Iceberg Surge During Last Deglaciation May Have Had Two Pulses
Seafloor sediment cores provide new clues that could help clarify the influence of ice sheet collapse on a period of ocean cooling marked by slowed deepwater circulation.
About 21,000 years ago at the peak of the last ice age, the vast Laurentide Ice Sheet covered most of Canada and parts of the northern United States. As Earth’s climate began to warm, glacial surges known as Heinrich events discharged huge numbers of icebergs through the Hudson Strait and into the North Atlantic Ocean, and the massive ice sheet eventually collapsed.
New research by Hodell et al. adds to evidence that a major surge known as Heinrich Event 1 actually consisted of two separate events of iceberg discharge from the Laurentide Ice Sheet. The findings could help illuminate the factors that influenced changing climate conditions at the time.
Evidence for Heinrich Event 1 consists of a layer of sediment consisting of rock and mineral fragments (detritus) that spans the seafloor from the Labrador Sea to a region off the coast of Portugal. During Heinrich events, discharged icebergs served as rafts for detritus that had been picked up in glacial beds before the ice entered the sea. As the icebergs melted, glacial debris sank to the bottom of the ocean, forming distinct layers that now serve as fingerprints of each surge of icebergs through the Hudson Strait.
To better understand Heinrich Event 1, the Integrated Ocean Drilling Program (IODP) drilled sediment cores at a seafloor site in the central North Atlantic, 2800 kilometers southeast of the mouth of the Hudson Strait. The researchers analyzed the structure of the cores using X-ray computed tomography (CT) and X-ray fluorescence scanning, which are high-resolution (1 millimeter), nondestructive methods, as well as more traditional approaches utilizing discrete sediment samples.
Core analysis revealed two distinct, dense bands that corresponded to Heinrich Event 1. The bands consisted of coarse-grained material embedded in fine-grained glacial “flour” derived from the grinding of bedrock by glacial erosion. The researchers propose that each band represents a separate pulse in which discharged icebergs melted and released glacial detritus. Radiocarbon dating suggests that the pulses occurred about 16,200 and 15,100 years ago.
These findings echo previously gathered evidence from other drill sites that also show two pulses associated with Heinrich Event 1. The researchers say the sediment layers could represent two stages of iceberg discharge from the same ice stream in the Hudson Strait that drained the Laurentide Ice Sheet or surges from two separate ice streams.
Some scientists have previously proposed that Heinrich Event 1 helped trigger a period of climate cooling known to have accompanied weakening of the Atlantic Meridional Overturning Circulation, which transports cold ocean water from high latitudes toward the equator. (Although Earth’s climate was warming overall, shorter cool periods alternated with warm periods, and regional conditions varied.)
However, the timing of the first pulse, as well as changes in the presence of certain plankton species in the cores, suggests that this cooling period had already begun prior to—and continued during—the first pulse. Instead, melting of European ice sheets may have triggered the cold conditions early in the last deglaciation. Still, the pulse of Laurentide icebergs may have driven later cooling.
More research is needed to clarify the timeline and influence of surging ice streams on the global climate system, the authors note.
-- Sarah Stanley, Freelance Writer,
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Anatomy of Heinrich Layer 1 and its role in the last deglaciation
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