A Warm, Stratified, and Restricted Labrador Sea Across the Middle Eocene and Its Climatic Optimum

Abstract

Several studies indicate that North Atlantic Deep Water (NADW) formation might have initiated during the globally warm Eocene (56-34 Ma). However, constraints on Eocene surface ocean conditions in source regions presently conducive to deep water formation are sparse. Here we test whether ocean conditions of the middle Eocene Labrador Sea might have allowed for deep water formation by applying (organic) geochemical and palynological techniques, on sediments from Ocean Drilling Program (ODP) Site 647. We reconstruct a long-term sea surface temperature (SST) drop from~30°C to~27°C between 41.5 to 38.5 Ma, based on TEX 86. Superimposed on this trend, we record~2°C warming in SST associated with the Middle Eocene Climatic Optimum (MECO;~40 Ma), which is the northernmost MECO record as yet, and another, likely regional, warming phase at~41.1 Ma, associated with low-latitude planktic foraminifera and dinoflagellate cyst incursions. Dinoflagellate cyst assemblages together with planktonic foraminiferal stable oxygen isotope ratios overall indicate low surface water salinities and strong stratification. Benthic foraminifer stable carbon and oxygen isotope ratios differ from global deep ocean values by 1-2‰ and 2-4‰, respectively, indicating geographic basin isolation. Our multiproxy reconstructions depict a consistent picture of relatively warm and fresh but also highly variable surface ocean conditions in the middle Eocene Labrador Sea. These conditions were unlikely conducive to deep water formation. This implies either NADW did not yet form during the middle Eocene or it formed in a different source region and subsequently bypassed the southern Labrador Sea.

Early Eocene Ocean Meridional Overturning Circulation: The Roles of Atmospheric Forcing and Strait Geometry

“…As shown in MIROC and NorESM, the high latitude (up to 75°N) of Baffin Bay can be a suitable region for deepwater formation as long as the surface forcing allows it, as will be discussed in Section 3.2.1. Deepwater formation and export in the Labrador Sea during the early and middle Eocene, however, is not supported by sedimentary or geochemical data, which imply relatively restricted and stagnant conditions in the basin (Cramwinckel et al., 2020; Kaminski et al., 1989).…”

Disentangling the Impact of Global and Regional Climate Changes During the Middle Eocene in the Hampshire Basin: New Insights From Carbonate Clumped Isotopes and Ostracod Assemblages

“…In particular, we argue that in the lower part of the section the water temperature was mitigated by the intermittent introduction of colder North Sea water within the basin, explaining the cool and less saline waters recorded, in average comparable to the Norwegian‐Greenland Sea (Inglis et al., 2015, 2020). This was reversed upward when total or partial closure of the English Channel led to an increase of regional water temperatures and salinity, probably also influenced by increased evaporation, closer to the Atlantic sites (Cramwinckel et al., 2020). A similar situation has been described by Davis and Elliott (1957) during the deposition of the London Clay formation in the lower Eocene, when the opening of a sea gateway in the northern part of the London Sea brought cool water into the Hampshire Basin and cold‐fauna specimens.…”

“…The trend is then interrupted by a warming event, identified as the MECO, characterized by an increase of surface temperatures from 4 to 6°C followed by a rapid decrease of global temperatures of ca. 3°C at its end (Bijl et al, 2009(Bijl et al, , 2010Bohaty et al, 2009;Boscolo Galazzo et al, 2014;Cramwinckel et al, 2018Cramwinckel et al, , 2020Inglis et al, 2015Inglis et al, , 2020Liu et al, 2009).…”

Slow-growing reef corals as climate archives: A case study of the Middle Eocene Climatic Optimum 40 Ma ago

“…In the course of the Eocene (56 to 34 Ma), equatorial SSTs declined by ~5°C, reaching approximately modern values during the late Eocene, while the mid and high latitudes remained 10° to 20°C warmer than at present ( 25 , 26 ). Superimposed on the long-term cooling trend were short transient warmings (“hyperthermals”), the last of which was the MECO that occurred during the Bartonian, ~40 Ma ago ( 27 , 28 ). With regard to other hyperthermals, the MECO was peculiar because of its long duration (0.4 Ma) effected by CO 2 from a flare-up of continental arc volcanism ( 29 , 30 ) and a delayed drawdown of atmospheric CO 2 ( 30 ).…”

“…With regard to other hyperthermals, the MECO was peculiar because of its long duration (0.4 Ma) effected by CO 2 from a flare-up of continental arc volcanism ( 29 , 30 ) and a delayed drawdown of atmospheric CO 2 ( 30 ). As result of the atmospheric p CO 2 rise, global ocean temperatures increased by 2° to 5°C, and surface waters acidified by up to ~0.2 pH units above middle Eocene background values ( 28 , 31 , 32 ).…”