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doi:10.2204/iodp.proc.320321.204.2012 Methods and materialsSites studiedOnly three sections have been recovered from the tropical Pacific that clearly show the “two-step” shift in lithology and geochemistry at the E/O boundary that we believe marks a truly complete stratigraphic section (Coxall et al., 2005). These three sites were also close to the paleoequatorial zone of high productivity during the E–O transition (Fig. F1; Table T1). Because of their completeness and because of their location within a very specific and important biogeographic and paleoceanographic zone, these three sites were selected for a detailed study of the E–O transition. Depths in sectionsThe work of Westerhold et al. (2012) greatly facilitated this study by providing a revised depth scale for all holes at each of the three sites. This allowed samples from any given hole to be placed in relative stratigraphic order with respect to samples from all other holes at that site. This depth adjustment was based on a decimeter-scale correlation using multisensor logger data from all cores at each of the sites covered in their study (IODP Sites U1331–U1334 and ODP Sites 1218–1220). In a further refinement that is critical to sites used in this study, Sites U1333 and U1334 were correlated to Site 1218 using the same correlation techniques. This allowed us to place all samples used in this study on a common (Site 1218) depth scale, thus greatly enhancing our ability to refine the radiolarian stratigraphy at these three sites. Nomenclature for the depth scales used in Tables T2, T3, T4, T5, T6, and T7 follow the usage described by Westerhold et al. (2012). SamplesBased on the correlation of multisensor logger data and paleomagnetic stratigraphy, 1 cm samples (one-quarter core) were taken over the stratigraphic interval spanning from ~40 to 30 Ma. Samples were taken from cores in individual holes that showed relatively complete and undisturbed recovery. There was some stratigraphic overlap of sampling when shifting from one hole to another in the sampling scheme. Sample spacing varied between ~20 and 50 cm. A total of 641 samples were examined from the three sites used in this study (Tables T2, T3, T4, T5, T6, and T7). The average sample spacing in the sites is ~35 cm. Samples were prepared following the procedures described in Sanfilippo et al. (1985). Sediment sample was placed in a beaker with 15% H2O2 to remove organic material and a ~20% HCl solution to remove the calcareous fraction from the sediment. The sample was sieved and washed through a 63 µm mesh sieve. If, upon visual inspection, the coarse residue was found to contain clumps of cemented clays and radiolarian fragments, the sample was treated for as long as 1 min in a concentrated solution of NaOH (pH = ~11), immersed briefly (~15 s) in an ultrasonic bath, and then re-sieved. This treatment usually disaggregated the cemented clumps and cleaned the radiolarian skeletons so that they could be more easily identified. Residues were randomly settled onto a slide (Moore, 1973), and then a 22 mm × 40 mm coverslip was mounted on top using Norland optical adhesive #61 as a mounting medium. Slides were studied under a transmitted light microscope at 100× magnification (10× eyepiece, 10× objective). An estimate of the number of radiolarian specimens on each slide was made by counting the number of specimens in one vertical traverse of the slide (one column, 1.4 mm wide) and in one horizontal traverse (one row, 1.5 mm wide). These values were multiplied by the number of rows and number of columns scanned and averaged to estimate the total number of specimens examined on the slide. The number of columns scanned was adjusted to assure that between ~5,000 and 10,000 specimens were examined. In these slides a total of 76 species, species groups, and specific variant forms were counted (see “Taxonomic notes,” below). In addition, all diatom valves and diatom girdles that appeared in the slides were counted. A qualitative estimate of preservation and abundance of radiolarians in the samples is given in the tables. Species illustrationNearly all of the species, species groups, and variant forms listed in “Taxonomic notes” and in Tables T2, T3, T4, T5, T6, and T7 are illustrated in Plates P1, P2, P3, P4, P5, P6, P7, P8, P9, and P10. Photographs were taken with a ProgRes C3 digital microscope camera. A total of 10 to 30 individual photographs (each at a slightly different focal depth) were taken of each specimen illustrated. These shots were then stacked using Helicon Focus software in order to achieve an in-focus image of the entire three-dimensional form. This software was also used to select out-of-focus images of surrounding radiolarian shells and fragments. Determining biostratigraphic datum levelsAlthough their sections are judged to be complete, the sediments of all three sites studied appear to contain varying amounts of reworked older microfossils. Many radiolarian species become extinct near the close of the Eocene, but radiolarian stratigraphy of the uppermost Eocene has always been plagued by the fact that the reworking of older microfossils makes the exact level of extinction of these species suspect. Nigrini et al. (2006) acknowledged this problem and indicated both the “last occurrence” and the “last continuous occurrence” of several upper Eocene species whose last occurrence in the samples may have been affected by reworking and immixing. Of the sites studied here, Site U1334 appears, on average, to have the fewest immixed older microfossils in the samples studied. To some extent this may result from its being drilled on the youngest crust of the three sites (~38 Ma). Here we take the last continuous occurrence of a species to be its last appearance datum (LAD), relying primarily on the last continuous occurrence of the species at Site U1334. Because we can accurately relate this depth to equivalent depths at all three sites through the Westerhold et al. (2012) correlations to Site 1218, we can evaluate the LADs, as well as first appearance datums (FADs), at all sites studied and often more tightly constrain the level of all datums (Table T8). The relatively dense sampling in the sites studied often aids us in determining the continuous versus the discontinuous presence of a species. However, there are two exceptions to this general guideline. First, if a species is very rare throughout its range, there may be samples within its range in which it is not found. Because such very rare species are statistically less likely to be reworked into younger sediments, the upper limit of its stratigraphic range can usually be determined unambiguously. There are only a few species studied that are both so rare and so intermittent in occurrence that their stratigraphic usefulness is doubtful (e.g., Pteropilium sp. aff. Pterocanium contiguum). The second exception is where the last continuous occurrence at one site is above that at the other sites. Usually in such cases data from Site U1334 defines the lowest last continuous occurrence and one of the other (or in rare cases, both) sites has continuous occurrences of the species above that level. For the purposes of defining the error bars of LADs with such a discrepancy, the last continuous occurrence is accepted for each site; thus, the error bars are broader for that datum. However, for the purposes of defining the amount of reworked material in each sample, the lowest last continuous occurrence found at the three sites is taken to be the correct level of the LAD of the species and samples from other sites containing specimens of this species above the equivalent depth of the lowest LAD are assumed to have been reworked. Species definitionWe generally adhere to the taxonomy found in Nigrini et al. (2006), and we do not try to define new species that appear in the samples studied. However, we are interested in tracing the details of how the Eocene fauna is decimated by major climatic and paleoceanographic changes near the E/O boundary and how the radiolarian fauna develops and expands in the lower Oligocene. In so doing the new species described in Nigrini et al. (2006) have been very helpful. In addition we have identified 12 transitional forms that help link the development of new species in both the upper Eocene and lower Oligocene (Tables T2, T3, T4, T5, T6, and T7):
Some of these forms appear only briefly in the section (over ~2–4 m of Site 1218 equivalent depth) and would likely have been missed in studies with coarser sample spacing. |
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