The Younger Dryas Cool Episode in the Gulf of Mexico

Abstract

Data are presented from Orca Basin piston core EN32‐PC4 in the Gulf of Mexico that confirm the existence of surface water cooling during the Younger Dry as chronozone (11–10 ka). Late glacial planktonic foraminiferal species made a reappearance between 11.4 and 9.8 ka, an episode also marked by distinctly higher oxygen isotopic values derived from the planktonic foraminifer Globigerinoides ruber. The presence of the Younger Dryas event in the gulf at 27°N demonstrates that surface water cooling extended to mid‐latitude regions in the North Atlantic. The cool surface water interval is bracketed by rapid shifts in δ18O related to changes in the influx of meltwater to the Gulf of Mexico. A chronology based upon seven accelerator radiocarbon dates indicates that cooling commenced over a ∼500 year period and ended in less than 200 years. These results are among the first deep‐sea sediment data documenting the climatic transitions bracketing the Younger Dryas with a rapidity observed in ice core records. A rapid decrease in δ18O values measured in the white form of Gs. ruber at 10.2 ka is explained by significant meltwater influx into the gulf and rapid increase in sea surface temperatures. Surprisingly, a similar decrease is not observed in the pink form of Gs. ruber, a summer surface water dweller in the gulf. This discrepancy may be explained by continued meltwater influx throughout the Younger Dryas during the summers only, such that there was no change in the δ18O of the pink form at the end of the episode. An additional possibility is that warming at the end of the Younger Dryas raised year‐average temperatures and summer temperatures remained constant. The coincidence of rapid shifts in δ18O with the Younger Dryas strongly suggests a dynamic causal relationship and therefore supports a model for the cause of the Younger Dryas cooling based on changes in the routing of Laurentide glacial meltwater.