996 resultados para 14C dating


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Thick, late Quaternary sediment sections were recovered at several sites on the leeward slope of Great Bahama Bank during Leg 166 of the Ocean Drilling Program. These sections have paleoceanographic records with potentially high temporal resolution. To make an initial assessment of the records corresponding to the Holocene highstand of sea level, we have identified and dated the sediments from the four upper slope sites (1004, 1005, 1008, and 1009) that were deposited during the period of time which spans the last glaciation through the Holocene. Age identifications are based upon the abundances of the Globorotalia menardii complex of planktonic foraminifera, the stable oxygen isotopic ratios of bulk sediment and the planktonic foraminifera Globogerinoides ruber, and AMS C-14 dating of bulk sediment. Comparison of these data with the sediment lithologic and geoacoustic properties shows that consistent stratigraphic relationships exist at each site: The uppermost interval of aragonite-rich sediments corresponds to the Holocene highstand of sea level (i.e. oxygen isotope stage 1) and these sediments are underlain by a relatively thin interval of aragonite-poor, partially lithified sediments which corresponds to the last glaciation when sea level was significantly lower than today (i.e. oxygen isotope stages 2-4). The Leg 166 upper slope sites possess carbonate accumulation and paleoceanographic proxy records with very high temporal resolution, with Sites 1004, 1008, and 1009 appearing to have the greatest stratigraphic integrity. Comparison of core and high-resolution seismic profile data establishes the Holocene nature of the uppermost seismic unit in the stratigraphic package of the western slope of Great Bahama Bank.

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Using the concept of 'orbital tuning', a continuous, high-resolution deep-sea chronostratigraphy has been developed spanning the last 300,000 yr. The chronology is developed using a stacked oxygen-isotope stratigraphy and four different orbital tuning approaches, each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data. Each approach yields a separate chronology. The error measured by the standard deviation about the average of these four results (which represents the 'best' chronology) has an average magnitude of only 2500 yr. This small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used. Excellent convergence between chronologies developed using each of five different paleoclimatological indicators (from a single core) is also obtained. The resultant chronology is also insensitive to the specific indicator used. The error associated with each tuning approach is estimated independently and propagated through to the average result. The resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr, in excellent agreement with the 2500-yr estimate. Transfer of the final chronology to the stacked record leads to an estimated error of +/-1500 yr. Thus the final chronology has an average error of +/-5000 yr.

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"Contract No. AT(40-1)-Gen-33."

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To determine the influence of fire and thermokarst in a boreal landscape, we investigated peat cores within and adjacent to a permafrost collapse feature on the Tanana River Floodplain of Interior Alaska. Radioisotope dating, diatom assemblages, plant macrofossils, charcoal fragments, and carbon and nitrogen content of the peat profile indicate ~600 years of vegetation succession with a transition from a terrestrial forest to a sedge-dominated wetland over 100 years ago, and to a Sphagnum-dominated peatland in approximately 1970. The shift from sedge to Sphagnum, and a decrease in the detrended tree-ring width index of black spruce trees adjacent to the collapse coincided with an increase in the growing season temperature record from Fairbanks. This concurrent wetland succession and reduced growth of black spruce trees indicates a step-wise ecosystem-level response to a change in regional climate. In 2001, fire was observed coincident with permafrost collapse and resulted in lateral expansion of the peatland. These observations and the peat profile suggest that future warming and/or increased fire disturbance could promote permafrost degradation, peatland expansion, and increase carbon storage across this landscape; however, the development of drought conditions could reduce the success of both black spruce and Sphagnum, and potentially decrease the long-term ecosystem carbon storage.