MioMG: Reconstructing robust ocean temperatures from Miocene foraminiferal Mg/Ca records

Abstract. Paleoclimate records from the Miocene epoch (~5.3 to ~23.0 Million years ago) present an opportunity to assess the Earth’s climate system under a warmer-than-modern mean state, when atmospheric CO₂ levels were within the range of those projected for the next century under intermediate climate change scenarios. Ocean temperature is a key climate system parameter because latitudinal temperature gradients provide critical information on the climate system and potential carbon cycle feedbacks in the geological past. Over the last several decades, magnesium-to-calcium (Mg/Ca) ratios in both planktic and benthic foraminiferal shells have been applied as a temperature proxy providing insight into spatial and temporal change in upper ocean and bottom water temperature across the Miocene. However, Miocene climate models struggle to reconstruct the reduced latitudinal temperature gradient inferred from available proxy records, including those based on Mg/Ca. Further, recent advances in planktic and benthic foraminiferal Mg/Ca-temperature paleothermometry show that non-thermal factors and seawater chemistry need to be considered when converting Mg/Ca to temperature. Here, we present the MioMG temperature compilation, led by the Mg/Ca working group within the international MioOcean community effort. Fifty-two Miocene foraminiferal Mg/Ca records from both planktic and benthic foraminifera were reevaluated and temperatures were recalculated to provide robust, consistent temperature estimates including uncertainties. Conversion of planktic and benthic Mg/Ca to temperature requires consideration of carbonate chemistry sensitivity for individual species, sensitivity of foraminiferal Mg/Ca to the Mg/Ca seawater composition (Mg/Ca_sw), and appropriate calibration. We compiled records of the seawater Mg/Ca composition (Mg/Ca_sw), based on a diverse set of proxies, and present this record for use in future application of Mg/Ca-derived Miocene temperature reconstructions. Miocene Mg/Ca records are derived from extant and extinct foraminiferal species. Further, preservation information is essential to determine the quality of reconstructions. Our recalculated upper ocean temperature (UOT) estimates showcase warm Middle Miocene temperatures and a cooling trend towards the Late Miocene. Revised bottom water temperature (BWT) estimates also depict warm Middle Miocene values followed by a cooling trend, which is consistent among sites located at different paleodepths. Overall, absolute temperature estimates are higher than previously published estimates due to application of corrections based on the updated MioOcean Mg/Ca_sw curve. The MioMG compilation is presented on an interactive Streamlit-based web platform allowing for exploration of the data for specialists and non-specialists to aid in application of dataset for Miocene researchers. The dataset is publicly available at Zenodo (https://doi.org/10.5281/zenodo.22258386, Sosdian et al. 2026).