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doi:10.2204/iodp.proc.302.201.2008

Introduction

The Lomonosov Ridge is a sliver of continental crust that rifted from the Barents–Kara Sea margin at ~57 Ma (see the “Expedition 302 summary” chapter). Since the onset of rifting, >400 m of Cenozoic sediments have been deposited as the ridge drifted toward its present position in the center of the Arctic Ocean. The Arctic Coring Expedition (ACEX) targeted four sites located on the central Lomonosov Ridge along seismic Line AWI91090 (Fig. F1) (Jokat et al., 1992). The coherent seismostratigraphy crossing these sites and correlations made between petrophysical properties measured on recovered sediments allowed construction of a single composite section that is described using four lithologic units (see the “Expedition 302 summary” chapter). The lithology of recovered sediments ranges from pyrite- and gypsum-enriched biosiliceous clays and oozes in the Paleogene to fossil-poor glaciomarine sediments in the Neogene (Figs. F2, F3).

The composite sedimentary column from ACEX was generated from overlapping recovery in the upper ~30 meters below seafloor (mbsf) from Holes M0002A, M0004A, M0003A, and M0004C (Fig. F2). The remainder of the composite section is composed of material from two nonoverlapping holes: M0002A and M0004A. In the ideal situation, duplicate or triplicate recovery at a single drilling site (from two to three holes) provides substantial overlap. In the construction of a composite section, the overlapping recovery not only ensures that there are no stratigraphic gaps but also allows for careful selection and inclusion of the highest quality cored intervals. The limited overlap between the ACEX sites prevented exclusion of disturbed sections from the composite section. Although major coring disturbances were systematically described and tabulated (see Table T24 in the “Sites M0001–M0004” chapter), more common coring disturbances, such as compression or stretching of material during piston coring (APC) or undercutting during extended core barrel (XCB) operations are not recorded. These less dramatic disturbances can equally affect the quality of multisensor core logger (MSCL) measurements, as calibration algorithms used to calculate bulk density (ρ_B) and compressional wave (P-wave) velocity require an accurate measurement of core diameter. Whole-core MSCL measurements, such as those performed during ACEX, are based on the diameter of the core liner but not the actual core.

Routine measurement of index properties on discrete samples during Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP) expeditions provides a means for assessing the quality of MSCL data. Index properties, which include bulk density, grain density, dry density, and porosity, are termed moisture and density (MAD) measurements. They are determined by applying basic phase relationships to measurements of mass and volume (Blum, 1997). Although collected at a much lower resolution (one sample per section versus one sample every 1–2 cm), MAD measurements provide a more accurate and precise measurement of ρ_B than the MSCL estimate, which is derived from empirical relationships between gamma ray attenuation and the bulk density of aluminum/water mixtures (Blum, 1997; Boyce, 1976). Comparison between MAD- and MSCL-derived ρ_B at equivalent core positions (depths) can provide a means for both assessing and improving the quality of the MSCL data.

Here MAD data are used to correct ρ_B records from the MSCL. Using basic phase relationships, the corrected ρ_B logs, and the average grain density (ρ_G) from the corresponding lithologic unit/subunit, high-resolution ρ_D and porosity (ϕ) records are generated. High-resolution ρ_D and ϕ are useful data sets for calculating mass accumulation rates and investigating compaction processes at the ACEX drill sites on the Lomonosov Ridge.

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