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doi:10.2204/iodp.proc.323.102.2011

Appendix

Physical properties calibration issues

Magnetic susceptibility

During drilling, problems were noticed with bias changes in magnetic susceptibility data. These problems were to be addressed on shore. Magnetic susceptibility loop sensor instability sometimes resulted in very low readings by both the STMSL and the WRMSL. When low readings were observed on board, cores were reanalyzed on the track loggers. Because the magnetic susceptibility loop sensors were not calibrated on board, the only way to correct the data sets may be to check magnetic susceptibility blank measurements before and after track measurements. Postcruise efforts to correct magnetic susceptibility data should be attempted.

Gamma ray attenuation

Calibration issues with GRA bulk density measurements occurred during the cruise. At Site U1341, both STMSL and WRMSL GRA bulk density data were found to be out of calibration; recalibration of the GRA sensor occurred in the middle of collecting data from Cores 323-U1341A-9H (WRMSL) and 13H (STMSL). Earlier calibration data were not available for either track. Both cores were reanalyzed after calibration. This comparison, along with comparison of the data overlap intervals in Holes U1341A, U1341B, and U1341C (in the CCSF-A depth scale, interpolated to 10 cm intervals and smoothed with a Gaussian window of 9 cm half-width to minimize offsets caused by small miscorrelations), suggests approximate correction factors of 0.016 (+0.002) subtracted from WRMSL GRA values measured prior to and including timestamp 2009-07-28 13:17:45.468 Universal Time Coordinated (UTC) and 0.351 (+0.006) subtracted from STMSL GRA values measured prior to and including timestamp 2009-07-28 02:30:07.312 UTC (Fig. AF1). As a result of this discovery, the GRA sensors on the WRMSL and the STMSL were recalibrated with an aluminum standard before measurements began in each hole.

Although calibration runs were performed after this discovery, out-of-calibration issues often recurred. Because of gas-expansion disruption of core sediment on the catwalk, holes were commonly drilled in the core liner about 10 cm apart. Sediment composed mainly of silty and clayey mud commonly extruded through the punctured holes in the core section during STMSL and WRMSL measurements. Because of the vertical placement of the GRA source (above the core tube) and detector (beneath the core tube), extruded mud tended to accumulate on the detector, which attenuated the GRA signal relative to its calibration (causing estimates of GRA density to be too high). When this was noticed, the extruded mud smeared on the scintillation detector was cleaned up and a GRA calibration standard was run before measurements began. It was not always possible to run calibrated measurements, so postcruise efforts to refine the calibration and correct the GRA bulk density data sets are probably necessary.

Natural gamma radiation

NGR data are very useful in detecting sedimentary cycles recorded by variations in clay content, which are invaluable in establishing stratigraphic correlations. However, systematic variation in NGR readings for core sections was observed during the cruise (Fig. AF2). Figure AF2 shows an example of the NGRL measurements recorded on board. In this figure, the NGR data from Hole U1343A are superimposed on the intervals in each section for all sections. There are two measurement issues: (1) a trend of decreasing intensity at both ends of the 1.5 m core section and (2) recordings of high-intensity peaks for each core section. The low counts for the section ends may be caused by the instrument's architecture. The second feature (two high-intensity lobes) may be related to calibration among the seven scintillation counters. The last calibration of the NGRL was carried out on 1 July 2009 near Victoria, British Columbia, Canada. No calibration of the instrument was possible during Expedition 323. The first calibration issue concerns background levels of NGR intensity. NGR flux in the Bering Sea is higher than in the equatorial Pacific Ocean; therefore, it might be necessary to correct for the higher background intensity of onboard measurements. The second calibration issue concerns an electrical shutdown and restart of the NGRL that occurred in the middle of the expedition.

Postcruise correction of NGR data would significantly enhance the usefulness of the NGR data sets. Table AT1 provides example corrections to NGR count data.

Color reflectance

Before each core was measured, the spectrophotometer was calibrated with a Labsphere-certified white reflectance standard and a black light trap with the light source shuttered. These standards were covered with the same plastic wrap used to cover the core section. For Sites U1339–U1341, the instrument was calibrated every 12 h with the same piece of plastic wrap covering the standards. Shipboard scientists observed the following calibration issues for color reflectance using the SHMSL: (1) reflectance data were affected by light leakage from the room lights into the sensor (depending on how flat the section half surface was) or light channeling along the plastic wrap (confirmed by the detection of Hg-emission lines from the laboratory's fluorescent lights); (2) reflectance data were not stable over 12 h timescales (for unknown reasons), requiring more frequent calibration; and (3) some of the plastic wrap was discovered to be yellowish, and different batches of plastic wrap were used on the white calibration and sediment core. Several unexplained excursions in b* data could possibly be explained by the use of yellowed plastic wrap. Unfortunately, the source of plastic was not documented for each core section, so it is not straightforward to check for these artifacts.

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