IODP Proceedings    Volume contents     Search


Discussion and conclusion

Rather than focusing on the detailed sedimentological processes along the NanTroSEIZE drilling transect, we aim to provide some quality assessment concerning the basic grain size data set presented.

Apart from the routine triplicate runs and their statistical analysis, we also dedicated analyses to assess the overall performance of the Coulter Counter LS200. Figure F9 illustrates results from 16 aliquots of the same core sample. A batch of eight aliquots of the first half of the sample were dispersed in the ultrasonic bath for 60 min plus 48 h of particle settling (Fig. F9A), whereas the second batch of eight samples were dispersed for only 30 min with 24 h of settling time (Fig. F9B). The data of all 16 samples show near-perfect agreement in the fine-grained (i.e., clay size) part of the spectrum and some minor deviations from 6 µm and above. Otherwise, the variability within the two batches was negligible, whereas the deviations between the two batches can be found. In fact, mean and median grain sizes of the specimens are shifted toward smaller values when time for disaggregation was increased. However, some of these differences in mean, median, and sorting (standard deviation) may be explained by initial differences when subsampling the specimen (i.e., minor natural variation within the sample chosen) and not so much from the treatment and grain size measurement.

In conclusion, the main findings of our study are as follows:

  • Precision and reproducibility of the method are satisfactory given the very narrow error bars of repeat measurements on aliquots of a larger sediment sample.

  • Results of the measurements with the Coulter counter LS200 laser particle analyzer are in good agreement with earlier shipboard qualitative visual core description.

  • The mass transport deposits have distinct particle size distribution patterns, whereas fault gouge and other material from structural features cannot easily be distinguished from surrounding sediment by grain size analysis alone.