Phosphogenesis and active phosphorite formation in sediments from the Arabian Sea oxygen minimum zone
Abstract: 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. 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.
Summary (4 min read)
- 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.
- 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.
- 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.
- 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.
- 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.
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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)....
"Phosphogenesis and active phosphori..." refers methods in this paper
...Diffusive porewater fluxes ( J) have been calculated with ( Berner, 1980 ):...
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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.