Tropical Sea Surface Temperatures Following the Middle Miocene Climate Transition From Laser‐Ablation ICP‐MS Analysis of Glassy Foraminifera

Michael G Nairn,Trevor R Bailey,Caroline H Lear,Simon Beavington‐Penney,Sindia M Sosdian

doi:10.1029/2020pa004165

Abstract

AbstractThe mid‐to‐late Miocene is proposed as a key interval in the transition of the Earth's climate state toward that of the modern‐day. However, it remains a poorly understood interval in the evolution of Cenozoic climate, and the sparse proxy‐based climate reconstructions are associated with large uncertainties. In particular, tropical sea surface temperature (SST) estimates largely rely on the unsaturated alkenone Uk37 proxy, which fails to record temperatures higher than 29˚C, the TEX86 proxy which has challenges around its calibration, and Mg/Ca ratios of poorly preserved foraminifera. We reconstruct robust, absolute, SSTs between 13.5 Ma and 9.5 Ma from the South West Indian Ocean (paleolatitude ∼5.5˚S) using laser‐ablation inductively coupled‐plasma mass spectrometer microanalysis of glassy planktic foraminiferal Mg/Ca. Employing this microanalytical technique, and stringent screening criteria, permits the reconstruction of paleotemperatures using foraminifera which although glassy, are contaminated by authigenic coatings. Our absolute estimates of 24–31°C suggest that SST in the tropical Indian Ocean was relatively constant between 13.5 and 9.5 Ma, similar to those reconstructed from the tropics using the Uk37 alkenone proxy. This finding suggests an interval of enhanced polar amplification between 10 and 7.5 Ma, immediately prior to the global late Miocene Cooling.

Highlights

The mid-late Miocene is an important interval in the evolution of global climate through the Cenozoic, representing a key period in the transition out of the warm, dynamic climate state of the Miocene Climatic Optimum (MCO) into a more stable unipolar icehouse world (Badger et al, 2013; Foster et al, 2012; Greenop et al, 2014; Sosdian et al, 2018)
Our Sunbird-1 sea surface temperature estimates from laser ablation (LA-)inductively coupled-plasma mass spectrometer (ICP-MS) Mg/Ca analyses are in good agreement with those using the δ18O paleo-thermometer on glassy foraminifera, supporting the use of LA-ICP-MS micro-analysis across multiple specimens for reconstructing paleotemperatures
This finding opens the potential for Mg/Ca paleothermometry on other challenging time intervals, and locations, where contaminant coatings have previously inhibited the geochemical analysis of primary foraminiferal calcite

Summary

Introduction

The mid-late Miocene is an important interval in the evolution of global climate through the Cenozoic, representing a key period in the transition out of the warm, dynamic climate state of the Miocene Climatic Optimum (MCO) into a more stable unipolar icehouse world (Badger et al, 2013; Foster et al, 2012; Greenop et al, 2014; Sosdian et al, 2018). Despite being characterized by similar to modern day atmospheric CO2 concentrations (Foster et al, 2012; Sosdian et al, 2018; Super et al, 2018), middle Miocene mean global temperatures were likely significantly warmer than the modern day (Pound et al, 2011; Rousselle et al, 2013). The LMC was preceded by a significant cooling of mid to high southern and northern latitudes, a heterogenous cooling at high northern latitudes, and a muted, limited cooling in the tropics (Herbert et al, 2016) This heterogenous cooling perhaps suggests an unusually high polar amplification factor for the interval immediately preceding the LMC.

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