Ocean circulation during Eocene extreme "greenhouse" climatic warmth

Cameron, Adele Jane (2016). Ocean circulation during Eocene extreme "greenhouse" climatic warmth. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f6c5

Abstract

The Early Eocene (≈ 47-56 Ma) 'greenhouse' climate represented the warmest climatic conditions witnessed in the last 90 million years, with peak Eocene warmth (the Early Eocene Climatic optimum, EECO) occurring around 50-52 Ma (Sexton et al. 2006; Bijl et al. 2009; Zachos et al. 2008; Littler et al. 2014) with subsequent global cooling thereafter (Sexton et al. 2006; Zachos et al. 2008). Ocean circulation plays a critical role in redistributing thermal energy across the planet and providing ventilation to the deepest parts of the ocean. Understanding how it may have operated in a globally warm world with little equator-to-pole gradients is paramount to understanding how it may respond to increasing temperatures today. The prevailing view of the early Eocene ocean was that deep-water formation was confined to the Southern Ocean, with little or no deep-water formation in the North Atlantic, unlike today. This study explores whether there is evidence for deep-water formation in the high latitude North Atlantic during the extreme climatic warmth of the early Eocene and its stability across transient climatic excursions. It also explores the strength and vigour of ocean circulation and whether this was influenced by the global decline in temperature following early Eocene peak-warmth. It utilises the neodymium isotopic signature of fossilised fish teeth (εNd) that is widely utilised to trace the movements of deep-water masses and can be used to reconstruct paleo-oceanic circulation along with detrital εNd that is an indicator of sediment provenance. It combines these with fish tooth rare earth element concentrations and sediment core XRF. Four key locations are utilised. Two on the Newfoundland margin in the West North Atlantic, and one from the high North Atlantic in the Labrador Sea, both ideal locations to identify the potential outputs of North Atlantic deep-water formation. The fourth site is Demerara Rise in the Equatorial Atlantic, chosen to monitor changes in the dominant source of deep-water sources from the North or the South.

This study demonstrates that throughout the late Paleocene and into the early Eocene, the εNd signature of deep-waters within the North Atlantic on the Newfoundland margin as recorded by fossilised fish teeth, are consistently more radiogenic (εNd ≈ -8) than deep waters found in this region today (e.g. North Atlantic Deep Water, NADW, εNd ≈ -13.5 (Lacan & Jeandel 2005a). These values are also more positive than any contemporaneous deep-water mass present elsewhere within the Atlantic, and are consistent with the formation of deep-water production within the high-latitude North Atlantic. Between 55 Ma and 54.5 Ma, the fish tooth εNd signature at Site U1403 at Newfoundland is at its most radiogenic (≈ -5.5), similar to other fish tooth εNd records from the Bay of Bengal and Goban Spur in the east Atlantic (Thomas et al. 2003). This excursion to positive εNd values is associated with a phase of extensive and explosive volcanism related to the emplacement of the North Atlantic Igneous Province (NAIP). Whilst the radiogenic values of deep-waters reaching the Newfoundland margin are consistent with deep-waters forming to the East of Greenland in the North Atlantic, unlike in modern day there is no contribution from a nonradiogenic deep-water source within the Labrador Sea, and no evidence to support export of this water-mass outwith the North Atlantic region, suggesting slower ocean-overturning.

During the peak warmth of the Early Eocene Climate Optimum (EECO, ≈ 50 Ma), this study finds that deep-water formation continued in the North Atlantic despite the extreme warmth, and as temperatures began to cool, an invigoration in ocean overturning is witnessed with further intensification around the Early Eocene/Middle Eocene boundary (≈ 47.8 Ma) associated with the onset of drift formation at Newfoundland. The εNd signature of deep waters at Newfoundland moves to more nonradiogenic values across this interval (εNd ≈ -9/-10), most likely due to the incorporation of more radiogenic waters from within the Labrador Sea being transported southward. Across the early/middle Eocene boundary at the onset of drift formation, the εNd signature of deep-waters at Newfoundland moves to more nonradiogenic values (εNd ≈ -11.5) for a period of 1.5 million years, before returning to values of 9/-10. This excursion is witnessed in fish teeth records from both Site U1403 and Site U1409 at Newfoundland, and is associated with a change in the detrital εNd signature from -14 to -15.5, indicative of increased transport of sediment material from further North within the Labrador Sea region, supporting a North to South movement of deep-water masses. However, currently there is no evidence of this excursion being witnessed in any other Atlantic water-mass records.

Within the equatorial Atlantic at Demerara Rise, the fish tooth sNd signature throughout the early Eocene is typically ≈ -11.5, more nonradiogenic than other Atlantic contemporaneous deep-water masses. This nonradiogenic signature does not appear to vary with changes in global temperature or to be sensitive to orbital-paced solar forcing. Detrital εNd values (εNd -16 to -18) are consistent with weathering products derived from the nearby Guyana shield. The deep-water arriving at Demerara Rise is therefore receiving additional inputs of nonradiogenic material, which may be by 'boundary exchange' along the South American coastline. The presence of this non-radiogenic water-mass in the Equatorial Atlantic supports there being slower ocean overturning than today.