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doi:10.2204/iodp.pr.342.2012 Preliminary scientific assessment: Newfoundland sediment driftsSummary of expedition objectivesExpedition 342 to the Newfoundland sediment drifts was a ground-breaking expedition that achieved both record IODP recovery in mid-Cretaceous to Miocene sediments as well as overall core recovery. In total, 5724 m of sediment was cored, of which 5413 m was recovered (average 94%; Table T2) in 25 APC/XCB holes across 10 sites (U1402 to U1411). Sediment of Paleogene age, our main objective, makes up the majority of our recovery including particularly spectacular records of the Eocene together with Paleocene/Eocene and Eocene/Oligocene boundaries. However, we also obtained some exceptional Cretaceous and Neogene sections that include the Cenomanian/Turonian boundary, a deepwater record of the Cretaceous/Paleogene boundary and, at three sites, the Oligocene/Miocene boundary. The nine drill sites form a depth transect between ~5 and 3 km water depth in clay-rich sediment drifts at the modern front between warm surface waters of the Gulf Stream flowing north and the southward-flowing Labrador Current. The intersection of these currents ponds icebergs drifting from Greenland over the site of the sinking of the Titanic a century ago. The sediments recovered at the drill sites are characterized, for the most part, by excellent age control and, over substantial intervals, they contain exquisitely well preserved microfossils that accumulated at much faster rates than in typical deep-sea settings. Expedition 342 was designed to examine four major objectives:
An interval of particular focus was to be the middle Eocene to early Oligocene where we expected to find expanded records of hyperthermal events and the middle Eocene climatic optimum. This history of the carbonate system, coupled with detailed geochemical studies, would allow us to test theories for the origin of hyperthermals—abrupt periods of greenhouse gas–fueled warming known to punctuate the Paleocene and Eocene (Galeotti et al., 2010; Quillévéré et al., 2008; Sexton et al., 2011). Natural experiments with global change like hyperthermals can enhance our understanding of the consequences of abrupt climate change for Earth's ecosystems, climate, and chemistry. Reconstruction of the CCD was expected to be enhanced by the expedition objective to obtain the first depth transect that captures dynamics of the truly deep ocean as well as the intermediate depths captured in pervious drilling programs (Bralower, 2002; Zachos, 2005).
Today, deepwater formation draws warm water into the Nordic seas keeping them warm. Expedition 342 was designed to show how far back this pattern of overturning circulation extends and its influence on climates of the past greenhouse world.
Drill cores through the Eocene–Oligocene transition are anticipated to yield highly resolved records of the events leading up to and following the greenhouse-to-icehouse transition, as well as a history of how Greenland and high northern latitudes responded to this event.
The material that we have recovered makes it possible to address all of these scientific objectives as well as some unanticipated objectives:
North Atlantic history of carbonate compensation depth changeOne main expedition objective was to reconstruct the Paleogene CCD in the North Atlantic for reference to recent high-fidelity records of the CCD obtained in the equatorial Pacific. Our deepest water site (Site U1403) is at a paleodepth of ~4.5 km 50 m.y. ago, whereas our shallowest one (Site U1408) can be backtracked to a paleodepth of 2.5 km at this time. This combination of sites yields a record of the history of CCD change over a 2 km depth range from the ocean abyss to mid–water depths. A main shipboard result of the expedition is that the long-term position of the CCD in the North Atlantic appears to have been markedly deeper during the Late Cretaceous through early Eocene than in the late Eocene through Miocene. A second result is that during the early Eocene, a time interval for which data are now available from both the Pacific and Atlantic Oceans, the CCD in the North Atlantic is positioned much deeper (by ~2 km) than in the equatorial Pacific. A third notable result is discovery of intermittent calcareous sediments in the Cretaceous, Paleocene, and early to middle Eocene at 4.5 km paleodepth suggesting a deep Atlantic CCD during these times. Still another notable result was to find frequent intervals in the Oligocene without carbonate, in contrast to the Pacific where the CCD reaches its deepest point in the Cenozoic during the Oligocene and early Miocene. We also find evidence of carbonate deposition events following the K/Pg mass extinction, the PETM, and the Eocene–Oligocene transition. North Atlantic history of ocean structure and sediment drift formationA second objective of our depth transect drilling strategy was to capture records of changes in deep-sea ventilation, vertical ocean structure, and circulation in the North Atlantic, both at the secular timescale comparing the Eocene greenhouse to the Oligocene icehouse baseline states and to test for reversals in deepwater formation patterns across abrupt extreme climate transitions. The sequences that we have recovered can be used to trace highly resolved geochemical records (e.g., δ13C and Nd isotopes) of the vertical structure of deep water in the North Atlantic, and these records can, in turn, be used to examine regional gradients using results from the expedition as a “northern end-member” for comparison with sites elsewhere in the world. It will also be possible to use the history of drift formation itself and information on bottom water flow strength (using, for example, sortable silt records) to monitor the strength of the Deep Western Boundary Current and estimate the rate of formation of deep water in the North Atlantic. High-resolution records of climate from rapidly accumulating sediment driftsA third major objective of Expedition 342 was to drill sediment drift deposits to recover sequences with high rates of accumulation in comparison to the modest rates of accumulation (~0.5–1 cm/k.y. in the Paleogene) typically encountered at pelagic sites. A major expedition highlight was the recovery of sequences with high sedimentation rates (up to 3 to 10 times higher) having excellent age control. Recovery of these sections will enable studies of the suborbital dynamics of past abrupt climate change, including both transitions into greenhouse and icehouse climate states, the full magnitudes of hyperthermal events, and rates of change in the carbonate compensation depth. We find that the thickest central sections through the various sediment drifts typically record the same depositional packages that are recovered in the thin noses and tails of these drifts, but these central sections are often massively expanded with clay. Times of peak accumulation of drift deposits drilled during the expedition include the middle Eocene, late Oligocene, and early Miocene. As anticipated, the task of creating spliced records of multiple holes at some of our sites was not always straightforward shipboard because of some intervals of low-amplitude change in some physical property data sets (most notably magnetic susceptibility) and lateral variations in stratigraphy. High-quality shipboard biostratigraphic, magnetostratigraphic, and other physical property data sets (e.g., NGR), however, suggest that this issue can be addressed using shore-based scanning-XRF work. Exceptional preservation of calcareous microfossils in sediment driftsA fourth major objective of Expedition 342 was the recovery of high–accumulation rate, clay-rich sediments containing well-preserved microfossils. A major expedition highlight is the recovery of intervals and sites containing assemblages of calcareous and siliceous microfossils exhibiting exceptional preservation. These include, in the case of calcareous flora and fauna, the intervals with the highest sedimentation rates, for example the Oligocene/Miocene boundary interval at Site U1405, the middle Eocene sequence at Sites U1408 and U1410, and the Eocene/Oligocene transition at Site U1411. The presence of holococcoliths and minute coccoliths is an indicator of high-quality preservation, and the conspicuous occurrence of these is coincident with observations of glassy planktonic foraminifers at Sites U1403, U1405, U1408, U1410, and U1411. It is notable that the benthic foraminifer record shows excellent preservation even when the quality of preservation is no longer exceptional in planktonic foraminifers and nannofossils. The best- preserved assemblages recovered are virtually unprecedented in deep-sea sections and have been previously found only in shelf environments with clay deposits (e.g., Tanzania, Gulf Coast). Expedition 342 material will allow shore-based studies of major expedition objectives (e.g., climate sensitivity to greenhouse gas forcing and floral and faunal turnover) to proceed effectively free of the usual concerns over taphonomic bias. Paleogene and Cretaceous hyperthermal and hypoxic eventsA major goal of the expedition was to acquire records of multiple hyperthermal events—abrupt intervals of greenhouse gas–fueled warming from the early Paleogene such as the PETM and ETM2. This objective was achieved at multiple sites across a depth transect of ~2 km, extending our knowledge of these events for the first time into near-abyssal water depths. We also recovered a complete record of the early Eocene–middle Eocene transition, a rarity in the drill hole record of the Atlantic. The early/middle Eocene boundary is typically unconformable or is represented by chert. Hence, we are now in a position to create the first highly detailed paleoceanographic records of the boundary interval. Furthermore, we recovered sediments at multiple sites from the middle Eocene climatic optimum and various carbonate accumulation events which should yield suborbitally resolved records of these events. An additional related, but unanticipated, result was the recovery at one site at shallow burial depth of a black shale sequence rich in Type II organic matter across the Cenomanian/Turonian boundary OAE 2, probably the biggest carbon cycle perturbation and global warming event in the last 100 m.y. A notable feature common to each of the Expedition 342 Paleocene/Eocene and Cenomanian/Turonian boundary sequences is the presence of siliceous sediments, including siliceous claystone, siliceous limestone, porcellanite, and chert. These Cretaceous and Paleogene hyperthermal and hypoxic events will be used to enhance our understanding of the consequences of abrupt climate change for Earth's ecosystems, climate, and chemistry, including the response of the hydrological cycle and terrestrial systems. Stability of Cenozoic ice sheets and early northern hemisphere glaciationAnother major objective of Expedition 342 was the recovery of paleoceanographic sections to assess competing hypotheses for the stability of Cenozoic ice sheets on Antarctica and the chain of events leading up to the intensification of northern hemisphere glaciation during the latest Pliocene–early Pleistocene. Major highlights of Expedition 342 were the recovery at multiple sites of high-latitude North Atlantic sections across three intervals of Cenozoic time that are key to the assessment of these problems, the middle Eocene interval (Chron C19; 40.5–42.5 Ma) on the run-up to the MECO, the late Eocene to early Oligocene transition (~38.5–32 Ma), and the latest Oligocene–earliest Miocene transition (~23.5–22.5 Ma). A potentially important discovery in this context is the presence, consistently across multiple sites, of angular lithic grains of the 63–150 µm size fraction in some of these intervals, most notably in lower Miocene and particularly in Oligocene strata. Astrochronology and calibration of the Cenozoic timescaleA further main objective of the expedition was to obtain records of the Eocene that can be used to link the astronomical timescale developed for the last ~40 m.y. to the floating timescale of the early Paleogene developed over a series of IODP and earlier drilling expeditions. Expedition 342 sites not only record well-developed cyclicity bundled into familiar orbital periods but also show promise for the development of magnetostratigraphy and refined biochronology. From this perspective, acquisition of sedimentary sections suited to generating an astronomically tuned record of the late Eocene and early middle Eocene is an extremely important expedition result that should make it possible to span existing gaps in our tuning efforts. For the Paleocene and Eocene, a shallow CCD has hampered efforts to obtain long, uninterrupted carbonate sequences that allow high-resolution paleoclimatic studies with traditional geochemical studies of stable isotope and trace element analysis. Shore-based studies of Expedition 342 sediment will make a major contribution to overcoming this particular problem. In addition, for the time intervals that have already been astronomically tuned elsewhere, the expedition will allow a comparison between low-latitude sites from the Pacific and Atlantic, offering the chance to decipher the processes controlling the amplification of, for example, ~41 k.y. obliquity versus ~100 and 405 k.y. eccentricity cycles and to test the hypothesis that there are different dominant astronomical forcing between Earth's warm and cool periods. |
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