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doi:10.2204/iodp.proc.342.105.2014 Site U14041R.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 U1404 (proposed Site JA-13A; 40°0.80′N, 51°48.60′W; 4710 m water depth) (Fig. F1) is the second deepest water site drilled on J-Anomaly Ridge. This site was positioned to capture a record of predominantly calcareous sediment 251 m shallower than the largely sub–carbonate compensation depth (CCD) record at Site U1403. Site U1404 is located to reconstruct the CCD history in the deepest expanded record of the plastered drift at the foot of J-Anomaly Ridge. We expect to obtain a record of shoaling excursions of the CCD during the Paleogene. The drift was expected to be composed of Paleogene pelagic sediment. Drilling at Site U1403 encountered ~150 m of upper to lower Eocene sediment with an average sedimentation rate of ~1.4 cm/k.y. above lower Eocene chert. Consequently, the ~500 m thick sediment package at Site U1404 is expected to have an average sedimentation rate of ~4.7 cm/k.y. The principal objective at Site U1404 was to obtain an expanded sequence of drift deposits just above the Eocene CCD. The Paleogene sediment package at Site U1404 is about the same thickness as other shallow sites on J-Anomaly Ridge. Hence, the stratigraphy at Site U1404 should be broadly representative of expanded Paleogene records on the Newfoundland ridges depth transect. Site U1404 should also be a sensitive recorder of large-amplitude CCD shoaling and deepening events including those not quite large enough to have triggered changes in calcium carbonate accumulation at Site U1403. We expected Site U1404 to be a sensitive recorder of greenhouse gas–driven shoaling of the CCD. The common occurrence of “clay beds” associated with hyperthermal events at shallow and mid-depth Paleogene sites is a reflection of periods when the CCD shoaled well above its average position (Ridgwell, 2007; Sexton et al., 2011; Zachos et al., 2005; Bohaty et al., 2009). Such CCD shoaling events are expected to result from abrupt increases in oceanic and atmospheric inventories of greenhouse gases at the onset of hyperthermal events. It is presently unclear how much oceanic carbonate chemistry changed associated with any of the Paleogene hyperthermals because no depth transects that capture the full magnitude of CCD excursions have been studied (Zachos et al., 2005). Large climate transients such as the Paleocene/Eocene Thermal Maximum (PETM) are hypothesized to have been triggered by large, rapid changes in greenhouse gas concentrations in the exogenic carbon cycle. Yet, huge uncertainty remains over the size of the carbon release involved, even for the best studied of these events, the PETM (estimates range from ~1200 to >4300 GT carbon, depending upon assumptions of the carbon source, the magnitude of the temperature anomaly, and the magnitude of the CCD excursion involved). Constraints are even poorer for smaller amplitude hyperthermals such as the “ELMO” and “X” events (Lourens et al., 2005), the numerous middle Eocene events described by Sexton et al. (2011), and the Middle Eocene Climate Optimum (MECO; Bohaty et al., 2009). A set of drill holes that capture the position of the CCD to within a few hundred meters should provide a tight constraint on the size of the CCD excursion for a wide range of hyperthermal events. Carbonate preservation is hypothesized to improve in the aftermath of hyperthermals in response to rebalancing oceanic bicarbonate inventories as a consequence of elevated weathering of seafloor carbonate and terrestrial silicate weathering during greenhouse gas–fueled hyperthermal events (Dickens et al., 1997; Zachos et al., 2005; Leon-Rodriguez, 2010). We expected to see improved carbonate preservation associated with “overshoots” of ocean alkalinity associated with the late stages of hyperthermal events. CCD deepening events involving similar overshoot dynamics are also hypothesized to be associated with the Cenozoic initiation of large ice sheets on Antarctica across the Eocene–Oligocene Transition (EOT) and also perhaps with their subsequent expansion across the Oligocene–Miocene transition (Coxall et al., 2005; Zachos and Kump, 2005; Holbourn et al., 2005; Merico et al., 2008). New long-term records of Cenozoic CCD in the equatorial Pacific Ocean also indicate major transient CCD deepening events during the middle Eocene (Lyle et al., 2002; Pälike et al., 2012). Large-amplitude CCD deepening events should be evident at Site U1404, both through increased abundances of carbonate and improved carbonate microfossil preservation. Our goal was to recover a sequence that is a record of sedimentation at Site U1404 from ~100 to 150 m above the local long-term average CCD for the Paleogene as reconstructed by Tucholke and Vogt (1979) so that deepening events can be well documented using modern coring and stratigraphic techniques. Seismic stratigraphy shows that the Paleogene sediment package downdip of Site U1404 thins considerably between a modern depth of 4800 and 4900 meters below sea level (mbsl) (Fig. F2), corresponding to paleodepths at 50 Ma of ~4600 mbsl using the subsidence reconstruction of Tucholke and Vogt (1979) (see Fig. F2 in the “Expedition 342 summary” chapter [Norris et al., 2014a]). In that reconstruction, the contemporaneous North Atlantic CCD is positioned at a paleodepth of ~4400–4300 mbsl and prominent CCD deepening is indicated below 5500 mbsl in the latest Cretaceous and early Paleocene. These CCD reconstructions, together with our drilling results from Site U1403, suggest that the CCD in the North Atlantic Ocean during the early Eocene was much deeper (by ~1.5 km) than in the contemporaneous equatorial Pacific Ocean (Pälike et al., 2012). We interpret the thinning of the Paleogene sediment package as an indication of the average position of the CCD in the early to late Eocene. The acoustic expression of the sequence drilled at Site U1404 comprises an ~80 m thick surficial, reflector-rich unit overlying a highly expanded (~400 m thick) Paleogene drift sediment package that shows almost no internal reflections, suggesting that the sediment pile has a fairly uniform composition (Figs. F2, F3). In detail, the upper half of the drift sequence at Site U1404 is virtually seismically transparent, whereas the lower half has a fuzzy acoustic expression but no well-developed internal reflections. Drilling at Site U1403 suggests that the fuzzy acoustic unit is of early Eocene age and has a higher carbonate and biogenic opal content than the overlying transparent upper Eocene and middle Eocene package. Therefore, we expect to find that this sequence also has a higher carbonate and siliceous sediment content than the overlying unit. The lack of distinct internal reflections within these two acoustic units suggests that they do not contain major unconformities or major changes in sediment composition. This uniformity has a number of possible explanations. One possibility is that CCD fluctuations were either too brief or too modest in amplitude to give rise to fluctuations in carbonate content prominent enough to exhibit a notable acoustic expression. Alternative explanations are that the entire drift package accumulated rapidly during an interval of relative CCD stasis or during an interval of CCD variability that took place at depths too shallow or too deep to influence this site. In addition to its utility for CCD reconstructions, Site U1404 will also be used to understand the history of deepwater currents. Most deep ocean drill sites are located at mid-ocean depths when we account for thermal subsidence. Hence, Site U1404, at a paleodepth of ~4300 mbsl at 50 Ma, is one of the few to recover a representative sequence of carbonate sediment deposited under true deep water. Furthermore, the site is well placed to record the history of deep water formed in the far North Atlantic, or even the Arctic, because the Deep Western Boundary Current is constrained to flow directly over or around the Newfoundland Ridges by geostrophic flow and the shape of the ocean basin. As the deep end of the J-Anomaly depth transect, the site should record the chemistry and flow history of abyssal waters in the Paleogene. Finally, Site U1404 may preserve records of late Eocene impacts. We provisionally identified a late Eocene impact deposit at Site U1403 in sediment corresponding to Chron C16, a horizon that could correlate with the impact blanket from the Chesapeake Impact structure. Assuming that there is no hiatus between the impact layer and the middle and lower Eocene fossiliferous sediments at Site U1403, it seems likely that a similar but more expanded sequence of sediment exists at Site U1404. |
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