No abstract

Stable isotope‐size data of four major planktonic foraminifera lineages from the Paleocene‐Eocene Thermal Maximum (PETM, ~56 Ma) at two locations (North Atlantic New Jersey shelf and Southern Ocean) are analyzed. By comparing changes in δ13C/δ18O‐size correlation prior to and during the PETM, we present evidence of significant changes in vital effects in photosymbiont‐bearing planktonic foraminifera during the PETM. The δ13C‐size data indicate divergent changes in δ13C vital effects in high‐latitude versus mid‐latitude populations, due likely to different responses in photosymbiotic activities. Combined δ18O‐size data and isotopic ranking indicates that some surface dwellers experienced changes that may be interpreted as depth migrations. Extreme temperatures (>32°C) may have exerted selective pressure and drove depth migrations. Species with flexible depth distribution were capable of adapting to rapid warming by vertical migration in the upper ocean, while populations restricted to near‐surface ocean may have undergone temporal and/or regional collapse during the peak warming. From a paleo‐proxy perspective, these biologic responses have the potential to obscure paleoceanographic signatures both regionally and globally.

The Miocene Climatic Optimum (MCO) from ~17 to 14 million years ago (Ma) represents an enigmatic reversal in Cenozoic cooling. A synthesis of marine paleotemperature records shows that the MCO was a local maximum in global sea surface temperature superimposed on a period from at least 19 Ma to 10 Ma, during which global temperatures were on the order of 10°C warmer than at present. Our high-resolution global reconstruction of ocean crustal production, a proxy for tectonic degassing of carbon, suggests that crustal production rates were ~35% higher than modern rates until ~14 Ma, when production began to decline steeply along with global temperatures. The magnitude and timing of the inferred changes in tectonic degassing can account for the majority of long-term ice sheet and global temperature evolution since 20 Ma.