Fifty years of the 137°E repeat hydrographic section in the western North Pacific Ocean

The 137°E repeat hydrographic section of the Japan Meteorological Agency across the western North Pacific was initiated in 1967 as part of the Cooperative Study of the Kuroshio and Adjacent Regions and has been continued biannually in winter and summer. The publicly available data from the section have been widely used to reveal seasonal to decadal variations and long-term changes of currents and water masses, biogeochemical and biological properties, and marine pollutants in relation to climate variability such as the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In commemoration of the 50th anniversary in 2016, this review summarizes the history and scientific achievements of the 137°E section during 1967–2016. Through the publication of more than 100 papers over this 50-year span, with the frequency and significance of the publication increasing in time, the 137°E section has demonstrated its importance for future investigations of physical–biogeochemical–biological interactions on various spatiotemporal scales, and thereby its utility in enhancing process understanding to aid projections of the impact of future climate change on ocean resources and ecosystems over the twenty-first century.     

Time-series observation of dissolved inorganic carbon and nutrients in the northwestern North Pacific

Time-series measurements of dissolved inorganic carbon (DIC) and nutrient concentrations were conducted in the northwestern North Pacific from October 2002 to August 2004. Assuming that data obtained in different years represented time-series seasonal data for a single year, vertical distributions of DIC and nutrients showed large seasonal variabilities in the surface layer (∼100 m). Seasonal variabilities in normalized DIC (nDIC) and nitrate concentrations at the sea surface were estimated to be 81–113 μmol kg⁻¹ and 12.7–15.7 μmol kg⁻¹, respectively, in the Western Subarctic Gyre. The variability in nutrients between May and July was generally at least double that in other seasons. In the Western Subarctic Gyre, estimations based on statistical analyses revealed that seasonal new production was 39–61 gC m⁻² and tended to be higher in the southwestern regions or coastal regions. The seasonal new productions in the northwestern North Pacific were two or more times higher than in the North Pacific subtropical gyre and the northeastern North Pacific. It is likely that this difference is due to spatial variations in the concentrations of trace metals and the species of phytoplankton present. In addition, from estimations of surface pCO₂ it was verified that the Western Subarctic Gyre is a source of atmospheric CO₂ between February and May and a sink for CO₂ between July and October.     

Deep ocean fluxes and their link to surface ocean processes and the biological pump

Intense studies of upper and deep ocean processes were carried out in the Northwestern Indian Ocean (Arabian Sea) within the framework of JGOFS and related projects in order to improve our understanding of the marine carbon cycle and the ocean’s role as a reservoir for atmospheric CO₂. The results show a pronounced monsoon-driven seasonality with enhanced organic carbon fluxes into the deep-sea during the SW Monsoon and during the early and late NE Monsoon north of 10°N. The productivity is mainly regulated by inputs of nutrients from subsurface waters into the euphotic zone via upwelling and mixed layer-deepening. Deep mixing introduces light limitation by carrying photoautotrophic organisms below the euphotic zone during the peak of the NE Monsoon. Nevertheless, deep mixing and strong upwelling during the SW Monsoon provide an ecological advantage for diatoms over other photoautotrophic organisms by increasing the silica concentrations in the euphotic zone. When silica concentrations fall below 2 μmol l⁻¹, diatoms lose their dominance in the plankton community. During diatom-dominated blooms, the biological pathway of uptake of CO₂ (the biological pump) appears to be more efficient than during blooms of other organisms, as indicated by organic carbon to carbonate carbon (rain) ratios. Due to the seasonal alternation of diatom and non-diatom dominated exports, spatial variations of the annual mean rain ratios are hardly discernible along the main JGOFS transect.Data-based estimates of the annual mean impact of the biological pump on the fCO₂ in the surface water suggest that the biological pump reduces the increase of fCO₂ in the surface water caused by intrusion of CO₂-enriched subsurface water by 50–70%. The remaining 30 to 50% are attributed to CO₂ emissions into the atmosphere. Rain ratios up to 60% higher in river-influenced areas off Pakistan and in the Bay of Bengal than in the open Arabian Sea imply that riverine silica inputs can further enhance the impact of the biological pump on the fCO₂ in the surface water by supporting diatom blooms. Consequently, it is assumed that reduced river discharges caused by the damming of major rivers increase CO₂ emission by lowering silica inputs to the Arabian Sea; this mechanism probably operates in other regions of the world ocean also.     

Seasonal evolution of hydrographic properties in the Antarctic circumpolar current at 170°W during 1997–1998

This paper discusses the seasonal evolution of the hydrographic and biogeochemical properties in the Antarctic Circumpolar Current (ACC) during the US Joint Global Ocean Flux (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in 1997–1998. The location of the study region south of New Zealand along 170°W was selected based on the zonal orientation and meridional separation of the physical and chemical fronts found in that region. Here we endeavor to describe the seasonal changes of the macronutrients, fluorescence chlorophyll_, particulate organic carbon (POC), and carbon dioxide (CO₂) in the upper 400 m of the ACC during the evolution of the seasonal phytoplankton bloom found in this area. While the ACC has extreme variability in the meridional sense (due to fronts, etc.), it appears to be actually quite uniform in the zonal sense. This is reflected by the fact that a good deal of the seasonal zonal changes in nutrients distributions at 170°W follow a pattern that reflects what would be expected if the changes are associated with seasonal biological productivity. Also at 170°W, the productivity of the upper waters does not appear to be limited by availability of phosphate or nitrate. While there is a significant decrease (or uptake) of inorganic nitrogen, phosphate and silicate associated with the seasonal phytoplankton bloom, none of the nutrients, except perhaps silicate (north of the silicate front) are actually depleted within the euphotic zone. At the end of the growing season, nutrient concentrations rapidly approached their pre-bloom levels. Inspection of the ratios of apparent nutrient drawdown near 64°S suggests N/P apparent drawdowns to have a ratio of 10 and N/Si apparent drawdowns to have a ratio of >4. These ratios suggest a bloom that was dominated by Fe limited diatoms. In addition, the surface water in the Polar Front (PF) and the Antarctic Zone (AZ) just to the south of the PF take up atmospheric CO₂ at a rate 2–3 times as fast as the mean global ocean rate during the summer season but nearly zero during the rest of year. This represents an important process for the transport of atmospheric CO₂ into the deep ocean interior. Finally, the net CO₂ utilization or the net community production during the 2.5 growing months between the initiation of phytoplankton blooms and mid-January increase southward from 1.5 mol C m⁻² at 55°S to 2.2 mol C m⁻² to 65°S across the Polar Frontal Zone (PFZ) into the AZ.     

Doubled CO2 impact on subarctic marine carbon cycle: a case study at Ocean Station P

Along with meteorological observations, complementary and systematic oceanographic observations of various physical, biological and chemical parameters have been made at Ocean Station P (OSP) (50°N, 145°W) since the early 1950s. These decadal time scale data have contributed to a better understanding of the physical, biological and chemical processes in the surface layer of the northeastern subarctic region of the Pacific Ocean. These data have demonstrated the importance of the North Pacific in the global carbon cycle and, in particular, the role of biological/chemical processes in the net exchange of CO₂ across the air–sea interface. Although we do not fully comprehend how climatic variations influence marine communities or marine biogeochemistry, previous studies have provided some basic understanding of the mechanisms controlling the seasonal and inter-annual variations of biological and chemical parameters (such as phytoplankton, bacteria, nitrate/ammonium concentration) at OSP, and how they affect the carbon cycling in the subarctic North Pacific. In this study, we investigate how these mechanisms might alter the seasonal variations of these parameters at OSP under a 2XCO₂ condition. We examine these influences using a new biological model calibrated by the climatological data from OSP. For the 2XCO₂ simulation, the biological model is driven off line (i.e., no feedback to the ocean/atmospheric model components) by the climatology plus 2XCO₂−1XCO₂ outputs from a global surface ocean model and the Canadian GCM. Under the 2XCO₂ condition, the upper layer ocean shows an increase in the entrainment rate at the bottom of the mixed layer for OSP during the late autumn and winter seasons, resulting in an increase in the f-ratio. Although there is an overall increase in the primary production (PP) by 3–18%, a decrease in the biomass of small phytoplankton and microzooplankton (due to mesozooplankton grazing) lowers the concentration of dissolved organic matter (DOM) by 4–25%. The model also predicts a significant increase in the concentrations of nitrate and ammonium, and in bacterial production during July and August. Doubling of the atmospheric CO₂ from 330 to 660 ppm forces the marine pCO₂ to increase by about 63%, much of which is driven by an increased flux of CO₂ from the atmosphere to the oceans.     

Seasonal and interannual variability of oceanic carbon cycling in the western and central tropical-subtropical pacific: A physical-biogeochemical modeling study

The oceanic carbon cycle in the tropical-subtropical Pacific is strongly affected by various physical processes with different temporal and spatial scales, yet the mechanisms that regulate air-sea CO₂ flux are not fully understood due to the paucity of both measurement and modeling. Using a 3-D physical-biogeochemical model, we simulate the partial pressure of CO₂ in surface water (pCO_2sea) and air-sea CO₂ flux in the tropical and subtropical regions from 1990 to 2004. The model reproduces well the observed spatial differences in physical and biogeochemical processes, such as: (1) relatively higher sea surface temperature (SST), and lower dissolved inorganic carbon (DIC) and pCO_2sea in the western than in the central tropical-subtropical Pacific, and (2) predominantly seasonal and interannual variations in the subtropical and tropical Pacific, respectively. Our model results suggest a non-negligible contribution of the wind variability to that of the air-sea CO₂ flux in the central tropical Pacific, but the modeled contribution of 7% is much less than that from a previous modeling study (30%; McKinley et al., 2004). While DIC increases in the entire region SST increases in the subtropical and western tropical Pacific but decreases in the central tropical Pacific from 1990 to 2004. As a result, the interannual pCO_2sea variability is different in different regions. The pCO_2sea temporal variation is found to be primarily controlled by SST and DIC, although the role of salinity and total alkalinity, both of which also control pCO_2sea, need to be elucidated by long-term observations and eddy-permitting models for better estimation of the interannual variability of air-sea CO₂ flux.     

Seasonal and interannual variability of carbon cycle in South China Sea: A three-dimensional physical-biogeochemical modeling study

The South China Sea (SCS) exhibits strong variations on seasonal to interannual time scale, and the changing Southeast Asian Monsoon has direct impacts on the nutrients and phytoplankton dynamics, as well as the carbon cycle. A Pacific basin-wide physical-biogeochemical model has been developed and used to investigate the physical variations, ecosystem responses, and carbon cycle consequences. The Pacific basin-wide circulation model, based on the Regional Ocean Model Systems (ROMS) with a 50-km spatial resolution, is driven with daily air-sea fluxes derived from the National Centers for Environmental Prediction (NCEP) reanalysis between 1990 and 2004. The biogeochemical processes are simulated with the Carbon, Si(OH)₄, Nitrogen Ecosystem (CoSINE) model consisting of multiple nutrients and plankton functional groups and detailed carbon cycle dynamics. The ROMS-CoSINE model is capable of reproducing many observed features and their variability over the same period at the SouthEast Asian Time-series Study (SEATS) station in the SCS. The integrated air-sea CO₂ flux over the entire SCS reveals a strong seasonal cycle, serving as a source of CO₂ to the atmosphere in spring, summer and autumn, but acting as a sink of CO₂ for the atmosphere in winter. The annual mean sea-to-air CO₂ flux averaged over the entire SCS is +0.33 moles CO₂ m⁻²year⁻¹, which indicates that the SCS is a weak source of CO₂ to the atmosphere. Temperature has a stronger influence on the seasonal variation of pCO₂ than biological activity, and is thus the dominant factor controlling the oceanic pCO₂ in the SCS. The water temperature, seasonal upwelling and Kuroshio intrusion determine the pCO₂ differences at coast of Vietnam and the northwestern region of the Luzon Island. The inverse relationship between the interannual variability of Chl-a in summer near the coast of Vietnam and NINO₃ SST (Sea Surface Temperature) index in January implies that the carbon cycle and primary productivity in the SCS is teleconnected to the Pacific-East Asian large-scale climatic variability.     

The Western North Pacific Playing a Key Role in Global Biogeochemical Fluxes

The oceanic biogeochemical fluxes in the North Pacific, especially its northwestern part, are discussed to prove their importance on a global scale. First, the air-sea exchange processes of chemical substances are considered quantitatively. The topics discussed are sea salt particles transported to land, sporadic transport of soil dust to the ocean and its role in the marine ecosystem, the larger gas transfer velocity of CO₂ indicating the effect of bubbles, and DMS and greenhouse gases other than CO₂. Next, chemical tracers are utilized to reveal the water circulation systems in the region, which are the Pacific Deep Water including its vertical eddy diffusivity, the North Pacific Intermediate Water and the Japan Sea Deep Water. Thirdly, the particulate transport process of chemical substances through the water column is clarified by analyzing the distribution of insoluble radionuclides and the results obtained from sediment trap experiments. Fourthly, the northern North Pacific is characterized by stating the site decomposing organic matter and Si playing a key role in the marine ecosystem. Both are induced by the upwelled Pacific Deep Water. Fifthly, the oceanic CO₂ system related to global warming is presented by clarifying the distribution of anthropogenic CO₂ in the western North Pacific, and roles of the upwelled Pacific Deep Water and the continental shelf zone in the absorption of atmospheric CO₂. Finally, Mn and other chemical substances in sediments are discussed as recorders of the early diagenesis and indicators of low biological productivity during glacial ages in the northwestern North Pacific. It is concluded that the western North Pacific is characterized mainly by the Pacific Deep Water bringing nutrients to the northern North Pacific, located at the exit of the global deep water circulation and, therefore, the region plays a key role in the global biogeochemical fluxes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Trace gas disequilibria during deep-water formation

We present high-precision measurements by a new isotope dilution technique of a suite of inert gases in the North Pacific. Remarkably smooth gradients in Ar, Kr and Xe from near equilibrium in intermediate waters to several percent undersaturated in deep waters were observed. The general pattern in the deepest waters was that Ar, Kr and Xe were undersaturated (Ar least and Xe most), while N₂ was close to equilibrium, and Ne was supersaturated. We propose that this pattern was produced by the interaction between the different physical properties of the gases (solubility and the temperature dependence of solubility) with the rapid cooling and high wind speeds that characterize deep-water formation regions. In a simple model of deep-water formation by convection, the saturations of the more temperature-sensitive gases were quickly driven down by rapid cooling and could not reequilibrate with the atmosphere before the end of the winter. In contrast, the gas exchange rate of the more bubble-sensitive gases (Ne and N₂) was able to meet or exceed the drawdown by cooling. Our simple convection model demonstrates that the heavier noble gases (Ar, Kr and Xe) are sensitive on seasonal timescales to the competing effects of cooling and air–sea gas exchange that are also important to setting the concentration of CO₂ in newly formed waters.     

Carbon dioxide in surface seawater of the eastern North Pacific Ocean (Line P), 1973–2005

Partial pressure of CO₂ (pCO₂) in surface seawater has been measured in the northeastern Pacific Ocean at Station P and along Line P since 1973. These data have been divided into ‘oceanic’ and ‘coastal/transition’ zones, and the seasonal and interannual variability and the long-term trends for each zone have been examined. The oceanic zone shows little seasonality in surface seawater pCO₂, with undersaturation throughout the year. A strong, biologically-driven seasonal cycle is offset by variation in temperature-dependent solubility of CO₂. The coastal/transition zone shows a decline in pCO₂ from winter–spring through summer and fall that is likely the result of seasonal stratification and convection rather than coastal upwelling. Interannual variability all along Line P is correlated with the multivariate ENSO index (MEI), with lower seawater pCO₂ associated with El Niño conditions. Correlations with the Pacific Decadal Oscillation Index are similar but weaker, in part because there are few data prior to the 1976 regime shift. The long-term trend in seawater pCO₂ in the oceanic zone is +1.36±0.16 μatm year^?1, indistinguishable from the atmospheric growth rate, and varies little among the seasons. In the coastal/transition zone a slow increase in the pCO₂ of surface seawater relative to that of the atmosphere has led to increasing undersaturation, particularly in spring. Aliasing of the seasonal and interannual variability due to sampling frequency may explain part of the observed trend in the coastal/transition zone, but real changes in physical or biological processes are also possible and require more detailed study.     

Seasonal and Interannual Variability in the Distribution of Surface Nutrients and Dissolved Inorganic Carbon in the Northern North Pacific: Influence of El Niño

The seasonal and interannual changes in surface nutrients, dissolved inorganic carbon (DIC) and total alkalinity (TA) were recorded in the North Pacific (30–54°N) from 1995 to 2001. This study focuses on the region north of the subarctic boundary (∼40°N) where there was extensive monthly coverage of surface properties. The nutrient cycles showed large interannual variations in the eastern and western subarctic gyres. In the Alaska Gyre the seasonal depletion of nitrate (ΔNO₃) increased from 8–14 μmol kg⁻¹ in 1995–1999 to 21.5 μmol kg⁻¹ in 2000. In the western subarctic the shifts were similar in amplitude but more frequent. The large ΔNO₃ levels were associated with high silicate depletions, indicating enhanced diatom production. The seasonal DIC:NO₃ drawdown ratios were elevated in the eastern and central subarctic due to calcification. In the western subarctic and the central Bering Sea calcification was significant only during 1997 and/or 1998, two El Ni?o years. Regional C/N stoichiometric molar ratios of 5.7 to 7.0 (>40°N) were determined based on the years with negligible or no calcification. The annual new production (NP_a) based on ΔNO₃ and these C/N ratios showed large interannual variations. NP_a was usually higher in the western than in the eastern subarctic. However, values of 84 gC m⁻²yr⁻¹ were found in the Alaska Gyre in 2000 which is similar to that in the most productive provinces of the northern North Pacific. There were also large increases in NP_a around the Alaska Peninsula in 1997 and 1998. Finally, the net removal of carbon by the biological pump was estimated as 0.72 Gt C yr⁻¹ in the North Pacific (>30°N). This revised version was published online in August 2006 with corrections to the Cover Date.     

Interannual variations of the air-sea carbon dioxide exchange in the different regions of the Pacific Ocean

Interannual variations of the air-sea CO 2 exchange from 1965 to 2000 in the Pacific Ocean are studied with a Pacific Ocean model.Two numerical experiments are performed,including the control run that is forced by climatological monthly mean physical data and the climate-change run that is forced by interannually varying monthly mean physical data.Climatological monthly winds are used in both runs to calculate the coefficient of air-sea CO 2 exchange.The analysis through the differences between the two runs shows that in the tropical Pacific the variation of export production induced by interannual variations of the physical fields is negatively correlated with that of the air-sea CO 2 flux,while there is no correlation or a weak positive correlation in the subtropical North and South Pacific.It indicates that the variation of the physical fields can modulate the variation of the air-sea CO 2 flux in converse ways in the tropical Pacific by changing the direct transport and biochemical process.Under the interannually varying monthly mean forcing,the simulated EOF1 of the air-sea CO 2 flux is basically consistent with that of sea surface temperature(SST) in the tropical Pacific,but contrary in the two subtropical Pacific Ocean.The correlation coefficient between the regionally integrated air-sea CO 2 flux and area-mean SST shows that when the air-sea CO 2 flux lags SST by about 5 months,the positive coefficient in the three regions is largest,indicating that in the tropical Pacific or on the longer time scale in the three regions,physical processes control the flux-SST relationship.     

Carbon dioxide partial pressures in North Pacific surface waters. 2. General late summer distribution

Observations of the equilibrium partial pressure of carbon dioxide in the surface waters of the North Pacific Ocean and Bering Sea indicate conditions of local upwelling or vertical mixing near the Aleutian Island passes, seasonal depletion of CO₂ in the sea surface by photosynthesis, and conditions of CO₂ supersaturation in the surface waters off the mouths of large rivers. Horizontal mixing has a large effect on the P_CO2 distribution. The area distribution of carbon dioxide in the surface waters of the Pacific Ocean from 19°N to 55°N latitude and in the Bering Sea is presented.     

Monsoon-driven biogeochemical processes in the Arabian Sea

Although it is nominally a tropical locale, the semiannual wind reversals associated with the Monsoon system of the Arabian Sea result annually in two distinct periods of elevated biological activity. While in both cases monsoonal forcing drives surface layer nutrient enrichment that supports increased rates of primary productivity, fundamentally different entrainment mechanisms are operating in summer (Southwest) and winter (Northeast) Monsoons. Moreover, the intervening intermonsoon periods, during which the region relaxes toward oligotrophic conditions more typical of tropical environments, provide a stark contrast to the dynamic biogeochemical activity of the monsoons. The resulting spatial and temporal variability is great and provides a significant challenge for ship-based surveys attempting to characterize the physical and biogeochemical environments of the region. This was especially true for expeditions in the pre-satellite era.Here, we present an overview of the dynamical response to seasonal monsoonal forcing and the characteristics of the physical environment that fundamentally drive regional biogeochemical variability. We then review past observations of the biological distributions that provided our initial insights into the pelagic system of the Arabian Sea. These evolved through the 1980s as additional methodologies, in particular the first synoptic ocean color distributions gathered by the Coastal Zone Color Scanner, became available. Through analyses of these observations and the first large-scale physical–biogeochemical modeling attempts, a pre-JGOFS understanding of the Arabian Sea emerged. During the 1990s, the in situ and remotely sensed observational databases were significantly extended by regional JGOFS activities and the onset of Sea-viewing Wide Field-of-View Sensor ocean color measurements. Analyses of these new data and coupled physical–biogeochemical models have already advanced our understanding and have led to either an amplification or revision of the pre-JGOFS paradigms. Our understanding of this complex and variable ocean region is still evolving. Nonetheless, we have a much better understanding of time–space variability of biogeochemical properties in the Arabian Sea and much deeper insights about the physical and biological factors that drive them, as well as a number of challenging new directions to pursue.     

Intercomparison of shipboard and moored CARIOCA buoy seawater fCO2 measurements in the Sargasso Sea

The ocean is an important sink for carbon and heat, yet high-resolution measurements of biogeochemical properties relevant to global climate change are being made only sporadically in the ocean at present. There is a growing need for automated, real-time, long-term measurements of CO₂ in the ocean using a network of sensors, strategically placed on ships, moorings, free-drifting buoys and autonomous remotely operated vehicles. The ground-truthing of new sensor technologies is a vital component of present and future efforts to monitor changes in the ocean carbon cycle and air–sea exchange of CO₂.A comparison of a moored Carbon Interface Ocean Atmosphere (CARIOCA) buoy and shipboard fugacity of CO₂ (fCO₂) measurements was conducted in the western North Atlantic during two extended periods (>1 month) in 1997. The CARIOCA buoy was deployed on the Bermuda Testbed Mooring (BTM), which is located 5 km north of the site of the US Joint Global Ocean Flux Study (JGOFS) Bermuda Atlantic Time-series Study (BATS). The high frequency of sampling revealed that temperature and fCO₂ responded to physical forcing by the atmosphere on timescales from diurnal to 4–8 days. Concurrent with the deployments of the CARIOCA buoy, frequent measurements of surface fCO₂ were made from the R/V Weatherbird II during opportunistic visits to the BTM and BATS sites, providing a direct calibration of the CARIOCA buoy fCO₂ data. Although, the in situ ground-truthing of the CARIOCA buoy was complicated by diurnal processes, sub-mesoscale and fine-scale variability, the CARIOCA buoy fCO₂ data was accurate within 3±6 μatm of shipboard fCO₂ data for periods up to 50 days. Longer-term assessments were not possible due to the CARIOCA buoy breaking free of the BTM and drifting into waters with different fCO₂-temperature properties. Strategies are put forward for future calibration of other in situ sensors.     

Latitudinal Distribution of CO2 Fugacity along 175°E in the North Pacific in 1992–1996

In order to examine latitudinal distribution and seasonal change of the surface oceanic fCO₂, we analyzed the data obtained in the North Pacific along 175°E during the NOPACCS cruises in spring and summer of 1992–1996. Except for around the equator where the fCO₂ was significantly affected by the upwelling of deep water, the latitudinal distribution of fCO₂ showed distinctive seasonal variation. In the spring, the fCO₂ decreased and then increased going southward with the minimum value of about 300 µatm around 35°N, while in the summer, the fCO₂ displayed high variability, showing minimum and maximum values at latitudes of around 44° and 35°N, respectively. It was also found that the fCO₂ was well correlated with the SST, but the relationship between the two was different for different hydrographic regions. In the subpolar gyre, the frontal regions between the Water-Mass Front and the Kuroshio bifurcation front, and between the Kuroshio bifurcation front and the Kuroshio Extension current, SST, DIC and TA influenced the seasonal fCO₂ change through seasonally-dependent biological activities and vertical mixing and stratification of seawater. In the central subtropical gyre and the North Equatorial current, the seasonal fCO₂ change was found to be produced basically by changes in SST and DIC. The summertime oceanic fCO₂ generally increased with time over the period covered by this study, but the increased rate was clearly higher than those expected from other measurements in the western North Pacific.     

Seasonal variability of dissolved oxygen,percent oxygen saturation,and apparent oxygen utilization in the Atlantic and Pacific Oceans

Using objectively analyzed seasonal fields of dissolved oxygen content, percent oxygen saturation, and apparent oxygen utilization (AOU), we describe the large-scale seasonal variability of oxygen for the Atlantic and Pacific Oceans in the upper 400 m. The winter minus summer basin zonal averages of AOU reveal a two-layer feature in both the Atlantic and the Pacific, for both hemispheres. Biological activity and seasonal stratification in the summer give the upper 50–75 m of the water column in each basin a lower AOU in summer than winter. Greater mixing of upper ocean waters in winter gives the 75–400 m layer lower AOU values in that season. The basin integral seasonal volumes of oxygen for both the North Atlantic and the North Pacific mirror what is occurring in the atmosphere, indicating that there is a seasonal flux of oxygen across the air–sea interface. Winter total O₂ volume in the ocean is above the annual mean; the summer volume is below. Larger seasonal differences in the total O₂ content are observed in the North Atlantic Ocean than the North Pacific Ocean. A seasonal net outgassing (SNO) of 8.3×10¹⁴ moles O₂ is calculated from basin means, which is 25% higher than previous results.     

Utility of an automatic water sampler to observe seasonal variability in nutrients and DIC in the Northwestern North Pacific

Concentrations of nutrients (NO₃, NO₂, Si(OH)₄, PO₄ and dissolved inorganic carbon (DIC) were measured in a series of seawater samples collected over approximately 15 months in 2005 and 2006 by an automatic water sampler (Remote Access Sampler, RAS) in the Northwestern North Pacific. Seasonal variability and concentrations of NO₃ + NO₂ (NO_x and Si(OH)₄ were comparable to previous shipboard observations, although there were small errors associated with measurements of PO₄ and DIC. Concentrations of these nutrients began to decrease in late April. After the end of June, NO_x and Si(OH)₄ decreased rapidly, with large fluctuations. After October, these nutrients increased again until late spring 2006. The ratio of the decrease of Si(OH)₄ to that of NO_x suggests that numbers of biogenic opal-producing creatures, such as diatoms, increased after the end of June. This conclusion was supported by a rapid increase in biogenic opal flux recorded in a sediment trap at 150 m. The relationship between NO_x concentrations at the RAS depth of 35 m and NO_x integrated over the upper 100 m was determined using previous shipboard hydrocast data. This relationship was used to estimate integrated mixed layer NO_x concentration from RAS data. Estimated new production based on seasonal drawdown of integrated NO_x averaged approximately 156 mg-C m⁻²day⁻¹ annually, which agrees with previous estimates. Thus, an automatic seawater sampler that documents annual maximum and minimum nutrient concentrations and episodic events such as storms and spring blooms, which might be missed by an ordinary research vessel, will contribute to time-series observations of nutrients and, by extension, biological pump activity.     

Alkalinity changes in the Sargasso Sea: geochemical evidence of calcification?

Strong seasonal patterns in upper ocean total carbon dioxide (TCO₂), alkalinity (TA) and calculated pCO₂ were observed in a time series of water column measurements collected at the US Joint Global Ocean Flux Study (JGOFS) BATS site (31 °50′N, 64 °10′W) in the Sargasso Sea. TA distribution was a conservative function of salinity. However, in February 1992, a non-conservative decrease in TA was observed, with maximum depletion of 25–30 μmoles kg⁻¹ occuring in the surface layer and at the depth of the chlorophyll maximum (˜ 80–100 m). Mixed-layer TCO₂ also decreased, while surface pCO₂ increased by 25–30 μatm. We suggest these changes in carbon dioxide species resulted from open-ocean calcification by carbonate-secreting organisms rather than physical processes. Coccolithophore calcification is the most likely cause of this event although calcification by foraminifera or pteropods cannot be ruled out. Due to the transient increase in surface pCO₂, the net annual transfer of CO₂ into the ocean at BATS was reduced. These observations demonstrate the potential importance of open-ocean calcification and biological community structure in the biogeochemical cycling of carbon.     

Total amount of oceanic excess CO2 taken from the North Pacific subpolar region

Concentrations of total carbonate, alkalinity and dissolved oxygen were obtained near the 1973 GEOSECS stations in the North Pacific subpolar region north of 40°N along 175°E between 1993 and 1994. A difference of excess CO₂ content between the GEOSECS and our expeditions was estimated. The maximum difference in water column inventory of excess CO₂ has increased by about 280 gC m^–2 above 2000 m depth which apparently means an uptake of excess CO₂ taken from air to sea during the last two decades. An averaged value of the annual flux of excess CO₂ at 75–1000 m depth was 8.63±2.01 gC m^–2yr^–1 in the North Pacific subpolar region. By introducing the annual flux of excess CO₂ into a two-box model for the North Pacific subpolar region, a penetration factor of excess CO₂ from air to sea was obtained to be 1.08×10^–2 gC m^–3ppm^–1 in the North Pacific subpolar region. Based on this factor, the surface concentration of excess CO₂ in the North Pacific subpolar region was estimated to be 68 mole I^–1, suggesting that the North Pacific subpolar region absorbed atmospheric excess CO₂ more than the saturated concentration of excess CO₂. Total amount of excess CO₂ taken from the North Pacific subpolar region by 1993 was estimated to be 36.2×10¹⁵ gC, which was equal to about one tenth of that released by human activities after the preindustrial era.