Dating Bulk Sediments from Limnic Deposits Using a Grain-Size Approach

Abstract

Radiocarbon measurements on bulk subaqueous sediments typically provide ages significantly older than actual time of deposition. This is generally caused by the presence of reworked organic compounds, which are depleted in 14C. To explore this issue of age heterogeneity, we collected 4 organic-rich samples from varying depths in a lake sediment core at the Gemündener Maar (Eifel, Germany), a lake of volcanic origin. We divided each sample into 5 standard grain-size fractions: gravel, sand, silt, clay, and 1 fraction smaller than 0.45 μm. These were cleaned separately using a standard acid-alkali-acid treatment. The highly organic gravel-size fraction provided the youngest 14C ages of all grain-size fractions and seems to be associated most closely with the time of deposition. By contrast, the silt and clay fractions show significantly older ages. If the investigated limnic sediment layer does not contain any identifiable terrestrial macrofossils, extracting and measuring coarser grain-size fractions instead of measuring bulk sediment samples will provide a better approximation of the time of sedimentation.

Climatic and environmental changes in the Yana Highlands of north‐eastern Siberia over the last c. 57 000 years, derived from a sediment core from Lake Emanda

“…Given that contamination with old carbon is much more common than contamination resulting in younger ages (Rothacker et al . 2013; Sirocko et al . 2013), it is assumed that the younger BETA ages are the better estimate of the time of deposition.…”

“…The age difference could be related to a size‐dependent trend, as organic sediment samples bear a higher risk of containing very small particles including planktonic and reworked material (Rothacker et al . 2013), as compared to OM samples sieved at >63‐μm mesh size. The two organic sediment samples were consequently discarded from the age‐depth model.…”

Stochastic alluvial fan and terrace formation triggered by a high-magnitude Holocene landslide in the Klados Gorge, Crete

“…to the environment is introduced by the low TOC and can result in smaller sample sizes prepared for the bulk radiocarbon measurement, which were affected with larger uncertainties after corrections for processing blanks and standards (Ruff et al, 2010). Nevertheless, empirical studies show that samples that contain a mixture of young and old carbon may overestimate the age of a deposit by 500-2000 years (Grimm et al, 2009;Rothacker et al, 2013). We recognise that the bulk sediment dating results contain inherent uncertainties and express reference timeframes rather than absolute ages for the processes due to a possible overestimation of the age.…”