Timing of iceberg scours and massive ice-rafting events in the subtropical North Atlantic

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Iceberg tracks in the seafloor: Icebergs can travel south all the way to Florida!

Illustration shows big grooves and other features on the seafloor.

This 3D perspective view of seafloor bathymetry, from multibeam sonar data collected offshore of South Carolina, show numerous scour marks carved by drifting icebergs. As iceberg keels plow into the seafloor, they dig deep grooves that push aside boulders and piles of sand and mud along their tracks. Sediment cores from nearby buried iceberg scours were used to determine when these icebergs travelled south along the coast.

A new study published in the journal Nature Communications by researchers from Woods Hole Oceanographic Institution (WHOI) and the U.S. Geological Survey (USGS) (including Dr. Jenna Hill of the Pacific Coastal and Marine Science Center) documents the long-ago drifting of enormous icebergs from Canada to as far south as the Florida Keys. The researchers found that more than 30,000 years ago, towering, bright white chunks of ice drifted south from the Hudson Bay in Canada, past Cape Hatteras in North Carolina, and all the way to Florida.

The icebergs were enormous, measuring about 300 meters (similar in stature to the Eiffel Tower!) and were ferried along by cold-water currents created during periods of rapid glacial melting. The fact that these icebergs were carried south, rather than north with the prevailing Gulf Stream, attests to the power of those meltwater currents. The ice left scour marks on the seafloor as it was pushed by the current. 

To figure out when and why these icebergs ended up in unexpected waters, a team of scientists collected sediment cores in areas offshore South Carolina. These seafloor sediments store chemical clues that the team used to reconstruct the past conditions. The scientists used radiocarbon dating on tiny shells (foraminifera) in the sediment to figure out when the icebergs left their mark on the seafloor.

The study demonstrates that when large volumes of ice melt from a glacier, it can create a current that brings cold, fresh water full of icebergs to far-off parts of the ocean. The transit of these erstwhile icebergs has implications for global ocean circulation and climate, the study argues, as glacial ice today is receding across much of the globe.

USGS and WHOI published their findings in the journal Nature Communications on 16 June 2021. Scientists Alan Condron (WHOI) and Jenna Hill (USGS) provide the first age constraints for the Floridian icebergs and suggest a new way to think about how icebergs and meltwater moved in the North Atlantic and impacted climate.

 

Photograph looking closely at sand and silt collected in a long wide tube.

A sediment core, collected from the seafloor offshore of South Carolina, shows a coarser sediment layer covering finer sediment with a very distinct boundary.

(Credit: Jenna Hill, USGS Pacific Coastal and Marine Science Center. Public domain.)

People stand around a long apparatus on the deck of a large ship.

Scientists from WHOI and USGS extract a core of sediment from a coring device.

(Credit: Jenna Hill, USGS Pacific Coastal and Marine Science Center. Public domain.)