Biotic response of calcareous nannoplankton to abrupt warming across the Paleocene/Eocene boundary reflects a primary response to climatically induced parameters including increased continental runoff of freshwater, global acidification of seawater, high sedimentation rates, and calcareous nannoplankton assemblage turnover. We identify ecophenotypic nannofossil species adapted to low pH conditions (Discoaster anartios, D. araneus, Rhomboaster spp.), excursion taxa adapted to the extremely warm climatic conditions (Bomolithus supremus and Coccolithus bownii), three species of the genus Toweius (T. serotinus, T. callosus, T. occultatus) adapted to warm, rather than cool, water conditions, opportunists adapted to high productivity conditions (Coronocyclus bramlettei, Neochiastozygus junctus), and species adapted to oligotropic and/or cool-water conditions that went into refugium during the PETM (Zygrablithus bijugatus, Calcidiscus? parvicrucis and Chiasmolithus bidens). Discoaster anartios was adapted to meso-to eutrophic, rather than oligotrophic, conditions. Comparison of these data to previous work on sediments deposited on shelf settings suggests that local conditions such as high precipitation rates and possible increase in major storms such as hurricanes resulted in increased continental runoff and high sedimentation rates that affected assemblage response to the PETM.

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The Paleocene‐Eocene Thermal Maximum (PETM) was an interval of extreme warmth that caused disruption of marine and terrestrial ecosystems on a global scale. Here we examine the sediments, flora, and fauna from an expanded section at Mattawoman Creek‐Billingsley Road (MCBR) in Maryland and explore the impact of warming at a nearshore shallow marine (30–100 m water depth) site in the Salisbury Embayment. Observations indicate that at the onset of the PETM, the site abruptly shifted from an open marine to prodelta setting with increased terrestrial and fresh water input. Changes in microfossil biota suggest stratification of the water column and low‐oxygen bottom water conditions in the earliest Eocene. Formation of authigenic carbonate through microbial diagenesis produced an unusually large bulk carbon isotope shift, while the magnitude of the corresponding signal from benthic foraminifera is similar to that at other marine sites. This proves that the landward increase in the magnitude of the carbon isotope excursion measured in bulk sediment is not due to a near instantaneous release of 12C‐enriched CO2. We conclude that the MCBR site records nearshore marine response to global climate change that can be used as an analog for modern coastal response to global warming.

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