Compound‐Specific Radiocarbon Analysis of (Sub‐)Antarctic Coastal Marine Sediments—Potential and Challenges for Chronologies

Abstract

In Antarctic and Subantarctic environments, 14 C-based age determination is often challenging due to unknown reservoir effects, low organic carbon contents of sediments, and high contributions of petrogenic (14 C-free) carbon in ice marginal settings. In this study, we evaluate possible benefits and challenges of compound-specific radiocarbon analysis (CSRA) as a tool for age determination of marine Antarctic and Subantarctic sediment sequences. We present a comprehensive data set of 14 C ages obtained on bulk organic carbon, carbonates, and on fatty acids (FA) from three coastal marine sediment cores from Subantarctic South Georgia and East Antarctica. Low molecular weight (LMW) FA represent the least 14 C-depleted fraction, indicating that the phytoplankton-derived compounds can be a means of dating sediments. In contrast, vascular plant-derived high molecular weight FA are systematically depleted in 14 C relative to the low molecular weight homologues, reflecting processes such as soil formation/erosion in the catchment. Comparative age-depth models show significant differences, depending on the material used for the respective models. While the land plant-derived FA may lead to an overestimation of the actual sediment age, LMW FA reveal complex aquatic reservoir effects. Bulk sedimentary organic carbon 14 C ages likely provide appropriate age estimates in settings with low petrogenic carbon input in the Antarctic, whereas CSRA has the potential to produce improved age control in settings with high contributions of petrogenic carbon.

DO RADIOCARBON AGES OF PLANT WAX BIOMARKERS AGREE WITH 14C-TOC/OSL-BASED AGE MODELS IN AN ARID HIGH-ALTITUDE LAKE SYSTEM?

“…A proxy group which is especially prone to such lag times is terrestrial organic biomarkers, e.g., long-chain n-alkanes derived from leafs and roots of terrestrial plants, which are frequently used as hydroclimatic indicators (Eglinton and Hamilton 1967;Castañeda and Schouten 2011). This is mainly because of significant residence time in soils due to high recalcitrance of compounds, their transport via rivers, as well as mixing processes, resuspension, and redistribution of material from floodplains from which it was reworked after a long accumulation time (Pearson and Eglinton 2000;Ohkouchi et al 2003;Smittenberg et al 2004;Mollenhauer et al 2005;Mollenhauer and Eglinton 2007;Eglinton and Eglinton 2008;Vonk et al 2014;Winterfeld et al 2018;Berg et al 2020;Bliedtner et al 2020).…”

Increased petrogenic and biospheric organic carbon burial in sub‐Antarctic fjord sediments in response to recent glacier retreat

“…2). A more depleted Δ 14 C value, thus, absence of bomb-14 C, for the lower limit of the terrigenous endmember is supported by a Δ 14 C value of À116‰ determined for C 26:0 fatty acid from a surface sediment sample in Little Jason Lagoon (Co1305), which confirms the mixed composition of terrigenous OM (Berg et al 2020) and is within the range of Δ 14 C values found in surface soils (Shi et al 2020). For the mixing models we, thus, use a Δ 14 C terr of À16 AE 138‰.…”

“…3). Yet, a predominantly aquatic origin of low‐molecular‐weight fatty acids in the marine sediments of Cumberland Bay and Little Jason Lagoon is supported by their compound‐specific 14 C ages in comparison to those of the concurrent high‐molecular‐weight fatty acids (Berg et al 2020). However, we cannot exclude low‐molecular‐weight fatty acid contributions from plants/peat a priori.…”

“…For the marine endmember (Δ 14 C marine ), we assume a Δ 14 C range from −85‰ to −133‰ (corresponding to a reservoir age of 645–1100 yr). A higher value of −85‰ is derived from C 16:0 fatty acid Δ 14 C isotopic composition obtained from surface sediments at site PS81/283 in Cumberland West Bay, which likely reflects the marine reservoir age in the inner fjord (Berg et al 2020). A lower Δ 14 C value is derived from benthic foraminifers from surface sediments in the fjord and on the shelf of South Georgia (Graham et al 2017; Berg et al 2020).…”

“…Based on our results, we show that OC terr is a minor contributor to the fjord sediments at present. A 14 C study on land plant derived fatty acids indicates that the terrigenous reservoir in Cumberland West Bay was characterized by centennial to millennial OC storage throughout the Holocene, indicating that the abundance of "old" (pre-Holocene) terrigenous OC is relatively minor (Berg et al 2020). Hence, re-exposed old OC terr is likely not a prominent source of OC during on-going glacier retreat.…”