The Last Interglacial Stage: Definitions and marine highstand, North America and Eurasia

Abstract

Delineation of the boundary between the Last Interglacial (LIG) and the last (Wisconsinan) Glacial Stage in North America represents a critical, yet unresolved issue. Subdivisions of the late Pleistocene are based on oxygen isotope, ice cover, and pollen stratigraphic data. Boundaries defined by isotope chronology hinge on complex interrelationships between δ18O in foraminifer tests, ice volumes stored on land, and coeval sea-level position. In the absence of adequate pollen-stratigraphic documentation, Pleistocene subdivision boundaries were harder to establish in North America than in Europe. Time-transgressive pollen zones revealed increased lengths of the climatically-floristically defined LIG from the European subarctic to the Mediterranean. Conflicting definitions of “Sangamon,” as representing only the last interglacial of minimum ice cover and higher temperatures or broadly defined, “sensu lato,” also incorporating early part of the Last (Wisconsinan) Glacial Stage persist in the North American literature. The exclusively interglacial age of the Sangamon Geosol, originally used in dating the Sangamonian Stage proved untenable. Designation of an “Eowisconsinan” interval corresponding to Susbtages MIS 5d-a also lacks merit. Despite climate- and vegetation-related discrepancies, pollen- and coastal deposit-based comparisons between Europe and North America during MIS 5 and the Holocene are useful in establishing the climate history of the North American Sangamonian and subsequent early Wisconsinan substages. An overarching MIS 5 cooling trend represented by scattered subarctic and high-mountain ice accumulation events followed the MIS 5e Eemian–Sangamonian temperature peak. Adoption of the general European practice that asymmetrically splits MIS 5 into a short MIS 5e interglacial and a long early Wisconsinan Glacial (MIS 5d-a) interval is preferred in North America as well. Subdivisions in the normalized δ18O curve that serve as the chronological framework and the wealth of European pollen data support this approach. While multiple pre-Sangamon Pleistocene marine-paralic intervals do occur on the NW Gulf coast, all pre-Sangamon Pleistocene marine and brackish-inshore deposits had been removed by erosion in the NE coastal plain. A single inshore-nearshore marine sediment and highstand interval is well-documented in this region. The LIG highstand sequence correlates with varied Eemian marine and paralic MIS 5e deposits encountered along northern and western European, Siberian, and additional shores. Apart from reliably dated Sangamonian S Florida coral reefs, identification and dating of LIG highstand deposits remain highly problematical in SE Atlantic shore terraces.