their ability to resist acid conditions. Palygorskite is
the more vulnerable species and kaolinite the more
resistant. The degradation of clay assemblages tends
to increase toward the central and deepest parts of
marine basins, in depressed morphological zones,
and at the base of the decimeter-to-meter thick
sapropels. The degradation of clay minerals under
organic conditions has occurred close to the sediment–water interface and appears to depend on the
chemical nature and evolution stage of the terrestrial
and marine organic matter.
Paleoenvironmental Expression
Clay mineral assemblages of sediments successively
deposited in marine basins express various environmental messages related to the geological history. A
few examples from recent Quaternary to late Cenozoic series will be considered here. Similar messages
may be preserved in much older series of Mesozoic
and even Paleozoic ages, provided that the diagenetic
imprint due to lithostatic overburden, geothermal
gradient, and fluid circulation has remained moderate. Clay-rich, low permeability sedimentary formations 2–3 km thick and submitted to normal heat
flow (c. 301C/km) are usually prone to preserve such
paleoenvironmental characteristics.
Climate
As clay minerals at the surface of the Earth are
dominantly formed through pedogenic processes
depending on climate and are particularly subjected
to surficial erosion and reworking, their assemblages
successively deposited in a given sedimentary basin
are a priori able to reflect successive climatic conditions that prevailed on adjacent land masses. This
implies that very little post-depositional, i.e., diagenetic changes have affected the clay assemblages
after their storage in sediments. This is observed to
be the case in many series drilled or cored in the
oceans. The climatic message borne by clay has been
documented by numerous investigations, and corroborated by the comparable range of variations
recorded in the nature and proportions of clay minerals in both present-day soils outcropping at various
latitudes and marine sedimentary columns. Marine
clay mineral assemblages basically express the type
and intensity of continental weathering, which depend predominantly on the ion leaching through the
action of humidity and temperature, and secondarily
on seasonal rainfall and drainage conditions.
Quaternary
glacial–interglacial
alternations
caused terrestrial alternation of physical and chemical weathering processes, and this was reflected in
the clay assemblages successively brought to marine
sediments through soil erosion and river or wind
transport. Sedimentary levels contemporary with
cold periods are usually characterized by more
abundant rock-derived minerals such as richly
crystalline illite, chlorite, smectite and associated
feldspars reworked from active physical weathering.
Warm, humid periods generally correspond to increased supply of soil-derived kaolinite and metal
oxides, poorly crystalline smectite and various random mixed layer clay minerals. For instance, the
terrigenous fraction of hemipelagic sediments deposited from 500 000 to 100 000 years ago in the
Northwestern Atlantic off New Jersey and dominantly derived from the erosion of Appalachian
highlands shows increased proportions of chlorite
in glacial isotopic stages, and of kaolinite in
interglacial stages. This is clearly expressed by the
kaolinite/chlorite ratio (Figure 6). Paleoclimatic
reconstructions from clay mineral data are available
for various geological periods, as for instance the
passage since about 40 Ma from a non-glacial world
dominated by chemical weathering (smectite, kaolinite) to a glacial world in which physical weathering was greater (chlorite, illite). The comparison
of climatic curves provided by clay minerals and
other indicators (oxygen isotopes, micro-faunas or -
floras, magnetic susceptibility, etc.) allows a better
understanding of the nature, intensity, and effect of
the different factors characterizing the terrestrial
and marine climate in given regions during given
geological intervals.
High resolution studies show that clay assemblages may express terrestrial climatic variations at a
centennial scale or even less, and that the influence of
Earth’s orbital parameters varies to different extents
according to the latitude. For example, the clay
minerals data of Quaternary North Atlantic deep sea
sediments were submitted to cross-correlation spectral analyses on 5.5–14 m-long cores encompassing
the last 300 000 years. The mineral composition
displays a general 100 000-year cyclic signal (eccentricity) in the whole 451–601N range, a 41 000-year
signal (obliquity) at highest latitudes related to
dominant aeolian supply, and a 23 000-year signal
(precession) at mid-latitudes related to dominant
transport by marine currents (Table 2).
The paleoclimatic expression by clay mineral
successions is direct or indirect, i.e., it either indicates
the climate that actually prevailed at a given period,
or reflects other events depending on climate:
migration of lithospheric plates across successive
climatic zones, varying extension of ice caps controlling the surficial erosion, variations in the marine
circulation regime due to changing latitudinal and
352 CLAY MINERALOGY
the more vulnerable species and kaolinite the more
resistant. The degradation of clay assemblages tends
to increase toward the central and deepest parts of
marine basins, in depressed morphological zones,
and at the base of the decimeter-to-meter thick
sapropels. The degradation of clay minerals under
organic conditions has occurred close to the sediment–water interface and appears to depend on the
chemical nature and evolution stage of the terrestrial
and marine organic matter.
Paleoenvironmental Expression
Clay mineral assemblages of sediments successively
deposited in marine basins express various environmental messages related to the geological history. A
few examples from recent Quaternary to late Cenozoic series will be considered here. Similar messages
may be preserved in much older series of Mesozoic
and even Paleozoic ages, provided that the diagenetic
imprint due to lithostatic overburden, geothermal
gradient, and fluid circulation has remained moderate. Clay-rich, low permeability sedimentary formations 2–3 km thick and submitted to normal heat
flow (c. 301C/km) are usually prone to preserve such
paleoenvironmental characteristics.
Climate
As clay minerals at the surface of the Earth are
dominantly formed through pedogenic processes
depending on climate and are particularly subjected
to surficial erosion and reworking, their assemblages
successively deposited in a given sedimentary basin
are a priori able to reflect successive climatic conditions that prevailed on adjacent land masses. This
implies that very little post-depositional, i.e., diagenetic changes have affected the clay assemblages
after their storage in sediments. This is observed to
be the case in many series drilled or cored in the
oceans. The climatic message borne by clay has been
documented by numerous investigations, and corroborated by the comparable range of variations
recorded in the nature and proportions of clay minerals in both present-day soils outcropping at various
latitudes and marine sedimentary columns. Marine
clay mineral assemblages basically express the type
and intensity of continental weathering, which depend predominantly on the ion leaching through the
action of humidity and temperature, and secondarily
on seasonal rainfall and drainage conditions.
Quaternary
glacial–interglacial
alternations
caused terrestrial alternation of physical and chemical weathering processes, and this was reflected in
the clay assemblages successively brought to marine
sediments through soil erosion and river or wind
transport. Sedimentary levels contemporary with
cold periods are usually characterized by more
abundant rock-derived minerals such as richly
crystalline illite, chlorite, smectite and associated
feldspars reworked from active physical weathering.
Warm, humid periods generally correspond to increased supply of soil-derived kaolinite and metal
oxides, poorly crystalline smectite and various random mixed layer clay minerals. For instance, the
terrigenous fraction of hemipelagic sediments deposited from 500 000 to 100 000 years ago in the
Northwestern Atlantic off New Jersey and dominantly derived from the erosion of Appalachian
highlands shows increased proportions of chlorite
in glacial isotopic stages, and of kaolinite in
interglacial stages. This is clearly expressed by the
kaolinite/chlorite ratio (Figure 6). Paleoclimatic
reconstructions from clay mineral data are available
for various geological periods, as for instance the
passage since about 40 Ma from a non-glacial world
dominated by chemical weathering (smectite, kaolinite) to a glacial world in which physical weathering was greater (chlorite, illite). The comparison
of climatic curves provided by clay minerals and
other indicators (oxygen isotopes, micro-faunas or -
floras, magnetic susceptibility, etc.) allows a better
understanding of the nature, intensity, and effect of
the different factors characterizing the terrestrial
and marine climate in given regions during given
geological intervals.
High resolution studies show that clay assemblages may express terrestrial climatic variations at a
centennial scale or even less, and that the influence of
Earth’s orbital parameters varies to different extents
according to the latitude. For example, the clay
minerals data of Quaternary North Atlantic deep sea
sediments were submitted to cross-correlation spectral analyses on 5.5–14 m-long cores encompassing
the last 300 000 years. The mineral composition
displays a general 100 000-year cyclic signal (eccentricity) in the whole 451–601N range, a 41 000-year
signal (obliquity) at highest latitudes related to
dominant aeolian supply, and a 23 000-year signal
(precession) at mid-latitudes related to dominant
transport by marine currents (Table 2).
The paleoclimatic expression by clay mineral
successions is direct or indirect, i.e., it either indicates
the climate that actually prevailed at a given period,
or reflects other events depending on climate:
migration of lithospheric plates across successive
climatic zones, varying extension of ice caps controlling the surficial erosion, variations in the marine
circulation regime due to changing latitudinal and
352 CLAY MINERALOGY