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:
- Laser-Ablation ICP-MS facilitates absolute sea surface temperature
reconstructions using foraminifera with diagenetic coatings.
- Tropical sea surface temperatures remained relatively stable at
24-31°C following the Miocene Climate Transition.
- Development of an increased latitudinal temperature gradient began
prior to the Late Miocene Cooling.
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
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 (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 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.
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.