Global Oceanic Source Research Articles

Biological N2O Fixation in the Eastern South Pacific Ocean and Marine Cyanobacterial Cultures

Despite the importance of nitrous oxide (N2O) in the global radiative balance and atmospheric ozone chemistry, its sources and sinks within the Earth’s system are still poorly understood. In the ocean, N2O is produced by microbiological processes such as nitrification and partial denitrification, which account for about a third of global emissions. Conversely, complete denitrification (the dissimilative reduction of N2O to N2) under suboxic/anoxic conditions is the only known pathway accountable for N2O consumption in the ocean. In this work, it is demonstrated that the biological assimilation of N2O could be a significant pathway capable of directly transforming this gas into particulate organic nitrogen (PON). N2O is shown to be biologically fixed within the subtropical and tropical waters of the eastern South Pacific Ocean, under a wide range of oceanographic conditions and at rates ranging from 2 pmol N L−1 d− to 14.8 nmol N L−1 d−1 (mean ± SE of 0.522±1.06 nmol N L−1 d−1, n = 93). Additional assays revealed that cultured cyanobacterial strains of Trichodesmium (H-9 and IMS 101), and Crocosphaera (W-8501) have the capacity to directly fix N2O under laboratory conditions; suggesting that marine photoautotrophic diazotrophs could be using N2O as a substrate. This metabolic capacity however was absent in Synechococcus (RCC 1029). The findings presented here indicate that assimilative N2O fixation takes place under extreme environmental conditions (i.e., light, nutrient, oxygen) where both autotrophic (including cyanobacteria) and heterotrophic microbes appear to be involved. This process could provide a globally significant sink for atmospheric N2O which in turn affects the oceanic N2O inventory and may also represent a yet unexplored global oceanic source of fixed N.

Optimal estimation of the surface fluxes of methyl chloride using a 3-D global chemical transport model

Abstract. Methyl chloride (CH3Cl) is a chlorine-containing trace gas in the atmosphere contributing significantly to stratospheric ozone depletion. Large uncertainties in estimates of its source and sink magnitudes and temporal and spatial variations currently exist. GEIA inventories and other bottom-up emission estimates are used to construct a priori maps of the surface fluxes of CH3Cl. The Model of Atmospheric Transport and Chemistry (MATCH), driven by NCEP interannually varying meteorological data, is then used to simulate CH3Cl mole fractions and quantify the time series of sensitivities of the mole fractions at each measurement site to the surface fluxes of various regional and global sources and sinks. We then implement the Kalman filter (with the unit pulse response method) to estimate the surface fluxes on regional/global scales with monthly resolution from January 2000 to December 2004. High frequency observations from the AGAGE, SOGE, NIES, and NOAA/ESRL HATS in situ networks and low frequency observations from the NOAA/ESRL HATS flask network are used to constrain the source and sink magnitudes. The inversion results indicate global total emissions around 4100 ± 470 Gg yr−1 with very large emissions of 2200 ± 390 Gg yr−1 from tropical plants, which turn out to be the largest single source in the CH3Cl budget. Relative to their a priori annual estimates, the inversion increases global annual fungal and tropical emissions, and reduces the global oceanic source. The inversion implies greater seasonal and interannual oscillations of the natural sources and sink of CH3Cl compared to the a priori. The inversion also reflects the strong effects of the 2002/2003 globally widespread heat waves and droughts on global emissions from tropical plants, biomass burning and salt marshes, and on the soil sink.

Nitrous oxide and methane in the Atlantic Ocean between 50°N and 52°S: Latitudinal distribution and sea-to-air flux

Abstract We discuss nitrous oxide (N2O) and methane (CH4) distributions in 49 vertical profiles covering the upper ∼300 m of the water column along two ∼13,500 km transects between ∼50°N and ∼52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N2O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH4 showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N2O concentrations coincident with very low levels of CH4, located between ∼23.5°N and ∼23.5°S; this feature reflects the upwelling of deep waters containing N2O derived from nitrification, as identified by an analysis of N2O, apparent oxygen utilization (AOU) and NO3−, and presumably depleted in CH4 by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to ∼42% of the Atlantic Ocean surface area were in the range 0.40–0.68 Tg N2O yr−1 and 0.81–1.43 Tg CH4 yr−1. Based on contemporary estimates of the global ocean source strengths of atmospheric N2O and CH4, the Atlantic Ocean could account for ∼6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N2O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH4 than previously thought.

Halocarbon and dimethyl sulphide studies around the Mascarene Plateau

Air-sea exchange is thought to be one of the major routes by which halocarbons and dimethyl sulphide reach the troposphere and stratosphere. Once there, in different ways, they participate in chemical reactions that have implications for ozone depletion and climate change. The gases are released by phytoplankton and other algae, but our present understanding of the sources and sinks is insufficient to establish a balanced global budget. Published data suggest that there are regions of coastal and ocean waters that constitute a major source, but, for halocarbons, in other regions the ocean is a net sink. For example, in many open oceanic areas, the rate of degradation of methyl bromide outweighs production. Here we present data from the Central Indian Ocean, a region considered to be low in terms of biological productivity. Little is known about trace-gas release from the Central Indian Ocean and without such data it is impossible to even hazard a guess at the global ocean source to the atmosphere.

Air‐sea flux of bromoform: Controls, rates, and implications

Bromoform (CHBr3) is the largest single source of atmospheric organic bromine and therefore of importance as a source of reactive halogens to the troposphere and lower stratosphere. The sea‐to‐air flux, originating with macroalgal and planktonic sources, is the main source for atmospheric bromoform. We review bromoform's contribution to atmospheric chemistry, its atmospheric and oceanic distributions and its oceanic sources and sinks. We have reassessed oceanic emissions, based on published aqueous and airborne concentration data, global climatological parameters, and information concerning coastal and biogenic sources. The goals are to attempt an estimate of the global source strength and partly to identify key regions that require further investigation. The sea‐to‐air flux is spatially and temporally variable with tropical, subtropical and shelf waters identified as potentially important source regions. We obtain an annual global flux of bromoform of ∼10 Gmol Br yr−1 (3–22 Gmol Br yr−1). This estimate is associated with significant uncertainty, arising from data precision and coverage, choice of air‐sea exchange parameterizations and model assumptions. Anthropogenic sources of ∼0.3 (to 1.1) Gmol Br yr−1 (as CHBr3) can be locally significant, but are globally negligible. Our estimate of the global oceanic source is three to four times higher than recent estimates based on the modeling of atmospheric sinks. The reasons for this discrepancy could lie with the limited regional and temporal data available and the broad assumptions that underlie our flux calculations. Alternatively, atmospheric sink calculations, often made on the basis of background CHBr3 levels, may neglect the influence of strong but highly localized sources (e.g., from some coastal and shelf regions). The strongly variable and poorly characterized source of CHBr3, together with its short atmospheric lifetime, complicates model‐based estimation of the distribution of reactive Br resulting from its atmospheric degradation. An integrated program of marine and atmospheric observations, atmospheric modeling and mechanistic studies of oceanic bromoform production is required to better constrain present and future Br delivery to the atmosphere.

Global significance of nitrous‐oxide production and transport from oceanic low‐oxygen zones: A modeling study

Recent studies of marine nitrons oxide have focused attention on the suboxic and low‐oxygen zones associated with ocean basin eastern boundaries. It has been suggested that complex N2O cycling mechanisms in these regions may provide a net source to the oceanic interior and a significant portion of the ocean‐atmosphere flux. In this study we evaluate the global significance of N2O formation in these regions. N2O is treated as a nonconserved tracer in an ocean general circulation model; a simple source function is developed which models N2O production as a function of organic matter remineralization and local oxygen concentration. Model results are evaluated against both surface and deep observational data sets. The oceanic oxygen minimum zones are predominantly found in the upper water column of tropical latitudes and overlain by regions of strong upwelling in the surface ocean. Simulations of increased N2O production under low‐oxygen conditions indicate that the majority of the N2O thus formed escapes directly to the atmosphere and is not subject to significant meridional transport. Results indicate that while enhanced N2O production in these regions cannot be held accountable for the majority of the sea‐air flux and interior distribution, it may, however, have significance for the local distribution and provide as much as 25–50% of the global oceanic source.

The solubility of a suite of low molecular weight organochlorine compounds in seawater and implications for estimating the marine source of methyl chloride to the atmosphere

Oceanic measurements made to determine the fluxes of methyl chloride between the sea surface and atmosphere have pointed to the possibility of significant errors in literature values for the solubility of this gas in seawater. In this work the Henry's Law constant for CH3Cl in seawater has been measured as lnH=8.83−2866/T, where T is the absolute temperature. Also reported are solubilities for CH2Cl2, CHCl3, C2HCl3 and C2Cl4. The influence of the new value for CH3Cl on estimates of its global oceanic source to the atmosphere is discussed. The net flux of CH3Cl from the ocean to the atmosphere is estimated to be in the range: 6.4–8×109 moles yr−1 (0.3–0.4 T g yr−1).

Variabilities in the fluxes and annual emissions of nitrous oxide from the Arabian Sea

Extensive measurements of nitrous oxide (N2O) have been made during April–May 1994 (intermonsoon), February–March 1995 (northeast monsoon), July–August 1995 and August 1996 (southwest monsoon) in the Arabian Sea. Low N2O supersaturations in the surface waters are observed during intermonsoon compared to those in northeast and southwest monsoons. Spatial distributions of supersaturations manifest the effects of larger mixing during winter cooling and wind‐driven upwelling during monsoon period off the Indian west coast. A net positive flux is observable during all the seasons, with no discernible differences from the open ocean to coastal regions. The average ocean‐to‐atmosphere fluxes of N2O are estimated, using wind speed dependent gas transfer velocity, to be of the order of 0.26, 0.003, and 0.51, and 0.78 pg (pico grams) cm−2 s−1 during northeast monsoon, intermonsoon, and southwest monsoon in 1995 and 1996, respectively. The lower range of annual emission of N2O is estimated to be 0.56–0.76 Tg N2O per year which constitutes 13–17% of the net global oceanic source. However, N2O emission from the Arabian Sea can be as high as 1.0 Tg N2O per year using different gas transfer models.

Ocean‐atmosphere exchange of methyl chloride: Results from NW Atlantic and Pacific Ocean studies

Measurements of methyl chloride concentrations and saturations in the NW Atlantic and Pacific Ocean from 40°N to 40°S are presented. Labrador Sea waters, analyzed in July, were at or below saturation, while warmer waters south of the Gulf Stream and all Pacific samples were consistently supersaturated. We find no correlation between methyl chloride concentrations and chlorophylla, but much of the variation in methyl chloride partial pressure appears to be accounted for by variations in sea surface temperature. We estimate ocean‐atmosphere fluxes of methyl chloride, using exchange velocities for spot wind speeds and climatological wind speeds. These indicate that none of the waters we sampled support the magnitude of fluxes which have hitherto been considered to be emitted by the ocean. A simple extrapolation yields a global efflux of 0.8 to 1.1 × 1010mol yr−1(0.4 to 0.6 Tg yr−1) supplied by warm waters and a high‐latitude influx of 0.2 to 0.5 × 1010mol yr−1(0.1 to 0.3 Tg yr−1). The net global ocean source to the atmosphere is 0.3 to 0.9 × 1010mol yr−1(0.2–0.4 Tg yr−1), which accounts for 5 to 12% of the total required to balance loss by reaction with hydroxyl radicals. Unless its atmospheric reactivity has been overestimated, we conclude that other sources of methyl chloride make major contributions to its atmospheric budget.

Global oceanic emissions of nitrous oxide

The global N2O flux from the ocean to the atmosphere is calculated based on more than 60,000 expedition measurements of the N2O anomaly in surface water. The expedition data are extrapolated globally and coupled to daily air‐sea gas transfer coefficients modeled at 2.8°×2.8° resolution to estimate a global ocean source of about 4 (1.2–6.8) Tg N yr−1. The wide range of uncertainty in the source estimate arises mainly from uncertainties in the air‐sea gas transfer coefficients and in the global extrapolation of the summertime‐biased surface N2O data set. The strongest source is predicted from the 40–60°S latitude band. Strong emissions also are predicted from the northern Pacific Ocean, the equatorial upwelling zone, and coastal upwelling zones occurring predominantly in the tropical northern hemisphere. High apparent oxygen utilization (AOU) at 100 m below the mixed layer is found to be correlated positively both to N2O production at depth and to the surface N2O anomaly. On the basis of these correlations, the expedition data are partitioned into two subsets associated with high and low AOU at depth. The zonally averaged monthly means in each subset are extrapolated to produce two latitude‐by‐month matrices in which monthly surface N2O is expressed as the deviation from the annual mean. Both matrices contain large uncertainties. The low‐AOU matrix, which mainly includes surface N2O data from the North Atlantic and the subtropical gyres, suggests many regions with positive summer deviations and negative winter deviations, consistent with a seasonal cycle predominantly driven by seasonal heating and cooling of the surface ocean. The high‐AOU subset, which includes the regions most important to the global N2O ocean source, suggests some regions with positive winter deviations and negative summer deviations, consistent with a seasonal cycle predominantly driven by wintertime mixing of surface water with N2O‐rich deep water. Coupled seasonal changes in gas transfer coefficients and surface N2O in these important source regions could strongly influence the global ocean source.