Abstract
High resolution seafloor mapping shows extraordinary evidence that massive (>300 m thick) icebergs once drifted >5,000 km south along the eastern United States, with >700 iceberg scours now identified south of Cape Hatteras. Here we report on sediment cores collected from several buried scours that show multiple plow marks align with Heinrich Event 3 (H3), ~31,000 years ago. Numerical glacial iceberg simulations indicate that the transport of icebergs to these sites occurs during massive, but short-lived, periods of elevated meltwater discharge. Transport of icebergs to the subtropics, away from deep water formation sites, may explain why H3 was associated with only a modest increase in ice-rafting across the subpolar North Atlantic, and implies a complex relationship between freshwater forcing and climate change. Stratigraphy from subbottom data across the scour marks shows there are additional features that are both older and younger, and may align with other periods of elevated meltwater discharge.
Highlights
High resolution seafloor mapping shows extraordinary evidence that massive (>300 m thick) icebergs once drifted >5,000 km south along the eastern United States, with >700 iceberg scours identified south of Cape Hatteras
The discovery of icebergs in this location has direct implications for understanding cryosphere–ocean–climate interactions as it suggests a narrow, buoyant, coastal boundary current must have flowed from the Northern Hemisphere ice sheets directly to the subtropical North Atlantic gyre (~20°N–40°N) and that south of Cape Hatteras this current was moving in the opposite direction to the northward flowing Gulf Stream at depth
Note that the ability of our model to capture the vertical structure and flow of these currents is implicit on using a “free-surface height” scheme and that they would not be resolved in models using a more traditional “rigid-lid” approach to study changes in meltwater input on climate.Our results show that if the SSH of the meltwater is larger than the SSH of the Gulf Stream at Cape Hatteras the meltwater will continue to flow south beyond this point and, in our model, this is the case for both the 2.5 and 5 Sv outburst floods, but not for smaller events (Supplementary Fig. 4)
Summary
High resolution seafloor mapping shows extraordinary evidence that massive (>300 m thick) icebergs once drifted >5,000 km south along the eastern United States, with >700 iceberg scours identified south of Cape Hatteras. For icebergs to reach the subtropical scour locations south of Cape Hatteras they must have drifted against the normal northward direction of flow over the continental slope; i.e., in the opposite direction to the Gulf Stream. The iceberg scours along the margin are interpreted to represent the plowing paths of iceberg keels transported more than 5000 km south along the United States continental margin to southern Florida in a cold, coastal boundary current derived from the former Laurentide Ice Sheet The discovery of icebergs in this location has direct implications for understanding cryosphere–ocean–climate interactions as it suggests a narrow, buoyant, coastal boundary current must have flowed from the Northern Hemisphere ice sheets directly to the subtropical North Atlantic gyre (~20°N–40°N) and that south of Cape Hatteras this current was moving in the opposite direction to the northward flowing Gulf Stream at depth. Research over the last 30 years has repeatedly shown that increases in freshwater (icebergs/meltwater) discharge to the subpolar North Atlantic can weaken the strength of the Atlantic meridional overturning circulation (AMOC) on multidecadal-to-millennial timescales by reducing North Atlantic Deepwater (NADW) formation[4,5]
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