1 2 3 Journal of Radioanalytical and Nuclear Chemistry An International Journal Dealing with All Aspects and Applications of Nuclear Chemistry Volume

Abstract We carried out time-series observations of 210 Po and 210 Pb radioactivity in the western North Pacific Ocean. The sinking fluxes of particulate organic carbon (POC) in the mesopelagic zone were estimated from 210 Po radioactivity during several seasons in the subarctic and subtropical regions of the western North Pacific. The seasonal changes of POC fluxes at a depth of 400 m were larger in the subarctic region than in the subtropical region. The annual mean POC flux at a depth of 400 m was larger in the subarctic region (57 mgC m -2 day -1 ) than in the subtropical region (36 mgC m -2 day -1 ). The annual mean of the e-ratio (the ratio of POC flux to primary productivity) in the subarctic region (18 %) was about twice the e-ratio in the subtropical region (10 %). These results imply that the efficiency of the biological pump is larger in the subarctic region than in the subtropical region of the western North Pacific

Challenges of Measuring Abyssal Temperature and Salinity at the Kuroshio Extension Observatory

The deep ocean is severely undersampled. Whereas shipboard measurements provide irregular spatial and temporal records, moored records establish deep ocean high-resolution time series, but only at limited locations. Here, highlights and challenges of measuring abyssal temperature and salinity on the Kuroshio Extension Observatory (KEO) mooring (32.3°N, 144.6°E) from 2013 to 2019 are described. Using alternating SeaBird 37-SMP instruments on annual deployments, an apparent fresh drift of 0.03–0.06 psu was observed, with each newly deployed sensor returning to historical norms near 34.685 psu. Recurrent salinity discontinuities were pronounced between the termination of each deployment and the initiation of the next, yet consistent pre- and postdeployment calibrations suggested the freshening was “real.” Because abyssal salinities do not vary by 0.03–0.06 psu between deployment locations, the contradictory salinities during mooring overlap pointed toward a sensor issue that self-corrects prior to postcalibration. A persistent nepheloid layer, unique to KEO and characterized by murky, sediment-filled water, is likely responsible for sediment accretion in the conductivity cell. As sediment (or biofouling) increasingly clogs the instrument, salinity drifts toward a fresh bias. During ascent, the cell is flushed, clearing the clogged instrument. In contrast to salinity, deep ocean temperatures appear to increase from 2013 to 2017 by 0.0059°C, whereas a comparison with historical deep temperature measurements does not support a secular temperature increase in the region. It is suggested that decadal or interannual variability associated with the Kuroshio Extension may have an imprint on deep temperatures. Recommendations are discussed for future abyssal temperature and salinity measurements

Western North Pacific Integrated Physical-Biogeochemical Ocean Observation Experiment (INBOX): Part 2. Biogeochemical responses to eddies and typhoons revealed from the S1 mooring and shipboard measurements

An interdisciplinary project called S1-INBOX (Western North Pacific Integrated PhysicalBiogeochemical Ocean Observation Experiment conducted around the S1 biogeochemical mooring site) was carried out during the summer of 2011 in the oligotrophic, subtropical North Pacific Ocean near biogeochemical mooring S1 (30° N, 145° E). Results from the S1 mooring during S1-INBOX revealed a large export flux at a depth of 200 m, a high chlorophyll a concentration in the deep chlorophyll maximum layer, and a high potential photochemical efficiency of photosystem II. These phenomena were associated with vertical uplift of isopycnal surfaces at the edge of a cyclonic eddy and a transition from the cyclonic eddy to an anticyclonic eddy. Shipboard biogeochemical surveys conducted during oligotrophic conditions in July 2011 revealed that the phytoplankton community in these waters was dominated by small species that are responsive to intermittent supplies of nutrients. Surface wind forcing because of Typhoons MA-ON and SONCA may have generated near-inertial oscillations. Diapycnal mixing associated with near-inertial waves was also related to high export fluxes, the indication being that propagation of near-inertial internal waves and subsequent mixing may have been important to biogeochemical phenomena during S1-INBOX

Mesopelagic particulate nitrogen dynamics in the subarctic and subtropical regions of the western North Pacific

Recently, new spatiotemporal-scale particle observations by autonomous profiling floats equipped with bio-optical sensors have revealed that, in addition to gravitational particle sinking, the downward transport of surface particles by physical mixing events, which has been overlooked, contributes to particulate organic carbon export. However, the subsequent behavior of these exported particles in the mesopelagic zone (e.g., particle fragmentation and degradation) remains unclear, although it may influence the efficiency of carbon transport to further depths. This study successfully depicted the new annual mean mesopelagic particulate nitrogen (PN) dynamics with multi-layer, steady-state suspended PN pools by reanalyzing seasonal data on the stable nitrogen isotopic compositions of both suspended and sinking particles, each with different profiles, from subarctic station K2 and subtropical station S1 in the North Pacific, which are both CO2 sinks but in different oceanic settings. As analytical conditions, we assumed that the net loss of sinking PN was entirely due to abiotic fragmentation of particle aggregates to non-sinking particles and that the apparent 15N enrichment associated with heterotrophic degradation in the suspended PN pools was vertically constant. The 15N mass balance for the PN supply to the uppermost mesopelagic pool, derived from such constraints, allowed estimating the PN export by the mixed-layer pump, which was 1.6 times greater at K2 than at S1. However, its contribution to the total export (including gravitational PN sinking) from the surface layer was approximately 20% at both stations. Moreover, the ratio of PN supplied to the uppermost pool by the mixed-layer pump and by the fragmentation of particle aggregates was also similar at both stations, approximately 1:1. Using these ratios, together with separate observations of the mixed-layer pump-driven flux, it may be possible to estimate the efficiency of the particulate organic carbon transport due to the biological gravitational pump responsible for carbon sequestration in the deep sea

Challenge for multifaceted data acquisition around active volcanoes using uncrewed surface vessel

Monitoring of volcanic eruptions, the atmosphere, and the ocean, along with their scientific understanding, can be achieved through multifaceted observations that include camera images, topographic deformations, elastic waves, geology, and the chemical constituents and temperatures of air and water. However, regions of increased volcanic activity and/or shallow waters are difficult to access by crewed ship due to danger or grounding risk. To overcome these difficulties, we used an uncrewed surface vessel (USV), Bluebottle, to operate multiple observation around oceanic volcanoes in the Bonin Island arc in May 2023. Even under adverse sea conditions, the USV successfully reached a remote observation site, Nishinoshima volcano, which is about 130 km away from Chichijima Island where the USV started out from. The USV conducted five days of observations at shallower than 500 m water depth around Nishinoshima Island, the first time after violent eruptions of Nishinoshima in June 2020. The USV is equipped with various sensors and data collection technologies, including a single-beam echosounder, oceanographic and meteorological sensors, an Acoustic Doppler Current Profiler, and a time-lapse camera. These tools have provided a multi-dimensional view of the underwater landscape and marine conditions near the volcano for the first time in the world. We obtained new bathymetric data, sub-bottom images, and measurements of temperature, salinity, and pH. This study leverages advanced technologies and innovative methodologies to enhance our understanding of marine and geological phenomena

Open ocean particle flux variability from surface to seafloor

The sinking of carbon fixed via net primary production (NPP) into the ocean interior is an important part of marine biogeochemical cycles. NPP measurements follow a log‐normal probability distribution, meaning NPP variations can be simply described by two parameters despite NPP’s complexity. By analyzing a global database of open ocean particle fluxes, we show that this log‐normal probability distribution propagates into the variations of near‐seafloor fluxes of particulate organic carbon (POC), calcium carbonate, and opal. Deep‐sea particle fluxes at subtropical and temperate time‐series sites follow the same log‐normal probability distribution, strongly suggesting the log‐normal description is robust and applies on multiple scales. This log‐normality implies that 29% of the highest measurements are responsible for 71% of the total near‐seafloor POC flux. We discuss possible causes for the dampening of variability from NPP to deep‐sea POC flux, and present an updated relationship predicting POC flux from mineral flux and depth

Seasonal variations in the nitrogen isotopic composition of settling particles at station K2 in the western subarctic North Pacific

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific western subarctic gyre (WSG) revealed seasonal changes in δ15N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting traps (DST; 100-200 m) and moored traps (MST; 200 and 500 m). All particles showed higher δ15N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ15N(SUS) of 0.4-3.1 ‰ in the euphotic zone (EZ). The δ15N(SUS) signature was reflected by δ15 N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ15 N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ15 N(DST) variations of 2.4-7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ15 N(DST) vs. PP regression to δ15 N(MST) of 1.9-8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. Moreover, the monthly export ratio at 500 m was calculated using both estimated PP and measured organic carbon fluxes. Results suggest a 1.6 to 1.8 times more efficient transport of photosynthetically-fixed carbon to the intermediate layers occurs in summer/autumn rather than winter/spring

Inorganic Radiocarbon in Time-Series Sediment Trap Samples: Implication of Seasonal Variation of 14C in the Upper Ocean

In order to verify sediment trap samples as indicators of upper ocean 14C concentrations, particulate inorganic radiocarbon (PIC-Delta-14C) collected by time-series sediment traps in the Sea of Okhotsk and the Bering Sea was measured by accelerator mass spectrometry (AMS). All of the PIC-Delta-14C measurements were <0%, in contrast to GEOSECS 14C data in the upper ocean from the northwestern North Pacific. This difference is attributed to the upwelling of deepwater that contains low Delta-14C of dissolved inorganic carbon (DICAI4C) and to the decrease over time of surface DIC-Delta-14C owing to the decrease of atmospheric Delta-14C values. In addition, PIC-Delta-14C values showed significant seasonal variability: PIC-Delta-14C collected in the fall was the greatest (-22 per mil on average), whereas PIC-Delta-14C collected in winter showed an average minimum of -48 per mil. It is likely that this difference was caused by changes in mixed layer thickness. Although some uncertainties remain, further study on PIC-Delta-14C will enable us to estimate seasonal variability in DIC-Delta-14C and air-sea CO2 exchange rate.This material was digitized as part of a cooperative project between Radiocarbon and the University of Arizona Libraries.The Radiocarbon archives are made available by Radiocarbon and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202