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doi:10.2204/iodp.proc.302.202.2008

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 302, also known as the Arctic Coring Expedition (ACEX), four sites were drilled near 88°N on the crest of the Lomonosov Ridge in the central Arctic Ocean (Fig. F1). These sites were located along a single multichannel seismic reflection profile (AWI-91090) acquired in 1991 (Fütterer, 1992), where a 450 m thick sequence of flat-lying Cenozoic sediments was predicted from seismic interpretations (Jokat et al., 1995). ACEX drilling penetrated 428 m into this sediment sequence, recovering 399 m of core (see the “Sites M0001–M0004” chapter). Although overlapping recovery was only achieved in the upper ~30 meters below seafloor (mbsf) (Fig. F2), the coherent seismostratigraphy imaged along Profile AWI-91090 allowed material recovered from four of the five sites to be confidently integrated into a composite profile and placed on a common depth scale.

Following conventions established during the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP), this common depth is the meters composite depth (mcd) scale. Where overlapping recovery is available, the mcd scale is derived by stratigraphically correlating downhole variations in physical, chemical, and magnetic properties of the recovered sediments (see the “Sites M0001–M0004” chapter). Stratigraphic alignment results in core-specific offsets to the mbsf depth scale, which is determined during drilling by measuring the amount of drill pipe deployed from the rig floor. In the absence of overlapping recovery, the published mcd scale introduces offsets to prevent overlap between subsequent cores from the same hole. Core-specific offsets required to calculate meters composite depth from the mbsf depth are compiled in an “affine” table (see Table T25 in the “Sites M0001–M0004” chapter).

The construction of composite depth scales helps circumvent a host of potential errors that arise when developing a stratigraphic reference frame with centimeter-scale precision from depth measurements made while drilling hundreds of meters into the seafloor from a nonstationary platform. These errors can be associated with the original rig floor measurements of the mbsf depth, as well as with deviation of the drill string from an assumed vertical axis in both the borehole and intervening seawater. Similarly, material related to drilling disturbances and core deformation from drilling (flow-in, small-scale sloughing of the borehole walls, etc.) is included as part of the recovered core but does not necessarily reflect the stratigraphic progression of the sediments. At the most basic level, curatorial procedures introduce a bias into the mbsf depth scale by assuming that the top of the core corresponds to the top of the cored interval. When <100% recovery is achieved, this can introduce substantial errors into the depth of a core. When recovery is >100%, material from subsequent cores overlaps when they are clearly not from the same stratigraphic position. A review of these and other factors associated with potential depth errors can be found in a number of DSDP and ODP publications (see Acton, Borton, et al., 2001, and references therein).

Unlike other ODP and IODP expeditions where multiple holes provide extensive overlapping recovery to facilitate the stratigraphic correlation procedure, ACEX only acquired overlapping recovery in the upper 30 mbsf (Fig. F2). Stratigraphically correlating this material proved difficult because of the large offsets encountered in near-surface sediments and the generally poor core quality. The highest quality cores came from holes where the advanced piston corer (APC) was successfully employed, in total eight cores. Alternatively, where the extended core barrel (XCB) was used to sample near-surface sediments, extensive core disturbance occurred.

Here we present a refinement to the published composite depth scale that alters offsets to cores in the uppermost 55 mbsf. Using additional data acquired from split cores and through the inclusion of two nearby cores collected from previous expeditions to the Lomonosov Ridge, the revised mcd (rmcd) scale more accurately reflects the true stratigraphic placement of the recovered material in the uppermost 55 mbsf and addresses uncertainties in the depths where there is either a lack of firm tie points or no overlapping recovery. Using the rmcd scale, we redefine the tie points for the spliced composite depth scale in the upper 27 rmcd and propose a series of additional ties that are required to make centimeter-scale adjustments to material from this interval that lies outside the spliced composite record.

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