EDIACARAN TO CAMBRIAN OCEAN PH AND CONTINENTAL WEATHERING CONDITIONS: IMPLICATIONS FROM SOUTHERN KAZAKHSTAN

From the aftermath of the late Cryogenian Marinoan glaciation (~635 Ma) to the Early Cambrian the Earth underwent major changes in (bio-)geochemical processes, climate and ocean-atmosphere interactions. This time period was intensely investigated within the last decades, yet the Karatau microcontinent (Kazakhstan, Central Asia) received little attention. The studied Kyrshabakty Section is located at the Malyi Karatau Range and hosts a >500 m succession of volcanics, siliciclastics and predominantly carbonates of Early Cryogenian to Ordovician age.

Here we present the first boron (δ¹¹B) and strontium (⁸⁷Sr/⁸⁶Sr) isotope dataset for the Karatau microcontinent to elucidate long-term modifications to ocean pH and weathering fluxes from the Ediacaran until the Early Cambrian (Stage 3, ~520 Ma). Within the Marinoan-age cap dolomites an ocean acidification event is indicated by a transient negative δ¹¹B excursion of 7‰. Corresponding ⁸⁷Sr/⁸⁶Sr as low as 0.7083 are close to published primary post-glacial seawater values of ~0.707. The middle part of the section records the tremendous negative Shuram-Wonoka carbon anomaly with δ¹³C_carb values down to -9.7‰ and radiogenic ⁸⁷Sr/⁸⁶Sr ratios scattering around 0.713. δ¹¹B data of this time period show a constant decline of ~9‰. Assuming a primary signal, a continuous decrease in ocean pH is indicated. At the Precambrian-Cambrian transition boron isotopes display a steady decrease in ocean pH by >0.7. Coeval strontium ratios show an increasing trend during the latest Ediacaran from 0.7086 to 0.7094 and decrease to 0.7092 within the Cambrian.

Overall, the Kyrshabakty Section provides valuable insights into changing ocean pH and weathering conditions over a time span of ~120 Ma: A general rise of seawater ⁸⁷Sr/⁸⁶Sr during the Ediacaran and several fluctuations in ocean pH until the Cambrian.