ERRONEOUSLY OLD RADIOCARBON AGES FROM TERRESTRIAL POLLEN CONCENTRATES IN YELLOWSTONE LAKE, WYOMING, USA

Christopher M Schiller,Mark B Abbott,Kathryn L Elder,Cathy Whitlock,Nels A Iverson

doi:10.1017/rdc.2020.118

Abstract

ABSTRACTAccelerator mass spectrometry (AMS) dating of pollen concentrates is often used in lake sediment records where large, terrestrial plant remains are unavailable. Ages produced from chemically concentrated pollen as well as manually picked Pinaceae grains in Yellowstone Lake (Wyoming) sediments were consistently 1700–4300 cal years older than ages established by terrestrial plant remains, tephrochronology, and the age of the sediment-water interface. Previous studies have successfully utilized the same laboratory space and methods, suggesting the source of old-carbon contamination is specific to these samples. Manually picking pollen grains precludes admixture of non-pollen materials. Furthermore, no clear source of old pollen grains occurs on the deglaciated landscape, making reworking of old pollen grains unlikely. High volumes of CO2are degassed in the Yellowstone Caldera, potentially introducing old carbon to pollen. While uptake of old CO2through photosynthesis is minor (F14C approximately 0.99), old-carbon contamination may still take place in the water column or in surficial lake sediments. It remains unclear, however, what mechanism allows for the erroneous ages of highly refractory pollen grains while terrestrial plant remains were unaffected. In the absence of a satisfactory explanation for erroneously old radiocarbon ages from pollen concentrates, we propose steps for further study.

Highlights

Lake sediments are the primary archive for reconstructing late-Quaternary terrestrial climate, vegetation, fire, and limnobiotic processes
The utility of radiocarbon dating pollen concentrates with accelerator mass spectrometry (AMS) has been well established in lacustrine settings where terrestrial plant remains are unavailable and where total organic carbon of sediments are potentially contaminated by refractory carbon and reservoir effects
Our objective in this study was to answer the following questions: (1) How do radiocarbon ages from different terrestrial materials in Yellowstone Lake sediments compare? (2) What is the carbon source for pollen concentrates from Yellowstone Lake sediments that might lead to erroneously old ages? To answer these questions, we explore a suite of Accelerator mass spectrometry (AMS) radiocarbon ages from several sediment cores from the northern basin of Yellowstone Lake and compare them with independent age controls and the radiocarbon ages of living plant material

Summary

Introduction

Lake sediments are the primary archive for reconstructing late-Quaternary terrestrial climate, vegetation, fire, and limnobiotic processes. The utility of radiocarbon dating pollen concentrates with accelerator mass spectrometry (AMS) has been well established in lacustrine settings where terrestrial plant remains are unavailable and where total organic carbon of (bulk) sediments are potentially contaminated by refractory carbon and reservoir effects. Pollen concentration through mechanical separation (Brown et al 1989; Long et al 1992; Mensing and Southon 1999), chemical maceration (Doher 1980), heavy liquid separation (Vandergoes and Prior 2003), and flow cytometry (Tennant et al 2013) selectively removes anachronous sources of carbon (e.g. carbonates, algal remains, humic acids) from the sample. Some studies suggest dating errors as a result of incomplete separation of non-pollen material (Regnéll 1992; Richardson and Hall 1994) or incorporation of reworked pollen grains (Mensing and Southon 1999; Zimmerman et al 2019), pollenconcentrate ages typically produce more reliable chronologies than bulk-sediment ages where reservoir effects are a concern (e.g. Brown et al 1989; Vandergoes and Prior 2003; Newnham et al 2007; Fletcher et al 2017).

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