Archives of Miocene bottom water temperatures (BWT) offer vital insights into future climate dynamics, with oceans playing a crucial role in absorbing significant amounts of heat. However, reconstructions of BWT for the middle Miocene are limited, especially in the Pacific Ocean, hindering our understanding of Miocene climate. Here, we present Mg/Ca records of benthic foraminifera Cibicidoides wuellerstorfi and Cibicidoides mundulus from seven Pacific sites for the middle Miocene (17-11 Ma), calibrate paleothermometer Mg/Ca in these two species with a new equation, and reconstruct BWT and δ¹⁸O_sw at various locations of the Pacific Ocean during the middle Miocene. Our data suggest that the BWT was high during the Middle Miocene Climatic Optimum (MCO), followed by a decrease during the Middle Miocene Climate Transition (MMCT), and a return to pre-MCO level after the MMCT. Additionally, the eastern and western Pacific Ocean exhibited asynchronous BWT changes, suggesting they were influenced by different water masses over extended periods. Our reconstructed δ¹⁸O_sw records from different sites are heterogenous, implying hydrological factors other than ice volume. The δ¹⁸O_sw records of the most stable and deeply located site suggest that ice volume between 17-11 Ma was comparable to or slightly smaller than preindustrial level, differing from previous findings.

middle Miocene,BWT,ice volume