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

TL;DR: In this paper, 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.

About: This article is published in Marine Geology.The article was published on 2000-09-15 and is currently open access. It has received 127 citations till now. The article focuses on the topics: Francolite & Phosphorite.

Summary

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.

Figures

Fig. 2. Porewater profiles for BC484 BC451, BC455: phosphate, fluoride, ammonium and alkalinity. The saturation state with respect to francolite in the porewaters was calculated as IAP (ion activity product) divided by theKsp (solubility product of francolite). The shaded areas indicate saturation.

Fig. 6. Model fits (solid lines) to the profiles of porewater phosphate, organic P (equal to Pres), Fe-bound P (PFe), authigenic P (equal to PNH4Cl 1 PCFA), and the sum of these three solid phase P fractions. The horizontal dotted line indicates the depth of bioturbation. The vertical solid line in the authigenic P profiles represents the PNH4Cl 1 PCFA concentration which is not related with in situ authigenic P formation (i.e. P associated with fish debris or carbonates, easily exchangeable P, or redeposited francolite). The vertical dotted lines represent the scenario where limited francolite formation takes place (i.e.ka 1026; leaving all other parameters the same).

Table 5 Phosphate production and removal rates for BC451 and BC455 as calculated with the diagenetic P model

Fig. 3. Profiles of extracted P phases (ppm): PNH4Cl (steps 11 2), PFe (step 3), PCFA (step 4), Pdet (step 5), Pres(steps 61 7), total P, as measured after total destruction (O), and the sum of all extracted P phases (X). Note the different scales for BC484.

Table 4 Values for the fixed and fitted parameters for BC451 and BC455 as used in the diagenetic P model

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

Fig. 1. Positions of the boxcore sample sites. The area where the present-day OMZ impinges on the continental slope is shaded.

Fig. 7. The P and Ca concentrations (ppm) in the solutions of the 8× NH4Cl sequential extraction for two sediment samples (BC451, 34.5 cm depth; BC455, 25.5 cm depth).

Fig. 4. Profiles of tripcore 484 (TC484) for total P (ppm), Corg/Ntot ratio (g/g), CaCO3 (wt%), Ti/Al (mg/g), Zr/Al (mg/g) and mean grainsize (mm) (bulk (O) and decarbonated (1) sediment). The dotted line indicates the depth above which the P concentration increases sharply. The top 10 cm of TC484 were lost during core recovery.

Table 2 The P sequential extraction scheme and extracted fractions

Table 6 Calculations for the downward flux of phosphate and fluoride (see Appendix A) (f — average porosity,z — depth of the of concentration gradient, dP/dz — average linear negative porewater phosphate gradient, dF/dz — average linear negative porewater fluoride gradient (for BC484 the phosphate/fluoride gradient was taken at a specific depth),JP — downward flux of phosphate (P removal rate by authigenic apatite formation),JF — downward flux of fluoride (F removal rate by authigenic apatite formation),JP(F) — P removal rate by authigenic apatite formation, based on the downward flux of fluoride,Jp model — the calculated authigenic P formation rate according to the diagenetic P model)

Fig. 5. Concentration of fish debris in BC484, BC451 and BC455 (numbers per gram sediment).

Table 3 Calibrated14C ages for non-coated foraminifers, coated foraminifers and phosphatised pellets in BC484 and pistoncore 484. The14C age for coated foraminifers is the age of the total particles, i.e. including carbonate and apatite C

Frequently asked questions

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.