Onset of North Atlantic Deep Water production coincident with inception of the Cenozoic global cooling trend

Abstract

Here we show that the onset of deep water overfl ow from the Norwegian-Greenland Sea into the North Atlantic, interpreted to represent the onset of a modern-style North Atlantic Deep Water mass, commenced close to the early to middle Eocene boundary. This fi nding is based on the identifi cation of a large, elongate contourite sediment drift, the "Judd Falls Drift," in the Faeroe-Shetland Basin, through detailed mapping of high-resolution two-dimensional and three-dimensional seismic data. This sediment drift covers an area of ~9000 km 2 in one of the critical present-day deep water gateways into the North Atlantic Ocean. Interpretation of the body as a contourite drift is based on standard seismic-stratigraphic diagnostic criteria including tridirectional onlap, lensoid sediment geometry, upslope progradational confi gurations of stacked onlap units, and an erosional base cut in a deepwater setting. The internal refl ection confi guration indicates that the drift was deposited under a southwest-fl owing current regime, which we propose represents the onset of North Atlantic Deep Water production, with continual deep water fl ow until at least the latest Eocene (ca. 35 million years ago [Ma]). Onset of drift deposition is dated by several key boreholes as occurring at ca. 50-49 Ma, across the early to middle Eocene boundary, and is coincident with independent geochemical evidence for major changes in the confi guration of deep-sea circulation patterns, global biological production, and the onset of Cenozoic climatic cooling. These temporal concurrences suggest a strong link between the initiation of North Atlantic Deep Water production and Cenozoic global climate evolution.

The demise of the early Eocene greenhouse – Decoupled deep and surface water cooling in the eastern North Atlantic

“…In contrast to Sexton et al (2006a), the increase of the interbasinal δ 13 C gradient is not obvious in our compilation (Fig. 6A) due to lack of the multi-site Indian Ocean data used by Sexton et al (2006a) and Hohbein et al (2012). At Site 401 we observed an increase in the surface to bottom-water gradient that coincides with major changes in deep-sea circulation as indicated by the increase in sedimentation rates and the onset of the post-EECO deep-water cooling trend (Fig.…”

“…7). In addition, intermediate water carbon isotope gradients between ocean basins also suggest increased formation of Southern Ocean intermediate waters in the Eocene (Hohbein et al, 2012). Sexton et al (2006a) argue that a strong correlation between interbasin isotope gradients points to a link between water mass aging and background ocean temperatures during Eocene warmth.…”

“…At almost the same time intensification of deep-water currents across this boundary is proposed by the contemporaneous onset of sediment drift deposits within a restricted sedimentary basin at the Greenland-Scotland Ridge, a key gateway for Arctic Ocean outflow into the North Atlantic (Hohbein et al, 2012). Also at Blake Nose in the western North Atlantic, winnowed foraminiferal sands have been deposited across the early-middle Eocene boundary disconformity (48-49 Ma, Norris et al, 2001b) contemporary to the increase in sedimentation rates at Site 401 as reflected by the expanded NP15 and NP16 interval.…”

Eocene cooling linked to early flow across the Tasmanian Gateway

“…3). Moreover, the intermediate deep water carbon isotope gradients between oceanic basins lend additional support for increased formation of Southern Ocean intermediate waters (43) during the middle Eocene, providing additional evidence for a connection between the temperature trends in benthic foraminiferal oxygen isotopes and in Southern Ocean surface waters.…”

“…This consistency in temperature inferences lends additional support to the South Pacific intermediate water formation taking place primarily during wintertime. Other studies provide evidence for an additional North Atlantic source for deep water formation (43) initiating around the early-to-middle Eocene boundary. The depth of this water mass would suggest a position below the carbonate compensation depth, and that makes it uncertain whether this deep water mass is represented in the data compilation of benthic foraminiferal oxygen isotopes (Fig.…”