Mordeckai Magaritz

13C minima follow extinction events: A clue to faunal radiation

Changes in the carbon isotope composition of carbonate rocks at time-boundary zones show similar patterns in all three major boundaries: Precambrian/Cambrian, Permian/Triassic, and Cretaceous/Tertiary. The δ 13 C values drop from a high level, reach a minimum after the time boundary, and then increase to a new level. These changes reflect variations in the exogenic carbon cycle that may correlate with variations in the total biomass. The pattern of changes may reflect a sequence of reduction in the biomass of the old fauna, a period when the old and new faunal populations reach similar size, and a proliferation episode that follows. The similarity of the isotopic patterns at each of the boundaries suggests that there occurred at the Precambrian/Cambrian not only a radiation event, during which the shelly fauna first appeared, but also an extinction of older fauna for which the record was lost.

Contaminated aquifers are a forgotten component of the global N2O budget

One of the chemical components contributing to the destruction of the ozone layer in the upper atmosphere consists of the nitrogen oxides formed from N2O (ref. 1). Prompted by the prevailing idea that the ocean is not a major source of N2O or a sink for N2O, estimates have been made of global fluxes from continental ecosystems2. Although most land areas are underlain by groundwater3, this medium has never been considered in global budgeting of N2O. A large number of aquifers around the world are contaminated by nitrogen compounds, and processes of nitrification and denitrification are reported to be operative in this environment3. These processes lead to the production of N2O (refs 4 and 5). Here we report that the concentration of N2O in phreatic aerobic aquifers contaminated by anthropogenic activities (disposal of human or animal waste, cultivation and fertilization) are up to three orders of magnitude higher than the concentration expected as a result of equilibrium with the atmosphere.

Precambrian-Cambrian boundary problem: carbon isotope correlations for Vendian and Tommotian time between Siberia and Morocco

At least three distinctive cycles are preserved in the carbon-isotope patterns for the Vendian/Tommotian interval of the Siberian platform and the Anti-Atlas Mountains of Morocco. In Siberia, an early Tommotian carbon cycle provides the first detailed test of correlations based on archaeocyathid biostratigraphy between classic sequences on the Aldan and Lena rivers, and demonstrates that the Early Cambrian zone boundaries are reasonably good time lines. The carbon signal is preserved in both limestone and dolomite and across regional faties variations. The δ^(13)C results from the late Vendian section in Morocco display a pattern similar to that found in Siberia, suggesting that carbon isotopes can be used to test stratigraphic correlations on an intercontinental scale.

MICROSCALE CHEMICAL HETEROGENEITY IN GROUNDWATER

Ronen, D., Magaritz, M., Gvirtzman, H. and Garner, W., 1987. Microscale chemical hetereogeneity in groundwater. J. Hydrol., 92: 173-178. Large variations in the concentration of Cl-, NO a and SO42 in the upper water layers of a polluted aquifer were found between samples that were vertically separated by as little as 3 cm. This was disclosed when a multilayer dialysis cell device was used to obtain undisturbed, real-time samples from test wells. This first time presentation of chemical fine structure provides new insights into mechanisms of contamination of an aquifer by surface inputs and raises the question of the validity of groundwater quality data based on single samples, however obtained.

Measurement of 129I concentrations in the environment after the Chernobyl reactor accident

Abstract The Chernobyl nuclear reactor accident which occurred on April 26, 1986 is known to have injected into the atmosphere a pulse of a large number of radionuclides. The activities of several radionuclides present in the subsequent fallout have been measured in different locations throughout Europe by gamma-ray and beta counting. We present here measurements of concentrations of the long-lived radionuclide 129I ( T 1 2 = 1.6 × 10 7 yr ) in environmental samples collected in Israel and Europe following the nuclear reactor accident. The measurements were performed by accelerator mass spectrometry, using the 14UD Rehovot Pelletron Accelerator. Concentrations of 129I in rainwater samples collected in the Munich (West-Germany) area and in Israel during the fallout period were measured to be 2.6 × 1010 and 1.2 × 109 atoms I respectively, while a 1982 rainwater sample from Israel shows a 129I concentration of 8.2 × 107 atoms I . Three measurements of the ratio 129I/131I gave a mean value of 21, from which an effective operating time of the reactor of 1.5 to 2 yr prior to the accident can be estimated. The possible use of anthropogenic 129I as a tracing tool for global environmental processes is discussed.

Anion exclusion during transport through the unsaturated zone

A 20-year chronological record of the flow of water and anions, along 27 m, in the unsaturated zone of a phreatic aquifer was reconstructed. Water was traced according to its tritium content, using the difference between the environmental tritium content of rain and irrigation water. Anions were traced using data about their sequential input to the overlying cultivated field and their concentration in the profile. Evidences of anion exlcusion were found along a 10-m thick clay loam layer. The vertical velocities of water molecules and anions (SO2−4 and Cl−) were calculated to be 0.7 ± 0.05 and 1.35 ± 0.05m yr−, respectively. The average thickness of the water films and the equivalent distance of exclusion, at a 15% gravimetric water content, were 25 and 12A, respectively. These field data fit and support the theoretical relationship between the water film thickness and the relative exclusion concentration proposed by Bresler (1973).

Stable isotope composition of land snail body water and its relation to environmental waters and shell carbonate

Abstract Day-to-day and within-day (diel) variations in δD and δ 18 O of the body water of the land snail, Theba pisana , were studied at a site in the southern coastal plain of Israel. Three phases of variation, which relate to isotopic changes in atmospheric water vapor, were distinguished: 1) on rain days, snail water becomes isotopically depleted approximately in the direction of the rain isotope values, but always less depleted in D as is atmospheric water vapor; 2) during the 1–3 days following a rain, the snail water becomes isotopically enriched along a line with slope δ 18 O space (this relates to an increasing influence of humidity derived from the Mediterranean Sea); 3) a period of relative stability of the isotopic composition persists until the next rain event. The isotopic variations can be explained by isotopic equilibration with atmospheric water vapor and/or uptake of dew derived therefrom. During the winter, when the snails are active, there is only very minor enrichment in 18 O relative to equilibrium with water vapor or dew, apparently as a result of metabolic activity. But this enrichment becomes pronounced after long periods of inactivity. Within-day variation in body water isotopic composition is minor on non-rain days. Shell carbonate is enriched in 18 O by ca. 1–2%. relative to equilibrium with body water. In most regions, the isotopic composition of atmospheric water vapor (or dew) is a direct function of that of rain. Because the isotopic composition of snail body water is related to that of atmospheric water vapor and the isotopic composition of shell carbonate in turn is related to that of body water, land snail shell carbonate 18 O should provide a reliable indication of rainfall 18 O. However, local environmental conditions and the ecological properties of the snail species must be taken into account.

Isotopic composition of formation waters from deep drillings in southern Israel

Abstract Formation water samples from deep drillings in southern Israel fall into three regional groups, each of which shows distinct chemical and isotopic characteristics. Waters from the Mediterranean coastal plain appear to be of marine lagoonal origin; some of these brines are associated with occurrences of oil. In the mountain region the top part of the sedimentary sequence has been flushed by fresher waters, apparently during Pleistocene time. In Jurassic and deeper-lying formations one encounters concentrated brines of seawater origin, with low Na/Ca ratios, which have undergone a process of ultrafiltration. In the Rift Valley proper, magnesium-rich brines have invaded the deeper formations, and are evidently of continental lake origin. The absence of oil occurrences in association with the CaCl 2 brines of the inland locations is interpreted as being due mainly to loss of oil as a result of the tectonic events associated with the formation of the Jordan Rift Valley.

Calcium carbonate nodules in soils: 13O/16O and 13C/12C ratios and 14C contents

Abstract δ 18 O values, δ 13 C values and 14 C ages were determined in ten CaCO 3 , nodule populations collected from soil and paleosols in the Israeli coastal plain. The selected soils were carbonate-free when formed and the nodules in them represent either reprecipitation of carbonate illuviated from overlying horizons (descending mode) or precipitated from a raised brackish water table (ascending mode). The 14 C ages represent the times of migration (illuviation) or eustatic movement (both climate-related) and the stable isotopic conditions reflect the environmental conditions at such times. The δ 13 C values of most populations were found to have a wide range and cannot be used to reconstruct former climates. This wide range is attributed to microvariations of pCO 2 in the soil. Because the δ 18 O ranges in the same samples are usually comparable to analytical uncertainties and because the δ 18 O mean values follow an age-correlated pattern, they permit us to place the soils in one of three categories. In two cases where nodules were soft both turned out to be young (∼ 1000 years) and to have wide ranges in δ 18 O values; the latter are apparently due to continual re-equilibration with new soil solutions. In three cases of the descending mode of nodule formation, 12,000–14,000 year-old hard nodules had very narrow δ 18 O whose means reflect the rainfall value at that time. The period between 12,000–14,000 years B.P. is independently known to be one of higher precipitation/evaporation ratio and of more intensive soil formation. These ascending-mode populations of hard nodules had narrow ranges in δ 18 O (reflecting a mixture of sea water and rainfall) and all turned out to be 3,000–4,000 years old. At this period the sea reached its highest elevation, resulting in raising the nearcoast water table.

Simultaneous changes in carbon isotopes, sea level, and conodont biozones within the Cambrian-Ordovician boundary interval at Black Mountain, Australia

Carbon isotope profiles of marine carbonate units within the Cambrian-Ordovician boundary section at Black Mountain, western Queensland, Australia, reveal cyclical changes in δ13C within the Georgina basin during the boundary interval. Minimum values in the δ13C profile are found in strata bearing lithostratigraphic evidence for sea-level drop and also correlate strongly with conodont assemblage zone boundaries. These relations indicate that direct mechanistic links existed between carbon cycles, changes in sea level, and conodont evolution.