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

Abstract

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 CO 2 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 CO 2 forcing (LaRiviere et al., 2012; Pagani et al., 1999), a characteristic which general circulation models struggle to simulate (Knorr et al., 2011; von der Heydt & 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 pCO 2 (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

The Miocene: The Future of the Past

“…Mg/Ca temperatures from the western tropical Pacific are about 30°C in the early Miocene (Sosdian et al, 2018) and, alongside a new record from ODP Site 761, show a cooling associated with the MMCT . These temperatures are further supported by Mg/Ca measurements from low latitude Miocene glassy foraminifera (Nairn et al, 2021). In the South China Sea, Mg/Ca surface temperatures over the interval 15.7 to 12.7 Ma range between 26°C and 30°C (values not corrected for changes in sea water chemistry) and exhibit only a slight cooling over the MMCT (Holbourn et al, 2010).…”

“…Tropical upwelling sites cooled in parallel with the mid- and high latitudes. Although the magnitude of cooling is least constrained in the tropics, where temperatures exceed the sensitivity range of UK′ 37 , recent Mg/Ca analyses on glassy foraminifera support the warmest UK′ 37 estimates, indicating that the magnitude of cooling has not been significantly underestimated by this proxy (Nairn et al, 2021). However, Mg/Ca SST suggest an average cooling of nearly 4°C between seven and 5.5 Ma in the South China Sea (Holbourn et al, 2018).…”

Warm, not cold temperatures contributed to a Late Miocene reef decline in the Coral Sea

“…Yet despite these conditions, this did not lead to coral reef loss in the Coral Sea, and there is some evidence that the corals had adapted to the suboptimal conditions by 11 Ma. During the Mid-Miocene Climatic Optimum (MMCO), tropical ocean temperatures likely were similar to, or higher then SSTs during the late Miocene [58][59][60] , and the surface ocean aragonite saturation state was even lower compared to the Late Miocene 54 . Despite this, reef growth was abundant and even showed signs of expansion on the Queensland Plateau during the MMCO 16 .…”

Revealing Their True Stripes: Mg/Ca Banding in the Paleogene Planktonic Foraminifera GenusMorozovellaand Implications for Paleothermometry

“…The use of microanalytical techniques (e.g., LA-ICP-MS, SIMS, nanoSIMS) in paleoclimatology to measure Mg/ Ca in individual chambers of single or multiple foraminifer tests has expanded in recent decades. Some authors have proposed Mg/Ca-temperature calibrations for a particular chamber within a species (e.g., Dueñas-Bohórquez et al, 2011) whilst others have emphasized the advantages of microspatial analysis for avoiding diagenetically altered or contaminated parts of the foraminifer test (e.g., Hollis et al, 2012Hollis et al, , 2015Kozdon et al, 2011Kozdon et al, , 2013Nairn et al, 2021). However, the observed large inter-and intra-test Mg/Ca variability in sample populations of individual foraminifera species means that a thorough assessment of the extent to which this affects the reproducibility of bulk test Mg/Ca is recommended (Nairn et al, 2021;Nürnberg, 1995;Sadekov et al, 2008).…”

“…There has also been an increased focus on using microanalytical techniques to measure Mg/Ca ratios in paleoclimate reconstructions, for example, measurements of Mg/Ca ratios by electron probe micro-analysis (EPMA; Kozdon et al, 2011Kozdon et al, , 2013 or averages of Mg/Ca profiles measured across individual chamber walls using laser ablation inductively coupled mass spectrometry (LA-ICP-MS; e.g., Nairn et al, 2021). Such targeted measurements fundamentally rely on an understanding of how Mg is distributed within the test wall (e.g., Sadekov et al, 2008).…”