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doi:10.2204/iodp.proc.302.105.2006

Discussion

In the warm part of the early Eocene, the sea level was 30–100 m higher than the modern sea level (Miller et al., 2005); thus, there were likely shallow-water connections to the North Atlantic, North Pacific, and central Tethys (see Fig. F18 in the “Sites M0001–M0004” chapter). The hydrologic cycle may have been operating at a rate almost twice that of today (Bice and Marotzke, 2001) because of the higher global temperatures during the Eocene. Thus, freshwater flow from the surrounding continents into the Arctic was probably substantially higher than today, whereas saltwater flow into the Arctic may have been very restricted by the very shallow passages connecting the Arctic with other ocean basins. As the climate cooled during the middle and late Eocene, sea level fell and the exchange of deeper waters with the surrounding open ocean became even more difficult. Thus, the Arctic Ocean of the Eocene probably had a brackish upper mixed layer and a strong vertical density gradient controlled by salinity. Nearly all identified Eocene microfossils indicate brackish water conditions. The minor exception to this is a short interval in the middle Eocene (Cores 302-M0002A-48X and 49X; ~44 Ma), in which a very few specimens of radiolarians are found. Radiolarians in the modern environment do not normally tolerate salinities less than ~20‰ (Boltovskoy et al., 2003).

Layering in the very dark gray (Eocene) sediments is generally in the form of very thin laminae. Near the bottom of Section 302-M0002A-46X-1, 112 cm, there is a sharp change in layering and color. The 3 cm thick light gray layer at this level is overlain by centimeter-scale couplets of gray and dark gray that extend to nearly the top of the overlying core (Core 302-M0002A-45X). This sharp color break at the base of the gray layer is thought to mark the base of the hiatus separating middle Eocene from lower Miocene sediments (see “Timescale and sedimentation rates” in the "Sites M0001–M0004" chapter).

Layering of the couplets above the gray layer appears tilted with some crosscutting of individual packages. The thickness of the layers and the angle of tilt increase upsection. In individual packages of tilted layers, the upper layering appears to have cut across part of the underlying layers. As shown in the seismic reflection data taken near the sites (see Fig. F7B in the “Sites M0001–M0004” chapter), the seismic facies lying just above the Eocene section consists of high-frequency anastomosing reflections. Both the character of the sediments overlying the uppermost part of the Eocene section and their apparent seismic signature suggest a dynamic sedimentary environment with a “cut and fill” character, which might indicate migrating sediment waves (Hall, 1979). Currents that could create such an energetic environment include strong baroclinic or boundary currents (Aagaard and Carmack, 1994), tidal-driven turbulence on top of and around the ridge, and internal wave oscillation on a surface with a high-density gradient. The gray to dark gray couplets in Cores 302-M0002A-45X and 46X contain reworked microfossils together with some dinoflagellates and pollen grains that have a distinct Miocene affinity (see “Micropaleontology” in the “Sites M0001–M0004” chapter). The presence of late Eocene and Oligocene microfossils in this interval gives evidence that some upper Eocene and lower Oligocene sediments were deposited in the region; however, they were largely eroded and removed from the section cored. The presumed oldest age of Miocene, together with the color and organic content of these couplets, indicates that the deeper waters in the Arctic remained relatively low in oxygen until well within the Miocene. Above the couplets, a more uniform gray layer (containing pyrite nodules) extends upsection for ~3 m before there is a diffuse transition to brownish, more oxidized sediments in Section 302-M0002A-44X-1, 95 cm. The transition to the brown oxidized section is also marked by a disappearance of palynomorphs.

Even as the Lomonosov Ridge continued to sink and after the anoxic character of the sediments disappeared, millimeter-scale sand lenses in the more oxygenated part of the section suggest a continued turbulent winnowing of the sediment. Under these conditions, it is not surprising that the average sedimentation rate (~1–3 cm/k.y.) is comparatively low for a hemipelagic section and that other hiatuses may be found within the Lomonosov Ridge section. Even so, the very large hiatus at the top of the (middle) Eocene interval is unusually pronounced and represents almost 30 m.y. of extremely low to non-deposition (see “Timescale and sedimentation rates” in the “Sites M0001–M0004” chapter).

The gradual cooling of the global climate through the middle and late Eocene, together with the major global climatic cooling at the Eocene/Oligocene boundary (Zachos et al., 2001), was very likely to have impacted the Arctic as well. Even in a very brackish ocean, severe cooling (especially seasonal cooling) may have altered the vertical structure of the near-surface waters and promoted some shallow convection. In addition, there are at least two major tectonic events (Figs. F1, F2) that may have contributed to changes in the deeper Arctic waters:

• Rifting of the Yermak and Morris Jesup Plateaus at ~33 Ma, and

• Opening of the Fram Strait to shallow-water flow between 10 and 15 Ma (Lawver, et al., 1990) (Fig. F2).

Rifting of the Yermak/Morris Jesup Plateaus was the necessary first step in offering a more open flow into and out of the Arctic Basin and is likely to have marked the tectonic linkage of spreading in the Arctic Ocean with that in the Norwegian-Greenland Sea (Talwani and Eldholm, 1977; Rona and Richardson, 1978; Myhre et al., 1992). Clearance of the edge of the European continental plate at ~15 Ma as it moved past northeastern Greenland left only the Yermak Plateau blocking the passage to deepwater flow between the Norwegian Sea and the Arctic Ocean. The opening of the Fram Strait to deep water flow is not believed to have taken place till the Yermak Plateau cleared the northeastern margin of Greenland around 7 Ma (Lawver, 1990). The subsidence of the Iceland-Faeroe Ridge around 15–19 Ma (Thiede and Eldholm, 1983; Thiede and Myhre, 1996; Wright and Miller, 1996; Wright, 1998) may have also played an important role in Arctic paleoceanography (Fig. F2), for it was only after both the Greenland-Faeroe Ridge and the Fram Strait became open to at least shallow-water flow that the relatively warm, salty surface waters of the North Atlantic could easily enter the Arctic Basin. Given the timing of these gateway openings, it seems most likely that the introduction of North Atlantic waters into the Arctic is associated with the shift from the gray (suboxic) sediments to the brown (oxidized) sediments found in and above the top of Core 302-M0002A-44X (~191 mcd) (see Fig. F5 in the “Sites M0001–M0004” chapter).

The earliest appearance of Northern Component Waters (NCW) in the deep North Atlantic occurred at ~19–20 Ma (Wright and Miller 1996; Wright, 1998). This could mark the time that the Greenland-Faeroe Ridge had subsided to a depth that allowed a vigorous exchange between the Norwegian Greenland Sea and the North Atlantic. However, we could also envisage a somewhat earlier intrusion of some North Atlantic near-surface waters into the Norwegian-Greenland Sea and then into the Arctic itself that led to convection within these basins and the development of strong boundary currents near a strengthened pycnocline. This may have slightly preceded sufficient deepening of the Greenland-Faroe Ridge that allowed a full exchange with the North Atlantic. The first extensive mixing of the salty North Atlantic waters with the fresher, colder Arctic waters would then have led to both a greater oxygenation of Arctic deep waters and nearly continuous NCW formation (Wright, 1998; Wright and Miller, 1996).

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