The use of sulfur hexafluoride (SF6) as a tracer of septic tank effluent in the Florida Keys

To determine the fate and movement of sewage derived contaminants and their possible interaction with surface waters in the Florida (USA) Keys, two types of experiments were conducted using SF₆ as an artificial tracer. The first type of experiment examined fluid flow from septic tanks placed in Miami Oolite on Big Pine Key, where there is a shallow freshwater lens overlying saline groundwaters. Here groundwater transport rates were constrained to be between 0.11 and 1.87 m/h, travelling in an easterly direction. The second type of experiment took place on Key Largo where there is no freshwater aquifer and the matrix of the aquifer is solely the more porous Key Largo limestone. Here we injected the tracer into a shallow well which was screened from 0.6 to 10 m. This allowed us to evaluate groundwater movement in the shallow upper portion of the aquifer, the area to which inputs by septic tanks occur. Groundwater transport rates in the Upper Keys were as great as 3.7 m/h and were controlled by the Atlantic tide. SF₆ laden groundwater plumes moved back and forth due to tidal pumping and reached nearby surface waters within 8 h.     

Definition and global correlation of the Santonian-Campanian boundary

Review of biostratigraphical evidence from different regions shows that criteria used by workers on various marine fossil groups to define the Santonian-Campanian boundary differ considerably in relative age and position. Probably the most widely recognizable of these criteria is the extinction of the distinctive crinoid Marsupites testudinarius (North America, Europe, Asia, north Africa, Australia), which, coincides exactly with two separate definitions of the boundary - appearances of the ammonite Placenticeras bidorsatum and the belemnite Gonioteuthis granulataquadrata - and may also coincide with a third - entry of the planktic foraminiferan Globotruncana elevata. A comparison of evidence from upper Santonian and lower Campanian successions in widely separated regions allows us to place a series of important biostratigraphical markers in correct order. Defining the boundary at the extinction of M. testudinarius corresponds to a ⁸⁷Sr/⁸⁶Sr of 0.707479, and a small positive excursion in δ¹³C. The base of magnetochron 33R, generally considered to coincide with, or fall just above the base of the Campanian, is shown to lie within the upper Santonian Uintacrinus socialis Zone.     

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

Pore water chemistry (total dissolved CO₂, NH₄, NO₃, NO₂, PO₄, Si(OH)₄, Ca, Mg, Fe, Mn, SO₄, H₂S and F, and titration alkalinity), solid phase chemistry (C_org, P_org, C_TOT, N_TOT, F, Si_OPAL and S_II), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20–40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C_org), very high porosity ( > 0.96 ml cm⁻³) and very low dry bulk density (< 0.1 g cm⁻³). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μM, orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation.

The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a “testable” model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites.     

Evaluating ground water-sea water interactions via resistivity and seepage meters

We investigated submarine ground water discharge and salt water-fresh water interactions at two locations along the shoreline of the Upper Gulf of Thailand to evaluate mechanisms of water and material transport into the coastal zone. Our data set illustrates the value of using a combined approach consisting of automatic seepage meters to monitor flow rates while assessing the conductivity (salinity) of the subterranean fluids via remote resistivity measurements. Negative correlations between electric conductivities of fluids measured directly inside seepage meter chambers and the remotely assessed resistivities of subsurface pore water show that such measurements may evaluate the spatial distribution of flow rates as well as the subterranean water quality in the coastal zone. Combined seepage and resistivity measurements may thus provide a more complete understanding of coastal ground water dynamics.     

Strontium isotope stratigraphy in the Late Cretaceous: intercontinentaI correlation of the Campanian/ Maastrichtian boundary

In the Boreal Chalk of northwestern Europe the base of the Maastrichtian Stage is defined by entry of the belemnite Belemnella . In the Kronsmoor section, in northwestern Germany, which is a standard section for the European Chalk, ⁸⁷Sr/⁸⁶Sr at the Campanian/ Maastrichtian (C/M) boundary is 0.707723±9 (10). An isotopic correlation of this boundary to the US Western Interior, for which a highly refined cephalopod biostratigraphy exists, suggests that this boundary, as defined at Kronsmoor, occurs within the Baculites jenseni zone. This correlationagrees with the latest placement based on biostratigraphic criteria. On⁸⁷Sr/⁸⁶Srcriteria the boundary at Kronsmoor correlates to the English Chalk at Trunch, Norfolk, UK, at a level 3.5 m lower than its position based on biostratigraphic criteria.
At Kronsmoor, ⁸⁷Sr/⁸⁶Sr increases in a quasi-linear manner through much of the section, suggesting that, averaged over intervals of 1 Myr, the gross sedimentation rate and temporal rate of change of ⁸⁷Sr/⁸⁶Sr were approximately constant through the section. For US samples, zonally-plotted values of ⁸⁷Sr/⁸⁶Sr define a quasi-linear trend, which suggests a moderate uniformity of zonal duration from the Baculites compressus zone (73.2±0.7 Ma, 10) to the Baculites grandis zone (70.1±0.7 Ma, 10).     

The Genesis solar wind sample return mission: Past,present, and future

The Genesis Discovery mission returned solar matter in the form of the solar wind with the goal of obtaining precise solar isotopic abundances (for the first time) and greatly improved elemental abundances. Measurements of the light noble gases in regime samples demonstrate that isotopes are fractionated in the solar wind relative to the solar photosphere. Theory is required for correction. Measurement of the solar wind O and N isotopes shows that these are very different from any inner solar system materials. The solar O isotopic composition is consistent with photochemical self‐shielding. For unknown reasons, the solar N isotopic composition is much lighter than essentially all other known solar system materials, except the atmosphere of Jupiter. Ne depth profiling on Genesis materials has demonstrated that Ne isotopic variations in lunar samples are due to isotopic fractionation during implantation without appealing to higher energy solar particles. Genesis provides a precise measurement of the isotopic differences of Ar between the solar wind and the terrestrial atmosphere. The Genesis isotopic compositions of Kr and Xe agree with data from lunar ilmenite separates, showing that lunar processes have not affected the ilmenite data and that solar wind composition has not changed on 100 Ma time scales. Relative to Genesis solar wind, ArKrXe in Q (the chondrite noble gas carrier) and the terrestrial atmosphere show relatively large light isotope depletions.     

Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary

We hypothesize that nutrient cycling in a Gulf of Mexico subterranean estuary (STE) is fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. We estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions. The results indicate a large variability in nutrient concentrations over tidal and seasonal time scales. At high tide, nutrient concentrations in shallow beach groundwater were low as a result of dilution caused by seawater recirculation. During ebb tide, the concentrations increased until they reached a maximum just before the next high tide. The dominant form of nitrogen was dissolved organic nitrogen (DON) in freshwater, nitrate in brackish waters, and ammonium in saline waters. Dissolved organic carbon (DOC) production was two-fold higher in the summer than in the winter, while nitrate and DON production were one order of magnitude higher. Oxic remineralization and denitrification most likely explain these patterns. Even though fresh SGD accounted for only ∼5% of total volumetric additions, it was an important pathway of nutrients as a result of biogeochemical inputs in the mixing zone. Fresh SGD transported ∼25% of DOC and ∼50% of total dissolved nitrogen inputs into the coastal ocean, with the remainder associated with a one-dimensional vertical seawater exchange process. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime SGD nutrient fluxes (40% higher for DOC and 60% higher for nitrogen in the summer compared to the winter). We suggest that coastal primary production and nutrient dynamics in the STE are linked.     

Measurement of the lunar neutron density profile

Anin situ measurement of the lunar neutron density from 20 to 400 g cm^?2 depth below the lunar surface was made by the Apollo 17 Lunar Neutron Probe Experiment (LNPE) using particle tracks produced by the¹⁰B (n,α)⁷Li reaction. Both the absolute magnitude and the depth profile of the neutron density are in good agreement with theoretical calculations by Lingenfelter, Canfield, and Hampel. However, relatively small deviations between experiment and theory in the effect of Cd absorption on the neutron density and in the relative¹⁴⁹Sm to¹⁵⁷Gd capture rates reported previously (Russet al., 1972) imply that the true lunar¹⁵⁷Gd capture rate is about one half of that calculated theoretically.     

Reconstructing 20thcentury lead pollution andsediment focusing in a peat land pool (Kempen, Belgium), via 210Pbdating

Pb-210 dating of two metal-polluted organic sedimentcores obtained near a former pyrometallurgical zinc smelter in Lommel, Belgiumhave been used to reconstruct atmospheric lead deposition rates during the20^th century. Independent knowledge concerning historical pollutionevents and ¹³⁷Cs fall-out profiles has allowed a criticalevaluation of the CRS, CIC and CF-CS models for the ²¹⁰Pb ageinterpretation. Resulting ages for the three models suggest that, in this case,the CIC model gives the most accurate interpretation of historical pollutionevents and atmospheric lead fall-out. The ²¹⁰Pbwater-sediment flux was estimated at 141–1158Bq·m^–2·yr^–1 for one site and62–106 Bq·m^–2·yr^–1 at theother site, during the last century. The large difference illustrates thatsediment focusing was important on a small spatial scale (10 m).The direction of focusing correlates with the predominant wind direction.Maximum atmospheric lead deposition rates were found to be 1.63 ± 0.59g·m^–2·yr^–1 around 1968 AD,which is 2 orders of magnitude larger than the Belgian average in 1980 AD, and5 orders larger than Holocene atmospheric lead deposition.