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

Abstract

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.

Middle to Late Holocene Variations in Salinity and Primary Productivity in the Central Baltic Sea: A Multiproxy Study From the Landsort Deep

“…A reservoir age (R) and a standard deviation of 900 and 500 14 C years, respectively, were used in this age-depth model. These constraints have been used to make our alternative age-depth model comparable to the one presented by Obrochta et al (2017) and are based on the regional bulk sediment R variability given by Lougheed et al (2017). All subsequent ages discussed refer to calibrated ages in years before AD 1950 (cal ka BP).…”

A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century

“…The apparent temporal fluctuations in the magnitude of the bulk sediment reservoir age as indicated by the downcore reversals in 14 C age ( Fig. 2; Lougheed et al, 2017) impeded utilization of the bulk sediment AMS-14 C dates for the construction of the age model. Because our data did not enable robust correction for such variations, we desisted from using bulk sediment 14 C dates as age constraints.…”

“…2; Table 3). The onset of medieval Pb pollution (at 900 AD, Lougheed et al, 2012) and the medieval pollution maximum (at 1200 AD, Lougheed et al, 2012Lougheed et al, , 2017Zillén et al, 2012) have previously been used as age constraints in sediments from the Baltic Proper. However, our study is, to our best knowledge, the first to identify the 1530 AD pollution peak and the pollution minima of 1350 and 1600 AD (Brännvall et al, 1999;Renberg et al, 2002) as age-depth points in Baltic Sea sediments.…”

Mt. Fuji Holocene eruption history reconstructed from proximal lake sediments and high-density radiocarbon dating

“…To investigate the influence of older carbon upon bulk radiocarbon dates (the so-called reservoir effect), bulk organic matter 14 C dates were performed at depths coinciding with the depths (1.915 mcd and 3.455 mcd) of the two widespread tephras of known calendar age. The calendar ages of the tephras were reverse calibrated into an expected 14 C age probability density function (PDF) according to the IntCal13 (Reimer et al, 2013) calibration curve using the methodology described in Lougheed et al (2017). The reservoir effect offset was calculated as the difference between the actual median 14 C ages according to the reverse calibration process.…”