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doi:10.2204/iodp.proc.342.109.2014 Site U14081R.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 U1408 (proposed Site SENR-21B; 41°26.3′N, 49°47.1′W; ~3022 m water depth) is a mid-depth site (~2575 meters below sea level [mbsl] paleodepth at 50 Ma) (Tucholke and Vogt, 1979) and the shallowest site drilled in the Expedition 342 Paleogene Newfoundland sediment drifts depth transect (Fig. F1). The site was positioned to capture a record of sedimentation ~2 km shallower than the largely sub–carbonate compensation depth (CCD) record drilled at Site U1403 (Figs. F2, F3). The location, well 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 U1408 were
Secondary objectives included the possible recovery of the Eocene/Oligocene boundary and the late Eocene impact at higher sedimentation rates than are typical in pelagic sedimentary sequences. Site U1408 is a companion site to Site U1407, where we employed an “offset” drilling strategy to obtain advanced piston corer (APC) records through a thicker section of the same sediment drift. Our strategy was designed to test whether Site U1408 would sample (1) a much more expanded sedimentary record of the same middle and lower Eocene sediment recovered by APC at Site U1407 or (2) a section of progressively younger sediment. Result 1 would indicate an overall sedimentary architecture in this Southeast Newfoundland sediment drift that is similar to the one encountered at J-Anomaly Ridge sediment drift. Result 2 would indicate an essentially layer-cake sedimentary sequence. The primarily calcareous sequence expected at Site U1408 should record changes in ocean alkalinity and carbonate production. Sites U1403 and U1404 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 boundary, the Paleocene/Eocene Thermal Maximum (PETM) and the Eocene–Oligocene transition (see the “Site U1403” and “Site U1404” chapters [Norris et al., 2014b, 2014c). Those events are 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 U1408 by decreases in carbonate preservation and decreasing carbonate content relative to clay or biosiliceous sediment, as we have already observed at Sites U1406 and U1407. As an upper-end site on the Newfoundland depth transect, Site U1408 could have few intervals in which the sediment is 100% carbonate, but also intervals where carbonate abundance falls in the record. Carbonate content was expected to be generally higher at sites in shallower water depth, such as the majority of the sites located on Southeast Newfoundland Ridge, including Sites U1407 and U1408. The high carbonate contents anticipated in sediment at Site U1408 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–U1406 further downslope. Ties between sites on Southeast Newfoundland Ridge and those on J-Anomaly Ridge will allow the isotope stratigraphy and biochronology developed for Sites U1406–U1408 to be exported to the lower ends of the depth transect. Site U1408 assumes greater importance in the depth transect because the lower and middle Eocene sedimentary sequence was expected to be more expanded than at any other Expedition 342 site. Ultimately, the goal was to use the combination of the lower and middle Eocene record at Sites U1408 and U1407 and the younger Paleogene record at Site U1406 to produce composite stable isotope and carbonate content records that can be tied to the more intermittent geochemical records at Sites U1403–U1405. Our aim was to match carbonate-rich intervals across all of the J-Anomaly Ridge sites with the sites on Southeast Newfoundland Ridge to create an orbital-resolution record of fluctuations in ocean chemistry and deep water origins. Site U1408 was proposed when we discovered that the subtle but recognizable internal stratigraphy of the acoustically transparent sequence on J-Anomaly Ridge was perhaps age diagnostic. Seismic data show an intermittent fuzzy reflector near the middle of the J-Anomaly Drift as well as a zone of fuzzy, slightly less transparent reflections in the lower half of the drift. Drilling on J-Anomaly Ridge showed that these two fuzzy reflector packages correspond to the Eocene/Oligocene boundary and lower and middle Eocene sediment. We used this interpretation of the seismic stratigraphy to propose sites that would preferentially target the lower half of the acoustically transparent seismic package. Site U1408 and its companion, Site U1407, were designed to recover APC records of the lower and middle Eocene sequence and to avoid drilling a large thickness of Miocene sediment such as that encountered at Sites U1404 and U1405. The seismic stratigraphy of Site U1408 shows a complex array of reflectors below the presumed Eocene sedimentary sequence. We proposed to core by APC through the acoustically transparent sequence and stop at the cherts that occur in the middle lower Eocene at Site U1407. This strategy should collect a sedimentary record at the thickest part of the Eocene section. We expected that Site U1408 would 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 U1408 should provide our highest deposition rate record of the early and middle Eocene as a counterpart to the largely sub-CCD record at Site U1403 and 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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