Surprise: Increased Hurricane Activity During the Younger Dryas

 

The vast ice sheet that covered Canada during the last Ice Age began to melt rapidly about 15,000 years ago, creating enormous glacial lakes. The largest glacial lake, known as Lake Aggasiz, was bigger than all of the present day Great Lakes combined.  The ice dam impounding this incredible volume of water collapsed 12,900 years ago, and a massive flood of cold freshwater, icebergs, and debris gushed into the North Atlantic via the St. Lawrence River.  This event caused a sudden drop in global temperatures and a reversal back to Ice Age conditions at northern latitudes because the influx of cold fresh water shut down ocean currents that brought tropically-heated salt water north.  The cold climate phase lasted for about 1500 years, and climate scientists refer to it as the Younger Dryas.

The colder ocean of the Younger Dryas should have spawned fewer hurricanes than the warmer oceans of today.  Hurricanes are a product of energy released from warm ocean water.  However, scientists discovered evidence hurricane activity increased off the coast of Florida during the Younger Dryas.  They discovered deposits of turbidite near the Dry Tortugas Islands, dating to the Younger Dryas.  Turbidite is sediment and rock resulting from underwater perturbations.  Earthquakes can cause turbidite formation, but this region is not prone to seismic activity.  Instead, hurricanes produced underwater currents that formed turbidite here.

Map of the Dry Tortugas–site of the study referenced in this blog entry.

Image showing how turbidite deposits are formed.

Scientists aren’t sure why hurricane activity increased during the Younger Dryas at this locality.  Some of their climate models suggest the oceans were much colder to the north and west of the Florida coast but only slightly colder than present day ocean temperatures off the modern Florida coast.  Perhaps the tropically-heated water that pooled near the equator spawned hurricanes that reached the Florida and south Atlantic coasts.

Increased hurricane activity contributed to the expansion of longleaf pine savannahs.  The wind felled forests, and the accompanying lightning-sparked fires maintained longleaf pine savannah ecosystems while repressing closed canopy hardwood forests.  Pleistocene megafauna became extinct during the Younger Dryas, even though longleaf pine savannahs are ideal habitat for grazers such as mammoths, bison, horses, giant tortoises, and many other species.

Reference:

Toomey, M. ; et. al.

“Increased Hurricane Frequency Near Florida during Younger Dryas Atlantic Meridional Overturning Circulation Slow Down”

Geology 45 October 2017

 

 

 

 

Tags: Dry Tortugas, Lake Aggasiz, turbidite, Younger Dryas