Surface Water Dissolved Inorganic Carbon Research Articles

Carbon isotopes in dissolved inorganic carbon as tracers of carbon sources in karst waters of the Plitvice Lakes, Croatia

Abstract Carbon isotope composition of dissolved inorganic carbon (DIC) was studied at springs, lakes and tributaries of the Plitvice Lakes. The Plitvice Lakes are a unique and complex karst lakes system consisting of 16 flow-through lakes connected by waterfalls and streams and characterized by intense calcium carbonate precipitation in the form of tufa and lake sediments. Two main springs have different 14 C concentrations due to different geology and their variations were controlled by flow rates. Further downstream, at lakes to the Korana River that outflows from the lakes, the 14 C activity and δ 13 C consistently increased down the flow. Carbon isotope composition from 2010 to 2015 was compared with the values measured 30 years earlier. The variation in δ 13 C DIC was accounted for seasonal change, while 14 C activity of surface water DIC decreased 7–12% during the 30 years. Using a semi-empiric model, it is calculated that the downstream increase is controlled by the exchange of DIC and atmospheric CO 2 carbon and by introduction of decomposed biogenic carbon from the top soil organic matter of the lakes surrounding area in almost equal proportions.

The seasonal cycle of carbonate system processes in Ryder Bay, West Antarctic Peninsula

The carbon cycle in seasonally sea-ice covered waters remains poorly understood due to both a lack of observational data and the complexity of the system. Here we present three consecutive seasonal cycles of upper ocean dissolved inorganic carbon (DIC) and total alkalinity measurements from Ryder Bay on the West Antarctic Peninsula. We attribute the observed changes in DIC to four processes: mixing of water masses, air–sea CO2 flux, calcium carbonate precipitation/dissolution and photosynthesis/respiration. This approach enables us to resolve the main drivers of the seasonal DIC cycle and also investigate the mechanisms behind interannual variability in the carbonate system. We observe a strong, asymmetric seasonal cycle in the carbonate system, driven by physical processes and primary production. In summer, melting glacial ice and sea ice and a reduction in mixing with deeper water reduce the concentration of DIC in surface waters. The dominant process affecting the carbonate system is net photosynthesis which reduces DIC and the fugacity of CO2, making the ocean a net sink of atmospheric CO2. In winter, mixing with deeper, carbon-rich water and net heterotrophy increase surface DIC concentrations, resulting in pH as low as 7.95 and aragonite saturation states close to 1. We observe no clear seasonal cycle of calcium carbonate precipitation/dissolution but some short-lived features of the carbonate time series strongly suggest that significant precipitation of calcium carbonate does occur in the Bay. The variability observed in this study demonstrates that changes in mixing and sea-ice cover significantly affect carbon cycling in this dynamic environment. Maintaining this unique time series will allow the carbonate system in seasonally sea-ice covered waters to be better understood.

Assessing the temporal evolution of dissolved inorganic carbon in waters exposed to atmospheric CO2(g): A laboratory approach

Summary Documenting the transformation of dissolved inorganic carbon (DIC) during the interaction of surface waters (e.g., rivers, lakes) with atmospheric CO 2(g) is vital for understanding carbon cycling. Investigations that mimic the continuum of changes in DIC concentrations and stable carbon isotope ratio of DIC (δ 13 C DIC ) to equilibrium with atmospheric CO 2(g) are difficult to conduct in natural settings because of multiple processes that occur in the water column, the interaction between water and sediments or rocks in stream channels and lake beds, as well as the variability in water residence times. Thus, laboratory simulations of the spectrum of DIC transformation provide insights which reduce the ambiguity in describing the mechanisms that control the behavior of DIC during surface water-atmospheric CO 2(g) interaction. To test how surface water-atmospheric CO 2(g) interaction affects DIC concentrations and δ 13 C DIC , we used three types of samples: (1) we prepared an artificial solution using NaHCO 3 where the DIC concentration is near chemical equilibrium and the δ 13 C DIC is far from isotopic equilibrium with atmospheric CO 2(g) , (2) natural groundwater where the DIC concentration and the δ 13 C DIC are both sufficiently far from chemical and isotopic equilibrium with atmospheric CO 2(g) and (3) lake water where the DIC concentration and the δ 13 C DIC are near chemical and isotopic equilibrium with atmospheric CO 2(g) . These samples allowed us to ascertain when only chemical or isotopic changes are occurring, or when both chemical and isotopic changes are occurring. The NaHCO 3 solution was prepared by dissolving ∼6 g of laboratory grade NaHCO 3 salt in 20 L of deionized water. Groundwater was collected from Stillwater, Oklahoma (36°08′22.20″N, 97°03′22.66″W) and lake water was collected from Lake McMurtry, Stillwater, Oklahoma (36°10′49.37″N, 97°10′52.9″W). The solution of NaHCO 3 , and groundwater (potential source of surface water) and lake water samples were exposed to the atmosphere in a laboratory setting for 850–1000 h until their DIC attained chemical and isotopic equilibrium with atmospheric CO 2(g) . All samples were prepared in duplicate and one set was agitated to simulate mixing in surface waters. The DIC concentrations of the NaHCO 3 samples increased without C loss and the δ 13 C DIC was enriched to a steady state for the mixed sample. The increase in the DIC concentrations was modeled as evaporation and not as CO 2(g) invasion since the pCO 2 was higher than atmospheric throughout the experiment. The enrichment in the δ 13 C DIC was modeled as equilibrium carbon isotopic exchange with atmospheric CO 2(g) . The DIC concentrations in the mixed groundwater sample initially decreased due to CO 2(g) outgassing and the accompanying enrichment in δ 13 C DIC was modeled as kinetic isotopic fractionation. After the initial decrease, the DIC concentrations increased continuously while the δ 13 C DIC was enriched to a steady state. Overall, the unmixed groundwater sample showed similar temporal δ 13 C DIC trends to the mixed groundwater sample, even though the unmixed sample did not achieve isotopic equilibrium with atmospheric CO 2(g) . Both the mixed and unmixed lake samples showed only small increases in temporal DIC concentrations and a slight initial decrease, followed by a small enrichment in the δ 13 C DIC during the experiment. The minor changes suggest that the lake samples were closer to chemical and carbon isotopic equilibrium with atmospheric CO 2(g) . The results of this study would apply in settings where the predominant process controlling carbon cycling is the interaction between the surface water DIC and atmospheric CO 2(g) .

Seasonality of CO<sub>2</sub> in coastal oceans altered by increasing anthropogenic nutrient delivery from large rivers: evidence from the Changjiang–East China Sea system

Abstract. Model studies suggested that human-induced increase in nutrient load may have stimulated primary production and thus enhanced the CO2 uptake capacity in the coastal ocean. In this study, we investigated the seasonal variations of the surface water's partial pressure of CO2 (pCO2sw) in the highly human-impacted Changjiang–East China Sea system between 2008 and 2011. The seasonality of pCO2sw has large spatial variations, with the largest extreme of 170 ± 75 μatm on the inner shelf near the Changjiang Estuary (from 271 ± 55 μatm in summer to 441 ± 51 μatm in autumn) and the weakest extreme of 53 ± 20 μatm on the outer shelf (from 328 ± 9 μatm in winter to 381 ± 18 μatm in summer). During the summer period, stronger stratification and biological production driven by the eutrophic Changjiang plume results in a very low dissolved inorganic carbon (DIC) in surface waters and a very high DIC in bottom waters of the inner shelf, with the latter returning high DIC to the surface water during the mixed period. Interestingly, a comparison with historical data shows that the average pCO2sw on the inner shelf near the Changjiang Estuary has decreased notably during summer, but has increased during autumn and winter from the 1990s to the 2000s. We suggest that this decadal change is associated with recently increased eutrophication. This would increase both the photosynthetic removal of DIC in surface waters and the respiratory release of DIC in bottom waters during summertime, thereby returning more DIC to the surface during the subsequent mixing seasons and/or episodic extreme weather events (e.g., typhoons). Our finding demonstrates that increasing anthropogenic nutrient delivery from a large river may enhance the sequestration capacity of CO2 in summer but may reduce it in autumn and winter. Consequently, the coastal ocean may not necessarily take up more atmospheric CO2 in response to increasing eutrophication, and the net effect largely depends on the relative timescale of air–sea gas exchange and offshore transport of the shelf water. Finally, the case we report for the Changjiang system may have general ramifications for other eutrophic coastal oceans.

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.

Inorganic carbon and biological oceanography above a shallow oxygen minimum in the entrance to the Gulf of California in the Mexican Pacific

The eastern Pacific intermediate oxygen minimum layer (OML) is particularly well‐developed and shoals close to the Mexican coast. We obtained hydrographic profiles including oxygen concentration [O], measured dissolved inorganic carbon (DIC), continuous surface pCO2 in water and air and took biological data south of the Gulf of California in March 2005. The core of the OML, with close to zero [O] was centered around 475 m with a thickness of the core varying between 80 m and 700 m and the upper limit of the core ranging from 500 m to 80 m. At the surface [O] was close to air‐saturation and showed no relation with the depth of the OML contrary to DIC and pCO2 concentrations. Below 50‐m depth the changes in DIC and Apparent Oxygen Utilization (AOU) yielding a molar ratio of δDIC = 0.79 x δAOU. When the OML shoaled, surface temperature, chlorophyll concentration [CHL], and depth‐integrated zooplankton increased, resulting in positive correlations among plankton biomass, pCO2, and DIC in surface waters. When [CHL] is recalculated as particulate organic carbon (POC), a linear relationship with the sum of DIC and POC is observed as expected for concomitant transport of DIC and inorganic nutrients supporting POC formation. Neglecting ventilation of carbon into the atmosphere and the production of dissolved organic carbon, the results suggested that ~16% of the carbon transported up from the OML was present in surface waters in the form of POC and the rest in inorganic form.

Effect of progressive acidification on stable carbon isotopes of dissolved inorganic carbon in surface waters

Acidification of surface waters by acid mine drainage (AMD) contamination or atmospheric deposition perturbs the carbonate equilibrium, with unknown effects to the isotope ratio of dissolved inorganic carbon (DIC). We progressively acidified samples of NaHCO 3, stream water, groundwater, and spring water contaminated by AMD (AMD spring) to a pH < 3 using H 2SO 4 under open conditions (exposed to the atmosphere) and closed conditions (isolated from the atmosphere). The δ 13C DIC of the acidified samples were enriched by 0.7‰ to 5.0‰ during the HCO 3 − dehydration phase and depleted by 0.6‰ to 2.3‰ during the phase after HCO 3 − was exhausted. The δ 13C CO 2 showed enrichment and depletion trends that were similar to those of DIC. The positive enrichment in δ 13C DIC during the HCO 3 − dehydration phase was driven by 1) kinetic fractionation of CO 2 during diffusion, or 2) a combination of fractionation accompanying HCO 3 − dehydration to CO 2(aq) followed by isotopic exchange of carbon between CO 2(aq) and HCO 3 −. We conclude that carbon isotope values in conjunction with concentrations of DIC species (CO 2(aq), HCO 3 −, and CO 3 2−) can be used to provide evidence for the effects of acidification on DIC in surface waters.

Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges

Coralline sponge skeletons are excellent tools for reconstructing the carbon isotope history of dissolved inorganic carbon (DIC) in tropical surface waters. Carbon isotope records from coralline sponges clearly reflect the industrial 12C increase in atmospheric CO2 with a precision that permits quantitative interpretations. We find from a set of δ13C records of four Caribbean sponge specimens that the isotopic response of surface water DIC to the changing isotopic composition of atmospheric CO2 varied dynamically during the last century, depending on the rate of atmospheric change. Three of our sponges provide 600 year long δ13C records. For the first time, we can reconstruct surface water δ13CDIC for the full history of the industrial CO2 release as well as the preceding preindustrial period back to the beginning of the Little Ice Age. This provides a well‐founded estimate of the anthropogenically uninfluenced, preindustrial background level of surface water 13C/12C ratios. Our records show small but systematic variations that appear to be linked to the climate fluctuations of the Little Ice Age.

An organic tracer for surface ocean radiocarbon

The Δ14C of surface water dissolved inorganic carbon (DIC) in the Southern California Bight was compared to Δ14C as recorded by the sterols in Santa Monica and Santa Barbara Basin sediments. All of the C26, C27, C28, and C29 sterols as well as dinosterol had 14C concentrations equal to surface water DIC, indicating that all of the major sterols were derived from phytoplanktonic production. There is no detectable terrestrial component. Their tracer capability was confirmed by comparing the “bomb 14C”‐derived change in surface water Δ14CDIC with the change in Δ14Csterol. The “prebomb” Δ14CDIC was −82‰, and prebomb sterols averaged −75±19‰. The Δ14C value in 1996 was +71‰. Eighteen measurements representing eight different sterols from the sediment‐water interface of both Santa Monica and Santa Barbara Basins averaged +62±23‰. When three of these values were eliminated because of suspected contamination, the remaining data averaged +71 ±12‰. The entire compound class could serve as an excellent proxy for the 14C concentration of ocean surface waters.

Surface-Water Seasonality from Stable Isotope Profiles of Littorina littorea Shells: Implications for Paleoenvironmental Reconstructions of Coastal Areas

Intra-shell stable oxygen and carbon isotope profiles (180, &13C) were established for the common intertidal gastropod Littorina littorea to test if this species, thriving on the border between land and sea, would be suitable for studies on the paleoseasonality of coastal areas. Comparison of the L. littorea data with isotope records of a Mytilus edulis bivalve from the same locality as the L. littorea shells, and with literature data for M. edulis and M. trossulus, indicates that L. littorea would be useful for stable isotope studies on the coastal marine environment, whether concerning variability in physical (temperature and salinity) or biochemical (phytoplankton productivity) processes. The &180 record representing the period prior to sexual maturity for each L. littorea shell is in agreement with the observed seasonality in surface-water temperature. This indicates that L. littorea precipitates shell calcite also during the temperature minimum in the winter. Responses in the shell 8180 to freshwater peaks in the spring suggest that oxygen isotope profiles of L. littorea also may be useful for detecting seasonal freshwater influx. The &13C values for L. littorea are relatively high (0%o to 2.5%o) as compared with Mytilus (below 0.5%o), which may suggest that the calcite of L. littorea is precipitated close to isotopic equilibrium with &13C of the dissolved inorganic carbon (DIC) of the water. This is probably related to a relatively low contribution of metabolic CO2 to the extrapallial fluid of L. littorea. A negative intrashell correlation between 8180 and 813C is found for L. littorea, reflecting the increased 813C of the surface-water DIC in the summer related to the preferential incorporation of 12C during phytoplankton photosynthesis. No correlation between 8180 and 813C is found for the M. edulis specimen, probably because metabolic CO2 overprints the variations in DIC 813C.

The dependence on temperature and salinity of dissolved inorganic carbon in East Atlantic surface waters

Recurring latitudinal patterns of the dissolved inorganic carbon (DIC) content and the fugacity of CO2 (fCO2) were observed in East Atlantic surface waters with strong gradients at hydrographic fronts. The dissolved inorganic carbon chemistry clearly displayed the effects of oceanic circulation and of persistent surface water processes. In two cases inorganic carbon components could be used as an indicator of the origin of hydrographic features. Surface water fCO2 below the atmospheric value, low DIC and low salinity north of the equator were ascribed to a combination of high rainfall and low wind speed in the Intertropical Convergence Zone and of biological uptake of CO2. Low surface water DIC and salinity delineated the Congo outflow. Along the cruise tracks calculated titration alkalinity (TA) had an almost linear relationship with salinity, while DIC had an apparent dependence on temperature and salinity. The latter dependence was tested by comparing observed DIC to DIC estimated from fCO2 and a reference value of TA normalised to salinity. Different scenarios of temperature, salinity, fCO2 and nutrient contents were applied. Changes of DIC were found to be indeed related to both temperature and salinity. The latitudinal distribution of DIC could be inferred with an accuracy of 17 μmol kg−1 and a standard deviation of 13 μmol kg−1 from in situ salinity, in situ temperature and the reference values of TA and nutrient contents normalised to in situ salinity (scenario D). The applied technique of estimating DIC from temperature and salinity is a powerful diagnostic tool to evaluate the spatial distribution of DIC.

Carbon isotope records from extant Caribbean and South Pacific sponges: Evolution of δ13C in surface water DIC

Stable isotope records of demosponges from the Caribbean and Coral Sea are described for the purpose of studying the influence of fossil fuel CO2 on the carbon isotopic composition of dissolved inorganic carbon (DIC) in surface water. The slow-growing sponges precipitate calcium carbonate in isotopic equilibrium with ambient sea water and are used to detect changes in δ13CDIC from pre-industrial times (early 19th century) to the present. We observed similar shapes and ranges in δ13C curves measured on Caribbean specimens collected from water depths of 25, 84 and 91 m as well as a specimen collected in shallow waters off New Caledonia. The records reveal a highly significant correlation with atmospheric δ13CCO2. δ13CDIC values for Caribbean and Coral Sea surface waters were calculated using the δ13C sponge records. While δ13C of atmospheric CO2 decreased by about 1.4‰ from the early 19th century to 1990, δ13CDIC of Caribbean and Coral Sea surface waters decreased by 0.9±0.2‰ and 0.7±0.3‰, respectively. No isotopic equilibrium between surface water DIC and atmospheric CO2 was observed, either during the pre-industrial steady state or during the last 100 years. The lower amount of depletion in the surface water δ13CDIC with respect to the atmospheric anthropogenic signal is explained by the dilution of the surface waters by biologically altered subsurface water DIC. The lower δ13C decrease in the Coral Sea points to a stronger influence of the subsurface water source compared to the Caribbean.