Mirai Cruise Research Articles

Source partitioning of dissolved inorganic carbon addition to Pacific Winter Water in the western Arctic Ocean

AbstractPacific Winter Water (PWW) with high dissolved inorganic carbon (DIC) is the key water mass in which subsurface acidification occurs in the western Arctic Ocean. To investigate and partition carbon sources added to PWW across the Chukchi shelf to the adjacent Canada Basin, we investigated the distributions of DIC and its stable isotope (δ13C‐DIC) with other hydrographic and biogeochemical parameters during the Mirai cruise in the late summer of 2021. Using a four‐end‐member mixing model, we deciphered the water masses and separated DIC changes induced by biological processes from those induced by conservative mixing. We demonstrated that DIC dynamics in PWW were mainly controlled by the biological decomposition of organic carbon (OC). A mass balance model analysis of DIC and δ13C‐DIC suggested that the apparent δ13C signature of the respired organic carbon (δ13COCx) within PWW was −22.2‰ ± 1.1‰ on the shelf and −25.6‰ ± 1.6‰ in the basin. Therefore, we concluded that marine‐origin OC was the dominant carbon source that was decomposed in the Chukchi shelf bottom water, while respired terrestrial‐origin carbon made a major contribution to DIC pool in the basin PWW. We proposed that terrestrial OC from the Chukchi and Beaufort coastal seas could be an important carbon source, which was associated with winter water formation on the shelves and influenced by Beaufort Gyre state shift and circulation changes. This unconventional finding has important ramifications for the prediction of the future state of ocean acidification in the western Arctic Ocean.

Global certified-reference-material- or reference-material-scaled nutrient gridded dataset GND13

Abstract. A global nutrient gridded dataset that might be the basis for studies of more accurate spatial distributions of nutrients in the global ocean was created and named GND13. During 30 cruises, reference materials of nutrients in seawater or their equivalents were used at all stations, and high-precision measurements were made. The precision of the nutrient analyses was better than 0.2 %. Data were collected from the hydrographic cruises in the JASMTEC R/V Mirai cruises, JMA cruise, CARINA, PACIFICA, and WGHC datasets from which nutrient data were available. Analyses were conducted at 243 crossover stations. Cruises that used certified reference materials or reference materials (CRMs/RMs) for seawater nutrient concentration measurements were used as a reference of an unbroken chain of comparison to determine correction factors which made nutrient concentrations obtained by other cruises SI traceable. Dissolved oxygen was secondarily quality-controlled using the same methodology as was used to create the nutrient gridded data product, but, lacking a traceable standard, the resulting oxygen data product is not SI traceable. Finally, a dataset of nitrate, phosphate, and silicate concentrations was created at latitude and longitude intervals of 0.5∘ and on 136 isobaric surfaces to depths of 6500 m as an SI-traceable dataset. This dataset has already been published at: https://doi.org/10.17596/0000001 (Aoyama, 2017).

Influence of warm-core eddy on dissolved methane distributions in the southwestern Canada basin during late summer/early fall 2015

We examined vertical profiles of methane (CH4) concentrations inside and outside of a warm-core eddy during a cruise of the R/V Mirai cruise (MR15-03 Leg. 1) in the Arctic Ocean in late summer/early fall 2015. Because a coherent mesoscale eddy is one of the possible mechanisms for transporting shelf water from the Chukchi Sea into the Canada Basin interior, we hypothesized that the warm-core eddy transported dissolved CH4 as well as nutrients and impacted the distribution of CH4. Seawater samples were therefore collected to obtain detailed vertical profiles of CH4 concentrations in the southwestern Canada Basin. We found high CH4 concentrations in bottom water on the shelf, which was located outside the warm-core eddy. The percent saturation in that water was as high as 947% of atmospheric CH4. We also found that sub-surface CH4 peaks within the radius of the maximum velocity of the eddy were broader at depth than those outside of the eddy. Lateral transport of the eddy was likely the controlling factor responsible for subsurface CH4 peaks within the warm-core eddy. Our study indicated that eddies play a crucial role in controlling and transporting CH4 laterally and vertically.

The 2013-15 temporal variation in the 129I concentration in seawater in the southern Canada Basin

129I concentrations were measured in surface waters and for two depth profiles in the Chukchi Borderland (74.5°N 162.0°W) and the southern Canada Basin (72.5°N 155.4°W) during the R/V Mirai cruises in 2014 and 2015. The 129I concentrations in the surface waters of the North Pacific Ocean and the Bering and Chukchi seas in 2014 and 2015 were 1.35 ± 0.19 and 1.18 ± 0.17 × 107 atoms L−1, respectively. The depth profiles of 129I were almost identical in 2014 and 2015. 129I concentrations were 10–20 × 107 atoms L−1 in the surface layer, less than 10 × 107 atoms L−1 (i.e., the minimum value) around 100–150 m, and 65–120 × 107 atoms L−1 below 300 m (below 200 m in 2015). Although the peak 129I concentrations exhibited slightly fluctuating values of 90–120 × 107 atoms L−1, the average 129I concentrations at water depths in ranging from 300 to 800 m were an almost constant (83–86 × 107 atoms L−1) from 2014 to 2015. Both the low 129I concentration level relative to other areas of the Arctic Ocean and the constant 129I concentrations over time may correspond to the ages of low discharge rate, until the mid-1990s, from the nuclear fuel-reprocessing facilities at Sellafield (U.K.) and La Hague (France) to the Atlantic Ocean.

Lithostratigraphy and paleoceanography in the Chukchi Rise of the western Arctic Ocean since the last glacial period

Paleoceanographic multi-proxies were investigated from two piston cores PC01 and PC04, along with two pilot cores PL01 and PL04, collected from the Chukchi Rise in the western Arctic Ocean during the R/V Mirai Cruise MR09-03. Both cores were composed of three lithologic units (Unit I: the Holocene brownish sandy mud, Unit II: the deglacial IRD layer, and Unit III: the glacial and gray thick mud with intervened IRD layers). The age estimate of the core units were decided by AMS 14C dates and confirmed by the correlation of the geochemical properties and ice-rafted debris (IRD) pattern with the well-dated core (P2) in the study area. The geochemical, isotopic, and mineralogical properties indicate different paleoceanographic conditions of three lithologic units in the Chukchi Rise: low primary production during the glacial period (Unit III), high terrigenous contribution during the deglacial period (Unit II), and increase in diatom productivity during the late Holocene (Unit I). In particular, most IRDs were specified as detrital carbonates (calcites and dolomites) by SEM-EDS examination. The sediment source from the northern North America and the transport pathway by the Beaufort Gyre were confirmed by the isotopic signature of bulk IRDs and high kaolinite content of fine-grained particles.

Impact of Arctic sea‐ice retreat on the recent change in cloud‐base height during autumn

Cloud‐base observations over the ice‐free Chukchi and Beaufort Seas in autumn were conducted using a shipboard ceilometer and radiosondes during the 1999–2010 cruises of the Japanese R/V Mirai. In comparison with cloud‐base heights in an ice‐covered case (the Surface Heat Budget of the Arctic Ocean project in 1998), our ice‐free results showed a 30% decrease (increase) in the frequency of low clouds with a ceiling below (above) 500 m. Temperature profiles revealed that the boundary layer was well developed over the ice‐free ocean in the 2000s, whereas a stable layer dominated during the ice‐covered period in 1998. The change in surface boundary conditions likely resulted in the difference in cloud‐base height, although it had little impact on air temperatures in the mid‐ and upper troposphere. Data from the 2010 R/V Mirai cruise were investigated in detail in terms of air‐sea temperature difference. Stratus clouds near the sea surface were predominant under a warm advection situation, whereas stratocumulus clouds with a cloud‐free layer were significant under a cold advection situation. The threshold temperature difference between sea surface and air temperatures for distinguishing the dominant cloud types was 3 K. Anomalous upward turbulent heat fluxes associated with the sea‐ice retreat have likely contributed to warming of the lower troposphere.

Relationship between time-series diatom fluxes in the central and western equatorial Pacific and ENSO-associated migrations of the Western Pacific Warm Pool

The water masses of the central and western equatorial Pacific can be divided into two parts: the Western Pacific Warm Pool (WPWP) and the Equatorial Upwelling Region (EUR). The behavior of the WPWP plays a significant role in global climate changes such as the El Niño-Southern Oscillation (ENSO), and it drastically modifies the oceanographic conditions in the area every few years. It is important to evaluate changes in time-series diatom fluxes during both the El Niño and the La Niña events. As a part of the Global Carbon Cycle and Related Mapping based on Satellite Imagery (GCMAPS) Program, time-series sediment trap moorings were deployed and recovered along the Equator at seven stations (Sites MT1–MT7) during five R/V Mirai cruises in the central and western Pacific during January 1999–January 2003. The entire length of this study is divided into two phases depending on the oceanographic conditions: the La Niña event (1999 and 2000); and the El Niño event (2002). Site MT3 was located in the WPWP and Sites MT5–MT7 were in the EUR. Annual means of total diatom fluxes increased towards the east in each year. The fluxes observed at Sites MT4–MT6 decreased from the La Niña event to the El Niño event. However, the fluxes observed at Site MT3 in 2001 and 2002 were higher than those in 2000. Total diatom fluxes showed different seasonal patterns at all sites. The diatom assemblages in the WPWP differed from those of the EUR. Pennate diatoms (e.g., Nitzschia bicapitata, Thalassionema nitzschioides) dominated in the WPWP, while the relative abundances of centric diatoms (e.g., Rhizosolenia bergonii, Azpeitia spp., Thalassiosira spp.) were higher than those of pennate diatoms in the EUR. The diatom fluxes during the La Niña event reflected seasonal oscillation of the WPWP in spatial extent. At Site MT3 during El Niño, terrestrial materials appeared to have been transported by subsurface currents, which might be a secondary influence on total diatom fluxes. The spatial extent of the WPWP reached Site MT7 in 2002, when total diatom fluxes decreased in the sediment traps located in the eastern region including Site MT7. Therefore, we conclude that the relationships between the ENSO and diatoms fluxes in the western and central equatorial Pacific can be explained by the geographic (west–east) expansion or contraction of the WPWP.

Vertical cloud properties in the tropical western Pacific Ocean: Validation of the CCSR/NIES/FRCGC GCM by shipborne radar and lidar

This study examined the vertical cloud structure over the tropical western Pacific Ocean using 95‐GHz radar and lidar data observed from September to December 2001 during the MR01‐K05 cruise of the research vessel Mirai. The cloud vertical structure was homogeneous between 6 and 10 km, and the maximum cloud occurrence was 20% and located at 12 km. The mean precipitation occurrence was 11.5% at 1 km. The cloud fraction, radar reflectivity factor, and lidar backscattering coefficient were simulated along the Mirai cruise track using the output from the Center for Climate System Research, University of Tokyo; National Institute for Environmental Studies; and Frontier Research Center for Global Change (CCSR/NIES/FRCGC) general circulation model (GCM). The original output showed the maximum cloud fraction at 15 km; however, after considering attenuation and the minimum sensitivity of the radar, the maximum shifted to 12 km. The model overestimated the cloud fraction above 8 km, with the simulated fraction more than twice as large as the observed fraction. The model overpredicted the frequency of deep convection reaching the upper atmosphere above 12 km. Further, it overestimated precipitation frequency. Simulated radar reflectivity was underestimated throughout the entire altitude range, whereas simulated and observed lidar backscattering were in good agreement above 12 km with subgrid‐scale treatment. The ice effective radius of 40 μm and ice water content were reasonable in thin clouds, but the radius was underestimated in other regions. The simulated liquid water content was overestimated.

Optical and chemical properties of marine aerosols over the central equatorial Pacific Ocean during the 2003 R/V Mirai cruise

The optical and chemical properties of atmospheric aerosols were measured over the central equatorial Pacific Ocean (from 150°E to 160°W longitude, near the equator) during the R/V Mirai cruise of January 2003. In the area from 165°E to 160°W along the equator, concentrations of anthropogenic chemical components were low; the average concentration of nonsea salt sulfate was 0.28 μg m−3, that of vanadium was less than the detection limit, and those of elemental carbon and organic carbon were 0.009 and 0.09 μg‐C m−3. The scattering coefficients were low (3.2 × 10−6 m−1 on average), and the absorption coefficients were also extremely low (0.06 × 10−6 m−1 on average). The mean value of single scattering albedo was 0.98, with a standard deviation of 0.01, indicating that the measured aerosols were transparent and had low variability. The measurement results suggest that nonsea salt sulfate and organic carbon were from natural sources. Sea salt particles and nonsea salt sulfate composed a large fraction (94%) of the water‐soluble components of fine particles. Thus the water‐soluble components and organic materials from natural sources were primarily responsible for the scattering of solar radiation. The measurement results give typical values of the optical and chemical properties of aerosols in the unpolluted central equatorial Pacific Ocean.

Increased surface seawater pCO2 in the eastern Bering Sea shelf: An effect of blooms of coccolithophorid Emiliania huxleyi?

Increased surface seawater pCO2 observed in the eastern Bering Sea shelf (long 168°W–160°W; lat 54°N–68°N) were examined in relation to blooms of the coccolithophorid Emiliania huxleyi, using data obtained from R/V Mirai cruises once or twice annually (August–October 1998–2002). A principal component analysis demonstrated that there are two spatial scales of pCO2 variability associated with E. huxleyi blooms. One is a small‐scale variation (∼0.2° in latitude), which shows a decrease of pCO2 by 18 μatm relative to surrounding waters. The other is a large‐scale variation (∼10.0° in latitude), with spatial gradient in pCO2 of up to 250 μatm. Detailed analyses show that the large‐scale variation is associated with diatom blooms, while the small‐scale variation is associated with coccolithophorid blooms with a calcification:photosynthesis = 1.0, and thus that seawater pCO2 is affected by the composition of marine ecosystems and its response to climate variability. SST rise also contributed to increased pCO2.

Melting Layer Cloud Observed during R/V Mirai Cruise MR01-K05

Abstract In this study, cloud profiling radar and lidar were used to determine the frequency distribution of the base heights of cloudy layers with little (or no) falling condensate particles. The data were obtained from stationary observations conducted from Research Vessel Mirai over the tropical western Pacific (around 1.85°N, 138°E) from 9 November to 9 December 2001. The observed cloudy layers had base heights predominantly in the range of 4.5–6.5 km. Almost all cloudy layers with a base in the range of 4.5–6.5 km had thickness thinner than 500 m, and the frequency peak of the base heights of measured cloudy layers is considered to represent the common occurrence of midlevel thin clouds. Midlevel thin clouds were frequently observed even during the active phase of the Madden–Julian oscillation (MJO). Composite analysis of radiosonde-derived relative humidity and temperature lapse rate indicates that the midlevel thin cloud in the MJO active period is generated via melting within the stratiform cloud, rather than by detrainment of surface-based convection.

Atmospheric Aerosols over the Western Pacific Ocean during the R/V Mirai Cruises in 2002

The optical and chemical properties of atmospheric aerosols were measured over the Pacific Ocean during two R/V Mirai cruises. One was the MR02-K02 cruise (winter cruise), crossing the western Pacific from Japan to 5°S between February 21 and March 31, 2002. The second was the MR02-K04 cruise (summer cruise), that covered the western Pacific Ocean, the Indian Ocean, and the seas around Indonesia from Japan to 10°S between June 25 and August 22, 2002. The surveys mainly encountered three kinds of air masses, maritime air masses (marine aerosol), air masses influenced by fossil fuel combustion (FF aerosol), and those influenced by biomass burning (BB aerosol). From 10°N to 20°N, marine aerosols were observed both in the summer and winter cruises with the value of the single scattering albedo (ω) above 0.98 in the summer and, 0.92 in the winter. The concentrations of anthropogenic components such as elemental carbon, vanadium, and arsenic were somewhat low. Sea salt particles play a dominant role in the variation of the scattering coefficients, and the optical thickness in this region. Between 30°N and 40°N, FF aerosols were observed on both cruises. With the absorptive aerosols, the mean ω in the winter cruise was 0.74, while with the transparent aerosols in the summer cruise, the mean ω was 0.92. This seasonal difference comes mainly from differences in the concentrations of elemental carbon. There were BB aerosols around Indonesia in the summer, with a mean ω of 0.72. This air mass contained the most absorptive aerosol measured on these cruises. The chemical compositions of BB particles were different from the other aerosols, with carbonous components and non-sea salt potassium in high concentrations and metal components, such as vanadium and arsenic, in low concentrations.

Effects of the anticyclonic eddies on water masses, chemical parameters and chlorophyll distributions in the Oyashio current region

Data from the R/V Mirai cruise (May‐June 2000) have been examined to discover how mesoscale processes associated with eddy dynamics directly affect the water masses, the distributions and the vertical fluxes of the dissolved oxygen, nutrients and dissolved inorganic carbon in the western subarctic Pacific. Using maps of the temperature, salinity, dissolved oxygen, nutrients, chlorophyll and sea-air pCO 2 difference we show that the boundaries of the anticyclone eddies in the study region were composed of high productivity coastal Oyashio water. The coastal waters were wrapped around the anticyclone eddies (thus creating a high productivity belt) and intruded inside of them. Using SeaWifs data we demonstrate that temporal variations in the position and the strength of anticyclone eddies advected the Kuril island coastal high productivity waters to the pelagic part, resulting in temporal variations of the chlorophyll in the Oyashio region. Computed vertical fluxes of the dissolved oxygen (DO), inorganic carbon (DIC) and silicate show that the anticyclonic eddies in the Kuroshio-Oyashio Zone are characterized by enhanced vertical fluxes of the DO and DIC between the upper (σ θ = 26.7‐27.0) and lower (σ θ = 27.1‐27.5) intermediate layer, probably due to the intrusions of the Oyashio waters into the eddies.