Climatic, oceanographic and environmental change in the early to middle Eocene

Kirby, Nicola (2023). Climatic, oceanographic and environmental change in the early to middle Eocene. University of Birmingham. Ph.D.

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Abstract

The early to middle Eocene (~56 – 37 Ma) ‘hothouse’ was a dynamic interval of Earth’s history. Global mean surface temperatures were much warmer than today, atmospheric pCO\(_{2}\) levels likely exceeded ~1,000 ppm at their highest, and large polar ice sheets did not exist. A number of global warming events of varying timescales and magnitudes were superimposed on background Eocene global warmth. Global warming events in the Eocene were likely forced by changing atmospheric pCO\(_{2}\) concentrations, however the exact mechanisms and carbon sources involved in each case are unknown. Constraining the parameters of Eocene climate change is important for informing estimates of Earth system sensitivity under greenhouse or hothouse conditions, which feeds into modelling studies of future warming.

In Chapter 2, new, high-resolution (~3 – 5 kyr) X-ray fluorescence records of sediment geochemistry are presented from IODP Site U1514, Mentelle Basin, Southern Indian Ocean, across the EECO. Site U1514 was located at a high palaeolatitude, ~60 °S, in the early Eocene. It is therefore well-situated to record the influence of changing Southern Ocean circulation, particularly any response to the early surface water connection through the Tasmanian Gateway, ~49 Ma. Changing balances between biogenic and lithogenic elements in Site U1514 sediments indicate local variations in hydrological cycling and weathering rates on the nearby Australian continent in response to sustained high temperatures of the EECO. The initiation of a surface water connection through the Tasmanian Gateway may have stimulated a ~1 Myr interval of increased local primary productivity in the southern Indian Ocean at the end of the EECO, however it is unlikely to have contributed significantly to post-EECO pCO\(_{2}\) drawdown.

In Chapter 3, a new benthic stable isotope (\(\delta^{18}\)O and (\(\delta^{13}\)C) record is presented across ~50 – 51 Ma from IODP Site U1514. These are the first climate records of early Eocene hyperthermal events P to U from the Indian Ocean, a relatively understudied ocean basin with respect to the early Eocene. Time-series analysis on these high-resolution (~4 kyr) records is used to assess the impact of orbital forcing on the timing and magnitude of early Eocene hyperthermals. Hyperthermal events P to U in the Indian Ocean are driven by the 100-kyr eccentricity cycle in line with previous studies of early Eocene hyperthermals, while transient obliquity influence may indicate weak high-latitude forcing periodically throughout the EECO. A longer XRF Ca/Fe record also shows the influence of the 405-kyr eccentricity cycle on hyperthermal variations. Orbital conditions do not correlate with hyperthermal magnitude, which may instead reflect local, site-specific variations in climate records.

In Chapter 4, a compilation of sea surface temperature records from 17 sites with a global geographic spread is presented across the Middle Eocene Climatic Optimum. Records are compiled from three proxies: planktic foraminiferal (\(\delta^{18}\)O, planktic foraminiferal Mg/Ca and isoprenoidal GDGTs (TEX\(_{86}\)). This compilation is the first such collection of all available (\(\delta^{18}\)O, Mg/Ca and TEX\(_{86}\) data to show the geographic variation of MECO warming, which may contribute to future estimations of global climate sensitivity for the middle Eocene. Proxies are biased to different extents due to post-depositional alteration, uncertainties in middle Eocene ocean chemistry and GDGT distribution quality. Surface warming is constrained to ~1 – 7.5 °C. The reduced latitudinal temperature gradient of the middle Eocene is reconstructed, though MECO warming does not appear to impact latitudinal temperature gradients based on the datasets presented in this compilation.

In summary, this thesis presents new stable isotope data (Chapter 3) alongside re-analysis of existing datasets (Chapters 2 and 4) from the early and middle Eocene to provide insight into the environmental dynamics of the early Eocene ‘hothouse’, with a particular focus on the relatively understudied Indian Ocean.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):