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Effects of sample mass and macrofossil type on radiocarbon dating of arctic and boreal lake sediments

College of Forest 'Resources, University of Washington, Seattle WA, USA; Quaternary Research Center, University of Washington, Seattle WA, USA; woswald{at}fas.harvard.edu

Patricia M. Anderson

Quaternary Research Center, University of Washington, Seattle WA, USA

Thomas A. Brown

Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore CA, USA

Linda B. Brubaker

College of Forest 'Resources, University of Washington, Seattle WA, USA

Feng Sheng Hu

Departments of Plant Biology and Geology, University of Illinois, Urbana IL, USA

Anatoly V. Lozhkin

North East Interdisciplinary Research Institute, Far East Branch, Russian Academy of Science, Magadan, Russia

Willy Tinner

Petra Kaltenrieder

Institute of Plant Sciences, University of Bern, Bern, Switzerland

Dating lake sediments by accelerator mass spectrometry (AMS) 14C analysis of terrestrial plant macrofossils overcomes one of the main problems associated with dating bulk sediment samples, i.e., the presence of old organic matter. Even so, many AMS dates from arctic and boreal sites appear to misrepresent the age of the sediment. To understand the nature of these apparent dating anomalies better, we conducted a series of 14C dating experiments using samples from Alaskan and Siberian lake-sediment cores. First, to test whether our analytical procedures introduced a sample-mass bias, we obtained ¹⁴C dates for different-sized pieces of single woody macrofossils. In these sample-mass experiments, statistically equivalent ages were found for samples as small as 0.05 mg C. Secondly, to assess whether macrofossil type influenced dating results, we conducted sample-type experiments in which 1⁴C dates were obtained for different macrofossil types sieved from the same depth in the sediment. We dated materials from multiple levels in sediment cores from Upper Capsule Lake (North Slope, northern Alaska) and Grizzly Lake (Copper River Basin, southern Alaska) and from single depths in other records from northern Alaska. In several of the experiments there were significant discrepancies between dates for different plant tissues, and in most cases wood and charcoal were older than other macrofossil types, usually by several hundred years. This pattern suggests that 14C dates for woody macrofossils may misrepresent the age of the sediment by centuries, perhaps because of their longer terrestrial residence time and the potential in-built age of longlived plants. This study identifies why some 14C dates appear to be inconsistent with the overall age-depth trend of a lake-sediment record, and it may guide the selection of 1⁴C samples in future studies.

Key Words: Radiocarbon dating • lake sediments • sample mass • plant macrofossils • AMS • chronology • palaeoecology • Siberia • Alaska • Holocene

The Holocene, Vol. 15, No. 5, 758-767 (2005)
DOI: 10.1191/0959683605hl849rr


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