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Paleomagnetic data combined with dinocyst stratigraphy suggest that the base of the Cenozoic section (the top of the rifting unconformity) lies near Core 302-M0004A-35X and has an age of ~56 Ma. Agglutinated benthic foraminifers indicate a neritic to inner neritic environment in Cores 302-M0004A-27X through 35X (i.e., from the base of the Cenozoic section up to the middle part of lithostratigraphic Unit 3 at 367 meters below seafloor) (see “Micropaleontology” in the “Sites M0001–M0004” chapter). The estimated age of this level is ~54 Ma. The Paleogene section is barren of benthic foraminifers above this level. Assuming that postrifting thermal cooling led to subsidence beginning at 54 Ma (i.e., somewhat after the reestablishment of deposition on the ridge), the model estimates 1–2 m.y. of subsidence would result in a basement deepening of a bit under 200 m, during which time ~40 m of sediment was added to the section. The upper part of lithostratigraphic Unit 3 (i.e., above Core 302-M0004A-27X) does not contain evidence of bioturbation or submillimeter-scale laminations seen in the lower part of the unit where benthic foraminifers are found (see “Lithostratigraphy” in the “Sites M0001–M0004” chapter). This gradual transition indicates a deepening of the site into less well oxygenated waters in the lowermost part of the oceanic mixed layer. The subsidence model used here indicates that this part of the section was deposited in paleowater depths between ~200 and ~350 m (Fig. F2). The paleosea level curve of Miller et al. (2005) indicates a lowstand at 53.5–54 Ma, followed by a rapid rise to a maximum in sea level between 53 and 52 Ma. The magnitude of this sea level rise is estimated to be 90 m (Miller at al., 2005).

The very organic rich sediments of lithologic Unit 2 are estimated to have been deposited at paleowater depths from ~350 to 650 m (at the base of the Eocene–Miocene hiatus) (Fig. F2). This depth range is consistent with the presence of an intense oxygen minimum zone below the upper mixed layer of the Eocene Arctic Ocean. In the modern, ice-covered Arctic, strong eddy mixing carries waters off the shelf into the ocean basin proper and mixes down to ~200 m (Aagaard and Carmack, 1989, 1994). In the Eocene, these eddies could have transported nutrient-rich, relatively freshwaters from the wide shelfal region of the Arctic out into the ocean basin. Here, high productivity and relative low oxygen bottom waters created the ideal environment for the production and preservation of laminated, organic-rich sediments. The paleowater depths estimated here, however, are not entirely consistent with the depositional environment interpretation based on the biostratigraphy (see “Micropaleontology” in the “Sites M0001–M0004” chapter). The planktonic assemblages preserved in the sediments are often described as being indicative of neritic or nearshore conditions, when in fact the Eocene shoreline was distant from the Lomonosov Ridge sites. This apparent discrepancy probably results from the very unusual environmental conditions of the Eocene Arctic, including highly productive, very brackish near-surface waters. These are the typical environmental conditions of surface waters found in nearshore areas of more normal open ocean basins.

A major hiatus in deposition (~30 m.y. in duration) separates the anoxic middle Eocene and older section from the overlying suboxic/oxic Neogene section. By the time sediment began to accumulate once again on the ridge in the Miocene, the sites on the ridge had subsided to ~1000 m paleowater depth (Fig. F2), and they have subsided only another ~200 m since that time. The presence of this large hiatus in the sedimentary record on the Lomonosov Ridge, together with the shift from the gray (suboxic) sediments just above the hiatus to the brown (oxidized) sediments found higher in the section (above the top of Section 302-M0002A-44X-1, 95 cm (~191 meters composite depth [mcd]) (see Fig. F5 in the “Sites M0001–M0004” chapter), lead us to speculate about what may have caused the hiatus. Did the ridge simply subside through an oxygen minimum zone to a depth at which an intense boundary current was active? Or were there marked changes in the vertical structure of the Arctic Ocean sometime during the time spanned by the hiatus that gave rise to such intense currents and changes in the oxygenation of the bottom waters?