Dynamic deep-water circulation in the northwestern Pacific during the Eocene: Evidence from Ocean Drilling Program Site 884 benthic foraminiferal stable isotopes (δ¹⁸O and δ¹³C)

Abstract

New benthic foraminiferal δ 18 O and δ 13 C data from Ocean Drilling Program (ODP) Site 884 (northwestern Pacific) add significant structure to a previous lower resolution record; our higher resolution detail and comparisons with published isotopic records provide new insights into paleoceanographic changes in the northwestern Pacific from the middle Eocene to the early Oligocene. From the early-middle until the mid-middle Eocene (ca. 49-43 Ma), a comparison of the Site 884 δ 18 O values with published ODP δ 18 O records from the Pacific (Site 1218), Atlantic (Site 1260), and Southern Ocean (Site 689) reveals that these basins were bathed by a common water mass, probably originating from the Southern Ocean, or by different water masses having a similar δ 18 O signature. From the mid-middle until the early-late Eocene (ca. 42-37 Ma), the Site 884 record reveals increasing complexity in deep-water circulation. The relatively low Site 884 δ 18 O values suggest that the northwestern Pacific was bathed by a water mass that was warmer than at the other Pacific, Atlantic, and Southern Ocean sites used in this study. Based on the relatively low δ 13 C values at Site 884, there are two possible scenarios for the origin of this northwestern Pacific water mass: (1) nutrient-rich surface waters downwelling at the higher latitudes of the North Pacific (Bering Sea); or (2) warm saline deep waters originating at low latitudes (possibly from the Tethys). In the late Eocene (ca. 36.5 Ma), the Site 884 δ 18 O and δ 13 C values increased, becoming more similar to Sites 1218 (Pacific), 1053 (Atlantic), and 689 (Southern Ocean), indicating an additional change in deep-water circulation, with a deep-water mass originating from the Southern Ocean bathing the northwestern Pacific.

Fish proliferation and rare-earth deposition by topographically induced upwelling at the late Eocene cooling event

“…The possibility that a deep water mass began to form around Antarctica in the late Eocene (ca. 36.5 Ma) and subsequently spread into the North Pacific is supported by the gradual increase of the oxygen isotope ratios of benthic foraminifera (δ 18 O bf ) over a period of about 2.5 million years 25 .…”

“…From the middle to late Eocene, the opening of the Southern Ocean Gateways-the Drake Passage and Tasman Rise-led to the development of a current around Antarctica shallower than the present Antarctic Circumpolar Current (ACC), the so-called proto-ACC 2,25 . The possibility that a deep water mass began to form around Antarctica in the late Eocene (ca.…”

Evidence for a heavily glaciated Antarctica during the late Oligocene “warming” (27.8–24.5 Ma): Stable isotope records from ODP Site 690

“…Current research suggests that Southern Ocean sourced bottom water was flowing into the northern Pacific Ocean as early as the late Eocene [ Borrelli and Katz , ]. Furthermore, a modern‐layered ocean structure developed during the early Oligocene (~30 Ma) [ Katz et al , ].…”

“…This was occurring at a time when equatorial seaways (i.e., Panama and Tethys) were still open, allowing water mass flow from the Atlantic to Pacific Oceans. Foraminiferal studies have suggested a warm, saline deepwater mass existed during the Eocene and Oligocene in the Atlantic and Pacific Oceans and had a low‐latitude source [ Borrelli and Katz , ; Kennett and Stott , ; Langton et al , ; Mead et al , ; Pak and Miller , ; Pekar et al , ; Via and Thomas , ].…”

“…This gradient began developing in the middle Eocene [ Cramer et al , ], reaching full extent in the early Oligocene. The development of this gradient has been linked to the strengthening and deepening of the Antarctic Circumpolar Current (ACC) during the late Eocene to early Oligocene [ Borrelli et al , ; Borrelli and Katz , ; Cramer et al , ; Katz et al , ; Langton et al , ; Scher et al , ; Scher and Martin , ], and development of a multilayered ocean structure by ~30 Ma as the high‐latitude Southern Ocean became thermally isolated [ Cramer et al , ; Katz et al , ; Scher et al , ]. This led to a differentiation between southern and northern deepwater sources [ Via and Thomas , ] and allowed the Atlantic and Pacific Ocean basins to be bathed by a new bottom water mass while the Southern Ocean became isolated and homogenized [ Scher et al , ].…”