Naomi Harada

Japan Agency for Marine-Earth Science Technology | JAMSTEC · Research and Development Center for Global Change

Southern Ocean deep-water circulation plays a vital role in the global carbon cycle. On geological time scales, upwelling along the Chilean margin likely contributed to the deglacial atmospheric carbon dioxide rise, but little quantitative evidence exists of carbon storage. Here, we develop an X-ray Micro-Computer-Tomography method to assess forami...

Nitrogen fixation is the major source of reactive nitrogen in the ocean and has been considered to occur specifically in low-latitude oligotrophic oceans. Recent studies have shown that nitrogen fixation also occurs in the polar regions and thus is a global process, although the physiological and ecological characteristics of polar diazotrophs are...

Plain Language Summary Exploring the year‐round utilization of nitrate and ammonium by phytoplankton is essential for understanding marine productivity. The nitrogen isotope ratio of sinking particles has long been used for reconstructing the availability of nitrogenous nutrients in the overlying surface water. However, the nitrogen isotope signal...

Phytoplankton production in the Arctic Ocean is increasing due to global warming‐induced sea ice loss, which is generally assessed through satellite observations of surface chlorophyll. Here we show that a diatom bloom can occur near the seafloor rather than at the surface in the open Arctic Ocean. Light can reach the seafloor underlying nutrient‐r...

We present a global atlas of downcore foraminiferal oxygen and carbon isotope ratios available at https://doi.org/10.1594/PANGAEA.936747 (Mulitza et al., 2021a). The database contains 2106 published and previously unpublished stable isotope downcore records with 361 949 stable isotope values of various planktic and benthic species of Foraminifera f...

Southern Ocean deep-water circulation plays an important role in the global carbon cycle. On geological time-scales, upwelling along the Chilean continental margin likely contributed to the deglacial atmospheric carbon dioxide rise, but little quantitative evidence exists of carbon storage. Here, we use a new X-ray Micro-Computer-Tomography method...

Time series monitoring of hydrography and marine settling particles supplies a large amount of information regarding the marine lower‐trophic ecosystem with respect to the changing Arctic Ocean. To investigate the interannual relationship between the settling particles and hydrographic conditions of the western Arctic Ocean, bottom‐tethered sedimen...

Urea sinks are mainly associated with assimilation by phytoplankton. However, recent studies have shown that there is a process by which nitrifiers convert urea‐derived nitrogen (urea‐N) into nitrate. We examined these two processes in the shelf and off‐shelf regions of the Arctic Ocean. Urea concentration was high near the bottom in the shelf regi...

We studied time‐series fluxes of radiolarian particles collected by two sediment traps deployed at the eastern (Sta. NAP12t) and western (Sta. CAP12t) sides of the Chukchi Borderland in the Chukchi Sea from 04 October 2012 to 18 September 2013. In order to elucidate the contribution of radiolarian skeletons to the biogenic silica flux, a three‐dime...

Biosynthesis of hydrocarbons is a promising approach for the production of alternative sources of energy because of the emerging need to reduce global consumption of fossil fuels. However, the suitability of biogenic hydrocarbons as fuels is limited because their range of the number of carbon atoms is small, and/or they contain unsaturated carbon b...

A re-investigation of phytoplankton assemblages on old membrane filters from polar–subpolar waters has revealed the presence of seven taxa within the Triparma strigata–Triparma verrucosa group (Bolidophyceae), including Triparma strigata f. strigata, Triparma strigata f. brevispina f. nov. and Triparma scotiaensis sp. nov. from the Scotia Sea (Sout...

A form of Triparma laevis (Parmales, Bolidophyceae), not formally described yet, has been reported in Southern Ocean phytoplankton surveys for more than 30 years. A re-examination of some filtered seawater samples collected off the Antarctic Peninsula in the 1960s by the USNS Eltanin provided an opportunity to make detailed morphological observatio...

Plankton samples have been routinely collected and preserved in formalin in many laboratories and museums for more than 100 years. Recently, attention has turned to use DNA information from formalin-fixed samples to examine changes in plankton diversity over time. However, no molecular ecological studies have evaluated the impact of formalin fixati...

The response of benthic foraminifera (BF) and radiolarian production in the central Okhotsk Sea (OS) to orbital and millennial changes of climate and environment over the last 136 kyr have been studied on the basis of the sediment core PC-7R, with available productivity, geochemical and lithological records and a modified age model. We used the cal...

Nitrogen fixation is the primary source of reactive nitrogen in the ocean. Most ecological models do not predict nitrogen fixation in the Antarctic Ocean because of the low availability of iron and high abundance of nitrogen. Here we extensively examined nitrogen fixation in the Antarctic Ocean, and found substantial nitrogen fixation (maximum: 44....

The Arctic Ocean is characterized as the greatly variable oceanic environment both seasonality and regionally. Such environmental variability would affect regional differences in the respective copepod community structures, though it has not been reported so far. In this study, we analyzed time-series zooplankton samples with focus on large copepod...

In general, strong wind events can enhance ocean turbulent mixing, followed by episodic nutrient supply to the euphotic zone and phytoplankton blooms. However, it is unclear whether such responses to strong winds occur in the ice‐free Canada Basin, where the seasonal pycnocline is strong and the nutricline is deep. In the present study, we monitore...

The Arctic Ocean has been experiencing rapid warming, which accelerates sea ice melt. Further, the increasing area and duration of sea ice-free conditions enhance ocean uptake of CO2. We conducted two shipboard experiments in September 2015 and 2016 to examine the effects of temperature, CO2, and salinity on phytoplankton dynamics to better underst...

Ocean acidification, induced by the increase in anthropogenic CO2 emissions, has a profound impact on marine organisms and biogeochemical processes1. The response of marine microbial activities to ocean acidification might play a crucial role in the future evolution of air–sea fluxes of biogenic gases such as nitrous oxide (N2O), a strong GHG and t...

The shallow Chukchi Sea is a gateway to the Arctic Ocean for Pacific‐origin waters. While a substantial portion of the Pacific‐origin waters flows through Barrow Canyon in the northeast corner of the Chukchi Sea, little is known on the hydrography of the surrounding regions in winter. We present profiles of wintertime hydrography on the Chukchi slo...

Phaeodaria, which comprise one group of large, single‐celled eukaryotic zooplankton, have been largely ignored by past marine biological studies because Phaeodaria and their delicate skeletons are liable to collapse. As a result, collection and quantification of specimens are difficult, and seasonal changes of phaeodarian abundance have not been th...

Nitrification is susceptible to changes in light and pH and, thus, could be influenced by recent sea ice reductions and acidification in the Arctic Ocean. We investigated the sensitivity of nitrification to light, pH, and substrate availability in a natural nitrifier community of the Arctic Ocean. Nitrification was active near the bottom of the she...

A drastic sea-ice reduction has been observed in the Pacific sector of the Arctic Ocean during the last few decades. However, it is still poorly understood how the future reduction in sea-ice cover will impact the microzooplankton communities within the Arctic marine ecosystems. To elucidate the relationship between hydrographic conditions and the...

In recent years, marine ecosystems have changed due to the drastic sea ice reduction in the Arctic Ocean, but the relationship between copepod phenology and environmental drivers is unclear. To reveal the relationship, seasonal changes in the flux (abundance of swimmers), population structure, lipid accumulation and gonad maturation of four dominan...

This paper describes the distribution of detrital minerals and sediment color in the surface sediments of the western Arctic Ocean and the northern Bering Sea and investigates the relationship between mineral composition and sediment provenance. This relationship was used to determine the provenance of western Arctic Ocean sediments deposited durin...

Strains of the coccolithophore Emiliania huxleyi (Haptophyta) collected from the subarctic North Pacific and Arctic oceans in 2010 were established as clone cultures and have been maintained in the laboratory at 15 °C and 32 ‰ salinity. To study the physiological responses of coccolith formation to changes in temperature and salinity, growth experi...

A continuous reconstruction of quantitative paleotemperatures in the Holocene was conducted by using alkenone sea surface temperature (SST) measurements from coastal sedimentary cores because of the strong correlation of SST with atmospheric temperature (AT) in the coastal bay area. This analysis enabled us to estimate a record of temperatures in w...

•Two new living Entactinaria species in the Arctic Ocean are described. •Taxonomic position of the order Entactinaria is discussed and the genus Joergensenium has been emended. •New species reflect distinct local environmental conditions in the Arctic Ocean. •Three-dimensional imaging using MXCT is a reliable tool for studying radiolarian skeletal...

Spatial and temporal variations in major phytoplankton populations such as diatoms in the changing Arctic Ocean have been well studied, whereas only a few monitoring studies have been conducted on minor siliceous flagellates. To discern the relationship between hydrographic conditions and the spatio-temporal distribution of silicoflagellates, ebrid...

Radiolarians in the Arctic Ocean have been studied lately in both plankton and sediment trap samples in the Chukchi Sea area. These studies have shed light on new radiolarian taxa, especially within the order Entactinaria, including two new species of Joergensenium, Joergensenium arcticum from the western Arctic Ocean, so far restricted to the Paci...

Aleutian Low variations provide vorticity, buoyancy, and heat-flux forcing to the North Pacific Ocean, which in turn cause changes in ocean circulation, mixed layer characteristics and sea ice coverage. In this process the white noise atmospheric characteristics are integrated dynamically and thermodynamically to generate red noise ocean spectra. U...

To examine seasonal changes in the mesozooplankton community, analyses were made on the swimmer samples (>1 mm) collected by a sediment trap mooring at 184 m depth on the Northwind Abyssal Plain in the western Arctic Ocean during October 2010–September 2011. The zooplankton swimmer flux ranged from 5 to 44 ind. m−2 day−1 and was greater during July...

Strains of the coccolithophore Emiliania huxleyi (Haptophyta) collected from the subarctic North Pacific and Arctic Oceans during the R/V MIRAI cruise in 2010 (MR10-05) were established as clone cultures and have been maintained in the laboratory at 15 °C and 32 ‰ salinity. To study the physiological responses of coccolith formation to changes in t...

The reduction of sea ice in the Arctic Ocean, which has progressed more rapidly than previously predicted, has the potential to cause multiple environmental stresses, including warming, acidification, and strengthened stratification of the ocean. Observational studies have been undertaken to detect the impacts on biogeochemical cycles and marine ec...

Seasonal and interannual variability in the biogenic particle sinking flux was recorded using multi-year bottom-tethered sediment trap mooring systems in the Northwind Abyssal Plain (Station NAP: 75° N, 162° W, 1975 m water depth) of the western Arctic Chukchi Borderland. Trapped particle flux at a median depth of 184 m had an obvious peak and domi...

Significance The Drake Passage (DP) represents the most important oceanic gateway along the pathway of the world’s largest current: the Antarctic Circumpolar Current (ACC). Resolving changes in the flow of circumpolar water masses through the DP is crucial for advancing our understanding of the Southern Ocean’s role in affecting ocean and climate c...

The future conditions of Arctic sea ice and marine ecosystems are of interest not only to climate scientists, but also to economic and governmental bodies. However, the lack of widespread, year-long biogeochemical observations remains an obstacle to understanding the complicated variability of the Arctic marine biological pump. Here we show an earl...

Winter ice cover of the Arctic Ocean makes year-round zooplankton sampling by plankton net a difficult task. Therefore, the collection of copepods with a sediment trap can be a powerful tool. In the present study, we analysed the seasonal changes in the population structures of five dominant planktonic copepods (Oncaea parila, Calanus hyperboreus,...

Seasonal and interannual variability in sinking flux of biogenic particles was reported by the multi-year bottom-tethered sediment trap measurements in the Northwind Abyssal Plain (Station NAP: 75° N, 162° W, 1975 m water depth) of the western Arctic Chukchi Borderland. Whereas the trapped particle flux had an obvious peak with the dominance of sea...

The vertical distribution of radiolarians was investigated using a vertical multiple plankton sampler (100– 0, 250–100, 500–250, and 1000–500 m water depths, 62 µm mesh size) at the Northwind Abyssal Plain and southwestern Canada Basin in September 2013. To investigate seasonal variations in the flux of radiolarians in relation to sea ice and water...

We studied time-series fluxes of diatom particles from 4 October 2010 to 18 September 2012 using bottom-tethered moorings with two sediment traps deployed at 180 and 1300 m depths at Station NAP (75° N, 162° W; 1975 m water depth) in the western Arctic Ocean. This paper discusses on the relationship of time-series diatom fluxes to satellite-based s...

The Drake Passage (DP) is the major geographic constriction for the Antarctic Circumpolar Current (ACC) and exerts a strong control on the exchange of physical, chemical, and biological properties between the Atlantic, Pacific, and Indian Ocean basins. Resolving changes in the flow of circumpolar water masses through this gateway is, therefore, cru...

The vertical distribution of radiolarians was investigated using a vertical multiple plankton sampler (100–0, 250–100, 500–250 and 1000–500 m water depths, 62 μm mesh size) at the Northwind Abyssal Plain and southwestern Canada Basin in September 2013. To investigate seasonal variations in the flux of radiolarians in relation to sea-ice and water m...

We studied time-series fluxes of diatom particles and their relationship to hydrographic variations from 4 October 2010 through 18 September 2012 using bottom-tethered sediment trap moorings deployed at Station NAP (75° N, 162° W; 1975 m water depth) in the western Arctic Ocean. We observed clear maxima of the diatom valve flux in November–December...

Past sea surface temperature (SST) and salinity (SSS) changes in the northeastern East Sea/Japan Sea since 19 cal ka BP have been reconstructed by using an alkenone unsaturation index and the oxygen isotope ratio of planktonic foraminifera Neogloboquadrina pachyderma(s) in Core MR0604-PC03B. Coretop alkenone temperature and calcification temperatur...

The future conditions of Arctic sea ice and marine ecosystems are of interest not only to climate scientists, but also to economic and governmental bodies. However, the lack of widespread, year-long biogeochemical observations remains an obstacle to understanding the complicated variability of the Arctic marine biological pump. Here we show an earl...

To examine seasonal changes in the mesozooplankton community, analyses were made on the swimmer samples (>1 mm) collected by a sediment trap mooring at 184 m depth on the Northwind Abyssal Plain in the western Arctic Ocean during October 2010–September 2011. The zooplankton swimmer flux ranged from 5 to 44 ind. m−2 day−1 and was greater during July...

To investigate the melting history of the Patagonian Ice Sheet (PIS) during the last deglaciation, we reconstructed Al2O3, flux in sediments from the Magellanes fjord (53°S) and southern Patagonia (55°S: Drake Passage) over the past 12 and 31 kiloyears (kyr), respectively. We confirm that the melting history of the PIS is recorded as changes in sed...