6 Isotope Geochemistry of Detrital and
Authigenic Clay Minerals in Marine
Sediments (Rb-Sr, K-Ar, O)
It was recognized early on in modern, marine research that most Holocene marine
sediments are of detrital origin (e.g. Biscaye 1965). These detrital sediments are
commonly very, fine grained and are derived from the weathering of rocks on the
continents. Biscaye (1965) suggested that the composition of the <2 p.m clay
fraction of this detritus may reflect the conditions of weathering on the continent
and so can be used to detect possible climatic fluctuations. The mineralogy of
detritat clay minerals can be used to determine both their place and nature of
origin as they give clues as to both the precursor rvck and the mode and intensity
of ~,eathering. The application of isotope methods to these detrital clay fractions
and a comparison of their isotopic ratios with those of river, ocean and pore
v,'aters ought to make it possible not only to determine the origin of the clay
minerals, but also to help us appreciate the chemical exchange processes between
clay minerals and their surroundings better.
6.1 D e t r i t a l C l a y Minerals
The isotope study of Dasch (1969) on the detrital silicate fraction of marine
sediments from the Atlantic represents a pioneering piece of work in the field of
low temperature isotope geochemistry. This study is still used today to prove that
clay minerals rarely form in isotopic equilibrium with their sedimentary
environment and so can not be used for dating sedimentary rocks (Savin and Lee
1988).
In actual fact, Dasch investigated the whole silicate fraction of the sediments
after treatment with HCI and removal of the soluble fraction. He determined
therefore the whole detrital fraction, which explains the large variations in
87Sr/S6Sr found in the investigated sediments. Dasch carried out isotopic analyses
Authigenic Clay Minerals in Marine
Sediments (Rb-Sr, K-Ar, O)
It was recognized early on in modern, marine research that most Holocene marine
sediments are of detrital origin (e.g. Biscaye 1965). These detrital sediments are
commonly very, fine grained and are derived from the weathering of rocks on the
continents. Biscaye (1965) suggested that the composition of the <2 p.m clay
fraction of this detritus may reflect the conditions of weathering on the continent
and so can be used to detect possible climatic fluctuations. The mineralogy of
detritat clay minerals can be used to determine both their place and nature of
origin as they give clues as to both the precursor rvck and the mode and intensity
of ~,eathering. The application of isotope methods to these detrital clay fractions
and a comparison of their isotopic ratios with those of river, ocean and pore
v,'aters ought to make it possible not only to determine the origin of the clay
minerals, but also to help us appreciate the chemical exchange processes between
clay minerals and their surroundings better.
6.1 D e t r i t a l C l a y Minerals
The isotope study of Dasch (1969) on the detrital silicate fraction of marine
sediments from the Atlantic represents a pioneering piece of work in the field of
low temperature isotope geochemistry. This study is still used today to prove that
clay minerals rarely form in isotopic equilibrium with their sedimentary
environment and so can not be used for dating sedimentary rocks (Savin and Lee
1988).
In actual fact, Dasch investigated the whole silicate fraction of the sediments
after treatment with HCI and removal of the soluble fraction. He determined
therefore the whole detrital fraction, which explains the large variations in
87Sr/S6Sr found in the investigated sediments. Dasch carried out isotopic analyses