92
Sources and Composition of Marine Sediments
control of oxygen abundance in the Earth's atmosphere. Carbon can be deposited as
carbonate or as organic carbon; sulfur as sulfate (gypsum) or as sulfide (pyrite); iron
as iron oxide or as iron sulfide. In each case, the second phase - which is favored by
anaerobism in the ocean - liberates oxygen to the system. Thus, an increase in
anaerobism leads to a decrease in oxygen consumption and to an increase in oxygen
supply. This negative feedback mechanism helps stabilize the oxygen content of
atmosphere and ocean.
An iron-mineral which has been much studied and discussed is glauconite. It is a
greenish silicate common in shallow marine areas. Chemically, it is a poorly crystallized mica, rich in potassium (7 %-8 %) and in iron (20 %-25 %). Geologists tend to
denote as "glauconite" any small green earthy pellet recovered from the sand fraction
of marine sediment. These pellets are commonly shaped like the interior of foraminifera or like fecal material, indicating their locus of growth. Apparently the association with decaying organic matter (fecal pellets, interior of shells) is a necessary
condition of growth: part of the iron in glauconite is reduced iron. A high concentration of iron in interstitial waters (at conditions intermediate between reduction of iron
oxide and precipitation of sulfide) appears to be favorable for glauconite formation,
as is the presence of the right kind of clay for conversion to glauconite mica. Glauconite is commonly found in association with phosphatic sediments, in high productivity regions along continental margins, as, for example, off Angola.
From fossil marine sediments we know types of marine iron deposits which are
not found today, such as the iron oolites abundant in the Jurassic formations of
England, eastem France, and southern Germany. These "minette" iron ores of Europe
have been mined for more than a century. The fact that their origin is still a mystery
once again illustrates how little we know about the chemistry of ancient seas.
3.9 Sedimentation Rates
Consider a fossil reef section exposed in a mountain valley, or a sequence of layers of
limestones or shales, telling about a certain period of Earth's history.
How long did it take to build that reef? How much time is recorded in those
layers?
The idea of geologic time, which is so fundamental to geology, is quite young.
Essentially, it starts with James Hutton (1726-1797), and its chief protagonists were
Charles Lyell (1797-1875) and Charles Darwin (1809-1882).
However, before radioactivity was discovered (in 1896, by Marie Curie) and applied to the geologic record, there was no way of telling just how much geologic time
might differ from the chronology which scholars had derived from the account in
Genesis. Nevertheless, some early guesses proved remarkably close T.M. Reade, in
1893, by observing present -day rates of denudation and of accumulation, estimated
an age for sediments of a time span over 7000 times greater than that admitted by
Bishop James Ussher's 6000 years (1581-1656). J. G. Goodchild, in 1897, estimated
704 million years, a remarkably lucky guess!
A general overview of sedimentation rates is given in Fig. 3.13.
Sources and Composition of Marine Sediments
control of oxygen abundance in the Earth's atmosphere. Carbon can be deposited as
carbonate or as organic carbon; sulfur as sulfate (gypsum) or as sulfide (pyrite); iron
as iron oxide or as iron sulfide. In each case, the second phase - which is favored by
anaerobism in the ocean - liberates oxygen to the system. Thus, an increase in
anaerobism leads to a decrease in oxygen consumption and to an increase in oxygen
supply. This negative feedback mechanism helps stabilize the oxygen content of
atmosphere and ocean.
An iron-mineral which has been much studied and discussed is glauconite. It is a
greenish silicate common in shallow marine areas. Chemically, it is a poorly crystallized mica, rich in potassium (7 %-8 %) and in iron (20 %-25 %). Geologists tend to
denote as "glauconite" any small green earthy pellet recovered from the sand fraction
of marine sediment. These pellets are commonly shaped like the interior of foraminifera or like fecal material, indicating their locus of growth. Apparently the association with decaying organic matter (fecal pellets, interior of shells) is a necessary
condition of growth: part of the iron in glauconite is reduced iron. A high concentration of iron in interstitial waters (at conditions intermediate between reduction of iron
oxide and precipitation of sulfide) appears to be favorable for glauconite formation,
as is the presence of the right kind of clay for conversion to glauconite mica. Glauconite is commonly found in association with phosphatic sediments, in high productivity regions along continental margins, as, for example, off Angola.
From fossil marine sediments we know types of marine iron deposits which are
not found today, such as the iron oolites abundant in the Jurassic formations of
England, eastem France, and southern Germany. These "minette" iron ores of Europe
have been mined for more than a century. The fact that their origin is still a mystery
once again illustrates how little we know about the chemistry of ancient seas.
3.9 Sedimentation Rates
Consider a fossil reef section exposed in a mountain valley, or a sequence of layers of
limestones or shales, telling about a certain period of Earth's history.
How long did it take to build that reef? How much time is recorded in those
layers?
The idea of geologic time, which is so fundamental to geology, is quite young.
Essentially, it starts with James Hutton (1726-1797), and its chief protagonists were
Charles Lyell (1797-1875) and Charles Darwin (1809-1882).
However, before radioactivity was discovered (in 1896, by Marie Curie) and applied to the geologic record, there was no way of telling just how much geologic time
might differ from the chronology which scholars had derived from the account in
Genesis. Nevertheless, some early guesses proved remarkably close T.M. Reade, in
1893, by observing present -day rates of denudation and of accumulation, estimated
an age for sediments of a time span over 7000 times greater than that admitted by
Bishop James Ussher's 6000 years (1581-1656). J. G. Goodchild, in 1897, estimated
704 million years, a remarkably lucky guess!
A general overview of sedimentation rates is given in Fig. 3.13.