Sampling of the OAE1a intervals for this study occurred postcruise at the Gulf Coast Core Repository at Texas A&M University (TAMU) in College Station, Texas, prior to sampling by other shipboard scientists. Therefore, sample intervals were chosen to maximize coverage of the lithologies tentatively differentiated via shipboard core descriptions while minimizing sample volume across critical organic-rich sections. Detailed visual core descriptions (VCDs) of the OAE1a intervals (Cores 198-1207B-43R and 44R, 198-1213B-8R, and 198-1214A-23R) were produced shipboard. These were used as sampling references at TAMU. Section 198-1214A-23R-1 was described in more detail during the sampling process. Representative samples of the various lithologies were selected for thin section preparation, including fragments of chert, organic-rich shale, chalk/limestone, and tuff. There were 36 samples selected, 21 from Cores 198-1207B-43R and 44R, 9 from Core 198-1213B-8R, and 6 from Core 198-1214A-23R (Fig. F3; Table T1).
Samples from DSDP Site 463 in the Mid-Pacific Mountains were also collected for this study from cores stored at the DSDP West Coast Repository at Scripps Institution of Oceanography in La Jolla, California (Table T2). Shipboard descriptions of these cores suggested that the OAE1a interval at Site 463 contained a significant amount of ash. Based on these published descriptions, 17 samples were chosen over a ~24 m interval (604.69–628.27 meters below seafloor [mbsf]).
Billets trimmed from the 53 samples were impregnated with blue-dyed epoxy for porosity recognition prior to preparation of standard petrographic thin sections. Detailed petrographic descriptions were performed on these thin sections (Table T1). Categories included biogenic (nannofossils, foraminifers, radiolarians, fish debris, and organic matter), volcanic (plagioclase crystals and altered vitric and microlitic fragments), and matrix components, as well as authigenic phases and porosity. The matrix often consisted of microporous mixtures of clay minerals, silt with authigenic pyrite, zeolites, carbonate, opal-CT, and quartz (chalcedony). Diagenetic phenomena were documented (e.g., replacement of radiolarians by pyrite or calcite, replacement of foraminifers by silica, cementation, and secondary porosity). The presence of lamination and intensity of bioturbation were also noted.
Pristine fragments of 19 representative samples from Leg 198 were selected for X-ray diffraction (XRD) analyses (Table T2). Both bulk and clay mineral separates were analyzed by K-T Geoservices, Dallas, Texas, who first disaggregated and split each sample. One-half of each sample was powdered and then pressure packed into an aluminum mount for random whole-rock analysis. The second half was ultrasonically dispersed and then centrifugally size fractionated to concentrate the <4 µm (clay-sized) fraction. Oriented mounts were prepared using vacuum-deposited membranes on glass slides. The slides were glycolated prior to analysis with a Rigaku automated powder diffractometer. Scans were conducted from 2º to 60ºq at a rate of 1º/min for the random mount, and 2º to 50ºq at a rate of 1.5º/min for the glycolated <4 µm fraction mount. Interpretations of mixed-layer clay ordering and expandability were performed by K-T Geoservices using the NEWMOD program created by R.C. Reynolds. Whole-rock mineral proportions were semiquantitatively determined from integrated peak areas (derived from peak-decomposition/profile-fitting methods), empirical reference intensity ratio (RIR) factors and, in the case of the phyllosilicates (clay and mica), the combined {00l} and {hkl} clay mineral reflections. The clay-fraction samples were analyzed in a similar manner, also using calculated RIR factor and comparison with simulated diffraction profiles generated by NEWMOD.