scispace - formally typeset

Journal ArticleDOI

Phosphogenesis and active phosphorite formation in sediments from the Arabian Sea oxygen minimum zone

15 Sep 2000-Marine Geology (Elsevier)-Vol. 169, Iss: 1, pp 1-20

AbstractIn this study, porewater chemistry, solid-phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the Arabian Sea. Three parameters, namely a decrease of the dissolved phosphate and fluoride concentrations with depth, saturation with respect to carbonate fluorapatite, and the presence of a solid-phase Ca-phosphate mineral, all indicate that phosphogenesis is currently taking place at all three sites. Authigenic apatite precipitation rates vary between 0.076 and 1.04 μmolP cm−2 yr−1, and are of the same order of magnitude as reported for other high productivity areas. Precipitation of an intermediate precursor precedes francolite formation in the continental slope sediments on the Karachi Margin. Results of a diagenetic P model indicate that phosphogenesis is induced by high rates of organic matter degradation. Dissolution of fish debris is likely to provide a substantial additional source of phosphate. Redox iron cycling does not influence phosphogenesis in these environments. Model results suggest that sediment mixing is essential in promoting early diagenetic phosphogenesis. The highest rate of francolite formation was observed in a boxcore taken on the Oman Margin, where it contributes to the formation of a Holocene phosphorite deposit. This observation contrasts with previous reports of only old phosphorites in this area. Phosphorites are presently forming on the Oman Margin as a result of: (a) deposition of older, reworked material from the continental shelf, which has undergone an earlier phase of phosphogenesis; (b) a high input of reactive P (fish debris and degradable organic matter); (c) a relatively low sediment accumulation rate; and (d) the absence of winnowing on this location. Holocene phosphorite deposits may be less common on the Oman Margin than in other coastal upwelling areas because of the narrowness of the shelf and the steepness of the slope, which limit the area where phosphorite formation may occur.

Topics: Francolite (56%), Phosphorite (54%), Continental shelf (53%), Oxygen minimum zone (52%), Authigenic (52%)

Summary (4 min read)

1. Introduction

  • Phosphogenesis is the early diagenetic precipitation of francolite, a carbonate fluorapatite mineral (CFA).
  • The authors investigate phosphogenesis in three boxcores from the Arabian Sea located within oxygen-depleted bottom waters, one recovered from the sediments underlying the Oman upwelling system, and two from the Pakistan Margin.
  • In contrast to previous reports, the authors present evidence for Holocene phosphorite formation on the Oman Margin.

2.1. Sediment sampling and core description

  • All three boxcores are located within the OMZ and underlie an area of high primary productivity.
  • Bottom water oxygen (BWO) concentrations were obtained from nearby conductivity temperature depth (CTD) stations.
  • 14C accelerator mass spectrometry (AMS) dating was performed on handpicked non-coated foraminifers (Globorotalia menardii), coated foraminifers and phosphorite pellets.

2.2. Porewater analysis

  • Porewater extractions were started on board within 24 h of core collection according to shipboard routine (De Lange, 1992a).
  • The boxcores were vertically sluiced into a glovebox, which was kept under lowoxygen conditions (O2 , 0.0005%) and at in situ bottom water temperature.
  • Alkalinity was calculated after titration using the Gran plot method (Gieskes, 1973).
  • Porewater fluoride concentrations were measured with an ionspecific electrode.

2.3. Solid-phase analysis

  • The porosity and dry bulk density (DBD) were calculated from the weight loss after drying at 608C, assuming a sediment density of 2.65 g cm23.
  • Relative errors for duplicate measurement were better than 3%, except for Zr and Ti (5%).
  • A separate sequential extraction consisting of eight times the 2 M NH4Cl step was performed for some sediment samples, where each extracted solution was analysed separately.
  • All extracted solutions were measured for P with ICP-AES.

2.4. Description of the model

  • A diagenetic model for P cycling developed by Slomp et al. (1996) was applied to the porewater and sequential extraction results for BC451 and BC455.
  • This steady state model describes the concentration change with depth of porewater phosphate and three forms of solid-phase P, namely organic P, Febound P and authigenic P.
  • The processes (1)–(4) are described as first-order reactions, with reaction rate constants kg, ks, km and ka, respectively.
  • Values ofkg, km, ka, JAx 0 and JGx 0 were varied to fit the model to the experimental data.
  • Extra weight was assigned to the data points in the upper part of each profile and the whole authigenic P profile.

3.1. Porewater

  • In BC451 and BC455, the phosphate concentrations are lower, and the decrease of phosphate with depth is less pronounced (BC451) or absent (BC455).
  • Fluoride concentrations decrease with depth in all three boxcores, whereas the ammonium concentrations and alkalinity increase almost linearly with depth (Fig. 2).
  • BWO concentrations for all three boxcores are below the detection limit (2 mM; Table 1).

3.2. Solid phase

  • The P concentration in this core gradually decreases with depth to ca. 4000 ppm.
  • Approximately 20% of the solid-phase P is present in the fraction smaller than 65mm, which constitutes 50 wt% of the bulk sediment.
  • In BC451 and BC455, the P fraction responsible for the increase with depth of total solid-phase P was extracted during steps 1 and 2 (Fig. 3).
  • The Corg/Ntot weight ratio in TC484 increases with depth till 18 cm, where it reaches a constant value of 10 (Fig. 4).
  • The fish debris concentration (numbers per gram of the 150–595mM sieve fraction) decreases with depth in BC484 (Fig. 5), and correlates reasonably well with the total P concentration.

3.3. Microscopic observations and calibrated14C ages of apatite macro particles in BC484

  • Microscopic observations and microprobe analysis allowed the identification of three types of apatite macro particles in BC484: coated foraminifers, phosphatised pellets and fish debris.
  • In the deeper part of the boxcore, coated foraminifers become less frequent.
  • Calibrated14C ages for coated foraminifers are higher than “clean” foraminifers in the same sediment interval (Table 3).
  • Their surface is blackish/brownish and usually smooth.
  • Thin slides of samples reveal no internal structures indicating that the pellets are probably composed of apatite micro crystals.

3.4. Application of the model

  • The porewater equilibrium concentration for francolite formation (Ca) may vary between 0.4 and 11mM for pH 4 (Atlas and Pytkowicz, 1977).
  • Here, a Ca concentration of 10mM was used for both cores, which equals the porewater phosphate concentration at greater sediment depth (Schenau, 1999).
  • The deposition rate of Fe-bound P (JMx 0) was estimated from the mass accumulation rate, an average reactive iron concentration of 6000 ppm (equal to the concentration in surface sediments below the OMZ) and an atomic Fe/P ratio of 20 for the newly deposited reducible iron particles (Schenau, 1999).
  • The model fits agree reasonably well with the measured data (Fig. 6), with the exception of the porewater profiles of BC451 and BC455.

4.1. Authigenic apatite formation

  • Three indicators have been studied to examine whether phosphogenesis is currently taking place in the sediments located within the OMZ of the Arabian Sea (Ruttenberg and Berner, 1993): (1) porewater phosphate and fluoride concentrations; (2) the saturation state of francolite; and (3) solid-phase authigenic P concentrations.
  • A decrease in porewater phosphate and fluoride concentration with depth is indicative for P and F removal to the solid phase (Jahnke et al., 1983; Ruttenberg and Berner, 1993).
  • The number of fish debris, as counted in the 150–595mm sieve fraction, however, does not clearly increase with depth (Fig. 5).
  • Therefore, the authors argue that the increase of the PNH4Cl fraction with depth in BC451 and BC455 is the result of precipitation of an authigenic Ca-phosphate mineral, which is more soluble than francolite.
  • Laboratory experiments have shown that francolite precipitation at high phosphate concentrations is a two-step process.

4.2. Implications of the model

  • The model results confirm that the increase of the solid-phase P with depth at stations BC451 and BC455 can be explained by early diagenetic phosphogenesis.
  • The calculated francolite formation rates correspond reasonably well with the downwardJP(F) fluxes (Table 6).
  • As a result, the model predicts that all degradable organic P is mineralised in the upper few centimetres of the sediment (Fig. 6).
  • This may also explain the discrepancy between the observed and the modelled phosphate porewater profiles.
  • Early diagenetic iron redox cycling has been shown to be important for phosphogenetic processes in certain marine environments (e.g.

4.3. Phosphorite formation in BC484

  • Many of these phosphorite deposits have been identified as lag deposits (Kolodny, 1981; Garrison and Kastner, 1990).
  • Beside redeposition processes, winnowing has been suggested to play an important role in the formation of phosphorites (e.g. Glenn and Arthur, 1988; Glenn et al., 1994).
  • Since phosphorite particles have a higher specific gravity than the surrounding detrital particles, bottom currents could wash away the finer, lighter particles and thus concentrate P in the top of the sediment.
  • Winnowing causes low sedimentation rates, which in turn may enhance the growth of phosphorite nodules by keeping them in the zone of active diagenesis.

4.3.1. Winnowing

  • The high phosphorus concentration in the top 20 cm of BC484 could be the result of winnowing, which is an important process affecting the continental slope sediments on the Oman Margin (Shimmield et al., 1990; Pedersen et al., 1992).
  • A comparison between BC484 and TC484 of the Corg and Ptot profiles revealed an offset of 10 cm.
  • These low Corg/Ntot ratios are common for many recent sediments that have not been subject to winnowing (e.g. Calvert et al., 1995; Van der Weijden et al., 1999).
  • O’Brien et al. (1990) found a close correlation between bottom water current velocity and the CaCO3 content of sediments on the East Australian continental margin.
  • The Ca concentration in the phosphorus-rich layer is in fact lower than deeper in the sediment, indicating that the top is not winnowed.

4.3.2. Redeposition

  • The continental slope of the Oman Margin is particularly steep (Prell and shipboard party of ODP Leg 117, 1990) and, therefore, redeposition processes are likely to occur.
  • The calibrated14C ages for “clean” (i.e. un-coated) foraminifers indicate normal sedimentation for the last 13,000 yr (Table 3).
  • The authors argue that the phosphatised material from in BC484 originates from two different sources.
  • The downward flux of fluoride may thus account for the high solid-phase P content in the top of BC484.
  • As a consequence, recent phosphorite formation on the Oman Margin may have remained unobserved thus far.

5. Conclusions

  • Porewater and solid-phase P speciation results indicate that phosphogenesis is occurring in the surface sediments located within the OMZ of the Arabian Sea.
  • The precipitation of a precursor precedes francolite formation in the sediments on the Karachi Margin.
  • Early diagenetic iron cycling does not significantly affect sedimentary P cycling in these environments.
  • This implies that dysoxic rather than fully anoxic bottom waters may be more effective in promoting early diagenetic phosphogenesis.

Did you find this useful? Give us your feedback

...read more

Content maybe subject to copyright    Report

Phosphogenesis and active phosphorite formation in sediments
from the Arabian Sea oxygen minimum zone
S.J. Schenau
*
, C.P. Slomp, G.J. De Lange
Department of Geochemistry, Institute of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands
Received 2 July 1999; accepted 22 June 2000
Abstract
In this study, porewater chemistry, solid-phase analysis and microscopic observations were combined to evaluate phospho-
genesis in three boxcores located within the intensive oxygen minimum zone of the Arabian Sea. Three parameters, namely a
decrease of the dissolved phosphate and fluoride concentrations with depth, saturation with respect to carbonate fluorapatite,
and the presence of a solid-phase Ca-phosphate mineral, all indicate that phosphogenesis is currently taking place at all three
sites. Authigenic apatite precipitation rates vary between 0.076 and 1.04 mmolP cm
2
yr
1
, and are of the same order of
magnitude as reported for other high productivity areas. Precipitation of an intermediate precursor precedes francolite forma-
tion in the continental slope sediments on the Karachi Margin. Results of a diagenetic P model indicate that phosphogenesis is
induced by high rates of organic matter degradation. Dissolution offish debris is likely to provide a substantial additional source
of phosphate. Redox iron cycling does not influence phosphogenesis in these environments. Model results suggest that sediment
mixing is essential in promoting early diagenetic phosphogenesis. The highest rate of francolite formation was observed in a
boxcore taken on the Oman Margin, where it contributes to the formation of a Holocene phosphorite deposit. This observation
contrasts with previous reports of only old phosphorites in this area. Phosphorites are presently forming on the Oman Margin as
a result of: (a) deposition of older, reworked material from the continental shelf, which has undergone an earlier phase of
phosphogenesis; (b) a high input of reactive P (fish debris and degradable organic matter); (c) a relatively low sediment
accumulation rate; and (d) the absence of winnowing on this location. Holocene phosphorite deposits may be less common
on the Oman Margin than in other coastal upwelling areas because of the narrowness of the shelf and the steepness of the slope,
which limit the area where phosphorite formation may occur. 2000 Elsevier Science B.V. All rights reserved.
Keywords: Phosphogenesis; Phosphorites; Oxygen minimum zone; Arabian Sea; Early diagenesis
1. Introduction
Phosphogenesis is the early diagenetic precipitation
of francolite, a carbonate fluorapatite mineral (CFA).
Authigenic apatite formation is an important sedimen-
tary sink for reactive phosphorus in the oceans
(Ruttenberg and Berner, 1993). The major element
composition of marine sedimentary apatite displays
little variation (e.g. Jarvis et al., 1994), and
approaches the simplified formula Ca
10
[(PO
4
)
6x
(CO
3
)
x
]F
2x
. In organic-rich sediments, francolite
precipitation is usually restricted to the uppermost
part of the sediment because the increase of carbonate
alkalinity with depth prohibits further formation of
apatite (Jahnke et al., 1983; Glenn and Arthur,
1988), and because francolite formation requires
fluoride diffusing from the overlying bottom water
(Froelich et al., 1983).
Marine Geology 169 (2000) 120
0025-3227/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.
PII: S0025-3227(00)00083-9
www.elsevier.nl/locate/margeo
* Corresponding author.
E-mail addresses: sjoerds@geo.uu.nl (S.J. Schenau), slomp@
geo.uu.nl (C.P. Slomp), gdelange@geo.uu.nl (G.J. De Lange).

For the evaluation of P cycling in the oceans, it is
important to understand the environmental conditions
influencing phosphogenesis. Saturation of the intersti-
tial water with respect to francolite is primarily
controlled by the flux of reactive phosphorus trans-
ferred to the sediment (e.g. Filippelli and Delaney,
1994). Phosphate production in the porewater may
originate from microbial degradation of organic
matter (e.g. Froelich et al., 1988; Ruttenberg and
Berner, 1993), desorption from iron oxides (e.g.
Sundby et al., 1992; Slomp et al., 1998) and dissolu-
tion of fish debris (Suess, 1981). A second prerequisite
for francolite precipitation is that sedimentary condi-
tions have to permit the build up of phosphate in the
porewater. High porosities enhance phosphate diffu-
sion to the bottom water and reduce the capacity to
retain reactive P in the sediment (Van Cappellen and
Berner, 1988; Filippelli and Delaney, 1994). Early
diagenetic iron cycling has been shown to promote
phosphogenesis under oxygenated bottom water
conditions (Sundby et al., 1992; Slomp et al., 1996).
In addition, redox dependent cycling of phosphate by
microorganisms (e.g. Ingall and Jahnke, 1994) and
microbial mat communities at the sediment water
interface (e.g. Williams and Reimers, 1983;
Krajewski et al., 1994) may play an important role
in regulating the interstitial phosphate concentrations.
Benthic P regeneration and consequent loss of phos-
phate to the water column appears to be more exten-
sive under oxygen-depleted bottom water conditions
(Ingall and Jahnke, 1994). As a consequence, phos-
phogenesis is often associated to oxic to suboxic
bottom water conditions (Heggie et al., 1990; Ingall
et al., 1993; Jarvis et al., 1994).
Coastal upwelling areas are known to have sedi-
ments with high authigenic phosphorus contents,
defined as phosphorites when containing more than
5 wt% P
2
O
5
(e.g. Cook, 1984). These high sedimen-
tary P concentrations are thought to result from high
francolite precipitation rates during early diagenesis
and processes of sediment reworking (e.g. redeposi-
tion, winnowing; Kolodny, 1981; Froelich et al.,
1988). Phosphorites have been found worldwide in
numerous geological formations (e.g. Filippelli and
Delaney, 1994; Fo
¨
llmi, 1996; Trappe, 1998), but
their recent formation is a relatively rare phenomenon
(Jahnke et al., 1983; Thomson et al., 1984; Froelich et
al., 1988; Heggie et al., 1990; Schuffert et al., 1994,
1998). In addition to upwelling areas, phosphogenesis
has been recognised in sediments of shallow continen-
tal margins (Ruttenberg and Berner, 1993; Reimers et
al., 1996; Louchouarn et al., 1997), the continental
slope underlying low productivity areas (Slomp et
al., 1996) and in deep-sea sediments (Lucotte et al.,
1994; Filippelli and Delaney, 1996).
The Arabian Sea is characterised by high seasonal
productivity (Qasim, 1982), which is caused by
monsoonal induced coastal and open-ocean upwelling
offshore Oman and Somalia. High vertical fluxes of
organic matter, in combination with sluggish ventila-
tion owing to the semi-enclosed configuration of the
Arabian Basin, result in an intensive oxygen mini-
mum zone (OMZ) between 150 and 1250 m water
depth, with oxygen concentrations 2 mM (e.g. Van
Bennekom and Hiehle, 1994). Ancient phosphorite
deposits, dating from the Miocene till Early Pleisto-
cene, have been found in ODP cores recovered from
the Oman Margin (Rao and Lamboy, 1995). Until now,
recent phosphorite formation in this area has never
been reported. This is surprising, as the environmental
conditions for the Oman upwelling area are in many
aspects similar to that of PeruChile and Namibia.
In this study, we investigate phosphogenesis in
three boxcores from the Arabian Sea located within
oxygen-depleted bottom waters, one recovered from
the sediments underlying the Oman upwelling system,
and two from the Pakistan Margin. We apply a diage-
netic model for sedimentary P cycling developed by
Slomp et al. (1996) to porewater and solid-phase P
profiles to gain a better insight into the processes
controlling phosphogenesis in these environments.
In contrast to previous reports, we present evidence
for Holocene phosphorite formation on the Oman
Margin.
2. Material and methods
2.1. Sediment sampling and core description
During the Netherlands Indian Ocean program
(1992) three boxcores were collected for this study:
BC484 was recovered from the Oman Margin from a
depth of 527 m, BC451 and BC455 from the Karachi
Margin, from depths of 495 and 1005 m, respectively
(Table 1; Fig. 1). All three boxcores are located within
S.J. Schenau, C.P. Slomp / Marine Geology 169 (2000) 1202

the OMZ and underlie an area of high primary produc-
tivity. In addition, a tripcore (TC484), taken on nearly
the same location as BC484 (at a distance of ^4 km),
was studied. Bottom water oxygen (BWO) concentra-
tions were obtained from nearby conductivity
temperature depth (CTD) stations.
14
C accelerator
mass spectrometry (AMS) dating was performed on
handpicked non-coated foraminifers (Globorotalia
menardii), coated foraminifers and phosphorite
pellets. AMS
14
C ages were calibrated according to
Stuiver et al. (1998) and corrected for a reservoir age
of 400 yr (Bard et al., 1990). Sedimentation rates,
v
,
(Table 1) were calculated from the ages of non-coated
foraminifers.
2.2. Porewater analysis
Porewater extractions were started on board within
24 h of core collection according to shipboard routine
(De Lange, 1992a). The boxcores were vertically
sluiced into a glovebox, which was kept under low-
oxygen conditions (O
2
0.0005%) and at in situ
bottom water temperature. Under a nitrogen pressure
of up to 7 bar, porewaters were extracted in Reeburgh-
type squeezers. The shipboard pH measurements and
nutrient analyses were performed within 12 h after the
extraction of the porewaters. Alkalinity was calcu-
lated after titration using the Gran plot method
(Gieskes, 1973). Phosphate and ammonium were
measured on a TRAACS 800 auto analyser, according
to automated methods of Strickland and Parsons
(1968) and Solarzano (1969), respectively. All
analyses were performed in duplicate. Porewater
fluoride concentrations were measured with an ion-
specific electrode. Relative errors were smaller than
2%.
S.J. Schenau, C.P. Slomp / Marine Geology 169 (2000) 120 3
Table 1
Position, water depth, sedimentation rate (
v
), organic carbon concentration in the top cm, oxygen concentration and bottom water temperature
of the sampled boxcores
Site Latitude Longitude Water depth
v
C
org
top cm T bottom [O
2
] bottom water
(N) (E) (m) (cm kyr
1
) (wt%) (C)(mM)
BC484 1930.0
0
5825.8
0
527 5.6 2.24 12.3 2
TC484 1929.8
0
5825.7
0
516
BC451 2341.4
0
6602.9
0
495 28 4.37 12.6 2
BC455 2333.0
0
6557.4
0
1005 16 3.43 8.7 2
Fig. 1. Positions of the boxcore sample sites. The area where the present-day OMZ impinges on the continental slope is shaded.

2.3. Solid-phase analysis
The porosity and dry bulk density (DBD) were
calculated from the weight loss after drying at 60C,
assuming a sediment density of 2.65 g cm
3
. After
removal of inorganic carbon with 1 M HCl, the
organic carbon (C
org
) content and total nitrogen
content (N
tot
) were measured with an NA 1500 NCS
analyser. Relative errors were smaller than 0.4%. For
the determination of total P, Ca, Al, Ti and Zr, 250 mg
of the sample was totally digested in 10 ml of a
6.5:2.5:1 mixture of HClO
4
(60%), HNO
3
(65%) and
H
2
O, and 10 ml HF (40%) at 90C. After evaporation
of the solutions at 190C on a sand bath, the dry resi-
due was dissolved in 50 ml 1 M HCl. The resulting
solutions were analysed with an inductively coupled
plasma atomic emission spectrometer (ICP-AES;
PerkinElmer Optima 3000). All results were checked
using international (SO1, SO3) and in-house standards.
Relative errors for duplicate measurement were better
than 3%, except for Zr and Ti (5%). Particle sizes
were determined on the bulk wet sediment and on
carbonate and organic matter free fractions, using a
Laser Particle Sizer (Malvern Series 2600).
The distribution of different phosphorus fractions in
sediment samples was examined with a six-step
sequential extraction scheme, which is an adaptation
of the SEDEX method developed by Ruttenberg
(1992) with additional steps for selective Ca-carbo-
nate extraction (Table 2). Approximately 250 mg of
dried sediment was subsequently washed with (1)
25 ml 2 M NH
4
Cl, pH 7(6× ), (2) 25 ml Na-acetate
solution, pH 6, (3) 25 ml citrate dithionite buffer
(CDB), pH 7.6, (4) 25 ml Na-acetate solution,
pH 4, (5) 25 ml 1 M HCl and (6) 20 ml HF/
HNO
3
/HClO
4
mixture. After extraction steps 25
the sediment was rinsed successively with 2 M
NH
4
Cl (pH 7) and demineralised water to prevent
readsorption of HPO
4
2
. The first extraction step
differs from the SEDEX method in that 2 M NH
4
Cl
is used instead of 1 M MgCl
2
to dissolve carbonates
prior to the other extraction steps (De Lange, 1992b).
This has the advantage that carbonates are dissolved
selectively, allowing a differentiation between franco-
lite and more soluble calcium-phosphate minerals
(Schenau and De Lange, 2000). A separate sequential
extraction consisting of eight times the 2 M NH
4
Cl
step was performed for some sediment samples,
S.J. Schenau, C.P. Slomp / Marine Geology 169 (2000) 1204
Table 2
The P sequential extraction scheme and extracted fractions
Step Extractant P phase extracted Reference
16× 25 ml 2 M NH
4
Cl (pH 7) P
NH4Cl
De Lange, 1992b
1 × 25 ml demin. water Exchangeable or loosely sorbed P
Carbonate associated P
Fish debris
22× 25 ml 1 M Na-acetate, buffered to pH 6 with acetic acid Residual carbonate associated P
1 × 25 ml 2 M NH
4
Cl (pH 7)
1 × 25 ml demin. water
31× 25 ml 0.15 M Na-citrate, 0.5 M NaCO
3
(pH 7.6), and
1.125 g Na-dithionite
P
Pfe
Ruttenberg, 1992
1 × 25 ml 2 M NH
4
Cl (pH 7) Easily reducible or reactive iron bound P
1 × 25 ml demin. water
42× 25 ml 1 M Na-acetate buffered to pH 4 with acetic acid P
cfa
Ruttenberg, 1992
1 × 25 ml 2 M NH
4
Cl (pH 7) Carbonate fluorapatite (CFA)
1 × 25 ml demin. water
51× 25 ml 1 M HCl P
det
Ruttenberg, 1992
1 × 25 ml demin. water Detrital apatite
6 20 ml HF/HNO
3
/HClO
4
P
res
Lord, 1982
P adsorbed to clay minerals
Organic P

where each extracted solution was analysed sepa-
rately. The Na-acetate (pH 6) extraction (step 2)
was added to ensure complete carbonate removal,
since incomplete dissolution of Ca-carbonates might
cause precipitation of gypsum in the subsequent Na-
dithionite extraction (step 3). All extracted solutions
were measured for P with ICP-AES. Reproducibility
was generally better than 5%, except for step 3 (10%).
The recovery with respect to the total P concentration
was 90, 83 and 90% for BC484, BC451 and BC455,
respectively. Fluoride concentrations in the extracted
solutions of BC484 were measured with an ion-speci-
fic electrode. For some samples, organic phosphorus
(P
org
) was determined according to the method of
Aspila et al. (1976).
Separate sediment samples were sieved into three
fractions (65150, 150595 and 595 mm). For
BC484, these fractions were weighed and, after total
digestion, analysed with ICP-AES. For all boxcore
samples fish debris was quantified in the 150
595 mm fraction by counting the number of fish frag-
ments in splits (using an Otto microsplitter). Coated
foraminifers, dark brownish pellets and fish debris in
the top sediment of BC484 were hand picked from the
150595 mm fraction. A few particles were
embedded in resin and analysed for Ca, P and F
contents using an electron microprobe-scanning elec-
tron microscope (JEOL 8600).
2.4. Description of the model
A diagenetic model for P cycling developed by
Slomp et al. (1996) was applied to the porewater
and sequential extraction results for BC451 and
BC455. This steady state model describes the concen-
tration change with depth of porewater phosphate and
three forms of solid-phase P, namely organic P, Fe-
bound P and authigenic P. As the sequential extraction
results for BC484 are not accurate for all P phases,
and some of the material may have been redeposited
(see Sections 3 and 4), the model was not applied to
this boxcore. The sediment column is divided into
three zones: an oxidised surface zone (I: 0 x
L
1
; a reduced sediment zone with bioturbation (II:
L
1
x L
2
; and a reduced sediment zone without
bioturbation (III: x L
2
: The processes included in
the model are: (1) phosphate release from organic P
due to organic matter degradation (zones I, II, III); (2)
reversible sorption of phosphate to iron oxides (zone
I); (3) phosphate release from Fe-bound P due to iron
oxide reduction (zones II, III); and (4) authigenic P
precipitation (zones II, III). Note that phosphate
release from fish debris dissolution is not included
in this model. The processes (1)(4) are described
as first-order reactions, with reaction rate constants
k
g
, k
s
, k
m
and k
a
, respectively. The set of differential
equations for the one-dimensional distribution of
porewater HPO
4
2
and the three particulate P forms
was solved analytically assuming continuity in
concentrations and fluxes at the boundaries between
the three depth zones and appropriate boundary condi-
tions for the system. The porewater equilibrium
concentrations for sorption and apatite precipitation
are C
s
and C
a
, respectively. The bottom water concen-
tration is C
0
. The asymptotic Fe-bound P and organic
P concentrations are equal to M
and G
. The mole-
cular (D
s
) and biodiffusion (D
b
) coefficients, sedimen-
tation rate (
v
), reaction rate constants and sediment
porosity (
f
) are assumed to be constant with depth in
each relevant layer. In addition, the fluxes of organic P
(J
Gx0
), Fe-bound P (J
Mx0
) and “authigenic” P (J
ax0
;
i.e. the P fraction associated with fish debris, Ca-
carbonates, easily exchangeable P and resuspended
authigenic apatite) from the water column to the sedi-
ment are assumed to be constant. Values of k
g
, k
m
, k
a
,
J
Ax0
and J
Gx0
were varied to fit the model to the
experimental data. Variance-weighted sums of
squares of the difference between the modelled and
experimental values were minimised for all four
components, i.e. phosphate, organic P, Fe-bound P
and authigenic P, simultaneously. Extra weight was
assigned to the data points in the upper part of each
profile and the whole authigenic P profile. The other
parameters (L
1
, L
2
, D
s
, D
b
,
v
, C
0
, C
s
, C
a
, k
s
, J
Mx0
,
M
and G
) were fixed on the basis of existing data.
3. Results
3.1. Porewater
The phosphate concentration in BC484 is charac-
terised by a sharp increase to 75 mM just below the
sediment surface, followed by a decrease to 20 mMat
the base of the boxcore (Fig. 2). In BC451 and BC455,
the phosphate concentrations are lower, and the
S.J. Schenau, C.P. Slomp / Marine Geology 169 (2000) 120 5

Figures (13)
Citations
More filters

Journal ArticleDOI
21 Jan 2005-Science
TL;DR: It is shown that apatite abundance in sediments on the Namibian shelf correlates with the abundance and activity of the giant sulfur bacterium Thiomargarita namibiensis, which suggests that sulfur bacteria drive phosphogenesis.
Abstract: Phosphorite deposits in marine sediments are a long-term sink for an essential nutrient, phosphorus. Here we show that apatite abundance in sediments on the Namibian shelf correlates with the abundance and activity of the giant sulfur bacterium Thiomargarita namibiensis, which suggests that sulfur bacteria drive phosphogenesis. Sediments populated by Thiomargarita showed sharp peaks of pore water phosphate ( /=50 grams of phosphorus per kilogram). Laboratory experiments revealed that under anoxic conditions, Thiomargarita released enough phosphate to account for the precipitation of hydroxyapatite observed in the environment.

326 citations


Journal ArticleDOI
01 Apr 2008-Elements
Abstract: The cycling of phosphorus, a biocritical element in short supply in nature, is an important Earth system process. Variations in the phosphorus cycle have occurred in the past. For example, the rapid uplift of the Himalayan-Tibet Plateau increased chemical weathering, which led to enhanced input of phosphorus to the oceans. This drove the late Miocene “biogenic bloom.” Additionally, phosphorus is redistributed on glacial timescales, resulting from the loss of the substantial continental margin sink for reactive P during glacial sea-level lowstands. The modern terrestrial phosphorus cycle is dominated by agriculture and human activity. The natural riverine load of phosphorus has doubled due to increased use of fertilizers, deforestation and soil loss, and sewage sources. This has led to eutrophication of lakes and coastal areas, and will continue to have an impact for several thousand years based on forward modeling of human activities.

312 citations


Book ChapterDOI
01 Jan 2007
Abstract: Publisher Summary This chapter discusses the elemental proxies for palaeoclimatic and palaeoceanographic variability in marine sediments. Physical and biological processes during deposition coupled with post-depositional chemical reactions yield a complex component mixture that can provide significant palaeoceanographic and palaeoclimatic information to complement and strengthen interpretations derived from the study of microfossils and the isotopic compositions of sedimentary components. The chapter reviews the application of sedimentary geochemistry to the reconstruction of climatic and oceanographic changes over the Cenozoic, with emphasis on the Late Pleistocene. Records from both pelagic regimes and ocean margin provinces are used to show how information on both ocean conditions and terrestrial climates can be assembled from the major, minor and trace element composition of sea-floor deposits. The chapter provides background sedimentary information, and describes some common data manipulation techniques that are used to construct and study palaeoclimatic and palaeoceanographic records in marine sediments.

294 citations


Book ChapterDOI
Abstract: Phosphorus is an essential nutrient for all life-forms It is a key player in fundamental biochemical reactions involving genetic material (DNA and RNA) and energy transfer (ATP) and in structural support of organisms provided by membranes (phospholipids) and bone (the biomineral hydroxyapatite) Photosynthetic organisms utilize dissolved phosphorus, carbon, and other essential nutrients to build their tissues using energy from the sun Biological productivity is contingent upon the availability of phosphorus to these simple organisms that constitute the base of the food web in both terrestrial and aquatic systems It begins with a brief overview of the various components of the global phosphorus cycle Estimates of the mass of important phosphorus reservoirs, transport rates (fluxes) between reservoirs Following the overview, various aspects of the global phosphorus cycle are examined in more depth, including a discussion of the most pressing research questions currently being posed and research efforts presently underway to address these questions

293 citations


Journal ArticleDOI
Abstract: Phosphorus (P) is a limiting nutrient for terrestrial biological productivity that commonly plays a key role in net carbon uptake in terrestrial ecosystems (Tiessen et al. 1984, Roberts et al. 1985, Lajtha and Schlesinger 1988). Unlike nitrogen (another limiting nutrient but one with an abundant atmospheric pool), the availability of “new” P in ecosystems is restricted by the rate of release of this element during soil weathering. Because of the limitations of P availability, P is generally recycled to various extents in ecosystems depending on climate, soil type, and ecosystem level. The release of P from apatite dissolution is a key control on ecosystem productivity (Cole et al. 1977, Tiessen et al. 1984, Roberts et al. 1985, Crews et al. 1995, Vitousek et al. 1997, Schlesinger et al. 1998), which in turn is critical to terrestrial carbon balances (e.g., Kump and Alley 1994, Adams 1995). Furthermore, the weathering of P from the terrestrial system and transport by rivers is the only appreciable source of P to the oceans. On longer time scales, this supply of P also limits the total amount of primary production in the ocean (Holland 1978, Broecker 1982, Smith 1984, Filippelli and Delaney 1994). Thus, understanding the controls on P weathering from land and transport to the ocean is important for models of global change. In this paper, I will present an overview of the natural (pre-human) and modern (syn-human) global P mass balances, followed by in-depth examinations of several current areas of research in P cycling, including climatic controls on ecosystem dynamics and soil development, the control of oxygen on coupled P and Carbon (C) cycling in continental margins, and the role that P plays in controlling ocean productivity on Cenozoic timescales. ### Natural (pre-human) phosphorus cycle The human impact …

272 citations


Cites background from "Phosphogenesis and active phosphori..."

  • ...Several recent studies suggest that P may exhibit a significant preferential regeneration compared to C during diagenesis of organic matter in low oxygen continental-margin sediments (Ingall et al. 1993, Compton et al. 1993, Ingall and Jahnke 1994, Schenau et al. 2000)....

    [...]


References
More filters

Book
01 Jan 1968

11,284 citations


Journal ArticleDOI
Abstract: The focus of this paper is the conversion of radiocarbon ages to calibrated (cal) ages for the interval 24,000-0 cal BP (Before Present, 0 cal BP = AD 1950), based upon a sample set of dendrochronologically dated tree rings, uranium-thorium dated corals, and varve-counted marine sediment. The 14C age-cal age information, produced by many laboratories, is converted to 14C profiles and calibration curves, for the atmosphere as well as the oceans. We discuss offsets in measured 14C ages and the errors therein, regional 14C age differences, tree-coral 14C age comparisons and the time dependence of marine reservoir ages, and evaluate decadal vs. single-year 14C results. Changes in oceanic deepwater circulation, especially for the 16,000-11,000 cal BP interval, are reflected in the Δ 14C values of INTCAL98.

4,252 citations



Book
01 Jan 1980
Abstract: Diagenesis refers to changes taking place in sediments after deposition. In a theoretical treatment of early diagenesis, Robert Berner shows how a rigorous development of the mathematical modeling of diagenetic processes can be useful to the understanding and interpretation of both experimental and field observations. His book is unique in that the models are based on quantitative rate expressions, in contrast to the qualitative descriptions that have dominated the field. In the opening chapters, the author develops the mathematical theory of early diagenesis, introducing a general diagenetic equation and discussing it in terms of each major diagenetic process. Included are the derivations of basic rate equations for diffusion, compaction, pore-water flow, burial advection, bioturbation, adsorption, radioactive decay, and especially chemical and biochemical reactions. Drawing on examples from the recent literature on continental-margin, pelagic, and non-marine sediments, he then illustrates the power of these diagenetic models in the study of such deposits. The book is intended not only for earth scientists studying sediments and sedimentary rocks, but also for researchers in fields such as radioactive waste disposal, petroleum and economic geology, environmental pollution, and sea-floor engineering.

2,800 citations


"Phosphogenesis and active phosphori..." refers methods in this paper

  • ...Diffusive porewater fluxes ( J) have been calculated with ( Berner, 1980 ):...

    [...]


Journal ArticleDOI
Abstract: The tracer-diffusion coefficient of ions in water, Dj0, and in sea water, Dj∗, differ by no more than zero to 8 per cent. When sea water diffuses into a dilute solution of water, in order to maintain the electro-neutrality, the average diffusion coefficients of major cations become greater but of major anions smaller than their respective Dj∗ or Dj0 values. The tracer diffusion coefficients of ions in deep-sea sediments, Dj,sed., can be related to Dj∗ by Dj,sed. = Dj∗ · αθ2, where θ is the tortuosity of the bulk sediment and a a constant close to one.

2,529 citations


Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Pii: s0025-3227(00)00083-9" ?

In this study, porewater chemistry, solid-phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the Arabian Sea. Authigenic apatite precipitation rates vary between 0. 076 and 1. 04 mmolP cm yr, and are of the same order of magnitude as reported for other high productivity areas. This observation contrasts with previous reports of only old phosphorites in this area. Model results suggest that sediment mixing is essential in promoting early diagenetic phosphogenesis.