1. PREFACE: DEPTH BELOW SEAFLOOR CONVENTIONS 1David J.W. Piper 2 and Roger D. Flood 3 |
| This preface briefly explains our
conventions for estimating depth below seafloor in
presenting downcore data. In general, we follow Ocean
Drilling Program (ODP) meters below seafloor (mbsf)
convention in positioning the top of recovered sediment
at the top of the cored interval and in calculating
sub-bottom depths for samples within a particular section
using the depth of the top of the section and the depth
in the section (see pp. 47-48 of Flood, Piper, Klaus, et
al., 1995). Sub-bottom depths determined using the ODP
convention are noted on figures as "mbsf."
Sediments recovered from many of the holes cored on Leg
155, however, expanded within the core liner, largely as
a result of pervasive biogenic methane gas. Core
expansion also results from elastic rebound and clay
swelling. Because core sections are cut and measured
after the sediment has expanded and the core length has
stabilized, the recorded length of a core can be greater
than the interval cored in the hole (as measured by pipe
advance), by as much as a meter per core in extreme
cases. In such circumstances in a downcore plot, data
from the core catcher would plot below any measurements
in the upper meter of the underlying core. For analyses
at intervals of a meter or greater, use of the standard
ODP convention and "mbsf" does not result in
anomalous downcore plots. For more closely spaced
analyses, however, expansion resulting from gas and other
phenomena must be corrected if downcore plots are to be
unambiguous where there is a nominal recovery of more
than 100%. We have provided a method that corrects for
gas expansion for all holes so that data analyses done by
different investigators can be correlated. For the presentation of closely spaced analyses, we have used a standard method for compressing recovered core to correspond to the cored interval in the hole. In figures, this scale is referred to as "expansion-corrected mbsf." The following steps have been used in calculating "expansion-corrected mbsf":
Excel (version 4) spreadsheets are provided on the CD-ROM (Tables 1-17) for calculating "expansion-corrected mbsf." To determine the "expansion-corrected mbsf," first scroll down the spreadsheet to the row of the core and section of interest. In this row, enter the interval depth into column I. Note that the spreadsheet calculates the corrected depth scale using units of meters in column I, rather than the commonly used centimeter units used in the identification of sample location. Once a value is entered in column I, the corresponding expansion-corrected mbsf depth is calculated and displayed in column J. Two errors can occur in this procedure. If the interval entered in the row for a specific core and section is longer than the length of the section, a zero value will appear in column J. The character string "#VALUE!" will appear in column J if the interval entered is associated with a void space in the selected section. Our procedure is a practicality to eliminate data overlap. It provides only a rough approximation to in situ depth, and the error will vary significantly from core to core. In particular, it uses the ODP convention of placing the top of the recovered section at the top of the cored interval. It does not take into account the 1- to 2-m gap between cores that has been recognized from triple coring in pelagic and hemipelagic sediment. As a result, many cores have not been sufficiently compressed to compensate for gas expansion and elastic rebound. In some holes, the availability of wireline log data has allowed short lengths of recovered core to be positioned more accurately than by the ODP convention, for example as shown by Pirmez et al. (this volume). Note that the graphic sedimentological columns presented in each site chapter and on the back-pocket foldout of Flood, Piper, Klaus, et al. (1995) were prepared by compressing cores with >100% recovery back to cored interval length after removal of large voids only. ACKNOWLEDGMENTSStan Johnson measured voids from photographs and prepared the Excel files. Laboratory work was supported by the Geological Survey of Canada. The manuscript benefited from reviews by Peter Blum and Kate Moran. REFERENCEFlood, R.D., Piper, D.J.W., Klaus, A., et al., 1995. Proc. ODP, Init. Repts., 155: College Station, TX (Ocean Drilling Program). APPENDIXNotes on Structure of the Spreadsheet The spreadsheets have been developed by modifying the shipboard coring summary tables. They are not particularly elegant; these notes are provided for users who may wish to modify them. The files will be more user-friendly and efficient if a template is created. The user can then paste sample identifiers or depths of an entire data set into the template and receive all corrected depths at once. This requires a routine that loops through sections of a particular core and performs the appropriate calculations. The following columns have been added to the shipboard coring summary spreadsheet: Column I: User entered interval (in
meters) at selected row of core and section. Provided that this condition is met, then
the expansion-corrected depth is determined using the
following protocol: Since EXCEL 4.0a will only allow seven levels of nested IF statements, cores that have 8 or more voids within a section have been split into two subsections. The subsection is divided at 1.0 m if the section is greater than 1.0 m long or at 0.5 m if the section is between 0.5 and 1.0 m in length. Date of initial receipt: 27 November 1995
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