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Mid-Late miocene climate constrained by a new laser ablation ICP-MS set up

Nairn, Michael 2018. Mid-Late miocene climate constrained by a new laser ablation ICP-MS set up. PhD Thesis, Cardiff University.
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Abstract

The unipolar icehouse world of the mid-late Miocene is a poorly understood interval in the evolution of Cenozoic climate. Widespread dissolution and poor preservation of carbonates in deep marine settings has resulted in large uncertainties in proxy-based climate reconstructions through the interval. Furthermore, models struggle to simulate the decoupling of low atmospheric CO2 (~250-350ppm) forcing in a warmer than modern world. This thesis uses the Sunbird-1 core, a clay-dominated sedimentary sequence from offshore East Africa, to improve the constraints on the global climate state of the mid-late Miocene. Prior to ~12 Ma, the Sunbird-1 site lay in a region of high productivity, with restricted, surface waters and an elevated supply of detrital organic matter invigorating carbon remineralisation in the water column. The redox environments associated with this high productivity setting resulted in the precipitation of diagenetic outer coatings on foraminifera, overprinting the primary Mg/Ca signal required for palaeotemperature reconstructions. The influence of these coatings decreased up-section, as the site subsided and experienced more open ocean conditions. This thesis develops and optimises a series of ablation parameters on the new Laser Ablation (LA-) ICP-MS system at Cardiff University. Employing medium to low fluences and repetition rates (3.5 Jcm-2 and 2.0 Hz) enables the collection of highly spatially resolved foraminiferal depth profiles suitable for assessing intra-test trace metal variability. By implementing the optimised LA-ICP-MS system this thesis reconstructs absolute sea surface temperature (SST) from Sunbird-1, indicating that applying a careful microanalytical approach can reconstruct palaeotemperatures from diagenetically altered foraminifera. This record suggests that tropical sea surface temperature remained relatively stable at 27-29⁰C from 13.3-9.4 Ma, presenting the mid-late Miocene as a key interval of increasing latitudinal temperature gradients towards that of the modern day climate state. These improved absolute sea surface temperature estimates, and the planktic foraminifera δ18O record, suggest a 39-48 m sea level equivalent increase in global ice volume through this interval. Furthermore, the planktic foraminiferal seawater δ18O record indicates that similar amplitude, short duration, fluctuations in global ice volume persisted through the mid-late Miocene. This implies that the East Antarctic Ice Sheet behaved dynamically following its expansion to a dry, land-based ice sheet during the Mid Miocene Climate Transition.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Ocean Sciences
Uncontrolled Keywords: Laser-ablation; LA-ICP-MS; Foraminifera; Miocene Paleoclimate; Trace Metal; Geochemistry;
Date of First Compliant Deposit: 27 March 2019
Last Modified: 28 Mar 2019 13:50
URI: http://orca-mwe.cf.ac.uk/id/eprint/121102

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