325
transgression
trace fossil ~ichnofossil.
trace metals ~trace elements.
tracer substance or property that marks water or sediment at a given location
and that can be followed during further transport. Such substances may be
artificially introduced like some radionuclides (radioactive tracers) or dyes
(rhodamine), or they may occur naturally (e.g., some isotopes). Radioactive
tracers are also used in measuring biological processes in the sea, e.g., 14(,
since its introduction by E. Steemann Nielsen in 1952, in measuring primary
production. Properties that can be used as tracers are potential temperature, salinity and potential vorticity. A water mass with well-defined properties is often
used to trace large-scale circulation in the ocean. Tracers that are non conservative (nutrients or unstable isotopes) are less suitable for studying longterm transport, but are sometimes helpful in combination with conservative
substances, to establish the transport time. Anthropogenic substances
recently introduced into the environment and detectable at very low concentrations are used more and more as tracers. As the time of introduction
into the environment is often known, they can also be used to determine the
"age" of the water. Examples are dissolved Si as water-mass tracer, Sr and Cs
radioisotopes as tracers of artificial inputs by mankind, Pb isotopes as tracers
for origin of gasoline-derived Pb in the oceans, 3Hf3He as tracer for bomb
testing inputs, chlorofluorocarbons derived from anthropogenic inputs as
tracers for downmixing of surface waters. ~cosmogenic nuclides and ~radioisotope.
trade winds winds on the equatorial side of the high-pressure cells in the subtropics. They are persistent in direction (NE in the northern, SE in the
southern hemisphere). ~climate.
transform fault fault with horizontal displacement only, marking a plate boundary
and terminating abruptly, at each end, at another plate boundary. Transform
faults have different names depending on the features they connect. Most
common are transform faults offsetting the axis of mid-ocean ridges (R-R
transform faults). ~plate tectonics.
transform plate boundary ~plate boundary.
transgression spread or extension of the sea over a land area, and the consequent evidence of such an advance, e.g., marine sediments deposited on former land surfaces. Transgressions may be worldwide as the result of global
sea-level rise or locally caused by the subsidence ofland. Compare coast and
regression.
transgression
trace fossil ~ichnofossil.
trace metals ~trace elements.
tracer substance or property that marks water or sediment at a given location
and that can be followed during further transport. Such substances may be
artificially introduced like some radionuclides (radioactive tracers) or dyes
(rhodamine), or they may occur naturally (e.g., some isotopes). Radioactive
tracers are also used in measuring biological processes in the sea, e.g., 14(,
since its introduction by E. Steemann Nielsen in 1952, in measuring primary
production. Properties that can be used as tracers are potential temperature, salinity and potential vorticity. A water mass with well-defined properties is often
used to trace large-scale circulation in the ocean. Tracers that are non conservative (nutrients or unstable isotopes) are less suitable for studying longterm transport, but are sometimes helpful in combination with conservative
substances, to establish the transport time. Anthropogenic substances
recently introduced into the environment and detectable at very low concentrations are used more and more as tracers. As the time of introduction
into the environment is often known, they can also be used to determine the
"age" of the water. Examples are dissolved Si as water-mass tracer, Sr and Cs
radioisotopes as tracers of artificial inputs by mankind, Pb isotopes as tracers
for origin of gasoline-derived Pb in the oceans, 3Hf3He as tracer for bomb
testing inputs, chlorofluorocarbons derived from anthropogenic inputs as
tracers for downmixing of surface waters. ~cosmogenic nuclides and ~radioisotope.
trade winds winds on the equatorial side of the high-pressure cells in the subtropics. They are persistent in direction (NE in the northern, SE in the
southern hemisphere). ~climate.
transform fault fault with horizontal displacement only, marking a plate boundary
and terminating abruptly, at each end, at another plate boundary. Transform
faults have different names depending on the features they connect. Most
common are transform faults offsetting the axis of mid-ocean ridges (R-R
transform faults). ~plate tectonics.
transform plate boundary ~plate boundary.
transgression spread or extension of the sea over a land area, and the consequent evidence of such an advance, e.g., marine sediments deposited on former land surfaces. Transgressions may be worldwide as the result of global
sea-level rise or locally caused by the subsidence ofland. Compare coast and
regression.