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doi:10.2204/iodp.proc.341.102.2014 Stratigraphic correlationExpedition 341 scientific objectives required recovery of complete stratigraphic sections to the extent possible. Continuous sedimentary sections cannot be recovered from a single borehole because gaps in recovery occur between successive cores, even when 100% or more nominal recovery is attained (Ruddiman et al., 1987; Hagelberg, et al., 1995). Construction of a complete stratigraphic section, referred to as a splice, requires combining intervals from two or more holes cored at the same site. To maximize the probability of bridging gaps between cores, we attempt to offset between holes the depths below the seafloor from which cores are recovered. This practice maximizes the probability that missing sedimentary sections from within a given hole are recovered in one or more adjacent holes. At least two complete holes, and in many cases three or more holes, are needed to recover a complete section in the APC portion of a site. Additional complete holes are cored to allow options for construction of alternate splices, where possible. Our methods for developing composite depths and splices followed the basic strategy that is now common practice on all high-resolution paleoceanographic expeditions. We used initial analyses of magnetic susceptibility and gamma density run on the STMSL to develop preliminary composite depths for purposes of making real-time drilling decisions (Mix, Tiedemann, Blum, et al., 2003). The depth scale was refined as more detailed information became available during drilling. Our goals for stratigraphic correlation, in priority order, were to
As a result of this stratigraphic correlation process, several different depth models are created. Table T1 and Figure F17 summarize these various IODP depth models and their ODP equivalents. Detailed discussion of the definitions of these depth scales, and the processes by which they are created, appear below. Composite depth scaleThe initial CSF-A depth scale is based on the length that the drill string is advanced core by core. This is equivalent to the ODP scale meters below seafloor (mbsf). The CSF-A scale is inaccurate because of ship heave (which is not compensated for during APC coring), tidal variations in sea level, and other sources of error. Before a splice can be constructed, the cores from the various holes must be stratigraphically correlated with each other. Such correlation transfers CSF-A depths into a composite depth scale referred to as core composite depth below seafloor (CCSF-A). The splice that results is known as core composite depth below seafloor method D (CCSF-D). Differences between these depth scales occur because features may be slightly offset between holes on the CCSF-A scale and the splice on the CCSF-D scale. These IODP depth scales are approximately equivalent to the ODP depth scale meters composite depth (mcd) and are further described below. The CCSF-A scale is built by assuming that the uppermost sediment (commonly referred to as the “mudline”) in the first core from one hole is the sediment/water interface. At each site, this selected core becomes the “anchor” in the composite depth scale and is usually the only one in which depths are the same on both the CSF-A and CCSF-A scales. From this anchor, core logging data are correlated among holes downsection. For each core, a (constant) depth offset, or affine value, chosen to best align observed lithologic variations to the equivalent cores in adjacent holes, is added to the CSF-A depth in sequence down the holes. It is not usually possible to align all features perfectly between holes, so where possible the affine values are chosen to maximize correlations at the levels that splice tie points are defined between holes. During Expedition 341, an initial composite depth scale was created using magnetic susceptibility measured with a loop sensor and GRA data from the STMSL. The final CCSF-A scale and the splice for each site used additional data from, for example, the WRMSL, which measures GRA density (a function of grain density and porosity) and P-wave velocity (on the P-wave logger [PWL]). GRA and magnetic susceptibility data from the WRMSL generally replaced the STMSL data for correlation purposes; however, the STMSL data are retained in the database as a useful check on the final data and because some damaged sections fit through the STMSL but could not be run on the WRMSL. In some cases, small depth offsets were found for features within core sections measured by the STMSL and WRMSL; a common cause of this offset is gas expansion in the cores between the STMSL and WRMSL analyses. In these cases, attempts were made to define composite depths using the last data measured prior to core splitting. This was not always possible, however; some depth mismatches (typically on the scale of centimeters, but perhaps tens of centimeters) may exist between composite depths defined by whole-round sensing and the depths at which particular features appear in split cores. Correlations based primarily on whole-round GRA and magnetic susceptibility data were augmented where needed by natural gamma radiation (NGR) data from the core logger and digital color parameters (L*a*b*) and magnetic susceptibility data obtained with a point sensor (MS-POINT) measured on the SHMSL. High-resolution color digital images analyzed on the SHIL, which provides color digital images, were incorporated into the correlation process using the Corelyzer software. Specific methods for WRMSL and SHMSL measurements are described in “Physical properties,” and SHIL measurements are described in “Lithostratigraphy.” Most core logging data were collected at 2.5, 5, or 10 cm intervals, depending on time availability and core flow; SHIL data were pixel based. Composite depth scale constructionThe core logging data were imported into the specialized shipboard software program Correlator (version 1.695; Mac) and linked to digital core images with the program Corelyzer (version 2.0.2; Mac). Correlator enables construction of a composite depth scale for each hole at a given site by depth-shifting individual cores to maximize the correlation of reproducible features in the core logging data. For hole-to-hole correlations and for plotting results, data were masked to avoid incorporating anomalous data influenced by edge effects at section boundaries, at core tops, or in voids where no sediment was present; however, all original data were retained in the LIMS database. Depth intervals within cores are not squeezed or stretched by Correlator; thus, it is not possible to align all the correlative features within each core. Differences between features in different holes may reflect small-scale differences in sedimentation and/or distortion caused by the coring and archiving processes. For example, the tops of APC cores may be stretched and the bottoms compressed, although this depends on lithology and the extent of lithification. In addition, sediment of unknown age occasionally falls from higher levels in the borehole onto the tops of cores as they are recovered, and as a result the tops of some cores are not reliable. Correlations among cores from adjacent holes are evaluated visually and statistically (by windowed cross-correlation). The depth offsets for each core that are necessary to convert CSF-A depths to the CCSF-A scale are recorded in an affine table for each site. The CCSF-A depth for any point within a core equals the CSF-A depth plus the affine offset. Correlation at finer resolution is not possible with Correlator because depth adjustments are applied linearly to individual cores. At this stage, no adjustments are made in the length of each core, such as numerically squeezing and stretching within cores. Finer scale adjustments of individual cores relative to the splice (e.g., Hagelberg et al., 1995; Pälike et al., 2005) or relative to logging data (e.g., Harris et al., 1995) can be done following the development of the composite section. Ideally, the base of the continuous CCSF-A scale is the bottom of the deepest core recovered from the deepest hole. In practice, however, the base often occurs where core recovery gaps align across all holes and below which constructing a splice is impossible. Deeper cores cannot be tied directly into the overlying continuous CCSF-A scale. These cores are appended, and CCSF-A depths are calculated by adding a constant offset, which is usually the largest affine value from each hole. An exception to this case occurs when some cores from two or more holes deeper than the base of the splice can be correlated with each other, allowing the generation of a “floating” CCSF-A scale and splice for some intervals deeper than the continuous CCSF-A scale. Corrected composite depth scaleThe length of the CCSF-A depth scale at a given site is typically ~10%–20% greater than the length of the cored interval in any one hole as indicated by the CSF-A depth scale. Although the exact reasons for this apparent expansion of the sediment column are not completely known, it is commonly attributed to rebound following release of overburden in the deeper sections, stretching during the coring process, gas expansion during the core recovery process, and other factors (Moran, 1997). Core composite depth below seafloor (CCSF-B) is a depth scale that is intended to correct the CCSF-A scale or the splice (CCSF-D) for empirically observed expansion. CCSF-B depths are produced by correcting CCSF-A or CCSF-D depths for the average affine growth value of the CCSF-A scale relative to the CSF-A scale over a sufficiently long interval that random variations in drill pipe advance due to ship heave, tides, and other factors are negligible. This scaling produces a complete composite stratigraphic sequence that is the same length as the total cored or drilled interval. The CCSF-B scale is a close approximation of actual drilling depths and is essential for comparison of core logging and downhole logging data and for estimation of sediment accumulation rates and mass fluxes. Because the CCSF-B scale provides an estimate of the sediment thickness before the sediment was removed from the borehole, the intervals measured between actual samples taken within each core section are better represented in the CCSF-A or CCSF-D (uncompressed) depth scales. SpliceThe splice is a composite stratigraphic section representing the complete record at a site. It is composed of core sections from adjacent holes such that coring gaps or sampling gaps, like those generated by taking of IW samples, in one hole are filled with core from an adjacent hole. The splice does not generally contain coring gaps, and an effort has been made to minimize inclusion of disturbed sections by examining core photographs. The splice guides core sampling for high-resolution studies. Tables and figures in each site chapter summarize the intervals from each hole used to construct the splice. The portion of the CCSF-A depth scale that is applied to the splice is referred to as the CCSF-D depth scale. Within the splice sections, CCSF-D is identical to CCSF-A. Note, however, that because of stretching and squeezing within cores, some features may not correlate precisely between the splice and samples taken off the splice, even though all samples have CCSF-A depths. Therefore, the final composite depth scale, CCSF-D, is only formally defined within the primary splice. The choice of splice tie points is a somewhat subjective exercise. Our method in the construction of a splice followed four rules. First, where possible, we avoided using the first and last sections of cores, where disturbance due to drilling artifacts (even if not apparent in core logging data) was most likely. Second, we attempted to incorporate those realizations of the stratigraphic section that in our judgment were most representative of the holes recovered. Third, we tried to minimize tie points (i.e., to use the longest possible sections within individual cores) in order to simplify sampling. Fourth, we tried to minimize use of intervals sampled during the expedition to leave room for high-resolution postexpedition sampling in the splice. Tidal effects on coring depthTidal influence on APC shot depth was previously documented by correlation of affine offset changes and tide height during ODP Leg 202 (Mix, Tiedemann, Blum, et al., 2003). During Expedition 341, we calculated tide corrections at 30 min intervals for all APC drill sites based on the Oregon State University Tidal Prediction Software (OTPS; copyright Oregon State University 2012, G. Egbert and L. Erofeeva, used by permission). Predicted tidal variations on Expedition 341 were up to 4 m over a diurnal and semidiurnal cycle, so proper tide corrections were needed to obtain complete composite sections and splices. The first APC core at each site established the reference depth for tidal corrections, and all following cores were adjusted for the offset of the predicted tide at the time of coring relative to the initial reference depth. During each coring operation, the drill string was advanced either less than the previous core’s recovery (for a falling tide) or greater than the previous core’s recovery (for a rising tide) to make this correction. The tide corrections used for Sites U1417–U1419 are illustrated in Figure F18. Age models and sedimentation ratesThe composite depth models (in CCSF-B depths where available) were applied to datums of known ages (see “Paleontology and biostratigraphy” and “Paleomagnetism”). Stratigraphic datums from multiple holes were merged to estimate the most representative depths and uncertainties in both depth and age of unique events at each site. We then developed minimum and maximum depth models, based on visual linear fits between equally spaced age increments that spanned the reasonable uncertainty range of the well-constrained datums, after rejecting obvious outliers. We considered these minimum and maximum models to represent the ±2σ uncertainties in the age:depth relationship. Given these models, we estimated sediment accumulation rates between age increments based on 500 Monte Carlo simulations in which Gaussian white noise of appropriate range was applied to the depth at each age level. The resulting 500 realizations of sedimentation rate estimates were used to calculate the central tendency (fiftieth percentile) and uncertainties spanning 68.2% probability (i.e., ±1σ). Note that these sedimentation rates are effectively averages and uncertainties over the selected age increments; they are not intended to represent instantaneous sedimentation rates. Uncertainties in sedimentation rate generally rose as narrower time increments were selected. Through an iterative process, the size of age increments at each site was selected such that variations in sedimentation rate were greater than their uncertainties. |
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