Lake water dissolved inorganic carbon dynamics revealed from monthly measurements of radiocarbon in the Fuji Five Lakes, Japan

Abstract

Lakes are sensitive recorders of anthropogenic activities, as human society often develops in their vicinity. Lake sediments thus have been widely used to reconstruct the history of environmental changes in the past, anthropogenic, or otherwise, and radiocarbon dating provides chronological control of the samples. However, specific values of radiocarbon in different carbon reservoirs due to the different pathways of radiocarbon from the upper atmosphere to the lake, called the radiocarbon reservoir age, is always difficult to evaluate because of dynamic processes in and around lakes. There are few systematic studies on radiocarbon reservoir ages for lakes owing to the complex radiocarbon transfer processes for lakes. Here, we investigate lake waters of the Fuji Five Lakes with monthly monitoring of the radiocarbon reservoir effects. Radiocarbon from dissolved inorganic carbon (DIC) for groundwater and river water is also measured, with resulting concentrations (Δ14C) at their lowest at Lake Kawaguchi in August 2018 (–122.4 ± 3.2‰), and at their highest at Lake Motosu in January 2019 (–22.4 ± 2.5‰), despite a distance of 25 km. However, winter values in both lakes show similar trends of rising Δ14C (about 20‰). Our lake water DIC Δ14C results are compared to previously published records obtained from sediments in Lake Motosu and Lake Kawaguchi. These suggest that total organic carbon and compound-specific radiocarbon found in sediments are heavily influenced by summer blooms of aquatic organisms that fix DIC in water. Thus, future studies to conduct similar analyses at the various lakes would be able to provide further insights into the carbon cycle around inland water, namely understanding the nature of radiocarbon reservoir ages.

Mixing Dynamics Within the Kuroshio Area Are Reflected in Dissolved Inorganic Radiocarbon Values

“…The DIC in the seawater samples was extracted as CO 2 using a water sample preparation line, as described by Ota et al. (2021) and Servettaz et al. (2019).…”

“…Extraction of CO 2 gas and preparation of graphitization for all samples was carried out at the Atmosphere and Ocean Research Institute (AORI), the University of Tokyo. The DIC in the seawater samples was extracted as CO 2 using a water sample preparation line, as described by Ota et al (2021) and Servettaz et al (2019). In brief, each seawater sample was acidified with 4 mL of 85% phosphoric acid to convert all forms of DIC into CO 2 .…”

First dataset of dissolved inorganic radiocarbon in the Tokara Strait

“…The bubbling device is assembled under an Argon (Ar) atmosphere. Then, by using a vacuum line system for water samples, 2 mL of phosphoric acid is added to seawater and bubbling is then carried out for 15 minutes in a glass tube filled with 60 kPa of helium (He) gas to remove the CO2 liberated from DIC (Ota et al, 2021). During the 15-minute of degassing processes under the He gas atmosphere, water vapor is initially removed by two cryogenic traps containing cooled ethanol at -120°C.…”