper sublayer in which particle concentration decreases up to a clear water minimum (CWM). Although BNLs have long been recognised in vertical traces of optical instruments lowered to abyssal depths, their influence on ocean biogeochemical cycles – on the cycling of particle-reactive metals in particular – remains poorly understood.

In this study, we characterize the BNLs observed between the New England continental shelf and Bermuda and explore their influence on the cycling of ²³⁰Th and ²³¹Pa – two naturally-occurring particle-reactive radionuclides that have found different applications in chemical oceanography and paleoceanography. To this end, we use concomitant measurements of temperature, salinity, particle concentration derived from light beam transmissometry, and ²³⁰Th and ²³¹Pa activities in the dissolved and particulate fractions, which have been collected along the western segment of the U.S. GEOTRACES GA03 transect. We estimate that the thickness of strong BNLs (particle concentration > 20 µg l^-1) varied from about 72 to 1358 m between different deep stations. At all stations, particle concentrations below the CWM were the highest in the BML, whose thickness ranged from 95 to 320 m, and decreased generally with height above the seafloor. A simplified model of particle-radionuclide cycling in the deep water column, which includes a particle source representing sediment resuspension at topographic reliefs and their subsequent lateral transport, is fitted to observed profiles of particle concentration and radionuclide activities at two selected stations. The model can reproduce simultaneously the increase of particle concentration with depth, the low dissolved activities in the BNLs, and the extremely large particulate activities near the bottom. Analysis of ²³⁰Th and ²³¹Pa budgets reveals that the behaviour of both radionuclides in the BNL is fundamentally different from that envisioned in reversible exchange theory. Sensitivity tests with the model suggest that lateral particle sources near continental slopes and similar reliefs can produce significant biases in the paleoceanographic applications of both radionuclides, including the ²³⁰Th-normalization method and the interpretation of sediment ²³¹Pa/²³⁰Th records.

How to cite: Chen, S. S., Marchal, O., Lerner, P., McCorkle, D., and Rutgers van der Loeff, M.: Benthic Nepheloid Layers along the U.S. GEOTRACES GA03 Transect in the Western North Atlantic: Characterization and Influence on 230Th and 231Pa Cycling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6510, https://doi.org/10.5194/egusphere-egu21-6510, 2021.

Boron isotope ratios, as measured in marine calcium carbonate, are a proven tracer of past seawater and calcifying fluid pH and thus a powerful tool for the reconstruction of past atmospheric CO₂ and monitoring of coral physiology. For such applications, understanding the inorganic baseline upon which foraminiferal vital effects or coral pH upregulation are superimposed should be an important prerequisite. Yet, investigations into boron isotope fractionation in synthetic CaCO₃ polymorphs have often reported variable and even conflicting results, implying that we may not fully understand pathways of boron incorporation into calcium carbonate.  Here we address this topic with experimental data from calcite and aragonite precipitated across a range of pH in the presence of both Mg and Ca. We confirm the results of previous studies that the boron isotope composition of inorganic aragonite precipitates closely reflects that of aqueous borate ion, but that calcites display a higher degree of scatter, and diverge from the boron isotope composition of borate ion at low pH. We discuss these findings with reference to the simultaneous incorporation of other trace and minor elements, and highlight a number of mechanisms by which crystal growth mechanisms may influence the concentration and isotope composition of boron in CaCO₃. In particular, we highlight the potential importance of surface electrostatics in driving variability in published synthetic carbonate datasets. Importantly for palaeo-reconstruction, however, these electrostatic effects are likely to play a much more minor role during natural precipitation of biogenic carbonates.

How to cite: Henehan, M., Klein-Gebbinck, C., Foster, G., Wyman, J., Hain, M., and Kim, S.-T.: No ion is an island: Multiple ions involved during boron incorporation into CaCO3, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10652, https://doi.org/10.5194/egusphere-egu21-10652, 2021.