PALEOCEANOGRAPHY, VOL. 23, PA1S12, doi:10.1029/2007PA001451, 2008 Click Here for Full Article Salinity of the Eocene Arctic Ocean from oxygen isotope analysis of fish bone carbonate Lindsey M. Waddell1 and Theodore C. Moore1 Received 12 March 2007; revised 27 November 2007; accepted 10 January 2008; published 22 March 2008. [1] Stable isotope analysis was performed on the structural carbonate of fish bone apatite from early and early middle Eocene samples (55 to 45 Ma) recently recovered from the Lomonosov Ridge by Integrated Ocean Drilling Program Expedition 302 (the Arctic Coring Expedition). The d18O values of the Eocene samples ranged from À6.84% to À2.96% Vienna Peedee belemnite, with a mean value of À4.89%, compared to 2.77% for a Miocene sample in the overlying section. An average salinity of 21 to 25% was calculated for the Eocene Arctic, compared to 35% for the Miocene, with lower salinities during the Paleocene Eocene thermal maximum, the Azolla event at 48.7 Ma, and a third previously unidentified event at 47.6 Ma. At the Azolla event, where the organic carbon content of the sediment reaches a maximum, a positive d13C excursion was observed, indicating unusually high productivity in the surface waters. Citation: Waddell, L. M., and T. C. Moore (2008), Salinity of the Eocene Arctic Ocean from oxygen isotope analysis of fish bone carbonate, Paleoceanography, 23, PA1S12, doi:10.1029/2007PA001451. 1. Introduction from the Arctic Ocean over a 2 year period [Aagaard and Carmack, 1989]. The anomaly corresponded with a signif- [2] The Arctic is relatively isolated from the world ocean icant freshening and cooling of North Atlantic Deepwater and receives a net surplus of freshwater through the hydro- [Brewer et al., 1983], and its occurrence raises concerns logic cycle, mostly as runoff. The freshwater surplus creates about the threat the Arctic could pose to NADW formation a low-salinity layer (32.5%) in the upper 50 m of the Arctic under future anthropogenic warming. water column, known as the Polar Mixed Layer, which [3] During the greenhouse climate of the early and mid limits vertical mixing and promotes sea ice formation. As a Eocene, plate tectonic reconstructions suggest that the result, the Arctic exhibits a strong salinity stratification and Arctic may have had even less of a deepwater connection an estuarine-like circulation pattern, with the lighter, fresher to the world ocean than it does today (Figure 1). Without waters of the Polar Mixed Layer overriding Atlantic waters major continental ice sheets at this time, sea level was entering through the Fram Strait. Low-salinity surface water substantially (30–100 m) higher than in modern times is exported to the Norwegian-Greenland Sea via the East [Miller et al., 2005a]; however the Fram Strait, the only Greenland Current, where it mixes with relatively warm, deep connection between the Arctic and the world ocean, saline water from the Atlantic and sets up the deep convec- may not have allowed for bidirectional exchange with the tion that produces the Denmark Strait Overflow Water Norwegian-Greenland Sea until 17.5 Ma [Jakobsson et al., (DSOW) and the Iceland Sea Overflow Water (ISOW), 2007] and may not have opened fully until the late Miocene the densest components of North Atlantic Deep Water [Lawver et al., 1990]. Thus with an intensified hydrologic (NADW), which enter the Atlantic basin through overflow cycle under a warm Eocene climate and fewer connections of the Greenland-Faroe Ridge. The Arctic plays a vital role to the world ocean, it might be expected that the near in global climate through its influence on the production of surface water of the Eocene Arctic was even fresher than NADW. However, deepwater formation in the Norwegian- it is today and that the outflow of low-salinity water from Greenland Sea is delicately balanced, and an increase in the the basin may have acted to prevent the formation of export of fresh water from the Arctic, whether through a deepwater in the high-latitude northern seas. reduction in surface salinity or an increase in sea ice [4] Recent evidence suggests significant shifts in deep- discharge, has the potential to cap convective regions and water and intermediate-water production between the high reduce thermohaline circulation [Aagaard and Carmack, southern and high northern latitudes in response to changes 1989]. An example of such an event is the ‘‘Great Salinity in global temperature during the late Cretaceous and Paleo- Anomaly’’ of the late 1960s. During this time a freshening gene. As the highest-latitude ocean basin, the exact role of of the surface waters north of Iceland may have resulted the Arctic in deep water production during this period is still from a 25% greater than average outflow of fresh water 13 uncertain. Modeling studies and d C data suggest that a significant intensification of the hydrologic cycle in re- sponse to extreme global warmth may have initiated a 1Department of Geological Sciences, University of Michigan, Ann switch in deepwater formation from the high southern to Arbor, Michigan, USA. the high northern latitudes [Bice and Marotzke, 2002; Nunes and Norris, 2006]. During peak warmth, the salinity of Copyright 2008 by the American Geophysical Union. 0883-8305/08/2007PA001451$12.00 Southern Ocean surface waters would have been signifi- PA1S12 1of14 PA1S12 WADDELL AND MOORE: SALINITY OF THE EOCENE ARCTIC OCEAN PA1S12 Figure 1. Paleoreconstruction of the Arctic region at 50 Ma showing the location of the Arctic Coring Expedition (ACEX) 302 sites examined in this study. Modified from the Expedition 302 Scientists [2006]. cantly reduced by an increase in high-latitude precipitation, the degree of isolation of the Arctic Ocean during the thereby inhibiting convection, but deepwater formation in Eocene. the North Pacific may have been enhanced by the import of higher-salinity subtropical surface waters from the North 2. Tectonic Setting Atlantic through the Central American Seaway. In the modeling studies of Bice and Marotzke [2002], deep con- [6] The modern Arctic Ocean contains two deep basins vection occurs in the high northern latitudes at the PETM separated by the Lomonosov Ridge: the Amerasia and despite an intensified hydrologic cycle because Arctic Eurasia Basins. The Amerasia Basin, in turn, is divided outflow is diverted directly into the northwestern Tethys into two subbasins, the Canada and Makarov, by the Alpha- by the Greenland-Faroe Ridge. Uplift of the North Atlantic Mendeleev Ridge, and the Eurasia Basin is divided into the region may have severed the Rockall Trough connection Nansen and Amundsen subbasins by the Gakkel Ridge, the between the Atlantic and Arctic and allowed mammal Arctic crustal spreading center. Although the tectonic evo- migration between North America and Europe across the lution of the Eurasia Basin is much better understood than Greenland-Faroe Ridge over an approximately 2 Ma period that of the Amerasia Basin, it is commonly accepted that the in the late Paleocene and early Eocene [Knox, 1998]. Canada subbasin of the Amerasia Basin opened in the early However, once a surface water connection was reestablished Cretaceous through seafloor spreading along a now extinct between the Arctic and the North Atlantic, we must be ridge. The spreading resulted in the counterclockwise rota- concerned with the possible role of Arctic outflow on tion of the Arctic Alaska and Chukotka terranes away from oceanic circulation. the Arctic Islands and toward their respective present-day [5] To better evaluate oceanographic conditions within locations in northern Alaska and Siberia [Lawver et al., the Arctic basin, and thus the possible impact of Arctic 2002]. By the mid Cretaceous, circulation between the outflow on global circulation, this study estimates the Arctic and Pacific had been obstructed by the rotated blocks 18 2À salinity of the Eocene Arctic Ocean using d OCO3 of [Johnson et al., 1994], and by the late Cretaceous, the fish apatite from cores recently recovered from the Lomo- Arctic connection to the Pacific Ocean was probably closed nosov Ridge by Integrated Ocean Drilling Program (IODP) as mammals were able to migrate between Asia and western Expedition 302, the Arctic Coring Expedition (ACEX). North America via the Beringia land bridge [Averianov and The results of this study will also allow an assessment of Archibald, 2003]. The complicated arrangement of micro- plates in easternmost Siberia, however, leaves some doubt 2of14 PA1S12 WADDELL AND MOORE: SALINITY OF THE EOCENE ARCTIC OCEAN PA1S12 about the exact nature of interbasin passages between the the Paleocene because dinocyst species of clear Tethyan Arctic and the Pacific during the early Cenozoic. affinity are not present in the flood deposits of the Soko- [7] Spreading in the Eurasia basin along the Gakkel lovsky Quarry of Kazakhstan. Radionova and Khokhlova Ridge commenced between Anomaly 24 and 25 [2000], however, have correlated thick Ypressian diatomite (57 Ma), close to the Paleocene/Eocene boundary and units extending from the Kara Sea to the north Turgay approximately contemporaneous with the onset of spreading region to siliceous sediments in the North Atlantic Ocean, in the southern Norwegian-Greenland Sea [Thiede and providing evidence for a connection between the North Myhre, 1996]. It is widely accepted that the Lomonosov Atlantic and Tethys through the Arctic and Turgay Strait Ridge is of continental origin and was rifted as a series of during the early Eocene. By the earliest middle Eocene, tilted fault blocks from the Barents/Kara Shelf during however, the connection between the Arctic and the West the opening of the Eurasia Basin [Johnson et al., 1994]. Siberian basin had apparently ceased to exist [Radionova Since rifting, the ridge has subsided to modern depths of and Khokhlova, 2000]. Shallow water exchange between approximately 1000–1600 m and has posed a great barrier to the Atlantic and Arctic Oceans through the Labrador deep circulation within the Arctic since its creation [Johnson Sea and Baffin Bay may have developed as early as the et al., 1994].
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