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Methods and materials

Sediment processing

All sediment samples used in this study were selected between 60 and 180 m core composite depth below seafloor (CCSF-A) along the composite splice at Site U1338 (Wilkens et al., 2013), thereby targeting the late Miocene to early Pliocene based on the shipboard age model (see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010]). Each 20 cm3 sample was freeze-dried and weighed and 5 cm3 was kept as an archive. The remaining 15 cm3 was washed through a 63 µm sieve to separate the fine (<63 µm) and coarse (>63 µm) fractions, both of which were then oven-dried at 45°C. When the foraminifers were selected, the >63 µm fraction was further dry sieved to isolate specific size fractions.

Scanning electron microscopy

C. mundulus and Gs. sacculifer were selected from the 250–500 µm and 250–355 µm size fractions, respectively, for SEM to assess the general preservation of these specific benthic and planktonic species throughout the late Miocene to early Pliocene section of the Site U1338 splice (Table T1). Two <63 µm fine fraction samples (c1 and c2) from 76.18 and 105.32 m CCSF-A were selected to investigate sedimentary composition and coccolith preservation.

Samples were mounted onto sticky carbon tape on a stub and were imaged using the Leo 1455 variable pressure (VP) SEM at the Natural History Museum (NHM; London, United Kingdom). The images were made at low vacuum using backscattered electrons in topography mode (BSE-TOPO) and therefore were not coated. The working distance for all images was set to 15–16 mm, with a spot size of 500 (Leo-specific setting) and an acceleration voltage of 15 kV.

As the spatial resolution of the BSE-TOPO approach was insufficient to resolve the smaller scale features of the fine fraction sediments, Sample c2 was coated with platinum (coating thickness = 3 nm) and imaged using the JEOL JCM-6000 NeoScope Benchtop SEM in high vacuum mode at Utrecht University (the Netherlands). Images were taken in SE mode at a working distance of 19 mm. The filament and probe currents were set to high, and the acceleration voltage was set at 10 or 15 kV, depending on the resolution required.

To determine the chemical composition of the different sedimentary components, EDS area maps were made of Sample c2. The EDS maps were all made with an acceleration voltage of 15 kV using high filament and probe currents.