Moussab/ORKG-training-evaluation-set huggingface dataset:
Research field: Oceanography
Title: Nitrous oxide cycling in the Arabian Sea
Abstract: Depth profiles of dissolved nitrous oxide (N2O) were measured in the central and western Arabian Sea during four cruises in May and July–August 1995 and May–July 1997 as part of the German contribution to the Arabian Sea Process Study of the Joint Global Ocean Flux Study. The vertical distribution of N2O in the water column on a transect along 65°E showed a characteristic double-peak structure, indicating production of N2O associated with steep oxygen gradients at the top and bottom of the oxygen minimum zone. We propose a general scheme consisting of four ocean compartments to explain the N2O cycling as a result of nitrification and denitrification processes in the water column of the Arabian Sea. We observed a seasonal N2O accumulation at 600–800 m near the shelf break in the western Arabian Sea. We propose that, in the western Arabian Sea, N2O might also be formed during bacterial oxidation of organic matter by the reduction of IO3 − to I−, indicating that the biogeochemical cycling of N2O in the Arabian Sea during the SW monsoon might be more complex than previously thought. A compilation of sources and sinks of N2O in the Arabian Sea suggested that the N2O budget is reasonably balanced.
Extracted statement — Region of data collection: Arabian Sea
Research field: Oceanography
Title: Nitrous oxide emissions from the Arabian Sea: A synthesis
Abstract: Abstract. We computed high-resolution (1o latitude x 1o longitude) seasonal and annual nitrous oxide (N2O) concentration fields for the Arabian Sea surface layer using a database containing more than 2400 values measured between December 1977 and July 1997. N2O concentrations are highest during the southwest (SW) monsoon along the southern Indian continental shelf. Annual emissions range from 0.33 to 0.70 Tg N2O and are dominated by fluxes from coastal regions during the SW and northeast monsoons. Our revised estimate for the annual N2O flux from the Arabian Sea is much more tightly constrained than the previous consensus derived using averaged in-situ data from a smaller number of studies. However, the tendency to focus on measurements in locally restricted features in combination with insufficient seasonal data coverage leads to considerable uncertainties of the concentration fields and thus in the flux estimates, especially in the coastal zones of the northern and eastern Arabian Sea. The overall mean relative error of the annual N2O emissions from the Arabian Sea was estimated to be at least 65%.
Extracted statement — Region of data collection: Arabian Sea
Research field: Oceanography
Title: Strong CO2emissions from the Arabian Sea during south-west monsoon
Abstract: The partial pressure of CO2 (pCO2) was measured during the 1995 South‐West Monsoon in the Arabian Sea. The Arabian Sea was characterized throughout by a moderate supersaturation of 12–30 µatm. The stable atmospheric pCO2 level was around 345 µatm. An extreme supersaturation was found in areas of coastal upwelling off the Omani coast with pCO2 peak values in surface waters of 750 µatm. Such two‐fold saturation (218%) is rarely found elsewhere in open ocean environments. We also encountered cold upwelled water 300 nm off the Omani coast in the region of Ekman pumping, which was also characterized by a strongly elevated seawater pCO2 of up to 525 µatm. Due to the strong monsoonal wind forcing the Arabian Sea as a whole and the areas of upwelling in particular represent a significant source of atmospheric CO2 with flux densities from around 2 mmol m−2 d−1 in the open ocean to 119 mmol m−2 d−1 in coastal upwelling. Local air masses passing the area of coastal upwelling showed increasing CO2 concentrations, which are consistent with such strong emissions.
Extracted statement — Region of data collection: Arabian Sea
Research field: Oceanography
Title: Nitrogen budgets following a Lagrangian strategy in the Western Tropical South Pacific Ocean: the prominent role of N&lt;sub&gt;2&lt;/sub&gt; fixation (OUTPACE cruise)
Abstract: <jats:p>Abstract. We performed N budgets at three stations in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (Feb. Mar. 2015) and quantified all major N fluxes both entering the system (N2 fixation, nitrate eddy diffusion, atmospheric deposition) and leaving the system (PN export). Thanks to a Lagrangian strategy, we sampled the same water mass for the entire duration of each long duration (5 days) station, allowing to consider only vertical exchanges. Two stations located at the western end of the transect (Melanesian archipelago (MA) waters, LD A and LD B) were oligotrophic and characterized by a deep chlorophyll maximum (DCM) located at 51 ± 18 m and 81 ± 9 m at LD A and LD B. Station LD C was characterized by a DCM located at 132 ± 7 m, representative of the ultra-oligotrophic waters of the South Pacific gyre (SPG water). N2 fixation rates were extremely high at both LD A (593 ± 51 µmol N m−2 d−1) and LD B (706 ± 302 µmol N m−2 d−1), and the diazotroph community was dominated by Trichodesmium. N2 fixation rates were lower (59 ± 16 µmol N m−2 d−1) at LD C and the diazotroph community was dominated by unicellular N2-fixing cyanobacteria (UCYN). At all stations, N2 fixation was the major source of new N (&gt; 90 %) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N2 fixation contributed circa 8–12 % of primary production in the MA region and 3 % in the SPG water and sustained nearly all new primary production at all stations. The e-ratio (e-ratio = PC export/PP) was maximum at LD A (9.7 %) and was higher than the e-ratio in most studied oligotrophic regions (~ 1 %), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6 % of the PC export at LD A, while there contribution was 5 and 
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Extracted statement — Season: Austral summer
Research field: Oceanography
Title: Chemoautotrophy in the redox transition zone of the Cariaco Basin: A significant midwater source of organic carbon production
Abstract: During the CARIACO time series program, microbial standing stocks, bacterial production, and acetate turnover were consistently elevated in the redox transition zone (RTZ) of the Cariaco Basin, the depth interval (~240–450 m) of steepest gradient in oxidation‐reduction potential. Anomalously high fluxes of particulate carbon were captured in sediment traps below this zone (455 m) in 16 of 71 observations. Here we present new evidence that bacterial chemoautotrophy, fueled by reduced sulfur species, supports an active secondary microbial food web in the RTZ and is potentially a large midwater source of labile, chemically unique, sedimenting biogenic debris to the basin's interior. Dissolved inorganic carbon assimilation (27–159 mmol C m−2 d−1) in this zone was equivalent to 10%–333% of contemporaneous primary production, depending on the season. However, vertical diffusion rates to the RTZ of electron donors and electron acceptors were inadequate to support this production. Therefore, significant lateral intrusions of oxic waters, mixing processes, or intensive cycling of C, S, N, Mn, and Fe across the RTZ are necessary to balance electron equivalents. Chemoautotrophic production appears to be decoupled temporally from short‐term surface processes, such as seasonal upwelling and blooms, and potentially is more responsive to long‐term changes in surface productivity and deep‐water ventilation on interannual to decadal timescales. Findings suggest that midwater production of organic carbon may contribute a unique signature to the basin's sediment record, thereby altering its paleoclimatological interpretation.
Extracted statement — Region of data collection: Cariaco Basin
Research field: Oceanography
Title: N2 Fixation in the Eastern Arabian Sea: Probable Role of Heterotrophic Diazotrophs
Abstract: Biogeochemical implications of global imbalance between the rates of marine dinitrogen (N2) fixation and denitrification have spurred us to understand the former process in the Arabian Sea, which contributes considerably to the global nitrogen budget. Heterotrophic bacteria have gained recent appreciation for their major role in marine N budget by fixing a significant amount of N2. Accordingly, we hypothesize a probable role of heterotrophic diazotrophs from the 15N2 enriched isotope labelling dark incubations that witnessed rates comparable to the light incubations in the eastern Arabian Sea during spring 2010. Maximum areal rates (8 mmol N m-2 d-1) were the highest ever observed anywhere in world oceans. Our results suggest that the eastern Arabian Sea gains ~92% of its new nitrogen through N2 fixation. Our results are consistent with the observations made in the same region in preceding year, i.e., during the spring of 2009.
Extracted statement — Region of data collection: Eastern Arabian Sea
Research field: Oceanography
Title: Dynamic responses of picophytoplankton to physicochemical variation in the eastern Indian Ocean
Abstract: Abstract Picophytoplankton were investigated during spring 2015 and 2016 extending from near‐shore coastal waters to oligotrophic open waters in the eastern Indian Ocean (EIO). They were typically composed of Prochlorococcus (Pro), Synechococcus (Syn), and picoeukaryotes (PEuks). Pro dominated most regions of the entire EIO and were approximately 1–2 orders of magnitude more abundant than Syn and PEuks. Under the influence of physicochemical conditions induced by annual variations of circulations and water masses, no coherent abundance and horizontal distributions of picophytoplankton were observed between spring 2015 and 2016. Although previous studies reported the limited effects of nutrients and heavy metals around coastal waters or upwelling zones could constrain Pro growth, Pro abundance showed strong positive correlation with nutrients, indicating the increase in nutrient availability particularly in the oligotrophic EIO could appreciably elevate their abundance. The exceptional appearance of picophytoplankton with high abundance along the equator appeared to be associated with the advection processes supported by the Wyrtki jets. For vertical patterns of picophytoplankton, a simple conceptual model was built based upon physicochemical parameters. However, Pro and PEuks simultaneously formed a subsurface maximum, while Syn generally restricted to the upper waters, significantly correlating with the combined effects of temperature, light, and nutrient availability. The average chlorophyll a concentrations (Chl a) of picophytoplankton accounted for above 49.6% and 44.9% of the total Chl a during both years, respectively, suggesting that picophytoplankton contributed a significant proportion of the phytoplankton community in the whole EIO.
Extracted statement — Region of data collection: Eastern Indian Ocean
Research field: Oceanography
Title: Dinitrogen Fixation Across Physico‐Chemical Gradients of the Eastern Tropical North Pacific Oxygen Deficient Zone
Abstract: The Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (<20 μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (>9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ.
Extracted statement — Region of data collection: Eastern tropical north Pacific Ocean
Research field: Oceanography
Title: High new production in the Bay of Bengal: Possible causes and implications
Abstract: We report the first measurements of new production (15N tracer technique), the component of primary production that sustains on extraneous nutrient inputs to the euphotic zone, in the Bay of Bengal. Experiments done in two different seasons consistently show high new production (averaging around 4 mmol N m−2 d−1 during post monsoon and 5.4 mmol N m−2 d−1 during pre monsoon), validating the earlier conjecture of high new production, based on pCO2 measurements, in the Bay. Averaged over annual time scales, higher new production could cause higher rate of removal of organic carbon. This could also be one of the reasons for comparable organic carbon fluxes observed in the sediment traps of the Bay of Bengal and the eastern Arabian Sea. Thus, oceanic regions like Bay of Bengal may play a more significant role in removing the excess CO2 from the atmosphere than hitherto believed.
Extracted statement — Sampling depth covered (m): Euphotic zone
Research field: Oceanography
Title: Nitrogen Uptake in the Northeastern Arabian Sea during Winter Cooling
Abstract: The uptake of dissolved inorganic nitrogen by phytoplankton is an important aspect of the nitrogen cycle of oceans. Here, we present nitrate () and ammonium () uptake rates in the northeastern Arabian Sea using tracer technique. In this relatively underexplored region, productivity is high during winter due to supply of nutrients by convective mixing caused by the cooling of the surface by the northeast monsoon winds. Studies done during different months (January and late February-early March) of the northeast monsoon 2003 revealed a fivefold increase in the average euphotic zone integrated uptake from January (2.3 mmolN ) to late February-early March (12.7 mmolN ). The -ratio during January appeared to be affected by the winter cooling effect and increased by more than 50% from the southernmost station to the northern open ocean stations, indicating hydrographic and meteorological control. Estimates of residence time suggested that entrained in the water column during January contributed to the development of blooms during late February-early March.
Extracted statement — Sampling depth covered (m): Euphotic zone
Research field: Oceanography
Title: Heterotrophic bacteria as major nitrogen fixers in the euphotic zone of the Indian Ocean
Abstract: Diazotrophy in the Indian Ocean is poorly understood compared to that in the Atlantic and Pacific Oceans. We first examined the basin‐scale community structure of diazotrophs and their nitrogen fixation activity within the euphotic zone during the northeast monsoon period along about 69°E from 17°N to 20°S in the oligotrophic Indian Ocean, where a shallow nitracline (49–59 m) prevailed widely and the sea surface temperature (SST) was above 25°C. Phosphate was detectable at the surface throughout the study area. The dissolved iron concentration and the ratio of iron to nitrate + nitrite at the surface were significantly higher in the Arabian Sea than in the equatorial and southern Indian Ocean. Nitrogen fixation in the Arabian Sea (24.6–47.1 μmolN m−2 d−1) was also significantly greater than that in the equatorial and southern Indian Ocean (6.27–16.6 μmolN m−2 d−1), indicating that iron could control diazotrophy in the Indian Ocean. Phylogenetic analysis of nifH showed that most diazotrophs belonged to the Proteobacteria and that cyanobacterial diazotrophs were absent in the study area except in the Arabian Sea. Furthermore, nitrogen fixation was not associated with light intensity throughout the study area. These results are consistent with nitrogen fixation in the Indian Ocean, being largely performed by heterotrophic bacteria and not by cyanobacteria. The low cyanobacterial diazotrophy was attributed to the shallow nitracline, which is rarely observed in the Pacific and Atlantic oligotrophic oceans. Because the shallower nitracline favored enhanced upward nitrate flux, the competitive advantage of cyanobacterial diazotrophs over nondiazotrophic phytoplankton was not as significant as it is in other oligotrophic oceans.
Extracted statement — Water column zone: Euphotic zone
Research field: Oceanography
Title: Contribution of picoplankton to the total particulate organic carbon (POC) concentration in the eastern South Pacific
Abstract: Abstract. Prochlorococcus, Synechococcus, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration, derived from the total particle beam attenuation coefficient (cp), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for Prochlorococcus, which was not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the cp signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particle beam attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both total chlorophyll a (i.e. mono + divinyl chlorophyll a) concentration and cp. Finally, picophytoeukaryotes contributed ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by size measurements (i.e. equivalent spherical diameter) on cells sorted by flow cytometry and optical theory. Although there are some uncertainties associated with these estimates, the new approach used in this work further supports the idea that picophytoeukaryotes play a dominant role in carbon cycling in the upper open ocean, even under hyper-oligotrophic conditions.
Extracted statement — Material/Method: Flow cytometry
Research field: Oceanography
Title: Inverse estimates of anthropogenic CO2uptake, transport, and storage by the ocean: AIR-SEA EXCHANGE OF ANTHROPOGENIC CARBON
Abstract: Regional air‐sea fluxes of anthropogenic CO2 are estimated using a Green's function inversion method that combines data‐based estimates of anthropogenic CO2 in the ocean with information about ocean transport and mixing from a suite of Ocean General Circulation Models (OGCMs). In order to quantify the uncertainty associated with the estimated fluxes owing to modeled transport and errors in the data, we employ 10 OGCMs and three scenarios representing biases in the data‐based anthropogenic CO2 estimates. On the basis of the prescribed anthropogenic CO2 storage, we find a global uptake of 2.2 ± 0.25 Pg C yr−1, scaled to 1995. This error estimate represents the standard deviation of the models weighted by a CFC‐based model skill score, which reduces the error range and emphasizes those models that have been shown to reproduce observed tracer concentrations most accurately. The greatest anthropogenic CO2 uptake occurs in the Southern Ocean and in the tropics. The flux estimates imply vigorous northward transport in the Southern Hemisphere, northward cross‐equatorial transport, and equatorward transport at high northern latitudes. Compared with forward simulations, we find substantially more uptake in the Southern Ocean, less uptake in the Pacific Ocean, and less global uptake. The large‐scale spatial pattern of the estimated flux is generally insensitive to possible biases in the data and the models employed. However, the global uptake scales approximately linearly with changes in the global anthropogenic CO2 inventory. Considerable uncertainties remain in some regions, particularly the Southern Ocean.
Extracted statement — Method: Green's function inversion method
Research field: Oceanography
Title: Sea–air CO&lt;sub&gt;2&lt;/sub&gt; fluxes in the Indian Ocean between 1990 and 2009
Abstract: <jats:p>Abstract. The Indian Ocean (44° S–30° N) plays an important role in the global carbon cycle, yet it remains one of the most poorly sampled ocean regions. Several approaches have been used to estimate net sea–air CO2 fluxes in this region: interpolated observations, ocean biogeochemical models, atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Indian Ocean sea–air CO2 fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea–air CO2 uptake of −0.37 ± 0.06 PgC yr−1 is consistent with the −0.24 ± 0.12 PgC yr−1 calculated from observations. The fluxes from the southern Indian Ocean (18–44° S; −0.43 ± 0.07 PgC yr−1 are similar in magnitude to the annual uptake for the entire Indian Ocean. All models capture the observed pattern of fluxes in the Indian Ocean with the following exceptions: underestimation of upwelling fluxes in the northwestern region (off Oman and Somalia), overestimation in the northeastern region (Bay of Bengal) and underestimation of the CO2 sink in the subtropical convergence zone. These differences were mainly driven by lack of atmospheric CO2 data in atmospheric inversions, and poor simulation of monsoonal currents and freshwater discharge in ocean biogeochemical models. Overall, the models and inversions do capture the phase of the observed seasonality for the entire Indian Ocean but overestimate the magnitude. The predicted sea–air CO2 fluxes by ocean biogeochemical models (OBGMs) respond to seasonal variability with strong phase lags with reference to climatological CO2 flux, whereas the atmospheric inversions predicted an order of magnitude higher seasonal flux than OBGMs. The simulated interannual variability by the OBGMs is weaker than that found by atmospheric inversions. Prediction of such weak interannual variability in CO2 fluxes by atmospheric inversions was mainly caused by a lack of atmospheric data in the Indian Ocean. The OBGM models suggest a small strengthening of the sink over the period 1990–2009 of −0.01 PgC decade−1. This is inconsistent with the observations in the southwestern Indian Ocean that shows the growth rate of oceanic pCO2 was faster than the observed atmospheric CO2 growth, a finding attributed to the trend of the Southern Annular Mode (SAM) during the 1990s.
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Extracted statement — Region of data collection: Indian Ocean