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An important find of Deep Sea Drilling Project (DSDP) Leg 12 was ice-rafted mineral grains in Pliocene sediments of the Labrador Sea and the Hatton-Rockall Basin. The oldest (first) occurrence of ice-rafted mineral grains in these sediments is associated with the evolutionary first appearance of the planktonic foraminifer Globorotalia inflata (d'Orbigny) and the extinction of the coccolith Reticulofenestra pseudoumbilica Gartner. This association yields a paleontologic age estimate of 3.0 m.y.B.P. for the start of low-elevation Northern Hemisphere glaciation extensive enough to produce icebergs in the North Atlantic. Results from subsequent drilling in the North Atlantic (DSDP Legs 37, 48, and 49) confirm that the first occurrence of ice-rafted mineral grains in North Atl ntic sediments is at about 3.0 m.y.B.P. and further show that, during the Pliocene, icebergs penetrated as far south as 45°N lat. (DSDP hole 410) but not as far south as 37°N lat. (DSDP holes 333 and 335).
The estimated age of 3.0 m.y.B.P. for the onset of Northern Hemisphere low-elevation glaciation derived from the North Atlantic is compatible with, although slightly younger than, the 3.2 m.y.B.P. estimate suggested by paleomagnetically controlled isotopic data from the Equatorial Pacific.
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