Reconstructing CO2 uptake capacity and pH dynamics in the Middle Ordovician Taebaeksan Basin, Korea

This study presents the first continuous carbonate boron isotope (δ¹¹B_carb) record from Middle Ordovician carbonate rocks, focusing on the Taebaek section in Korea. The δ¹¹B_carb values analyzed from the Makgol, Jigunsan, and Duwibong formations range between 7.5 ‰ and 17.8 ‰, falling within the published Paleozoic range. Samples exhibit minimal diagenetic alteration, as evidenced by weak correlations between isotopic factors and elemental ratios (δ¹¹B_carb, δ¹³C_carb, δ¹⁵N_bulk, B/Ca, Mn/Sr, and Na/Ca) as well as mineralogical observations used to select unaltered micrite portions. Using the δ¹¹B_carb data, we reconstruct pH levels (8.4 to 9.1) which suggest that this shallow shelf environment acted as a CO₂ sink during the Middle Darriwilian, despite periods of acidification coinciding with sea level rise and expansion of anoxic zones. Weakening of the basin's CO₂ uptake capacity via reduced CaCO₃ precipitation and photosynthesis accompanied these oceanographic changes. Our findings provide new insights into the dynamics of CO₂ accumulation and pH in Middle Ordovician shallow marine settings in relation to sea level fluctuations. This work lays the foundation for further research into ocean chemistry and carbon cycle perturbations during this important period in Earth's history.