Subtropical iceberg scours and meltwater routing in the deglacial western North Atlantic

Abrupt centennial-to-millennial shifts inNorthernHemisphere climate during the last deglaciation are thought to have been triggered by the discharge of large volumes of meltwater and icebergs to the subpolar North Atlantic1. Here we show that meltwater and icebergswere also transporteddirectly from the Laurentide icemargin to the subtropicalNorthAtlantic in a narrow coastal current. We present high-resolution bathymetric data from south of Cape Hatteras showing numerous scours that we interpret as relict iceberg keel marks. This indicates that icebergs up to 300m thick drifted to southern Florida (24.5 N). In simulations with an ocean circulation model, duringdeglaciation, freshwater and icebergs routinely reached as far south as 32.5 N, in a period of less than four months. The southernmost scours formed only during periods of high meltwater discharge from theNorthernHemisphere ice sheets. In the simulations, such extreme periods of meltwater release led to a reversal of the typically northward surface flow in the nearshore subtropical western North Atlantic. We therefore suggest that significant volumes of iceberg-laden meltwater routinely bypassed subpolar regions and spread across the subtropical North Atlantic. The retreat of the NorthernHemisphere ice sheets during the last deglaciation (20–6 kyr ago) was accompanied by the discharge of meltwater and icebergs to the North Atlantic1. During this period, the onset of several prominent cold episodes coincided with times of particularly high freshwater release to theNorthAtlantic, suggesting that a freshening of the ocean weakened the strength of the Atlantic meridional overturning circulation1,2 and reduced the transport of heat to Europe and North America. It is typically believed that freshwater runoff was routed to the subpolar North Atlantic, with little penetration south of ∼40N (ref. 1). More recently, iceberg scours have been identified outside this region, on the Scotian shelf and Bay of Fundy3–5, Massachusetts and New Jersey margins6–8, as well as offshore of South Carolina (∼32.5N; ref. 9). In addition, ice-rafted debris (IRD) in marine sediment records from Blake Plateau (∼31N, 79W) and Bermuda Rise (32N, 65W) show that icebergs drifted to the subtropics during Heinrich events10,11. Determining how far south of the subpolar gyre icebergs and meltwater penetrated is vital for understanding the sensitivity of North Atlantic Deep Water formation and climate to past changes in high-latitude freshwater runoff. Here we use a combination of high-resolution multibeam bathymetry data and high-resolution numerical ocean circulation modelling to highlight a new ocean circulation pathway capable of rapidly transporting icebergs and meltwater along the east coast of North America to the subtropical North Atlantic. Analysis of high-resolution, multibeam bathymetry data reveals numerous iceberg scours along the continental slope from Cape Hatteras (∼35N) to the Florida Keys (∼24.5N; see Methods, Figs 1 and 2 and Supplementary Fig. 1). The identified iceberg keel marks are found in water 170–380m deep, and are traceable for >30 km. The scour marks, oriented south-southwest along regional bathymetric contours, exhibit lateral berms that record the ploughing of iceberg keels along the sea floor and semi-circular terminal grounding pits where icebergs came to rest (Fig. 2). The scours observed offshore of South Carolina are found in water 170–380m deep; farther south the scours are found in shallower water ∼215–280m deep (Supplementary Fig. 1). The keel marks decrease in size and abundance moving south along the margin, concordant with increased iceberg melt farther from the ice calving margin. The scour depressions offshore of South Carolina (∼32.5N) are ∼10–100m wide and 10–20m deep9, whereas the Florida margin scours (31N–24.5N) are narrower (10–50m wide) and 2–5m deep. The scour features are best preserved where shoaling platforms came into contact with the icebergs. Elsewhere, oversteepened slopes along large stretches of the margin limited seafloor–iceberg interaction and allowed unhindered iceberg transport over long distances. Both the scour morphology and correlation with stratigraphic controls on the New Jersey margin indicate that the South Carolina and Florida scours were probably formed by icebergs calving from the Laurentide Ice Sheet (LIS) following the Last Glacial Maximum8 (LGM). Relative sea level constraints at this time (70–120m lower than modern12) imply that iceberg keel depths ranged from 50–310m to generate the observed scour distribution. Icebergs at this subtropical latitude would therefore have been comparable in size to those calving from the modern-day Greenland Ice Sheet margin. As iceberg melt rate is inversely related to size13, icebergs from the LIS must have been considerably larger to reach this far south without completely melting. Indeed, seafloor scours in Hudson Bay show that extremely large, tabular ‘megabergs’ (keel drafts >650m) were sourced from the Canadian margin following the LGM (ref. 14). The south-southwest orientation of the iceberg scours, reduction in incision depth, and decrease in scour frequency with increasing distance from the former LIS (Fig. 2 and Supplementary Fig. 1) imply that icebergs drifted south along the continental margin in regionswhere themodernGulf Streamconsistently flows northward at>1m s−1. Here we use a high-resolution (1/6,∼18 km), coupled global ocean/sea-ice model configured for the LGM (Methods and Supplementary Fig. 2) to investigate the ocean circulation dynamics necessary to transport icebergs from the LIS to subtropical latitudes. The model is able to resolve both narrow coastal boundary currents andnearshore shelf and slope circulation patterns that are important for realistically simulating freshwater transport in the ocean15,16 (Supplementary Fig. 3).