Coastal dysoxia accompanies Early Miocene warming based on benthic foraminiferal and sedimentary records from Oregon

Abstract

Abstract The Newport Member of the Astoria Formation, exposed on the central Oregon coast, was deposited at shallow marine depths during the Early Miocene global warming that led to the Middle Miocene Climate Optimum. This record thus provides the opportunity to examine a coastal environment over past warming and ask if the low oxygen conditions observed in recent years on the modern Oregon shelf is a recent phenomenon or a repeated symptom of global warming. Although δ18O variation can be influenced by freshwater flux, if due to temperature δ18O values suggest 2.4–4.5 °C of local warming during deposition of the Newport Member. Species of Buliminacea, Fursenkoina, and Nonionella, abundant in organic-rich, low-oxygen sediments in modern seas, increased in proportional abundance after warming began. In addition, the offset in δ13C values between epifaunal to shallow infaunal taxa and a deeper infaunal species is greater in sediments with higher abundances of these taxa, further supporting increased organic carbon flux. An increase in laminated sediments coincident with these faunal and geochemical changes suggests dysoxic conditions accompanied the increase in organic carbon. Taken together, this multiproxy record suggests that high-organic, low-oxygen, environments developed in shallow waters during Early Miocene warming, perhaps driven by similar upwelling mechanisms thought to drive hypoxia on the modern Oregon coast.