Low 14C Activity Research Articles

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

Low 14C activities in groundwater of the confined part of the Aquia aquifer in southeastern Maryland suggest that most of this water infiltrated at least 30,000 years ago. However, radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange, possibly with secondary calcite deposits in the formation, leading to overestimated 14C ages. Whereas the geochemistry of the Aquia aquifer complicates the application of the widely used 14C dating method, the accumulation of radiogenic He seems to provide a viable alternative for establishing a chronology. The quasi-linear increase of He concentrations with flow distance observed in the Aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic He. U and Th concentrations in Aquia sand were measured in order to determine the accumulation rate of 4He with sufficient confidence to establish a He time scale.Concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration. These noble gas temperatures (NGTs) clearly show the presence of water that infiltrated under much cooler conditions than at present. NGTs are correlated with chloride concentrations, corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal. In contrast, the stable isotope ratios δ180 and δD do not provide a clear record of past climatic changes in the Aquia aquifer and the correlation between NGTs and stable isotope ratios is weak. The NGT record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum (LGM) were (9.0 ± 0.6) °C colder than during the Holocene. This difference is slightly lower than estimates derived from pollen data for this region, but considerably larger than the rather uniform cooling of about 5°C indicated by noble gas studies in more southern locations of North America. The larger cooling is ascribed to the influence of the Laurentide ice sheet, which at its maximum extension came as close as 250 km to our study site.

Radiocarbon and Uranium-Series Dating of the Plitvice Lakes Travertines

Radiocarbon and uranium-series ages of the calcareous deposits of the Plitvice Lakes show that travertines were deposited during three warm, humid, interglacial oxygen isotope stages. According to our measurements, only calcite crystals or crystal aggregates represent reliable material for both 230Th/234U and 234U/238/U dating. Compact old travertine in the form of sandstone is less reliable; it can be dated by both methods provided that its detrital contamination is not significant, demonstrated by very low 14C activity (< 1.5–2.0 pMC) and a high 230Th/232Th ratio. Old porous travertine contaminated with recent carbonates and Th-bearing clay (pMC > 5, 230Th/232Th < 5) gives erroneous results by both methods. Stage 1 (Holocene) deposition is shown primarily by 14C dating corroborated by sedimentological and palynologic studies as well as by both 230Th/234U and 234U/238U disequilibrium methods. The intensive growth of travertine barriers coincided with significant climate warming in the Holocene. Stage 5 deposition is confirmed by the 230Th/234U dating of crystalline calcite aggregates embedded in the travertine matrix and by concordant 230Th/234U and 234U/238U ages, assuming that the 234U/238U activity ratio of 1.88 observed in modern streams and in Holocene deposits can be extended to past epochs. The travertine deposition period was very short, peaking ca. 120 ± 10 ka bp. Stage 11 deposition is indicated by 234U/238U dating only, the period being within the 234U decay range, but not that of 230Th. Stage 11 travertine was deposited ca. 420 ± 50 ka bp. We did not find travertine samples with U-series ages indicating a growth period during relatively warm Stages 7 and 9; due to the scarcity of old travertine outcrops, these and possibly other stages cannot be excluded on the basis of presented data. All of these isotopic dating results concur with the field relation of the travertine complex of the Plitvice Lakes.

Exemple d'etude geochimique et isotopique de circulations aquiferes en terrain volcanique sous climat semi-aride (Amurga, Gran Canaria, Iles Canaries)

A hydrochemical and isotopic study of a groundwater flow system in volcanic media has been carried out in the southern part of Gran Canaria (Canary Islands). Monthly samples of precipitation were collected over a two year period at stations distributed all over the island. Sampling of groundwaters took place mainly from drillholes in the Amurga phonolitic massif but also from wells and springs discharging from the volcanic and related detrital formations. The δ 2H vs. δ 18O relationship of the precipitation is close to the “global meteoric water line”. Two distinct altitudinal isotopic gradients are observed. On the warm and dry southern slope, the combination of a contribution of saharan dusts and seasprays and of partial evaporation explains the high content of dissolved salts (Cl −, SO 2− 4) in precipitation. Additional processes may be invoked as sources of groundwater mineralization: leaching of silicate minerals during infiltration and/or from hydrolysis during the long residence time of the waters in the volcanic rocks. Salt content is increased through a concentration of the percolating solutions by evaporation, as shown by their stable isotope contents ( 18O, 2H). The groundwaters in the phonolitic aquifer are well differentiated from the deeper groundwaters, which are recharged at an altitude of about 1000m above sea level, or even less, if exceptional episodes of rain have a lower heavy isotope content. The interpretation of the 13C contents of soil CO 2, sampled throughout the area, and of the 13C and 14C contents of total dissolved inorganic carbon (TDIC) leads to the concept of an incorporation of pure biogenic carbon under open-system conditions in the Amurga waters. For these groundwaters, high estimates of residence time (ca. 11000 years) can be calculated from the low 14C activities. In the neighbouring valleys, the input of deep “dead” volcanic carbon and the occurrence of mixing do not allow an exact quantification of residence time values. However, they seem to be shorter than those measured for the Amurga waters.

Characterization and isotopic composition of organic and inorganic carbon in the Milk River Aquifer

Carbon isotope analyses were performed by accelerator mass spectrometry on fractions of dissolved organic carbon (DOC) in groundwater from the Milk River aquifer, Alberta, Canada. The DOC was separated into high molecular weight (HMW) and low molecular weight (LMW) fractions for analysis. Characterization information and 14C activities suggest that the HMW fractions (primarily humic substances) originate in the interstitial waters of the soil zone in the area of recharge and are transported through the porous media of the aquifer. Transformations of HMW fractions down the flow path, including an increase in carbon, decrease in oxygen, and a pronounced increase in the aliphatic nature of this fraction, are consistent with decreasing 14C activity. The LMW fractions show a dominant kerogen origin. This fraction of the DOC has low 14C activities (generally <1 percent modern carbon), and gas chromatograph/mass spectrometry analyses identified a homologous series of substituted alkanes, further evidence that these compounds may have originated from organic‐rich shales in the aquifer and/or confining beds. Geochemical and stable isotope measurements of the dissolved inorganic carbon (DIC) indicate that microbial activity is occurring in the Milk River aquifer. The alteration of these DIC δ13C ratios by microbial activity render corrections for groundwater age in this aquifer difficult, if not impossible. Although 14C analyses of DOC fractions will not replace other groundwater dating techniques, these analyses do provide additional information on groundwater age and perhaps a more complete interpretation of groundwater age than DIC alone.

Variation of Concentration, 14C Activity and 13C/12C Ratios of CO2 in Air Samples from Kitt Peak, Arizona

Air was sampled with 5L flasks at Kitt Peak (2100m elev) from 1983 through 1984 at approximately monthly intervals, occasionally supplemented with air samples from urban Tucson ca 75km away (760m elev). The Kitt Peak CO2 concentrations, represented by a yield measurement, fluctuated ca 25% over the monitoring period. The δ13C values (uncorrected for N2O) varied from ca −7.6 to −9.0, with high values (and low CO2 yields) in the late summer consistent with hemispheric seasonal biosphere effects. Tucson air has lower δ13C values and possibly greater CO2 yield suggesting a local fossil-fuel effect. 14C activity of four Kitt Peak samples range from 1.158±.007 to 1.223±.008 as uncorrected fraction of modern, below free air activity of ca 1.250 for 1984 even after correcting for fractionation. The slightly low 14C activity and δ13C values suggest the Kitt Peak air is not quite 100% clean and there may be a local/regional fossil-fuel contribution, but CO2 concentrations are similar to background atmospheric values.

Dietary carbon sources for deep-sea organisms as inferred from their organic radiocarbon activities

A small but significant depletion of radiocarbon activity ( Δ 14 C 0 00 ) is present in surface, meso-, bathy- and abyssopelagic fishes and crustaceans collected from depth in the North Central Pacific (NCP) and in the Northeast Pacific (NEP) in the 1960s and 1970s compared to average values for dissolved inorganic carbon (DIC) and zooplankton found in the euphotic zone during this period. These low 14C activities are postulated to result from (1) the 14C gradient in DIC in the euphotic zone, and (2) the incorporation of low activity, pre-bomb peakcarbon (pre-1969) from the surface ocean into chronologically old organisms. Based on the relative increase of 14C activity in surface and deep organisms, maximum estimates of the turnover times range from 1< to 9 years for the organic carbon in surface and mesopelagic animals and 4–18 years for bathy- ad abyssopelagic animals from the NCP and NEP. These data suggest that the main source of dietary carbon for deep-sea organisms is from rapidly sinking organic detritus from the surface and from active (animal mediated) transport of living carbon. There may be a minor contribution (15%) to bathypelagic organisms from low 14C activity chemolithotrophically der ived organic carbon, from heterotrophic uptake of mid-depth DIC or from recycled carbon.

Radiocarbon 14C Differentiation of Sparkling and Carbonated Wines

Specific 14C-activities, percent of modern 14C-activity, and calculated percent of fermentation CO2 are presented for CO2 contained in commercial sparkling wines, labeled as champagne or produced by the bulk (charmat) process. These data are given for the production years 1976-1982. The survey encompassed effervescent wines produced in Spain, Italy, West Germany, California, and New York. Addition of synthetic CO2 to approximately 40 samples represented as sparkling wines was indicated by low 14C-activities of CO2 in these wines. Data for 14C-activity were also presented for the ethanol distilled from sparkling wines for the years 1977-1980. In all cases, the 14C-activity of ethanol was appropriate to the year of vintage.

Isotope hydrology of groundwaters in the Pampa del Tamarugal, Chile

The water resources in northern Chile are extremely scarce and development is limited by this. This paper discusses the isotope hydrology of groundwaters in the Pampa del Tamarugal — an area of extreme aridity (rainfall < 1 mm/yr.) — whose groundwater resources are mined for the town of Iquique, as well as for industrial and agricultural purposes. The aim of the project was to obtain information on modern recharge and to delineate, if possible recharge environments. To obtain the necessary background information a precipitation survey in the high Andes, as well as spring- and surface-water studies were carried out. The results show that a well-defined meteoric water line exists where σ 2 H = (7.8 σ 18 O + 10.3)% , and altitude effects depend on air-mass movements and cannot be defined without a broader regional and detailed sampling programme. However, it is still possible to assign maximum altitudes of recharge to springs in the Andes and at the eastern border of the Pampa del Tamarugal. Comparison of these data with groundwater compositions show, that these groundwaters originate from infiltrating surface water rather than directly infiltrated precipitation. A dependence of individual groundwater systems on specific quebradas (river valleys) is recognized. However, low 14C activities indicate that most of the waters pumped today are fossil and at least several hundreds if not thousands of years old. Some minor subsurface recharge does occur at the foot of the Andes, especially at Pica where high-altitude waters discharge, but even there groundwater appears to be a diminishing resource.

Groundwater recharge and palaeoclimate in the Sirte and Kufra basins, Libya

Stable and radio-isotope results (C, H, O) for groundwaters from the Sirte and northern Kufra basins are used to determine the recharge history during the Holocene and late Pleistocene. Radiocarbon ages have been corrected on the basis of their stable carbon isotope ratios and on environmental samples from the areas, and two groups may be recognised: (1) low 14C activity groundwaters (13000–34000 yr. BP) with δ 13C-5.6 to −11.7‰; and (2) higher 14C activity groundwaters (5000–7800 yr. BP) enriched in 13C up to δ 13 C = −3.2‰ . There is a general correlation of age with depth. A well defined freshwater (< 50 mg/l Cl −) channel can be traced within the aquifer for some 130 km through the region, which is considered to represent recharge from a former wadi. This water with an age of ± 7800 yr. BP is chemically and isotopically distinct from the regional groundwaters and provides direct evidence of a significant recharge event during the Holocene. The stable isotope (O and H) composition of groundwater from the Kufra and Sirte basins are all related by an evaporative line with slope δ D = 4.5 δ 13O − 35 with an intercept on the meteoric line of -11‰. This suggests a recharge source continuing into the Holocene from air masses, analogous to current heavy monsoon rain derived from south of the Sahara. The spatial and temporal distribution of groundwaters in relation to the evaporative line suggests a progressive change in character of the recharge which is controlled by a shift towards strongly convective rainfall during the Holocene. The direct hydrogeological and geochemical evidence supports climatic models proposed by several workers in which discrete humid episodes during the Holocene are inferred.

Hydrocarbons in western North Atlantic surface sediments

High concentrations of hydrocarbons (500–3000 μg/g dry weight) together with a complex and wide molecular weight range of composition, and low 14C activity of δ 14 C = −833 ± 50%. strongly indicate that fossil fuel hydrocarbons make up the bulk of hydrocarbons in and near the New York Bight dump site areas. The composition and concentrations of hydrocarbons in Hudson Channel sediments suggest some transport of dump site hydrocarbons to areas in the channel midway to the edge of the continental shelf. Sediments at 16 stations in other areas of the continental shelf, continental slope and abyssal plain have hydrocarbon concentrations and compositions indicating that no more than 1 μg/g dry weight of fossil fuel hydrocarbons are present. N-alkanes from land sources were among the most predominant hydrocarbons in all samples except those in the New York Bight region. Their presence in abyssal plain surface sediments documents transport of some land derived organic matter to these deep ocean areas. Two 25-carbon cycloalkenes are among the more predominant hydrocarbons in continental shelf surface sediments except for the dump site areas. These two compounds are present in lower concentrations in slope sediments and are not detected in abyssal plain sediments.

Dietary amino requirements for Heliothis zea determined by dietary deletion and radiometric techniques

Feeding tests using a diet containing 18 allo free, l-amino acids as the major source of nitrogen showed that valine, leucine, lysine, arginine, histidine, isoleucine, threonine, tryptophan, phenylalanine, and methionine were indispensable for larval growth. Although larval growth rates, pupal weights, and survival to the pupal and adult stages on the amino acid diet were comparable to those obtained on a casein-wheat germ diet, most adults reared on the amino acid diet showed wing and/or body deformities. To verify further the larval dietary amino acid requirements, amino acid synthesis from glucose-U- 14C was investigated. Fourth instar larvae were reared on a synthetic diet containing glucose-U- 14C and the specific activities of the individual amino acids isolated from the whole carcass and from the protein extracted from the carcass were determined. The results of the radiometric study agreed with those obtained from feeding tests in that the radioactive carbon incorporated into the carbon chain of valine, leucine, lysine, arginine, histidine, isoleucine, threonine, and methionine showed sufficiently low 14C activity that they could be classified as indispensable nutrients. The relatively high labelling in aspartic acid, glutamic acid, glycine, serine, alanine, and cysteine indicated that the insect is capable of synthesizing these amino acids from glucose. Results of the radiometric study were inconclusive regarding the dietary need for phenylalanine, tyrosine, and tryptophan: however, feeding studies showed the phenylalanine and tryptophan were indispensable and tyrosine was dispensable. Proline was shown to be dispensable by the deletion technique, and the relatively low 14C labelling in proline was accounted for by dilution of the 14C with unlabelled components of the diet during the indirect synthetic route from glucose to proline rather than by limited synthesis of proline from glucose.

A section of14C activities of sea water between 9°s and 66°s in the South-West Pacific Ocean

Summary Since late 1956 14C activities of 16 surface and 22 subsurface samples of sea water have been determined. The distribution of these activities in a section extending from 9° S to 66° S through the South West Pacific Ocean is described and compared with ideas of the nature and movements of water masses inferred from conventional oceanographic data. It is confirmed that upwelling of Deep Water occurs in a divergence region near 64° S and that sinking occurs near the Antarctic Convergence at 61° S. The upwelling water at the divergence has a low 14C activity and water sinking at the convergence does so while the 14C content is still far removed from equilibrium with that in the atmosphere. The activity of this sinking water is about 6% to 14% lower than that of water sinking near Greenland, so that great care must be taken in interpreting the activity of a sample as the time spent since leaving the surface, unless there is only one possible source of sinking water. However, if the distribution of 14C...

The Antarctic convergence South of New Zealand

Summary A detailed profile of temperature and salinity across the upper layers of the Antarctic Convergence worked to the north of the Ross Sea during February 1958 is discussed in terms of water movements and mixing across this major oceanic water mass boundary. The introduction of Pacific Deep Water into near surface waters is found to dominate the hydrological pattern, its influence apparently enhanced in this sector of the Southern Ocean by the position of the Pacific Antarctic Ridge in relation to deep water movements, This results in water of relatively high salinity for the Antarctic zone being found in this sector, and also introduces water of low 14C activity from the very “old” Deep water into the surface pattern resulting in a significant carbon-14 depletion of Antarctic surface water.