The Reservoir Age Effect Varies With the Mobilization of Pre-Aged Organic Carbon in a High-Altitude Central Asian Catchment

Abstract

Lake sediments provide excellent archives to study past environmental and hydrological changes at high temporal resolution. However, their utility is often restricted by chronological uncertainties due to the “reservoir age effect” (RAE), a phenomenon that results in anomalously old radiocarbon ages of total organic carbon (TOC) samples that is mainly attributed to the contribution of pre-aged carbon from aquatic organisms. Although the RAE is a well-known problem especially in high altitude lakes, detailed studies analyzing the temporal variations in the contribution of terrestrial and aquatic organic carbon (OC) on the RAE are scarce. This is partially due to the complexity of isolating individual compounds for subsequent compound-specific radiocarbon analysis (CSRA). We developed a rapid method for isolating individual short-chain (C16 and C18) and long-chain (>C24) saturated fatty acid methyl esters (FAMEs) by using high-pressure liquid chromatography (HPLC). Our method introduces only minor contaminations (0.50 ± 0.22 µg dead carbon on average) and requires only few injections (≤10), therefore offering clear advantages over traditional preparative gas chromatography (prep-GC). Here we show that radiocarbon values (Δ14C) of long-chain FAs, which originate from terrestrial higher plant waxes, reflect carbon from a substantially pre-aged OC reservoir, whereas the Δ14C of short-chain FAs that originate from aquatic sources were generally less pre-aged. 14C ages obtained from the long-chain FAs are in closer agreement with 14C ages of the corresponding bulk TOC fraction, indicating a high control of pre-aged terrestrial OC input from the catchment on TOC-derived 14C ages. Variations in the age offset between terrestrial and aquatic biomarkers are related to changes in bulk sediment log(Ti/K) that reflect variations in detrital input from the catchment. Our results indicate that the chronological offset between terrestrial and aquatic OC in this high-altitude catchment is mainly driven by temporal variations in the mobilization of pre-aged OC from the catchment. In conclusion, to obtain accurate and process-specific lake sediment chronologies, attention must be given to the temporal dynamics of the RAE. Variations in the apparent ages of aquatic and terrestrial contributions to the sediment and their mass balance can substantially alter the reservoir age effect.

The East Gotland Basin (Baltic Sea) as a candidate Global boundary Stratotype Section and Point for the Anthropocene series

“…This delay can be attributed to atmosphere-ocean 14 C equilibration time (Waters et al, 2019). Also, post-depositional, vertical migration within sediments, sediment remobilization, as well as sediment inwash from the drainage basin are typical potential issues affecting radionuclide profiles (Davis et al, 1984;He et al, 1996;Owens and Walling, 1996;Mattila et al, 2006;Ilus et al, 2007;Hancock et al, 2014;Waters et al, 2019;Haltia et al, 2021;Schroeter et al, 2021).…”

“…Between 1870 and 1950 CE, older-than-expected 14 C ages (1892 ± 80 year BP; n = 7) were obtained (Supplemental Table S2). This indicates an admixture of relatively older organic matter most likely due to the inwash of pre-aged terrestrial organic matter from the drainage basin (Howarth et al, 2013; Schroeter et al, 2021), or sediment reworking (Moros et al, 2020). The 206/207 Pb and ΣPAH profiles from core EMB201/7-4 (Figure 2b and d) support the absence of sediment reworking as they are comparable to existing records from lakes within the Baltic Sea drainage basin (Renberg et al, 2001, 2002).…”

“…This delay can be attributed to atmosphere-ocean 14 C equilibration time (Waters et al, 2019). Also, post-depositional, vertical migration within sediments, sediment remobilization, as well as sediment inwash from the drainage basin are typical potential issues affecting radionuclide profiles (Davis et al, 1984; Haltia et al, 2021; Hancock et al, 2014; He et al, 1996; Ilus et al, 2007; Mattila et al, 2006; Owens and Walling, 1996; Schroeter et al, 2021; Waters et al, 2019). These issues likely explain the detection of 137 Cs since 1917 ± 6 CE (33.3 cm depth) because it is very mobile in sediments compared to 241 Am, as well as the detection of radionuclides in the sediments until the present-day.…”

Soil Erosion Caused the Increasing Holocene Radiocarbon Reservoir Effect of Lake Kanas in the Altai Mountains

“…They include crossdating approaches with 14 C dating and other dating methods (e.g., 210 Pb and 137 Cs) to evaluate the lake reservoir effect (e.g., Lan et al 2018;Xu et al 2021); optical luminescence dating (e.g. Wilkins et al 2012;An et al 2018); U-series dating (e.g., Hall et al 2001;Fan et al 2014); counting annual laminations (e.g., Tlan et al 2005;Zhou et al 2009;Bonk et al 2015;Zhang et al 2021); and comparative AMS 14 C dating of different materials from the same sedimentary layer to assess the reservoir effect, such as fatty acids (Schroeter et al 2021), TOC, and terrestrial plant remains (e.g., Huang et al 2021a). Although AMS 14 C dating of terrestrial plant material is a reliable means of quantifying lake sediment ages, terrestrial plant remains are usually scarce in lake sediments in arid central Asia and hence the method is rarely applied.…”