Michael G. Nairn¹, Caroline H. Lear¹, Sindia M. Sosdian¹, Trevor R Bailey², and Simon Beavington-Penney³

¹School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK,

²Department of Natural Sciences, Amgueddfa Cymru, National Museum Wales, Cathays Park, Cardiff,

³BG Group, 100 Thames Valley Park Drive, Reading, RG6 1PT, UK*

Corresponding author: Michael G. Nairn (NairnMG@cardiff.ac.uk)

*Current address: Department of Earth and Environmental Sciences, The University of Manchester, Williamson Building, Oxford Road, Manchester, M13 9PL, UK.

Key Points:

Abstract

The mid-to-late Miocene is proposed as a key interval in the transition of the Earth’s climate state towards 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 U^k₃₇ 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 (LA-) ICP-MS 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 24-31⁰C 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 U^k₃₇ 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.

1 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). This has been used to suggest a decoupling of global temperature and atmospheric CO₂forcing (LaRiviere et al. , 2012; Pagani et al. , 1999), a characteristic which general circulation models struggle to simulate (Knorr et al. , 2011; von der Heydt and Dijkstra , 2006). It has also been suggested that the late Miocene was an additional important key step in the transition to our modern climate state, as high latitudes cooled more than low latitudes, leading to a marked steepening of latitudinal temperature gradients (Herbert et al. , 2016).

The late Miocene Cooling (LMC) between ~ 7.5 and 5.5 Ma was a global phenomenon (Herbert et al. , 2016) perhaps associated with decreasing atmospheric pCO₂ (Stoll et al. , 2019). The increase in the equator to pole surface temperature gradients was not associated with an increase in the benthic foraminiferal oxygen isotope record, implying that it occurred in the absence of a large increase in continental ice volume (Herbert et al. , 2016). Polar amplification in the LMC is consistent with estimates for other time intervals (e.g., Cramwinckel et al. (2018)). However, the LMC was also 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. Potential changes in the Earth System that could impact the magnitude of polar amplification include sea ice extent, vegetation induced changes in albedo, cloud cover, or ocean-atmosphere heat transport. Constraining the magnitude and timing of the steepening of latitudinal temperature gradients is therefore important for understanding the factors driving the late Miocene surface cooling specifically, and Earth System feedbacks more generally. Ideally, this would be achieved through a combined data-modelling approach using multi-proxy temperature reconstructions spanning a range of latitudes to increase confidence in calculated changes in temperature gradients.

Despite the significance of this climate interval, the evolution of global sea surface temperatures (SST) and hence temperature gradients during the mid-late Miocene is relatively poorly constrained due to a paucity of complete well-preserved sedimentary successions (Lunt et al. , 2008). The widespread carbonate dissolution, which dramatically reduced the sediment carbonate content and preservation quality in deep marine sediments, is termed the middle-late Miocene carbonate crash (Farrell et al. , 1995; Jiang et al. , 2007; Keller and Barron , 1987; Lübbers et al. , 2019; Lyle et al. , 1995). In addition to these dissolution issues, the majority of foraminifera-bearing Miocene sections are comprised of carbonate rich sediments which have undergone some degree of recrystallisation. The oxygen isotopic composition of planktic foraminifera that have undergone recrystallisation in seafloor sediments has been shown to be biased to colder temperatures (Pearson et al. , 2001). While planktic foraminiferal Mg/Ca appears to be less affected than δ¹⁸O, the impact of recrystallisation on reconstructed Mg/Ca sea surface temperatures remains an additional source of uncertainty (Sexton et al. , 2006). As a consequence, many mid-late Miocene absolute sea surface temperature reconstructions are restricted to estimates based on the unsaturated alkenone proxy and the TEX₈₆ proxy (Herbert et al. , 2016; Huang et al. , 2007; LaRiviere et al. , 2012; Rousselle et al. , 2013; Seki et al. , 2012; Zhang et al. , 2014). These records show a cooling in the late Miocene which begins around 10 Ma at high northern and southern latitudes. However, significant cooling in the tropics is not apparent in the alkenone records until ~7.5 Ma, while atmospheric pCO₂reconstructions also suggest a significant decline from this time (Sosdian et al. , 2018; Stoll et al. , 2019). At face value therefore, these records imply an interval of enhanced polar amplification between 10 Ma and 7.5 Ma in the absence of significant drawdown of CO₂ or increase in ice volume (Herbert et al. , 2016; Sosdian et al. , 2018). One significant caveat to this interpretation is that the Uk₃₇ alkenone proxy becomes saturated above 28⁰C (Müller et al. , 1998) and the late Miocene tropical SSTs prior to 7.5 Ma are at this limit (Herbert et al. , 2016). Therefore, an alternative interpretation of the data would be that the high latitudes and the tropics cooled synchronously from ~10 Ma, but the initial cooling in the tropics was not able to be recorded by the Uk₃₇ alkenone proxy. Corroboration of the absolute Uk₃₇ alkenone temperatures by an independent proxy would therefore confirm the timing of the global late Miocene Cooling and the possible interval of enhanced polar amplification between 10 Ma and 7.5 Ma.

Here we present a new planktic foraminiferal Mg/Ca record from the Sunbird-1 industry well cored offshore Kenya by BG Group. Critically, middle to late Miocene sediments in Sunbird-1 are hemipelagic clays, which has resulted in glassy preservation of the foraminifera. However, the foraminifera are coated with metal-rich authigenic coatings, which are not removed by standard cleaning techniques. Planktic foraminifera were therefore analyzed by laser ablation ICP-MS to obtain Mg/Ca from the primary foraminiferal test and hence enable estimation of absolute SSTs.

2 Materials and Methods

2.1 Site location, stratigraphy, and age control

This study utilizes 91 cuttings, spanning 273 meters at burial depths ranging from 630 m to 903 m, recovered by BG Group from the Sunbird-1 well offshore Kenya (04° 18’ 13.268” S, 39° 58’ 29.936” E; 723.3 m water depth) (Figure 1, Supplementary Table S1). Sedimentation at Sunbird-1 through the studied interval (9.5-13.5 Ma) is dominated by clays; the fraction of the sediment >63µm averages 11.5% (Supplementary Table S1), much lower than typical carbonate-rich deep-water sites. The impermeable nature and chemical composition of clay-rich sediment reduces diagenetic alteration of primary foraminiferal calcite, making them ideal targets for geochemical analysis (Pearson et al. , 2001; Sexton et al. , 2006). Tests displaying the desired exceptional preservation appear glassy and translucent under reflected light, and SEM imaging shows retention of the foraminiferal original microstructure (Pearson and Burgess , 2008). This style of preferential glassy preservation, as displayed in the Sunbird-1 well, is rare to absent in published records from Miocene foraminifera.