Chronologies for Recent Peat Deposits Using Wiggle-Matched Radiocarbon Ages: Problems with Old Carbon Contamination

Abstract

Dating sediments which have accumulated over the last few hundred years is critical to the calibration of longer-term paleoclimate records with instrumental climate data. We attempted to use wiggle-matched radiocarbon ages to date 2 peat profiles from northern England which have high-resolution records of paleomoisture variability over the last ~300 yr. A total of 65 14C accelerator mass spectrometry (AMS) measurements were made on 33 macrofossil samples. A number of the age estimates were older than expected and some of the oldest ages occurred in the upper parts of the sequence, which had been dated to the late 19th and early 20th century using other techniques. We suggest that the older 14C ages are the result of contamination by industrial pollution. Based on counts of spheroidal carbonaceous particles (SCPs), the potential aging effect for SCP carbon was calculated and shown to be appreciable for samples from the early 20th century. Ages corrected for this effect were still too old in some cases, which could be a result of fossil CO2 fixation, non-SCP particulate carbon, contamination due to imperfect cleaning of samples, or the reservoir effect from fixation of fossil carbon emanating from deeper peat layers. Wiggle matches based on the overall shape of the depth-14C relationship and the 14C minima in the calibration curve could still be identified. These were tested against other age estimates (210Pb, pollen, and SCPs) to provide new age-depth models for the profiles. New approaches are needed to measure the impact of industrially derived carbon on recent sediment ages to provide more secure chronologies over the last few hundred years.