Surface water mixing estimated from 228Ra and 226Ra in the northwestern North Pacific

Surface distributions of 228Ra in seas and oceans globally: Implications for water circulation and contaminant transport

Seawater movements are challenging to track and monitor for the transport of various soluble materials. Since the 1960s, naturally occurring radium isotopes (Ra), particularly 228Ra, with a half-life of 5.75 y, have provided unique insights into oceanic seawater circulations within a 30 y timescale. Since the 1980s, especially in the 2000s, frequent research expeditions and improvements in analytical techniques have enabled the determination of fine-scale lateral variations in 228Ra/226Ra ratio and 228Ra concentration. These results describe ocean-, sea-, and basin-scale seawater circulations and current mixing, including seasonal variations. Additionally, the source areas of Ra in seawater (i.e., coastal and shallow shelf areas) often overlap with areas containing contaminants released by human activities. Notably, the surface current systems inferred from the distribution of 228Ra closely explained the transport patterns of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant. Databases of 228Ra/226Ra ratios and 228Ra concentrations have the potential to predict flow pathways and timescales for various soluble contaminants in ocean and sea environments.

Migration of the FDNPP-derived 134Cs and 137Cs along with 226Ra and 228Ra concentrations across the northwestern North Pacific Ocean

We examined lateral distributions of ¹³⁴Cs, ¹³⁷Cs, ²²⁶Ra, and ²²⁸Ra in the surface seawaters around the Kuril Islands and the Kamchatka Peninsula in the northwestern North Pacific Ocean during June 2014. The sampling area included three water current areas, the Oyashio Current, the current from the Okhotsk Sea, and the coastal current along the east Kamchatka Peninsula. ²²⁶Ra and ²²⁸Ra distributions differed along the three currents. Low levels of ¹³⁴Cs were detected in the surface waters of the Oyashio Current (0.09-0.35 mBq/L), but it was <∼0.1 mBq/L at the surface along the other two currents. This indicates that the distribution of Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived radiocesium in surface waters off the Kamchatka and along the Kuril Islands is predominantly governed by the Oyashio current system.

Submarine groundwater discharge and nutrient loadings in Tolo Harbor, Hong Kong using multiple geotracer-based models, and their implications of red tide outbreaks

Multiple tracers, including radium quartet, (222)Rn and silica are used to quantify submarine groundwater discharge (SGD) into Tolo Harbor, Hong Kong in 2005 and 2011. Five geotracer models based on the end member model of (228)Ra and salinity and mass balance models of (226)Ra, (228)Ra, (222)Rn, and silica were established and all the models lead to an estimate of the SGD rate of the same order of magnitude. In 2005 and 2011, respectively, the averaged SGD based on these models is estimated to be ≈ 5.42 cm d(-1) and ≈2.66 cm d(-1), the SGD derived DIN loadings to be 3.5 × 10(5) mol d(-1) and 1.5 × 10(5) mol d(-1), and DIP loadings to be 6.2 × 10(3) mol d(-1) and 1.1 × 10(3) mol d(-1). Groundwater borne nutrients are 1-2 orders of magnitude larger than other nutrient sources and the interannual variation of nutrient concentration in the embayment is more influenced by the SGD derived loadings. Annual DIP concentrations in the harbor water is positively correlated with the precipitation and annual mean tidal range, and negatively correlated with evapotranspiration from 2000 to 2013. Climatologically driven SGD variability alters the SGD derived DIP loadings in this phosphate limited environment and may be the causative factor of interannual variability of red tide outbreaks from 2000 to 2013. Finally, a conceptual model is proposed to characterize the response of red tide outbreaks to climatological factors linked by SGD. The findings from this study shed light on the prediction of red tide outbreaks and coastal management of Tolo Harbor and similar coastal embayments elsewhere.

Delivery mechanism of (134)Cs and (137)Cs in seawater off the Sanriku Coast, Japan, following the Fukushima Dai-ichi NPP accident

To assess the delivery mechanism of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we examined vertical profiles of (134)Cs, (137)Cs, and (228)Ra concentrations and the (228)Ra/(226)Ra ratio in the water columns off the Sanriku Coast in the northwestern Pacific Ocean, in July 2012, along with their surface lateral variations in July 2009. Radiocesium concentrations exhibited maximum peaks (3-5 mBq/L for (134)Cs) at depths of 100-200 m, accompanied by high (228)Ra concentrations (0.6-0.8 mBq/L) in comparison with shallower depths (∼0.4 mBq/L). Taking the circulation patterns of currents in the area into account, it was inferred that radioactive depositions were supplied to the (228)Ra-rich Tsugaru Warm Current Water (TWCW) in the offshore area of the Sanriku Coast following the FDNPP accident, and that after the spring of 2011, this water (∼26.5σθ) was covered by lower density surface water, which helped intrude its way to depths of 100-200 m.

A Southern Ocean trigger for Northwest Pacific ventilation during the Holocene?

Holocene ocean circulation is poorly understood due to sparsity of dateable marine archives with submillennial-scale resolution. Here we present a record of mid-depth water radiocarbon contents in the Northwest (NW) Pacific Ocean over the last 12.000 years, which shows remarkable millennial-scale variations relative to changes in atmospheric radiocarbon inventory. Apparent decoupling of these variations from regional ventilation and mixing processes leads us to the suggestion that the mid-depth NW Pacific may have responded to changes in Southern Ocean overturning forced by latitudinal displacements of the southern westerly winds. By inference, a tendency of in-phase related North Atlantic and Southern Ocean overturning would argue against the development of a steady bipolar seesaw regime during the Holocene.

Spatial variations of 226Ra, 228Ra, 137Cs, and 228Th activities in the southwestern Okhotsk Sea

We collected 14 water column seawater samples in the southwestern Okhotsk Sea and 7 surface samples around the northern area of Hokkaido Island, Northern Japan, and employed low-background γ-spectrometry with convenient minimal radiochemical processing to determine the activities of (226)Ra (half-life t(1/2)=1600 y), (228)Ra (5.75 y), (137)Cs (30.2 y), and (228)Th (1.91 y) in the samples. Activities of (226)Ra (~2.3 mBq/L), (228)Ra (~0.7 mBq/L), and (137)Cs (~1 mBq/L) of surface waters on the Okhotsk Sea side show notable differences from those on the Japan Sea side (Soya Warm Current Water; SWCW) (~1.5 mBq/L; 1.5-2 mBq/L; 1.4-1.6 mBq/L), indicating their different origins and lateral mixing patterns. All of these nuclides exhibit unique vertical profiles; activities of soluble (226)Ra, (228)Ra, (137)Cs, and reactive (228)Th exhibit small variations from 50 to 500 m depth ((226)Ra, ~2.2 mBq/L; (228)Ra, ~0.4 mBq/L; (137)Cs, ~1 mBq/L; (228)Th, ~0.13 mBq/L). These profiles can be explained by the convective mixing of surface water such as the East Sakhalin Current Water (ESCW) to this layer.

Determination of 226Ra and 228Ra in slightly mineralised natural waters

A radiochemical method for simultaneous separation of (226)Ra and (228)Ra from natural waters by precipitating the radionuclides in the form of chromates that have low solubility in weak acetic acid has been described. For analytical purposes the change into soluble state was achieved through high-temperature melting the radium chromates precipitate with sodium and potassium carbonates at certain ratios. The chemical yield for radium-226 amounted to 87.1 ± 1.4% at the efficiency of counting 92.8 ± 0.7%. Calculated in series of 20 parallel determinations, reproducibility of the method was 7%. The chemical yield in separating radium-228 made up 63.8 ± 1.1%.