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doi:10.2204/iodp.proc.340.202.2015

Results

The composite depth scale for Site U1396 was constructed from 0.0 to 153.13 m CCSF-A. Section 340-U1396C-1H-1 was selected as the mudline core and serves as the anchor for the rest of the composite depth scale. All other cores are offset relative to Section 1H-1 by correlating variations in core logging data between holes on a core-by-core basis using Correlator. Because of the highly disturbed nature of Core 340-U1396A-2H, it was not possible to correlate this core to the other holes. Core lengths, offsets, and the CSF-A and CCSF-A depth scales are listed in Table T1. Core 340-U1396C-13H was a partial stroke and experienced basal flow-in of ~4 m of pumiceous sand (Jutzeler et al., 2014). Oversampling of this unit and its incorporation in calculating the target top depth of Core 340-U1396C-14H in the CSF-A depth scale meant that the interval in Hole U1396C that could potentially be used to bridge Cores 340-U1396A-13H and 14H was not recovered. Therefore, deeper Cores 14H and 15H and Cores 340-U1396C-14H and 15H can be correlated to each other, but they cannot be directly tied into the overlying continuous CCSF-A scale, resulting in a floating CCSF-A scale below Core 340-U1396A-14H. Linear regression of the CSF-A and CCSF-A scales for all holes up to and including Core 340-U1396C-13H (N = 25, r = >0.99) allows estimation of the depth offset required to integrate Core 340-U1396A-14H (which serves as the anchor for the floating CCSF-A scale) into the overlying CCSF-A scale. Although these cores are assigned a CCSF-A depth, they must be treated with caution because these depths are predicted using the offset behavior of the overlying sections and not directly assigned. Linear regression of the CSF-A and CCSF-A scales for each hole reveal growth factors of 1.1096 and 1.0992 for Hole U1396A and Hole U1396C, respectively (Fig. F2). This calculation results in an ~11% increase in the CCSF-A scale relative to CSF-A depths for Hole U1396A and ~10% for Hole U1396C.

Without nonlinear manipulation of individual cores, it is not possible to align all features between holes. However, significant improvement is observed in the correlation of MS between holes using the CCSF-A scale (R² = 0.52) over the CSF-A scale (R² = 0.14) (Fig. F3). MS for Holes U1396A–U1396C on the newly developed CCSF-A depth scale is shown in the lower panels of Figure F4. The spliced record is shown in the upper panel and is color coded (and labeled) by the hole that the data in the splice is from; splice tie points are listed in Table T2. The goal of creating this splice was to ensure the most complete and representative intervals were identified and integrated into one continuous record. Although in-splice data correlate well at the tie points, off-splice data should not be expected to directly correlate between holes. Physical property data should be consulted to assess where off-splice data fits into the spliced CCSF-D depth scale.

Using the shipboard paleomagnetic reversal record (see the “Site U1396” chapter [Expedition 340 Scientists, 2013]), mass accumulation rates for the site can now be calculated. First, however, the CCSF-A scale must be corrected for the growth factor. Linear regression through the core tops of all three holes at Site U1396 indicates that a scaling factor of 0.9057 (90.57%) is required to compress the CCSF-A depths into a scale that has the same total depth as the interval drilled (Fig. F5). The compressed CCSF-B depth scale is listed in Table T1 alongside the CSF-A and CCSF-A scales. The declination of the shipboard paleomagnetic record was azimuthally orientated with the FlexIt tool (see the “Site U1396” chapter [Expedition 340 Scientists, 2013]), and polarity reversal horizons were identified by the 180° change in declination that accompanies a magnetic polarity reversal. These horizons were recorded at Site U1396 as the maximum depth (the last point at which stable polarity was recorded), the minimum depth (the first point at which stable antipodal polarity was established), and the midpoint (average) of the two. These three depths are reported on the CCSF-A depth scale in Table T3. The Cobb Mountain Subchron (C1r.2r-1n) between 1.173 and 1.185 Ma (Ogg, 2012) was difficult to identify in this relatively low resolution record, so the record remains untied between 1.072 and 1.778 Ma. It is also important to note that no event-depth correction was made for the assumed instantaneous deposition of tephra whose eruption frequency may have been greater between 3.5 and 4.5 Ma than after 3.5 Ma (Stinton et al., 2014) relative to the slower accumulation of carbonate-dominated autochthonous background sediment. Using the geomagnetic polarity timescale of Ogg (2012) and the CCSF-B depth scale and linearly interpolating between tie points, the resulting age model is shown in Figure F5; sedimentation rates (with low and high estimates derived from the minimum and maximum depths) are shown in Figure F5 alongside the LR04 benthic oxygen isotope stack (Lisiecki and Raymo, 2005) for reference. The average sedimentation rate over the whole record is 30.7 m/My, however mid–late Pliocene and early Pleistocene sedimentation rates (4.493–2.128 My; 41.6 m/My) are higher than rates since the early Pleistocene (<2.128 My; 18.7 m/My).

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