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doi:10.2204/iodp.proc.341.105.2014

Stratigraphic correlation

The composite depth scale at Site U1419 is constructed from 0.0 to 205.79 m core composite depth below seafloor (CCSF-A). The splice consists of one complete and continuous interval from the mudline to 112.10 m core composite depth below seafloor (CCSF-D). The CCSF-A and CCSF-D depth scales are defined in “Stratigraphic correlation” in the “Methods” chapter (Jaeger et al., 2014a).

The splice ranges from the top of Core 341-U1419B-1H (the mudline) to the base of Core 341-U1419E-17H (Tables T9, T10). To the extent possible, the splice was constructed from Holes U1419D and U1419E (six intervals each), although five intervals were included from Hole U1419B and two were included from Hole U1419C. Hole U1419A was avoided in the splice when possible because it was sampled at sea; nevertheless, it was necessary to include two short intervals of Hole U1419A in the splice.

Weather was calm and ship heave was negligible while coring Site U1419, but the sediments were relatively rich in lonestones, as expected for an ice-proximal environment. This lithology proved challenging for core recovery and interhole correlation. Intervals of core disturbance, incomplete recovery, and use of the half APC coring system all presented complications in the development of composite depths and a spliced record. In particular, we observed many instances of flow-in, generally (but not always) near the base of cores, and some fall-in of rocky debris in core tops. Some parts of the splice are unique in an individual hole and are not well verified.

Correlations between holes were accomplished using the Integrated Ocean Drilling Program Correlator software (version 1.695), and all the splice tie points were checked with digital line-scan images using Corelyzer (version 2.0.2), linked to Correlator. During coring, real-time development of composite depths and guidance for coring operations relied on Special Task Multisensor Logger (STMSL) gamma ray attenuation (GRA) bulk density and magnetic susceptibility (MS) data. The final composite depth scale (CCSF-A) and the splice (CCSF-D scale) are based primarily on the stratigraphic correlation of MS and GRA density from the Whole-Round Multisensor Logger (WRMSL) (Figs. F16, F17), as well as whole-round natural gamma radiation (NGR) from the Natural Gamma Ray Logger and reflectance spectroscopy color data from the Section Half Multisensor Logger (SHMSL). Of these variables, MS offered the most reliable tool for correlation at Site U1419; the other variables served primarily as verification data and were generally less useful for purposes of hole-to-hole correlation. Anomalously low GRA density was used primarily as an indicator of core disturbance. MS from the loop sensors was checked with point MS data, although the presence of small lonestones in intervals of diamict made the point-sensor data relatively noisy.

The CCSF-A and CCSF-D scales were constructed by assuming that the uppermost sediment (the mudline) in Core 341-U1419B-1H represented the sediment/water interface. An approximate mudline was also recovered in Core 341-U1419A-1H, confirming the fidelity of the top of the recovered interval. Core 341-U1419B-1H serves as the “anchor” in the composite depth scale and is the only core with depths that are the same on the CCSF-A and CCSF-D scales. From this anchor we worked downhole, matching the variations in core logging data on a core-by-core basis using Correlator.

In the splice, one of the included intervals (341-U1419D-9H-3, 51.2 cm, to 9H-7, 69.2 cm; 55.33–60.06 m CCSF-A) contains possible disturbance from void collapse during core processing and archiving on the catwalk, but this appeared to be the least disturbed option for this interval of the splice. Several tie points are tentative. In particular, the tie point between 341-U1419C-6H-7, 102.7 cm, and 341-U1419D-9H-3, 51.2 cm (55.33 m CCSF-D), is weak because of probable coring disturbance at shallower depths in Sections 341-U1419D-9H-1 through 9H-3. The ties between 341-U1419D-9H-7, 69.2 cm, and 341-U1419B-7H-1, 146.1 cm (60.06 m CCSF-D); 341-U1419E-12H-5, 88.2 cm, and 341-U1419A-9H-1, 83.7 cm (79.00 m CCSF-D); and 341-U1419B-9H-6, 135.1 cm, and 341-U1419E-14H-1, 54.7 cm (89.34 m CCSF-D), all have very little overlap, which increases the probability of a temporal gap due to nonrecovery. The tie point between 341-U1419A-10H-6, 65.7 cm, and 341-U1419E-15H-1, 26.2 cm (96.96 m CCSF-D), requires a splice to a relatively short core (although the variations in MS appear to be reproduced in Core 341-U1419B-10H). Similarly, the tie point between 341-U1419E-15H-3, 17.3 cm, and 341-U1419C-12H-1, 51.9 cm (99.49 m CCSF-D), has very little overlap and leaves the chance of a temporal gap. It is possible that the interval from 96.96 to 99.49 m CCSF-D is a condensed replication of the interval from ~100 to 103.6 m CCSF-A; this part of the splice will need to be checked with postcruise data.

Within the splice, the composite CCSF-A depth scale is (by definition) identical to the CCSF-D depth scale. Note that CCSF-D rigorously applies only to the spliced interval. Intervals outside the splice, although available with CCSF-A composite depth assignments, should not be expected to correlate precisely with fine-scale details within the splice or with other holes because of variation in the relative spacing of features in the recovered intervals from different holes. Such apparent depth differences may reflect coring artifacts or fine-scale spatial variations in sediment accumulation and preservation at and below the seafloor.

The cumulative offset between the CSF-A depth scale and the CCSF-A and CCSF-D depth scales is nonlinear (Fig. F18). The affine growth factor (a measure of the fractional stretching of the composite section relative to the drilled interval; see “Stratigraphic correlation” in the “Methods” chapter [Jaeger et al., 2014a]) is 1.145 on average (i.e., the sediment is expanded by ~15% relative to the interval drilled) at Site U1419 in the APC-cored interval from 0 to 72.52 m CCSF-A or CCSF-D. Anomalies around this relatively uniform affine growth relationship are plausibly explained by partial recovery in lonestone-rich sediments (implying uncertainty in what portion of the cored interval is represented by recovered sediment) and uncertainty in the application of tide corrections for drilling depths (for example, between Cores 341-U1419A-1H and 2H).

Within the interval from 72.52 to 137.50 m CCSF-A, the affine growth factor is 1.279 (i.e., the sediment is expanded by ~28% relative to the interval drilled). Such a high rate of expansion at Site U1419 likely reflects either coring artifacts in the borehole or gas expansion of sediments during recovery (see “Geochemistry” for discussion of methane at Site U1419). Below 137.50 m CCSF-A, intervals of possible correlation were found between a few cores, but the intervals were not considered sufficiently continuous nor were the correlations certain enough to warrant adjustment of their affine values on a core-by-core basis. In this interval, the affine value is held at a constant 28.66 m. This does not imply that there is no sediment expansion at depths below 137.50 m CCSF-A, just that we have no constraint on its extent.

Calculation of mass accumulation rates based on the CCSF-A or CCSF-D scale must correct for the affine growth factor. To facilitate this process, we developed an additional depth model, CCSF-B, which compressed the CCSF-A and CCSF-D scales into a scale that has the same total depth of sediment column as the interval actually drilled (see “Stratigraphic correlation” in the “Methods” chapter [Jaeger et al., 2014a]). The following three equations define transformation of the CCSF-A or CCSF-D depth scale into CCSF-B depths (Fig. F18):

From 0 to 72.52 m CCSF-A/CCSF-D:

From 72.52 to 137.50 m CCSF-A/CCSF-D:

For 137.50 m CCSF-A/CCSF-D:

The depth boundaries between these equations are chosen to coincide with a splice point (between 341-U1419D-10H-6, 6.79 cm, and 341-U1419E-12H-1, 40.83 cm, at 72.52 m CCSF-A) and at the top of Core 341-U1419E-23H (137.50 m CCSF-A).

Initial age model

Normal geomagnetic polarity in all cores implies that all sediments recovered are younger than 781 ka. The uppermost part of the sequence can be correlated with site survey Core EW0408-85JC, a radiocarbon-dated piston core from the same site (Davies et al., 2011), based on MS. The interval of relatively low MS from 0 to 6.4 m CCSF-A (0 to 5.5 m CCSF-B) at Site U1419 is assigned to Holocene and Late Pleistocene ages (0–15 ka), implying an average interglacial sedimentation rate of 0.4 m/k.y. Oxygen isotopes of foraminifers picked at sea from the core catcher sample and analyzed immediately postcruise at Oregon State University (USA), indicate that the maximum age at the base of the site (205.79 m CCSF-A) is <60,000 y, implying an average glacial sedimentation rate of 3.8 m/k.y.

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