Uranium-series and AMS 14C studies of modern phosphatic pellets from Peru shelf muds

Burnett, William C.; Baker, Kaysee; Chin, Philip A.; McCabe, W.J.; Ditchburn, R.G.

doi:10.1016/0025-3227(88)90091-6

Cited by 31 publications

(10 citation statements)

References 19 publications

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“…(1980 ) suggests the age to be about 100 ka BP. The AMS age of phosphorite 1 (18 720 years BP) is much younger than the age obtained by the U‐series method, as has been found elsewhere by Burnett et al . (1988 ) for the Peru margin phosphorites.…”

Section: Discussionsupporting

confidence: 57%

A comparative study of Pleistocene phosphorites from the continental slope off western India

Rao¹,

Naqvi²,

Kumar³

et al. 2000

Sedimentology

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Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light‐brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod‐shaped apatite microparticles. Apatite also occurs as coatings on these particles. The P2O5 content of the phosphorite is 29%, and the CO2 content of the CFA is about 4·5%. The rare‐earth element (REE) abundance (ΣREE=2·02 μg g–1) is lower than in other modern phosphorites. The 87Sr/86Sr ratio and ɛNd value of this sample are 0·70921 and –9·9 respectively. The 14C age found through accelerator mass spectrometry (AMS) dating (18 720 ± 120 years BP) is much younger than that determined by the U‐series method (100 ka). The second type, phosphorite 2, comprises a friable, light‐brown nodule consisting of CFA as the only major mineral, with a CO2 content of the CFA of 4·5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P2O5 content is 33%, and REE contents (ΣREE = 0·18 μg g–1) are lower than in phosphorite 1. The age of phosphorite 2 is >300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during slow sedimentation under low‐energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P2O5 content of the phosphorites are found to be dependent on the nature of the initial substrates and physico‐chemical conditions during phosphatization. The CO2 content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment–water interface may control REE concentrations in phosphorites.

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Section: Discussionsupporting

confidence: 57%

A comparative study of Pleistocene phosphorites from the continental slope off western India

Rao¹,

Naqvi²,

Kumar³

et al. 2000

Sedimentology

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“…Some nodules grow more slowly than the sedimentation rates of the enclosing sediment; thus, they should be quickly buried beneath the reactive zone near the seafloor, unless they are maintained in this zone by burrowing organisms or winnowing (Burnett, 1980;Burnett et al, 1982). In contrast, phosphatic pellets may form more rapidly, in as little as 10 years or less after growth is initiated (Burnett et al, 1988).…”

Section: Previous Work On Peruvianmentioning

confidence: 99%

Phosphatic Sediments and Rocks Recovered from the Peru Margin during ODP Leg 112

Garrison¹,

Kastner²

1990

Proceedings of the Ocean Drilling Program

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Coring during Ocean Drilling Program (ODP) Leg 112 recovered phosphatic materials from six sites in forearc basins on the shelf and upper slope west of Peru. Three types of phosphates occur. F-phosphates are friable, light colored micronodules and peloids of carbonate fluorapatite (CFA); these formed by precipitation of CFA in laminated diatom muds deposited within the oxygen-minimum zone. Phosphatic sands, termed P-phosphates, consist of phosphatic peloids, coated grains, and fish debris, often having an admixture of fine siliciclastic grains. These sands occur in thin layers and burrowed beds up to 2 m thick; they may record high energy conditions and, in places, they occur above unconformities. Most abundant are dark and dense phosphates, herein called D-phosphates, that occur as nodules, gravels, and hardgrounds. These phosphates formed through complicated cycles of CFA precipitation during early diagenesis, erosion and exhumation, and reburial and rephosphatization processes associated with changing energy conditions that may reflect the effects of changes in sea level on the Peru shelf. CFA cements in P-and D-phosphates often replaced microbial structures, but our data do not reveal whether this microbial involvement was passive or active. The three phosphate types record a time and energy spectrum, with F-phosphates at the lower end, D-phosphates at the upper. Phosphates along the Peru margin occur in sediments as old as middle Miocene but are most abundant in Pliocene and especially Quaternary upwelling sediments. F-phosphates are most common in deeper water, outer-shelf/upper-slope sites, whereas D-and P-phosphates tend to predominate at shallower shelf sites more subject to episodic high energy conditions, especially during lowstands of sea level. Although lowstands of sea level probably led to concentration of phosphatic particles by winnowing, most CFA precipitation and cementation may have occurred, at least during Quaternary time, during interglacial highstands.

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“…Since then, researchers have increasingly recognized that intervals of sequence condensation also frequently were marked by the occurrence and, in some instances, a high (even minable) abundance of authigenic minerals (e.g., Glenn, 1990). Of continuing debate, however, is how much sediment reworking and winnowing contributes to the condensation process (e.g., Rod, 1946;Jenkyns, 1971;Kennedy and Garrison, 1975;Krajewski, 1984;Kidwell, 1991;Burnett et al, 1988;Glenn and Arthur, 1990;Föllmi, 1990;Snyder et al, 1990).…”

Section: Significance Of Condensed Sectionsmentioning

confidence: 99%

High-Resolution Sequence Stratigraphy, Condensed Sections, and Flooding Events off the Great Barrier Reef: 0-1.5 Ma

Glenn¹,

Kronen²,

Symonds³

et al. 1993

Proceedings of the Ocean Drilling Program

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Examination of seismic reflection and core data between Sites 819 and 821 provides information about patterns of sedimentation that result from repetitive fluctuations in relative sea level and climatic perturbations. On the basis of sequencestratigraphic interpretations, we identify nine sequences bounded by sequence boundaries; each sequence contains lithologically distinct and relatively thin units that have been interpreted as condensed sections. Because of very high mixed carbonate-siliciclastic sedimentation rates in this region, resolution of third-, fourth-, and fifth-order changes of sea level is recognized. One predominant condensed section is identified within each sequence and is associated with the maximum flooding surface of that sequence. These condensed sections are marked by increases in abundance of quartz, clay, and shallow-water bioclasts; increases in nonreworked pristine glauconites; decreases in total carbonate abundance; and increases in magnetic susceptibility. Increases in glauconite have been interpreted to indirectly reflect relative decreases in sedimentation rates, whereby sediment residence time is prolonged within suboxic bacterial degradation zones. Increases in bioclasts within transgressive systems tracts are attributed to relatively rapid buildups of shallow-water reefal communities that accompanied punctuated flooding events; during these times shallow-water carbonate growth accelerated as reefal communities built upward rapidly to keep up with rising sea level. Increases in quartz and clay contents during maximum flooding are thought to be tied to sediment starvation, whereby background clays and quartz contents are concentrated. Decreased accommodation potential during relative highstand phases promoted progressive highstand systems tract progradation and sedimentation of reworked glauconites and terrigenous sands, silts, and clays. These depositional phases also are marked by increases in bioclastic sands that accompanied the progradational, seaward building events.

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Uranium-series and AMS 14C studies of modern phosphatic pellets from Peru shelf muds

Cited by 31 publications

References 19 publications

A comparative study of Pleistocene phosphorites from the continental slope off western India

A comparative study of Pleistocene phosphorites from the continental slope off western India

Phosphatic Sediments and Rocks Recovered from the Peru Margin during ODP Leg 112

High-Resolution Sequence Stratigraphy, Condensed Sections, and Flooding Events off the Great Barrier Reef: 0-1.5 Ma

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