221
hypothetical calculation showing the important
role of iron oxyhydroxides for the P-cycle is
given in Section 7.4.3.3. By studying the single
P-reservoirs in marine sediments, Ruttenberg
(1993) calculated the total burial flux of reactive/
bio-available P in the range of 8 – 18.5⋅10
10
mol yr
-1
,
which results in a reassessment of the average
residence time of oceanic phos-phate as 1638 kyr.
Finally, it is obvious that the processes described above have a strong impact on the sedimentary ratio between organic carbon and phosphorus (either as organic P also or as “reactive” P,
which means the sum of authigenic, oxide-associated and organic P). While the initial organic
matter can be approximated with a ratio close to
Redfield (106:1), analysis on CFA gave values as
low as 4:1 (Ruttenberg 1993). Therefore, several
studies have discussed variations in the different
types of C/P ratios as indicating temporal changes
in the preservation efficiency of deposited organic
matter (e.g. Ingall and Van Cappellen 1990; cf.
Section 12.3.3), in the intensity of phosphorus
regeneration relative to carbon (e.g. Slomp et al.
2004), or as a generally useful approach to
describe the geochemical behavior of sedimentary
P (Anderson et al. 2001).
6.3.2.2 Authigenic Formation of Phosphorites
There is no consistent definition for the use of the
term phosphorites. Suggestions reach from a limiting P content of 6 wt.% (van Cappellen and
Berner 1988) to a threshold value of 18 wt.% P 2 O 5 ,
as representative for authigenic and biogenic
phosphate minerals (Jarvis et al. 1994). However,
the formation of secondary P phases from initially
more labile-P in marine sediments (see above) is
one major sink for phosphorus on Earth, because
this general process results in sequestration of P
from the nutrient cycle in the water column (e.g.
Compton et al. 2000).
Research on the benthic phosphorus cycle and
phosphorites formation in particular was intensified especially in the seventies to early nineties
of the last century (e.g. Burnet 1977; Burnett and
Froelich 1988; Burnett and Riggs 1990; Nolton and
Jarvis 1990; Föllmi 1996; Glenn et al. 2000). All
present result clearly substantiate that the process of phosphogenesis (= the authigenic formation of carbonate fluorapatite - CFA) is highly
complex and not completely understood. However,
hydroxy-apatite may be the primary authigenic Pmineral phase. This initial apatite is forming intermediately under reducing conditions within the
uppermost few centimeters of sediment. Following
a recent study, the metabolic cycle of large sulfur
bacteria may play a key role for this process
(Schulz and Schulz 2005; cf. next Section). Subsequent dissolution-re-precipitation processes are
thought to be responsible for the transfer into the
more stable and complex carbonate-fluoride variety francolite, which is most common in phosphaterich sediments and rocks (e.g. McClellan 1980;
Kolodny and Luz 1992). The chemical composition
of francolite is very complex and variable (Jarvis
et al. 1994). Based on radiocarbon dating of
phosphatic pellets from the Peru shelf, their formation can be very quick, on time scales of only a
few years (Burnett and Froelich 1988). To investigate the different stages of phosphogenesis,
many interdisciplinary studies have been performed, which include the sequential extractions
mentioned before, the formation kinetics of special
apatite crystals (e.g. Van Cappellen and Berner
1991), or the stable isotopic composition of
sedimentary apatite reflecting the variability of
environmental conditions (e.g. summarized in
Kolodny and Luz 1992). Nevertheless, control
factors of phosphogenesis in different environments are still under debate, particularly with
regard to the overall benthic C-cycle.
Because CFA is forming at the expense of
organic P, high productive areas or at least
organic-rich sediments favor phosphogenesis.
Therefore, it is not surprising that sites of
present-day phosphorite formation are found
along continental margins where the organic
detrital input is sufficient for intense microbial
activity and suboxic to anoxic conditions close to
the sediment surface. This is particularly the case
in regions of intense coastal upwelling and below
permanent oxygen minimum zones (Fig. 6.13).
6.3.2.3 Release of Phosphate by Bacterial
Activity
In contrast to the different sulfur and nitrogen
species, phosphorus in marine environments
occurs almost exclusively in the oxidation state +5.
Thus, with few exceptions (Schink and Friedrich
2000), bacteria cannot gain energy by the oxidation of reduced phosphorus species and cannot
use phosphate as an electron acceptor. Nevertheless, phosphorylation, the addition of a phosphate group to another compound, plays a major
6.3
The Role of Oxygen, Nitrate and Phosphorus in Marine Sediments
hypothetical calculation showing the important
role of iron oxyhydroxides for the P-cycle is
given in Section 7.4.3.3. By studying the single
P-reservoirs in marine sediments, Ruttenberg
(1993) calculated the total burial flux of reactive/
bio-available P in the range of 8 – 18.5⋅10
10
mol yr
-1
,
which results in a reassessment of the average
residence time of oceanic phos-phate as 1638 kyr.
Finally, it is obvious that the processes described above have a strong impact on the sedimentary ratio between organic carbon and phosphorus (either as organic P also or as “reactive” P,
which means the sum of authigenic, oxide-associated and organic P). While the initial organic
matter can be approximated with a ratio close to
Redfield (106:1), analysis on CFA gave values as
low as 4:1 (Ruttenberg 1993). Therefore, several
studies have discussed variations in the different
types of C/P ratios as indicating temporal changes
in the preservation efficiency of deposited organic
matter (e.g. Ingall and Van Cappellen 1990; cf.
Section 12.3.3), in the intensity of phosphorus
regeneration relative to carbon (e.g. Slomp et al.
2004), or as a generally useful approach to
describe the geochemical behavior of sedimentary
P (Anderson et al. 2001).
6.3.2.2 Authigenic Formation of Phosphorites
There is no consistent definition for the use of the
term phosphorites. Suggestions reach from a limiting P content of 6 wt.% (van Cappellen and
Berner 1988) to a threshold value of 18 wt.% P 2 O 5 ,
as representative for authigenic and biogenic
phosphate minerals (Jarvis et al. 1994). However,
the formation of secondary P phases from initially
more labile-P in marine sediments (see above) is
one major sink for phosphorus on Earth, because
this general process results in sequestration of P
from the nutrient cycle in the water column (e.g.
Compton et al. 2000).
Research on the benthic phosphorus cycle and
phosphorites formation in particular was intensified especially in the seventies to early nineties
of the last century (e.g. Burnet 1977; Burnett and
Froelich 1988; Burnett and Riggs 1990; Nolton and
Jarvis 1990; Föllmi 1996; Glenn et al. 2000). All
present result clearly substantiate that the process of phosphogenesis (= the authigenic formation of carbonate fluorapatite - CFA) is highly
complex and not completely understood. However,
hydroxy-apatite may be the primary authigenic Pmineral phase. This initial apatite is forming intermediately under reducing conditions within the
uppermost few centimeters of sediment. Following
a recent study, the metabolic cycle of large sulfur
bacteria may play a key role for this process
(Schulz and Schulz 2005; cf. next Section). Subsequent dissolution-re-precipitation processes are
thought to be responsible for the transfer into the
more stable and complex carbonate-fluoride variety francolite, which is most common in phosphaterich sediments and rocks (e.g. McClellan 1980;
Kolodny and Luz 1992). The chemical composition
of francolite is very complex and variable (Jarvis
et al. 1994). Based on radiocarbon dating of
phosphatic pellets from the Peru shelf, their formation can be very quick, on time scales of only a
few years (Burnett and Froelich 1988). To investigate the different stages of phosphogenesis,
many interdisciplinary studies have been performed, which include the sequential extractions
mentioned before, the formation kinetics of special
apatite crystals (e.g. Van Cappellen and Berner
1991), or the stable isotopic composition of
sedimentary apatite reflecting the variability of
environmental conditions (e.g. summarized in
Kolodny and Luz 1992). Nevertheless, control
factors of phosphogenesis in different environments are still under debate, particularly with
regard to the overall benthic C-cycle.
Because CFA is forming at the expense of
organic P, high productive areas or at least
organic-rich sediments favor phosphogenesis.
Therefore, it is not surprising that sites of
present-day phosphorite formation are found
along continental margins where the organic
detrital input is sufficient for intense microbial
activity and suboxic to anoxic conditions close to
the sediment surface. This is particularly the case
in regions of intense coastal upwelling and below
permanent oxygen minimum zones (Fig. 6.13).
6.3.2.3 Release of Phosphate by Bacterial
Activity
In contrast to the different sulfur and nitrogen
species, phosphorus in marine environments
occurs almost exclusively in the oxidation state +5.
Thus, with few exceptions (Schink and Friedrich
2000), bacteria cannot gain energy by the oxidation of reduced phosphorus species and cannot
use phosphate as an electron acceptor. Nevertheless, phosphorylation, the addition of a phosphate group to another compound, plays a major
6.3
The Role of Oxygen, Nitrate and Phosphorus in Marine Sediments