https://hdl.handle.net/10481/31820 Sat, 18 Apr 2026 02:45:03 GMT 2026-04-18T02:45:03Z https://hdl.handle.net/10481/112761 Mg isotope fractionation during microbial dolomite formation in the Khor Al-Adaid sabkha, Qatar Tatzel, Michael; Paytan, Adina; Carter, Samantha; Frick, Daniel A.; Oelze, Marcus; Martinez Ruiz, Francisca; DiLoreto, Zach A.; Griffith, Elizabeth M.; Dittrich, Maria; Bontognali, Tomaso R. R.; Sánchez-Román, Mónica The processes governing dolomite [CaMg(CO3)2] formation remain among the most debated topics in sedimentary geology. Although primary dolomite can precipitate at low temperatures in certain modern environments, its scarcity today contrasts sharply with its abundance in ancient rocks—a discrepancy known as the ‘dolomite problem’. Dolomite typically forms through two pathways: (1) primary precipitation during early diagenesis, often influenced by microbial activity and organic matter and (2) secondary replacement of preexisting carbonates during burial at higher temperatures. In this study, we investigate Mg isotope fractionation in a modern sabkha in southern Qatar to evaluate its potential as a tracer of dolomite formation processes. We analysed δ26Mg and δ44Ca in surface- and pore waters, authigenic clays and organic- and leached dolomite-containing fractions. Ca isotopes reveal an ~1‰ fractionation between pore water–organic matter and dolomite, consistent with a two-step, biologically mediated formation pathway. Contrary, only minor 26Mg enrichment in the organic fraction relative to pore water suggests that Mg isotopes alone provide limited evidence for such microbial mediation. Dolomite δ26Mg values (~−2.15‰) align with predictions for temperature-dependent inorganic precipitation. Overall, the results indicate that microbial activity probably influences dolomite formation indirectly by altering local water chemistry rather than having a distinct Mg isotopic fractionation. These findings refine the application of Mg isotopes as proxies for dolomite genesis and offer new insights into carbonate diagenesis in saline environments. MT is grateful for funding by the DFG (395854813). MSR acknowledges financial support from the Dutch Research Council (NWO) Projects GEOBIOCARBON: OCENW.KLEIN.037 and Beatriz Galindo Senior Fellowship (No. BG23-00132) funded by the Spanish Ministry of Science, Innovation and Universities (MICIU). FMR acknowledges funding by MICIU/AEI/10.13039/501100011033 (grant PID2023-147440OB-C22). Maria Dittrich (University of Toronto) would like to acknowledge the Qatar National Research Fund (member of Qatar Foundation) for the Grant NPRP12S-0313-190349. The project was supported by the National Sciences and Engineering Research Council of Canada (NSERC Discovery Grant, RGPIN-06184) and the Canada Foundation for Innovation and Ontario Research Fund (Leaders Opportunity Fund, grant number 22404). Tomaso R.R. Bontognali acknowledge the Qatar National Research Fund (member of Qatar Foundation) for the Grant NPRP12S-0313-190349, NPRP13S-0207-200291. https://hdl.handle.net/10481/112761 https://hdl.handle.net/10481/112676 Blackening of copper pigments in wall paintings: impact of the fresco technique and the chemical composition of the pigments Jiménez Desmond, Daniel; Arizzi, Anna; Ricci, Chiara; Piccirillo, Anna; Pozzi, Federica; Abdelkader Fernández, Víctor K.; Pozo Antonio, José Santiago This study addresses the blackening of copper-based pigments in fresco paintings. Historical evidence shows that copper carbonates, silicates and acetates were widely used despite often blackening, usually due to copper oxide formation in humid, highly alkaline conditions. To clarify the causes, eight fresco-applied copper pigments were studied after blackening. Physical, mineralogical and chemical analysis suggested that moisture, high alkalinity and carbonation promoted oxide formation. However, crystalline oxides alone did not explain the observed blackening. Instead, XPS analysis indicated poorly crystalline or amorphous reduced copper species as additional contributors. Pigment composition was found to be critical: carbonate- and acetate-based pigments showed instability, whereas silicate ones remained comparatively stable. Pigment origin and impurities further influenced alteration pathways, with copper-acetate verdigris exhibiting the most complex behaviour. These results highlight the multifactorial nature of copper pigment degradation and underline the importance of pigment identification for conservation strategies in fresco paintings. https://hdl.handle.net/10481/112676 https://hdl.handle.net/10481/112622 Microbial and clay-mediated dolomite formation in a saline playa-lake: Implications for biosignatures on Mars Sánchez-Román, Mónica; Naim, Zeina; Sánchez Navas, Antonio; Waajen, Annemiek C.; Yao, Tingting; Sankar Gupta, Karthick Sai; Nieto García, Fernando This study investigates the authigenesis of palygorskite in association with dolomite, as well as the alteration of detrital clays into vermiculite and Mg/Fe smectites, within the upper sediments of the semi-arid saline playa lake system of Laguna Fuente de Piedra (southern Spain). A combination of analytical techniques, including XRD, SEM-EDS, HRTEM-AEM, ICP-MS/OES, NMR, and 16S rRNA gene sequencing, was used to characterize sediment mineralogy, microtextures, geochemistry, and microbial assemblages. The results reveal an assemblage of authigenic dolomite and clays embedded within an extracellular polymeric substance (EPS) matrix, consistent with microbially influenced mineralization at low temperature. These findings provide new insight into how clay minerals modulate microenvironments (e.g., ion activity, fluid and pore-water composition) and offer reactive surfaces that facilitate dolomite precipitation, while also acting as substrates for microbial colonization and EPS preservation in inland saline lake settings (e.g., playa lakes, continental sabkhas). Because evaporitic to alkaline lacustrine environments and Mg-rich carbonates and clay minerals have been documented or inferred for ancient Martian basins, the co-occurrence of Mg-carbonates with Fe/Mg-rich clays constitutes a testable mineralogical target for biosignature investigations on Mars. In particular, when evaluated together with diagnostic microtextures and complementary geochemical indicators, this assemblage may help distinguish purely abiotic precipitation from microbially influenced mineralization in Martian sedimentary records. This work was supported by the Dutch Research Council (NWO) under project numbers OCENW.KLEIN.037 and ENW.GO.001.033 awarded to MSR. MSR also acknowledges financial support from the Beatriz Galindo Senior Grant (No. BG23-00132), funded by the Spanish Ministry of Science, Innovation and Universities (MICIU). E.M. Marín Martín is kindly acknowledged for her kind assistance and help in the lab. https://hdl.handle.net/10481/112622 https://hdl.handle.net/10481/112614 Presence of sulfate does not inhibit low-temperature dolomite precipitation Romanek, Christopher S.; Jiménez López, Concepción; Rodríguez Navarro, Alejandro; Sánchez-Román, Mónica A set of free-drift experiments was undertaken to synthesize carbonates of mixed cation content (Fe, Ca, Mg) from solution at 25 and 70 °C to better understand the relationship between the mineralogy and composition of these phases and the solutions from which they precipitate. Metastable solid solutions formed at 25 °C which are not predicted from the extrapolation of higher temperature equilibrium assemblages; instead, solids formed that were intermediary in chemical composition to known magnesite–siderite and dolomite solid solutions. A calcite–siderite solid solution precipitated at 25 °C, with the percentage of CaCO3 in the solid being proportional to the aqueous Ca/Fe ratio of the solution, while Mg was excluded from the crystal structure except at relatively high aqueous Mg/Ca and Mg/Fe ratios and a low Ca content. Alternatively, at 70 °C Mg was the predominant cation of the solid solutions. These results are compatible with the hypothesis that the relative dehydration energies of Fe, Ca and Mg play an important role in the formation of mixed cation carbonates in nature. CJL acknowledges support from grants CGL2004-03910 and CGL2007-63859 from MEC (Spain) and the Fulbright/MEC Program. NS acknowledges funding support from NSF EAR CAREER 0208036 and ACS PRF 47792-AC2. The contribution of MC was carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA), partly supported by JPL’s Research and Technology Development Program (grant 01STCR-R.07.023.011). This work was also supported in part by NASA’s Astrobiology Institute, NASA’s Ancient Martian Meteorite program, and the US Department of Energy through Financial Assistance Award No. DE-FC09-96-SR18546 to the University of Georgia Research Foundation. https://hdl.handle.net/10481/112614 https://hdl.handle.net/10481/112318 Micritized Grains and Peloids: The Taphonomic Adventures of Carbonate Grains in Shallow Marine Settings Sánchez-Román, Mónica https://hdl.handle.net/10481/112318