Abstract
Abstract. The middle Miocene climate transition (MMCT), around 14 Ma, was associated with a significant climatic shift, but the mechanisms triggering the event remain enigmatic. We present a clumped isotope (Δ47) bottom-water temperature (BWT) record from 16.0 to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean and compare it to existing BWT records from different latitudes. We show that BWTs in the Southern Ocean reached 8–10 ∘C during the Miocene climatic optimum. These high BWT values indicate considerably warmer bottom-water conditions than today. Nonetheless, bottom-water δ18O (calculated from foraminiferal δ18O and Δ47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset further demonstrates that BWTs at Site 747 were variable with an overall cooling trend across the MMCT. Notably, a cooling of around 3–5 ∘C preceded the stepped main increase in benthic δ18O, interpreted as global ice volume expansion, and appears to have been followed by a transient bottom-water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom-water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Feedbacks required for substantial ice growth and/or tectonic processes may have contributed to the observed decoupling of global ice volume and Southern Ocean BWT.
Highlights
During the Cenozoic Era, Earth’s climate transitioned from a state of expansive warmth with very limited ice to colder conditions and permanent ice sheets at the poles (Zachos et al, 2001)
From ∼ 16.0 to ∼ 15.3 Ma, we analysed stable isotope compositions of both C. mundulus and C. wuellerstorfi, allowing for a direct assessment of species-specific effects on the isotopic compositions of these two different epifaunal species (Fig. 2a and c). δ18O values measured on C. mundulus and C. wuellerstorfi appear indistinguishable, whereas a consistent offset of up to ∼ 0.5 ‰ exists between the δ13C values of these species at Site 747
Even if the δ18O signal was influenced by bottom-water temperature (BWT) only, the gradual ∼ 0.5 ‰ increase in benthic δ18O would correspond to a cooling of roughly 2 ◦C (e.g. Marchitto et al, 2014)
Summary
During the Cenozoic Era (the last 65 Myr), Earth’s climate transitioned from a state of expansive warmth with very limited ice to colder conditions and permanent ice sheets at the poles (Zachos et al, 2001). A substantial increase in benthic foraminiferal oxygen isotope ratios (δ18O) during the MMCT has been interpreted as reflecting a combination of decreasing bottom-water temperatures (BWTs) and ice sheet expansion (increasing bottom-water δ18O) occurring in the Southern Hemisphere (Lear et al, 2015; Lewis et al, 2007). Several studies propose a degree of decoupling between BWT and global ice volume during the middle Miocene (Billups and Schrag, 2002; Lear et al, 2010, 2015; Shevenell et al, 2008) These studies are based on deconvolving the bottomwater δ18O (δ18Obw) and temperature signals in benthic foraminiferal δ18O with independent temperature estimates based on benthic foraminiferal Mg/Ca ratios. Their results indicate a middle Miocene decrease in BWT of ∼ 0.5–3 ◦C
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