A high-resolution δ18O record from the northeastern Gulf of Mexico (Ocean Drilling Program Leg 100, hole 625B) provides a history of surface waters for the past 5.35 m.y. Negative δ18O anomalies between 0.3‰ and 3.0‰ in planktic foraminifera beginning at ∼2.30 Ma can only have resulted from the discharge of isotopically light glacial meltwater from the Mississippi River. Meltwater anomalies implicate either the periodic presence of North American, mid-latitude, glacial ice since the late Pliocene or a drainage system very different from that of the present.

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