As a potentially serious consequence of global warming, a substantial loss of dissolved oxygen in the world's ocean is reported not only in declining concentration but also in the spatial expansion of the oxygen minimum zones (OMZs). Similar to the modern situation, oceanic deoxygenation in upper ocean was often associated with warmer climatic conditions in the geological past, e.g. the Paleocene-Eocene Thermal Maximum and interglacial periods. However, Hess et al. (2023) reported a counterintuitive phenomenon —— a well-oxygenated upper layer with no OMZs during the warmer Miocene climatic optimum (MCO), and a strong oxygen-deficient zone expanding to surface layer during the middle Miocene climatic transition (MMCT) which is very unlikely under rapid air-sea gas exchange in the eastern tropical Pacific. Here, based on previous studies, we apply GDGT proxies from site U1337 in the eastern tropical Pacific to infer microbial community changes which could reflect subtle redox changes in the water column. Methane Index, Ring Index and GDGT-0/crenarchaeol all display anomalously high index values during MCO and a sharp decrease around 13.6 Ma following relatively low and stable values. We interpret high MI, RI and GDGT-0/cren values as increased production of methanogenic and methanotrophic archaea, thus reflecting the expansion of oxygen-deficient zones during the MCO. 

GDGT, oxygen minimum zones, eastern tropical Pacific, the middle Miocene climatic transition