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Exploring the Depths of the Black Sea's Curious Chemistry

by Dr. George Luther, University of Delaware College of Earth, Ocean, and Environment

Image of Laboratory aboard the R/V Knorr
Scientists conduct chemistry experiments in the lab aboard the R/V Knorr.

Understanding the chemistry of the Black Sea requires an understanding of the major processes at work in the system. A key one is the decomposition of organic matter — the break down of dead plant and animal life. This process drives the transition from oxic conditions, where oxygen is present in the water, to anoxic conditions, where there is no oxygen.

Bacteria enhance the decomposition process, but they need certain chemicals in order to do their thing. In the sediments and water column of the Black Sea, bacteria first use oxygen (O2), which is converted to water (H2O) during the break down of microscopic plants (phytoplankton) and microscopic animals (zooplankton).

After the oxygen is depleted, the bacteria use other natural oxidants in a stepwise progression based on decreased energy yield from the reactions. These natural oxidants are nitrate (NO3-), manganese dioxide (MnO2), iron oxides (e.g., FeOOH), and sulfate (SO4 2-), which are reduced to ammonia (NH3), dissolved manganese (Mn2+), iron (Fe2+), and hydrogen sulfide (H2S), respectively. The order of oxidants used for decomposition of organic matter is listed below. The colored chemical species can be measured with the solid-state microelectrodes we built here at the University of Delaware.

1.   O2 H2O  
2.   NO3- NH3  
3.   IO3- I-  
4. organic matter + MnO2 --------> Mn2+ + CO2
5.   FeOOH Fe2+  
6.   SO4 2- H2S  

When oxygen is present, the zone is termed oxic. When sulfide is present, the zone is termed anoxic, and the zone in between is called suboxic. In the Black Sea, the upper 150 meters (492 feet) has oxygen; then there is a zone 40 meters (131 feet) below which is suboxic, and the bottom waters (1,900 meters; nearly 1.2 miles) is anoxic. Thus, the Black Sea is the world’s largest anoxic basin.

Classical Measurement

To understand organic matter decomposition and the cycling of elements in the ocean, marine scientists take analytical measurements of all these dissolved compounds, as well as any solid-phase materials. These measurements provide information on the fluxes and transport of materials in the environment as well as the processes. For example, the release of dissolved manganese and iron signals the release of other trace metals since trace metals are included in the oxidized solid phases of manganese and iron. The release of hydrogen sulfide (H2S) — a toxin to fish and crustaceans — indicates nutrient overenrichment (eutrophication) and water column stratification.

The classical approach to the study of water column properties and reaction kinetics is based on placing a device called a CTD (shown at left) over the ship's side and sampling the water at specific depths. The CTD houses up to 24 bottles and has sensors to determine depth, temperature, and salinity.

The water is then brought back on deck, filtered through a 0.2-micrometer plastic filter for the solid particles and analyzed for each chemical separately. This is time consuming and offers limited spatial resolution — on the order of 5 meters (16.4 feet) — depending on the sea state (wave motion). This classical method also can create sampling artifacts if atmospheric oxygen is not excluded during sample processing. For example, oxygen from air reacts with H2S, Fe2+ and Mn2+ to re-form oxidized phases.

Membrane Microelectrodes

Some water constituents, notably gases, have been determined with meter resolution from a ship using (micro)electrodes covered with a gas permeable membrane. However, these electrodes can only measure one of the numerous target species (O2, H2S, pH) at a time, which means that scientists need several electrodes and analyzers.

Solid-State Microelectrodes Designed at University of Delaware

Recently, we have designed solid-state (micro)electrodes to measure the target species (O2, Mn2+, Fe2+, H2S) simultaneously and at (sub)meter vertical resolution from a ship. Also, iodide (I-) and molecular forms of iron sulfide (FeS) and iron organic compounds have been detected. This resolution allows us to more precisely determine the major processes related to organic matter decomposition and the flux of material between oxic, suboxic, and anoxic zones.

Microelectrode Construction

The electrodes for water column work are housed in a durable plastic called PEEK. The gold wire is soldered to a conductor wire, then placed into the PEEK and sealed with a non-conductive epoxy. The tips are carefully polished, then electrochemically plated with mercury for the measurement of the target chemicals. The electrode is placed into the water with a reference electrode for the application and measurement of voltage and a counter electrode for the measurement of current.

A curve of current versus voltage results yielding ‘s’-shaped waves as shown in the figure below. The amount of current indicates that a chemical species is present and its amount, whereas the position in potential of the ‘s’ shape wave indicates the type of chemical species. The electrodes are cleaned electrochemically after each measurement in situ.

Image of Figure


Data from the Black Sea

The figure below shows a representative plot of oxygen and sulfide from the Black Sea using our electrode technology during the 2001 cruise at a central station. In the southwest Black Sea, mixing of Mediterranean Sea waters coming through the Strait of Istanbul (Bosporus) with surface Black Sea waters creates a fingering effect in the middle suboxic zone that disrupts the smooth profiles shown.

Image of Figure 4

Portable Field Equipment

The photo here shows the Analytical Instrument Systems (AIS) electrochemical analyzer encased in the greenish pressure housing on the right. It is operated on battery power for field measurements with minimal disturbance of the environment. The analyzer is mated to our voltammetric electrodes and to the Monterey Bay Aquarium Institute’s pump-profiler so that water could be pumped aboard ship for other measurements as we made real-time measurements. Thus, it is now possible to determine the health or status of a body of water in real time.

Technology Transfer

Many scientists from the U.S. and around the world have visited our laboratory or have invited us to their laboratories to teach them our technology since 1994. U.S. colleagues include Robert Aller (Stony Brook), Dr. Michelle Lorah (USGS, Baltimore, MD), John Morse (Texas A&M), and Clare Reimers (Oregon State Univ.) Foreign colleagues include Dr. Bjørn Sundby (University of Quebec, Canada), Dr. Gert De Lange (University of Utrecht, The Netherlands), Dr. David Rickard (University of Wales, United Kingdom), Dr. Sylvia Schnell (Max Planck Institute for Terrestrial Microbiology , Germany), Drs. Bo Barker Jørgensen and Marcus Huettel (Max Planck Institute for Marine Microbiology, Germany), Dr. Pierre Anschutz (University of Bordeaux, France), Dr. Anneli Gunnars (University of Stockholm, Sweden), Dr. Bruce Williamson (National Institute of Water & Atmospheric Research, New Zealand), and Dr. Maria dos Santos Afonso (University of Buenos Aires, Argentina).

© Copyright 2003, University of Delaware College of Earth, Ocean, and Environment