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doi:10.2204/iodp.sp.342.2011

Scientific objectives

Expedition 342 is focused on Paleogene records on the Newfoundland ridges. Although there is an extensive Cretaceous record of both drifts and fossil reefs evident in the seismic record, we do not have time to do justice to Cretaceous objectives without sacrificing our studies of the Paleogene system. Furthermore, although we are likely to obtain a record of the majority of the Paleogene during the expedition, a particular interval of focus is the middle Eocene to Oligocene, where there are thick sediment drift deposits that should preserve unusually expanded records of the transition from the greenhouse world of the Eocene climatic optimum to the glaciated world of the Oligocene. Drilling the Newfoundland sediment drifts will allow us to address key problems in paleoceanography, paleoclimate, and biotic evolution.

1. Recover data on the history of the Paleogene carbonate compensation depth and forcing factors for Paleogene hyperthermals.

One of the primary objectives of Expedition 342 is to provide data on the history and dynamics of the Paleogene carbon cycle. A particular goal is to use the carbon isotope data sets in combination with depth reconstructions of the carbonate lysocline to provide constraints for carbon cycle models. An extremely successful result of ODP Leg 208 (Zachos, Kroon, Blum, et al., 2004) was the acquisition of a ~2000 m depth transect that permitted estimation of the magnitude of CCD fluctuations in the Paleogene, particularly the Paleocene/Eocene Thermal Maximum (PETM) (Zachos et al., 2005). This depth transect showed that the CCD excursion associated with the PETM was >2000 m and suggested that the amount of CO₂ added to the biosphere during the PETM must have been much greater than previously documented (Panchuk et al., 2008). In turn, these data have been interpreted to suggest that the source of greenhouse gases could not have been primarily gas hydrates because the combined magnitude of the temperature excursion, the CCD fluctuation, and the size of the δ¹³C anomaly all suggest a carbon source more similar to marine organic carbon than bacterial methane (Panchuk et al., 2008). This example emphasizes how important it is to capture the magnitude of these carbonate preservation excursions because they provide a direct estimate of the change in ocean undersaturation and hence the transient change in CO₂ storage during extreme climate events. Further, when combined with δ¹³C data sets, the size of the CCD excursion can be used to study the source of isotopically light carbon transferred during such events and, ultimately, climate sensitivity to CO₂ forcing (e.g., Pagani et al., 2006a; Goodwin et al., 2009; Dunkley Jones et al., 2010).

Understanding the spatial and depth dimensions of the CCD change is clearly key to modeling the likely sources and amounts of carbon added to the biosphere during the PETM, but our current observational data do not yet provide a clear picture of either variable. If the methane hydrate hypothesis is untenable for the PETM and other hyperthermals, then it becomes even more important to identify a viable alternative—a task that would be made much easier if we had a clear idea of the total amount of CO₂ put into the ocean and atmosphere during the PETM and other Paleogene hyperthermal events. A well-resolved depth transect that could be used to identify the full depth range of the CCD excursion is critical to resolving the amount and type of carbon introduced into the biosphere during Paleogene hyperthermals. Recent evidence that the PETM and Paleogene global warming events may be orbitally forced or at least conditioned (Lourens et al., 2005; Galeotti et al., 2010; Sexton et al., 2011) implies that the source of carbon was not an impact or one-time volcanic eruption (Kent et al., 2003; Svensen et al., 2004). Thus, long astronomically calibrated records of CCD changes through the warm Eocene may be critical to unraveling the origins of the PETM and other Paleogene events (e.g., the middle Eocene climatic optimum, a prominent reversal in the long-term global cooling trend from peak Cenozoic warmth in the early Eocene) (Bohaty et al., 2009; Bilj et al., 2010).

In addition to reconstructing the sources and fate of carbon that produces extreme climate events, we also need to know the rate and full magnitude of these events and the response of marine biota. In most deep-sea records, slow sedimentation rates mean that the full magnitude of the events may have been bioturbated or dissolved away (Zeebe and Zachos, 2007). As has long been capitalized on by the Pleistocene paleoclimate community, the first solution to the problem of resolving transient events is to target sites where sedimentation rates are higher than the ocean average. Expedition 342 is aimed first and foremost at obtaining highly expanded records suitable for studies of Paleogene climate by targeting drift sediments of hypothesized Oligocene–middle Eocene age. The depth transect approach and acquisition of expanded carbonate-rich sections near the limits of their northern extent will allow us to study faunal and floral processes such as population turnover and latitudinal and stratigraphic range expansion during intervals of ocean acidification and global warming (e.g., Gibbs et al., 2006; Edgar et al., 2010).

The record on the Newfoundland ridges should allow a companion study to Walvis Ridge drilling (Leg 208) to be conducted under the flow path of northern-source deep waters. We have not previously obtained a good record of the history of the carbonate chemistry of the Deep Western Boundary Current because the Leg 208 transect on Walvis Ridge is likely influenced mainly by southern source waters by virtue of its location on the eastern side of the South Atlantic Basin. Furthermore, we can see in the Newfoundland seismic data that the toe of J Anomaly Ridge (at ~5 km depth) lies close to the average depth of the CCD because the drift package thins markedly at this point. Therefore, proposed drilling on J Anomaly Ridge, combined with the shallower sites on Southeast Newfoundland Ridge, should position us to recover a depth transect spanning ~4 km. It is of course possible that the amplitude of CCD shift during the most extreme Paleogene transient events could still exceed our transect, but we are better placed to obtain a record of the full magnitude of CCD excursions than previous studies. Our proposed transect is particularly suited to resolving fluctuations in the carbonate chemistry of the truly deep ocean—a big change from most Paleogene sites, which rarely have been targeted at paleodepths below ~2500 m.

The carbonate content of deep-sea sediments is a sensitive measure of the productivity, weathering, and fluxes of carbon within the biosphere. The CCD reflects changes in carbon erosion, deposition, and silicate weathering and in association with δ¹³C of marine and terrestrial proxies (benthic foraminifers, terrestrial biomarkers, and planktonic-benthic gradients) can be used to gauge changes in the size and distribution of carbon reservoirs (Higgins and Schrag, 2006; Leon-Rodriguez and Dickens, 2010). However, although the general form of CCD changes is moderately well known for the various ocean basins (Peterson et al., 1992), we have little information on the rates and amplitude of change in the CCD on short timescales, which are so important to understanding transient episodes such as the various extinction events, hyperthermals, and glacial events of the Paleogene.

Although the PETM marks a pronounced global warming and ocean acidification event, the obverse is true of the EOT, where the Pacific Ocean appears to undergo pronounced de-acidification associated with the onset of major Cenozoic polar ice sheets (Fig. F12) (Coxall et al., 2005; Merico et al., 2008). Ultimately, ice sheet initiation appears to have been triggered by an orbitally forced interval of cool summers, but some other factor, probably long-term drawdown in atmospheric carbon dioxide levels, must have conditioned the climate system (DeConto and Pollard, 2003; Coxall et al., 2005; DeConto et al., 2008). Records from the tropical Pacific (ODP Leg 199 [Lyle, Wilson, Janecek, et al., 2002]) demonstrate that the CCD deepened permanently (by ~1 km) in the earliest Oligocene—much faster than previously documented—in two 40 k.y. “jumps” and in lockstep with the stepwise onset of Antarctic glaciation, as recorded in benthic foraminifer δ¹⁸O (Coxall and Wilson, 2011; Coxall et al., 2005). However, it is unclear to what extent this lockstep behavior represents the global picture because sufficiently complete EOT depth transects from other ocean basins and latitudes are not available. Comparatively low-resolution records across a highly condensed and reworked Eocene/Oligocene boundary from Walvis Ridge (Leg 208 [Zachos, Kroon, Blum, et al., 2004]) have been interpreted in terms of a much smaller (~200 m) and nonpermanent CCD deepening. Thus, it is not clear whether CCD behavior in the equatorial Pacific represents the global picture, driven by wholesale change in deep-sea CO₃ ion concentration, or is to some extent a regional signal driven by changes in export production with important implications for our understanding of contemporaneous carbon cycling (Dunkley Jones et al., 2008; Coxall and Wilson, 2011).

Additional large, but temporary, CCD deepening events are also documented in the middle Eocene of the equatorial Pacific (Lyle et al., 2005). These calcium carbonate accumulation events have been interpreted (by analogy with the EOT and on the basis of discontinuous stable isotope records) to be a lithologic expression of large-scale bipolar continental glaciation (Tripati et al., 2005, 2007), but this interpretation is contested (Edgar et al., 2007; Eldrett et al., 2007, 2009; DeConto et al., 2008). The middle Eocene to lower Oligocene depth transect on the Newfoundland ridges presents an ideal opportunity to generate high-resolution records of changes in mass accumulation of CaCO₃ needed to test the hypothesis that the CCD changes recently documented in the equatorial Pacific are globally representative and intimately associated with the onset of continental glaciation. Together with the floral, faunal, and geochemical proxy records such as compound-specific δ¹³C_org and δ¹¹B in foraminiferal calcite (e.g., Pagani et al., 2005; Pearson et al., 2009) that could be generated, it would be possible to test for biotic turnover and pCO₂ drawdown across these intervals, with important implications for our understanding of the causes and consequences of Cenozoic climate deterioration.

2. Determine the flow history of the Atlantic Deep Western Boundary Current.

Deep ocean circulation is critical to defining the global climatic regime and its stability for two reasons. First, the sinking of surface waters distributes thermal energy around the globe. Second, the chemistry and nutrient composition of the dominant deepwater masses influence the partitioning of CO₂ between the deep ocean and the atmosphere. The pattern, strength, and stability of thermohaline circulation arise from a combination of wind forcing (Toggweiler and Samuels, 1995) and thermal or salinity density contrasts.

Numerous models suggest that thermohaline circulation can exist in multiple equilibrium states (e.g., Marotzke and Willebrand, 1991), with the potential for catastrophic disruption or collapse. For example, in the recent past the warmth and stability of interglacial climates is notably distinct from the rapid and abrupt cycles of warming and cooling that pepper the cold of glacial periods (e.g., Dansgaard et al., 1993). It has been proposed that enhanced climate instability during cold periods may originate from salinity feedbacks, which set up multiple thermohaline modes (Keeling and Stephens, 2001; Rahmstorf, 1995; Stommel and Arons, 1960), or from instabilities that lead to periodic purges and the collapse of large ice sheets (Alley and Clark, 1999). Analogues to Pleistocene ice sheet–driven cycles could exist in Oligocene climate records where geochemical studies suggest that both poles may have been glaciated, although other evidence suggests that only Antarctica supported major ice sheets (DeConto et al., 2008; Edgar et al., 2007). Salinity-driven feedbacks are a potential source of rapid climate shifts in Eocene and older records before the large-scale development of polar ice caps.

The proposed drilling transect lies directly under the flow path of the Atlantic Deep Western Boundary Current and therefore is ideally placed to monitor the chemistry and temperature of waters exiting the Nordic basins in the Paleogene. Therefore, the J Anomaly and Southeast Newfoundland Ridge sites are perfectly placed to act as end-members for studying the contribution of northern source waters to the rest of the global ocean. We already have a South Atlantic/Southern Ocean end-member site (ODP Site 690, Weddell Sea) and Pacific end-member sites (Shatsky Rise, ODP Leg 198), but we have nothing equivalent in the North Atlantic except for the subtropical Blake Nose (ODP Leg 171B) sites, many of which are located in water depths that are too shallow to truly record Deep Western Boundary Current properties.

Why do we need end-members? If all we had were equatorial sites (far from the high-latitude sites of deepwater formation) we could still determine whether Atlantic waters were being exported into the Pacific and vice versa, but we would not know where those waters originated. Barring the unexpected revival of models of warm saline deep water (Brass et al., 1982; Friedrich et al., 2008), it seems most likely that deep waters dominantly form at high latitudes (Bice and Marotzke, 2002; Bice et al., 1997), as is the case today and as is predicted in ocean circulation models for the Paleogene. Hence, in order to offer any concrete observational data to test ocean modeling for past warm climates, we must know where deep waters are sourced and how deepwater production varies during different climate states. Drilling Newfoundland Ridge sediment drifts offers a first-rate opportunity to obtain a true North Atlantic end-member for these studies. The large depth range covered by the proposed depth transect also will allow studies of the full range of ocean properties from the abyss to thermocline waters, really for the first time anywhere in the Paleogene.

In order to capture such nuances and rapid shifts in circulation patterns, it is critical to obtain records that allow us to reconstruct climate at suborbital temporal resolution. The Newfoundland sediment drifts offer an unprecedented opportunity to achieve this goal from sites close to the source of northern overflow waters in the Paleogene and to resolve the changes in circulation that accompany various Paleogene extreme climate events.

A case in point is the inferred reversal in deepwater overturning during the PETM (Nuñes and Norris, 2006; Tripati and Elderfield, 2005). A compilation of δ¹³C data from benthic foraminifers suggests that the “aging-gradient” in δ¹³C between Southern Ocean and Northern Hemisphere sites abruptly reverses during this interval of global warming (Fig. F13) (Nuñes and Norris, 2006). Southern Ocean overturning in the late Paleocene is interpreted to give way to northern overturning during the PETM and then gradually revert to Southern Ocean deepwater formation over the next ~100–150 k.y. An alternative explanation is that the reconstructed gradient reversal is a stratigraphic artifact created by the effects of dissolution on sediment accumulation in northern Atlantic sites (Zeebe and Zachos, 2007). A reversal in deep ocean circulation is seen in global climate model experiments of the PETM (e.g., Bice and Marotzke, 2001) and may have occurred during other Paleogene climate events. Drilling on the Newfoundland ridges can be used to look for such reversals in deepwater formation during a host of extreme climate events using three techniques. First, we can use depth transects to produce highly resolved records of the vertical structure of deep waters at critical sites such as the Newfoundland Ridges. Second, we can examine geographic gradients in benthic foraminifer geochemistry (e.g., δ¹³C) using the Newfoundland sites as a northern end-member for comparison with sites elsewhere in the world. Third, we can use the history of drift formation itself to qualitatively monitor the strength of the Deep Western Boundary Current and estimate the rate of formation of deep waters in the North Atlantic.

Depth transects have been shown to produce highly resolved records of changes in the source of deep waters in the Pleistocene (e.g., ODP Leg 154 [Curry, Shackleton, Richter, et al., 1995]) and the Paleogene (Legs 198, 207, and 208). For example, the hypothesized change in deepwater circulation during the PETM may have similarities to changes in deepwater formation during Pleistocene interstadials, in which outflows from the Nordic seas shift from deepwater formation to intermediate water formation as North Atlantic overturning waxes and wanes. In the Pleistocene, reduced North Atlantic overturning is associated with the intrusion of Antarctic Bottom Water well into the northern North Atlantic beneath Glacial North Atlantic Intermediate Water (Oppo and Lehman, 1993). We can look for similar changes in deepwater source areas by geochemical analysis of benthic foraminiferal calcite along depth transects. Appropriately well-preserved material can also be used to generate proxy records for the total CO₃^2– content of the deep and intermediate waters using planktonic foraminifer weight and percent fragmentation.

Our deepest sites lie within the carbonate lysocline in the Paleogene. None of our sites are strictly below the depth of carbonate accumulation, but the deepest site may have very little carbonate given the condensed nature of the transparent (Eocene) horizon. Foraminifer preservation will clearly suffer in sediments that have been subjected to extensive calcite dissolution and may well preclude foraminifer-based geochemical studies, particularly in planktonic foraminifers. However, if the sites can be correlated (by physical property records) to well-dated upslope sites (as was done during ODP Leg 154), we can use the deepwater site to pin the depth of the CCD and therefore evaluate the full magnitude of CCD variations throughout the Paleogene. The deepwater sites should also preserve a record of more refractory substrates such as fish teeth that can be used in Nd isotope studies of deep ocean flow paths (e.g., Via and Thomas 2006; Scher et al., 2011; Thomas et al., 2003).

An additional approach for identifying patterns of deepwater formation relies on dating drift sedimentary packages and analyzing their thicknesses and distributions in seismic profiles. The dates we currently have for the sediment drifts on the Newfoundland ridges strongly suggest that major drift formation began in the early Eocene with the deposition of thick acoustically transparent sequences and then changed character in the early Oligocene to a more mud-wave dominated system. Verification of early Eocene drift formation would require a major reassessment of our inferences of the relationship between the rates of deepwater formation and polar glaciation because it has been generally held that drift formation began at the major onset of Antarctic glaciation in the earliest Oligocene (Davies et al., 2001).

Studies of deposition rates, sediment sources, and grain size would do much to evaluate the origins and current regimes under which these drifts formed, as well as provide precise dates for changes in drift formation that could be compared with geochemical evidence for changes in deepwater sources. To achieve these primary goals we propose to combine proxies from sedimentology, clay mineralogy, and organic/isotope geochemistry to be applied specifically to the current-sensitive silt fraction. Continuous granulometric records will provide direct information on the variability of current strength and thus the history of the Deep Western Boundary Current. Clay mineral assemblages are excellent tracers for provenance studies, and organic matter composition will support the identification of source areas of fine-grained material and the presence of allochthonous organic matter from higher northern latitudes. As a working hypothesis we consider that periods of enhanced strength of the Deep Western Boundary Current should have resulted in stronger advection of a higher latitude component and generally coarser grain sizes in the silt fraction, whereas periods of reduced current activity should primarily record more local sediment sources and surface-ocean signatures as well as finer grained sediments.

3. Obtain high-resolution records of the onset and development of Cenozoic glaciation.

The canonical view of the onset of Cenozoic glaciation is that it took place in two main steps: (east) Antarctic ice sheets were established around the time of the EOT (~33 Ma), whereas Northern Hemisphere glaciation was not triggered until ~3–7 Ma (e.g., Miller et al., 1987; Zachos et al., 2001). However, on the basis of sediments recovered in the last phase of ODP and the initial phase of IODP, it has been suggested that this view of Earth’s climate history may need revision. Dropstones have been reported from the Arctic (IODP Expedition 303) in sediments of ~45 Ma age (Backmann et al., 2005). Discontinuous δ¹⁸O records in bulk and benthic foraminiferal calcite from the equatorial Pacific (ODP Leg 199) have been interpreted in terms of extensive ice sheet development in both hemispheres, together with a huge (>150 m) eustatic sea level fall around 42 Ma (Tripati et al., 2005, 2007). This interpretation is controversial (Lear et al., 2004; Edgar et al., 2007; DeConto et al., 2008). Records from the Norwegian-Greenland Sea and the Arctic Lomonosov Ridge suggest that winter sea ice formation was initiated around the start of the middle Eocene and that isolated alpine outlet glaciers existed on Greenland by the late Eocene (Eldrett et al., 2007; Stickley et al., 2009). In contrast with early interpretations (Lear et al., 2000), it is now also clear that the magnitude of the δ¹⁸O increase across the EOT (~33.5 Ma; Fig. F11) is too large to reflect ice growth on Antarctica alone (Coxall et al., 2005), and there is growing evidence for contemporaneous global cooling (Lear et al., 2004, 2008; Liu et al., 2009; Eldrett et al., 2009).

Expedition 342 provides an opportunity to shed new light on these aspects of Paleogene climate in the high northern latitudes in unprecedented stratigraphic detail. The middle to upper Eocene and lower Oligocene sections targeted here will allow us to generate the high-resolution records of changes in sedimentation rate, clay mineral assemblage, and occurrence and provenance of ice-rafted debris (IRD) that are needed. A potential problem exists with differentiating between putative Paleogene IRD and sediment eroded from the continental margin. In Pleistocene strata, IRD is often identified because of its coarse grain size (sand-sized and larger) and its distinctive provenance (e.g., “red-coated grains” from Labrador; basalt from Iceland) (Bond and Lotti, 1995; Hemming et al., 1998). IRD in the silt fraction has been identified by a number of techniques, including modeling of end-member components using a wide spectrum of grain-size analyses (Weltje, 1997; Weltje and Prins, 2003; Prins et al., 2002). IRD might also be identified by combining grain-size (especially sand content) and mineralogical information during “cold” events inferred from light stable isotope studies and geochemical assessment of provenance.

Clay minerals are excellent tracers of provenance in the modern North Atlantic and should have broadly similar distributions in the Paleogene. Continental-sourced clays should dominate the west Greenland and Canadian margins, whereas volcanic-sourced clays should be more typical of the Paleogene flood basalt province in eastern Greenland. In the modern North Atlantic, smectite, illite, chlorite, and kaolinite constitute the major proportion of the clay fraction. Smectite has been regarded as a major tracer for Iceland-Faeroe-derived material (Fagel et al., 1996) and could be expected from the Greenland volcanic province in the Paleogene. Where smectites are lacking, chlorite is the typical high-latitude clay from old metamorphic sources. A lack of volcanogenic and weathered basalt-derived clays (nontronite and amorphous minerals) would suggest a rather restricted contribution from eastern Greenland and mid-ocean-ridge sources. Clay mineral type (e.g., montmorillonite versus beidellite) and percentage data and Sm-Nd ratios in the clay fraction of Reykjanes Ridge sediments indicate a dominant terrigenous contribution from young continental crust that may derive from Europe and/or the Arctic (Fagel et al., 2001; Innocent et al., 2000). Today, the clay fraction may also contain terrigenous calcites and dolomites deriving from glacier milk in the Hudson Bay area. Such a signal could be transported for much longer distances than sand-sized IRD.

4. Extend the astronomical calibration of biostratigraphic and magnetostratigraphic markers and the geological timescale.

A detailed geological timescale that can be applied widely has always been a primary goal. Indeed, a highly resolved timescale is becoming increasingly critical in studies of Paleogene paleoceanography as we extend work on millennial timescales into deep time and require increasingly high-resolution correlation to evaluate the evolution of interbasin geochemical gradients. Determination of the rates at which Earth processes take place and how these rates change are key not only to developing an understanding of Earth history but also to accurately describing the nature and rate of the processes themselves.

The discovery of orbitally driven variations in Earth’s climate, and their preservation in the marine sedimentary record, has been the latest advance in chronostratigraphy. The application of this technique is not without assumptions, including the predictability of the exact beat of the climatic metronome (Pälike et al., 2000, 2004). However, if constrained by a paleomagnetic reversal stratigraphy, and if the same ages are obtained for geologic boundaries in different regions with different sensitivities to each of the different orbital forcings, we can develop a strong confidence in the timescale. A complete orbitally tuned Cenozoic timescale can provide estimates of process rates that have comparable precision throughout the Cenozoic.

We are working toward an astronomically tuned Cenozoic timescale. There is a semianchored astronomical timescale back to 30 Ma (mostly from ODP Leg 154). No magnetostratigraphy was recovered during Leg 154 (Curry, Shackleton, Richter, et al., 1995), but investigations from ODP Legs 177 (Gersonde, Hodell, Blum, et al., 1999) and 199 have resulted in verification and refinement of results from Leg 154 (e.g., Billups et al., 2004; Wade and Pälike, 2004), as well as an extension of the tuning across the EOT (Coxall et al., 2005). Several other ODP legs and IODP expeditions have provided suitable data for work of this sort—most recently from the Pacific Equatorial Age Transect (Expedition 320/321 [Pälike, Lyle, Nishi, Raffi, Gamage, Klaus, and the Expedition 320/321 Scientists, 2010]). Additional records are available for the time intervals of 35–42 Ma (Pälike et al., 2001; ODP Legs 171B and 177), ~45–56 Ma (ODP Leg 207; Westerhold and Röhl, 2009), 53–57 Ma (Norris and Röhl, 1999; Lourens et al., 2005; ODP Legs 171B, 198, and 208), and 62–65 Ma (ODP Legs 165 [Sigurdsson, Leckie, Acton, et al., 1997], 171B, 198, and 208). From this perspective, acquisition of an astronomically tuned record of the late Eocene and the early middle Eocene would be extremely important to span existing gaps in our tuning efforts and to provide single-site, high-resolution data that allow splicing, verification, and extension of previous efforts.

For the Paleocene and Eocene, a shallow CCD has hampered efforts to get long, uninterrupted carbonate sequences that allow high-resolution paleoclimatic studies with traditional geochemical studies (e.g., stable isotope analysis). The tectonic setting of the sites targeted by Expedition 342 is likely to overcome this problem because the proposed sites track above the CCD for the critical time slices needed, with potentially very expanded Paleocene, Eocene, and Oligocene sedimentary deposits. In addition, for the time intervals that have already been astronomically tuned, a comparison between low-latitude sites from the Pacific and Atlantic will be complemented by the North Atlantic setting of Expedition 342, 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 forcings between Earth’s warm and cool periods.

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