Simone Metz

A Decline in Lead Transport by the Mississippi River

Inputs of pollutant lead to the Gulf of Mexico from the Mississippi River have declined by about 40 percent within the past decade. This decrease has been determined from annual lead loads of the Mississippi River and from the lead record in Mississippi Delta sediments. The observed trend is consistent with reduced consumption of lead in gasoline in the United States. More than 90 percent of the riverborne lead is associated with suspended sediments. Most of this particle-bound lead is deposited within 50 kilometers of the river mouth and is not easily leached at pH values above 3.

Chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids

Abstract Concentrations of trace metals in hydrothermal solutions from the southern Juan de Fuca Ridge (SJFR) and the TAG Hydrothermal Field on the Mid-Atlantic Ridge are influenced by the combined effects of source rock composition, brine–seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys. Copper, Mo, and Co show predicted, sharp decreases in concentration for vent fluids as temperatures decrease below 350°C; however, concentrations of Cu in 363°C fluids at the TAG site are well above values determined from seawater/basalt interactions due to subseafloor-refining processes and remineralization of chalcopyrite. Concentrations of Zn, Cd, Pb, As, and Ga do not show any temperature dependence between 214 to 363°C. However, Zn values do correlate well with Cl on an area-by-area basis due to subseafloor mixing of brine with altered seawater. Zinc levels also correlate well with concentrations of Cd, Pb, and As for all vent fluid samples, as well as with basalt and Zn-rich sulfides, implying some continuity in behavior from source rock to vent fluid to sulfide minerals. Gallium values correlate with Zn and Cl levels for the SJFR, but are high relative to Zn at the TAG site due to subseafloor refining processes. Concentrations of Tl follow Cl, K, and Rb, consistent with the behavior predicted for Tl + . Overall, the Cl- and Zn-rich fluids from the SJFR are characteristic of a brine-dominated system formed during recent volcanism. In contrast, high-temperature fluids from the TAG area are representative of a system that has evolved over the past 100,000 yr with high concentrations of Cu, Co, and Ga that result from higher temperatures and complex refining processes within a large vent mound.

Transport of particulate organic carbon by the Mississippi River and its fate in the Gulf of Mexico

This study was designed to determine the amount of particulate organic carbon (POC) introduced to the Gulf of Mexico by the Mississippi River and assess the influence of POC inputs on the development of hypoxia and burial of organic carbon on the Louisiana continental shelf. Samples of suspended sediment and supporting hydrographic data were collected from the river and >50 sites on the adjacent shelf. Suspended particles collected in the river averaged 1.8±0.3% organic carbon. Because of this uniformity, POC values (in μmol l−1) correlated well with concentrations of total suspended matter. Net transport of total organic carbon by the Mississippi-Atchafalaya River system averaged 0.48×1012 moles y−1 with 66% of the total organic carbon carried as POC. Concentrations of POC decreased from as high as 600 μmol l−1 in the river to <0.8 μmol l−1 in offshore waters. In contrast, the organic carbon fraction of the suspended matter increased from <2% of the total mass in the river to >35% along the shelf at ≥10 km from the river mouth. River flow was a dominant factor in controlling particle and POC distributions; however, time-series data showed that tides and weather fronts can influence particle movement and POC concentrations. Values for apparent oxygen utilization (AOU) increased from ∼60 μmol l−1 to >200 μmol l−1 along the shelf on approach to the region of chronic hypoxia. Short-term increases in AOU were related to transport of more particle-rich waters. Sediments buried on the shelf contained less organic carbon than incoming river particles. Orgamic carbon and δ13C values for shelf sediments indicated 3 that large amounts of both terrigenous and marine organic carbon are being decomposed in shelf waters and sediments to fuel observed hypoxia.

A comparison of the scavenging of phosphorus and arsenic from seawater by hydrothermal iron oxyhydroxides in the Atlantic and Pacific Oceans

Abstract Studies of hydrothermal plumes on the Juan de Fuca Ridge and the Mid-Atlantic Ridge indicate that newly-formed Fe oxyhydroxides, formed as a consequence of hydrothermal venting, readily scavenge P and As from seawater in proportion to the dissolved concentrations of these elements. The Fe content of the suspended matter decreases from ∼40% near the vents to

Trace metals in hydrothermal solutions from Cleft segment on the southern Juan de Fuca Ridge

Concentrations of trace metals in Fe- and Cl-rich hydrothermal solutions from the southern Juan de Fuca Ridge (SJFR) have been determined and corrected for residual precipitates formed in the sampler. Precipitate corrections for Ag, Cd, Cu, Mo, Pb, Sb, and Zn were variable, contributing an average of 20% to total solution concentrations whereas corrections for Co averaged less than 5% and essentially no corrections were required for Fe and Mn. Values for Cu, Co, and Mo in these solutions showed a strong dependence on temperature with sharp decreases in concentrations as temperatures decreased to less than 320°C. In addition, and unlike most other metals studied, all vent fluids from the SJFR were almost completely depleted in Mo relative to seawater values of about 110 nmol kg−1. In contrast to the Cu-Co-Mo group, concentrations of Ag, Cd, Sb, and Pb correlated well with those for Zn and are presumed to follow a distribution that is influenced less by temperature over the 246 to 332°C range encountered in this study and more by the combined chemical processes that control Zn levels along the transport pathway from the deep reaction zone to the vent orifice.

Time-based correlation of biogenic, lithogenic and authigenic sediment components with anthropogenic inputs in the Gulf of Mexico NECOP study area

Hypotheses related to variability in seasonal hypoxic conditions, coastal nutrient enhancement, and off-shelf transport of carbon on the Louisiana continental shelf were tested by characterization of biogenic, lithogenic, and authigenic components from two shelf and one Mississippi Canyon sediment cores. The authigenic-phase glauconite occurs above detection limits only in the core from the hypoxic area. A major increase in glauconite concentration was coincident with the onset (≈1940) of the increased use of commercial fertilizers in the United States. In the same hypoxic-area core, benthic foraminifera species diversity decreases upcore from approximately the turn of the century to the present in a manner concurrent with glauconite and fertilizer increases. A subset of opportunistic benthic foraminifera species, known to become more prominent in stressed environments (i.e., hypoxic), increased upcore from ∼52% of the total population at core bottom to ≈90% at core top. These benthic foraminifera population and diversity changes were not apparent in a “control” core outside the area of documented hypoxia. Seaward of the shelf, in the Mississippi Canyon, coincident increases in sediment accumulation rate, percentages of coarse fraction and of organic carbon at core top indicate increased offshelf transport of carbon and other components. Quartz percentages indicate that episodic down-canyon transport has been active to core bottom (prior to the mid 1800s).

Field and laboratory studies of metal uptake and release by hydrothermal precipitates

Data for suspended particles from hydrothermal plumes on the Mid-Atlantic Ridge (MAR) support uptake of dissolved V and As during formation of Fe oxides and release of Cu and Cd during alteration of sulfides. To complement these field observations, we carried out laboratory time series experiments to determine the stability of V, As, Cu, and Cd in precipitates from hydrothermal plumes on the MAR and Juan de Fuca Ridge. For V and As, the metal/Fe ratio in plume particles remained uniform after sampling. In contrast to V and As, particulate Cu and Cd values decreased relative to Fe over time in laboratory studies using both plume particles and mound-derived sulfides. The combined field and laboratory results show that scavenging is an important removal mechanism for seawater V and As, whereas oxidative dissolution of hydrothermal precipitates in seawater adds Cu and Cd to the ocean.

Assessing the Potential for Enhanced Bio Accumulation of Heavy Metals from Produced Water Discharges to the Gulf of Mexico

More than 140 million m3 of produced water per year are discharged to surface water from more than 3000 production platforms in the northern Gulf of Mexico (Reilly et al., 1991; LeBlanc, 1994). Concentrations of heavy metals in this produced water can be enriched by factors of 10 to greater than 10,000 relative to ambient seawater (Boesch and Rabalais, 1989; Ray and Engelhardt, 1992). Concern for the fate and effects of such discharges continues to prompt a variety of research and monitoring programs.