2. THE CONCEPTION OF NEPTUNE AS A STEPPING STONE TOWARD THE MICROPALEONTOLOGIST'S DREAM OF THE IDEAL WORLD

2.1. Stratigraphic and geographic coverage

Marine sediments provide more or less continuous, laterally extensive and correlatable geological archives. The first choice that we made was to limit the database to deep-sea sediments, thereby excluding land sections. Hence the name given by D. Lazarus to the database: Neptune, the Roman god of the sea. The largest amount of data on deep-sea sediments come from ocean drilling, and we began our work by systematically searching through the Initial Reports of the Deep Sea Drilling Project (DSDP) and the Initial and the Proceedings of the Ocean Drilling Program, Scientific Results. Based on a variety of criteria, we rated the holes drilled by DSDP and by ODP up to Leg 135 (the latest leg available in 1995, when I expanded the database to the whole Cenozoic). Ratings were given for each fossil group separately. No rating meant that biostratigraphy was not available, a rating of ‘M’ (medium) indicated the presence of biostratigraphy limited to a few markers and often the absence of detailed range charts. A rating of ‘H’ (high) was used for detailed biostratigraphic reports with extensive range charts. Comments on core recovery, preservation, etc. were also added at this point. Given our long-range goal of using the data for a micropaleontological database, we selected to include in Neptune mainly holes that were marked as high or medium priority for more than one biostratigraphic group. Other hole selection criteria included good core recovery, relatively continuous coring, the length of the stratigraphic interval covered, as well as good microfossil preservation. The recognition of magnetostratigraphy, which could be used for further age control, was also determinant in the selection.

To date, Neptune includes stratigraphic data for the whole Cenozoic (last 65 million years). Several reports are available on Cretaceous sediments and some on Jurassic sediments. However, the K/T boundary represents a major stratigraphic boundary that marks a dramatic faunal and floral assemblage turnover. I feel, therefore, justified in the choice of limiting the coverage to the Cenozoic. At the other end of the spectrum, upper Pleistocene and Holocene sediments are not well represented in DSDP and ODP reports and, therefore, in Neptune. This is mainly due to the limited resolution of marine biostratigraphy for recent sediments, the relatively coarse sampling used in most reports and to loss of the upper few meters of sediments in early coring work.

Final additions or changes to the list of holes were done after plotting the geographic location of the selected holes. We aimed to have a broad geographic coverage and at least one complete section for each biogeographic province (Figure 2.1). The coverage of shelf to abyssal sediments was equally considered: the range of water depths of the sediment/water interface represents a statistically representative subset of all the holes drilled by DSDP and ODP as of 1995 (Spencer-Cervato 1998). However, shallow water (shallower than 1000 m) sediments are underrepresented in the DSDP and ODP collection and are, therefore, underrepresented also in Neptune. In total, we did include 165 holes (Table 2.1). More holes would have been desirable and we have possibly excluded holes of considerable importance. This was due to time limitations and the project’s goals of creating a ‘micropaleontological database’.

2.2. Chronology

The next step consisted in establishing an internally consistent chronology for the selected holes. Because magnetostratigraphic data were available only for some of the holes, biochronology represented the best and often the only way to provide an age model for the holes. Biochronology provides a series of ‘calibrated events’ which essentially mark the first and last appearance of biostratigraphic markers (taxa). Ideally, these events have been correlated in several locations to an independent stratigraphic method, like magnetostratigraphy or oxygen isotope stratigraphy. These scales have in turn been calibrated to absolute chronology in millions of years through complex procedures. The magnetostratigraphic scale used initially for Neptune was Berggren et al. (1985). We subsequently updated our chronology to Berggren et al. (1995b), which is based on Cande and Kent’s magnetostratigraphy (1992, 1995). Berggren et al.’s chronology was chosen because it is the most updated and most comprehensive time scale published to date - it includes biochronological data for several hundred Cenozoic events. Oxygen isotope stratigraphy (in turn calibrated to a magnetostratigraphic scale) was used for only a few of the calibrated events used in Neptune.

Through this two-step approach, numerical ages in million of years (Ma) are given to biostratigraphic events. We assumed that these events are geologically instantaneous and occur simultaneously throughout a given region of the globe (i.e., are globally synchronous and at least regionally widespread, and not dependent on local environment or sediment facies). Berggren et al. (1985) and its recent updates (Berggren et al., 1995a, b) were the source of biochronological events for planktic foraminifera and calcareous nannoplankton. Various regional calibrations were used for siliceous plankton (for radiolarians: Hays and Opdyke 1967; Hays 1970; Theyer et al. 1978; Johnson and Nigrini 1985; Sanfilippo et al. 1985; Goll and Bjørklund 1989; Nigrini 1991; Harwood et al. 1992; Caulet 1991; for diatoms: Barron 1981, 1985a, b; Berggren et al. 1985; Fenner 1984; Koizumi and Tanimura 1985; Gersonde and Burckle 1990; Mikkelsen 1990; and Harwood and Maruyama 1992). Paleogene siliceous plankton biochronology is less well established than the one for the Neogene, so most of the events used were biozonation boundary markers.

Published biochronological events were used to construct the chronology of each hole. Templates were assembled with all the events that we found in the cited references (Table 2.2). These templates (ASCII files to be used in MS Excel) contained the description of the event, an identification code, and the age interval of the calibration. An excerpt from one of these files is shown in Table 2.3.

2.3. Taxonomy

The articles published in the DSDP and ODP reports are an immense source of evolutionary and biostratigraphic data. Although we were aware of many discrepancies in the subjective nature of taxa and taxonomic names (e.g., Gradstein et al. 1985), we assumed that these factors would be manageable by use of simple synonymy lists in our study. A very extensive taxonomic literature is available for marine plankton, and taxa and nomenclature are quite well defined among the most common microfossil groups. This can be used to reasonably standardize taxonomic usage. Thus if taxon Ab is called Ab by one author but Bb by another, we could standardize the data by creating an equivalence Bb = Ab in the database. Moreover, the holes that we selected had been extensively studied for biostratigraphy and some of them represented classical micropaleontological studies. We, however, had to assume that taxon names in all the selected holes were uniformly used, in other words, that taxon Ab described in Hole 289 was identical to taxon Ab described in Hole 747A. More than 8800 taxon names have been used in the selected holes.

2.4. Biostratigraphy

Most micropaleontological studies are limited to one or perhaps two fossil groups. Biostratigraphic studies in DSDP and ODP reports include diatoms, radiolarians, calcareous nannoplankton, planktic and benthic foraminifera, dinoflagellates, silicoflagellates. We decided to consider only planktic organisms and out of the several groups described in the Reports, we selected the four groups that are most abundant in deep-sea sediments, most regularly described in the biostratigraphic literature, and for which extensive event calibration is available: diatoms, radiolarians, calcareous nannoplankton and planktic foraminifera. This selection includes two siliceous (diatoms and radiolarians) and two calcareous (nannoplankton and foraminifera) plankton groups, and at the same time two phytoplankton (diatoms and nannoplankton) and two zooplankton (radiolarians and foraminifera) groups. This approach has several advantages: it would allow us to compare evolutionary trends in multiple groups, but mainly it allowed us to have a better biochronological control on the age models. Planktic foraminifera are probably the most used microfossils for biostratigraphy, and with this approach we were able to compare their resolution and accuracy to the other groups.

The templates were filled in with actual occurrences of the events for each hole. We went through the published range charts or lists of markers and located the events present in the templates. The list of references to the individual reports is given in Table 2.4. Each event was normally recorded as occurring between two samples within the stratigraphic section. Samples were either recorded as meters below seafloor (mbsf) or as actual sample names, in core-section-interval within section in centimeter format. The sample names were then automatically translated into mbsf by the plotting software. No systematic attempt was made to search the general literature for additional stratigraphic data, although biostratigraphic data for some critical holes (e.g., DSDP 558 and 563) were extracted from charts published outside the DSDP reports. The creation of biostratigraphic files from the templates was initially subdivided among the project participants. In the later phase of the project, I was solely responsible for this task. This eliminated some of the discrepancies in the event identification due to subjective interpretations of range charts in terms of First Occurrence (FO) and Last Occurrence (LO).

The first and last occurrence of a taxon were identified when the taxon was not recorded in two or more samples above or below the first or last recorded occurrence. Because the precision of the actual FO or LO depends on the sample spacing, we recorded each event as the stratigraphic interval between the two samples bracketing the event.

Paleomagnetic stratigraphy was recorded as a set of paleomagnetic polarity interval identifications, as given by the original author. In some cases, it became necessary to revise the original identification scheme to achieve an optimal fit between biostratigraphy and paleomagnetic polarity patterns. However, this was usually apparent only when the events were plotted.

The biostratigraphic files prepared for each group were pasted together and used in the construction of age models. There is a varietyare several of methods available to process stratigraphic event data, including Shaw’s plots (Shaw 1964) and Probabilistic Stratigraphy (Hay 1972). However, the most used method of stratigraphic correlation for deep-sea sediments is the age vs. depth plot method. A plot is made of the depth occurrences of previously age-calibrated events in each hole and a line is drawn to correlate depth to age. Although various curve-fitting methods can be used, we have chosen to manually fit a series of straight line segments of varying slopes to the data.

To handle the large volume of data plotting and analysis we used a special-purpose, age-depth plotting program written by Lazarus (1992). The program, written for Macintosh computers, reads the stratigraphic data files and produces an age- vs. versus-depth scatter plot of the data points. The program allows us to draw a line of correlation through the points interactively on the computer screen. Automatic correlation methods were attempted but proven unreliable because they were too easily affected by data outliersdata outliers too easily affected them. The manual construction of the line of correlation allows us to take into consideration recovery gaps and changes in preservation or lithology that may affect the reliability of the age vs. depth plot. The age models are thus subjective and, with a few exceptions, the scatter of data allows for two or more possible interpretations. The use of two or more biostratigraphic groups was intended to minimize the bias introduced by an a priori selection of ‘good’ or ‘bad’ events.

Age models were initially constructed by several project participants. To eliminate discrepancies in the selection of the line of correlation due to subjective preferences, all Neogene DSDP age models were subsequently revised by Dave Lazarus (Lazarus et al. 1995a) and later by myself (after the addition of Paleogene data and ODP holes, and the update of the chronology). A personal rating of the quality of the age models is given in Table 2.1. Although the results of all these efforts still do not guarantee that the age models are optimally reliable, I hope that they represent a far more consistent and updated data set than available prior to the beginning of the project.

The established chronology provided age control on the 30,000 samples described in the DSDP and ODP reports for the selected holes. Information on the micropaleontological content of these samples is available as range charts. These charts give information on the presence or absence of a taxon, and usually describe its abundance. Properly formatted MS Excel range charts were either extracted from the DSDP CD-ROM by the Neptune database program, typed by us, or provided directly from ODP (Table 2.4). These were then imported into Neptune and represent the bulk of data available. We planned to use this information for various studies (species occurrence patterns, longevity and diversity, identification of temporal distribution of biogeographic provinces) which are described in Chapter 4.

The age/depth plots and the age models (text files) are given in the Appendix A. The stratigraphic data files used to construct the age vs. depth plots, are not published here because of space considerations and the complexity of having such a large number of files and links. They are, however, available from the author.

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