Search

Human activities are changing the Arctic environment at an unprecedented rate resulting in rapid warming, freshening, sea ice retreat and ocean acidification of the Arctic Ocean. Trace gases such as nitrous oxide (N2O) and methane (CH4) play important roles in both the atmospheric reactivity and radiative budget of the Arctic and thus have a high potential to influence the region’s climate. However, little is known about how these rapid physical and chemical changes will impact the emissions of major climate-relevant trace gases from the Arctic Ocean. The combined consequences of these stressors present a complex combination of environmental changes which might impact on trace gas production and their subsequent release to the Arctic atmosphere. Here we present our current understanding of nitrous oxide and methane cycling in the Arctic Ocean and its relevance for regional and global atmosphere and climate and offer our thoughts on how this might change over coming decades. Supplementary Information The online version contains supplementary material available at 10.1007/s13280-021-01633-8.

Dimethyl sulphide (DMS) and carbon monoxide (CO) are climate-relevant trace gases that play key roles in the radiative budget of the Arctic atmosphere. Under global warming, Arctic sea ice retreats at an unprecedented rate, altering light penetration and biological communities, and potentially affect DMS and CO cycling in the Arctic Ocean. This could have socio-economic implications in and beyond the Arctic region. However, little is known about CO production pathways and emissions in this region and the future development of DMS and CO cycling. Here we summarize the current understanding and assess potential future changes of DMS and CO cycling in relation to changes in sea ice coverage, light penetration, bacterial and microalgal communities, pH and physical properties. We suggest that production of DMS and CO might increase with ice melting, increasing light availability and shifting phytoplankton community. Among others, policy measures should facilitate large-scale process studies, coordinated long term observations and modelling efforts to improve our current understanding of the cycling and emissions of DMS and CO in the Arctic Ocean and of global consequences.

Air–sea carbon dioxide ( CO2 ) flux is often indirectly estimated by the bulk method using the air–sea difference in CO2 fugacity ( Δf CO2 ) and a parameterisation of the gas transfer velocity ( K ). Direct flux measurements by eddy covariance (EC) provide an independent reference for bulk flux estimates and are often used to study processes that drive K . However, inherent uncertainties in EC air–sea CO2 flux measurements from ships have not been well quantified and may confound analyses of K . This paper evaluates the uncertainties in EC CO2 fluxes from four cruises. Fluxes were measured with two state-of-the-art closed-path CO2 analysers on two ships. The mean bias in the EC CO2 flux is low, but the random error is relatively large over short timescales. The uncertainty (1 standard deviation) in hourly averaged EC air–sea CO2 fluxes (cruise mean) ranges from 1.4 to 3.2 mmol m - 2 d - 1 . This corresponds to a relative uncertainty of ∼ 20 % during two Arctic cruises that observed large CO2 flux magnitude. The relative uncertainty was greater ( ∼ 50 %) when the CO2 flux magnitude was small during two Atlantic cruises. Random uncertainty in the EC CO2 flux is mostly caused by sampling error. Instrument noise is relatively unimportant. Random uncertainty in EC CO2 fluxes can be reduced by averaging for longer. However, averaging for too long will result in the inclusion of more natural variability. Auto-covariance analysis of CO2 fluxes suggests that the optimal timescale for averaging EC CO2 flux measurements ranges from 1 to 3 h, which increases the mean signal-to-noise ratio of the four cruises to higher than 3. Applying an appropriate averaging timescale and suitable Δf CO2 threshold (20 µatm ) to EC flux data enables an optimal analysis of K .

Air-sea carbon dioxide (CO2) flux is generally estimated by the bulk method using upper ocean CO2 fugacity measurements. In the summertime Arctic, sea-ice melt results in stratification within the upper ocean (top ∼10 m), which can bias bulk CO2 flux estimates when the seawater CO2 fugacity is taken from a ship's seawater inlet at ∼6 m depth (fCO2w_bulk). Direct flux measurements by eddy covariance are unaffected by near-surface stratification. We use eddy covariance CO2 flux measurements to infer sea surface CO2 fugacity (fCO2w_surface) in the Arctic Ocean. In sea-ice melt regions, fCO2w_surface values are consistently lower than fCO2w_bulk by an average of 39 μatm. Lower fCO2w_surface can be partially accounted for by fresher (≥27%) and colder (17%) melt waters. A back-of-the-envelope calculation shows that neglecting the summertime sea-ice melt could lead to a 6%–17% underestimate of the annual Arctic Ocean CO2 uptake.

The flux of terrigenous organic carbon through estuaries is an important and changing, yet poorly understood, component of the global carbon cycle. Using dissolved organic carbon (DOC) and fluorescence data from 13 British estuaries draining catchments with highly variable land uses, we show that land use strongly influences the fate of DOC across the land ocean transition via its influence on the composition and lability of the constituent dissolved organic matter (DOM). In estuaries draining peatland-dominated catchments, DOC was highly correlated with biologically refractory “humic-like” terrigenous material which tended to be conservatively transported along the salinity gradient. In contrast, there was a weaker correlation between DOC and DOM components within estuaries draining catchments with a high degree of human impact, that is, relatively larger percentage of arable and (sub)urban land uses. These arable and (sub)urban estuaries contain a high fraction of bioavailable “protein-like” material that behaved nonconservatively, with both DOC removals and additions occurring. In general, estuaries draining catchments with a high percentage of peatland (≥18%) have higher area-specific estuarine exports of DOC (>13 g C m−2 yr−1) compared to those estuaries draining catchments with a high percentage (≥46%) of arable and (sub)urban land uses (

A key step in assessing the global carbon budget is the determination of the partial pressure of CO2 in seawater (pCO2 (sw)). Spatially complete observational fields of pCO2 (sw) are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of pCO2 (sw) using satellite observations. Within these schemes, satellite chlorophyll a (Chl a) is often used as a proxy for the biological drawdown or release of CO2. Chl a does not however quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP). In this study, pCO2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and either satellite derived NCP, net primary production (NPP) or Chl a to compare which biological proxy is the most accurate. Estimates of pCO2 (sw) using NCP, NPP or Chl a were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl a or NPP. Reducing the uncertainties in the satellite biological parameters to estimate pCO2 (sw), illustrated further improvement and greater differences for NCP compared to NPP or Chl a. Using NCP to estimate pCO2 (sw) showed that the South Atlantic Ocean is a CO2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region becomes a sink for CO2. These results highlight that using NCP improved the accuracy of estimating pCO2 (sw), and changes the South Atlantic Ocean from a CO2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will further improve our ability to quantify the global ocean CO2 sink.

Surface ocean CO2 measurements are used to compute the oceanic air–sea CO2 flux. The CO2 flux component from rivers and estuaries is uncertain. Estuarine and coastal water carbon dioxide (CO2) observations are relatively few compared to observations in the open ocean. The contribution of these regions to the global air–sea CO2 flux remains uncertain due to systematic under-sampling. Existing high-quality CO2 instrumentation predominantly utilise showerhead and percolating style equilibrators optimised for open ocean observations. The intervals between measurements made with such instrumentation make it difficult to resolve the fine-scale spatial variability of surface water CO2 at timescales relevant to the high frequency variability in estuarine and coastal environments. Here we present a novel dataset with unprecedented frequency and spatial resolution transects made at the Western Channel Observatory in the south west of the UK from June to September 2016, using a fast response seawater CO2 system. Novel observations were made along the estuarine–coastal continuum at different stages of the tide and reveal distinct spatial patterns in the surface water CO2 fugacity (fCO2) at different stages of the tidal cycle. Changes in salinity and fCO2 were closely correlated at all stages of the tidal cycle and suggest that the mixing of oceanic and riverine end members determines the variations in fCO2. The observations demonstrate the complex dynamics determining spatial and temporal patterns of salinity and fCO2 in the region. Spatial variations in observed surface salinity were used to validate the output of a regional high resolution hydrodynamic model. The model enables a novel estimate of the air–sea CO2 flux in the estuarine–coastal zone. Air–sea CO2 flux variability in the estuarine–coastal boundary region is dominated by the state of the tide because of strong CO2 outgassing from the river plume. The observations and model output demonstrate that undersampling the complex tidal and mixing processes characteristic of estuarine and coastal environment bias quantification of air-sea CO2 fluxes in coastal waters. The results provide a mechanism to support critical national and regional policy implementation by reducing uncertainty in carbon budgets.

The flux of CO2 between the atmosphere and the ocean is often estimated as the air–sea gas concentration difference multiplied by the gas transfer velocity (K660). The first order driver for K660 over the ocean is wind through its influence on near surface hydrodynamics. However, field observations have shown substantial variability in the wind speed dependencies of K660. In this study we measured K660 with the eddy covariance technique during a ~ 11,000 km long Southern Ocean transect. In parallel, we made a novel measurement of the gas transfer efficiency (GTE) based on partial equilibration of CO2 using a Segmented Flow Coil Equilibrator system. GTE varied by 20% during the transect, was distinct in different water masses, and related to K660. At a moderate wind speed of 7 m s−1, K660 associated with high GTE exceeded K660 with low GTE by 30% in the mean. The sensitivity of K660 towards GTE was stronger at lower wind speeds and weaker at higher wind speeds. Naturally-occurring organics in seawater, some of which are surface active, may be the cause of the variability in GTE and in K660. Neglecting these variations could result in biases in the computed air–sea CO2 fluxes.

Air-sea carbon dioxide (CO2) flux is often indirectly estimated by the bulk method using the in-situ air-sea difference in CO2 fugacity and a wind speed dependent parameterisation of the gas transfer velocity (K). In the summer, sea-ice melt in the Arctic Ocean generates strong shallow stratification with significant gradients in temperature, salinity, dissolved inorganic carbon (DIC) and alkalinity (TA), and thus a near-surface CO2 fugacity (fCO2w) gradient. This gradient can cause an error in bulk air-sea CO2 flux estimates when the fCO2w is measured by the ship’s underway system at ~5 m depth. Direct air-sea CO2 flux measurement by eddy covariance (EC) is free from the impact of shallow stratification because the EC CO2 flux does not rely on a fCO2w measurement. In this study, we use summertime EC flux measurements from the Arctic Ocean to back-calculate the sea surface fCO2w and temperature and compare them with the underway measurements. We show that the EC air-sea CO2 flux agrees well with the bulk flux in areas less likely to be influenced by ice melt (salinity > 32). However, in regions with salinity less than 32, the underway fCO2w is higher than the EC estimate of surface fCO2w and thus the bulk estimate of ocean CO2 uptake is underestimated. The fCO2w difference can be partly explained by the surface to sub-surface temperature difference. The EC estimate of surface temperature is lower than the sub-surface water temperature and this difference is wind speed-dependent. Upper-ocean salinity gradients from CTD profiles suggest likely difference in DIC and TA concentrations between the surface and sub-surface water. These DIC and TA gradients likely explain much of the near-surface fCO2w gradient. Accelerating summertime loss of sea ice results in additional meltwater, which enhances near-surface stratification and increases the uncertainty of bulk air-sea CO2 flux estimates in polar regions.

The flux of terrigenous organic carbon across estuaries is an important and changing component of the global carbon cycle, but it is poorly understood. It has been proposed that estuaries can act either as a transporter of terrestrial dissolved organic carbon (DOC) to the ocean or as a reactor system in which DOC can be buried or transformed into carbon dioxide and released to the atmosphere. However, there is no clear understanding of the factors that drive estuaries to behave in one way or the other. Here we present the results from a study conducted in thirteen British estuaries which drain catchments of diverse land-uses under different hydrological conditions. Our data show that land-use influences the composition of the dissolved organic matter (DOM), the mixing dynamics of DOC and the quantity of DOC exported off the estuaries. Estuaries, whose catchments are less intensively managed and represent more natural ecosystems (average proportion of arable and (sub)-urban land-use ~12 %), contain a higher proportion of biologically-refractory “humic-like” DOM, which is transported conservatively across the salinity gradient. In contrast, estuaries whose catchments are more intensively managed (average proportion of arable and (sub)-urban land-use ~32 %) contain a high fraction of “protein-like” DOM which is transported non-conservatively, and thus suggest the existence of additions and removal processes across the salinity gradient. Furthermore, estuaries with more intensively managed catchments tend to export more DOC to coastal areas than they receive from rivers. Our results indicate that future changes in land-use have the potential to alter aquatic fluxes of terrigenous DOM and the fate of the constituent carbon.

Uncertainty in the CO2 gas transfer velocity (K660) severely limits the accuracy of air-sea CO2 flux calculations and hence hinders our ability to produce realistic climate projections. Recent field observations have suggested substantial variability in K660, especially at low and high wind speeds. Laboratory experiments have shown that naturally occurring surface active organic materials, or surfactants, can suppress gas transfer. Here we provide direct open ocean evidence of gas transfer suppression due to surfactants from a ~11,000 km long research expedition by making measurements of the gas transfer efficiency (GTE) along with direct observation of K660. GTE varied by 20% during the Southern Ocean transect and was distinct in different watermasses. Furthermore GTE correlated with and can explain about 9% of the scatter in K660, suggesting that surfactants exert a measurable influence on air-sea CO2 flux. Relative gas transfer suppression due to surfactants was ~30% at a global mean wind speed of 7 m s-1 and was more important at lower wind speeds. Neglecting surfactant suppression may result in substantial spatial and temporal biases in the computed air-sea CO2 fluxes.

A comprehensive in situ dataset of chlorophyll a (Chl a; N = 18,001), net primary production (NPP; N = 165) and net community production (NCP; N = 95), were used to evaluate the performance of Moderate Resolution Imaging Spectroradiometer on Aqua (MODIS-A) algorithms for these parameters, in the South Atlantic Ocean, to facilitate the accurate generation of satellite NCP time series. For Chl a, five algorithms were tested using MODIS-A data, and OC3-CI performed best, which was subsequently used to compute NPP. Of three NPP algorithms tested, a Wavelength Resolved Model (WRM) was the most accurate, and was therefore used to estimate NCP with an empirical relationship between NCP with NPP and sea surface temperature (SST). A perturbation analysis was deployed to quantify the range of uncertainties introduced in satellite NCP from input parameters. The largest reductions in the uncertainty of satellite NCP came from MODIS-A derived NPP using the WRM (40%) and MODIS-A Chl a using OC3-CI (22%). The most accurate NCP algorithm, was used to generate a 16 year time series (2002 to 2018) from MODIS-A to assess climate and environmental drivers of NCP across the South Atlantic basin. Positive correlations between wind speed anomalies and NCP anomalies were observed in the central South Atlantic Gyre (SATL), and the Benguela Upwelling (BENG), indicating that autotrophic conditions may be fuelled by local wind-induced nutrient inputs to the mixed layer. Sea Level Height Anomalies (SLHA), used as an indicator of mesoscale eddies, were negatively correlated with NCP anomalies offshore of the BENG upwelling fronts into the SATL, suggesting autotrophic conditions are driven by mesoscale features. The Agulhas bank and Brazil-Malvinas confluence regions also had a strong negative correlation between SLHA and NCP anomalies, similarly indicating that NCP is forced by mesoscale eddy generation in this region. Positive correlations between SST anomalies and the Multivariate ENSO Index (MEI) in the SATL, indicated the influence of El Niño events on the South Atlantic Ocean, however the plankton community response was less clear. UK Natural Environment Research Council | Ref. NERC; NE / L002434 / 1 European Space Agency | Ref. AMT4SentinelFRM (ESRIN / RFQ / 3-14457 / 16 / I-BG) European Space Agency | Ref. AMT4OceanSatFlux (4000125730/18 / NL / FF / gp) NERC International Opportunity Fund Grant Satellite estimates of marine net community production in the South Atlantic from Sentinel-3 | Ref. SemSAS; NE / P00878X / 1 P&D ANP / BRASOIL | Ref. 48610.011013 / 2014-66 Oceanographic Institute of the University of São Paulo | Ref. FAPESP 2015 / 01373-0 Oceanographic Institute of the University of São Paulo | Ref. CNPq 442926 / 2015-4 Oceanographic Institute of the University of São Paulo | Ref. FAPESP 2014 / 50820-7 Oceanographic Institute of the University of São Paulo | Ref. CNPq 565060 / 2010-4 NERC | Ref. NE / R015953 / 1

Results from a 1D setup of the European Regional Seas Ecosystem Model (ERSEM) biogeochemical model were compared with new observations collected under the UK Shelf Seas Biogeochemistry (SSB) programme to assess model performance and clarify elements of shelf-sea benthic biogeochemistry and carbon cycling. Observations from two contrasting sites (muddy and sandy) in the Celtic Sea in otherwise comparable hydrographic conditions were considered, with the focus on the benthic system. A standard model parameterisation with site-specific light and nutrient adjustments was used, along with modifications to the within-seabed diffusivity to accommodate the modelling of permeable (sandy) sediments. Differences between modelled and observed quantities of organic carbon in the bed were interpreted to suggest that a large part (>90%) of the observed benthic organic carbon is biologically relatively inactive. Evidence on the rate at which this inactive fraction is produced will constitute important information to quantify offshore carbon sequestration. Total oxygen uptake and oxic layer depths were within the range of the measured values. Modelled depth average pore water concentrations of ammonium, phosphate and silicate were typically 5–20% of observed values at the muddy site due to an underestimate of concentrations associated with the deeper sediment layers. Model agreement for these nutrients was better at the sandy site, which had lower pore water concentrations, especially deeper in the sediment. Comparison of pore water nitrate with observations had added uncertainty, as the results from process studies at the sites indicated the dominance of the anammox pathway for nitrogen removal; a pathway that is not included in the model. Macrofaunal biomasses were overestimated, although a model run with increased macrofaunal background mortality rates decreased macrofaunal biomass and improved agreement with observations. The decrease in macrofaunal biomass was compensated by an increase in meiofaunal biomass such that total oxygen demand remained within the observed range. The permeable sediment modification reproduced some of the observed behaviour of oxygen penetration depth at the sandy site. It is suggested that future development in ERSEM benthic modelling should focus on: (1) mixing and degradation rates of benthic organic matter, (2) validation of benthic faunal biomass against large scale spatial datasets, (3) incorporation of anammox in the benthic nitrogen cycle, and (4) further developments to represent permeable sediment processes. Electronic supplementary material The online version of this article (doi:10.1007/s10533-017-0367-0) contains supplementary material, which is available to authorized users.

We undertook a seasonal study of benthic N-cycling on the Celtic Sea continental shelf in 2015, augmented by an earlier cruise in 2014. Two cruises in 2015 were centred before and after the Spring phytoplankton bloom and a further cruise was carried out in late summer. Five sites covering the mud to sand continuum were visited on all cruises, where we determined ammonium-oxidation, anammox and denitrification rates, expression of anammox and denitrification genes, N-nutrient fluxes and sediment porewater profiles of N-nutrients. Highest process rates were found during the post-bloom and late summer periods. The Celtic Sea was overwhelmingly a source of inorganic-N to the overlying water column. The efflux of nitrate was controlled by the magnitude of ammonium-oxidation. The latter accounted for 10–16% of total Oxygen consumption in cohesive sediments and 35–56% in sandy sediments. Ammonium oxidation rates in the range of 0.001–2.288 mmol m−2 days−1 were inversely correlated with sediment porosity and positively correlated with organic matter content (OM) which together explained 66% of the variance in rates. N-removal was dominated by anammox (0.003–0.636 mmol m−2 days−1), rather than denitrification (0.000–0.034 mmol m−2 days−1). This finding was supported by the corresponding gene expression data. The expression of hydrazine oxidoreductase (anammox) was significantly correlated with anammox and total N-removal rates. Anammox was positively correlated with porosity and OM, whilst denitrification was correlated with OM. The N-requirement of these processes was largely met through nitrification (ammonium-oxidation) rather than influx from the overlying water column. We estimated that N-removal via denitrification and anammox removed 6–9% of the organic-N deposited at the sea-floor from the overlying water column. The Celtic Sea system was thereby losing N which must be replenished on an annual basis in order to sustain productivity.

Shelf seas play an important role in the global carbon cycle, absorbing atmospheric carbon dioxide (CO2) and exporting carbon (C) to the open ocean and sediments. The magnitude of these processes is poorly constrained, because observations are typically interpolated over multiple years. Here, we used 298500 observations of CO2 fugacity (fCO2) from a single year (2015), to estimate the net influx of atmospheric CO2 as 26.2 ± 4.7 Tg C yr−1 over the open NW European shelf. CO2 influx from the atmosphere was dominated by influx during winter as a consequence of high winds, despite a smaller, thermally-driven, air-sea fCO2 gradient compared to the larger, biologically-driven summer gradient. In order to understand this climate regulation service, we constructed a carbon-budget supplemented by data from the literature, where the NW European shelf is treated as a box with carbon entering and leaving the box. This budget showed that net C-burial was a small sink of 1.3 ± 3.1 Tg C yr−1, while CO2 efflux from estuaries to the atmosphere, removed the majority of river C-inputs. In contrast, the input from the Baltic Sea likely contributes to net export via the continental shelf pump and advection (34.4 ± 6.0 Tg C yr−1).

The transport of dissolved organic matter (DOM) across the land-ocean-aquatic-continuum (LOAC), from freshwater to the ocean, is an important yet poorly understood component of the global carbon budget. Exploring and quantifying this flux is a significant challenge given the complexities of DOM cycling across these contrasting environments. We developed a new model, UniDOM, that unifies concepts, state variables and parameterisations of DOM turnover across the LOAC. Terrigenous DOM is divided into two pools, T1 (strongly-UV-absorbing) and T2 (non- or weakly-UV-absorbing), that exhibit contrasting responses to microbial consumption, photooxidation and flocculation. Data are presented to show that these pools are amenable to routine measurement based on specific UV absorbance (SUVA). In addition, an autochtonous DOM pool is defined to account for aquatic DOM production. A novel aspect of UniDOM is that rates of photooxidation and microbial turnover are parameterised as an inverse function of DOM age. Model results, which indicate that ~ 5% of the DOM originating in streams may penetrate into the open ocean, are sensitive to this parameterisation, as well as rates assigned to turnover of freshly-produced DOM. The predicted contribution of flocculation to DOM turnover is remarkably low, although a mechanistic representation of this process in UniDOM was considered unachievable because of the complexities involved. Our work highlights the need for ongoing research into the mechanistic understanding and rates of photooxidation, microbial consumption and flocculation of DOM across the different environments of the LOAC, along with the development of models based on unified concepts and parameterisations.

Air-water CH4 and CO2 fluxes were directly measured using the eddy covariance technique at the Penlee Point Atmospheric Observatory on the southwest coast of the United Kingdom from September 2015 to August 2016. The high frequency, year-long measurements provide unprecedented detail into the variability of these Greenhouse Gas fluxes from seasonal to diurnal and to semi-diurnal timescales. Depending on the wind sector, fluxes measured at this site are indicative of air-water exchange in coastal seas as well as in an outer estuary. For the open water sector when winds were off the Atlantic Ocean, annual CH4 emission averaged ~ 0.05 mmol m−2 d−1. Open water CH4 flux was near zero in December and January, probably due to reduced biological production of CH4. At times of high rainfall and river flow rate, CH4 emission from the estuarine-influenced Plymouth Sound sector was several times higher than emission from the open water sector. The implied CH4 saturation, derived from the measured fluxes and a wind speed dependent gas transfer velocity parameterization, of over 1000 % in the Plymouth Sound is within range of in situ dissolved CH4 measurements near the mouth of the river Tamar. CO2 flux from the open water sector was generally from sea-to-air in autumn and winter and from air-to-sea in late spring and summer, with an annual mean flux of near zero. CO2 flux from the Plymouth Sound sector was more positive, consistent with a higher dissolved CO2 concentration in the estuarine waters. A diurnal signal in CO2 flux and implied dissolved pCO2 are clearly observed for the Plymouth Sound sector and also evident for the open water sector during biologically productive periods. These observations suggest that coastal CO2 efflux may be underestimated if the sampling strategy is limited to daytime only. Combining the fluxes with in situ dissolved pCO2 measurements within the flux footprints allows us to estimate the CO2 transfer velocity. The gas transfer velocity vs. wind speed relationship at this coastal location agrees reasonably well with previous open water parameterizations in the mean, but demonstrates considerable variability. We discuss the influences of biological productivity and bottom-driven turbulence on coastal air-water gas exchange.

Pteropods are among the first responders to ocean acidification and warming, but have not yet been widely explored as carriers of marine paleoenvironmental signals. In order to characterize the stable isotopic composition of aragonitic pteropod shells and their variation in response to climate change parameters, such as seawater temperature, pteropod shells (Heliconoides inflatus) were collected along a latitudinal transect in the Atlantic Ocean (31° N to 38° S). Comparison of shell oxygen isotopic composition to depth changes in the calculated aragonite equilibrium oxygen isotope values implies shallow calcification depths for H. inflatus (75 m). This species is therefore a good potential proxy carrier for past variations in surface ocean properties. Furthermore, we identified pteropod shells to be excellent recorders of climate change, as carbonate ion concentration and temperature in the upper water column have dominant influences on pteropod shell carbon and oxygen isotopic composition. These results, in combination with a broad distribution and high abundance, make the pteropod species studied here, H. inflatus, a promising new proxy carrier in paleoceanography.

Continental shelf sediments are globally important for biogeochemical activity. Quantification of shelf-scale stocks and fluxes of carbon and nutrients requires the extrapolation of observations made at limited points in space and time. The procedure for selecting exemplar sites to form the basis of this up-scaling is discussed in relation to a UK-funded research programme investigating biogeochemistry in shelf seas. A three-step selection process is proposed in which (1) a target area representative of UK shelf sediment heterogeneity is selected, (2) the target area is assessed for spatial heterogeneity in sediment and habitat type, bed and water column structure and hydrodynamic forcing, and (3) study sites are selected within this target area encompassing the range of spatial heterogeneity required to address key scientific questions regarding shelf scale biogeochemistry, and minimise confounding variables. This led to the selection of four sites within the Celtic Sea that are significantly different in terms of their sediment, bed structure, and macrofaunal, meiofaunal and microbial community structures and diversity, but have minimal variations in water depth, tidal and wave magnitudes and directions, temperature and salinity. They form the basis of a research cruise programme of observation, sampling and experimentation encompassing the spring bloom cycle. Typical variation in key biogeochemical, sediment, biological and hydrodynamic parameters over a pre to post bloom period are presented, with a discussion of anthropogenic influences in the region. This methodology ensures the best likelihood of site-specific work being useful for up-scaling activities, increasing our understanding of benthic biogeochemistry at the UK-shelf scale. Electronic supplementary material The online version of this article (doi:10.1007/s10533-017-0366-1) contains supplementary material, which is available to authorized users.

We present over 900 carbonate system observations collected over four years (2007–2010) in the Western English Channel (WEC). We determined CO2 partial pressure (pCO2), Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC) along a series of 40 km transects, including two oceanographic stations (L4 and E1) within a sustained coastal observatory. Our data follow a seasonal pattern of CO2 undersaturation from January to August, followed by supersaturation in September–October and a return to near-equilibrium thereafter. This pattern is explained by the interplay of thermal and biological sinks in winter and spring–summer, respectively, followed by the breakdown of stratification and mixing with deeper, high-CO2 water in autumn. The drawdown of DIC and inorganic N between March and June with a C:N ratio of 8.7–9.5 was consistent with carbon over-consumption during phytoplankton growth. Monthly mean surface pCO2 was strongly correlated with depth integrated chlorophyll a highlighting the importance of subsurface chlorophyll a maxima in controlling C-fluxes in shelf seas. Mixing of seawater with riverine freshwater in near-shore samples caused a reduction in TA and the saturation state of calcite minerals, particularly in winter. Our data show that the L4 and E1 oceanographic stations were small, net sinks for atmospheric CO2 over an annual cycle (−0.52±0.66 mol C m−2 y−1 and −0.62±0.49 mol C m−2 y−1, respectively).

Dissolved methane and nitrous oxide in seawater were measured along a 6700 km transect of the North-West Passage between the North Atlantic Ocean and Beaufort Sea in the Arctic Ocean. Over- and under-saturation with respect to atmospheric equilibrium were observed for both gases. Methane and nitrous oxide were in the range of 58–528% and 82–181% saturation, respectively. Under-saturation was attributed to melt-water with low methane and nitrous oxide, while over-saturation was found under multi-year sea-ice. Elevated methane was also found in the vicinity of the marginal ice zones and the Mackenzie River plume. Our data support both water column and sedimentary sources of methane and nitrous oxide. We found first-order methane oxidation in surface seawater with a rate constant of 3.8 × 10−3 h−1. Based on these results and a conceptual model, we suggest that future sea-ice retreat may decrease the residence times of methane and nitrous oxide in the surface Arctic Ocean and thus enhance the sea–air flux of these climatically active gases.

Methane (CH4) concentrations were measured in the water column, in sediment porewaters, and in atmospheric air, in the Ria de Vigo, NW Spain, during both the onset (April 2003) and at the end of (September 2004) seasonal upwelling. In addition, CH4 concentration and stable isotopic signatures (δ13CH4) were measured in porewaters, and sediment methanogenesis and aerobic oxidation of CH4 were determined in sediment incubations. Surface water column CH4 (2 m depth) was in the range 3–180 nmol l− 1 (110–8500% saturation) and followed a generally landward increase but with localised maxima in both the inner and middle Ria. These maxima were consistent with CH4 inputs from underlying porewaters in which CH4 concentrations were up to 3 orders of magnitude higher (maximum 350 μmol l− 1). Surface water CH4 concentrations were approximately three times higher in September than in April, consistent with a significant benthic CH4 flux driven by enhanced sediment methanogenesis following the summer productivity maximum. CH4 and δ13CH4 in sediment porewaters and in incubated sediment slurries (20 °C) revealed significant sediment CH4 oxidation, with an apparent isotopic fractionation factor (rc) of ∼ 1.004. Using turbulent diffusion models of air–sea exchange we estimate an annual emission of atmospheric CH4 from the Ria de Vigo of 18–44 × 106 g (1.1–2.7 × 106 mol). This estimate is approximately 1–2 orders of magnitude lower than a previous estimate based on a bubble transport model.

Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and a methodology to quantify all major components of the global carbon budget, including their uncertainties, based on the combination of a range of data, algorithms, statistics, and model estimates and their interpretation by a broad scientific community. We discuss changes compared to previous estimates, consistency within and among components, alongside methodology and data limitations. CO2 emissions from fossil fuel combustion and cement production (EFF) are based on energy statistics and cement production data, respectively, while emissions from land-use change (ELUC), mainly deforestation, are based on combined evidence from land-cover-change data, fire activity associated with deforestation, and models. The global atmospheric CO2 concentration is measured directly and its rate of growth (GATM) is computed from the annual changes in concentration. The mean ocean CO2 sink (SOCEAN) is based on observations from the 1990s, while the annual anomalies and trends are estimated with ocean models. The variability in SOCEAN is evaluated with data products based on surveys of ocean CO2 measurements. The global residual terrestrial CO2 sink (SLAND) is estimated by the difference of the other terms of the global carbon budget and compared to results of independent dynamic global vegetation models forced by observed climate, CO2, and land-cover-change (some including nitrogen-carbon interactions). We compare the mean land and ocean fluxes and their variability to estimates from three atmospheric inverse methods for three broad latitude bands. All uncertainties are reported as ±1σ, reflecting the current capacity to characterise the annual estimates of each component of the global carbon budget. For the last decade available (2004-2013) EFF was 8.9 ± 0.4 GtC yr-1, ELUC 0.9 ± 0.5 GtC yr-1, GATM 4.3 ± 0.1 GtC yr-1, SOCEAN 2.6 ± 0.5 GtC yr-1, and SLAND 2.9 ± 0.8 GtC yr-1. For year 2013 alone, EFF grew to 9.9 ± 0.5 GtC yr-1, 2.3% above 2012, continuing the growth trend in these emissions, ELUC was 0.9 ± 0.5 GtC yr-1, GATM was 5.4 ± 0.2 GtC yr-1, SOCEAN was 2.9 ± 0.5 GtC yr-1, and SLAND was 2.5 ± 0.9 GtC yr-1. GATM was high in 2013, reflecting a steady increase in EFF and smaller and opposite changes between SOCEAN and SLAND compared to the past decade (2004-2013). The global atmospheric CO2 concentration reached 395.31 ± 0.10 ppm averaged over 2013. We estimate that EFF will increase by 2.5% (1.3-3.5%) to 10.1 ± 0.6 GtC in 2014 (37.0 ± 2.2 GtCO2 yr-1), 65% above emissions in 1990, based on projections of world gross domestic product and recent changes in the carbon intensity of the global economy. From this projection of EFF and assumed constant ELUC for 2014, cumulative emissions of CO2 will reach about 545 ± 55 GtC (2000 ± 200 GtCO2) for 1870-2014, about 75% from EFF and 25% from ELUC. This paper documents changes in the methods and data sets used in this new carbon budget compared with previous publications of this living data set (Le Quéré et al. 2013, 2014). All observations presented here can be downloaded from the Carbon Dioxide Information Analysis Center (doi:10.3334/CDIAC/GCP-2014)., 0, SCOPUS: ar.j, info:eu-repo/semantics/published

48 pages, 9 figures, 10 tables.-- Proyecto Carbochange, Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and a methodology to quantify all major components of the global carbon budget, including their uncertainties, based on the combination of a range of data, algorithms, statistics, and model estimates and their interpretation by a broad scientific community. We discuss changes compared to previous estimates as well as consistency within and among components, alongside methodology and data limitations. CO2 emissions from fossil fuels and industry (EFF) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on combined evidence from land-cover-change data, fire activity associated with deforestation, and models. The global atmospheric CO2 concentration is measured directly and its rate of growth (GATM) is computed from the annual changes in concentration. The mean ocean CO2 sink (SOCEAN) is based on observations from the 1990s, while the annual anomalies and trends are estimated with ocean models. The variability in SOCEAN is evaluated with data products based on surveys of ocean CO2 measurements. The global residual terrestrial CO2 sink (SLAND) is estimated by the difference of the other terms of the global carbon budget and compared to results of independent dynamic global vegetation models forced by observed climate, CO2, and land-cover change (some including nitrogen–carbon interactions). We compare the mean land and ocean fluxes and their variability to estimates from three atmospheric inverse methods for three broad latitude bands. All uncertainties are reported as ±1σ, reflecting the current capacity to characterise the annual estimates of each component of the global carbon budget. For the last decade available (2005–2014), EFF was 9.0 ± 0.5 GtC yr−1, ELUC was 0.9 ± 0.5 GtC yr−1, GATM was 4.4 ± 0.1 GtC yr−1, SOCEAN was 2.6 ± 0.5 GtC yr−1, and SLAND was 3.0 ± 0.8 GtC yr−1. For the year 2014 alone, EFF grew to 9.8 ± 0.5 GtC yr−1, 0.6 % above 2013, continuing the growth trend in these emissions, albeit at a slower rate compared to the average growth of 2.2 % yr−1 that took place during 2005–2014. Also, for 2014, ELUC was 1.1 ± 0.5 GtC yr−1, GATM was 3.9 ± 0.2 GtC yr−1, SOCEAN was 2.9 ± 0.5 GtC yr−1, and SLAND was 4.1 ± 0.9 GtC yr−1. GATM was lower in 2014 compared to the past decade (2005–2014), reflecting a larger SLAND for that year. The global atmospheric CO2 concentration reached 397.15 ± 0.10 ppm averaged over 2014. For 2015, preliminary data indicate that the growth in EFF will be near or slightly below zero, with a projection of −0.6 [range of −1.6 to +0.5] %, based on national emissions projections for China and the USA, and projections of gross domestic product corrected for recent changes in the carbon intensity of the global economy for the rest of the world. From this projection of EFF and assumed constant ELUC for 2015, cumulative emissions of CO2 will reach about 555 ± 55 GtC (2035 ± 205 GtCO2) for 1870–2015, about 75 % from EFF and 25 % from ELUC. This living data update documents changes in the methods and data sets used in this new carbon budget compared with previous publications of this data set (Le Quéré et al., 2015, 2014, 2013). All observations presented here can be downloaded from the Carbon Dioxide Information Analysis Center (doi:10.3334/CDIAC/GCP_2015)., NERC provided funding to C. Le Quéré, R. Moriarty, and the GCP through their International Opportunities Fund specifically to support this publication (NE/103002X/1). G. P. Peters and R. M. Andrew were supported by the Norwegian Research Council (236296). J. G. Canadell was supported by the Australian Climate Change Science Programme. S. Sitch was supported by EU FP7 for funding through projects LUC4C (GA603542). R. J. Andres was supported by US Department of Energy, Office of Science, Biological and Environmental Research (BER) programmes under US Department of Energy contract DE-AC05- 00OR22725. T. A. Boden was supported by US Department of Energy, Office of Science, Biological and Environmental Research (BER) programmes under US Department of Energy contract DE-AC05-00OR22725.J. I. House was supported by the Leverhulme foundation and the EU FP7 through project LUC4C (GA603542). P. Friedlingstein was supported by the EU FP7 for funding through projects LUC4C (GA603542) and EMBRACE (GA282672). A. Arneth was supported by the EU FP7 for funding through LUC4C (603542), and the Helmholtz foundation and its ATMO programme. D. C. E. Bakker was supported by the EU FP7 for funding through project CARBOCHANGE (284879), the UK Ocean Acidification Research Programme (NE/H017046/1; funded by the Natural Environment Research Council, the Department for Energy and Climate Change and the Department for Environment, Food and Rural Affairs). L. Barbero was supported by NOAA’s Ocean Acidification Program and acknowledges support for this work from the National Aeronautics and Space Administration (NASA) ROSES Carbon Cycle Science under NASA grant 13-CARBON13_2-0080. P. Ciais acknowledges support from the European Research Council through Synergy grant ERC-2013-SyG-610028 “IMBALANCE-P”. M. Fader was supported by the EU FP7 for funding through project LUC4C (GA603542). J. Hauck was supported by the Helmholtz Postdoc Programme (Initiative and Networking Fund of the Helmholtz Association). R. A. Feely and A. J. Sutton were supported by the Climate Observation Division, Climate Program Office, NOAA, US Department of Commerce. A. K. Jain was supported by the US National Science Foundation (NSF AGS 12-43071) the US Department of Energy, Office of Science and BER programmes (DOE DE-SC0006706) and NASA LCLUC programme (NASA NNX14AD94G). E. Kato was supported by the ERTDF (S-10) from the Ministry of Environment, Japan. K. Klein Goldewijk was supported by the Dutch NWO VENI grant no. 863.14.022. S. K. Lauvset was supported by the project “Monitoring ocean acidification in Norwegian waters” from the Norwegian Ministry of Climate and Environment. V. Kitidis was supported by the EU FP7 for funding through project CARBOCHANGE (264879). C. Koven was supported by the Director, Office of Science, Office of Biological and Environmental Research of the US Department of Energy under contract no. DE-AC02-05CH11231 as part of their Regional and Global Climate Modeling Program. P. Landschützer was supported by GEOCarbon. I. T. van der Lann-Luijkx received financial support from OCW/NWO for ICOS-NL and computing time from NWO (SH-060-13). I. D. Lima was supported by the US National Science Foundation (NSF AGS-1048827). N. Metzl was supported by Institut National des Sciences de l’Univers (INSU) and Institut Paul Emile Victor (IPEV) for OISO cruises. D. R. Munro was supported by the US National Science Foundation (NSF PLR-1341647 and NSF AOAS-0944761). J. E. M. S. Nabel was supported by the German Research Foundation’s Emmy Noether Programme (PO1751/1-1) and acknowledges Julia Pongratz and Kim Naudts for their contributions. Y. Nojiri and S. Nakaoka were supported by the Global Environment Research Account for National Institutes (1432) by the Ministry of Environment of Japan. A. Olsen appreciates support from the Norwegian Research Council (SNACS, 229752). F. F. Pérez were supported by BOCATS (CTM2013-41048-P) project co-founded by the Spanish government and the Fondo Europeo de Desarrollo Regional (FEDER). B. Pfeil was supported through the European Union’s Horizon 2020 research and innovation programme AtlantOS under grant agreement no. 633211. D. Pierrot was supported by NOAA through the Climate Observation Division of the Climate Program Office. B. Poulter was supported by the EU FP7 for funding through GEOCarbon. G. Rehder was supported by BMBF (Bundesministerium für Bildung und Forschung) through project ICOS, grant no. 01LK1224D. U. Schuster was supported by NERC UKOARP (NE/H017046/1), NERC RAGANRoCC (NE/K002473/1), the European Space Agency (ESA) OceanFlux Evolution project, and EU FP7 CARBOCHANGE (264879). T. Steinhoff was supported by ICOS-D (BMBF FK 01LK1101C) and EU FP7 for funding through project CARBOCHANGE (264879). J. Schwinger was supported by the Research Council of Norway through project EVA (229771). T. Takahashi was supported by grants from NOAA and the Comer Education and Science Foundation. B. Tilbrook was supported by the Australian Department of Environment and the Integrated Marine Observing System. B. D. Stocker was supported by the Swiss National Science Foundation and FP7 funding through project EMBRACE (282672). S. van Heuven was supported by the EU FP7 for funding through project CARBOCHANGE (264879). G. R. van der Werf was supported by the European Research Council (280061). A. Wiltshire was supported by the Joint UK DECC/Defra Met Office Hadley Centre Climate Programme (GA01101) and EU FP7 Funding through project LUC4C (603542). S. Zaehle was supported by the European Research Council (ERC) under the Eu ropean Union’s Horizon 2020 research and innovation programme (QUINCY; grant agreement no. 647204). ISAM (PI: Atul K. Jain) simulations were carried out at the National Energy Research Scientific Computing Center (NERSC), which is supported by the US DOE under contract DE-AC02-05CH11231. Contributions from the Scripps Institution of Oceanography were supported under DoE grant DE-SC0012167 and by Schmidt Philanthropies. This is NOAA-PMEL contribution number 4400

Carbon monoxide (CO) atmospheric mixing ratios and surface-water concentrations were determined during Atlantic Meridional Transect cruise number 10, April–May 2000. Atmospheric CO increased from south (mean=74±9 ppbv) to north (mean=151±19 ppbv) with a steep increase around the intertropical convergence zone. Surface-water CO (0.2–2.6 nmol L −1 ) showed pronounced diurnal variations with afternoon maxima exceeding pre-dawn minima 5–7 fold. Modest regional variations, as indicated by maximum daily CO concentrations, were also observed. Highest CO maxima occurred at ∼11.5°N, where high solar irradiance was combined with elevated coloured dissolved organic matter (CDOM) levels and modest winds, while lowest CO maxima occurred during periods of high winds and lowest solar irradiance near the western European margin at 45°N. Atlantic Ocean CO emissions were estimated to be 1.5±1.1 Tg CO-C yr −1 based on near-instantaneous atmospheric CO, sea-surface CO and windspeeds from the cruise. However, as spatial and temporal variability in both terms was considered to be unique to the timing and path of the cruise, the mean Atlantic diel cycle of sea-surface CO concentration was estimated by pooling all cruise data into 1-h sections, yielding a mean of 0.94 nmol L −1 ; and diurnal variations from 0.4 to 1.6 nmol L −1 . Using the mean diurnal cycle, the Atlantic and global open-ocean sources of CO to the atmosphere were estimated to be 0.9±0.6 and 3.7±2.6 Tg CO-C yr −1 , respectively. Therefore it is our contention that IPCC-2001 (Prather, M., Ehhalt, D., Dentener, F., Derwent, R., Dlugokencky, E., Holland, E., Isaksen, I., Katima, J., Kirchhoff, V., Matson, P., Midgley, P., Wang, M., 2001. Chapter 4: Atmospheric chemistry and greenhouse gases. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K., Johnson, C.A. (Eds.), Climate Change 2001: The Scientific Basis. Contribution of working group 1 to the third assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, pp. 239–287; 21 Tg CO-C yr −1 ) overestimates the source of atmospheric CO from the global ocean by around 5 fold.

We report over 600 absorption spectra (250–800 nm) of unfiltered surface waters

22 páginas, 9 figuras, 10 tablas.-- D. C. E. Bakker ... et al.-- This work is distributed under the Creative Commons Attribution 3.0 License.-- Proyecto Carbochange, The Surface Ocean CO2 Atlas (SOCAT), an activity of the international marine carbon research community, provides access to synthesis and gridded fCO2 (fugacity of carbon dioxide) products for the surface oceans. Version 2 of SOCAT is an update of the previous release (version 1) with more data (increased from 6.3 million to 10.1 million surface water fCO2 values) and extended data coverage (from 1968–2007 to 1968–2011). The quality control criteria, while identical in both versions, have been applied more strictly in version 2 than in version 1. The SOCAT website (http://www.socat.info/) has links to quality control comments, metadata, individual data set files, and synthesis and gridded data products. Interactive online tools allow visitors to explore the richness of the data. Applications of SOCAT include process studies, quantification of the ocean carbon sink and its spatial, seasonal, year-to-year and longerterm variation, as well as initialisation or validation of ocean carbon models and coupled climate-carbon models, Support for SOCAT has been received from the University of East Anglia (UK), the Bjerknes Centre for Climate Research, the Geophysical Institute at the University of Bergen (Norway), the Climate Observation Division of the Climate Program Office of the US National Oceanic and Atmospheric Administration, the University of Washington, Oak Ridge National Laboratory (USA), PANGAEA ® Data Publisher for Earth and Environmental Data (Germany), the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (Germany), the National Institute for Environmental Studies (Japan), the Korean Institute of Ocean Science and Technology, the European Union projects CarboChange (FP7 264879) and GEO- CARBON (FP7 283080), the UK Ocean Acidification Research Programme (NE/H017046/1; funded by the Natural Environment Research Council, the Department for Energy and Climate Change and the Department for Environment, Food and Rural Affairs), the Research Council of Norway (CARBON-HEAT), the Scientific Committee on Oceanic Research (SCOR, US, OCE-0938349), the US National Science Foundation (OCE-1068958), Swedish Research Council projects (2009–2994; 2008–6228; 2009–2994) and the Swedish National Space Board (RESCUE project; REmote Sensing ocean Carbon UptakE).

The Arctic marine environment holds large stores of methane (CH4) in the form of methane hydrate and, potentially, permafrost. The dynamic interactions between these carbon (C) reservoirs and the Arctic CH4 cycle are discussed in this chapter. A number of papers and books have recently reviewed dive...