5 Isotopic Composition of Seawater Past and
Present (Sr, Nd, Pb, Os, Ce)
Most material that has undergone weathering on the continent ends up in the sea
either in suspended or in dissolved form, where it is subsequently deposited either
as detritus or as a chemical precipitate, respectively. Therefore, seawater
represents the most important environment for the formation of sediments.
Isotopic information about the marine environment today can help towards
making material mass balances for the oceans as well as shedding light on
circulation patterns. Isotopic information about past oceans can help us not only to
date sediments by establishing secular trends in isotopic ratios through time but
also to constrain the various parameters which may influence an isotopic ratio at
any one time, such as tectonics, weathering rates, ocean spreading .rates,
cosmogenic input, anthropogenic contamination or biological evolution. An
exciting aspect of this research is how various isotopic systems, each of which
may reveal different information to us. can be brought together in conjunction
with more traditional geological techniques, to bear on any one specific question.
Research in this area has been made easier by the efforts of the ODP ( Ocean
Drilling Project ). Seawater has been sampled on numerous occasions at
geographically various locations and as a result, the isotopic and chemical
homogeneity of the oceans is well known for the major and trace elements. Some
elements possess isotopic ratios, which are more or less identical in open seawater
worldwide (e.g. St, S), whereas others vary so much that they can be used as
tracers for circulation patterns (e.g. O, Nd, Pb, Ce). Biologically related isotopic
fractionation processes only affect the stable isotopes, (e.g. C, O and S).
Therefore, the isotopic homogeneity in seawater of the much heavier radiogenic
elements is related to how quickly an element can be removed from seawater
relative to the size of the whole reservoir only. This useful parameter, reservoir
size/flux, is referred to as the 'residence time' and its significance will be
explained more fully using Sr and Nd as examples. For information about past
oceans, it is necessary to analyze sedimentary minerals that formed in isotopic
equilibrium with coeval seawater, i.e. authigenic minerals. The oldest sediments
that have been drilled offshore are from the Pacific Ocean and are no more than
160 million years old. If well preserved, these minerals can yield important
Present (Sr, Nd, Pb, Os, Ce)
Most material that has undergone weathering on the continent ends up in the sea
either in suspended or in dissolved form, where it is subsequently deposited either
as detritus or as a chemical precipitate, respectively. Therefore, seawater
represents the most important environment for the formation of sediments.
Isotopic information about the marine environment today can help towards
making material mass balances for the oceans as well as shedding light on
circulation patterns. Isotopic information about past oceans can help us not only to
date sediments by establishing secular trends in isotopic ratios through time but
also to constrain the various parameters which may influence an isotopic ratio at
any one time, such as tectonics, weathering rates, ocean spreading .rates,
cosmogenic input, anthropogenic contamination or biological evolution. An
exciting aspect of this research is how various isotopic systems, each of which
may reveal different information to us. can be brought together in conjunction
with more traditional geological techniques, to bear on any one specific question.
Research in this area has been made easier by the efforts of the ODP ( Ocean
Drilling Project ). Seawater has been sampled on numerous occasions at
geographically various locations and as a result, the isotopic and chemical
homogeneity of the oceans is well known for the major and trace elements. Some
elements possess isotopic ratios, which are more or less identical in open seawater
worldwide (e.g. St, S), whereas others vary so much that they can be used as
tracers for circulation patterns (e.g. O, Nd, Pb, Ce). Biologically related isotopic
fractionation processes only affect the stable isotopes, (e.g. C, O and S).
Therefore, the isotopic homogeneity in seawater of the much heavier radiogenic
elements is related to how quickly an element can be removed from seawater
relative to the size of the whole reservoir only. This useful parameter, reservoir
size/flux, is referred to as the 'residence time' and its significance will be
explained more fully using Sr and Nd as examples. For information about past
oceans, it is necessary to analyze sedimentary minerals that formed in isotopic
equilibrium with coeval seawater, i.e. authigenic minerals. The oldest sediments
that have been drilled offshore are from the Pacific Ocean and are no more than
160 million years old. If well preserved, these minerals can yield important