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doi:10.2204/iodp.proc.320321.102.2010

Reconstruction of geologic age during Expedition 320/321

A major task aboard the JOIDES Resolution during the PEAT expedition was to establish an age model for each site. These age models will be further refined during postcruise shore-based work. It is anticipated that the final limit of resolution in the PEAT site age models will depend mainly on sedimentation rate and thickness of the bioturbated layer at each site. Under these premises, and assuming average sedimentation rates of 1–2 cm/k.y. for the PEAT biogenic sediments, a sample resolution of 10 cm offers a relative time resolution of 5–10 k.y. Ultimately, after postcruise work the goal is to constrain the geologic age of any such sample to a precision that is better than ±50 k.y. in any part of the age span of the PEAT cores (i.e., from ~53 Ma to the present). The age resolution provided during the shipboard work is considerably less precise and depends on how well sedimentation rates can be constrained.

The reconstruction of geologic age and sedimentation rates in PEAT sediments involves several different data sets. The first crucial step in this process was to select a framework timescale for Eocene through recent times, the age span of the PEAT sediments. Geologic timescales are being constantly improved and refined as new methods and approaches are developed and new data are generated. This also holds true for Cenozoic timescales. Existing timescales for the Cenozoic era are built using a combination of geomagnetic polarity data and biostratigraphy (e.g., Berggren et al., 1995) and, more recently, cyclostratigraphy tuned to the orbital metronome (e.g., Shackleton et al., 1995; Lourens et al., 2004; Pälike et al., 2006b). The chosen timescale for the Pacific Equatorial Age Transect has been pieced together from three different timescales because at present this combination is considered to represent the best available organization and subdivision of Cenozoic time for marine sediments. The three data sets are arranged chronologically to encompass the past 60 m.y.: Neogene time (0–23 Ma) is based on work presented by Lourens et al. (2004), Oligocene and Eocene time (23–42.5 Ma) is based on work presented by Pälike et al. (2006b), and Eocene through late Paleocene time (42.5–60 Ma) is based on work presented by Cande and Kent (1995). Details about how these three data sets were merged are discussed in "Magnetostratigraphy."

The PEAT timescale is summarized in a series of figures (Figs. F2, F3, F4). These graphs show how the geomagnetic polarity zones (chrons and subchrons) are correlated to formal geochronologic units, Cenozoic epochs (Pleistocene through Paleocene), and standard, albeit informal, subdivisions of these epochs (early, middle, late; see Salvador, 1994). In order to avoid overloading the figures, we have omitted corresponding chronostratigraphic units (series) as well as the next higher rank (above epoch) in the geochronologic hierarchy (period) and the next lower rank (age).

Another key data set used during shipboard work for reconstruction of geologic age consists of micropaleontological bioevents. Shipboard PEAT expertise covers benthic and planktonic foraminifers, calcareous nannofossils, diatoms, and radiolarians. Biostratigraphers acquired whole-assemblage data in terms of abundance and preservation for the different microfossil groups. The rate of taxonomic evolution, defined as the number of evolutionary appearances and extinctions per unit time, determines the biostratigraphic resolution attainable. In a few cases, distinct abundance crossovers among two taxa at the species or genus level, often ancestor/descendant, are employed as biostratigraphic marker events as well as coiling direction changes (in planktonic foraminifers). All bioevents used during the PEAT expeditions have been calibrated either to magnetostratigraphic polarity zones (chrons and subchrons) and/or astrochronologically tuned cyclostratigraphies in numerous marine sediment cores/sections gathered at different geographic locations. Subsequently, calibration ages have been integrated into a single age estimate considered to be presently the best available age for each bioevent. Original calibration ages of some bioevents are derived from different (pre-PEAT) timescales. We have therefore recalculated these, using linear interpolation, in order to adjust them to our chosen PEAT framework timescale. Each bioevent thus has a given age estimate expressed in Ma. All bioevents used during the PEAT expeditions are shown in Figures F2 and F3, which also show how individual bioevents relate to geochronologic epochs and geomagnetic polarity zones. Bioevents and their associated best-age estimates are listed in Tables T2, T3, T4, and T5. References presenting published calibrations (in some cases prior to adjustment to the corrected PEAT timescale) can be found in BIOSTRAT in "Supplementary material."

Biostratigraphic data are unique in the sense that they represent a continuum of organic evolution among the planktonic and benthic assemblages. The various assemblages and their component taxa are thus uniquely confined to certain geological time intervals. Thus, we can arrange the stratigraphic column in terms of relative time based on the order in which microfossil taxa appear and become extinct. This crucial biostratigraphic character is employed to guide the interpretation of magnetostratigraphic and cyclostratigraphic data, which in contrast can only be used to tell geologic age if we know to which part of the stratigraphic column they belong. Biostratigraphers provided this guiding input to cyclo- and magnetostratigraphers.

Geologic age information derived from bio- and magnetostratigraphers is integrated by stratigraphic correlators, who in turn generate age-depth plots and calculate sedimentation rates (see "Stratigraphic correlation and composite section") for each PEAT site. The core depth below seafloor is estimated using a core composite depth scale (CCSF-A) (see "Stratigraphic correlation and composite section").

A few days after the PEAT expedition phase ended in late June 2009, the International Union of Geological Sciences Executive Committee decided to ratify the suggestion by the International Commission on Stratigraphy to make the following changes to the subdivision of Cenozoic time: (1) the Quaternary is established as a third system/period; (2) the Quaternary is divided into two series/epochs (Pleistocene and Holocene); (3) the lower boundary of the Pleistocene series is moved to encompass the Gelasian stage, which previously belonged to the Pliocene series, thereby shortening the Pliocene epoch and Neogene period by 782 k.y.; and (4) the Neogene/Quaternary (Pliocene/Pleistocene) boundary has an age of 2.588 Ma according to the August 2009 version of the International Stratigraphic Chart (stratigraphy.science.purdue.edu). As these changes occurred after the PEAT timescale was established and extensively used by the two expeditions, we adhere to the PEAT subdivision of late Cenozoic time, following Lourens et al. (2004).

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