Improving the reliability of bulk sediment radiocarbon dating

Radiocarbon dated chronologies are the most extensively applied dating technique when investigating the last ∼45,000 years. In lake and marine sediments, a single macrofossil or several microfossils are the preferred sample material because they have a ¹⁴C age that accurately reflects the time of deposition. However, absence of macro- or microfossils are a recurring challenge. Often, it is necessary to use bulk sediment samples for radiocarbon dating, but they frequently yield ages clearly exceeding the depositional timeframe due to occurrence of organic material of older origin. Until now, it has not been possible to assess if the dating result of a bulk sediment sample is reliable, or to adequately explain the mechanisms behind age disagreement between bulk and macro- and microfossils. In this study, we investigate the age offset between paired bulk and macrofossil ages in sediment cores from three lakes in SE and E Greenland. Furthermore, we distinguish three quantifiable main carbon sources in the samples: a) recent terrestrial organic fragments, b) recent aquatic organic matter, and c) reworked land-derived old carbon, through pyrolysis organic geochemistry, organic petrographic microscopy, and isotopic fractionization. Our results show that the offset between bulk (humic fraction) and macrofossil radiocarbon ages in three similar lakes range from 9 ¹⁴C yr to 20.1 ¹⁴C kyr. We observe a reduced age offset between bulk- and macrofossil ages, and thereby a higher reliability of bulk (humic fraction) ages, correlated with increased values of TOC, C/N ratio, and high CO₂ release in pyrolysis (S3 value). We recommend that future studies presenting bulk sediment chronologies apply pyrolysis organic geochemistry, organic petrographic microscopy, and isotopic fractionization as demonstrated here, to select reliable sediment intervals for bulk dating and retrieve more robust results.