Applied Geochemistry (v.17, #1)

Organic geochemistry of the oils from the southern geological Province of Cuba by J.O Grimalt; P.G Campos; L Berdie; J.O Lopez-Quintero; L.E Navarrete-Reyes (1-10).
The aliphatic hydrocarbon composition (acyclic isoprenoids, hopanoids and steroids) of oils from the most productive fields in the southern geological Province of Cuba have been studied. This province is defined by its position with respect to the Cretaceous overthrust belt generated during the formation of oceanic crust along the axis of the proto-Caribbean Basin. The relative abundances of 18α(H)-22,29,30-trisnorneohopane, gammacerane and diasteranes suggest that Pina oils are related to the carbonate oils from the Placetas Unit in the northern province (low T s/(T s+T m) and C27,29 rr/(rr+sd) ratios). The Cristales and Jatibonico oils exhibit some differentiating features such as higher T s/(T s+T m) and absence of gammacerane. The oils from this province do not exhibit significant differences in either hopane, C32 22S/(S+R) and C30 αβ/(αβ+βα), or sterane, C29 αα 20S/(S+R), maturity ratios. However, the relative content of 5α(H),14β(H),17β(H)-cholestanes (C29 ββ/(ββ+αα) ratio) indicates that Pina oils are more mature than Cristales and Jatibonico oils. Several of these oils (Cristales, Jatibonico and Pina 26) are heavily biodegraded, lacking n-alkanes, norpristane, pristane and phytane (the two former oils do not contain acyclic isoprenoid hydrocarbons). Other biodegradation products, the 25-norhopanes, are found in all the oils. Their occurrence is probably due to mixing of severely biodegraded oil residues with undegraded crude oils during accumulation in the reservoir.

Previously the dominance of base cations by Na+ in river water in upland catchments with low weathering rates and influenced by marine-derived aerosols has been suggested as a quantitative index of weathering rate upstream of the sampling point. Using data for 59 sites from a study of the River Dee catchment in NE Scotland, the index has been fully calibrated against catchment weathering rates and net alkalinity production, derived through input output budget methods, for both upland and agricultural catchments and over a wide range of parent materials. It is shown that the relationship between Na+ dominance and weathering rate is logarithmic, rather than linear as initially suggested. The excellent correlations highlight the potential use of this Na+ dominance index for the direct quantification of catchment susceptibility to acidification at fine spatial resolution, using a few simple and inexpensive measurements. Stronger correlations were observed between the % Na+ dominance and net annual flux of alkalinity than between % Na+ dominance and weathering rate derived from summation of base cation fluxes. This demonstrates the importance of mechanisms controlling the transport of base cations out of catchments, namely in association with organic matter and with anthropogenically derived SO4 2−. These processes are shown to reduce the residual alkalinity derived through weathering. The partial neutralization of organic acidity by internally generated alkalinity has implications in the context of using the mass balance approach for setting critical loads for catchments.

Mercury interactions in a simulated gold placer by J William Miller; John E Callahan; James R Craig (21-28).
A simulated stream sediment bed was constructed in a laboratory to determine whether dissolved Hg could be transported through sediment and deposited as amalgam on Au grains. Metallic Hg was placed in a sump at one end of a tank filled with gravel (quartz sand, granules, and pebbles), and Au grains were buried in the gravel at the other end. Water was circulated in a continuous closed loop over the Hg and through the gravel that included the Au grains for more than 850 days. The Hg content of the water increased from nil at the beginning to approximately 0.5 μg/l after approximately 22 days. The Hg content on the rims of Au grains went from nil to approximately 0.2 wt.% over 22 days reaching a maximum 0.48 wt.% Hg after 14 days. Subsequent measurements indicated a persistent decrease of the maximum Hg on Au grains to ⩽0.19 wt.% Hg at 552 days and to ⩽0.05 wt.% at 851 days. Deposition of Hg on the Au grains indicates that amalgams can form without actual contact of Hg on Au in stream sediments. Why Hg first deposited on Au and then dissolved from the Au is unknown, but a paucity of microbiota early in the experiment and subsequent development of microbiota that could facilitate dissolution of Hg is suspected. The simulated Au placer, with its coarse sediments and free water flow, is analogous to streams that have measurable (>0.2 μg/l) Hg in the water and no amalgams on Au grains within the sediments. An example of a Au mining region with similar water concentrations of Hg would be the Amazon Basin, although information on the presence of amalgam rims on Au grains is lacking, as for most regions. Lower but still measurable (⩽0.55 μg/l Hg) concentrations of Hg in stream water and a lack of amalgams on Au grains occur in Au placers near Talladega, Alabama. The opposite case would be streams with less-than-measurable (<0.2 μg/l) Hg in the water but amalgams on Au grains, where conditions are less aerated and Hg would be more likely to remain in the substrate. This situation is analogous to Au placers in the North Carolina Piedmont (South Mountains, Robbins, and High Point), where Hg is not detected in stream water (<0.2 μg/l) and Au grains possess amalgamated rims. Mercury concentrations in the air over the tank (41–465 ng/m3) varied inversely with barometric pressure (1012–1033 mb @ SL), with a positive response to light, which is consistent with the work of other researchers. The positive photo effect on Hg concentration in the air was obvious at lower barometric pressures (∼ave. 1015 mb @ SL) but subdued or nonexistent at higher barometric pressures (∼ave 1025 mb @ SL). Mercury concentrations were as much as 3 times as high during daylight hours compared to nighttime concentrations over the tank at relatively low barometric pressures. The Hg content of the water remained relatively low (<1 μg/l) through the first 200 days and then abruptly increased where it oscillated between 4 and 17 μg/l to the end of the experiment (851 days). Meanwhile, the rate of loss of Hg from the tank averaged approximately 1 μg/cm2 day with a high of 1.54 μg/cm2 day. Apparently the release of Hg is little affected by the Hg content of the water, as long as a minimum amount of Hg is maintained in the water. The release rate of Hg from the tank experiment is approximately 10 times higher than those reported by other workers but probably represents a maximum due to ideal, oxidizing, high water flow conditions in the tank.

During the EU funded project BASYS (Baltic Sea System Study) short (Niemistö-type) and long (box and piston cores) sediment cores were taken which cover sedimentation during the past 8 ka. The uppermost part of the sedimentary sequence was chosen for a detailed geochemical study and freeze dried samples were analysed for about 20 elements but only the elements Mn and Ca are discussed. An age model was constructed using radiometric dating results by 210Pb/137Cs and 14C AMS. Significant correlation exists along the cores between very high Mn and moderately high Ca due to occrrences of the mineral rhodochrosite (kuthnahorite), a complex Mn(Ca) carbonate. This mineral is thought to be produced when salt water meets the pool of dissolved Mn at the bottom of the Gotland Basin. During favourable hydrographic conditions, e.g. strong northwesterly winds, salt water from the North Sea invades even the deepest parts of the central Baltic. Mn2+ which is produced mainly by the dissolution of ferromanganse oxides/oxyhydroxides in the water colum and in the course of destruction of organic matter in the sediments, combines with HCO3 2- and Ca2+ in the seawater to form rhodochrosite. After burial, this mineral stays in the sediment and is seen as light-coloured layers. A certain cyclicity in the upper 1.5 m of the cores was observed in that about 200–300 a periods of elevated Mn–Ca are followed by periods with lower Mn–Ca of similar duration. An explanation for the observed cyclicity may be sea level variations: during rising sea level (transgression) more and more saline water is pushed into the deep basin of the Baltic Sea and if conditions are favourable (high dissolved Mn) the mineral rhodochrosite is precipitated.

Chemical reactions and porosity changes during sedimentary diagenesis by Philippe Gouze; Anne Coudrain-Ribstein (39-47).
It is of interest to simulate the porosity changes associated with fluid flow and water–rock reactions in sedimentary systems. The chemical species taken into account are: {H2O, H+, Ca++, HCO3 , Mg++, Al(OH)4 , H4SiO4, K+, Na+, Cl}. The solution is assumed to be locally at equilibrium with different mineral assemblages including at least calcite, dolomite and one Mg–Al-silicate. These assemblages buffer the partial pressure of CO2 (Pco2 ) which increases with temperature (T) as is commonly observed in natural systems. The nature of the Mg–Al-silicate and the presence or absence of other at-equilibrium minerals (i.e. the feldspars) do not modify the Pco2T relationship. Considering complete transfer between the solution and the solid phases, computed relative porosity increases with increasing temperature for all the systems that were investigated. However, the direction and the amount of the clay transfer depend on the presence of feldspars. Furthermore, it is shown that computing mass transfer using a prescribed Pco2, as is commonly done, leads to very different values of mineral transfers. Hence, it must be concluded that prescribing a Pco2 value, even if the proper Pco2T relationship is used, leads to erroneous results.

A large database of dissolved-fraction groundwater concentrations for Ag, As, Ba, Be, Cd, Co, Cr, Cu, Hg, Ni, Pb, Sb, Se, Tl, Sn, V and Zn was constructed using public domain data. Data analysis techniques for multiply-censored sample populations were used to extract salient information regarding sample populations from the raw data. The results of this work show that trace element concentrations are approximately log-normally distributed. Median values for <90% censored sample populations range from 0.2 to 35 μg l−1. Minimum to maximum concentrations range as high as 7 orders of magnitude for certain elements (e.g. Zn). This work provides a rapid, cost-effective screening tool for evaluating groundwater quality based on observed trace element concentration data.

Grain size effect on multi-element concentrations in sediments from the intertidal flats of Bohai Bay, China by Chaosheng Zhang; Lijun Wang; Guosheng Li; Shuangshuang Dong; Jingrong Yang; Xiuli Wang (59-68).
In the intertidal flats of Bohai Bay, China, the spatial distribution of the grain size of sediments becomes coarser from landward to seaward. To study the grain size effect on multi-element concentrations in sediments, samples along the coastline and 3 horizontal cross-sections were collected from the intertidal flats of Bohai Bay, China, in 1998. Total concentrations of 49 elements, including alkali, alkaline earth, trace metal, rare and rare earth elements, were measured by ICP–AES, ICP–MS and INAA. The results show that sediment texture plays a controlling role on the concentrations and their spatial distribution. Principal component analysis and cluster analysis were carried out based on the grain sizes of the sediments, and the samples were classified into 3 groups: mud, silt-mud and silt. Significant differences among the element concentrations in the 3 groups were observed, and the concentrations of the elements in each group are reported in this study. Most of the elements have their highest concentrations in the fine grained mud samples, in comparison with the silt and silt-mud samples, with clay minerals possibly playing an important role. In contrast, concentrations of Ba, Sr, Hf, and Zr are elevated in the coarse silt samples. Ba and Sr may reside in feldspars, while Zr and Hf are present in zircons. Landward to seaward spatial variation of element concentrations in the sediments is observed, which is in line with the spatial distribution of the grain size and is related to the seawater hydrological dynamics in the intertidal flats.