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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
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 (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.

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Citations
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Journal ArticleDOI
01 Apr 2008-Elements
TL;DR: In this article, the cycle of the Earth's phosphorus cycle has been studied and the effect of human activities on the cycling of the phosphorus cycle is discussed, which has led to increased use of fertilizers, deforestation and soil loss, and sewage sources.
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.

383 citations

Book ChapterDOI
01 Jan 2007
TL;DR: In this article, the elemental proxies for palaeoclimatic and palaeoceanographic variability in marine sediments are discussed, and the application of sedimentary geochemistry to the reconstruction of climatic and oceanographic changes over the Cenozoic is discussed.
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.

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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.

352 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present an overview of the natural (pre-human) and modern (synhuman) 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 cycling in continental margins, and the role that P plays in controlling ocean productivity on Cenozoic timescales.
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 …

325 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)....

    [...]

Book ChapterDOI
TL;DR: A brief overview of the various components of the global phosphorus cycle is given in this paper, including a discussion of the most pressing research questions currently being posed and research efforts presently underway to address these questions.
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

311 citations

References
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Journal ArticleDOI
TL;DR: In 1987, the Australian Bureau of Mineral Resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern Australia between 28 and 32°S.
Abstract: During 1987, the Australian Bureau of Mineral Resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern Australia between 28 and 32°S. The objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements, nitrogen, phosphorus, sulphur and iron in the sediments, and to investigate the roles which these processes played in the formation of the modern phosphorites. Bacterial productivities, sulphate-reduction rates, sedimentary oxygen and pore-water concentrations of nitrate, ammonia, phosphate, iron, sulphate and fluoride were measured at sea. The highest rates of microbial productivity were found in the surficial (0-20 mm) sediments of the modern phosphorite zone in 350-460 m water depth. These rates were about double those in shallower shelf (<300 m) sediments and 3-4 fold those rates in mid-slope (600-1000 m) sediments. Aerobic and anaerobic oxidation rates of organic matter, calculated from sediment oxygen profiles and sulphate-reduction rates were highest in the surface sediments in the modern phosphorite zone. The recycling of sedimentary iron, via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic/anoxic boundary results in a near-surface sedimentary trap for iron in the phosphorite zone sediments. Phosphate released from organic matter in the interfacial sediments, and fluoride from seawater, are scavenged by iron oxyhydroxides in the top few centimetres of sediment. Phosphorus, in this way, is decoupled from organic carbon in the near-surface sediments and linked to the redox cycling of iron. Phosphate and fluoride scavenged onto iron oxyhydroxides, and concentrated in the surficial sediments, are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve. The recycling process releases phosphate and fluoride for incorporation into apatite; fluoride is depleted from pore waters at depths <18 cm, phosphorite nodules form within anoxic sediments at depths <18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer. Combinations of rapid sediment mixing rates, a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten-fold that of the mid-slope sediments. Long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate, while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules. Phosphorite nodules are not found in mid-slope sediments probably because of combinations of relatively rapid sedimentation rates, ineffective iron, phosphorus and fluoride recycling and trapping mechanisms, plus dilution and dissemination of any incipient apatite.

109 citations

Journal ArticleDOI
01 May 1983-Geology
TL;DR: Bacterial mats play an important role in the sedimentation history of many marine biogenic regimes as discussed by the authors, and they are a source of the organic matter that in time is thermally altered to hydrocarbons.
Abstract: Bacterial mats play an important role in the sedimentation history of many marine biogenic regimes. Observations of the living mats suggest that in shallow waters they are dominated by photosynthetic cyanobacteria, while in deeper waters the mats are dominated by nonphotosynthetic sulfur-oxidizing bacteria. Fossilized remains of analogous mat structures from the Miocene Monterey Formation are commonly associated with oil-producing and phosphate-rich zones. Preliminary observations suggest that bacterial mats may be a source of the organic matter that in time is thermally altered to hydrocarbons.

104 citations


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

  • ...Williams and Reimers, 1983; Krajewski et al., 1994) may play an important role in regulating the interstitial phosphate concentrations....

    [...]

Journal ArticleDOI
01 May 1999
TL;DR: The presence of a strongly developed oxygen minimum zone (OMZ; [O 2 ] 2+ and Fe 2+ showed that reduction of manganese and iron oxides plays a minor role in sediments lying within the OMZ as discussed by the authors.
Abstract: The presence of a strongly developed oxygen minimum zone (OMZ; [O 2 ] 2+ and Fe 2+ show that reduction of manganese and iron oxides plays a minor role in sediments lying within the OMZ and a larger role in sediments lying below the OMZ.

103 citations


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

  • ...L1 cm 0.1 0.2 See text L2 cm 2.8 3.8 Van der Weijden et al., 1999 ...

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  • ...Calvert et al., 1995; Van der Weijden et al., 1999 )....

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Journal ArticleDOI
TL;DR: In this paper, sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200-350 m) sediments and relatively rapidly accumulating, iron oxide-deficient and organic-rich (tOC>0.8%) deep water (460-650 m) sediment sediments.
Abstract: Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

94 citations

Journal ArticleDOI
TL;DR: In this paper, a new method is presented, which differentiates P associated with biogenous apatite (Pfish) from other P fractions, and it enables the first time to quantify fish debris in sediment records and to assess their importance for the marine phosphorus cycle.
Abstract: Burial of fish bones, which consist primarily of hydroxyapatite, has been recognized as a mechanism to remove reactive phosphorus from the oceans. In this study, a new method is presented, which differentiates P associated with biogenous apatite (Pfish) from other P fractions. The method, consisting of a sequential chemical extraction with 2 M NH4Cl, has been successfully tested on standard materials. It enables us, for the first time, to quantify fish debris in sediment records and to assess their importance for the marine phosphorus cycle. The NH4Cl extraction has been applied to sediment samples from the Arabian Sea. Preservation of fish debris is significantly higher in sediments located above 1,200 m water depth than it is for deeper sediments. The distribution of Pfish contents in surface sediments is predominantly governed by the extent of fish debris regeneration, which is related to differences in water depth and sedimentation rates. In addition, a good correlation between high sedimentary Pfish contents and low oxygen bottom water concentrations suggests that the presence of the intense oxygen minimum zone may account for the enhanced preservation of fish debris in continental slope sediments.

90 citations


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

  • ...Fish debris, which consists primarily of hydroxyapatite, is, in contrast to authigenic or detrital apatite, highly soluble in 2 M NH4Cl ( Schenau and De Lange, 2000 )....

    [...]

  • ...This has the advantage that carbonates are dissolved selectively, allowing a differentiation between francolite and more soluble calcium-phosphate minerals ( Schenau and De Lange, 2000 )....

    [...]

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.