Export Production in the Equatorial and North Pacific Derived from Dissolved Oxygen, Nutrient and Carbon Data

A global ocean inverse model that includes the 3D ocean circulation as well as the production, sinking and remineralization of biogenic particulate matter is used to estimate the carbon export flux in the Pacific, north of 10°S. The model exploits the existing large datasets for hydrographic parameters, dissolved oxygen, nutrients and carbon, and determines optimal export production rates by fitting the model to the observed water column distributions by means of the “adjoint method”. In the model, the observations can be explained satisfactorily with an integrated carbon export production of about 3 Gt C yr⁻¹ (equivalent to 3⋅10¹⁵ gC yr⁻¹) for the considered zone of the Pacific Ocean. This amounts to about a third of the global ocean carbon export of 9.6 Gt C yr⁻¹ in the model. The highest export fluxes occur in the coastal upwelling region off northwestern America and in the tropical eastern Pacific. Due to the large surface area, the open-ocean, oligotrophic region in the central North Pacific also contributes significantly to the total North Pacific export flux (0.45 Gt C yr⁻¹), despite the rather small average flux densities in this region (13 gC m⁻²yr⁻¹). Model e-ratios (calculated here as ratios of model export production to primary production, as inferred from satellite observations) range from as high a value as 0.4 in the tropical Pacific to 0.17 in the oligotrophic central north Pacific. Model e-ratios in the northeastern Pacific upwelling regions amount to about 0.3 and are lower than previous estimates. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temporal Evolution of the North Pacific CO2 Uptake Rate

The recent changes in the North Pacific uptake rate of carbon have been estimated using a number of different techniques over the past decade. Recently, there has been a marked increase in the number of estimates being submitted for publication. Most of these estimates can be grouped into one of five basic techniques: carbon time-series, non-carbon tracers, carbon tracers, empirical relationships, and inverse calculations. Examples of each of these techniques as they have been applied in the North Pacific are given and the estimates summarized. The results are divided into three categories: integrated water column uptake rate estimates, mixed layer increases, and surface pCO₂ increases. Most of the published values fall under the water column integrated uptake rate category. All of the estimates varied by region and depth range of integration, but generally showed consistent patterns of increased uptake from the tropics to the subtropics. The most disagreement between the methods was in the sub-arctic Pacific. Column integrated uptake rates ranged from 0.25 to 1.3 mol m⁻²yr⁻¹. The mixed layer uptake estimates were much more consistent, with values of 1.0–1.3 μmol kg⁻¹yr⁻¹ based on direct observations and multiple linear regression approaches. Surface pCO₂ changes showed the most obvious regional variability (0.5–2.5 μatm yr⁻¹) reflecting the sensitivity of these measurements to differences in the physical and biological forcing. The different techniques used to evaluate the changes in North Pacific carbon distributions do not completely agree on the exact magnitude or spatial and temporal patterns of carbon uptake rate. Additional research is necessary to resolve these issues and better constrain the role of the North Pacific in the global carbon cycle. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spatiotemporal Decreases of Nutrients and Chlorophyll-a in the Surface Mixed Layer of the Western North Pacific from 1971 to 2000

Using time series of hydrographic data in the wintertime and summertime obtained along 137°E from 1971 to 2000, we found that the average contents of nutrients in the surface mixed layer showed linear decreasing trends of 0.001∼0.004 μmol-PO₄ l⁻¹ yr⁻¹ and 0.01∼0.04 μmol-NO₃ l⁻¹ yr⁻¹ with the decrease of density. The water column Chl-a (CHL) and the net community production (NCP) had also declined by 0.27∼0.48 mg-Chl m⁻² yr⁻¹ and 0.08∼0.47 g-C-NCP m⁻² yr⁻¹ with a clear oscillation of 20.8±0.8 years. These changes showed a strong negative correlation with the Pacific Decadal Oscillation Index (PDO) with a time lag of 2 years (R = 0.89 ± 0.02). Considering the recent significant decrease of O₂ over the North Pacific subsurface water, these findings suggest that the long-term decreasing trend of surface-deep water mixing has caused the decrease of marine biological activity in the surface mixed layer with a bidecadal oscillation over the western North Pacific.     

Rates of calcium carbonate dissolution and organic carbon decomposition in the North Pacific ocean

Recent carbonate data collected in the North Pacific were combined with the data in the literature in order to understand more clearly the carbonate chemistry in the North Pacific. Our analyses show that inorganic CaCO₃ dissolution contributes about 26% of the total inorganic CO₂ increase of deep water, after leaving the Southern Ocean. The calcium and alkalinity data indicate a CaCO₃ dissolution rate of 0.060±0.010 and 0.053±0.005 µ mol kg^–1 yr^–1 respectively, for waters deeper than 2,000 m in reference to the Weddell Sea Deep Water. The organic carbon decomposition rate is 0.107±0.012 µ mol kg^–1 yr^–1 while the oxygen consumption rate is 0.13±0.002 µ mol kg^–1 yr^–1. These results which are based on the direct comparison of two water masses agree well with other estimates which are based on methods such as the one-dimensional-diffusion-advection model. Comparison of data along the two sections at 165°E and 150°W shows no significant difference in the ratio of the CaCO₃ dissolution rate and the organic carbon decomposition rate. The eastern section, however, has a higher TCO₂ input than the western section because of the older age of the deep water along the eastern section.     

Increase of anthropogenic CO2 in the Pacific Ocean over the last two decades

The multiple-parameter linear regression method (Monitoring global ocean carbon inventories. Ocean Observing System Development Panel, Texas A&M University, College Station, TX, 1995, 54pp; Global Biogeochem. Cycles 13 (1999) 179) is used to compare inorganic carbon data from the GEOSECS CO₂ survey in the Pacific Ocean in 1973 to the WOCE/JGOFS global CO₂ survey in the 1990s. A model of total dissolved inorganic carbon (DIC) as a function of five variables (AOU, θ, S, Si, and PO₄) has been developed from the recent CO₂ survey data (namely CGC91 and CGC96) in the Pacific Ocean. After correcting for a systematic DIC offset of −30.3±7 μmol kg⁻¹ from the GEOSECS data, the residual DIC based on this model as computed from GEOSECS data has been used to estimate the anthropogenic CO₂ penetration in the Pacific Ocean. In the Northeast Pacific, we obtained an increase of CO₂ of 21.3±7.9 mol m⁻² over the period from GEOSECS in 1973 to CGC91 in 1991. This gives a mean anthropogenic CO₂ uptake rate of 1.3±0.5 mol m⁻² yr⁻¹ over this 17 year time period. In the South Pacific, north of 50°S between 180° and 120°W region, the integrated anthropogenic CO₂ inventory is estimated to be 19.7±5.7 mol m⁻² over the period from GEOSECS in 1974 to CGC96 in 1996. The equivalent mean CO₂ uptake rate is estimated to be 0.9±0.3 mol m⁻² yr⁻¹ over the 22 years. These results are compared with the isopycnal method (Nature 396 (1998) 560) to estimate the anthropogenic CO₂ signal in the Northeast Pacific (30°N, 152°W) at the crossover region between CGC91 and GEOSECS. The results of the isopycnal method are consistent with those derived from the MLR method. Both methods show an increase in anthropogenic CO₂ inventory in the ocean over two decades that is consistent with the increase expected if the ocean uptake has kept pace with the atmospheric CO₂ increase.     

East-west distribution of POC fluxes estimated from <Superscript>234</Superscript>Th in the northern North Pacific in autumn

Observations of primary productivity, ²³⁴Th, and particulate organic carbon (POC) were made from west to east across the northern North Pacific Ocean (from station K2 to Ocean Station Papa) during September–October 2005. Primary productivities in this region varied longitudinally from approximately 236 to 444 mgC m⁻²d⁻¹ and clearly indicate the West High East Low (WHEL) trend. We estimated east-west variations in the POC flux from the surface layer (0–100 m) by using ²³⁴Th as a tracer. POC fluxes in the western region (44–53 mgC m⁻²d⁻¹) were higher than those in the eastern region (21–34 mgC m⁻²d⁻¹). However, the export ratios (e-ratios) ranged from approximately 8% to 16% and did not show the WHEL trend. Contrary to our expectation, no relation between POC flux (or e-ratio) and diatom biomass (or dominance) was apparent in autumn in the northern North Pacific.     

Increase in anthropogenic CO2 in the Atlantic Ocean in the last two decades

Data from the first systematic survey of inorganic carbon parameters on a global scale, the GEOSECS program, are compared with those collected during WOCE/JGOFS to study the changes in carbon and other geochemical properties, and anthropogenic CO₂ increase in the Atlantic Ocean from the 1970s to the early 1990s. This first data-based estimate of CO₂ increase over this period was accomplished by adjusting the GEOSECS data set to be consistent with recent high-quality carbon data. Multiple Linear Regression (MLR) and extended Multiple Linear Regression (eMLR) analyses to these carbon data are applied by regressing DIC with potential temperature, salinity, AOU, silica, and PO₄ in three latitudinal regions for the western and eastern basins in the Atlantic Ocean. The results from MLR (and eMLR provided in parentheses) indicate that the mean anthropogenic CO₂ uptake rate in the western basin is 0.70 (0.53) mol m^?2 yr^?1 for the region north of 15°N; 0.53 (0.36) mol m^?2 yr^?1 for the equatorial region between 15°N and 15°S; and 0.83 (0.35) mol m^?2 yr^?1 in the South Atlantic south of 15°S. For the eastern basin an estimate of 0.57 (0.45) mol m^?2 yr^?1 is obtained for the equatorial region, and 0.28 (0.34) mol m^?2 yr^?1 for the South Atlantic south of 15°S. The results of using eMLR are systematically lower than those from MLR method in the western basin. The anthropogenic CO₂ increase is also estimated in the upper thermocline from salinity normalized DIC after correction for AOU along the isopycnal surfaces. For these depths the results are consistent with the CO₂ uptake rates derived from both MLR and eMLR methods.     

Particulate carbon and nitrogen fluxes and compositions in the central equatorial Pacific

Mass, carbon, and nitrogen fluxes and carbon and nitrogen compositions were determined for particulate samples from plankton net tows, shallow floating sediment traps, intermediate and deep moored sediment traps, and sediment cores collected along 140°W in the central equatorial Pacific Ocean during the US JGOFS EqPac program. Mass, particulate organic carbon (POC), and particulate inorganic carbon (PIC) fluxes measured by the floating sediment traps during the Survey I (El Niño) and Survey II (non-El Niño) cruises follow essentially the same pattern as primary production: high near the equator and decreasing poleward. POC fluxes caught in free-floating traps were compared with alternative estimates of export fluxes, including ²³⁴Th models, new production, and other sediment trap studies, resulting in widely differing estimates. Applying ²³⁴Th corrections to the trap-based fluxes yielded more consistent results relative to primary production and new production. Despite factors of five differences in measured fluxes between different trap types, POC : ²³⁴Th ratios of trap material were generally within a factor of two and provided a robust means of converting modeled ²³⁴Th export fluxes to POC export fluxes. All measured fluxes decrease with depth. Trap compositional data suggest that mineral “ballasting” may be a prerequisite for POC settling. POC remineralization is most pronounced in the epipelagic zone and at the sediment–water interface, with two orders of magnitude loss at each level. Despite seawater supersaturation with respect to calcium carbonate in the upper ocean, 80% of PIC is dissolved in the epipelagic zone. Given the time-scale differences of processes throughout the water column, the contrasting environments, and the fact that only 0.01% of primary production is buried, sedimentary organic carbon accumulation rates along the transect are remarkably well correlated to primary production in the overlying surface waters. POC to particulate total nitrogen (PTN) ratios for all samples are close to Redfield values, indicating that POC and PTN are non-selectively remineralized. This constancy is somewhat surprising given conventional wisdom and previous equatorial Pacific results suggesting that particulate nitrogen is lost preferentially to organic carbon.     

Oxygen dynamics in the North Atlantic subtropical gyre

Dissolved oxygen (DO) in the ocean is a tracer for most ocean biogeochemical processes including net community production and remineralization of organic matter which in turn constrains the biological carbon pump. Knowledge of oxygen dynamics in the North Atlantic Ocean is mainly derived from observations at the Bermuda Atlantic Time-series Study (BATS) site located in the western subtropical gyre which may skew our view of the biogeochemistry of the subtropical North Atlantic. This study presents and compares a 15 yr record of DO observations from ESTOC (European Station for Time-Series in the Ocean, Canary Islands) in the eastern subtropical North Atlantic with the 20 yr record at BATS. Our estimate for net community production of oxygen was 2.3±0.4 mol O₂ m⁻² yr⁻¹ and of oxygen consumption was −2.3±0.5 mol O₂ m⁻² yr⁻¹ at ESTOC, and 4 mol O₂ m⁻² yr⁻¹ and −4.4±1 mol m⁻² yr⁻¹ at BATS, respectively. These values were determined by analyzing the time-series using the Discrete Wavelet Transform (DWT) method. These flux values agree with similar estimates from in-situ observational studies but are higher than those from modeling studies. The difference in net oxygen production rates supports previous observations of a lower carbon export in the eastern compared to the western subtropical Atlantic. The inter-annual analysis showed clear annual cycles at BATS whereas longer cycles of nearly 4 years were apparent at ESTOC. The DWT analysis showed trends in DO anomalies dominated by long-term perturbations at a basin scale for the consumption zones at both sites, whereas yearly cycles dominated the production zone at BATS. The long-term perturbations found are likely associated with ventilation of the main thermocline, affecting the consumption and production zones at ESTOC.     

Nearshore scavenging phenomenon elucidated by <Superscript>234</Superscript>th/<Superscript>238</Superscript>u disequilibrium in the coastal waters off Western Taiwan

Dissolved and particulate ²³⁴Th activities in surface seawater were determined at 27 stations along the coastline of western Taiwan during 19–23 November 2004. Contrasting scavenging settings were observed between the northern and southern regimes of the nearshore water off western Taiwan, separated by the Cho-Shui River. The northern regime is characterized by a large quantity of suspended load contributed by northward transport of a suspension plume from the Cho-Shui River, while the southern regime, low in suspended load and high in chlorophyll concentration, is a system controlled by biological activity. A scavenging model that takes account of the physical transport was used to estimate the ²³⁴Th budget in order to estimate the scavenging and removal rates from the nearshore water. The scavenging and removal rates ranged from 21 to 127 dpm m⁻³d⁻¹ and from 36 to 525 dpm m⁻³d⁻¹, for dissolved and particulate ²³⁴Th, respectively. The removal fluxes of particulate organic carbon (POC) and particulate organic nitrogen (PON) were estimated by multiplying the particulate ²³⁴Th removal flux to the organic carbon/²³⁴Th and nitrogen/²³⁴Th ratios in suspended particles, which ranged from 4.5 to 275.2 mmol-C m⁻²d⁻¹ and from 1.3 to 50.1 mmol-N m⁻²d⁻¹, respectively. These fluxes resulted in residence times of 1∼20 days for the POC in the surface water of nearshore water off western Taiwan.     

Thermocline ventilation and oxygen utilization rates in the subtropical North Pacific based on CFC distributions during WOCE

Thermocline ventilation rates for the subtropical North Pacific are determined using a 1-dimensional (meridional) along-isopycnal advective–diffusive model tuned to chlorofluorocarbon (CFC) concentrations measured along 152°W in 1991 during WOCE P16. Mean southward advection rates in the subtropics range from 1.03 to 0.56 cm s^-1 between σ_θ=25.5 and 26.6. Model-derived ventilation times for the subtropical gyre increase from about 10 to 27 years for that isopycnal range. Oxygen utilization rates (OURs) determined using the advective-diffusive model decrease with depth from 6.6 to 3.2 μmol kg^-1 yr^-1 between σ_θ=25.5 and 26.6. Extrapolation of the OUR versus depth trend to the base of the euphotic zone with the 1/Z power function of Martin et al. (1987) and integration from 500 to 100 m depth implies a carbon export rate from the overlying euphotic zone of 2.2±0.5 moles C m^-2 yr^-1 at 30°N, 152°W. Analysis of the WOCE radiocarbon and salinity distributions indicates that zonal and cross-isopycnal transport terms would have to be considered in modeling these tracers in the subtropical North Pacific.     

Cold deep water in the South China Sea

Two deep channels that cut through the Luzon Strait facilitate deep (>2000 m) water exchange between the western Pacific Ocean and the South China Sea. Our observations rule out the northern channel as a major exchange conduit. Rather, the southern channel funnels deep water from the western Pacific to the South China Sea at the rate of 1.06 ± 0.44 Sv (1 Sv = 10⁶ m³s⁻¹). The residence time estimated from the observed inflow from the southern channel, about 30 to 71 years, is comparable to previous estimates. The observation-based estimate of upwelling velocity at 2000 m depth is (1.10 ± 0.33) × 10⁻⁶ ms⁻¹, which is of the same order as Ekman pumping plus upwelling induced by the geostrophic current. Historical hydrographic observations suggest that the deep inflow is primarily a mixture of the Circumpolar Deep Water and Pacific Subarctic Intermediate Water. The cold inflow through the southern channel offsets about 40% of the net surface heat gain over the South China Sea. Balancing vertical advection with vertical diffusion, the estimated mean vertical eddy diffusivity of heat is about 1.21 × 10⁻³ m²s⁻¹. The cold water inflow from the southern channel maintains the shallow thermocline, which in turn could breed internal wave activities in the South China Sea.     

Rates of C,N, P and Si recycling and denitrification at the US Mid-Atlantic continental slope depocenter

Benthic fluxes of O₂, titration alkalinity (TA), total inorganic carbon (TIC), Ca²⁺, NO₃⁻, NH₄⁺, PO₄³⁻, and Si(OH)₄ were measured by in situ benthic flux chamber incubations at 13 locations on the North Carolina continental slope. The majority of measurements were made at water depths of approximately 700–850 m, in the previously identified upper slope depocenter. This region is characterized by extremely high organic matter deposition rates and near saturation bottom water oxygen concentrations. Measured benthic fluxes of TA are reasonably correlated with O₂ benthic fluxes. Because bottom waters are supersaturated with respect to calcite and aragonite at these shallow water depths, these results demonstrate the importance of metabolically driven dissolution in this region. Subtraction of the calcium carbonate dissolution contributions from the TIC benthic fluxes suggests rates of organic matter remineralization ranging from 0.97 to 3.9 mol C m⁻² yr⁻¹ at the depocenter sites, a factor of 3–10 greater than estimated for the adjacent continental rise and upper slope areas. Because biological primary production in the overlying waters does not follow this pattern, these extremely high values are most likely supported by lateral inputs of highly reactive organic matter. Mass balance calculations indicate that despite the oxygenated bottom water conditions, 68% of the organic nitrogen released during organic matter remineralization processes is ultimately denitrified. The release of PO₄³⁻ from the depocenter sediments is equivalent to or larger than that predicted from the remineralization of Redfield organic matter. This implies either that PO₄³⁻ is preferentially released in this setting and that the accumulating sediments must be depleted in PO₄³⁻ relative to organic carbon or that another, non-organic, phase is contributing PO₄³⁻ to the system. The molar ratio of the Si benthic flux and organic carbon remineralization rate ranges from 0.30 to 0.86. This is significantly greater than the ratio reported for most pelagic diatoms. Possible reasons for this high ratio include the deposition of benthic diatoms that may have a larger Si : C ratio than pelagic diatoms, the near-bottom lateral input of partially reworked organic matter that may have an elevated Si : C ratio relative to fresh diatoms, preferential loss of carbon in sinking particulates or the release of Si from non-opaline materials.     

Holocene accumulation of organic carbon at the Laptev Sea continental margin (Arctic Ocean): sources, pathways, and sinks

Composition and accumulation rates of organic carbon in Holocene sediments provided data to calculate an organic carbon budget for the Laptev Sea continental margin. Mean Holocene accumulation rates in the inner Laptev Sea vary between 0.14 and 2.7 g C cm⁻² ky⁻¹; maximum values occur close to the Lena River delta. Seawards, the mean accumulation rates decrease from 0.43 to 0.02 g C cm⁻² ky⁻¹. The organic matter is predominantly of terrigenous origin. About 0.9 × 10⁶ t year⁻¹ of organic carbon are buried in the Laptev Sea, and 0.25 × 10⁶ t year⁻¹ on the continental slope. Between about 8.5 and 9 ka, major changes in supply of terrigenous and marine organic carbon occur, related to changes in coastal erosion, Siberian river discharge, and/or Atlantic water inflow along the Eurasian continental margin. Received: 26 October 1998 / Revision accepted: 15 June 1999     

Distribution and burial of organic carbon in sediments from the Indian Ocean upwelling region off Java and Sumatra,Indonesia

Sediments were sampled and oxygen profiles of the water column were determined in the Indian Ocean off west and south Indonesia in order to obtain information on the production, transformation, and accumulation of organic matter (OM). The stable carbon isotope composition (δ¹³C_org) in combination with C/N ratios depicts the almost exclusively marine origin of sedimentary organic matter in the entire study area. Maximum concentrations of organic carbon (C_org) and nitrogen (N) of 3.0% and 0.31%, respectively, were observed in the northern Mentawai Basin and in the Savu and Lombok basins. Minimum δ¹⁵N values of 3.7‰ were measured in the northern Mentawai Basin, whereas they varied around 5.4‰ at stations outside this region. Minimum bottom water oxygen concentrations of 1.1 mL L^?1, corresponding to an oxygen saturation of 16.1%, indicate reduced ventilation of bottom water in the northern Mentawai Basin. This low bottom water oxygen reduces organic matter decomposition, which is demonstrated by the almost unaltered isotopic composition of nitrogen during early diagenesis. Maximum C_org accumulation rates (CARs) were measured in the Lombok (10.4 g C m^?2 yr^?1) and northern Mentawai basins (5.2 g C m^?2 yr^?1). Upwelling-induced high productivity is responsible for the high CAR off East Java, Lombok, and Savu Basins, while a better OM preservation caused by reduced ventilation contributes to the high CAR observed in the northern Mentawai Basin. The interplay between primary production, remineralisation, and organic carbon burial determines the regional heterogeneity. CAR in the Indian Ocean upwelling region off Indonesia is lower than in the Peru and Chile upwellings, but in the same order of magnitude as in the Arabian Sea, the Benguela, and Gulf of California upwellings, and corresponds to 0.1–7.1% of the global ocean carbon burial. This demonstrates the relevance of the Indian Ocean margin off Indonesia for the global OM burial.     

Sedimentation rates and heavy metal pollution of sediments in the Seto Inland Sea

Sedimentation rates in ten sediment cores from Hiroshima Bay in the Seto Inland Sea of Japan were determined with the |2210|0Pb technique, and heavy metals were analyzed. The sedimentation rates vary from 0.18 to 0.33 g cm|2-2|0 yr|2-1|0. The highest sedimentation rates were observed in the northern part of the bay at the mouth of Ota River, while lower sedimentation rates not more than 0.20 g cm⁻² yr⁻¹ were observed at stations close to narrow water-ways, or where water depth was shallow. The contents of copper and zinc in the sediment cores began to increase around 1930 as a result of increased human activity, and have remained almost unchanged since 1970 possibly because of regulation of pollutant discharge. The natural background values of copper and zinc in the sediment of this bay range from 16 to 27 mgkg⁻¹ and 70 to 105 mg kg⁻¹, respectively. The total amounts of anthropogenic copper and zinc deposited in the sediments since about 1930 are estimated to be 0.5–2.7 ton km⁻² and 2.2–14.5 ton km⁻², respectively. At the present-day, the anthropogenic loads of copper and zinc to the sediments of the whole bay are 26 ton yr⁻¹ and 183 ton yr⁻¹, and these values constitute 39% and 48% of the total sedimentary loads at the present-day, respectively.     

Accumulation and Export of Dissolved Organic Carbon in Surface Waters of Subtropical and Tropical Pacific Ocean

Dissolved organic carbon (DOC) concentrations in surface waters of the Pacific Ocean during October–November, 1995, were determined using a high-temperature combustion method. The DOC in the surface mixed-layer was approximately homogeneous with a concentration between 55 and 89 μmol C l⁻¹. This homogeneity indicates that there is a strong control of the vertical distribution of DOC by mixing processes. The DOC concentrations in the mixed-layer in the subtropical region were up to 27 μmol C l⁻¹ higher than in the tropical region. This difference reflects the subtropical accumulation and the tropical export of DOC. There is a significant positive correlation between DOC and chlorophyll a concentrations in the mixed-layer of the North Pacific subtropical region, suggesting that phytoplankton is the primary source of DOC accumulated in this region. Calculations using simple box models suggest that DOC export in the tropical region (0–50 m depth, 10°N-10°S, along 160°W) occurs primarily by poleward advection at a rate of 0.5–3 mmol C m⁻²day⁻¹. A comparison with estimates of the export rate of particulate organic carbon published in previous studies leads us to conclude that DOC export may contribute less to the carbon budget in the tropical region than has recently been supposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean

Most marginal seas in the North Pacific are fed by nutrients supported mainly by upwelling and many are undersaturated with respect to atmospheric CO₂ in the surface water mainly as a result of the biological pump and winter cooling. These seas absorb CO₂ at an average rate of 1.1 ± 0.3 mol C m⁻²yr⁻¹ but release N₂/N₂O at an average rate of 0.07 ± 0.03 mol N m⁻²yr⁻¹. Most of primary production, however, is regenerated on the shelves, and only less than 15% is transported to the open oceans as dissolved and particulate organic carbon (POC) with a small amount of POC deposited in the sediments. It is estimated that seawater in the marginal seas in the North Pacific alone may have taken up 1.6 ± 0.3 Gt (10¹⁵ g) of excess carbon, including 0.21 ± 0.05 Gt for the Bering Sea, 0.18 ± 0.08 Gt for the Okhotsk Sea; 0.31 ± 0.05 Gt for the Japan/East Sea; 0.07 ± 0.02 Gt for the East China and Yellow Seas; 0.80 ± 0.15 Gt for the South China Sea; and 0.015 ± 0.005 Gt for the Gulf of California. More importantly, high latitude marginal seas such as the Bering and Okhotsk Seas may act as conveyer belts in exporting 0.1 ± 0.08 Gt C anthropogenic, excess CO₂ into the North Pacific Intermediate Water per year. The upward migration of calcite and aragonite saturation horizons due to the penetration of excess CO₂ may also make the shelf deposits on the Bering and Okhotsk Seas more susceptible to dissolution, which would then neutralize excess CO₂ in the near future. Further, because most nutrients come from upwelling, increased water consumption on land and damming of major rivers may reduce freshwater output and the buoyancy effect on the shelves. As a result, upwelling, nutrient input and biological productivity may all be reduced in the future. As a final note, the Japan/East Sea has started to show responses to global warming. Warmer surface layer has reduced upwelling of nutrient-rich subsurface water, resulting in a decline of spring phytoplankton biomass. Less bottom water formation because of less winter cooling may lead to the disappearance of the bottom water as early as 2040. Or else, an anoxic condition may form as early as 2200 AD. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inputs of organic carbon from Ria de Aveiro coastal lagoon to the Atlantic Ocean

Land/ocean boundaries constitute complex systems with active physical and biogeochemical processes that affect the global carbon cycle. An example of such a system is the mesotidal lagoon named Ria de Aveiro (Portugal, 40°38′N, 08°45′W), which is connected to the Atlantic Ocean by a single channel, 350 m wide. The objective of this study was to estimate the seasonal and inter-tidal variability of organic carbon fluxes between the coastal lagoon and the Ocean, and to assess the contribution of the organic carbon fractions (i.e. dissolved organic carbon (DOC) and particulate organic carbon (POC)) to the export of organic carbon to the Ria de Aveiro plume zone. The organic carbon fractions fluxes were estimated as the product of the appropriate fractional organic carbon concentrations and the water fluxes calculated by a two-dimensional vertically integrated hydrodynamic model (2DH). Results showed that the higher exchanges of DOC and POC fractions at the system cross-section occurred during spring tides but only resulted in a net export of organic carbon in winter, totalling 85 t per tidal cycle. Derived from the winter and summer campaigns, the annual carbon mass balance estimated corresponded to a net export of organic carbon (7957 = 6585 t yr⁻¹ POC + 1372 t yr⁻¹ DOC). On the basis of the spring tidal drainage area, it corresponds to an annual flux of 79 g m⁻² of POC and 17 g m⁻² of DOC out of the estuary.     

O<Subscript>2</Subscript> consumption rate and its isotopic fractionation factor in the deep water of the Philippine Sea

Concentration and stable isotopic compositions (δ ¹⁸O) of dissolved O₂ were measured in seawater samples collected from the Philippine Sea in June 2006. The in-situ O₂ consumption rate and the isotopic fractionation factor (α _r ) during dissolved O₂ consumption were obtained from field observations by applying a vertical one-dimensional advection diffusion model to the deep water mass of about 1000–4000 m. The average O₂ consumption rate and α _r were, respectively, 0.11 ± 0.07 μmol kg⁻¹yr⁻¹ and 0.990 ± 0.001. These estimated values agree well with values from earlier estimations of Pacific deep water. The in-situ O₂ consumption rates are two or more times higher north of 20°N, although the value of α _r was not significantly different between the north and south. Its levels varied rapidly in the water mass of less about 2000 m depth. These results suggest that organic matter from the continent imparts a meaningful contribution to the upper water in the northern part of the area; it might produce the strong O₂ minimum that is evident in the water mass from about 1000–2000 m in the northern part of the Philippine Sea.