Dissolved Inorganic Carbon 14C Research Articles

Recharge and residence times of groundwater in hyper arid areas: The confined aquifer of Calama, Loa River Basin, Atacama Desert, Chile

Groundwater recharge in hyper arid areas often depends on surface water infiltration and diffuse recharge of highly evaporated precipitation only contribute under favorable conditions. This happens in the Calama basin two-aquifer system, in the Central Andean area of northern Chile. A conceptual model of the groundwater system and its relationship with the Loa River is defined. We focus on the confined aquifer of the Calama basin, combining hydrodynamic, hydrogeochemical and isotopic methods. Radiocarbon (14C) activity data of dissolved inorganic carbon (DIC), in conjunction with chemical data, are applied to evaluate groundwater residence time within the confined aquifer. The Loa River recharges the Calama basin aquifers in its northeastern part, with water that has chemical and isotopic characteristics inherited from the arid environment and volcanic rocks in its upper basin. In the central and northeastern part of the confined aquifer, minor variations in chloride concentration suggest that the deep aquifer is well confined. The δ18O and δ2H values in groundwater of the confined aquifer show an increasing isotopic fractionation from the recharge area (around −10‰ δ18O) to those in the discharge area (between −8.5‰ and −8‰) in the southwestern part of the aquifer. The 14C activity continuously decreases down flow from the recharge by the Loa River. Adjusted DIC radiocarbon ages indicate a groundwater travel time between 1500 and 4000 years in the confined aquifer of Calama. Despite the limitations and uncertainties of radiocarbon in DIC to estimate groundwater transit times for the confined aquifer and considering complementary chemical and isotopic constraints, the DIC 14C provides acceptable values. The approach may be applicable in other confined aquifers in hyper-arid climates in which the formation of aquifer systems linked to river damming by geological action took place. This information is needed for sound management of the scarce groundwater resources.

Carbon isotopes of dissolved inorganic carbon reflect utilization of different carbon sources by microbial communities in two limestone aquifer assemblages

Abstract. Isotopes of dissolved inorganic carbon (DIC) are used to indicate both transit times and biogeochemical evolution of groundwaters. These signals can be complicated in carbonate aquifers, as both abiotic (i.e., carbonate equilibria) and biotic factors influence the δ13C and 14C of DIC. We applied a novel graphical method for tracking changes in the δ13C and 14C of DIC in two distinct aquifer complexes identified in the Hainich Critical Zone Exploratory (CZE), a platform to study how water transport links surface and shallow groundwaters in limestone and marlstone rocks in central Germany. For more quantitative estimates of contributions of different biotic and abiotic carbon sources to the DIC pool, we used the NETPATH geochemical modeling program, which accounts for changes in dissolved ions in addition to C isotopes. Although water residence times in the Hainich CZE aquifers based on hydrogeology are relatively short (years or less), DIC isotopes in the shallow, mostly anoxic, aquifer assemblage (HTU) were depleted in 14C compared to a deeper, oxic, aquifer complex (HTL). Carbon isotopes and chemical changes in the deeper HTL wells could be explained by interaction of recharge waters equilibrated with post-bomb 14C sources with carbonates. However, oxygen depletion and δ13C and 14C values of DIC below those expected from the processes of carbonate equilibrium alone indicate considerably different biogeochemical evolution of waters in the upper aquifer assemblage (HTU wells). Changes in 14C and 13C in the upper aquifer complexes result from a number of biotic and abiotic processes, including oxidation of 14C-depleted OM derived from recycled microbial carbon and sedimentary organic matter as well as water–rock interactions. The microbial pathways inferred from DIC isotope shifts and changes in water chemistry in the HTU wells were supported by comparison with in situ microbial community structure based on 16S rRNA analyses. Our findings demonstrate the large variation in the importance of biotic as well as abiotic controls on 13C and 14C of DIC in closely related aquifer assemblages. Further, they support the importance of subsurface-derived carbon sources like DIC for chemolithoautotrophic microorganisms as well as rock-derived organic matter for supporting heterotrophic groundwater microbial communities and indicate that even shallow aquifers have microbial communities that use a variety of subsurface-derived carbon sources.

Ventilation time scales of the North Atlantic subtropical cell revealed by coral radiocarbon from the Cape Verde Islands

AbstractWe present coral‐ and sclerosponge‐based reconstructions of the 14C content in North Atlantic dissolved inorganic carbon (DIC) during the last ~100 years from the subtropical cells (STCs). These waters are sensitive to the dynamics of the shallow overturning meridional circulation that transports heat and water masses from the subtropics to the tropics. We use these records to investigate the circulation patterns of the off‐equatorial upwelling regions of the STCs, which are not well understood. Coral and sclerosponge skeletons provide long time series of ocean DIC 14C content, a tracer of oceanic circulation, effectively extending the observational record back in time. Sclerosponge data from the Bahamas were used to extend the existing subtropical 14C time series to the 21st century. Coral 14C data from the Cape Verde Islands (1890–2002) captured the 14C signature of water brought to the surface in the off‐equatorial regions of the STC present near the West African coast. We observe a unique postbomb trend at Cape Verde that is similar to the upwelling regions in the Pacific, and we interpret this trend as the result of the slow penetration of bomb 14C into the interior ocean as part of the STC circulation. Using a multibox mixing model we constrain the time history of bomb 14C in the eastern tropical Atlantic, and we estimate a 20 year time scale for ventilation of the thermocline in this area of the ocean. The similarity between the Atlantic and Pacific 14C‐based records of upwelling suggests that both are caused by bomb 14C penetration rather than more complex explanations that invoke changes in thermocline depth (e.g., related to El Niño–Southern Oscillation variability) or changes in the strength of the subtropical cells. Our results offer constraints for models of tropical ocean circulation and anthropogenic CO2 uptake that attempt to reproduce the characteristics of the shallow wind‐driven circulation in the Atlantic.

Radiocarbon in dissolved organic carbon of the South Pacific Ocean

AbstractMarine dissolved organic carbon (DOC) originates mainly from primary production using dissolved inorganic carbon (DIC) that has young 14C ages. Paradoxically, the 14C age of deep DOC ranges from 4000 to 6400 14C years, indicating that a portion of DOC survives multiple, deep ocean mixing cycles. Here we show that 14C ages of DOC from the deep South Pacific are equal to those from the deep north central Pacific. This is contrary to DIC 14C ages that increase from south to north in the deep Pacific. We hypothesize that DOC in the South Pacific is influenced by input of ancient DOC from hydrothermal flanks and ridges of the East Pacific Rise. We show that DOC Δ14C values in the deep Pacific are not controlled by aging during northward transport of deep waters, indicating that the deep oceanic carbon cycle needs reassessment.

Hydrogeochemistry and groundwater origin of the Basses-Laurentides sedimentary rock aquifer system, St. Lawrence Lowlands, Québec, Canada

A comprehensive hydrogeochemical study was carried out in the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Quebec over a 1500 km2 study area. Groundwater samples were collected at 153 sites, characterizing all geological and hydrogeological units to a maximum depth of 140 m. Groundwater was analyzed for major, minor and trace inorganic constituents, stable isotopes δ 2H, δ 18O, and δ 13C of dissolved inorganic carbon (DIC), and some samples were analyzed for 3H, and 14C of DIC. The regional distribution of groundwater types shows that the hydrogeological conditions exert a dominant control on the major ions chemistry of groundwater. Preferential recharge areas are characterized by tritiated Ca-Mg-HCO3 groundwater, and confined conditions by submodern Na-HCO3 and Na-Cl groundwater types. Two groundwater end-members are identified in the aquifer system, modern meteoric water and Pleistocene Champlain Sea water. The region displays significant variations of groundwater geochemistry and quality controlled by glaciation, Champlain Sea invasion, lithological rock diversity, and flow system scales. This situation leads to varied groundwater types and origins within a restricted area.

Partitioning microbial respiration between jet fuel and native organic matter in an organic-rich long time-contaminated aquifer

The relative importance of jet fuel biodegradation relative to the respiration of natural organic matter in a contaminated organic-rich aquifer underlying a fire training area at Tyndall Air Force Base, Florida, USA was determined with isotopic measurements. Thirteen wells were sampled and analyzed for BTX (benzene, toluene, xylene), dissolved inorganic carbon (DIC) and CH 4 concentrations, and δ 13C and 14C of DIC. Results range from non-detectable to 3790 ppb, 1.4–24 mM, 0.2–776 μM, +5.8‰ to −22‰, and from 52 to 99 pmc, respectively. Residual fuel was confined to two center wells underlying the fire training area. DIC and CH 4 concentrations were elevated down-gradient of the contamination, but also at sites that were not in the apparent flow path of the contaminated groundwater. DIC exhibited greatest δ 13C enrichment at highest DIC and CH 4 concentrations indicating that CH 4 production was an important respiration mode. Radiocarbon-depleted DIC was observed at sites with high hydrocarbon concentrations and down-gradient of the site. The results indicate that while natural attenuation was not rapidly reducing the quantity of free product overlying the aquifer at the site of contamination, it was at least constraining its flow away from the spill site. Apparently under the conditions of this study, BTX was degraded as rapidly as it was dissolved.

The use of isotopes to identify landfill gas effects on groundwater.

An evaluation of the source of volatile organic compounds in groundwater samples was performed at a landfill in southern California. The 3H (tritium) content of the water in leachate and water from the gas-collection system (condensed water and entrained water droplets) and the delta 13C and 14C content of the inorganic carbon in landfill gas CO2, leachate, and gas-collection system water were used to characterize the dissolved inorganic carbon (DIC) inside the landfill, while the same parameters were monitored in groundwater samples from affected monitoring wells and an unaffected well. Tritium levels from leachate and gas-collection system condensate ranged from approximately 2000 TU to over 4000 TU, orders of magnitude higher than unaffected groundwater. The average 14C content of DIC in the landfill pore-water samples was 121 pMC and the 14C content of unaffected groundwater DIC was 93 pMC, while the 14C content of the dissolved inorganic carbon in groundwater with VOC detections ranged from 105 to 119 pMC. The delta 13C of DIC in pore water was consistently above 0 per thousand and the delta 13C of unaffected groundwater DIC was -20.3 per thousand, while the delta 13C of DIC in affected groundwater samples was increased from -17.3 to -13.2 per thousand. The increases in both delta 13C and 14C in landfill gas-impacted groundwater DIC generally correlated with the number of volatile organic compounds detected and their concentrations. Based on the tritium and DIC 14C levels in leachate and water from the gas-collection system compared to those of unaffected water, significant increases in the tritium content of the water would be expected to accompany VOC detections and increases in delta 13C and 14C caused by landfill water. The results rule out landfill water as the VOC source, leaving landfill gas as the source. The identities and concentrations of the specific VOCs in affected groundwater samples varied among wells as well as between two leachate samples, ruling out the use of a VOC "fingerprint" for leachate or landfill gas to be compared to groundwater VOC concentrations.

Dating Quaternary lacustrine sediments in the McMurdo Dry Valleys, Antarctica

Reports of erroneously old 14C dates for modern Antarctic materials have thrown doubt into 14C chronologies. The carbon reservoir effect purported to exist in Quaternary lacustrine sediments of the McMurdo Dry Valleys was investigated by studying 14C distribution, and testing alternate dating techniques. Our results show that the carbon reservoir effect is not pervasive. Stream and near-shore microbial mats and dissolved inorganic carbon (DIC) in the surface waters of Lake Fryxell are in equilibrium with modern 14CO 2. The surface waters of Lake Hoare and Lake Bonney, however, have DIC 14C ages of 1650 and 2080 yr B.P., respectively. These older age estimates are suggested to be due to the direct input of large amounts of glacial melt with relict DIC. On the other hand, Lake Fryxell receives only a minor component of its inflow directly from a glacier, while a large component must travel long distances in numerous shallow ephemeral streams after leaving local valley glaciers. This mode of melt-water input allows the water to equilibrate with modern CO 2 before entering the lake. Bottom-water 14C ages for Lake Hoare closely match surface sediment ages, supporting the widely published period ∼1200 yr B.P. (after a 1650 yr reservoir correction) when most dry valley lakes apparently evaporated to small brine ponds and/or disappeared completely. Lake Bonney bottom-water is ∼8000 yr B.P. Carbon dating is shown to be a viable technique for lake edge deposits, and possibly lake bottom deposits where a correction to the sediment surface age can be obtained. However, we conclude that deep-water paleolake deposits can not be reliably dated using 14C alone because of an inability to determine the age of the reservoir correction (i.e. accounting for the initial carbon reservoir, plus the age of the bottom water). A suite of alternative and complimentary dating techniques were tested on modern and late Holocene lacustrine deposits. These include thermoluminescence (TL), 210Pb, 137Cs, and paleomagnetism. Of these techniques, TL (or the more sensitive optically stimulated luminescence) holds the most promise for correcting lake sediment 14C ages. TL dating of the modern Lake Hoare sediments showed an ∼1000 yr relict signal. This signal is unaffected by the age of the lake water. Both 210Pb and 137Cs occurred in very low levels in the sediments and do not appear to be viable dating techniques for the perennially ice-covered lakes of the McMurdo Dry Valleys. Paleomagnetism was not suited to the coarse-grained nature of Lake Hoare bottom sediments, but could be a useful alternative given a finer sediment type.

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.