Accelerator Mass Spectrometry Ages Research Articles

A high‐resolution diatom record of the last deglaciation from the SE Norwegian Sea: Documentation of rapid climatic changes

Although the pulsating nature and the abruptness of the last deglaciation are well documented in marine and land records, very few marine records have so far been able to capture the high‐frequency climatic changes recorded in the Greenland ice core Dye 3. We studied high‐resolution sediment cores from SE Norwegian Sea, which display a detailed climatic record during the last deglaciation comparable to that of Dye 3. Accelerator mass spectrometry age control of the cores enables us to correlate this record in detail with continental records. The results indicate that the surface waters of the SE Norwegian Sea were seasonally ice free after 13,400 B.P. The Bølling/Allerød interstadial complex (13,200–11,200 B.P.) was a climatically unstable period with changing Arctic‐Subarctic conditions. This period was punctuated by four progressively more severe sea surface temperature (SST) minima: between 12,900–12,800 B.P. (BCP I); 12,500–12,400 B.P. (BCP II); 12,300–12,000 B.P. (OD I); and 11,800–11,500 B.P. (OD II). The Younger Dryas (YD) (11,200–10,200 B.P.) represents the severest and most prolonged cold episode of this series of climatic deteriorations. It was bounded by very rapid SST changes and characterized by Arctic‐Polar conditions. The first true warm Atlantic water incursion to the SE Norwegian Sea took place around 10,100 B.P., followed by a brief cooler condition between 9900–9600 B.P. (YD II). The early Holocene climatic optimum occurred between 8000–5000 B.P. A conceptual model is proposed where meltwater fluxes are suggested to cause the observed instability in the SST record.

The Mount Edgecumbe tephra deposits, a marker horizon in southeastern Alaska near the Pleistocene-Holocene boundary

Late Pleistocene tephra deposits found from Sitka to Juneau and Lituya Bay are assigned to a source at the Mount Edgecumbe volcanic field, based on similarity of glass compositions to nearvent deposits and on thinning away from Kruzof Island. The sequence of near-vent layers is basaltic andesite and andesite at the base, rhyolite, and mixed dacite and rhyolite on top. The only breaks in the tephra sequence are two 1-mm-thick silt partings in a lake-sediment core, indicating a depositional interval from basaltic andesite to dacite of no more than about a millennium. Tephra deposits at sites >30 km from the vent are solely dacite and rhyolite and are 10,600 to 11,400 14C yr old based on interpretation of 18 radiocarbon ages, including 5 by accelerator mass spectrometry (AMS). Basaltic andesite and andesite deposits nearer the vent are as much as 12,000 yr old. Discrepancy among radiocarbon ages of upland tephra deposits provisionally correlated as the same grainfall is resolvable within ±2 σ of analytical uncertainty. Comparison of bulk and AMS ages in one sediment core indicates a systematic bias of +600 to +1100 yr for the bulk ages; correlation of tephra deposits among upland and lacustrine sites implies an additional discrepancy of 200–400 yr between upland (relatively too young) and lacustrine ages. In any case, the Mount Edgecumbe tephra deposits are a widespread, latest Pleistocene stratigraphic marker that serves to emphasize the uncertainty in dating biogenic material from southeastern Alaska.

Younger dryas climatic reversal in northeastern USA? AMS ages for an old problem

Late-glacial macrofossils from a 10-m core from Alpine Swamp, New Jersey, were radiocarbon dated using accelerator mass spectrometry (AMS). The arrival of the first trees to the area following deglaciation is indicated by maximum percentages of spruce pollen and a date of 12,290 ± 440 yr B.P. on a single spruce needle. Subsequent spread of deciduous hardwoods was followed by the expansion of boreal taxa, including spruce ( Picea), fir ( Abies), larch ( Larix laricina), paper birch ( Betula papyrifera), and alder ( Alnus). Three AMS dates on paper birch seeds and a spruce needle during this boreal expansion indicate that it took place between 11,000 and 10,000 yr B.P. The timing of this vegetational shift and its correlation with late-glacial pollen stratigraphy from many sites in southern New England indicate that a climatic reversal correlative with the Younger Dryas characterized the North Atlantic seaboard of the United States.