Alan M. Shiller

Dissolved trace elements in the Mississippi River: Seasonal, interannual, and decadal variability

Abstract A monthly trace element sampling of the lower Mississippi River, utilizing ultra-clean methods, was conducted from October 1991 to December 1993. Dissolved concentrations were determined for Fe, Mn, Zn, Ph, V, Mo, U, Cu, Ni, Cd, Rb, and Ba. The results show significant seasonal dissolved concentration changes for a number of elements. Specifically, dissolved Mn and Fe are found to increase rapidly in the fall and then decrease in the spring. Zn and Pb follow a similar seasonal trend, though with lower percentage concentration changes. V, Mo, and U follow an opposite seasonal trend to Mn and Fe. The data do not allow a complete determination of the causes of this variability. However, changes in the adsorption process do not appear to play an important role. Hydrologic factors are also of minimal importance for most elements, with the probable exception of Ba and U. I suggest here that redox processes, occurring both in-stream and in source regions, play an important role in determining seasonal variations in dissolved trace elements. No evidence was found of significant dissolved trace element concentration changes over the past decade and interannual variability in mean concentrations was generally small. The time series also encompassed a period of extreme flooding in the U.S. Midwest. However, the flooding did not significantly influence dissolved trace element concentrations in the lower Mississippi River.

Variability of dissolved trace metals in the mississippi river

Abstract The concentrations of eight dissolved trace metals were measured in the lower Mississippi River seven times at various flow stages during a two-year interval. Using trace metal sampling and analysis techniques demonstrated to be reliable, these metals are shown to occur at levels lower than frequently reported. There are systematic relations between the metals and discharge which can serve as predictors of metal variability. Anthropogenic contributions cannot be assessed accurately from these data but do not appear to cause the concentrations of most of these metals to be significantly higher than observed in less disturbed systems, with the possible exception of Ni and Cd.

Dissolved vanadium in rivers and estuaries

Abstract New measurements of dissolved vanadium in a variety of rivers and estuaries are presented here. The data indicate that the average concentration of dissolved vanadium in major rivers entering the ocean is ∼ 15 nmol/kg. Weathering rate and type of source rock, rather than solution chemistry or anthropogenic influences, appear to be the important factors in determining fluvial dissolved vanadium concentrations. Laboratory experiments suggest that in oxic waters vanadium is found predominantly in its most oxidized, anionic form (V(V)). Complexation with organic matter and formation of large colloidal species appear to be unimportant. Adsorption also appears to be a less important influence for vanadium than for some other trace elements such as zinc. In estuaries, vanadium behaves as a bioactive element, showing a close correspondence with the distribution of phosphate. The extent of estuarine vanadium removal is presently uncertain but may be low due to rapid remineralization of this element. Based on the river flux, the oceanic residence time for dissolved vanadium is estimated to be 100,000 years.

Analytical intercomparison results from the 1990 Intergovernmental Oceanographic Commission open-ocean baseline survey for trace metals: Atlantic Ocean

“Dissolved” (< 0.4 μm filtered) and “total dissolvable” (unfiltered) trace element samples were collected using “clean” sampling techniques from four vertical profiles in the eastern Atlantic Ocean on the first IOC Trace Metals Baseline expedition. The analytical results obtained by 9 participating laboratories for Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, and Se on samples from station 4 in the northeast Atlantic have been evaluated with respect to accuracy and precision (intercomparability). The data variability among the reporting laboratories was expressed as 2 × SD for a given element and depth, and was comparable to the 95% confidence interval reported for the NASS seawater reference standards (representing analytical variability only). The discrepancies between reporting laboratories appear to be due to inaccuracies in standardization (analytical calibration), blank correction, and/or extraction efficiency corrections.Several of the sampling bottles used at this station were not adequately pre-cleaned (anomalous Pb results). The sample filtration process did not appear to have been a source of contamination for either dissolved or particulate trace elements. The trace metal profiles agree in general with previously reported profiles from the Atlantic Ocean. We conclude that the sampling and analytical methods we have employed for this effort, while still in need of improvement, are sufficient for obtaining accurate concentration data on most trace metals in the major water masses of the oceans, and to enable some evaluation of the biogeochemical cycling of the metals.

Dissolved vanadium in rivers: effects of silicate weathering

New measurements of vanadium in 15 small California catchments as well as some tributaries of the Mississippi River are presented here. These measurements complement previously published fluvial vanadium data and allow a re-examination of prior conclusions about the geochemistry of vanadium in rivers. The data suggest a best estimate of the fluvial flux of dissolved vanadium to be 0.52 Gmol year−1, though considerable uncertainty remains in this number. Dissolved vanadium in rivers appears to be derived largely from the weathering of silicate rocks which leads to a general correlation between dissolved V and Si in rivers. However, rock type can modify this basic correlation as can solubility limits on Si. The nature of the erosional regime may play some role in modifying fluvial V/Si ratios, but the effect is weak enough to be obscured by other factors. Fluvial vanadium transport is also potentially affected both by organic matter (through complexation and reduction) and by the effects of oxygen depletion in lakes, reservoirs and fresh water sediments. However, these factors require additional study. The development of methodology to determine vanadium redox speciation in the field would aid the further understanding of the processes affecting the transport of this element through the environment.

Mississippi river methods comparison study: Implications for water quality monitoring of dissolved trace elements

Recent reports have questioned the validity of dissolved trace element concentrations reported by the U.S. Geological Surveys National Stream Quality Accounting Network (NASQAN) as well as by other water-quality monitoring programs. To address these concerns and to evaluate the NASQAN protocols, the U.S. Geological Survey undertook the Mississippi River Methods Comparison Study. We report here the major results and implications of this study. In particular, we confirm the possible inaccuracy of previous NASQAN dissolved trace element data. The results suggest that all steps of the NASQAN protocol (sampling, processing, and analysis) require revision, though the sample filtration step appears to be of particular concern

Manganese in surface waters of the Atlantic Ocean

A map of the surface water distribution of manganese in the Atlantic Ocean has been created using 494 data points. Most of these data have been reported in the literature previously, though 93 new analyses are included. Despite this amount of data, many regions of the Atlantic have sparse coverage including much of the South Atlantic. In general the map conforms with previous conclusions regarding the input and removal of Mn in surface ocean waters. Highest concentrations are seen in coastal regions and areas affected by aeolian input; lowest concentrations are observed in high latitude regions. Although some advective features are seen, the lack of stronger advective signatures suggests that the Mn surface ocean residence time is likely close to the low end of the range of previous estimates (i.e., on the order of a few years).

The Role of Biologically-Enhanced Pore Water Transport in Early Diagenesis: An Example from Carbonate Sediments in the Vicinity of North Key Harbor, Dry Tortugas National Park, Florida

Biologically enhanced pore water irrigation affects the course of early diagenesis in shallow marine sediments, as illustrated here for the carbonate sediments from North Key Harbor, Dry Tortugas National Park, Florida. Whereas macrofaunal activity at the study site extends approximately 15 cm below the water-sediment interface, measured O 2 microprofiles only show O 2 penetration to depths of a few mm. This apparent discrepancy can be explained by considering the 3-D O 2 distribution in the burrowed sediments. Calculations based on an idealized tube model for burrow irrigation show that measureable O 2 concentrations are limited to the immediate vicinity of burrows. Given the observed burrow density (705 ± 15 m -2 ), a randomly positioned O 2 microprofile has a high probability (>90%) to fall outside the reach of radial O 2 diffusion from burrows. Hence, the shallow penetration depths recorded at the site do not exclude a much deeper supply of O 2 in the sediment via the burrows. Other characteristic features observed in the upper 15-20 cm of the sediments, in particular, the absence of SO 4 2- depletion and the presence of subsurface maxima in the profiles of NH 4 + and TCO 2 , are also the result of pore water irrigation. These features are reproduced by the multicomponent reactive transport model STEADYSED1. Results of the model simulations indicate that bacterial SO 4 2- reduction is the dominant pathway of organic carbon degradation, but that consumption of SO 4 2- in the sediments is compensated by its enhanced transport by irrigation. Thus, depth profiles of SO 4 2- may be poor indicators of the importance of SO 4 2- reduction in irrigated sediments.

Dissolved Vanadium on the Louisiana Shelf: Effect of Oxygen Depletion

Abstract New measurements of dissolved vanadium in waters of the Louisiana Shelf affected by outflow from the Mississippi/Atchafalaya River system are presented here. These measurements complement previously published estuarine vanadium data and allow a reexamination of prior conclusions. In estuarine and coastal regions it appears that the most significant vanadium depletions occur in association with reducing conditions. These reducing conditions are frequently driven by anthropogenic eutrophication. Sedimentary inputs also appear to be a factor in affecting the flux of vanadium to the ocean in certain environments. In contrast to previous results we find no compelling evidence of biological removal of vanadium from estuarine surface waters. Given the uncertainties, it is difficult to accurately estimate the natural flux of dissolved vanadium from the land to the open ocean. Nonetheless, increasing coastal anthropogenic eutrophication could substantially alter the natural fluvial vanadium input as well as possibly shift the primary locus of oceanic vanadium removal.

The Geochemistry of Gallium Relative to Aluminum in Californian Streams

Abstract The possibility that the chemically similar elements Ga and Al might be fractionated during the weathering process was examined. Gallium and Al were determined in waters, source rocks, and soils in a variety of weakly basic small stream drainages in California. As expected from the low mobility of these two elements (and in agreement with previous work), little difference was found between Ga/Al ratios in source rocks and weathered residuals. However, dissolved Ga/Al ratios were consistently higher than the Ga/Al ratios of solids from the same stream basin. Evidence suggests that neither anthropogenic effects nor presence of trace sulfide minerals are likely explanations for the dissolved Ga/Al enrichment. Comparisons of Ga/Al ratios with major elements indicate that the nature and intensity of chemical weathering plays an important role in determining the dissolved Ga/Al ratio. Organic complexation of Al can also affect the dissolved Ga/Al ratio, even in weakly basic streams.