sensors give a two-dimensional record of ocean currents to several hundred meters. Acoustic techniques are also used to track subsurface floats and transmit information between ships and sensors at depth. Yet, sound is an underemployed tool in oceanography. Significant advances should be made during the next decade in physical, biological, and geological oceanography as a result of thoughtful application of acoustic principles and techniques for direct probing and information transfer. Because of its inherent global nature, the acoustic monitoring of ocean climate is a strong candidate for a global ocean observing system.
The air-sea interface provides a variety of challenges and opportunities to the oceanographer using acoustic techniques. The formation and subsequent collapse of bubbles are important sources of sound whose monitoring could provide an estimate of wave breaking intensity from which gas transfer rates could be inferred. Such measurements give important insight into poorly understood sea surface physics. The passive measurement of rainfall-generated sound is a way to measure precipitation in the open ocean. Direct measurement of precipitation is difficult to obtain and generally inaccurate. Better measurements of precipitation over the ocean are important because of its effect on the global heat and water budgets.
Some of the oceanographic applications of underwater sound are simple. Others will require improvements in our understanding of the physics of sound propagation in the sea and improved signal processing techniques and instrumentation.
Studies of chemicals dissolved in seawater, adsorbed on suspended particles, incorporated in living or nonliving organic material, and buried in seafloor sediments have yielded much information about Earth processes and past conditions. Environmental conditions are imprinted on particles that fall to the seafloor and are buried over time. With adequate understanding about processes that affect chemical concentrations and forms after deposition, sediments recovered by seafloor drilling can illuminate Earth's environmental history for millions of years into the past. In addition, modern ocean processes can be studied by measuring the concentrations of trace elements and compounds in seawater. For example, measurement of trace element distributions is a major tool used by physical oceanographers to study ocean circulation.