During Ocean Drilling Program (ODP) Leg 178, two sites were drilled in the Palmer Deep, a basin on the continental shelf of the Antarctic Peninsula (Barker, Camerlenghi, Acton, et al., 1999). The Palmer Deep contains three subbasins, the deepest of which is over 1400 m deep, roughly 1000 m deeper than the surrounding shelf (Fig. F1). Thick Holocene sedimentary sections were recovered from both drill sites. Site 1098 lies within Basin I, the smallest and shallowest of the three subbasins, at a water depth of 1011 m. Site 1099 lies within Basin III, the largest and deepest of the subbasins, at a water depth of 1400 m.
Multiple holes were cored at both sites with the advanced piston corer (APC), with the goal of recovering as complete a section as possible given time limitations. At Site 1098, three holes were cored through the roughly 46-m-thick sedimentary fill overlying acoustic basement. Hole 1098A was drilled to 45.9 meters below seafloor (mbsf) with 45.78 m of sediment recovered (99.74% recovery); Hole 1098B was drilled to 43.0 mbsf with 44.66 m of sediment recovered (103.86% recovery); and Hole 1098C was drilled to 46.7 mbsf with 46.3 m of sediment recovered (99.14% recovery).
At Site 1099, two holes were cored through the upper 107.5 m of the ~260-m-thick sediment fill overlying basement. Owing to time constraints, the interval was effectively cored only once with the two holes overlapping only 2.3 m in the meters below seafloor depth scale. The upper 62.3 m of section was cored in Hole 1099A, and the interval from 60.0 to 107.5 mbsf was cored in Hole 1099B.
The primary goal of this study is to construct a common depth scale for the three holes at Site 1098. Secondarily, we illustrate that a common depth scale cannot be accurately constructed for the two holes at Site 1099.
The depth to the top of each core is originally estimated from a drill-pipe measurement. These drill-pipe measurements, along with curation procedures, establish a unique depth scale in meters below seafloor for the cores from each hole (e.g., see the "Explanatory Notes" chapter in Barker, Camerlenghi, Acton, et al., 1999).
The mbsf depths differ from true depths for several reasons. Ship motion, heave, and deviation of the drill hole from vertical can all cause errors in drill-pipe measurements. Depth errors may also result from biases in core recovery estimates, which commonly exceed 100% for APC cores. These artificially high recovery percentages probably result from decompression of sediments (Farrell and Janecek, 1991; Hagelberg et al., 1995; MacKillop et al., 1995; Moran, 1997), entrance of excess sediment into the core barrel as some of the sediment displaced by the walls of the coring shoe is forced inward (p. 93-96 of Hvorslev, 1949), and curation practices, in which soupy core material commonly occurring at the top of many cores is curated as part of the core. In reality, much of the soupy material results from sediment falling into the hole or from sediment being stirred at the bottom of the hole. This happens as the roller-cone bit, which is part of the bottom-hole assembly (BHA), advances from the top of the previously recovered core to the top of the core that is next to be recovered. If the ship heaves upward as the piston strokes into the sediment, then the debris in the hole can be recovered. Additional expansion of the upper part of each core can occur because the top of the core is exposed to circulating water, particularly as the water jets from the BHA are cleaning out the hole. Incomplete recovery also results in potential depth errors because ODP curation convention assumes the top of the core corresponds to the top of the cored interval. Similarly, duplicate recovery within a hole, where the piston corer repenetrates the same sediment sequence either by piercing the side wall of the borehole or by a lateral shift of the BHA in very water saturated and unconsolidated sediment, can result in depth biases of several meters, as was shown by Robinson (1990). Depth errors also result from core deformation, such as "suck-in" that occurs when sediment is sucked up into the core liner. Smaller depth errors result from minor core distortion that occurs to some degree in most APC cores, such as bowed or sheared sediment near the core liner caused by friction as the sediment passes through the coring shoe and into the core liner (p. 93-100 of Hvorslev, 1949).
Given these factors, one would expect misalignment of correlative features between holes at a site. Thus, a continuous horizontal feature collected in several holes at a site, which in the absence of local bathymetric variations would have the same true depth in each hole, will likely have different mbsf depths. The offset of such features in the mbsf depth scale may be only several centimeters or could be a few meters, though rarely more than 10 m, based on observations from past Deep Sea Drilling Project (DSDP) and ODP legs (e.g., Leg 94 [Ruddiman et al., 1987; Ruddiman, Kidd, Thomas, et al., 1987], Leg 108 [Ruddiman, Sarnthein, Baldauf, et al., 1988], Leg 111 [Alexandrovich and Hays, 1989], Leg 115 [Robinsion, 1990], Leg 117 [Murray and Prell, 1991; deMenocal et al., 1991], Leg 121 [Farrell and Janecek, 1991], Leg 138 [Hagelberg et al., 1992, 1995], Leg 154 [Curry, Shackleton, Richter, et al., 1995], Leg 162 [Jansen, Raymo, Blum, et al., 1996], Leg 167 [Lyle, Koizumi, Richter, et al., 1997], Leg 172 [Keigwin, Rio, Acton, et al., 1998], Leg 175 [Wefer, Berger, Richter, et al., 1998], and Leg 177 [Gersonde, Hodell, Blum, et al., 1999]).
A common or composite depth scale overcomes many of the inadequacies of the mbsf depth scale, allowing core data from one hole to be compared or combined ("spliced") directly with core data from other holes at the same site (e.g., Ruddiman et al., 1987; Ruddiman, Sarnthein, Bauldauf, et al., 1988; Alexandrovich and Hays, 1989; Robinson, 1990; Farrell and Janecek, 1991; Hagelberg et al., 1992, 1995; Curry, Shackleton, Richter, et al., 1995; Jansen, Raymo, Blum, et al., 1996; Lyle, Koizumi, Richter, et al., 1997; Keigwin, Rio, Acton, et al., 1998; Wefer, Berger, Richter, et al., 1998; Gersonde, Hodell, Blum, et al., 1999). The construction of composite depth scales is a natural outcome of efforts to obtain complete recovery of sedimentary sections by piston coring two or more offset holes at a site, which began with the use of the hydraulic piston corer during DSDP Leg 68 (Prell, Gardner, et al., 1982) and has become standard practice since APC coring began during DSDP Leg 94 (p. 8-9 of Ruddiman, Kidd, Thomas, et al., 1987; see also Hagelberg et al., 1995, for a history of composite section development). Cores from a single hole will nearly always have some intervals with poor or no recovery or with recovery of sediments that are disturbed during drilling. Even when core recovery is quoted as 100% or higher, gaps of several meters can occur between cores (e.g., Shackleton et al., 1984; Ruddiman et al., 1987; Robinson, 1990; Farrell and Janecek, 1991). By splicing or stacking core intervals from different holes at a site that has been multiply cored, a complete or nearly complete stratigraphic section can be constructed.