Climate changes in the pre-Hirnantian Late Ordovician based on δ18Ophos studies from Estonia

Results of conodont-based δ18Ophos studies from two core sections in Estonia (Mehikoorma-421 and Valga-10) indicate that the pre-Hirnantian Late Ordovician was characterized by a general cooling trend: a rapid climate cooling in the early Sandbian and a less intense cooling in the late Sandbian through Katian. During this time interval, sea-surface temperatures probably decreased by up to 9o C. This fits with earlier ideas of a colder Late Ordovician climate before the Hirnantian. On the background of general cooling trend in the pre-Hirnantian Late Ordovician, tendencies in the temperature changes were quite variable. Several episodes of SST decrease and increase of different magnitudes and durations are evident from the δ18Ophos curves in the studied sections. In the interval sampled in detail, 7 cooling events are recognized and tentatively called (in stratigraphical order): Early Kukruse, Haljala, Keila, Early Nabala, Vormsi, Early Pirgu and Middle Pirgu CE-s. Duration of these events and amplitudes of changes in seawater temperatures increased from Sandbian to late Katian. All cooling events may have triggered the waxing of polar and subpolar Gondwana icecaps and are reflected in the sedimentary succession by major sea-level lowstands recognized also outside the study region. Results of our research do not support a simple, straightforward correlation between temperature trends inferred from δ18Ophos measurements and changes in the δ13C record. Comparison of δ18Ophos and δ13C curves reveal that these two variables behave quite independently: in some intervals their trends fit quite well, in others they might show completely the opposite. For this comparison we provide also a new, detailed δ13C curve from the Valga-10 core section.