Trace elements, rare earth elements and isotopes of poorly preserved fossils from lower Cretaceous carbonates (Eastern Black Sea): Implications for early diagenetic alteration

Abstract

Poorly preserved belemnite samples are often disregarded in palaeoclimatology and palaeoceanography research within the Eastern Pontides (NE Türkiye), which is part of the Alp-Himalayan Belt that experienced extensive orogenic and magmatic activity. Late Jurassic–Early Cretaceous carbonates are widely exposed in the Eastern Pontides, having undergone a complex diagenetic history since deposition. Belemnite rostra samples from the uppermost part of the carbonate sequence (Aptian–Albian strata) exhibit diagenetic overprints. These poorly preserved belemnite samples can serve as excellent archives for basin early diagenetic evolution. However, the diagenetic evolution of these poorly preserved belemnite samples has not been fully studied within the context of the diagenetic evolution of the basin. Thus, this study presents a comprehensive dataset including isotope analyses (Sr/C/O) and trace element, including REE chemistry of these belemnite samples to elucidate the basin's early diagenetic history. The studied samples are commonly dark grey and show visible signs of diagenetic processes, including dissolution, silicification and pyrite mineralisation. These samples show high Fe (1259–5176 ppm; ave. 2859 ppm) and Mn (77–387 ppm; ave. 232 ppm), low Sr/Mn (0.63–12.33; ave. 3.47) and low Sr/Rb (42–568, ave. 200) values, consistent with a diagenetic alteration. Interestingly, carbon and strontium isotopes of belemnite samples correlate well with those from well-preserved belemnite rostra from the Aptian–Albian strata. They exhibit a diagnostic sea water REE_N signature, including mildly enriched Gd/Gd* ratios of 1.04–1.48 (ave. 1.24) and high La/La* of 1.54–5.50 (ave. 2.51) and Ce/Ce* of 0.46–0.60 (ave. 0.51). However, slight LREE enrichment is observed, with La/LuN ratios ranging from 0.81 to 1.74 (average: 1.21) and La/YbN ratios from 0.9 to 1.79 (average: 1.22). A slight positive Eu anomaly is also present, with Eu/Eu* ratios between 1.15 and 1.55 (average 1.36). Y/Ho ratios range from 35.00 to 45.00 (average 39.84), and Sm/Yb ratios from 1.70 to 4.05 (average 2.42). These geochemical characteristics, along with δ¹⁸O values that deviate from typical marine carbonate signatures, suggest a subsequent low-temperature hydrothermal alteration, potentially triggered by hydrothermal activity associated with an Early Cretaceous magmatic event. This finding underscores the significance of analysing REEs in poorly preserved belemnite rostra, offering valuable insights into early diagenetic pathways, dissolution/precipitation processes and water–rock interaction.