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Sites and samples

Sites U1331–U1333 (Fig. F1) are southeast of Hawaii in the central Pacific Ocean (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]). The positions are 12°04.09′N, 142°09.70′W in 5116 m water depth (Site U1331); 11°54.71′N, 141°02.74′W in 4935 m water depth (Site U1332); and 10°31.00′N, 138°25.17′W in 4865 m water depth (Site U1333). At all three sites, three holes were drilled (A, B, and C). The sedimentary sequences are ~190 m thick at Site U1331, ~152 m thick at Site U1332, and ~182 m thick at Site U1333.

The three study sites lie above oceanic crust of the Pacific plate that formed in the past at the East Pacific Rise (Pälike et al., 2010). Site U1331 targeted basalt formed at ~53 Ma, Site U1332 targeted basalt formed at ~50 Ma, and Site U1333 targeted basalt formed at ~46 Ma (Pälike et al., 2010). This is important because, given tectonic reconstructions for the region, the seafloor at the sites should have been near the Equator and much shallower during portions of the early to middle Eocene. Thus, although well below the CCD at present-day, the sites might contain intervals of carbonate deposited above the CCD in the past.

Recovered sediment at the sites was divided into units based on lithology (Pälike et al., 2010). For this study, we mostly focus on lithologic Unit IV at Site U1332 and Units III and IV at Site U1333. These are nominally 60–80 m thick sediment sections that correspond to the middle and upper Eocene (Figs. F2, F3). Unit IV at Site U1332 comprises clayey radiolarian ooze, radiolarian ooze, radiolarian nannofossil ooze, nannofossil radiolarian ooze, nannofossil ooze, and porcellanite (see the “Site U1332” chapter [Expedition 320/321 Scientists, 2010c]). At Site U1333, Unit III consists of similar lithologies, whereas Unit IV comprises nannofossil ooze and limestone (see the “Site U1333” chapter [Expedition 320/321 Scientists, 2010d]). A thin (~2 m) interval of lower Eocene nannofossil ooze and zeolite clay (Unit V) was recovered at Site U1331 (see the “Site U1331” chapter [Expedition 320/321 Scientists, 2010b]).

We collected 373 “tubes” (10 cm3 each) of sediment (Table T1). The samples span between 208.8 and 212.1 m core composite depth below seafloor, method A (CCSF-A) in Hole U1331C (n = 8), between 76.6 and 151.5 m CCSF-A in Hole U1332A (n = 100), between 74.3 and 157.5 m CCSF-A in Hole U1332B (n = 106), and between 132.5 and 203.5 m CCSF-A in Hole U1332A (n = 159). The CCSF-A depth scale results from aligning cores from multiple holes at a drill site (see the “Methods” chapter [Expedition 320/321 Scientists, 2010a]; Westerhold et al., 2012). Samples were taken from every core section in units of interest, with increased sampling resolution (every 10–15 cm) across the CAEs. Samples were selected to avoid disturbed intervals related to coring.