Database for time-series stations K2 and S1

These data consist of the radioactivities of ²¹⁰Po and ²¹⁰Pb and concentrations of POC and PON obtained from seawater samples collected with Niskin bottles attached to CTD sensors and from in situ pumping systems at stations K2 and S1. Data from both stations are arranged as one MS Excel file.

Global warming resulting from the increase in concentrations of greenhouse gases such as carbon dioxide is of great concern to the world community. In the last few decades, the carbon cycle in the ocean has been studied to clarify the exchange of carbon dioxide between the atmosphere and the ocean. One important issue is the quantification of the role played by the biological pump: how much atmospheric CO₂ is assimilated in the sunlit layer (euphotic zone) and how much carbon is exported to the deep ocean.

Since October 2002, we have made time-series observations at station K2 in the northwestern North Pacific Ocean. High fluxes associated with the biological pump have been estimated from sediment trap data (Honda et al., 2006 and 2009) and the seasonal variability of nutrient concentrations (Honda and Watanabe, 2007; Kawakami et al., 2007). Kawakami and Honda (2007) reported that seasonal variation in the flux of particulate organic carbon (POC) as estimated from ²³⁴Th was large (54–179 gC m^–2 d^–1), and annual POC fluxes were estimated to be 31 gC m^–2 y^–1 at station K2. The short-lived radionuclide ²³⁴Th (half-life, 24.1 days) serves as a valuable tracer for studying the rates of particle-associated scavenging and subsequent particle export from the euphotic zone (Coale and Bruland, 1985; Buesseler, 1998). However, because ²³⁴Th becomes the radioactive equivalent of ²³⁸U at a depth of approximately 100 m, ²³⁴Th analysis is not suitable for estimating particle fluxes in the mesopelagic and bathypelagic zones of the ocean. ²¹⁰Po–²¹⁰Pb, which are not at equilibrium at intermediate depths, are used as tracers to estimate particle fluxes in the mesopelagic and bathypelagic zones (Shimmield et al., 1995; Kawakami et al., 2009; Stewart et al., 2010). In particular, the ²¹⁰Po–²¹⁰Pb analysis, which requires only about 10 L of seawater, is relatively easy to conduct.

In addition to our observations at station K2, we started time-series observations at station S1 in the subtropical western North Pacific in January 2010. In this dataset, we present vertical profiles of ²¹⁰Po, ²¹⁰Pb, POC, and particulate organic nitrogen (PON) above 1000 m depth at stations K2 and S1. These data will help further understanding of particle dynamics within the epipelagic and mesopelagic ocean.

Samples were collected at stations K2 and S1 during the cruises of R/V Mirai beginning in October 2008 and October 2010, respectively. The cruises were MR08-05 (Oct–Nov 2008), MR10-06 (Oct–Nov 2010), MR11-02 (Feb–Mar 2011), MR11-03 (Apr–May 2011), MR11-05 (Jun–Aug 2011) , and MR12-02 (Jun–Jul 2012) .

Two aliquots of seawater samples (10 L each) were used for determination of the radioactivities of ²¹⁰Po and ²¹⁰Pb. The samples were collected from 16 depths between 10 and 1000 m by Niskin bottle samplers attached to CTD sensors (SBE 911plus Sea-Bird Electronics Inc.) that record salinity, temperature, and depth. In only 2012, seawater samples (10 L each) were collected from 16 depths between 10 and 4000 m. The seawater samples for dissolved ²¹⁰Po measurement were filtered through polypropylene cartridge filters with a pore size of 0.8µm. Samples were filtered on board the research vessel immediately after collection. After filtration, water samples were acidified to pH 1 by the addition of concentrated HCl solution, and 100 mg of Fe (as FeCl₃) was added. After the samples were shaken and allowed to stand for about half a day, ammonium was added to the water samples to precipitate iron. The co-precipitated ²¹⁰Po and iron in the water samples were collected by using the centrifugal technique. Particulate samples for the analysis of ²¹⁰Po, POC, and PON content were taken from the same depths as the seawater samples by using an in situ pumping system (Large Volume Water Transfer System WTS-6-1-142LV04, McLane Inc.). At each depth, 200–1000 L of seawater was filtered through a pre-combusted (450^oC, 4h) glass-fiber filter with a nominal pore size of 0.7µm.

For POC and PON measurements, a stainless-steel cork borer was used to cut out subsamples with a diameter of 21 mm from the filter samples collected by in situ pumping. To measure particulate ²¹⁰Po, the rest of the sample was digested with a mixture of concentrated HCl and HNO₃ (3:1, v/v). The polonium separated from the seawater and particles was dissolved in approximately 50 mL of 1 M HCl, and 1–2 g of ascorbic acid were added for reduction of iron. The solution, including polonium, was heated to 70–90 °C, and the polonium was adsorbed from solution onto a silver plate for 3 h. The plates were alpha-counted with silicon surface barrier detectors (Octéte, Seiko EG＆G Co. Ltd.) to determine ²¹⁰Po activity.

For ²¹⁰Pb measurements, the same procedure was applied to the acidified water samples 18 months later, when ²¹⁰Po has come to radioactive equilibrium with ²¹⁰Pb. The analytical precisions (1σ) for dissolved ²¹⁰Po, particulate ²¹⁰Po, and ²¹⁰Pb determinations were about 4%, 3%, and 5%, respectively. ²²⁶Ra radioactivity was calculated from silicate concentration (²²⁶Ra dpm L^–1 = 0.0012 × silicate [µmol kg^–1] + 0.084; Ku et al., 1980).

Samples for POC and PON analysis were stored in a freezer until analysis. POC and PON were measured with an elemental analyzer (Model 2400II, Perkin Elmer Inc.). Before measurement, the samples were treated with concentrated HCl vapor for 24 h to remove calcium carbonate and dried at 50^oC for 3 h. The standard deviation of the POC and PON measurements was usually less than 3% of the sample mean values.

Data obtained from cruises were electronically compiled in an MS Excel file (“210Po_data_v2.xlsx”). The data include depth, potential temperature (Theta), salinity, potential density (Sigma-theta), POC, PON, particulate ²¹⁰Po (P-210Po) activity, dissolved ²¹⁰Po (D-210Po) activity, total ²¹⁰Pb (T-210Po) activity, and ²²⁶Ra activity. The measurements of depth, potential temperature, salinity, and potential density were made with CTD sensors. The error in particulate ²¹⁰Po, dissolved ²¹⁰Po, and total ²¹⁰Pb measurements was estimated from the counting error.

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