Structure and stratigraphy of J Anomaly Ridge and Southeast Newfoundland Ridge

The J Anomaly and Southeast Newfoundland Ridges (Fig. F1) formed along the axis of the mid-Cretaceous Mid-Atlantic Ridge in a fashion analogous to the modern Reykjanes Ridge and Iceland (Tucholke and Ludwig, 1982). The tops of both ridges were above sea level in the Aptian and subsequently subsided to abyssal depths by the Late Cretaceous (Tucholke and Vogt, 1979). Aptian rudist platform carbonates were drilled at Deep Sea Drilling Project (DSDP) Site 384 (now at 3900 m) (Tucholke, Vogt, et al., 1979), and buried reeflike seismic features are present on the flank of J Anomaly and Southeast Newfoundland Ridges. Pelagic carbonates began to blanket the tops of the ridges by ~75–80 Ma (Tucholke, Vogt, et al., 1979) and accumulated to a total thickness of nearly 1.5 km by the late Neogene.

There are five principal sedimentary sequences on the Newfoundland ridges, bounded by reflection Horizons A–D (Fig. F3). The uppermost sequence of Pleistocene age displays well-defined internal reflections and mantles the northern side of Southeast Newfoundland Ridge. Nearly all piston cores collected from the Newfoundland ridges (Figs. F4, F5) are of Pleistocene age, including “long piston” Core MD95-2027. The presence of thick Pleistocene sections on the north side of the Newfoundland ridges may reflect the accumulation of iceberg-transported sediment derived from the Hudson Bay and Greenland. To the south, on Southeast Newfoundland and J Anomaly Ridges, the Pleistocene cover is nearly absent and is represented by only a thin (3–5 m thick) layer of Pleistocene foraminifer sand mixed with glacially transported sediments. This thin Pleistocene cover over our target sediments likely reflects the barrier imposed by the warm Gulf Stream to icebergs drifting southward along the Newfoundland margin. Pleistocene glacial sand protects the Paleogene sediment drifts from extensive erosion and preserves them in unconsolidated condition, as seen in piston Cores KNR179-1-13PC and KNR179-1-15PC from Southeast Newfoundland Ridge.

The second sequence, bounded by reflection Horizons A and B, displays poorly defined, discontinuous reflections (Fig. F3) and is probably of Oligocene and Neogene age on the basis of its similar acoustic character to other drifts in the North Atlantic (Davies et al., 2001). In some areas, the discontinuous reflections can be resolved as fields of sediment waves, suggesting that much of the unit was deposited under strong, directional bottom currents. Large parts of the southern flank of Southeast Newfoundland Ridge and the northern end of J Anomaly Ridge are covered by this sequence, which has thicknesses of >700 m. We have no cores that firmly date this sequence because of failure of the hydraulic winch during our site survey cruise.

The third sequence, bounded by reflection Horizons B and C, is seismically transparent and has a poorly defined contact with the overlying sequence of discontinuous reflections (Fig. F3). The absence of a strong reflector between these seismic units suggests that seismic Unit 3 has a conformable relationship with overlying Unit 2. Piston cores and seismic ties to DSDP Site 384 show that this sequence is of early Eocene age (nannofossil Zones NP14 and NP15) and younger. Its great thickness (up to 800 m; Fig. F6) suggests an unusually expanded sequence of lower and middle Eocene sediments on J Anomaly Ridge. Furthermore, the gradational relationship of this sequence into overlying strata may mean that there is a complete EOT in the drilling transect on Southeast Newfoundland Ridge (Fig. F7). Piston core samples show the main lithology in the lower to middle Eocene section is a clay-rich, white to yellow nannofossil ooze. The absence of strong internal reflections suggests that the sequence is not punctuated by major hiatuses but was deposited steadily like many modern Pliocene–Pleistocene drifts in the North Atlantic. This sequence thins below ~4.5 km present water depth (~4 km in the Eocene), reflecting reduced sedimentation rates in the lysocline and below the calcite compensation depth (CCD). Thinning of the Eocene package at ~4 km paleodepth is broadly consistent with the position of the Eocene CCD estimated from prior North Atlantic drilling (Fig. F8) (Tucholke and Vogt, 1979).

The fourth sequence, bounded by reflection Horizons C and D (Fig. F3) of Cretaceous/Paleogene age, is >500 m thick and crops out on the base of J Anomaly Ridge (e.g., Site JA-1; Fig. F6) and in numerous places on the crest of Southeast Newfoundland Ridge, including outcrops in moats around several seamounts. These strata also display driftlike morphology, albeit of smaller size than the Eocene drifts, and mostly well-defined parallel reflections like the Pleistocene cover section. This sequence was drilled at DSDP Site 384 (Fig. F9), which recovered Campanian to lowermost Eocene beige calcareous ooze and soft chalk with excellent magnetic stratigraphy, well-preserved foraminifers and calcareous nannofossils, as well as radiolarians in the late Paleocene (Berggren et al., 2000). The unconformities at the Cretaceous/Paleogene boundary and the Paleocene/Eocene boundary at DSDP Site 384 are to be expected because the site is located in a condensed section on top of a ridge. All recent Paleogene/Cretaceous drilling expeditions (ODP Legs 171B, 198, 199, 207, and 208 [Norris, Kroon, Klaus, et al., 1998; Bralower, Premoli Silva, Malone, et al., 2002; Lyle, Wilson, Janecek, et al., 2002; Erbacher, Mosher, Malone, et al., 2004; Zachos, Kroon, Blum, et al., 2004]) recovered one or more of these boundary sections despite their absence in older DSDP holes upon which the new drilling legs were based.

The fifth sequence, underlying reflection Horizon D, displays dense but parallel reflections (Fig. F3) and crops out on the northwest slope of J Anomaly Ridge, the crest and flanks of Southeast Newfoundland Ridge, and apparently in the pelagic caps of several seamounts. It consists of several discrete seismic sequences separated by possible unconformities that are indicated by truncations of reflectors. The entire seismic package is likely of mid-Cretaceous to early Late Cretaceous age on the basis of seismic ties to DSDP Site 384. Some of these sequences lap up against seismically identified buried reefs and are as thick as 450 m. The reefs and surrounding sediments are probably Barremian/Albian in age on the basis of results from DSDP Site 384 (Tucholke and Vogt, 1979) and Sr isotope stratigraphy (P. Wilson, unpubl. data), in keeping with the estimated ages of buried reefs off Florida (Hutchinson et al., 1995; Norris et al., 2001a). Representative summaries of our interpretations of the sequence of sedimentary packages on J Anomaly and Southeast Newfoundland Ridges are given in Figures F10 and F11, respectively.

The modern Deep Western Boundary Current

The area east of the Grand Banks is a region critical to understanding the history of deepwater circulation in the North Atlantic because it is the gateway between bottom water sources in the Norwegian-Greenland and Labrador Seas and the main basins of the North Atlantic to the south. Denmark Straight overflow water is the main deepwater mass, centered at ~3500 m and overlain by Labrador Sea water at ~1500 m (Pickart et al., 1999). Southeast Newfoundland Ridge is a major barrier to deep southward flow, and it diverts the Deep Western Boundary Current offshore into the path of the northeasterly flow of the Gulf Stream. The deepest part of the bottom current follows submarine contours around the southeastern end of the ridge and continues west around J Anomaly Ridge and along the Nova Scotian continental rise (Fig. F1). Shallower portions of the current follow contours around the crest of Southeast Newfoundland Ridge and also interact with seamounts on the ridge, forming local moats and drifts.

The Gulf Stream actually reaches the seafloor over Southeast Newfoundland Ridge and may contribute to bottom scouring. East of the ridge, Meinen and Watts (2000) found that the mean North Atlantic Current clearly extends to the bottom. Their measured bottom currents are strong enough to suspend sediments but probably not strong enough to cause extensive erosion. Still, we must bear in mind the possibility that erosion on J Anomaly and Southeast Newfoundland Ridges is related to a southward-flowing deep boundary current and/or a northward-flowing surface current that regionally extends to the bottom.

History of the Deep Western Boundary Current

Something akin to modern North Atlantic Deep Water appears not to have been produced until the late Miocene or early Pliocene, when a combination of tectonic subsidence of the Greenland-Scotland Ridge and Northern Hemisphere refrigeration began to form cold, dense overflow waters in the Nordic seas and the Labrador Sea (Oppo et al., 1995; Wright and Miller, 1993; Wright et al., 1992). However, northern-component deep waters clearly formed in the North Atlantic throughout the Neogene and Oligocene, judging from geochemical differences between Atlantic and Indo-Pacific waters (Wright et al., 1992). Intensification of deepwater formation in the North Atlantic is proposed to account for acceleration of the Deep Western Boundary Current during the Oligocene, leading to widespread erosion along continental margins and formation of the seismic reflection, Horizon Au in the western North Atlantic; subsequent current-controlled sedimentation formed major sediment drifts throughout the North Atlantic (Arthur et al., 1989; Miller and Fairbanks, 1985; Miller et al., 1987; Tucholke and Mountain, 1986; Tucholke, 1979; Tucholke and Vogt, 1979).

Direct evidence for significant flow in a deep boundary current before the Oligocene is sparse (Davies et al., 2001). Tucholke and Mountain (1986) suggested that the Eirik and Gloria Drifts south of Greenland may have begun to grow in the middle Eocene, based on interpreted ages of deep reflections in the drifts. The presence of onlapping reflectors and depositional structures on Blake Nose along the mid-Atlantic margin suggest erosion by an intermediate water mass centered above 2000 m water depth in the early Eocene (Katz et al., 1999; Norris et al., 2001a, 2001b). The seismic record from Blake Nose also shows evidence of condensed sections and slumps on the tip of Blake Nose (~2600 m) that could indicate deeper erosional flow along the Blake Escarpment in the early Eocene (Norris, Kroon, Klaus, et al., 1998). Erosion on Blake Nose may have been caused by shallow parts of the Deep Western Boundary Current rather than by the northward-flowing Gulf Stream. The area of erosion is more than 100 km east of the main flow of the Gulf Stream, which is constrained by the location of the Florida Straight and Suwanee Channel. In the deep basin, Mountain and Miller (1992) presented seismic evidence for late Paleocene bottom currents over the Bermuda Rise that could have a source analogous to Antarctic Bottom Water. Although limited, all of these data suggest that both a southern-source water mass and a northern-source water mass may have been present in the deep North Atlantic and circulated strongly enough to control seafloor deposition and erosion during the relatively warm climates of the early Paleogene. This eventuality raises the possibility that our drilling operations can expand upon some of the extraordinary paleoclimate results yielded during IODP coring in the Arctic (e.g., Brinkhuis et al., 2006; Moran et al., 2006; Pagani et al., 2006b; Sluijs et al., 2006).

Newfoundland sediment drifts

One of the main advantages of drilling the Newfoundland sediment drift complex is the near-absence of Neogene sedimentary cover. Most areas were swept by sufficiently strong currents during the later Cenozoic to prevent extensive deposition of younger strata on the southern side of the ridges or in patches around the seamounts. Although we do not have firm dates on when these strong currents were initiated, they are probably a post-Oligocene feature (on the basis of sedimentation rate changes at ODP Site 1267 in the Newfoundland Basin) and may reflect the full development of North Atlantic Deep Water, possibly in combination with a strengthened Gulf Stream. Before this time, the remarkable thickness, absence of internal reflections, and drift morphology suggest that the Paleogene section is likely to be hugely expanded, with sedimentation rates much higher than the 1–2 cm/k.y. typically encountered in the deep sea (Figs. F6, F7). A similar transition from drift deposition to nondeposition was encountered on Blake Plateau off the Florida-South Carolina margin during ODP Leg 171B (Norris, Kroon, Klaus, et al., 1998) in the latest Eocene. On Blake Nose, the Eocene and older sections were unusually expanded, with deposition rates as high as 6 cm/k.y. throughout the Eocene (Norris et al., 2001a, 2001b). We view the Leg 171B results as a favorable prognosis for the outcome of drilling the Newfoundland ridges.

The primary drilling targets for Expedition 342 are in plastered drifts that exist in three places: (1) the southern toe and eastern flank of J Anomaly Ridge, (2) the southern flank of Southeast Newfoundland Ridge, and (3) the north-facing slopes of seamounts on Southeast Newfoundland Ridge. In addition, we target the pelagic cap of one of the seamounts to obtain the shallow end-member of the depth transect.

Drift sedimentation clearly has a complex history on the Newfoundland ridges, with an initial phase of drift formation on the eastern flank of J Anomaly Ridge in the Late Cretaceous (probably starting in the Campanian or early Maastrichtian, judging from DSDP Site 384 drilling results [Berggren et al., 2000; Tucholke and Ludwig, 1982]) and continuing through most or all of the Paleocene. DSDP Site 384 was spot cored in a highly condensed section, and the Paleocene/Eocene boundary was not obtained. However, acoustic character does change at some point in the late Paleocene or early to middle Eocene with the creation of an acoustically transparent layer (seismic Unit 3). This transparent seismic unit is our primary drilling target because the absence of internal reflectors suggests that it is a conformable sequence with a good likelihood of being correlative across the depth transect.

Drift morphology suggests that the primary drift deposits formed mostly under a southward-flowing bottom current in the Eocene. This current formed plastered drifts on the north faces of seamounts on Southeast Newfoundland Ridge, as well as a long episodically growing and accreting ridge system between J Anomaly Ridge and Southeast Newfoundland Ridge. The southeastern flank of Southeast Newfoundland Ridge has a very thick seismically transparent drift (part of seismic Unit 3) that is overlain by a younger drift deposit that displays complex internal reflectors suggestive of sediment waves (seismic Unit 2). We provisionally assign the transition between these drift packages to the EOT.

The considerable thickness of the middle Eocene to ?upper Eocene seismically transparent interval (seismic Unit 3) means that traditional methods of coring Paleogene targets will have to give way to more focused drilling objectives. Most previous Paleogene and Cretaceous drilling was designed to obtain records spanning tens of millions of years. In contrast, previous drilling legs that targeted Pleistocene drifts typically cored the upper parts of the sediment packages in order to obtain highly expanded records of the late Pleistocene. Our goal will be to follow the “Pleistocene strategy” in which we will core expanded drift records at the expense of long time series.