Makio C. Honda

Seasonal changes in planktonic foraminifera in the northwestern north Pacific Ocean : sediment trap experiments from subarctic and subtropical gyres

Abstract Upper ocean environments such as seawater temperature, salinity, thermal structure in the water column, light intensity and food supply affect the assemblage of planktonic foraminifera. Since detailed information on planktonic foraminifera was not available for the northwestern North Pacific, we examined seasonal changes in fluxes and composition of planktonic foraminifera there. Data were collected by sediment traps deployed at three sites (Site 50N (50°01′N, 165°02′E), Site KNOT (43°58′N, 155°03′E), and Site 40N (39°60′N, 165°00′E)) in this area in order to better understand the relationship between the foraminiferal assemblage and surface-ocean environments. Although each planktonic foraminiferal species showed specific seasonal variations in flux, some exhibited similar flux profiles. In order to define these more accurately, correlations among species were calculated and the foraminiferal species classified into four groups: (1) Group A (subpolar species: Neogloboquadrina pachyderma , Globigerina quinqueloba , Globigerina bulloides , and Globigerinita glutinata ); (2) Group B (subtropical and tropical species: Globigerinoides ruber and Globigerinoides sacculifer ); (3) Group C (post-upwelling species: Neogloboquadrina dutertrei ); and (4) Group D (deep-water species: Globorotalia scitula and Globorotalia truncatulinoides ). The common environments for each period, based on foraminiferal production and composition, were observed among the three sites. Based upon predominant foraminiferal groups, total foraminiferal fluxes (TFFs), organic matter (OM) fluxes and hydrographic conditions, including sea-surface temperatures (SST) and thermal structure, the surface-ocean environments in the northwestern North Pacific could be generalized into five types. Type I is characterized by a dominance of Group B, with low TFFs under high SST, and Type II is marked by a high flux of Group A. On the other hand, Type III shows low TFFs and low OM fluxes due to low insolation during winter. Type IV represents a high flux of Group A, and Group D at Site 40N, under a well-mixed surface ocean, and Type V is characterized by a prominent peak of Group C under a developed thermocline. A comparison of annual mean foraminifera fluxes at 50°N between the northwestern (Site 50N) and the northeastern North Pacific (Station PAPA) during the normal “cold” mode demonstrated higher fluxes of foraminifera at Site 50N due to higher nutrient concentrations. The results also showed that G. bulloides is not always a proxy for upwelling and that N. pachyderma is not solely controlled by seawater temperature in the northwestern Pacific, where the surface water is enriched in nutrients. The fluxes and composition of foraminifera suggest that similar ocean environments are expected in large areas of the northern North Pacific, from Site KNOT to Station PAPA during the “warm” mode, which is affected by El Nino events.

Enhanced role of eddies in the Arctic marine biological pump

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 early winter maximum of sinking biogenic flux in the western Arctic Ocean and illustrate the importance of shelf-break eddies to biological pumping from wide shelves to adjacent deep basins using a combination of year-long mooring observations and three-dimensional numerical modelling. The sinking flux trapped in the present study included considerable fresh organic material with soft tissues and was an order of magnitude larger than previous estimates. We predict that further reductions in sea ice will promote the entry of Pacific-origin biological species into the Arctic basin and accelerate biogeochemical cycles connecting the Arctic and subarctic oceans.

Decadal change of dissolved inorganic carbon in the subarctic western North Pacific Ocean

Dissolved inorganic carbon (DIC) was measured from 1992 to 2008 at two time-series sites in the subarctic western North Pacific; this region is a source of atmospheric CO2 in winter due to vertical mixing of deep waters rich in DIC. To estimate the decadal DIC increase resulting from CO2 uptake from the atmosphere, we corrected DIC for the contribution of biological activity below the temperature minimum (Tmin) layer (¡«100 m), which is the remnant of the mixed layer from the preceding winter. Decadal DIC increases in the Tmin layer and upper intermediate water (1.3¨C1.5 ¦Ìmol kg-1 yr-1; 100¨C200 m) were higher than those expected from oceanic equilibration with increasing atmospheric CO2 and those previously reported in the open North Pacific. The increase in water column CO2 was estimated to be 0.40 ¡À 0.08 mol m-2 yr-1. The decadal DIC change in the Tmin layer affects winter CO2 emission. The increase of atmospheric xCO2 in winter (2.1 ± 0.0 ppm yr-1) is higher than that of oceanic CO2 (0.7 ¡À 0.5 ppm yr-1) that calculated from DIC and total alkalinity in the Tmin layer. This difference suggests reduction of CO2 emission in winter is possibly controlled by the increase of total alkalinity.

Disturbance of deep-sea environments induced by the M9.0 Tohoku Earthquake

The impacts of the M9.0 Tohoku Earthquake on deep-sea environment were investigated 36 and 98 days after the event. The light transmission anomaly in the deep-sea water after 36 days became atypically greater (∼35%) and more extensive (thickness ∼1500 m) near the trench axis owing to the turbulent diffusion of fresh seafloor sediment, coordinated with potential seafloor displacement. In addition to the chemical influx associated with sediment diffusion, an influx of 13C-enriched methane from the deep sub-seafloor reservoirs was estimated. This isotopically unusual methane influx was possibly triggered by the earthquake and its aftershocks that subsequently induced changes in the sub-seafloor hydrogeologic structures. The whole prokaryotic biomass and the development of specific phylotypes in the deep-sea microbial communities could rise and fall at 36 and 98 days, respectively, after the event. We may capture the snap shots of post-earthquake disturbance in deep-sea chemistry and microbial community responses.

Radiocarbon of sediment trap samples from the Okinawa trough: lateral transport of 14C-poor sediment from the continental slope

Abstract Radiocarbon of carbonate (PIC) and of organic carbon (POC) in sediment trap samples from the Okinawa trough was measured by AMS. Concentrations of 14 C in PIC and POC ( Δ 14 C -PIC and Δ 14 C -POC) ranged from approximately +40‰ to −80‰ and average over the entire 2 years was approximately −32‰. These values are much lower than Δ 14 C values of dissolved inorganic carbon ( Δ 14 C -DIC) in the upper 200 m of the water column (+100‰ on average). Δ 14 C -PIC and Δ 14 C -POC showed seasonal variability over 2 years with lower values in winter and higher values in summer. In the 1994–1995 period, Δ 14 C -PIC was also low in spring. Variations in Δ 14 C -PIC and Δ 14 C -POC were positively correlated with concentrations of inorganic and organic carbon, respectively, and negatively correlated with concentration of Al. This suggests that variability in Δ 14 C -PIC and Δ 14 C -POC were associated with the input of lithogenic materials. Assuming Δ 14 C -PIC and Δ 14 C -POC for two end members (settling particles produced in the overlying water column, and laterally transported materials produced outside of the overlying water column, which originated from the continental slope of the East China Sea), contributions of laterally transported materials to the sediment trap samples were estimated for each collecting period. The contribution of laterally transported materials ranged from approximately 50%–90% and the annual average of flux of old carbon was ca. 5 mg m−2 day−1 in 1993 and 10 mg m−2 day−1 in 1994–1995.

Evidence for the offshore transport of terrestrial organic matter due to the rise of sea level: The case of the East China Sea Continental Shelf

Accelerator mass spectrometry (AMS) 14 C dating and analyses of the stable isotope ratio of organic carbon (δ 13 C org ) were carried out on two piston cores obtained from the continental slope and the Okinawa Trough in the East China Sea (ECS). These cores covered the last ca. 15 14 C kyr BP (17 to 18 cal kyr BP). Mass and organic carbon (org. C) accumulation rates decreased gradually after the last deglaciation period. On the other hand, δ 13 C org showed distinct depletion from ca. 14 to 12 14 C kyr BP (17.5 to 13.2 cal kyr BP). The depletion relates to the large supply of terrestrial org. C from the ECS shelf triggered by the seas intrusion onto the shelf area because of the rise in sea level.

Seasonal variability of primary production and phytoplankton biomass in the western Pacific subarctic gyre: Control by light availability within the mixed layer

A distinct seasonal variation of primary production was revealed from shipboard observations conducted from 2005 to 2013 at time series station K2 in the western Pacific subarctic gyre (WSG). The mean depth-integrated primary production was highest (569 ± 162 mg C m−2 d−1) in summer and lowest (101 ± 16 mg C m−2 d−1) in winter. Strong winter mixing enriched the mixed layer (ML) with nutrients that were not fully consumed during the remainder of the year, the result being that the WSG was a high-nutrient, low-chlorophyll (HNLC) region. The deep ML reduced primary production by reducing light availability in winter, whereas primary production was enhanced by strong light availability in the shallower ML as summer progressed. However, primary production was often attenuated by a reduction of light availability attributable to dense sea fog in summer. We found a significant relationship between primary production and light availability in this HNLC region. However, chlorophyll a was less variable seasonally than primary production. The highest depth-integrated chlorophyll a was observed in summer (54.6 ± 13.4 mg m−2), but chlorophyll a remained high in winter (45.3 ± 7.7 mg m−2). Reduced light availability depressed primary production, but a reduction of the chlorophyll a concentration was prevented by a relaxation of grazing in the deep ML during winter. We found that light availability exerted an important control on the seasonal variability of primary production and phytoplankton biomass in the WSG.

Helium anomalies suggest a fluid pathway from mantle to trench during the 2011 Tohoku-Oki earthquake

Geophysical evidence suggests that fluids along fault planes have an important role in generating earthquakes; however, the nature of these fluids has not been well defined. The 2011 magnitude 9.0 Tohoku-Oki earthquake ruptured the interface between the subducting Pacific plate and the overlying Okhotsk plate. Here we report a sharp increase in mantle-derived helium in bottom seawater near the rupture zone 1 month after the earthquake. The timing and location indicate that fluids were released from the mantle on the seafloor along the plate interface. The movement of the fluids was rapid, with a velocity of ~4 km per day and an uncertainty factor of four. This rate is much faster than what would be expected from pressure-gradient propagation, suggesting that over-pressurized fluid is discharged along the plate interface.

Observation of the dissolution process of Globigerina bulloides tests (planktic foraminifera) by X‐ray microcomputed tomography

We performed a 9 day dissolution experiment with tests of the planktic foraminifer Globigerina bulloides at pH 6.7 ± 0.1 in water undersaturated with respect to calcite. The initial stage of the dissolution process, which is not recognizable from the surface structure of the tests, was quantitatively evaluated by X-ray microcomputed tomography (XMCT). XMCT revealed three distinct test structures: early-developed calcite formed during the juvenile stage of G. bulloides, an inner calcite layer, and an outer calcite layer. The test ultrastructure was observed by scanning electron microscopy, and CT number evaluated the density distribution in the test. The early-developed calcite and inner calcite layer had low CT numbers (500–1300; low density, porous) and were sensitive to dissolution, whereas the outer calcite layer had high CT numbers (<1300; high density) and resisted dissolution. Both the modes and the frequencies of the CT numbers decreased with progress of dissolution. Changes in the CT number histogram with progress of dissolution were quantified in terms of the percentage of calcite volume accounted for by low-density calcite (% Low-CT-number calcite). A clear linear relationship (R2 = 0.87) between % Low-CT-number calcite and % Test weight loss was found. This relationship indicates that the amount of test dissolution can be estimated from the distribution of CT numbers. We propose that XMCT measurements will be useful for quantitatively estimating the amount of carbonate loss from foraminiferal tests by dissolution.

Bomb radiocarbon invasion into the northwestern North Pacific

Invasion of the bomb radiocarbon was investigated in the northwestern North Pacific, including the subarctic (Oyashio region), the subtropical (Kuroshio region), and the subarctic/subtropical mixing (transition) areas. After the GEOSECS Pacific expedition in 1973, the bomb radiocarbon decreased in the Oyashio region and increased in the Kuroshio region. The former was caused by rapid ventilation of the Oyashio water. The latter was owing to accumulation of the bomb radiocarbon in the intermediate water of the Kuroshio region. The reverse trends in the temporal variation of radiocarbon between the Oyashio and the Kuroshio regions lead to virtually small temporal change of the bomb radiocarbon in the transition area where the two waters mix. The temporal changes of the bomb radiocarbon suggest that the North Pacific Intermediate Water is formed in the northwestern North Pacific.