Variability Of Dissolved Inorganic Carbon Research Articles

Recent Trends and Variability in the Oceanic Storage of Dissolved Inorganic Carbon

AbstractSeveral methods have been developed to quantify the oceanic accumulation of anthropogenic carbon dioxide (CO2) in response to rising atmospheric CO2. Yet, we still lack a corresponding estimate of the changes in the total oceanic dissolved inorganic carbon (DIC). In addition to the increase in anthropogenic CO2, changes in DIC also include alterations of natural CO2. Once integrated globally, changes in DIC reflect the net oceanic sink for atmospheric CO2, complementary to estimates of the air‐sea CO2 exchange based on surface measurements. Here, we extend the MOBO‐DIC machine learning approach by Keppler et al. (2020a, https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.nodc%3A0221526) to estimate global monthly fields of DIC at 1° resolution over the top 1,500 m from 2004 through 2019. We find that over these 16 years and extrapolated to cover the whole global ocean down to 4,000 m, the oceanic DIC pool increased close to linearly at an average rate of 3.2 ± 0.7 Pg C yr−1. This trend is statistically indistinguishable from current estimates of the oceanic uptake of anthropogenic CO2 over the same period. Thus, our study implies no detectable net loss or gain of natural CO2 by the ocean, albeit the large uncertainties could be masking it. Our reconstructions suggest substantial internal redistributions of natural oceanic CO2, with a shift from the midlatitudes to the tropics and from the surface to below ∼200 m. Such redistributions correspond with the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. The interannual variability of DIC is strongest in the tropical Western Pacific, consistent with the El Nio Southern Oscillation.

Subantarctic Mode Water Biogeochemical Formation Properties and Interannual Variability

AbstractSubantarctic mode water (SAMW) is a key water mass for the transport of nutrients, oxygen, and anthropogenic carbon into the ocean interior. However, a lack of biogeochemical observations of SAMW properties during wintertime formation precluded their detailed characterization. Here we characterize for the first time SAMW properties across their entire wintertime formation regions based primarily on biogeochemical profiling floats. Observations show that the SAMW properties differ between the two main formation regions in the Pacific and Indian sectors of the Southern Ocean. SAMW formed in the Pacific is colder, fresher, and higher in oxygen, nitrate, and dissolved inorganic carbon (DIC) than its Indian Ocean counterpart. The relationship between potential density and biogeochemical water properties is nearly identical between the two formation regions; property differences thus predominantly reflect the difference in mean densities of SAMW formed in each region. SAMW is undersaturated in oxygen during formation, which will impact calculations of derived quantities that assume preformed oxygen saturation. SAMW is at or above atmospheric pCO2 during wintertime and therefore not a direct sink of contemporary carbon dioxide during the formation period. Results from the Biogeochemical Southern Ocean State Estimate suggest anti‐correlated interannual variability of DIC, nitrate, and oxygen between the central and southeastern Pacific formation regions similar to previously established patterns in mixed layer physical properties. This indicates that the mean properties of SAMW will vary depending on which sub‐region has a stronger formation rate, which is in turn linked to the Southern Annual Mode and the El‐Niño Southern Oscillation.

Contrasting projections of the ENSO-driven CO2 flux variability in the equatorial Pacific under high-warming scenario

Abstract. The El Niño–Southern Oscillation (ENSO) widely modulates the global carbon cycle. More specifically, it alters the net uptake of carbon in the tropical ocean. Indeed, over the tropical Pacific less carbon is released by oceans during El Niño, while the opposite is the case for La Niña. Here, the skill of Earth system models (ESMs) from the latest Coupled Model Intercomparison Project (CMIP6) to simulate the observed tropical Pacific CO2 flux variability in response to ENSO is assessed. The temporal amplitude and spatial extent of CO2 flux anomalies vary considerably among models, while the surface temperature signals of El Niño and La Niña phases are generally well represented. Under historical conditions followed by the high-warming Shared Socio-economic Pathway (SSP5-8.5) scenarios, about half the ESMs simulate a reversal in ENSO–CO2 flux relationship. This gradual shift, which occurs as early as the first half of the 21st century, is associated with a high CO2-induced increase in the Revelle factor that leads to stronger sensitivity of partial pressure of CO2 (pCO2) to changes in surface temperature between ENSO phases. At the same time, uptake of anthropogenic CO2 substantially increases upper-ocean dissolved inorganic carbon (DIC) concentrations (reducing its vertical gradient in the thermocline) and weakens the ENSO-modulated surface DIC variability. The response of the ENSO–CO2 flux relationship to future climate change is sensitive to the contemporary mean state of the carbonate ion concentration in the tropics. We present an emergent constraint between the simulated contemporary carbonate concentration with the projected cumulated CO2 fluxes. Models that simulate shifts in the ENSO–CO2 flux relationship simulate positive bias in surface carbonate concentrations.

Eutrophication Amplifies the Diel Variability of Carbonate Chemistry in an Equatorial, Semi-Arid, and Negative Estuary

This study presents high-resolution data on diel variations of carbonate chemistry in a semi-arid estuary (Jaguaribe River) in NE Brazil, which has witnessed decreasing annual rainfall and freshwater inputs due to climate change and river damming. In addition, the estuary has been suffering with increasing discharges from shrimp farm and urban effluents. We monitored surface water and atmospheric CO2 partial pressure (pCO2), temperature, salinity, and wind speed with continuous real-time measurements during two eulerian surveys in October 2017 (33 h) and September 2018 (44 h), during spring tides in the dry season. Additionally, pH, total alkalinity (TA), dissolved inorganic carbon (DIC), carbonate (CO32–), and saturation state of calcite (Ωcal) and aragonite (Ωara) were monitored hourly. Higher salinity (>38) during ebb tides confirmed the hypersalinity and negative estuarine circulation. TA and DIC concentrations in the estuary were higher than in the adjacent coastal ocean due to evaporation, showing positive correlation with salinity and negative correlation with tidal height. Measured TA and DIC concentrations were slightly higher than those calculated by the conservative evaporation model, suggesting their production in the estuary by aerobic and anaerobic processes. CO32–, Ωcal, and Ωara showed a clear semi-diurnal (tidal-driven) and diel (24 h; biological-driven) patterns: lowest values occurred at flood tide during night-time (respectively, 185 μmol kg–1, 4.3 and 2.8), whereas highest occurred during ebb tide and daytime (respectively, 251 μmol kg–1, 5.7 and 3.8). DIC/TA ratios were higher at night-time supporting a diel control (linked to solar irradiance) of the carbonate buffering capacity. pCO2 was oversaturated comparing to the atmosphere (512–860 μatm) and the estuary was a source of CO2, with fluxes ranging from 2.2 to 200.0 mmol C m–2 d–1 (51.9 ± 26.7 mmol C m–2 d–1), which are higher than emissions normally found in low-inflow, marine-dominated estuaries. The diel variability of DIC indicated a net heterotrophic metabolism averaging −5.17 ± 7.39 mmol C m–2 h–1. Eutrophication amplifies the diel variability of the CO2 system generating large differences between daytime and night-time. The results highlight the importance of considering diel variability when estimating CO2 fluxes and carbonate chemistry in eutrophic, semi-arid, and tidally dominated estuaries under rapid environmental changes, and may represent future conditions in estuaries worldwide experiencing warming, increasing aridity and eutrophication.

Effects of Wind-Driven Lateral Upwelling on Estuarine Carbonate Chemistry

Estuaries are productive ecosystems that support extensive vertebrate and invertebrate communities, but some have suffered from an accelerated pace of acidification in their bottom waters. A major challenge in the study of estuarine acidification is strong temporal and spatial variability of carbonate chemistry resulting from a wide array of physical forces such as winds, tides and river flows. Most past studies of carbonate system dynamics were limited to the along channel direction, while lateral dynamics received less attention. Recent observations in Chesapeake Bay showed strong lateral asymmetry in the partial pressure of carbon dioxide (pCO2) and air-sea CO2 flux during a single wind event, but comparable responses to different wind events has yet to be investigated. In this work, a coupled hydrodynamic-carbonate chemistry model is used to understand wind-driven variability in the estuarine carbonate system. It is found that wind-driven lateral upwelling ventilates high DIC (Dissolved Inorganic Carbon) and CO2 deep water and raises surface pCO2, thereby modifying the air-sea CO2 flux. The upwelling also advects low pH water onto the adjacent shoals and reduces the aragonite saturation state Ω_arag in these shallow water environments, producing large temporal pH fluctuations and low pH events. Regime diagrams are constructed to summarize the effects of wind events on temporal pH and Ω_arag fluctuations and the lateral gradients in DIC, pH and pCO2 in the estuary. This modeling study provides a mechanistic explanation for the observed wind-driven lateral variability in DIC and pCO2 and reproduces large pH and Ω_arag fluctuations that could be driven by physical forcing. Given that current and historic mainstem Bay oyster beds are located in shallow shoals affected by this upwelling, a large fraction of the oyster beds (100-300 km2) could be exposed to carbonate mineral under-saturated 〖(Ω〗_arag<1) conditions during wind events. This effect should be considered in the management of acidification-sensitive species in estuaries.

Nutrient and dissolved inorganic carbon variability in the North Pacific

A compilation of surface water nutrient (phosphate, nitrate, and silicate) and partial pressure of CO2 (pCO2) observations from 1961 to 2016 reveals seasonal and interannual variability in the North Pacific. Nutrients and calculated dissolved inorganic carbon (DIC) reach maximum concentrations in March and minimum in August. Nutrient and DIC variability is in-phase (anti-phase) with changes in the mixed layer depth (sea surface temperature) north of 30 °N, and it is anti-phase (in-phase) with changes in Chl-a north of 40 °N (in 30 °N–40 °N). Seasonal drawdown of nutrients and DIC is larger toward the northwest and shows a local maximum in the boundary region between the subarctic and subtropics. Stoichiometric ratios of seasonal drawdown show that, compared to nitrate, silicate drawdown is large in the northwestern subarctic including the Bering and Okhotsk seas, and drawdown of carbon is larger toward the south. Net community production in mixed layer from March to July is estimated to be more than 6 gC/m2/mo in the boundary region between the subarctic and subtropics, the western subarctic, the Gulf of Alaska, and the Bering Sea. Nutrient and DIC concentrations vary with the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation which cause changes in horizontal advection and vertical mixing. The DIC trend is positive in all analysis area and large in the western subtropics (> 1.0 μmol/l/yr). Averaged over the analysis area, it is increasing by 0.77 ± 0.03 μmol/l/yr (0.75 ± 0.02 μmol/kg/yr).

Temporal changes in ventilation and the carbonate system in the Atlantic sector of the Southern Ocean

The Southern Ocean is the most important area of anthropogenic carbon (Cant) uptake in the world ocean, only rivalled in importance by the North Atlantic Ocean. Significant variability on decadal time-scales in the uptake of Cant in the Southern Ocean has been observed and modelled, likely with consequences for the interior ocean storage of Cant in the region, and implications for the global carbon budget. Here we use eight cruises between 1973 and 2012 to assess decadal variability in Cant storage rates in the southeast Atlantic sector of the Southern Ocean. For this we employed the extended multiple linear regression (eMLR) method. We relate variability in DIC (dissolved inorganic carbon) storage, which is assumed to equal anthropogenic carbon storage, to changes in ventilation as observed from repeat measurements of transient tracers. Within the Antarctic Intermediate Water (AAIW) layer, which is the dominant transport conduit for Cant into the interior ocean, moderate Cant storage rates were found without any clear temporal trend. In Subantarctic Mode Water (SAMW), a less dense water mass found north of the Subantarctic Front and above AAIW, high storage rates of Cant were observed up to about 2005 but lower rates in more recent times. The transient tracer data suggest a significant speed-up of ventilation in the summer warmed upper part of AAIW between 1998 and 2012, which is consistent with the high storage rate of Cant. A shift of more northern Cant storage to more southern storage in near surface waters was detected in the early 2000s. Beneath the AAIW the eMLR method as applied here did not detect significant storage of Cant. However, the presence of the transient tracer CFC-12 all through the water column suggests that some Cant should be present, but at concentrations not reliably quantifiable. The observed temporal variability in the interior ocean seems at a first glance to be out of phase with observed surface ocean Cant fluxes, but this can be explained by the time delay for the surface ocean signal to manifest itself in the interior of the ocean.

Observed small spatial scale and seasonal variability of the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; system in the Southern Ocean

Abstract. The considerable uncertainties in the carbon budget of the Southern Ocean are largely attributed to unresolved variability, in particular at a seasonal timescale and small spatial scale (~ 100 km). In this study, the variability of surface pCO2 and dissolved inorganic carbon (DIC) at seasonal and small spatial scales is examined using a data set of surface drifters including ~ 80 000 measurements at high spatiotemporal resolution. On spatial scales of 100 km, we find gradients ranging from 5 to 50 μatm for pCO2 and 2 to 30 μmol kg−1 for DIC, with highest values in energetic and frontal regions. This result is supported by a second estimate obtained with sea surface temperature (SST) satellite images and local DIC–SST relationships derived from drifter observations. We find that dynamical processes drive the variability of DIC at small spatial scale in most regions of the Southern Ocean and the cascade of large-scale gradients down to small spatial scales, leading to gradients up to 15 μmol kg−1 over 100 km. Although the role of biological activity is more localized, it enhances the variability up to 30 μmol kg−1 over 100 km. The seasonal cycle of surface DIC is reconstructed following Mahadevan et al. (2011), using an annual climatology of DIC and a monthly climatology of mixed layer depth. This method is evaluated using drifter observations and proves to be a reasonable first-order estimate of the seasonality in the Southern Ocean that could be used to validate model simulations. We find that small spatial-scale structures are a non-negligible source of variability for DIC, with amplitudes of about a third of the variations associated with the seasonality and up to 10 times the magnitude of large-scale gradients. The amplitude of small-scale variability reported here should be kept in mind when inferring temporal changes (seasonality, interannual variability, decadal trends) of the carbon budget from low-resolution observations and models.

High-Frequency Spectrophotometric Measurements of Total Dissolved Inorganic Carbon in Seawater

A new spectrophotometric method was developed to achieve continuous measurements of total dissolved inorganic carbon (DIC) in seawater. It uses a countercurrent flow design and a highly CO2-permeable membrane (Teflon AF 2400) to achieve flow-through CO2 equilibration between an acidified sample and an indicator solution with a fast response time of ~22 s. This method improves the spatiotemporal resolution by more than 1 order of magnitude compared to the existing spectrophotometric method. The flow-through equilibration allows for continuous (~1 Hz) detection and real-time data smoothing. The method had a short-term precision of ± 2.0 μmol kg(-1) for a given flow-through sample. It achieved a field precision of ± 3.6 μmol kg(-1) and successfully captured high DIC variability down to minute scales. Measurements by the new method over the typical range of oceanic DIC showed good agreement with measurements made by an established method (mean differences -1.6 to 0.3 μmol kg(-1) with 1σ ± 6.0-6.7 μmol kg(-1)). This level of precision and accuracy is comparable to that of the existing spectrophotometric method. The characteristics of the new method make it particularly suitable for high-frequency, submerged measurements required for mobile observing platforms in the ocean. It can also be adapted for high-frequency, spectrophotometric measurements of seawater CO2 fugacity.

The Suess effect in Fiji coral δ13C and its potential as a tracer of anthropogenic CO2 uptake

In the context of increasing anthropogenic CO2 emissions, determining the rate of oceanic CO2 uptake is of high interest. Centennial-scale changes in δ13C of the surface water dissolved inorganic carbon (DIC) reservoir have been shown to be influenced by the carbon isotopic composition of atmospheric CO2. However, the availability of direct oceanic δ13C measurements is limited and methods for reconstructing past δ13C variability of the oceanic DIC are needed. Geochemical reconstructions of DIC variability can help in understanding how the ocean has reacted to historical changes in the carbon cycle. This study explores the potential of using temporal variations in δ13C measured in five Fijian Porites corals for reconstructing oceanic δ13C variability. A centennial-scale decreasing δ13C trend is observed in these Fiji corals. Other studies have linked similar decreasing δ13C trends to anthropogenic changes in the atmospheric carbon reservoir (the “13C Suess effect”). We conclude that solar irradiance is the factor influencing the δ13C cycle on a seasonal scale, however it is not responsible for the centennial-scale decreasing δ13C trend. In addition, variations in skeletal extension rate are not found to account for centennial-scale δ13C variability in these corals. Rather, we found that water depth at which a Fijian Porites colony calcifies influences both δ13C and extension rate mean values. The water depth-δ13C relationship induces a dampening effect on the centennial-scale decreasing δ13C trend. We removed this “water depth effect” from the δ13C composite, resulting in a truer representation of δ13C variability of the Fiji surface water DIC (δ13CFiji-DIC). The centennial-scale trend in this Fiji coral composite δ13CFiji-DIC time-series shares similarities with atmospheric δ13CCO2, implicating the 13C Suess effect as the source of the this coral δ13C trend. Additionally, our study finds that the δ13C variability between the atmosphere and the ocean in this region is not synchronous; the coral δ13C response is delayed by ~10years. This agrees with the previously established model of isotopic disequilibrium between atmospheric δ13CCO2 and oceanic surface water DIC.

Temporal and spatial variability of dissolved inorganic carbon in a boreal stream network: Concentrations and downstream fluxes

Carbon dioxide (CO2) and dissolved inorganic carbon (DIC) concentrations and export were analyzed throughout a 67 km2 boreal stream network in northern Sweden. 700 DIC and CO2 samples from 14 subcatchments were collected in 2006 and 2007. All sites were consistently supersaturated in CO2 with respect to the atmosphere. Temporal variability of DIC and CO2 concentration was best correlated with discharge, with concentrations generally diluting at high discharge. However, the variability in CO2 concentration was also dependent on the specific pH range of the stream, as variability was greatest in acidic headwater streams and lowest in larger circumneutral streams. In the larger ones the increase in the CO2 proportion of DIC at increased discharge counteracts the dilution of CO2. The shift toward proportionally more CO2 of the DIC at higher discharge is caused by decline in pH. Spatial patterns showed that DIC and CO2 concentrations were best correlated with peatland coverage of the subcatchment. The highest concentrations were found in headwater streams draining peatlands. The downstream export of DIC from the catchment outlet constitutes 19% of the total downstream export of carbon (DIC + DOC), or 0.7 (±0.09) g C m−2 yr−1. This study demonstrates the importance of including fluvial fluxes of inorganic carbon in landscape carbon budgets via runoff, and also highlights the need to account for stream evasion of CO2 to the atmosphere in such estimates since it can be larger than the downstream DIC export.

Sensitivity of sea-to-air CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux to ecosystem parameters from an adjoint model

Abstract. An adjoint model is applied to examine the biophysical factors that control surface pCO2 in different ocean regions. In the tropical Atlantic and Indian Oceans, the annual cycle of pCO2 in the model is highly dominated by temperature variability, whereas both the temperature and dissolved inorganic carbon (DIC) are important in the tropical Pacific. In the high-latitude North Atlantic and Southern Oceans, DIC variability mainly drives the annual cycle of surface pCO2. Phosphate addition significantly increases the carbon uptake in the tropical and subtropical regions, whereas nitrate addition increases the carbon uptake in the subarctic Pacific Ocean. The carbon uptake is also sensitive to changes in the physiological rate parameters in the ecosystem model in the equatorial Pacific, North Pacific, North Atlantic, and the Southern Ocean. Zooplankton grazing plays a major role in carbon exchange, especially in the HNLC regions. The grazing parameter regulates the phytoplankton biomass at the surface, thus controlling the biological production and the carbon uptake by photosynthesis. In the oligotrophic subtropical regions, the sea-to-air CO2 flux is sensitive to changes in the phytoplankton exudation rate by altering the flux of regenerated nutrients essential for photosynthesis.

Daily variability of dissolved inorganic radiocarbon in Sargasso Sea surface water

Surface water samples were collected daily in June 2000 at a site in the Sargasso Sea to observe variability of Δ 14C values in dissolved inorganic carbon (DIC). Temperature, salinity, DIC concentration, alkalinity, and δ 13C and Δ 14C values of DIC were measured in the samples. Ten Δ 14C measurements averaged 81 ± 8‰ and had a range of 24‰ over the sixteen-day cruise. Δ 14C values were more variable during the latter half of the cruise. Salinity and temperature measurements in the mixed layer throughout the cruise indicate that there were changes in water mass that occurred at our site. We conclude that the daily range of DIC Δ 14C values in the surface ocean at our site is several times greater than the annual change in surface waters in the Sargasso Sea during the last two decades of the 20th century. This points to the importance of obtaining multiple measurements of the surface ocean to adequately define the true variability of DIC Δ 14C measurements.

Interannual and decadal variability of the oceanic carbon sink in the North Atlantic subpolar gyre

The evaluation of interannual and decadal variations of air-sea CO2 fluxes represents important step for understanding the changes in the global carbon cycle. In this study we analyse the variations of sea surface dissolved inorganic carbon (DIC) and total alkalinity (TA) in the North Atlantic over the period 1993.2003 (SURATLANT Program). The analysis focuses on the subpolar gyre (53°.N-62°.N/45.W-20°.W). Large interannual variability of DIC and air-sea CO2 fluxes is observed mostly during summer. In the extreme case, this region was a CO2 source in 2003 explained by a dramatic warming and the absence of late-summer bloom. At the decadal scale, DIC and TA concentrations appeared stable indicating a complex balance between primary production, vertical mixing, horizontal transport and anthropogenic CO2.We also found that winter fCO2 has increased at a rate of +2.8 μatm yr-1 between 1993 and 2003, due to strong surface warming (1.5.°C over 10 yr) particularly since winter 1995 when the North Atlantic Oscillation index moved into a negative phase. This resulted in a decrease of carbon uptake in the North Atlantic subpolar gyre, a trend also suggested for the period 1972.1989 but not captured by current class atmospheric inverse models.

LAKE DISSOLVED INORGANIC CARBON AND DISSOLVED OXYGEN: CHANGING DRIVERS FROM DAYS TO DECADES

Dissolved inorganic carbon (DIC) and dissolved oxygen (DO) are commonly measured to compute metabolism of aquatic ecosystems. However, concentrations of DIC and DO depend on many factors in addition to ecosystem metabolism, such as water temperature, gas exchange with the atmosphere, abiotic chemical reactions, and inputs in precipitation, groundwater, and surface water. We used 20-year time series from seven lakes to understand how DIC and DO concentrations are controlled as a function of time scale. Diel cycles of both solutes are controlled primarily by metabolism, exchange with the atmosphere, and temperature. At seasonal and annual scales, metabolism is important, but physical processes associated with spring and autumn mixing, as well as solute loading from the watershed, have comparably large effects. At decadal scales, effects of metabolism are negligible. Controls of the two solutes diverge, with variance in DIC explained largely by solute inputs and variance in DO explained largely by water temperature. Like other indicators in many ecosystems, variability of DIC and DO is strongly scale dependent and associated with different drivers depending on the time scale of the analysis.

C and O isotope compositions of modern fresh-water mollusc shells and river waters from the Himalaya and Ganga plain

The aim of this paper is to unfold the relationship between the O and C isotope compositions of modern fresh-water mollusc shells and water in order to refine the basis of interpretation for paleoenvironnemental reconstruction in the sub-Himalayan river basins. Large number of mollusc shells and associated host water from both running water and closed body of water were analysed including intra-shell variability in a few cases. The O isotopic compositions of river waters in the Himalaya and Ganga plain have a large range, from − 18‰ in the north of the high range up to − 8‰ to − 4‰ in the Ganga plain. δ 18O of rivers are also seasonally variable, especially in foothills rivers whereas the seasonal contrast is smoothed out for the Main Himalayan rivers having large catchments. O isotopic compositions of bulk shells (Aragonite) vary between − 15‰ and − 5‰. Average δ 18O Ara values are consistent with precipitation at equilibrium with host waters at a temperature range of 20–25 °C suggesting that shell growth may be favoured during non-monsoon conditions. Shells collected along the Main Himalayan rivers have δ 18O values uniformly distributed within − 11‰ and − 6‰ reflecting the minimal seasonal contrast shown by these rivers. In contrast, O isotopic compositions of shells from foothills rivers vary only by 4‰. This shows that, depending on the type of river where the molluscs grow, the information in term of δ 18O amplitude will be different for identical climatic conditions. In closed or pond water bodies significant enrichment in 18O due to evaporation is observed. The C isotopic compositions of river dissolved inorganic carbon (DIC) decrease downstream from 0‰ to − 10‰ reflecting input of soil derived alkalinity and plant productivity in the river. δ 13C of shells are systematically lower than compositions calculated for equilibrium with river DIC indicating that in addition to DIC, a significant fraction of carbon is derived from metabolic sources. Intra-shell δ 13C are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year.

Physical controls on dissolved inorganic radiocarbon variability in the California Current

We present depth profiles of Δ 14C and δ 13C of dissolved inorganic carbon (DIC) at Station M in the Eastern North Pacific. Several seasonal profiles are presented for the time period between 1991 and 1996. Comparison with GEOSECS data clearly shows changes in ocean radiocarbon profiles since 1973. The Δ 14C of DIC shows the most variability at depths of 450, 85, and 25 m, and the lowest variability at depths of 1600 and 2500 m. The largest variability in DIC Δ 14C occurs at 450 m, a depth marked by large fluctuations in the radiocarbon signatures of the source waters. The likely controls of DIC Δ 14C variability are physical changes in the circulation of the California Current System. A simple two-box model is used to show the importance of wind driven mixing at the surface. We discuss the likely effects of mesoscale eddies and ENSO on the DIC Δ 14C values at this site. We also show that remineralization of organic carbon (dissolved or particulate) is not responsible for the variability in the Δ 14C of DIC observed at Station M.

Carbon time series in the Norwegian sea

Depth profiles of carbon parameters were obtained monthly from 1991 to 1994 as the first time series from the weathership station M located in the Norwegian Sea at 66°N 2°E. CO 2 was extracted from acidified seawater by a flushing procedure, with nitrogen as the carrier gas. The pure CO 2 gas was measured using a manometric technique, and the gas was further used for 13C and 14C measurements. The precision of the dissolved inorganic carbon (DIC) was better than ±6‰. Satisfactory agreement was obtained with standard seawater from Scripps Institution of Oceanography. The partial pressure of CO 2 (pCO 2) was measured in the atmosphere and surface water, beginning in October 1991. The most visible seasonal variation in DIC, 13C and pCO 2 was due to the plankton bloom in the upper 50–100 m. Typical values for surface water in the winter were: 2.140±0.012 mmol kg -1 for DIC, 1.00±0.04‰ for δ 13 C and 357±15 μatm for pCO 2, and the corresponding values in the summer were as low as 2.04 mmol kg -1, greater than 2.1‰, and as low as 270–300 μatm. The values for deep water are more constant during the year, with DIC values of about 2.17±0.01 mmol kg -1, and δ 13 C values between 0.97 and 1.14‰. A simple one-dimensional biological model was applied in order to investigate possible short-term variability in DIC caused by the phytoplankton growth and depth variations of the wind-mixed layer. The simulated seasonal pattern was in reasonable agreement with the observed data, but there were significant temporal variations with shorter time interval than the monthly measurements. As a supplement to the measurements at station M, some representative profiles of DIC, δ 13 C , Δ 14C, salinity and temperature from other locations in the Nordic Seas and the North Atlantic Ocean are also presented. The results are also compared with some data obtained ( Δ 14 C ) by the TTO expedition in 1981 and the GEOSECS expedition in 1972. The carbon profiles reflect the stable deep water in the Greenland and Norwegian Basins, and the relatively young bottom water just south of Iceland.

High turnover of inorganic carbon in kelp habitats as a cause of ?13C variability in marine food webs

In a study to assess qualitatively the importance of organic matter derived from kelp production in the Aleutian Islands of subarctic Alaka, replicated samples of autotrophic sources and primary and secondary consumer organisms were sampled for δ13C among sources, sites, (treatment) islands, and years. Unanticipated variation in the δ13C of kelps occurred among overtly similar sites at different islands. Variation in the δ13C of the surface canopy-forming kelp Alaria fistulosa was particularly extreme, ranging from-15.5 to-28.0‰ compared to the understory kelps, Laminaria spp. A. fistulosa δ13C varied by as much as 6 to 7‰ among similar sites at a given island within years, and by as much as 3 to 4‰ between years at the same sampling site. In serveral cases, δ13C variation was weakly tracked by some consumer organisms, suggesting that even detritus pathways through the food web can be localized and tightly coupled. Dynamic cycles in the concentration and δ13C of dissolved inorganic carbon (DIC) and aqueous CO2 concentration ([CO2]aq) were measured at three sites on one island. The δ13C or organic carbon fixed by A. fistulosa, calculated from diurnal DIC concentration and δ13C measurements, varied by 15‰ and varied inversely with [CO2]aq concentrations. Local DIC variability, probably resulting from high productivity and decreased turbulence in dense kelp habitats, provides a possible mechanism of variation in kelp δ13C. The short-term variability in the δ13C of organic carbon fixed by kelps indicates that sampling methodology and design must assess this potential variation in marine macrophyte δ13C before making assumptions about the transfer of δ13C-invariate organic matter to higher trophic levels. On the positive side, a predictable relationship between [CO2]aq concentration and kelp δ13C offers a potentially robust means to assess productivity effects on CO2 limination in kelps and other complex aquatic macrophyte habitats.