This article focuses on chemical sensors that are based on absorbance spectroscopy. These sensors exploit the principles of a widely used optical method for chemical analysis, involving the property of a chemical species to absorb light and diminish the intensity of the probing radiation. These sensors integrate the various components of the conventional instrumentation into a miniature system, and simplify the measurement process for a large number of chemical species. The various measurement modes and instrumentation components are presented and discussed, and some examples are described.

Differential diffusion is a turbulent process characterized by a more effective vertical mixing of temperature (T) than salinity (S) in ocean regions where mean gradients of both contribute to a stable density gradient. Because the molecular diffusivity of T is roughly 100 times that of S, T leaks out of a water particle displaced upward more rapidly than does S: a resulting buoyancy force acting on the salt-heavy remnant moves it downward, partially reversing initially upward flux and leaving the net upward transfer of S smaller than that of T. Differential diffusion in this sense has been documented in laboratory experiments and direct numerical simulations, using ranges of stratification and turbulent intensities that include those typical of the ocean. In all cases, the ratio of the turbulent diffusivity of S relative to T, d≡KS/KT, approaches 1 for strong turbulence but falls significantly below 1 for turbulence typical of episodic mixing in the stratified ocean interior. Various pieces of indirect observational evidence suggest that differential diffusion is active in at least some ocean regions. Because ocean general circulation models are sensitive to parametrization of small-scale mixing, differential diffusion at microscales may affect ocean structure and variability at much larger spatial and temporal scales.

Breaking deep-water surface waves play an important role in many aspects of air–sea interaction, including transfer of momentum, gases and heat, dissipation of wave energy, and the generation of sea spray and aerosols. Of further practical importance are their dispersion of nutrients and pollutants and the generation of ambient sound. Breaking waves are most commonly associated with whitecaps. Wave breaking occurs at scales ranging from centimeters up to the dominant waves but microbreakers cause no visible air entrainment. Nonlinear wave hydrodynamics seem to form the path to wave breaking. Mean turbulence levels in a wind-driven sea are 1–2 orders of magnitude larger than would occur in an equivalent flow along a rigid boundary. The turbulence enhancement is largest close to the surface, with the depth of the wave enhanced layer reaching a few meters. Instantaneous levels of turbulence energy dissipation beneath actively breaking waves can reach more than 4 orders of magnitude enhancement, but turbulence decays rapidly. The concept of relating wave breaking dynamics to the length and propagation speed of breaking crests is suitable for remote observation techniques and may prove to be a valuable tool in solving some of the outstanding issues related to wave breaking.

Acoustic scintillation thermography (AST) was developed and applied to map areas of seepage of warm water (diffuse flow) from the seafloor in submarine hydrothermal fields. The AST technique measures decorrelation of successive acoustic pulses from a stationary sonar transducer over a two-way travel path produced by random travel-time changes related to sound speed variations caused by propagation through turbulent temperature fluctuations in a benthic boundary layer of warm water. We apply the AST method to map diffuse hydrothermal flow over areas from meters to thousands of meters long.

Below the warm upper layers of the ocean lie the deep and bottom waters comprising about three-fourths of the total volume of the oceans. The motion of this water is largely governed by forces acting at the surface and hydrodynamic instabilities of the upper ocean general circulation. However, the ocean’s bottom relief plays a very important role in determining the pathways for the deep circulation and, to a great extent, determines the mixing between different water masses. In addition, the deepest parts of the ocean are separated into smaller basins by ridges through which deep passages and fracture zones permit the flow of bottom water. These constrictions can act in an analogous fashion to dams in rivers to control the properties of the flow connecting the basins. In this article, the theory for such control will be reviewed taking into account the rotation of the Earth. A number of important deep passages will be described in which reasonably accurate measurements of flow have been made that can be compared with theoretical predictions. Broader implications, which include enhanced local mixing, the determination of regionally averaged mixing rates, and the monitoring of long-term property changes, are also discussed.

The distributions of upper ocean heat and fresh water (or salinity) have profound consequences on the atmosphere, including the weather and climate that influence many aspects of human societies. While their mean states are reasonably well described (or so we think), the difficulty of making high-accuracy measurements at high spatial and temporal resolution, uniformly distributed around the oceans, limits our knowledge of seasonal, and shorter, variations and constrains our ability to monitor changes over years and decades. This is more the case for salinity than for heat. The freshwater distribution influences the stability of the upper ocean, especially in polar regions. The increasing stability of the ocean at high latitudes associated with enhanced ice melt is believed to portend disruption to the global thermohaline circulation. Improvements in our ability to measure the heat and freshwater budgets and of the fluxes that control them, that will result from better remote sensing and in situ capabilities, will lead to a better understanding of the ocean, atmosphere, and climate system. This will be aided by the development of more realistic and more accurate numerical models capable of simulating the upper ocean and its interactions with the overlying atmosphere.

Well-dated marine sediment cores that span the last 50000 years frequently show considerable variability in both sediment and geochemical variables. In the North Atlantic, a series of ice- and meltwater events are spaced c. 7000 years apart (called Heinrich events) and represent periodic collapse of the Laurentide Ice Sheet with possible contributions from other Northern Hemisphere ice sheets. Some marine proxies also show higher frequency events that match the Dansgaard–Oeschger events first noted from ice core records from the Greenland Ice Sheet. In the last 12000years, the interval formally referred to as the Holocene, there are indications that various marine sediment proxies record ∼1500-year millennial-scale variability that might be linked to variations in the solar output. However, the range of variability during the Holocene is much reduced from that which is observed during marine isotope stages 2 and 3.

The use of acoustics to measure near-bed sediment transport processes has gained increasing acceptance within the sedimentological community over the past two decades. The idea of using sound to study fundamental sediment processes in the underwater environment is attractive, and, in concept, straightforward. A pulse of high-frequency sound is transmitted downward from a directional sound source usually mounted a meter or two above the bed. As the pulse propagates down toward the bed, sediment in suspension backscatters a proportion of the sound and the bed generally returns a strong echo. The signal backscattered from the suspended sediments can be used to obtain vertical profiles of the suspended concentration and particle size and profiles of the three orthogonal components of flow. The strong echo from the bed can be used to measure the bed forms. Further, the profiles can be obtained with sufficient spatial and temporal resolution to allow near-bed turbulence and intrawave sediment processes to be probed; this coupled with the bedform morphology observations provides sedimentologists and coastal engineers with an extremely powerful tool to advance understanding of sediment entrainment and transport. All of this is delivered with almost no influence on the processes being observed, because sound is the instrument of measurement.

Both absolute and elapsed time can be measured to very high precision in sediments that contain climatic signals paced by regular changes in the Earth's orbit, because the orbital changes occur at well-known frequencies. The process of matching climatic signals recorded in sediments to a time template of orbital pacing is referred to as orbital tuning. Tuning involves techniques that range from simple pattern recognition to sophisticated methods of spectral (frequency) analysis. Although orbital tuning derives from statistical analyses of late Pleistocene Ice Age cycles, it has been successfully applied to sedimentary sequences over much of the 200My.

The microscopic plants and animals of our oceans known as plankton fix 100 million tons of CO2 per day, produce half the oxygen we breathe, support almost all marine life, impact human health through harmful algal blooms, contribute to cloud formation, and help to move carbon into the deep ocean. Because of these critical ecosystem roles and the sensitivity of plankton to their environment, impacts of climate change on plankton reverberate throughout marine ecosystems. Plankton have exhibited some of the largest range shifts in response to global warming of any marine or terrestrial group, expanding their distribution poleward as temperatures warm. There have also been striking examples of changes in plankton abundance as waters warm, with warm-water species increasing in abundance and replacing cold-water forms. Changes in phenology, or the timing of repeated seasonal activities, are greater for plankton than for terrestrial plants and animals. The timing of phytoplankton blooms appears to have advanced more than for zooplankton, which may disrupt the synchrony between primary and secondary production, leading to inefficient transfer of energy to fish. The increased concentration of CO2 in our oceans is turning them more acidic, potentially making it more difficult for some plankton to build and maintain their calcium carbonate shells. Observations and modeling work suggest that the large tropical oceans are becoming warmer and morestratified, with fewer nutrients in surface waters, leading to smaller phytoplankton cells. As more trophic linkages are needed to transfer energy from small phytoplankton to higher trophic levels, this ultimately results in fewer fish, marine mammals, and seabirds. This could also cause less carbon to be drawn down into the deep ocean, leading to less CO2 diffusing from the atmosphere into the oceans, and elevated atmospheric concentrations. Ultimately, impacts of climate change on plankton will not only determine the future of marine ecosystems but will also influence the pace and extent of climate change globally.