First dataset of dissolved inorganic radiocarbon in the Tokara Strait

Abstract

Radiocarbon (¹⁴C) has been widely used to understand the ages in archeology and paleo-environmental sciences. In marine environments, the dissolved inorganic radiocarbon (expressed as DIC Δ¹⁴C) of seawater has been used as a reliable tracer in the research of carbon cycling and studies in global to regional water mixing. Here, we present the first high-resolution dataset of DIC Δ¹⁴C values in the Tokara Strait collected at eight stations during a cruise on the Research Vessel (R/V) Hakuho-Maru in March 2022. The DIC Δ¹⁴C ranges from –211‰ to 28‰ in the upper 1200 m depth of the Tokara Strait. High and modern values (bomb ¹⁴C; DIC Δ¹⁴C ≥ 0‰) were observed above ~400 m depth at station T3 and above ~200 m depth at other stations. These are indicative of the influence of anthropogenic carbon from above-ground nuclear bomb tests conducted in the Pacific Ocean during the 1940s to 1960s. The dataset also includes hydrographic information (temperature, salinity, and density), all of which may help interpret the DIC Δ¹⁴C variations in the Tokara Strait. These datasets are expected to improve our understanding of water mixing processes and the carbon cycle in the Tokara Strait, which has important implications for understanding climate variability.

Introduction

The oceans play an important role in mitigating global warming by absorbing about 30% of total anthropogenic CO₂ emissions through air-sea exchange (Gruber et al., 2009). This affects the distribution of the effects of global warming across different geographical regions (Manabe and Stouffer, 2007). Understanding of upper ocean dynamics, including variations in ocean currents and hydrographic properties, is crucial to accurately predict the extent of global warming for the coming decades (Broecker et al., 1995; Druffel et al., 2008). The Kuroshio is a major western boundary current in the North Pacific that transports warm equatorial water poleward. It exerts a significant influence on both ocean circulation and ecosystems within the North Pacific Ocean (NPO), and on the climate of East Asia due to its warm and saline characteristics (Saito, 2019). The Tokara Strait, situated at approximately 30°N and 130°E, is located between the islands of Amami Oshima and Tanegashima, south of Kyushu, Japan, and has a northeast–southwest orientation (Fig. 1). It serves as the primary conduit for the Kuroshio to return from the East China Sea to the NPO. Due to rough bathymetry in the Tokara Strait, turbulent mixing occurs around the shallow seamounts and islands positioned along the route of the Kuroshio in the Tokara Strait (Nagai et al., 2017; Tsutsumi et al., 2017), which influences Japan’s climate and shapes distribution and diversity patterns of biota (Kawai et al., 2013; Hasegawa et al., 2021). In addition, the ongoing Kuroshio Large-Meander (LM), in which the Kuroshio takes an offshore detour and reaches 30°N at its southernmost latitude, began in August 2017 and has had a significant impact on Japan’s climate and fisheries (Qiu et al., 2023). Changes in the position of the Kuroshio in the Tokara Strait are considered crucial for understanding variations in the Kuroshio meander south of Japan (Feng et al., 2000; Kawabe, 2005). Therefore, understanding water mixing processes, local variations in ocean currents, and hydrographic properties in the Tokara Strait is essential for assessing the regional impacts of Kuroshio path variations associated with regional climate change.

Fig. 1.

Map showing locations of water samples (blue circles) collected from the Tokara Strait in March 2022

Dissolved inorganic carbon (DIC) is the largest pool of carbon in the ocean. It constitutes around 95% of total carbon stored in the ocean (38,000 Pg C; Key et al., 2004). The radiocarbon content of DIC (expressed as DIC Δ¹⁴C) in seawater is recognized as an important tracer in oceanographic studies (Levin and Hesshaimer, 2000; Sweeney et al., 2007; Hirabayashi et al., 2017; Kubota et al., 2022). Following the above-ground nuclear bomb tests in the Pacific Ocean during the 1940s–1960s, a substantial amount of bomb-produced ¹⁴C entered the ocean from the atmosphere through air-sea exchange in surface waters and gradually permeated deeper through water advection and diffusion (Hua et al., 2013, 2022; Yokoyama et al., 2022a, 2022b). The characteristics of the DIC Δ¹⁴C distribution underline its importance for the study of oceanic mixing processes and anthropogenic carbon effects caused by the above-ground nuclear bomb tests. Several global ocean radiocarbon studies, such as the Geochemical Ocean Section Study (GEOSECS), the World Ocean Circulation Experiment (WOCE), and the Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP), include DIC Δ¹⁴C as a key parameter. However, to the best of our knowledge, no records of DIC Δ¹⁴C data are available for the Tokara Strait, as observations of DIC Δ¹⁴C in global surveys within the Kuroshio region are primarily focused along fixed transects in the Kuroshio Extension area, such as the P10 line (~147°E). Here, we present the first high-resolution DIC Δ¹⁴C measurements in the Tokara Strait, which are valuable for studying the local water mixing processes occurring in the Tokara Strait and assessing the impact of anthropogenic carbon in this region.

Methods

Sample collection

The water samples were collected at 8 stations of the Tokara Strait during a cruise on the R/V Hakuho-Maru in March 2022 (Fig. 1). To determine the radiocarbon content of DIC in the seawater, the seawater samples were taken by the CTD (Conductivity, Temperature, and Depth sensors) rosette sampler with 24-Niskin bottles (12-liter type). These seawater samples were collected at 11 different depths, ranging from 10 m to a maximum depth of 1200 m (Table 1). At Station T3, the deepest site in this study which reaches the bottom of the Tokara Strait, seawater samples were collected throughout the entire depth. However, at other stations with shallower depths, the maximum sampling depth did not exceed 400 m. At each station, the hydrographic characteristics (temperature, salinity, and density) were determined using a Sea-bird SBE-9 plus CTD. All the DIC Δ¹⁴C and hydrographic data are summarized in Table 1.

Table 1.

Summary description of the sampling stations, hydrographic data (temperature, salinity, and density), and DIC Δ¹⁴C values for water collected at the Tokara Strait

Lab. No.	Date [yyyy/m/d]	Stn.	Latitude [°N]	Longitude [°E]	Bot.Depth [m]	Sampling depth [m]	Temperature [°C]	Salinity	σθ [kg/m3]	Δ14C [‰]
YAUT-072916	2022/3/9	T1	28.526	129.774	88	10	20.15	34.73	24.52	23.2 ± 2.9
YAUT-072917	2022/3/9	T1	28.526	129.774	88	50	20.01	34.73	24.56	25.1 ± 2.2
YAUT-072918	2022/3/9	T2	28.867	129.893	302	10	20.24	34.71	24.48	24.9 ± 1.9
YAUT-072919	2022/3/9	T2	28.867	129.893	302	50	20.23	34.71	24.48	21.4 ± 1.8
YAUT-072923	2022/3/9	T2	28.867	129.893	302	100	20.07	34.73	24.54	26.6 ± 2.7
YAUT-072924	2022/3/9	T2	28.867	129.893	302	150	19.48	34.76	24.72	25.0 ± 2.1
YAUT-073902	2022/3/9	T2	28.867	129.893	302	200	18.08	34.73	25.06	25.0 ± 6.1
YAUT-073903	2022/3/9	T2	28.867	129.893	302	290	16.53	34.68	25.39	22.4 ± 5.2
YAUT-073915	2022/3/9	T3	29.164	130.018	1402	10	20.73	34.64	24.29	25.6 ± 6.3
YAUT-073916	2022/3/9	T3	29.164	130.018	1402	50	20.67	34.63	24.31	16.3 ± 4.7
YAUT-073917	2022/3/9	T3	29.164	130.018	1402	100	20.37	34.69	24.44	21.1 ± 3.6
YAUT-073918	2022/3/9	T3	29.164	130.018	1402	150	20.35	34.72	24.46	25.5 ± 3.9
YAUT-073919	2022/3/9	T3	29.164	130.018	1402	200	20.24	34.74	24.51	28.0 ± 2.8
YAUT-073923	2022/3/9	T3	29.164	130.018	1402	300	17.91	34.73	25.10	28.4 ± 2.7
YAUT-073924	2022/3/9	T3	29.164	130.018	1402	400	13.50	34.48	25.90	24.1 ± 2.8
YAUT-073925	2022/3/9	T3	29.164	130.018	1402	600	8.71	34.32	26.64	–56.5 ± 2.6
YAUT-073926	2022/3/9	T3	29.164	130.018	1402	800	4.68	34.37	27.22	–159.7 ± 4.6
YAUT-073928	2022/3/9	T3	29.164	130.018	1402	1000	3.50	34.42	27.38	–194.0 ± 3.5
YAUT-073929	2022/3/9	T3	29.164	130.018	1402	1200	3.01	34.47	27.47	–211.1 ± 3.4
YAUT-073904	2022/3/9	T4	29.494	130.147	409	10	21.62	34.58	24.01	14.2 ± 4.1
YAUT-073905	2022/3/9	T4	29.494	130.147	409	50	21.62	34.58	24.01	19.1 ± 9.9
YAUT-073906	2022/3/9	T4	29.494	130.147	409	100	21.61	34.58	24.01	14.2 ± 5.1
YAUT-073909	2022/3/9	T4	29.494	130.147	409	150	21.27	34.62	24.14	20.4 ± 3.7
YAUT-073911	2022/3/9	T4	29.494	130.147	409	200	19.73	34.74	24.64	21.4 ± 4.8
YAUT-073912	2022/3/9	T4	29.494	130.147	409	300	14.96	34.57	25.66	12.5 ± 5.7
YAUT-073913	2022/3/9	T4	29.494	130.147	409	400	13.48	34.48	25.91	1.1 ± 6.5
YAUT-073931	2022/3/10	T5	29.834	130.288	223	10	21.67	34.58	23.99	21.6 ± 3.9
YAUT-073932	2022/3/10	T5	29.834	130.288	223	50	21.57	34.59	24.03	17.5 ± 4.4
YAUT-073933	2022/3/10	T5	29.834	130.288	223	100	20.72	34.65	24.31	21.5 ± 4.0
YAUT-073936	2022/3/10	T5	29.834	130.288	223	150	20.37	34.67	24.42	25.0 ± 4.1
YAUT-073937	2022/3/10	T5	29.834	130.288	223	200	18.19	34.68	24.99	22.9 ± 3.6
YAUT-073938	2022/3/10	T6	30.158	130.420	246	10	21.33	34.57	24.08	15.4 ± 2.9
YAUT-073939	2022/3/10	T6	30.158	130.420	246	50	20.47	34.59	24.32	20.1 ± 3.9
YAUT-074102	2022/3/10	T6	30.158	130.420	246	100	18.58	34.61	24.83	20.2 ± 2.5
YAUT-074103	2022/3/10	T6	30.158	130.420	246	150	17.95	34.61	24.99	24.4 ± 3.2
YAUT-074104	2022/3/10	T6	30.158	130.420	246	200	17.82	34.61	25.03	14.0 ± 2.8
YAUT-074105	2022/3/10	P1	30.586	130.662	177	10	18.60	34.63	24.84	19.3 ± 3.3
YAUT-074106	2022/3/10	P1	30.586	130.662	177	50	18.50	34.63	24.87	24.0 ± 2.9
YAUT-074109	2022/3/10	P1	30.586	130.662	177	100	18.31	34.62	24.91	21.5 ± 2.8
YAUT-074111	2022/3/10	P1	30.586	130.662	177	150	16.96	34.60	25.22	20.8 ± 3.6
YAUT-074112	2022/3/10	P2	30.833	130.669	177	10	18.54	34.63	24.86	25.5 ± 2.9
YAUT-074113	2022/3/10	P2	30.833	130.669	177	50	18.40	34.63	24.89	22.4 ± 3.5
YAUT-074115	2022/3/10	P2	30.833	130.669	177	100	16.91	34.60	25.23	19.0 ± 3.0
YAUT-074116	2022/3/10	P2	30.833	130.669	177	150	15.90	34.56	25.44	13.4 ± 2.6

The water samples for DIC Δ¹⁴C analysis were slowly transferred into 250 mL glass bottles from the Niskin bottles. After a three-fold volume overflow to prevent air contamination, the samples were poisoned with 50 μL of a saturated mercuric (II) chloride (HgCl₂) solution for preservation. Subsequently, the glass bottles were capped tightly with a grease-coated, ground-glass stopper and sealed with parafilm to prevent the exchange of CO₂ with the atmosphere. All DIC Δ¹⁴C samples were stored in the dark at room temperature until analysis. The maximum storage period until analysis was 2 months.

The ¹⁴C Analysis

Extraction of CO₂ gas and preparation of graphitization for all samples were carried out at the Atmosphere and Ocean Research Institute (AORI), the University of Tokyo. The water samples were first processed using the bubbling method (McNichol et al., 1994; Servettaz et al., 2019). 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. Subsequently, the evolved CO₂ is trapped by a liquid nitrogen trap at –196°C and quantified at the nanometric level. Finally, the purified CO₂ samples are reduced to graphite using iron powder and pure hydrogen at 630°C for 6 hours, following the method described by Yokoyama et al. (2007, 2010). The DIC Δ¹⁴C values of these samples were then analyzed using a single stage Accelerator Mass Spectrometer (YS-AMS: National Electrostatic Corporation, SAMS) at the AORI, University of Tokyo (Yamane et al., 2019; Yokoyama et al., 2019, 2022b). ¹²C and ¹³C are measured as well to correct for isotope fractionation effects. The ¹⁴C/¹²C measurement precision on standard material is better than 0.1% (Yokoyama et al., 2022b).

The ¹⁴C measurements can effectively date carbon that originated up to ~50,000 years before the present (BP; years before CE 1950). ¹⁴C content in units of percent modern carbon (pMC) obtained by the abc program (version 7.0) from NEC is shown in equation 1 (Stenström et al., 2011):

  

$${}^{14}\mathrm{C} content \left(pMC\right)=\frac{{\left(\frac{{}^{14}C}{{}^{12}C}\right)}_{SN}}{{\left(\frac{{}^{14}C}{{}^{12}C}\right)}_{ON}}\times 100\%$$(1)

where (¹⁴C/¹²C)_SN and (¹⁴C/¹²C)_ON are the ¹⁴C/¹²C ratios of samples and standards measured by the AMS and are normalized by the AMS δ¹³C value (see equation 2 for δ¹³C notation) measured simultaneously. The AMS δ¹³C value for the sample was normalized to a value of –25‰ (VPDB: Vienna Pee Dee Belemnite), which is the δ¹³C value of the wood (Stenström et al., 2011), to remove the influence of isotopic fractionation during natural processes such as photosynthesis, assimilation, and respiration. This normalized ¹⁴C value is then compared to 95% of the ¹⁴C/¹²C ratio of reference standards (OxI, NBS: When other standards are used, e.g., NIST oxalic acid SRM4990C, the specific activity values are used to correct for OxI equivalent values) that are also measured by AMS and are corrected to their δ¹³C values (Yokoyama et al., 2019). The δ¹³C values are expressed relative to the international standard VPDB according to equation 2:

  

$$\mathrm{\delta }{}^{13}\mathrm{C}=[\frac{{\left(\frac{{}^{13}C}{{}^{12}C}\right)}_{sample}}{{\left(\frac{{}^{13}C}{{}^{12}C}\right)}_{standard}}-1]\times 1000‰$$(2)

The term “measured fraction modern”, denoted as F_m, was introduced by Donahue et al. (1990) as shown in equation 3:

  

$$F_m=\frac{pMC}{100\%}$$(3)

In this study, results of the ¹⁴C analyses are reported as Δ¹⁴C, expressed in per mille (‰), as shown in equation 4 (Stuiver and Polach, 1977; Stenström et al., 2011):

  

$$∆{}^{14}\mathrm{C}=({F_{m}e}^{\left(\right(1950-x)/8267)}-1)\times 1000\mathrm{‰}$$(4)

where x is the year of the sample measurement.

Errors for ¹⁴C analyses are given as 1σ standard deviation of at least 10 repeated measurements on the same sample. Errors in Δ¹⁴C measurements based on repeated analyses of the same seawater sample (n = 6) were ±3.8‰ (1σ).

Results

Vertical profiles and sectional distributions of water temperature, salinity, density, and DIC Δ¹⁴C values measured at eight sampling stations in the Tokara Strait are shown in Fig. 2. The T-S diagram of all stations in the Tokara Strait is plotted in Fig. 3. Thermocline and halocline of stations at the Tokara Strait fluctuates significantly from station to station, which indicates the highly dynamic ocean condition in the Tokara Strait (Fig. 2a–d). Seawater of stations T6, P1, and P2 were characterized by lower temperature and lower salinity (Fig. 3). This could be due to the influence of terrestrial water due to the shallow water depth and/or marked differences in surface temperature and salinity between the north and south sides of the Kuroshio axis, as the Kuroshio transports warm, saline water from the tropical Pacific Ocean. The DIC Δ¹⁴C values ranged from –211‰ to 28‰ from surface to 1200 m depth in the Tokara Strait (Fig. 2g and h). This wide range of DIC Δ¹⁴C values enables us to observe the mixing behaviors of seawater. For instance, a lower DIC Δ¹⁴C value (1‰) was found at a depth of 400 m at station T4, compared to the value (24‰) at station T3. This suggests the influence of upwelling deeper waters, possibly related to the bottom topography at station T4. High and modern values (bomb ¹⁴C; DIC Δ¹⁴C ≥ 0‰) were observed above ~400 m depth at station T3 and above 200 m depth at other stations. This is mainly caused by the bomb Δ¹⁴C signal in the atmosphere being mixed into the upper ocean through air-sea exchange. Thus, the DIC Δ¹⁴C distribution in the Tokara Strait presented here shows great significance for studying water mixing processes and anthropogenic carbon effects. Further detailed discussions of DIC Δ¹⁴C variations in the Tokara Strait as well as the whole region of the western NPO will be presented in a future paper.

Fig. 2.

Depth profiles and sectional distributions of water temperature (a, b), salinity (c, d), density (e, f), and DIC Δ¹⁴C values (g, h) for sampling stations in the Tokara Strait in March 2022. The letters below the figures (b), (d), (f), and (h) show the sampling stations.

Fig. 3.

The potential temperature vs. salinity (T-S) diagram for sampling stations in the Tokara Strait in March 2022. Isoclines (σ₀) are included in the figures. The dashed lines correspond to the CTD data at each station. The colors of the circles indicate the DIC Δ¹⁴C values for water samples.

Acknowledgments

We thank the crew of the R/V Hakuho-Maru (KH-22-5) for their cooperation and help on board. We are grateful to Dr. T. Aze and Dr. C. Sawada (AORI) for their support on AMS measurements. This work was supported by JSPS KAKENHI (20H00193; 23KK0013; 23H02541) and UTokyo FSI project (Implementation of the Subtropical Kuroshio Research and Education Center for Evaluating Japanese Environmental Changes and Ecosystems in Anthropocene). H. Lan acknowledges the financial support from China Scholarship Council (No. 202108050131).

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