202
Numerous papers on phosphorites and their genesis were
collected and summarized in several volumes and articles
(e.g. Baturin 1982; Garrison et a1. 1987; Notholt and Jarvis
1990; Föllmi et a1. 1991; Glenn et a1. 1994; Trappe 1994,
1998). These and further publications deal with a variety of
problems, e.g. physicochemical considerations for the precipitation of phosphorites, their contents of trace elements,
and their radiometric dating. Other topics are the burial
rates, budget, and recycling of phosphorus in the ocean
(e.g. Föllmi 1995; Filippelli 1997; Schuffert et a1. 1998),
paleoceanographic reconstructions for the depositional environment of phosphorites, the role of diagenesis and replacement of carbonate in phosphorite genesis, effects of
repeated reworking and weathering, economic evaluations,
etc. Some of these problems are still not very weil understood. We can touch here only a few general points.
For some time it was believed that francolite can be
precipitated direct1y from nutrient-rich sea water. The
requirements for this process were thought to be a
high phosphorus concentration in the upwelling sea
water, decreasing partial pressure of CO 2 , and increasing temperature and pH. This view is now
mostly rejected, although the formation of
phosphoritic hardgrounds is not clear. Most workers
now assurne that phosphorus is released from organic
matter and skeletal parts within the uppermost layer
of organic-rich sediments and that phosphate is precipitated during early diagenesis.
Buried organic matter contains considerable amounts of
phosphorus (often around 1%), which is released by microbial decomposition. In addition, the phosphorus in bones
(e.g., fish), fecal pellets, and coprolites may be dissolved
and thus increase the phosphorus concentration in interstitial waters to values much higher than in sea water. Some
workers authors have pointed out the signifieanee of mierobial mats for the uptake of phosphorus and the precipitation of phosphate (O'Brien et a1. 1981; Lueas andPrevot
1984; Martin-Algarra and Sanchez-Navas 1995; Wilby et
a1. 1996), whieh can also take place in lagoonal environments (Soudry and Lewy 1988).
Phosphorus is also provided by the suspended load of
rivers, partieularly those draining regions with lateritic
soils (Glenn and Arthur 1990). In this case, phosphorus is
predominantly adsorbed to iron hydroxides and released
later, when the river load has been deposited in a shallow
sea.
The uppermost sediment zone with phosphorus release
appears to be characterized by a weakly reducing, mildly
anaerobic environment (Fig. 5.7b). Henee, nitrate and some
ferric iron can be redueed, thus also providing ferroan iron
for the generation of glauconite. Sulfate reduction and the
preeipitation of iron sulfide commonly do not yet take
place at this shallow burial depth. These processes require
strongly redueing eonditions, which ean affeet the uppermost sediment only in a very weak eurrent regime. In this
case, pyrite may be precipitated simultaneously with phosphate.
Reprecipitation of phosphorus takes place within the
sediment or directly below the sediment-water interface (Fig. 5.7b). Due to the concentration gradient
Chapter 5 Oceanic Sediments
between the pore water and sea water, phosphorus
tends to migrate upward by diffusion, and fluoride
from sea water (with its higher concentration) downward into the sediment. Poorly soluble carbonate
fluorapatite will precipitate preferentially where
(1) Phosphate nuclei are already available (e.g., m
fecal pellets or coprolites).
(2) Calcium concentration in the pore water is also
high, and/or
(3) Calcium carbonate is present.
The phosphate forms ooid-like pellets, nodules, and
hardgrounds or crusts. Coatings on other materials
are also common, particularly on the surface of carbonates and calcareous shells, which can be more or
less replaced by phosphate in the course of time. Besides the upwelling zone off Baja Califomia mentioned below, modem phosphogenesis takes place
beneath coastal upwelling off -Namibia, Peru, and
some other continental margins, mainly on the western sides of continents. Under these conditions, most
of the present-day global P burial is accomplished,
but upwelling water is not the only mechanism for
phosphogenesis.
Secondary Enrichment of Phosphorite
Processes. The phosphorus content in the primary
organic matter of marine sediments, including those
beneath zones of high fertility, is limited. A sediment
layer of a certain thickness can produce only a small
quantity of phosphate. Such a situation can be observed in some present-day and ancient sediments
which were deposited more or less continuously in
zones of upwelling. They contain thin layers of small
phosphate grains or phosphatized lenses and laminae
consisting of fecal pellets, microbial filaments, or
foraminifera.
These observations were made in the Miocene Monterey
Fonnation of California interpreted as a result of eoastal
upwelling (Garrison et a1. 1987; Föllmi et a1. 1991). In
modern, organie-rieh mud on the upper continental slope
off Baja California, Mexico, aseries of thin (1-3 em), partially indurated phosphorite layers has been found
(Sehuffert et a1. 1998). These hardgrounds formed in a
poorly oxygenated benthie environment during the last 15
ka.
Significant concentrations of phosphorite can be produced only in an environment alternating between
- Sediment accumulation and in situ phosphogenesis, and
- Winnowing and reworking (Fig. 5.8b and c, see
also Baturin 1982).
Numerous papers on phosphorites and their genesis were
collected and summarized in several volumes and articles
(e.g. Baturin 1982; Garrison et a1. 1987; Notholt and Jarvis
1990; Föllmi et a1. 1991; Glenn et a1. 1994; Trappe 1994,
1998). These and further publications deal with a variety of
problems, e.g. physicochemical considerations for the precipitation of phosphorites, their contents of trace elements,
and their radiometric dating. Other topics are the burial
rates, budget, and recycling of phosphorus in the ocean
(e.g. Föllmi 1995; Filippelli 1997; Schuffert et a1. 1998),
paleoceanographic reconstructions for the depositional environment of phosphorites, the role of diagenesis and replacement of carbonate in phosphorite genesis, effects of
repeated reworking and weathering, economic evaluations,
etc. Some of these problems are still not very weil understood. We can touch here only a few general points.
For some time it was believed that francolite can be
precipitated direct1y from nutrient-rich sea water. The
requirements for this process were thought to be a
high phosphorus concentration in the upwelling sea
water, decreasing partial pressure of CO 2 , and increasing temperature and pH. This view is now
mostly rejected, although the formation of
phosphoritic hardgrounds is not clear. Most workers
now assurne that phosphorus is released from organic
matter and skeletal parts within the uppermost layer
of organic-rich sediments and that phosphate is precipitated during early diagenesis.
Buried organic matter contains considerable amounts of
phosphorus (often around 1%), which is released by microbial decomposition. In addition, the phosphorus in bones
(e.g., fish), fecal pellets, and coprolites may be dissolved
and thus increase the phosphorus concentration in interstitial waters to values much higher than in sea water. Some
workers authors have pointed out the signifieanee of mierobial mats for the uptake of phosphorus and the precipitation of phosphate (O'Brien et a1. 1981; Lueas andPrevot
1984; Martin-Algarra and Sanchez-Navas 1995; Wilby et
a1. 1996), whieh can also take place in lagoonal environments (Soudry and Lewy 1988).
Phosphorus is also provided by the suspended load of
rivers, partieularly those draining regions with lateritic
soils (Glenn and Arthur 1990). In this case, phosphorus is
predominantly adsorbed to iron hydroxides and released
later, when the river load has been deposited in a shallow
sea.
The uppermost sediment zone with phosphorus release
appears to be characterized by a weakly reducing, mildly
anaerobic environment (Fig. 5.7b). Henee, nitrate and some
ferric iron can be redueed, thus also providing ferroan iron
for the generation of glauconite. Sulfate reduction and the
preeipitation of iron sulfide commonly do not yet take
place at this shallow burial depth. These processes require
strongly redueing eonditions, which ean affeet the uppermost sediment only in a very weak eurrent regime. In this
case, pyrite may be precipitated simultaneously with phosphate.
Reprecipitation of phosphorus takes place within the
sediment or directly below the sediment-water interface (Fig. 5.7b). Due to the concentration gradient
Chapter 5 Oceanic Sediments
between the pore water and sea water, phosphorus
tends to migrate upward by diffusion, and fluoride
from sea water (with its higher concentration) downward into the sediment. Poorly soluble carbonate
fluorapatite will precipitate preferentially where
(1) Phosphate nuclei are already available (e.g., m
fecal pellets or coprolites).
(2) Calcium concentration in the pore water is also
high, and/or
(3) Calcium carbonate is present.
The phosphate forms ooid-like pellets, nodules, and
hardgrounds or crusts. Coatings on other materials
are also common, particularly on the surface of carbonates and calcareous shells, which can be more or
less replaced by phosphate in the course of time. Besides the upwelling zone off Baja Califomia mentioned below, modem phosphogenesis takes place
beneath coastal upwelling off -Namibia, Peru, and
some other continental margins, mainly on the western sides of continents. Under these conditions, most
of the present-day global P burial is accomplished,
but upwelling water is not the only mechanism for
phosphogenesis.
Secondary Enrichment of Phosphorite
Processes. The phosphorus content in the primary
organic matter of marine sediments, including those
beneath zones of high fertility, is limited. A sediment
layer of a certain thickness can produce only a small
quantity of phosphate. Such a situation can be observed in some present-day and ancient sediments
which were deposited more or less continuously in
zones of upwelling. They contain thin layers of small
phosphate grains or phosphatized lenses and laminae
consisting of fecal pellets, microbial filaments, or
foraminifera.
These observations were made in the Miocene Monterey
Fonnation of California interpreted as a result of eoastal
upwelling (Garrison et a1. 1987; Föllmi et a1. 1991). In
modern, organie-rieh mud on the upper continental slope
off Baja California, Mexico, aseries of thin (1-3 em), partially indurated phosphorite layers has been found
(Sehuffert et a1. 1998). These hardgrounds formed in a
poorly oxygenated benthie environment during the last 15
ka.
Significant concentrations of phosphorite can be produced only in an environment alternating between
- Sediment accumulation and in situ phosphogenesis, and
- Winnowing and reworking (Fig. 5.8b and c, see
also Baturin 1982).