Conodont apatite record of marine Sr isotopic composition in the advent of the Hangenberg Event (Late Devonian)

The Devonian-Carboniferous boundary constitutes one of the major gaps of the Phanerozoic oceanic radiogenic Sr curve. This gap is of particular interest in the ongoing discussion to redefine the Devonian/Carboniferous boundary, as isotopic Sr stratigraphy could provide an auxiliary correlation tool. We present herein the first latest Famennian radiogenic Sr data from three monogeneric sets of conodont elements collected from the well-studied Kowala Quarry section in central Poland. The minimal alteration of this conodont dataset, supported by the good fit with published brachiopod data from coeval intervals, makes this record highly reliable. The Kowala Quarry material provides updated seawater 87 Sr/ 86 Sr ratio trend along the few million years preceding the Hangenberg Event. The decreasing 87 Sr/ 86 Sr ratios trend from the Bispathodus costatus (~ 0.70832 ± 0.00003) to the Siphonodella (Eosiphodella) praesulcata Zone (0.70822 ± 0.00003) suggest the early Carboniferous decreasing trend started as early as the late Fammenian. Comparison with early Fammenian trends indicate seawater 87 Sr/ 86 Sr ratio reached a maximum during the late or middle Famennian. We attempt to correlate this new dataset to the GTS 2020 and discuss two different age models based on available biostratigraphic, radiometric and astrochronological data. Both age models suggest higher rates of 87 Sr/ 86 Sr ratio decrease during the S. (E.) praesulcata Zone. We explore the cause of this decreasing trends using one-dimensional mass balance modelling of oceanic Sr. The modelling experiments reject the hypothesis of the coeval Kola-Dniepr Large Igneous Province emplacement as the main driver of this 87 Sr/ 86 Sr shift. It is more likely explained by a global and sustained imbalance of oceanic Sr, which may include a combination of orogenic belt waning, increased volcanic arc activity and accretion, increased hydrothermalism, as well as a small contribution of LIP emplacement.