Bulk sediment 14C dating in an estuarine environment: How accurate can it be?

Due to a lack of marine macrofossils in many sediment cores from the estuarine Baltic Sea, researchers are often forced to carry out ¹⁴C determinations on bulk sediment samples. However, ambiguity surrounding the carbon source pathways that contribute to bulk sediment formation introduces a large uncertainty into ¹⁴C geochronologies based on such samples, and such uncertainty may not have been fully considered in previous Baltic Sea studies. We quantify this uncertainty by analyzing bulk sediment ¹⁴C determinations carried out on densely spaced intervals in independently dated late-Holocene sediment sequences from two central Baltic Sea cores. Our results show a difference of ~600 ¹⁴C yr in median bulk sediment reservoir age, or R(t)_bulk, between the two core locations (~1200 ¹⁴C yr for one core, ~620 ¹⁴C yr for the other), indicating large spatial variation. Furthermore, we also find large downcore (i.e., temporal) R(t)_bulk variation of at least ~200 ¹⁴C yr for both cores. We also find a difference of 585 ¹⁴C yr between two samples taken from the same core depth. We propose that studies using bulk sediment ¹⁴C dating in large brackish water bodies should take such spatiotemporal variation in R(t)_bulk into account when assessing uncertainties, thus leading to a larger, but more accurate, calibrated age range.