Chisato Yoshikawa

Climate-carbon cycle feedback analysis: Results from the C4MIP model intercomparison

Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850–2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0.1° and 1.5°C. All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models.

Geographical distribution of the feedback between future climate change and the carbon cycle

[i] We examined climate-carbon cycle feedback by performing a global warming experiment using MIROC-based coupled climate-carbon cycle model. The model showed that by the end of the 21st century, warming leads to a further increase in carbon dioxide (CO 2 ) level of 123 ppm by volume (ppmv). This positive feedback can mostly be attributed to land-based soil-carbon dynamics. On a regional scale, Siberia experienced intense positive feedback, because the acceleration of microbial respiration due to warming causes a decrease in the soil carbon level. Amazonia also had positive feedback resulting from accelerated microbial respiration. On the other hand, some regions, such as western and central North America and South Australia, experienced negative feedback, because enhanced litterfall surpassed the increased respiration in soil carbon. The oceanic contribution to the feedback was much weaker than the land contribution on global scale, but the positive feedback in the northern North Atlantic was as strong as those in Amazonia and Siberia in our model. In the northern North Atlantic, the weakening of winter mixing caused a reduction of CO 2 absorption at the surface. Moreover, weakening of the formation of North Atlantic Deep Water caused reduced CO 2 subduction to the deep water. Understanding such regional-scale differences may help to explain disparities in coupled climate-carbon cycle model results.

Nitrogen isotope ratios of nitrate and N* anomalies in the subtropical South Pacific

Nitrogen isotopic ratios of nitrate (δ15N– NO3−) were analyzed above 1000 m water depth along 17°S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The δ15N– NO3− and N* values were as high as 17‰ and as low as −18 μmol N L−1, respectively, at depths around 250 m east of 115°W, but were as low as 5‰ and as high as +1 μmol N L−1, respectively, in subsurface waters west of 170°W. The relationships among NO3− concentrations, N* values, δ15N– NO3− values, and oxygen and nitrite concentrations suggest that a few samples east of 90°W were from suboxic and nitrite-accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high-δ15N– NO3− and negative-N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between δ15N– NO3− and N* values, as well as Trichodesmium abundances and size-specific nitrogen fixation rates at the surface, suggest that the low-δ15N– NO3− and positive-N* subsurface waters between 160°E and 170°W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the δ15N values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific.

Origin and fluxes of nitrous oxide along a latitudinal transect in western North Pacific: Controls and regional significance

Nitrous oxide (N2O) is an atmospheric trace gas playing an important role in both radiative forcing and stratospheric ozone depletion. The oceans are the second most important natural source of N2O. The magnitude of the flux of this source is poorly constrained. Moreover, the relative importance of the microbial processes leading to the formation or the consumption of N2O in oceans remains unclear. We present here fluxes and isotope and isotopomer signatures of N2O measured at three stations located along a latitudinal transect in subtropical and subarctic western North Pacific. These results indicate that about 30% to 55% of the oceanic flux of N2O to the atmosphere originates from the deep euphotic and shallow aphotic zones. The sea-to-air fluxes of N2O calculated using an isotope mass balance model indicate that the emission rate of N2O in subarctic waters is about 2 times higher than in oligotrophic subtropical waters suggesting that nutrient-rich water coming from the western subarctic gyre stimulates the N2O production. Moreover, isotopomer analysis has revealed that in shallow water N2O originates from nitrification and nitrifier denitrification processes, and its distribution in the water column is partly controlled by the incident solar radiation. The results of this study contribute to better constrain the global N2O budget and provide important information to better predict the future evolution of the oceanic emissions of N2O.

Comparison of carbon cycle between the western Pacific subarctic and subtropical time-series stations: highlights of the K2S1 project

A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.

Hadal water biogeochemistry over the Izu–Ogasawara Trench observed with a full-depth CTD-CMS

Full-depth profiles of hydrographic and geochemical properties at the Izu–Ogasawara Trench were observed for the first time using a CTD-CMS (conductivity– temperature–depth profiler with carousel multiple sampling) system. Additionally, comparative samplings were done at the northern Mariana Trench using the same methods. A well-mixed hydrographic structure below 7000 m was observed within the Izu–Ogasawara Trench. Seawater samples collected from this well-mixed hadal layer exhibited constant concentrations of nitrate, phosphate, silicate, and nitrous oxide as well as constant nitrogen and oxygen isotopic compositions of nitrate and nitrous oxide. These results agree well with previous observations of the Izu–Ogasawara hadal waters and deep-sea water surrounding the Izu–Ogasawara Trench. In turn, methane concentrations and isotopic compositions indicated spatial heterogeneity within the well-mixed hadal water mass, strongly suggesting a local methane source within the trench, in addition to the background methane originating from the general deep-sea bottom water. Sedimentary compound releases, associated with sediment resuspensions, are considered to be the most likely mechanism for generating this significant CH4 anomaly.

Basin-scale distribution of NH4+ and NO2− in the Pacific Ocean

We used more than 25,000 nutrient samples to elucidate for the first time basin-scale distributions and seasonal changes of surface ammonium (NH4+) and nitrite (NO2−) concentrations in the Pacific Ocean. The highest NH4+, NO2−, and nitrate (NO3−) concentrations were observed north of 40°N, in the coastal upwelling region off the coast of Mexico, and in the Tasman Sea. NH4+ concentrations were elevated during May–October in the western subarctic North Pacific, May–December in the eastern subarctic North Pacific, and June–September in the subtropical South Pacific. NO2− concentrations were highest in winter in both hemispheres. The seasonal cycle of NH4+ was synchronous with NO2−, NO3−, and satellite chlorophyll a concentrations in the western subtropical South Pacific, whereas it was synchronous with chlorophyll-a but out of phase with NO2− and NO3− in the subarctic regions.

Nitrate Isotope Distribution in the Subarctic and Subtropical North Pacific

Nitrogen isotopic composition of nitrate (δ15NNitrate) is widely used as a tracer of ocean-internal nitrogen cycling (consumption and regeneration) and ocean-external nitrogen inputs and losses (N2-fixation; fixation of N2 gas into bioavailable nitrogen such as ammonia by diazotrophs, and denitrification; microbial respiration using nitrate as an electron acceptor). When the phytoplankton assimilates nitrate, nitrogen isotopes are fractionated. A δ15NNitrate value increases, in conjunction with nitrate depletion, due to an isotopic effect during nitrate assimilation by phytoplankton. When denitrification occurs in the water column, a δ15NNitrate value extremely increases due to a strong isotopic effect. N2-fixation produces fixed nitrogen with a δN value of ~0‰, as nitrogen fixers take up N2 gas with little isotopic effect. This fixed nitrogen with low δN value is eventually converted into low-δ15NNitrate through degradation of nitrogenous organic compounds called remineralization and subsequent nitrification. Those signatures of δ15NNitrate in the euphotic zone are conserved in nitrogenous organic compounds and transfers to the sinking particles and deep-sea sediments. Here we determined δ15NNitrate and δ 18ONitrate along 47°N in the subarctic North Pacific and 149°E in the western North Pacific. In the western subarctic gyre, known as High Nutrient, Low Chlorophyll (HNLC) region, the δ15NNitrate and the differences between δ 15NNitrate and δ18ONitrate, or Δ(15-18), in the intermediate and deep waters were significantly lower than the surrounding area. Between the western subarctic gyre and the Alaskan gyre, there was an observed 0.4‰ increase in δ15NNitrate and a 0.6‰ increase in Δ(15-18) associated with a 7.2 μM decrease in the nitrate concentration at the surface. These results suggest that the N-depleted nitrate is generated by nitrified nitrate from remineralization of organic matter synthesized by partial consumption of surface nitrate pool, and the N enrichment toward the east is affected by the increase in utilization of surface nitrate pool. Assuming Rayleigh distillation kinetics, the increase in utilization between the western subarctic gyre and the Alaskan gyre, going from 29% to 85% utilization, corresponded to a change in δN of organic matter from 2.6‰ to 4.9‰. This study also revealed that the N-depleted nitrate in the surface water of the western subtropical gyre is generated by N2-fixation, whereas the N-enriched nitrate in the intermediate water at the western margin of North America is generated by water-column denitrification. The δN sediment record in the western subarctic North Pacific is expected to reflect the past changes in the HNLC region, but may also be controlled by water-column denitrification and N2-fixation.