The Cenozoic sediments of the Equatorial Pacific provide an ideal and unique record of oceanographic conditions and climate change over the last 50 million years of Earth’s history. Previous studies utilizing data and samples from ODP Leg 199 have provided great insight into astronomically calibrated time-scales which control depositional sequences and lithologies of the deep Pacific waters. IODP Expeditions 320 and 321 aimed to further our understanding of the time-scales, processes and geological signatures in this environment by recovering a more detailed and higher resolution record of data and samples through this important geological record.
This study uses physical properties and wireline logging data together with detailed sedimentological descriptions from IODP Expedition 320 to investigate heterogeneity and cyclicity in the physical properties across a time-transect of six sites. The application of statistical techniques for the numerical quantification of heterogeneity to these data shows that the various discrete time periods (age units) studied along the transect (lower Eocene through upper Miocene) return consistent values allowing these age units to be traced laterally based on contrasts in heterogeneity values. Heterogeneities in bulk density, magnetic susceptibility, and natural gamma ray data are seen to vary with unit thickness and can be related to lithology, and the presence / abundance of bioturbated intervals and carbonate turbidite beds. Numerical heterogeneities are also shown to be consistent across three scale of investigation; wireline (meter scales), track (cm scales) and discrete (mm-cm scale) datasets. These results could significantly impact future sampling strategies for similar sites by guiding the minimum sampling density required to ensure resolving high-resolution heterogeneities and cyclicity.
Analysis of cyclicity within the physical properties data using the Fourier transform and semi-variogram analysis, with depth-scale converted to a time-series based on shipboard paleontological analysis, shows a number of large wavelength cycles have been captured, ranging from 100ka to 6Ma. Further analysis of the location and intensity of bioturbation and the occurrence of carbonate turbidites at two key sites reveals that cyclicity of similar wavelengths can still be resolved, suggesting that it may be possible to identify and remove the bioturbated layers and/or turbidite beds prior to a more detailed study of Milankovitch cyclicity.