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doi:10.2204/iodp.proc.320321.201.2012

Material and methods

Magnetic susceptibility and GRA bulk density data from the Whole-Round Multisensor Logger and virtual geomagnetic pole (VGP) data calculated from natural remanent magnetization measurements in a superconducting rock magnetometer collected at Expedition 320 sites (see the “Expedition 320/321 summary” chapter [Expedition 320/321 Scientists, 2010a]) were used to refine depth offsets and to revise the shipboard composite section for Sites U1331, U1332, U1333, and U1334. Similarly, magnetic susceptibility, GRA, and VGP data from Leg 199 sites (Shipboard Scientific Party, 2002a, 2002b, 2002c; Pälike et al., 2005) were used to refine offsets and splices at Sites 1218, 1219, and 1220. Refinements are also based on detailed high-resolution X-ray fluorescence (XRF) core scanning data (T. Westerhold et al., unpubl. data; D. Liebrand et al., unpubl. data).

In 2006–2007 new classification and nomenclature for depth scale types were defined for IODP (see IODP Depth Scales Terminology at www.iodp.org/program-policies/). The new methods and nomenclature for calculating sample depth in a hole has changed to be method specific to ensure that data acquisition, mapping of scales, and construction of composite scales and splices are unequivocal. Because this study integrates data with different depth scale nomenclatures, we will describe in detail the classifications and definitions of depth scales as used here for Leg 199 and Expedition 320. A much more detailed definition of IODP depth scales used during Expedition 320/321 is given in the “Methods” chapter (Expedition 320/321 Scientists, 2010b).

For this study, the most important depth is the core depth below seafloor. This depth for each drilled core is based on the actual length of the recovered core and the drillers depth. It is defined as core depth below seafloor (CSF) for Expedition 320 and meters below seafloor (mbsf) for Leg 199. For consistency, we suggest the use of “mbsf (m CSF-A)” for Expedition 320 sites and “mbsf” for Leg 199 sites. Each point in the core can now be located by adding the offset between sample and core top to the drilling depth below seafloor (DSF) of the top of the core. To construct an initial continuous stratigraphic reference during the expedition, individual cores were depth shifted to maximize correlation between multiple adjacent holes and spliced together into a composite record. The new shipboard depth scale of the spliced section is defined as the composite core depth below seafloor (CCSF-A) for Expedition 320 and meters composite depth (mcd) for Leg 199. For consistency we suggest the use of “mcd (m CCSF-A)” for Expedition 320 and “mcd” for Leg 199. The addendum “-A” to Expedition 320 cores to the CCSF denotes that individual cores were shifted vertically without permitting expansion or contraction of the relative depth scale within any core.

Postcruise, we evaluated and revised the shipboard spliced composite section, establishing new core offsets and refined the shipboard splice if necessary. Intervals having significant disturbance or distortion were not used for composite section construction. For construction of the revised records, we tried to maintain those tie points given in the shipboard composite, where possible. Changes in the position of tie points in the revised spliced record have been highlighted as bold letters in the splice tables of each site. The new refined depth scale is defined as the revised composite core depth below seafloor (revised CCSF-A) for Expedition 320 and revised meters composite depth (rmcd) for Leg 199. For consistency we suggest the use of “rmcd (m revised CCSF-A)” for Expedition 320 and “rmcd” for Leg 199. Correction to the rmcd depth scales of Leg 199 (Pälike et al., 2005) are indicated by the corrected revised meters composite depth (corrected rmcd).

After assembling the new composite records, we adjusted sedimentary sections outside the revised composite splice by squeezing and stretching to conform to the overall rmcd and rmcd (m revised CCSF-A) depth scales. To indicate this adjustment we added the prefix “adjusted” to the rmcd (m revised CCSF-A) and the corrected rmcd if necessary. This mapping procedure allows data and samples located outside the spliced composite record to be placed in the new revised composite depth scale at each site.

Finally, to integrate all available data we correlated magnetic susceptibility, GRA, and VGP data between sites using the time series analysis program AnalySeries (Paillard et al., 1996). All tables and cleaned composite records of magnetic susceptibility and GRA data from Sites 1218, 1219, 1220, U1331, U1332, U1333, and U1334 and VGP data from Sites 1218, 1219, and 1220 are available online in the WDC-MARE PANGAEA database (doi.pangaea.de/10.1594/PANGAEA.757215).

Composite core images were created as an additional aid in site-to-site correlation using an approach modified from that described in Wilkens et al. (2009). Individual section images collected by the shipboard Section Half Imaging Logger (SHIL) during Expedition 320 were initially assigned a core depth below seafloor (CSF-A) depth range based on site coring data. Section images were then mapped onto a single image of an entire core. Core image depth ranges were then shifted by constant offsets to revised CCSF-A depths based on the revised composite splice for each site (e.g., Table T1). The final step in creating a single image of the entire revised composite section involved another mapping of depth intervals from individual core images defined in site splice tables (e.g., Table T2) onto a composite image. As the resolution of the original SHIL images is on the order of 50 µm, each mapping step included image interpolation to a coarser scale—250 µm for the individual core images and 1 mm for the composite image. Leg 199 was the first use of an earlier version of the shipboard digital imaging system (DIS), and unfamiliarity with its operation led to inconsistent image exposures. Rather than attempt to correct the DIS images, we elected to use the digitized core photos available online at the IODP data website to construct composite site images. Images of each section of each core were digitally cut from the core images and then combined as described above. As the core photo images were much lower resolution (nominally 2.5 mm), there was no need to downsize while mapping. A slight unevenness in lighting of the core photos (darker around the perimeter) produced an artifact when cutting and combining section images from digitized core photos into total core images. Apparent 1.5 m wavelength banding is particularly evident in lighter colored sediments. An example can be seen in the composite image of Site 1218 between 150 and 200 corrected rmcd.

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