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doi:10.2204/iodp.proc.342.107.2014 Site U14061R.D. Norris, P.A. Wilson, P. Blum, A. Fehr, C. Agnini, A. Bornemann, S. Boulila, P.R. Bown, C. Cournede, O. Friedrich, A.K. Ghosh, C.J. Hollis, P.M. Hull, K. Jo, C.K. Junium, M. Kaneko, D. Liebrand, P.C. Lippert, Z. Liu, H. Matsui, K. Moriya, H. Nishi, B.N. Opdyke, D. Penman, B. Romans, H.D. Scher, P. Sexton, H. Takagi, S.K. Turner, J.H. Whiteside, T. Yamaguchi, and Y. Yamamoto2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1406 (proposed Site JA-6A; 40°21.0′N, 51°39.0′W; 3798.9 m water depth) is a mid-depth site (~3300 meters below sea level [mbsl] paleodepth at 50 Ma) (Tucholke and Vogt, 1979) in the middle of the Expedition 342 Paleogene Newfoundland sediment drifts depth transect (Fig. F1). The site is positioned to capture a record of sedimentation >1.1 km shallower than the largely sub–carbonate compensation depth (CCD) record drilled at Site U1403 (Figs. F2, F3, F4). The location above the average late Paleogene CCD should be sensitive to both increases and decreases in carbonate burial, whether these reflect variations in dissolution related to changes in the CCD, changes in carbonate production, or variations in background noncarbonate sedimentation. Our primary scientific objectives for drilling Site U1406 were
The primarily calcareous sequence expected at Site U1406 should record changes in ocean alkalinity and carbonate production. Sites U1403–U1405 were mainly positioned to capture large-amplitude CCD deepening events such as the carbonate budget “overshoots” that are thought to be associated with the most extreme climate perturbations of the Cenozoic, such as those involved with the Cretaceous/Paleogene (K/Pg) boundary, the Paleocene/Eocene Thermal Maximum (PETM), and the EOT (see the “Site U1403” and “Site U1404” chapters [Norris et al., 2014b, 2014c]). These events were expected to be recorded at deepwater sites as stratigraphically thin intervals of calcareous sediment in otherwise noncalcareous sediment. In contrast, transient shoaling of the CCD in generally carbonate-rich sequences should be recorded at Site U1406 by decreases in carbonate preservation and decreasing carbonate content relative to clay or biosiliceous sediment. As a mid-depth site on the Newfoundland depth transect, Site U1406 should have few intervals in which the sediment is 100% carbonate, but also few intervals in which no carbonate appears in the record. Carbonate content is expected to be generally higher at sites in shallower water depth, such as the majority of the sites located on Southeast Newfoundland Ridge. The high carbonate contents anticipated in sediment at Site U1406 should permit the construction of detailed stable isotope records and calcareous microfossil biostratigraphy that can be tied by physical property records and magnetochronology to Sites U1403–U1405 further downslope. Ties between sites on J-Anomaly Ridge will allow the isotope stratigraphy and biochronology developed for Site U1406 to be extended to the entire lower half of the depth transect. Stable isotope records are likely to be developed for Sites U1404 and U1405 for intervals of good carbonate preservation in, for example, the early Miocene–Oligocene and the EOT, and it should be possible to match them with more complete Site U1406 records. In addition, coring at Site U1404 also recovered a partial record of carbonate burial events in the late Eocene and late middle Eocene. Our aim is to match these carbonate-rich intervals across all of the J-Anomaly sites in which they occur to create an orbital-resolution record of fluctuations in ocean chemistry and deepwater origins. Coupled with provenance and grain size studies of the sediments, we should also be able to identify times of deposition of lithic fragments and test competing hypotheses for the onset and development of extensive northern hemisphere ice sheets (see the “Site U1403” and “Site U1404” chapters [Norris et al., 2014b, 2014c]). Site U1406 was proposed as a replacement for proposed Site JA-5A. This substitution was made to avoid drilling a large thickness of Miocene sediment such as that encountered at Sites U1404 and U1405. Instead, Site U1406 was proposed to drill directly through an outcropping of the basal part of the acoustically uniform, largely transparent seismic sequence that was expected to be mostly of Oligocene to early Eocene age, based on prior drilling of nearby Deep Sea Drilling Project (DSDP) Site 384 and previous Expedition 342 sites. One piece of gray silt recovered from the Site 384 drill bit after drilling operations were terminated contained a moderately preserved upper Miocene assemblage (Discoaster quinqueramus Zone) and must have originated somewhere within the uncored upper 50 m of the sedimentary section (Tucholke and Vogt, 1979). Spot coring of the sequence below indicated a sequence of middle Eocene to Cretaceous pelagic ooze and chalk overlying Lower Cretaceous reef sediment and basalt (Tucholke and Vogt, 1979). The exact seismic expression of Site 384 is not entirely clear because Leg 43 drilled in the vicinity of a single-channel Glomar Challenger seismic line of low quality and could only report that the line was “near” the drill site. Expedition 342 single-channel seismic lines show a reflector package that broadly resembles the Glomar Challenger line and suggests that Site 384 was drilled on the edge of the transparent acoustic unit before penetrating a lower Eocene, Paleocene, and Cretaceous succession of chert and chalk that is represented by a package of relatively dense reflectors. Therefore, we interpret the lowermost 50 m of the transparent acoustic unit at Site U1406 to be of middle Eocene and early Eocene age. We conclude that Site U1406 will provide an expanded record of primarily calcareous ooze and chalk of rough age-equivalence to sites in deeper water on J-Anomaly Ridge. In particular, Site U1406 should provide a carbonate-rich record through the middle to late Eocene as a counterpart to the largely sub-CCD record drilled through this interval at Site U1404. Site U1406 will thereby improve age- and water-depth control on the behavior of the CCD in the North Atlantic during this key interval of the Cenozoic when the CCD in the equatorial Pacific was at its most dynamic (Lyle, Wilson, Janecek, et al., 2002; Pälike et al., 2012).
Publication: 3 March 2014 |
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