Oceanographic Controls on Sedimentary and 

Geochemical Facies on the Peru Outer Shelf and 

Upper Slope

By Michael A. Arthur and Walter E. Dean

Open-File Report 2013–1105

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013 

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Suggested citation:

Arthur, M.A., and Dean, W.E., 2013, Oceanographic controls on sedimentary and geochemical facies 

on the Peru outer shelf and upper slope: U.S. Geological Survey Open-File Report 2013–1105, 38 p., 

http://pubs.usgs.gov/of/2013/1105/

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copyright owners to reproduce any copyrighted material contained within this report.

Cover. Shipboard scientific party for Cruise SL10-92 of the Research Vessel Seward Johnson and the 

Deep Submersible Vessel Johnson Sea-Link II in October and November 1992. Daniel Schwartz, 

Captain of the R/V Seward Johnson, is on the far right.
iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 2

Materials and Methods .................................................................................................................................................. 4

Acquisition of Samples............................................................................................................................................... 4

Inorganic Geochemical Analyses............................................................................................................................... 4

Carbon Analyses........................................................................................................................................................ 4

Factor Analyses ......................................................................................................................................................... 6

Characteristics of the Water Column and Distribution of Biota ...................................................................................... 6

Lithofacies Distributions................................................................................................................................................. 8

Geochemical Facies Distributions.................................................................................................................................11

Results from the 12°S Transect................................................................................................................................11

Results from the 13.5°S Transect.............................................................................................................................15

Discussion ....................................................................................................................................................................25

Geochemical Indicators of Bioproductivity ................................................................................................................25

Organic Carbon Concentration and Preservation..................................................................................................25

Phosphorus Distribution and Phosphorite Formation ............................................................................................26

Geochemical Indicators of Redox Conditions ...........................................................................................................27

Organics and Metals .............................................................................................................................................27

Origin of Glaucony Pellets.....................................................................................................................................28

Geochemical Indicators of Current Effects................................................................................................................29

Phosphorite ...........................................................................................................................................................29

Glaucony ...............................................................................................................................................................31

Summary of Controls on Geochemical Facies Patterns ...............................................................................................32

Conclusions..................................................................................................................................................................33

Acknowledgments ........................................................................................................................................................34

References ...................................................................................................................................................................34

Figures

Figure 1. Map of the coast of Peru showing bathymetry; the average position of the oxygen minimum 

zone; the locations of selected salinity, temperature, and depth casts; and the locations of 

DS/V Johnson Sea-Link II submersible dive transects at 12°S and 13.5°S in October and 

November 1992.......................................................................................................................................... 3

Figure 2. Representative salinity, temperature, depth, and dissolved oxygen profiles taken at select 

locations on the 12°S and 13.5°S transects of the R/V Seward Johnson expedition on the Peru 

continental margin...................................................................................................................................... 5

Figure 3. Map of bathymetry and bottom-current measurements made along two dive transects on the 

Peru margin ............................................................................................................................................... 7

Figure 4. Examples of the progressive formation of phosphorite crusts and pavements on the Peru 

margin........................................................................................................................................................ 9

Figure 5. Cross sections showing the lithology and characteristics of surface sediments as a function of 

water depth along two transects on the Peru margin ................................................................................10
iv

Figure 6. Plots of concentration of percent organic carbon, Rock-Eval pyrolysis Hydrogen Index in 

milligrams hydrocarbon per grams organic carbon, and percent calculated CaCO3-calcite in 

surface sediments versus water depth along two transects on the Peru margin.......................................13

Figure 7. Plots of factor loadings versus water depth along the 12°S transect for factor 1, organic carbon 

and metals, factor 2, phosphorite, and factor 3, clastic, from Q-mode factor analysis of 

chemical composition of surface sediments..............................................................................................14

Figure 8. Plots of factor loadings versus water depth along the 13.5°S transect for factor 1, organic 

carbon and metals, factor 2, glaucony, factor 3, phosphorite, and factor 4, clastic, from Q-mode 

factor analysis of chemical composition of surface sediments ..................................................................17

Figure 9. Plots of percent aluminum versus percent iron and percent potassium for all samples along the 

13.5°S transect .........................................................................................................................................18

Figure 10. Plots of percent total calcium versus percent total phosphorus for all samples along the 12°S 

transect and the 13.5°S transect...............................................................................................................23

Figure 11. Plot of percent aluminum versus percent total silicon and calculated percent excess Si versus 

water depth for the 13.5°S transect ..........................................................................................................24

Figure 12. Hypothetical sequence of phosphorite crust formation on the Peru margin ..............................................30

Tables

Table 1. Correlation coefficients between loadings for factors 1–4 and concentrations of major elements 

and trace elements in surface sediments on the 12°S transect ................................................................12

Table 2. Correlation coefficients between loadings for factors 1–4 and concentrations of major elements 

and trace elements in surface sediments on the 13.5°S transect .............................................................16

Table 3. Composition of Q-mode factor analysis end-member samples and samples of glaucony pellets, 

phosphorite crusts and nodules, and dolomitic crusts collected along the 13.5°S transect on 

the Peru margin.........................................................................................................................................19

Table 4. Detrital and excess concentrations of selected trace elements in surface sediments ...............................27

Abbreviations Used in This Report

Al aluminum

As arsenic

Ba barium

Ca calcium

CaCO3 calcium carbonate

Cd cadmium

Ce cerium

CFA carbonate-fluorapatite (phosphorite)

cm centimeter

Co cobalt

Cr chromium

STD salinity, temperature, and depth

Cu copper

DO dissolved oxygen

Fe iron

g gram

Ga gallium
v

HAP hydroxyapatite

HC hydrocarbon

HI Hydrogen Index (Rock-Eval)

IC inorganic carbon

ICP-AES inductively coupled, argon-plasma, atomic emission spectrometry

K potassium

kHz kilohertz

La lanthanum

Li lithium

m meter

Mg magneseium

mg milligram

mm millimeter

Mn manganese

Mo molybdenum

Na sodium

Nb niobium

Nd neodymium

Ni nickel

OC organic carbon

ODP Ocean Drilling Program

OMZ oxygen-minimum zone

P phosphorus

Pb lead

s second

Sc scandium

Si silicon

Sr strontium

TC total carbon

Th thorium

Ti titanium

V vanadium

VPDB Vienna Pee Dee Belemnite marine carbonate standard

XRF X-ray fluorescence spectrometry

Y yttrium

Zn zinc

μm micrometer

μM micromoles per kilogram
Oceanographic Controls on Sedimentary and 

Geochemical Facies on the Peru Outer Shelf and 

Upper Slope

Michael A. Arthur1 and Walter E. Dean2

Abstract

Concentrations and characteristics of organic matter in surface sediments deposited under 

an intense oxygen-minimum zone (OMZ) on the Peru margin were mapped and studied in 

samples from deck-deployed box cores and push cores acquired by submersible on two east-west 

transects spanning depths of 75 to 1,000 meters (m) at 12°S and 13.5°S. On the basis of sampling 

and analyses of the top 1–2 centimeters (cm) of available cores, three main belts of sediments 

were identified on each transect with increasing depth: (1) muds rich in organic carbon (OC); (2) 

authigenic phosphatic mineral crusts and pavements; and (3) glaucony facies.

Sediments rich in OC on the 12°S transect were mainly located on the outer shelf and 

upper slope (150–350 m), but they occurred in much shallower water (approximately 100 m) on 

the 13.5°S transect. The organic matter is almost entirely marine as confirmed by Rock-Eval 

pyrolysis and isotopic composition of OC. Concentrations of OC are highest (up to 18 percent) in 

sediments within the OMZ where dissolved oxygen (DO) concentrations are <5 micromoles per 

kilogram (μM). Even at these low concentrations of DO, however, the surface sediments from 

within the OMZ are dominantly unlaminated. Concentrations of DO may have the dominant 

effect on organic matter characteristics, but reworking of fine-grained sediment and organic 

matter by strong bottom currents with velocities as high as 30 centimeters per second (cm/s) on 

the slope between 150 and 300 m and redeposition on the seafloor in areas of lower energy and 

higher DO concentration also exert important controls on OC concentration and degree of 

oxidation in this region. 

Phosphate-rich sediments and crusts occurred at depths of about 300 to 550 m on both 

transects. Nodular crusts of sediment cemented by carbonate-fluorapatite (CFA; phosphorite) or 

dolomite form within the OMZ. These phosphorite crusts evolve through cementation from light 

olive-green, stiff but friable, phosphatized claystone “protocrusts” through dense, dark 

phosphorite crusts, cemented breccias, and pavements. The degree of phosphatization and 

thickness of the crusts depend on the rates of sediment supply and on the strength and frequency 

of currents that re-expose crusts on the seafloor. Phosphorite crusts and pavements on the Peru 

margin can only become buried and incorporated into the geologic record once bottom currents 

slacken sufficiently to allow fine-grained sediment to accumulate.

Glaucony-rich surface sediments, relatively undiluted by other components, were found 

mainly in deeper water on the 13.5°S transect (750 m to at least 1,067 m). These sediments 

 

1 Pennsylvania State University–Department of Geosciences 2 U.S. Geological Survey
2

consist almost entirely of sand-size glaucony pellets. These widespread glaucony sands formed 

in place and were then concentrated and reworked by strong currents that winnowed away the 

fine-grained matrix. Although the glaucony occurs in sand-size pellets, the pellets are made up of 

aggregates of authigenic, platy, micaceous clay minerals. Glaucony is predominantly a potassium

(K), sodium (Na), iron (Fe), magnesium (Mg) aluminosilicate with an approximate formula of

(K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2. The glaucony on the 13.5°S transect forms by alteration 

of one or more original “framework” minerals (carbonate and [or] aluminosilicates) to form 

pellital aggregates of Fe-, K-, and Mg-rich clay minerals. Because Fe, K, and Mg are derived 

from seawater, sedimentation rates must be extremely slow in order for the original framework 

minerals to remain in contact with seawater. The close association of glaucony and phosphorite 

indicates a delicate balance between the slightly oxidizing conditions at the base of the OMZ that 

form glaucony and the slightly reducing conditions that mobilize iron and phosphate to form 

phosphorite.

Introduction

Systematic studies of sedimentary and geochemical facies distributions on upwelling 

margins involving mapping and sampling of facies types using submersible and sediment coring 

are needed to understand the effects of climate and sea level change on facies patterns and 

geochemical fluxes and to construct models for interpreting sedimentary sequences in ancient 

basins characterized by wind-driven upwelling and strong oxygen-minimum zones (OMZ) that 

impinge on basin slopes. The Peru margin, in particular, is a target for such investigations 

because it is characterized by upwelling of nutrient-rich Pacific intermediate waters that drives 

high biologic productivity, high fluxes of organic carbon (OC) to the seafloor, and the 

development of an intense midwater (100–700 meters [m]) OMZ. This region is climatically 

sensitive as evidenced by its sensitivity to El Niño–Southern Oscillation events and the Pacific 

Decadal Oscillation (Dettinger and others, 1998; Brown, 2011). Previous expeditions to the Peru 

margin have documented the processes involved in the formation of carbonate-fluorapatite 

(CFA; phosphorite) and dolomite crusts (Burnett, 1977; Froelich and others, 1988; Kulm and 

others, 1988; Garrison and Kastner, 1990; Burnett and others, 2000). Sediments in this region are 

characterized by relatively low fluxes of detrital clastic (aluminosilicate) material. They are 

dominated instead by authigenic and biogenic components related directly or indirectly to the 

accumulation of high concentrations of organic matter in response to coastal upwelling and 

relatively low concentrations of dissolved oxygen within an intense OMZ.

The investigation of the formation of authigenic minerals, particularly phosphorite crusts, 

was the objective of the R/V Seward Johnson expedition with the submersible DS/V Johnson 

Sea-Link II in October and November 1992. Our expedition took place during the protracted 

1991–1992 El Niño conditions, so it is difficult to assert that the distribution of dissolved oxygen 

(DO), temperature, fauna, and surface-sediment characteristics that we observed are normal for 

the margin. A variety of deck-operated coring devices; profiles of salinity, temperature, depth, 

and dissolved oxygen (STD-DO); benthic flux chambers; pump-filter stations; and submersible 

deployments were used for direct and remote sampling and observation of the seafloor and water 

column along two east-west transects across the Peru margin at about 13.5°S and 12°S and at a 

depth range of 75 to 1,000 m (fig. 1). Here we report results of these surveys and initial

sedimentological and geochemical studies of the sediments at the surface (1–2 centimeters [cm]).
3

Figure 1. Map of the coast of Peru showing bathymetry (in meters); the average position of the oxygen 

minimum zone (OMZ; <5 micromoles per kilogram or <1.0 milligram per liter [parts per million]

dissolved oxygen); the locations of selected salinity, temperature, and depth (STD) casts; and the 

locations of DS/V Johnson Sea-Link II submersible dive transects at 12°S and 13.5°S in October and 

November 1992.

The main objectives of this paper are (1) to examine the distribution and characteristics of 

surface sediments, particularly surface-sediment geochemistry and mineralogy, as a function of 

water depth, and (2) to relate these parameters to conditions resulting from prevailing 

oceanographic processes including DO concentrations, organic-matter flux, and bottom-current 

patterns. We emphasize the extensive early diagenesis at or just below the seafloor that is fueled 

by high fluxes of OC and low bottom-water DO content at the sediment-water interface. The 

rapid mobilization of iron and phosphorus as well as silica from the dissolution of biogenic opal 

leads to the formation of authigenic minerals at the surface. In addition, we recognize a 

surprisingly strong role of currents on slope sedimentation along the margin that results in 

winnowing and concentration of certain sediment components, as well as long sedimentexposure times at the seafloor across the upper slope. These currents and important redox 

reactions promote the occurrence of three major depth-related facies belts: (1) a shallow belt of 

OC-rich, calcareous and diatomaceous muds, relatively enriched in detrital clay minerals and 

stabilized by bacterial mats; (2) an intermediate-depth phosphatic facies, consisting primarily of 

grains, crusts and nodules of phosphorite, and some dolomite cemented crusts; and (3) a deeper 

region of glaucony sand. Though some authors prefer to restrict “glauconite” to the specific ironpotassium phyllosilicate (mica) mineral and use the term “glaucony” for the facies (Smith and 

others, 1998), we use the term “glaucony” for the mineral grains and the facies, as suggested by 

Odin (1985). Here, as in most other localities, glaucony occurs as sand-size, rounded aggregates

of very fine-grained scaly particles. The dominant authigenic origin of sediments on the Peru 

margin and the importance of bottom currents on slope sedimentation have important 

implications for sediment accumulation on continental margins with strong OMZs.
4

Materials and Methods

Acquisition of Samples

Samples of sediment in 36 box cores, 26 gravity cores, and 18 multicores deployed from 

the R/V Seward Johnson and box and push cores deployed during 31 submersible dives with 

DS/V Johnson Sea-Link II were collected for this investigation (Arthur and others, 1998). The

submersible dives resulted in high-resolution video coverage of bottom operations, sediment 

types, and organisms and the recovery of 108 push cores, 32 pore-water cores, 25 small box 

cores, a large number of bottom-sediment slurp samples in buckets, and numerous samples of 

crusts, nodules, and other materials using the mechanical arm of the submersible. For this study 

of surface-sediment characteristics, we collected samples at intervals of 0–1 cm and 1–2 cm from 

most submersible-deployed push cores and at intervals of 0–2 cm from selected deck-deployed 

box cores. Bottom-current directions and velocities were measured from the submersible at 

numerous stations using an external current meter. Water-column salinity, temperature, depth, 

and DO profiles (fig. 2) were measured at numerous stations using a Seabird Electronics (SBE19) STD-DO system. Bottom reflectivity characteristics were monitored in some areas using a 

33-kilohertz (kHz) bottom profiling system on the R/V Seward Johnson.

Inorganic Geochemical Analyses

Splits of surface-sediment samples from box and submersible push cores for geochemical 

analyses were air dried and ground in a ceramic mill to pass a 100-mesh sieve. Concentrations of 

30 major, minor, and trace elements [aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), 

sodium (Na), potassium (K), titanium (Ti), phosphorus (P), manganese (Mn), arsenic (As), 

barium (Ba), cadmium (Cd), cerium (Ce), cobalt (Co), chromium (Cr), copper (Cu), gallium 

(Ga), lanthanum (La), lithium (Li), molybdenum (Mo), niobium (Nb), neodymium (Nd), nickel 

(Ni), lead (Pb), scandium (Sc), strontium (Sr), thorium (Th), vanadium (V), yttrium (Y), and zinc 

(Zn)] in 81 samples from the 13.5°S transect and 66 samples from the 12°S transect were 

determined by inductively coupled, argon-plasma, atomic emission spectrometry (ICP-AES; 

Briggs, 2002). Concentrations of 10 major elements [silicon (Si), Al, Fe, Mg, Ca, Na, K, Ti, P,

and Mn] in 68 of the 81 samples from the 13.5°S transect were measured by wavelengthdispersive X-ray fluorescence spectrometry (XRF; Taggart and others, 1987). For most elements,

the precision of the method, determined by analyzing U.S. Geological Survey rock standards and 

duplicating 10 percent of the samples, was better than 10 percent of the amount present and 

usually better than 5 percent of the amount present for elements in concentrations greater than 10 

times the detection limit.

Carbon Analyses

Concentrations of total carbon (TC) and inorganic carbon (IC) were determined by 

coulometry (Engleman and others, 1985) in splits of the geochemistry samples. Carbonate in the

untreated sample is reacted with phosphoric acid to liberate CO2, which is then titrated in a 

coulometer cell to measure inorganic carbon. Percent calcium carbonate (CaCO3) is calculated 

by dividing percent IC by 0.12, the fraction of IC in CaCO3. Total carbon is measured by 

liberating CO2 by combustion of an untreated sample at 1,050°C in a stream of oxygen and 

titrating the CO2. Concentrations of organic carbon (OC) were determined by the difference 

between TC and IC. The accuracy and precision for both TC and IC, determined from hundreds 
5

of replicate standards (reagent grade CaCO3 and a Cretaceous OC-rich marlstone), usually are 

better than 0.10 weight percent. Methods for characterization of Peru margin organic matter by 

Rock-Eval pyrolysis and carbon-isotope techniques have been presented in Arthur and others 

(1998).

Figure 2. Representative salinity, temperature, depth, and dissolved oxygen (STD-DO) profiles taken at 

select locations on the 12°S and 13.5°S transects of the R/V Seward Johnson expedition on the Peru 

continental margin. Only the longer profiles at the deeper sites (>750 meters) show the full extent of the 

oxygen minimum zone (100–700 m).
6

Factor Analyses

In order to determine what elements are associated with specific sediment components, 

multivariate analysis of the inorganic geochemical data and the OC and IC for each transect was 

analyzed by multivariate Q-mode factor analysis. In addition, Q-mode analysis objectively 

determines geographic groupings of samples across each of the transects based on geochemical 

similarities. The computer program used is a modified version of the extended CABFAC 

program described by Klovan and Miesch (1976). All samples (0–2 cm in box cores, 0–1 cm and

1–2 cm in push cores) were used for the Q-mode factor analyses. However, because the values 

for geochemical concentrations ranged widely, from fractions of a percent to thousands of parts 

per million (ppm), each value for a variable was scaled prior to analysis to a fraction of each 

variable’s range. Thus the scaled values ranged from 0 to 1. The factor analyses essentially 

reduced 32 geochemical variables to 4 (13.5°S transect) or 3 (12°S transect) composite 

geochemical variables, or factor loadings, that explained most of the variance in the scaled data.

To determine what geochemical variables had the most influence on each factor, the factor 

loadings (composite variables) were correlated with the concentration of each element. The 

results of these correlation analyses are given in table 1 for the 12°S transect and table 2 for the 

13.5°S transect.

Characteristics of the Water Column and Distribution of Biota

Casts for STD-DO (fig. 2) indicate that DO concentrations were similar along both 

transects, ranging from greater than (>) 200 micromoles per kilogram (μM) in surface waters to 

less than (<) 5 μM (essentially zero according to calibrations on board ship using microwinkler 

titrations) within the OMZ (fig. 2). The top of the OMZ in October–November 1992 was at <50 

m on the shelf in water depths <100 m, and at about 100 m over the slope (fig. 2). The bottom of 

the OMZ (>5 μM DO) typically was at about 700 m over the slope.

Persistent, high-velocity bottom currents (up to 30 centimeters per second [cm/s]) 

characterize much of the Peru slope. The strongest currents measured in this study were 

associated with the southward (poleward) flowing Peru Countercurrent and were most persistent 

on the upper slope between about 150 and 300 m (fig. 3). Current directions and magnitudes

were more variable on the shelf and lower part of the upper slope. Also, current magnitudes and 

directions were much more variable on the 12°S transect (fig. 3A) than on the 13.5°S transect 

(fig. 3B). There was a minor influence of the northward flowing surface Peru Current on the 

shallow shelf on both transects (fig. 3A and B). The strongest currents occurred on the upper 

slope at depths of 200–250 m on the 12°S transect (fig. 3A) and 150–250 m on the 13.5°S

transect (fig. 3B). Submersible observations showed that these upper-slope depths were 

characterized by very high concentrations of particulates in the water column and traction 

transport of larger particles on the seafloor.
7

Figure 3. Map of bathymetry and bottom-current measurements made along two dive transects on the 

Peru margin. A, 12°S transect, October 27–November 5, 1992. B, 13.5°S transect, October 15–25, 

1992. Bathymetric contour interval (dashed line) is 50 meters. Each black dot represents a 

measurement of the current. The length of the bar radiating from each dot is proportional to the speed 

of the current in centimeters per second (cm/s; see scale), and the azimuth of the bar is the direction of 

the current. 0,0 = zero speed, zero direction.
8

A variety of fish, octopi, crabs, and other organisms were observed in the water column 

near the top and bottom of the intense midwater OMZ. High concentrations of euphausiids were 

observed consistently near the top of the OMZ on both transects, even at DO concentrations of 

5–10 μM. These dense concentrations of euphausiids commonly were observed on the 33 kHz

“fish finder” records. Relatively few benthic metazoans were observed on the inner shelf (70–

100 m) where the sediment surface commonly was covered by thick, white mats of Thioploca

(sulfide-oxidizing bacteria). Abundant polychaete worms occurred in the upper few centimeters 

of sediment on the inner shelf. No benthic metazoans were observed at water depths of 150–350 

m on the upper slope, indicating that low DO concentrations in the core of the OMZ apparently 

limit the occurrence of benthic organisms. However, even though DO concentrations remained 

low (<5 μM to water depths of as much as 700 m, the abundance and diversity of organisms 

increased substantially below 350 m, and was very high at depths below 700 m (>10 μM DO).

Thioploca mats and polychaete worms, when they were present, inhabited the upper 5 cm or 

more of surface muds at these deeper mid-slope sites. Well-developed laminations were rarely 

observed in submersible- or deck-deployed cores, even though no molluscs or other large 

burrowing organisms were present. Fossil and living benthic foraminifera were very abundant, 

however, and often were concentrated at the surface as a winnowed lag deposit.

Lithofacies Distributions

Green-gray to dark olive-gray gelatinous, calcareous, diatomaceous muds are 

characteristic of surface sediments on the Peru outer shelf and upper slope. Phosphorite crusts 

begin as layers of sediment just below the sediment-water interface that are weakly cemented by 

CFA and less commonly by dolomite with polychaete worm burrows still preserved 

(“protocrusts”; fig. 4A). Layers of protocrust were observed from the submersible cropping out

on the seafloor where they had been undercut by bottom currents. With time, the protocrusts

become completely cemented and most of the polychaete burrows are filled with CFA (fig. 4B). 

The crusts are periodically broken up by the activity of organisms and bottom currents and are 

recemented to form mature phosphorite pavements (fig. 4C).

On the 12°S transect, phosphorite nodules and crusts were observed between about 200 

and 500 m (fig. 5A) and extensive phosphorite pavements from 500 m to at least 700 m.

Phosphorite nodules, crusts, and pavements were found on the 13.5°S transect from about 150 m

to 600 m (fig. 5B). The deeper regions of the upper slope on the 13.5°S transect, sampled down 

to 1,050 m, typically were composed of homogeneous glauconitic sands, silts, and muds 

(fig. 5B).
9

Figure 4. Examples of the progressive formation of phosphorite crusts and pavements on the Peru 

margin. A, Weakly cemented sediment (protocrust) with uncemented polychaete worm burrows. B,

Strongly cemented slab of phosphorite with black manganese oxide coating and mostly filled and 

cemented polychaete worm burrows. C, Mature phosphorite pavement composed of broken fragments 

of well-cemented phosphorite crust that have been recemented.
10

Figure 5. Cross sections showing the lithology and characteristics of surface sediments as a function of 

water depth along two transects on the Peru margin. A, 12°S transect. B, 13.5°S transect. Star shows 

the location of Callao, Peru.
11

Geochemical Facies Distributions

Results from the 12°S Transect

Table 1 shows that concentrations of OC, Ni, V, Cd, Mo, Cu, Na, Cr, Zn, and Co, in

order of decreasing correlation coefficient between element concentration and factor 1 loadings,

had the most influence in grouping samples from the 12°S transect into factor 1. These OC- and 

CaCO3-rich, factor 1 sediments occur on the outer shelf and upper slope between about 150 and 

400 m (figs. 6A and C; fig. 7A). Factor 1 accounted for 67 percent of the variance in the scaled

data.

Grouping of samples into factor 2 was influenced mostly by Sr, Ca, P, and IC, in order of 

deceasing correlation coefficients between element concentration and factor 2 loadings (table 1). 

The higher concentrations of these elements in factor 2 sediments are the result of higher 

amounts of CFA. The elements Ca, P, and IC are part of the main structure of CFA; Sr 

substitutes for Ca in the crystal lattice. Factor 2 sediments are concentrated in a band between 

about 350 and 750 m (fig. 7B), down slope from the band of factor 1 sediments. Factor 2 

accounted for 17 percent of the variance in the scaled data.

Factor 3 sediments have the highest content of detrital clastic material (aluminosilicates) 

undiluted by either organic matter (as in factor 1 sediments) or CFA (as in factor 2 sediments). 

Concentrations of many elements influenced the grouping of samples into factor 3; most 

important were the lithophile elements Fe, Al, Ga, Th, Ti, Li, Sc, Ce, Nb, K, Nd, As, Mn, La, Zn, 

and Pb, in order of decreasing correlation coefficients between element concentration and factor 

3 loadings (table 1). Factor 3 sediments mostly occur at depths >750 m, below the zones of 

sediments enriched in organic matter and CFA (fig. 7C). Figure 7C shows that a few samples 

with factor 3 characteristics occur at 100 m and 400–500 m. Factor 3 accounted for 8 percent of 

the variance in the scaled data. 

Grouping samples into factor 4 was based mainly on concentrations of Na and Mg 

(table 1), probably due to concentrations of these elements precipitated as sea salts from 

interstitial seawater during drying because the sediments were not washed before being analyzed. 

Therefore, factor 4 loadings are not plotted in Figure 7. Note that these two elements also make 

minor contributions to factor 1 (table 1) because OC-rich sediments usually have higher 

porosities and, therefore, higher sea-salt concentrations when dried.
12

Table 1. Correlation coefficients between loadings for factors 1–4 and concentrations of major elements 

(in weight percent) and trace elements (in parts per million) in surface sediments on the 12°S transect. 

Correlation coefficients in bold type indicate those elements that had the most influence in determining 

each factor.

Element Factor 1 Factor 2 Factor 3 Factor 4

Aluminum -0.02 -0.33 0.93 -0.44

Calcium -0.66 0.92 -0.30 -0.52

Iron -0.19 -0.11 0.94 -0.44

Potassium 0.11 -0.51 0.80 0.07

Magnesium 0.45 -0.59 -0.15 0.88

Sodium 0.53 -0.58 -0.34 0.90

Phosphorus -0.61 0.92 -0.26 -0.56

Titanium 0.05 -0.52 0.90 -0.19

Manganese -0.18 -0.33 0.66 -0.36

Arsenic 0.31 -0.44 0.76 -0.15

Barium 0.02 0.13 0.31 -0.11

Cadmium 0.91 -0.51 -0.07 0.18

Cerium -0.32 -0.13 0.83 -0.55

Cobalt 0.51 -0.24 0.14 -0.24

Chromium 0.52 -0.12 0.03 -0.57

Copper 0.84 -0.62 0.23 0.16

Gallium -0.05 -0.29 0.92 -0.38

Lanthanum -0.41 0.04 0.65 -0.67

Lithium 0.17 -0.43 0.86 -0.16

Molybdenum 0.89 -0.55 0.03 0.22

Niobium 0.28 -0.57 0.82 -0.21

Neodymium -0.44 0.20 0.69 -0.75

Nickel 0.94 -0.52 -0.22 0.17

Lead 0.33 -0.53 0.52 0.16

Scandium -0.07 -0.25 0.86 -0.47

Strontium -0.64 0.95 -0.27 -0.54

Thorium -0.27 -0.24 0.90 -0.29

Vanadium 0.92 -0.50 -0.03 0.04

Yttrium -0.37 0.18 0.06 -0.54

Zinc 0.52 -0.56 0.55 0.03

Inorganic carbon -0.46 0.30 -0.28 -0.14

Organic carbon 0.95 -0.56 -0.24 0.37
13

Figure 6. Plots of concentration of percent organic carbon (% OC), Rock-Eval pyrolysis hydrogen index 

(HI) in milligrams hydrocarbon per gram organic carbon (mgHC/gOC), and percent calculated CaCO3-

calcite (see text for method of calculation) in surface sediments versus water depth along two transects 

on the Peru margin. A–C, 12°S transect. D–F, 13.5°S transect. Dashed red lines are 5-point moving 

averages of the data.
14

Figure 7. Plots of factor loadings versus water depth along the 12°S transect for (A) factor 1, organic 

carbon and metals, (B) factor 2, phosphorite, and (C) factor 3, clastic, from Q-mode factor analysis of 

chemical composition of surface sediments. Dashed red lines are 5-point moving averages of the 

elemental data.
15

Results from the 13.5°S Transect

Factor 1 sediments on the 13.5°S transect have characteristics similar to those of factor 1 

sediments on the 12°S transect, with high concentrations of OC, Cd, Mo, and several other trace 

metals (table 2). The OC- and metal-rich factor 1 sediments on the 13.5°S transect are

concentrated mainly on the inner shelf at about 100 m with a zone of lesser factor 1 loadings 

between 550 and 700 m (fig. 8A). In contrast, the OC- and metal-rich factor 1 sediments on the

12°S transect are on the outer shelf and upper slope between 150 and 300 m (fig. 7A). These 

inner shelf sediments contain OC concentrations as high as 17 percent (fig. 6B). Concentrations 

of OC in the 550–700 m slope zone are mostly <10 percent, but this zone also contains elevated 

concentrations of CaCO3 (fig. 6B). Factor 1 accounted for 72 percent of the variance in the scaled

data.

Grouping of samples into factor 2 was influenced primarily by Fe, K, Cr, and Mg, in 

order of deceasing correlation coefficients between element concentration and factor 2 loadings 

(table 2). The higher concentrations of the elements making up factor 2 (table 3) occur because

these sediments contain high concentrations of glaucony, mainly as sand-sized aggregates of 

authigenic, micaceous clay minerals. These glaucony-rich factor 2 sediments are mainly found 

on the slope from 550 m to at least 1,067 m (the deepest sample) (fig. 8B), although samples 

with relatively high loadings for factor 2 are also found on the inner shelf at about 100 m. Factor 

2 accounted for 11 percent of the variance in the scaled data.

Glaucony is predominantly a K, Na, Fe, Mg aluminosilicate with an approximate formula 

of (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2. The dominant cation in glauconite is Fe, and an 

estimate of the percentage of glaucony in surface sediments on the 13.5°S transect can be 

obtained by subtracting out the amount of Fe in the detrital fraction, assuming that the detrital 

fraction contains the same element concentrations as average upper continental crust, and 

assuming that the iron present in excess of the detrital Fe is in glaucony (fig. 9A). A similar

calculation also can be made for detrital and excess K (fig. 9B). There are excellent correlations 

between Fe and Al and between K and Al for samples between 100 and 400 m on the 13.5°S 

transect where there is little glaucony (fig. 8B). For these samples, we assume that all of the Fe 

and K is in the detrital fraction and that the detrital fraction has the composition of upper 

continental crust. The regression equations for these samples are %Fe=0.48%Al–0.028, r= 0.943 

(fig. 9A) and %K=0.23%Al–0.335, r= 0.930 (fig. 9B). Assuming that the slope of the ironaluminum regression equation (0.48) is the ratio of iron to aluminum in the detrital fraction, the 

concentration of “detrital Fe” can be calculated by multiplying the aluminum concentration by 

0.48, close to the Fe/Al ratio in upper continental crust (0.45; Wedepohl, 1995) (fig. 9A). The 

concentration of “excess Fe” (assumed to be glaucony Fe) can be calculated as the difference 

between total Fe and detrital Fe. Chemical analysis of relatively pure glaucony sand in box cores 

BC50 and BC43 from the western end of the 13.5°S transect (>77°W longitude; water depths 

>750 m) support this calculation. For example, the concentrations of Fe, K, and Mg in glaucony 

in sediments from BC50 and BC43 are very similar to those in the factor 2 end-member sediment 

sample and are very different from the other end-member sediments (table 3). End-member 

glaucony contains about 20% Fe, 5% K, and 2% Mg (table 3; fig. 9). Using 20% excess Fe as an 

end member, the amount of glaucony in any sample can be determined by the amount of excess 

Fe in that sample. The average percent glaucony in all samples is 15.6 percent, range 0–100

percent.
16

Table 2. Correlation coefficients between loadings for factors 1–4 and concentrations of major elements 

(in weight percent) and trace elements (in parts per million) in surface sediments on the 13.5°S 

transect. Correlation coefficients in bold type indicate those elements that had the most influence in 

determining each factor.

Element Factor 1 Factor 2 Factor 3 Factor 4

Aluminum 0.45 -0.61 -0.57 0.92

Calcium -0.47 -0.13 0.97 -0.38

Iron -0.60 0.97 -0.03 -0.28

Potassium -0.56 0.97 -0.13 -0.16

Magnesium -0.02 0.51 -0.55 0.12

Sodium 0.89 -0.67 -0.56 0.15

Phosphorus -0.49 -0.07 0.95 -0.38

Titanium 0.40 -0.51 -0.62 0.94

Manganese 0.52 -0.66 -0.57 0.87

Arsenic -0.14 -0.17 0.34 0.10

Barium 0.58 -0.71 -0.53 0.76

Cadmium 0.94 -0.67 -0.50 0.15

Cerium 0.20 -0.61 -0.21 0.88

Cobalt 0.28 -0.37 -0.29 0.55

Chromium -0.60 0.87 0.18 -0.31

Copper 0.79 -0.51 -0.71 0.53

Gallium -0.02 0.13 -0.59 0.81

Lanthanum -0.07 -0.45 0.19 0.65

Lithium 0.41 -0.39 -0.65 0.88

Molybdenum 0.91 -0.63 -0.52 0.19

Niobium 0.51 -0.52 -0.56 0.72

Neodymium -0.13 -0.40 -0.04 0.79

Nickel 0.41 -0.30 0.10 -0.27

Lead 0.31 -0.26 0.29 0.29

Scandium -0.05 0.05 -0.42 0.79

Strontium -0.44 -0.17 0.96 -0.33

Thorium 0.19 -0.35 -0.52 0.93

Vanadium 0.83 -0.44 -0.41 0.03

Yttrium -0.49 0.07 0.68 -0.04

Zinc 0.67 -0.54 -0.69 0.70

Inorganic carbon -0.27 -0.13 0.62 -0.32

Organic carbon 0.88 -0.58 -0.46 0.04
17

Figure 8. Plots of factor loadings versus water depth along the 13.5°S transect for (A) factor 1, organic 

carbon and metals, (B) factor 2, glaucony, (C) factor 3, phosphorite, and (D) factor 4, clastic, from Qmode factor analysis of chemical composition of surface sediments. Dashed red lines are 5-point 

moving averages of the elemental data. (OC, organic carbon)
18

Figure 9. Plots of percent aluminum (Al) versus (A) percent iron (Fe) and (B) percent potassium (K) for all 

samples along the 13.5°S transect. See text for methods of calculations of detrital and excess 

concentrations. Red dashed lines are the regression lines for the composition of upper continental crust

(UCC) (Wedepohl, 1995).
19

Table 3. Composition of Q-mode factor analysis end-member samples and samples of glaucony pellets, phosphorite crusts and nodules, and 

dolomitic crusts collected along the 13.5°S transect on the Peru margin.

[ID, identification; F., factor; JSL, DS/V Johnson Sea-Link II; BC, box core; OC, organic carbon; IC, inorganic carbon; m, meter; %, percent; ppm, parts per 

million]

Sample ID Latitude Longitude Water 

depth (m)

Si SiO2 Al Al2O3 Fe Fe2O3 Mg MgO Ca CaO Na Na2O K

% % % % % % % % % % % % %

End members

F. 1: JSL 3348-3 13°31.00' S 76°43.06' W 183 13 28 3.8 7.2 1.6 2.2 1.4 2.3 1.8 2.5 4.3 5.8 0.66

F. 2: JSL 3351-1 13°29.87' S 77°02.07' W 823 22 48 1.4 2.6 20 28 2.1 3.6 0.4 0.5 0.9 1.2 4.51

F. 3: JSL 3356-1 13°29.77' S 76°51.98' W 386 4.9 10 1.5 2.8 0.8 1.1 0.8 1.3 28 39 1.0 1.3 0.12

F. 4: JSL 3345-1 13°31.36' S 76°19.67' W 75 20 44 6.3 12 3.3 4.7 1.9 3.2 1.7 2.4 5.2 7.1 1.31

Glaucony pellets

BC50 13°30.00' S 77°03.00' W 898 22 48 1.2 2.3 20 28 2.2 3.6 0.4 0.5 0.9 1.2 4.27

BC43 13°30.65' S 77°01.83' W 781 23 48 1.3 2.5 20 29 2.2 3.6 0.3 0.4 0.4 0.6 5.40

Crusts and nodules

D-527999 13°39.79' S 76°47.80' W 256 6.4 14 1.6 2.9 1.2 1.8 0.6 1.0 28 39 1.2 1.7 0.55

D-528002 13°29.72' S 76°47.89' W 255 7.4 16 1.9 3.6 2.4 3.4 0.7 1.1 26 36 1.2 1.7 0.74

D-528003 13°29.55' S 76°47.79' W 255 7.7 17 2.0 3.9 1.4 2.0 0.8 1.3 25 35 1.2 1.6 0.61

D-528004 13°30.47' S 76°55.42' W 528 6.5 14 1.7 3.2 1.6 2.2 0.6 1.0 27 38 1.3 1.7 0.58

D-528006 13°30.72' S 76°54.63' W 488 5.5 12 1.3 2.4 2.3 3.3 0.7 1.1 28 39 1.2 1.6 0.66

D-528000 13°29.55' S 76°47.79' W 255 19 41 5.0 9.4 2.1 3.0 4.6 7.7 10 13 1.8 2.5 1.29

D-528001 13°29.55' S 76°47.79' W 255 12 27 3.3 6.3 2.3 3.2 4.7 7.8 16 22 1.5 2.0 0.85

D-528005 13°30.72' S 76°54.63' W 488 14 30 3.5 6.7 1.7 2.4 6.0 9.9 13 19 1.3 1.8 0.86

End members:

Factor 1 end member (OC-metals) JSL Dive 3348, submersible push core 3

Factor 2 end member (glaucony) JSL Dive 3351, submersible push core 1

Factor 3 end member (phosphorite) JSL Dive 3356, submersible push core 1

Factor 4 end member (detrital) JSL Dive 3345, submersible push core 1

Glaucony: deck-deployed box cores 50 and 43

Crusts and nodules:

D-527999 mature phosphate nodule from JSL Dive 3350

D-528002 mature phosphate crust from JSL Dive 3352

D-528003 bioturbated phosphate crust from JSL Dive 3352

D-528004 bioturbated phosphate crust from JSL Dive 3354

D-528006 mature phosphate nodule from JSL Dive 3355

D-528000 dolomitic crust from JSL Dive 3352

D-528001 dolomitic, phosphatic crust from JSL Dive 3352

D-528005 dolomitic, phosphatic crust from JSL Dive 3355
20

Table 3. Composition of Q-mode factor analysis end-member samples .—Continued

[ID, identification; F., factor; JSL, DS/V Johnson Sea-Link II; BC, box core; OC, organic carbon; IC, inorganic carbon; m, meter; %, percent; ppm, parts per 

million]

Sample ID Latitude Longitude Water 

depth (m)

K2O Ti TiO2 P P2O5 Mn Mn0 As Ba Cd Ce Co

% % % % % ppm % ppm ppm ppm ppm ppm

End members

F. 1: JSL 3348-3 13°31.00' S 76°43.06' W 183 0.79 0.17 0.28 0.34 0.78 160 0.020 15 260 54 21 8

F. 2: JSL 3351-1 13°29.87' S 77°02.07' W 823 5.43 0.05 0.08 0.07 0.17 13 0.002 11 37 1 3 4

F. 3: JSL 3356-1 13°29.77' S 76°51.98' W 386 0.14 0.07 0.12 12 28 71 0.009 10 98 5 13 7

F. 4: JSL 3345-1 13°31.36' S 76°19.67' W 75 1.58 0.31 0.51 0.11 0.25 460 0.060 19 350 18 36 20

Glaucony pellets

BC50 13°30.00' S 77°03.00' W 898 5.15 0.05 0.08 0.08 0.19 100 0.013 12 36 1 3 5

BC43 13°30.65' S 77°01.83' W 781 6.50 0.05 0.08 0.08 0.18 21 0.003 17 16 1 3 5

Crusts and nodules

D-527999 13°39.79' S 76°47.80' W 256 0.66 0.07 0.11 12 28 70 0.009 17 120 7 17 8

D-528002 13°29.72' S 76°47.89' W 255 0.89 0.10 0.17 11 25 130 0.017 32 120 5 12 8

D-528003 13°29.55' S 76°47.79' W 255 0.73 0.11 0.18 10 25 130 0.017 17 140 5 21 15

D-528004 13°30.47' S 76°55.42' W 528 0.70 0.10 0.16 11 25 110 0.014 29 120 6 19 39

D-528006 13°30.72' S 76°54.63' W 488 0.80 0.07 0.12 12 28 98 0.013 21 89 8 42 7

D-528000 13°29.55' S 76°47.79' W 255 1.55 0.25 0.41 0.25 0.58 350 0.045 27 360 1 29 12

D-528001 13°29.55' S 76°47.79' W 255 1.03 0.19 0.32 2.88 6.6 230 0.030 55 220 16 20 30

D-528005 13°30.72' S 76°54.63' W 488 1.04 0.17 0.28 0.81 1.9 190 0.025 17 260 1 26 43

End members:

Factor 1 end member (OC-metals) JSL Dive 3348, submersible push core 3

Factor 2 end member (glaucony) JSL Dive 3351, submersible push core 1

Factor 3 end member (phosphorite) JSL Dive 3356, submersible push core 1

Factor 4 end member (detrital) JSL Dive 3345, submersible push core 1

Glaucony: deck-deployed box cores 50 and 43

Crusts and nodules:

D-527999 mature phosphate nodule from JSL Dive 3350

D-528002 mature phosphate crust from JSL Dive 3352

D-528003 bioturbated phosphate crust from JSL Dive 3352

D-528004 bioturbated phosphate crust from JSL Dive 3354

D-528006 mature phosphate nodule from JSL Dive 3355

D-528000 dolomitic crust from JSL Dive 3352

D-528001 dolomitic, phosphatic crust from JSL Dive 3352

D-528005 dolomitic, phosphatic crust from JSL Dive 3355
21

Table 3. Composition of Q-mode factor analysis end-member samples —Continued

[ID, identification; F., factor; JSL, DS/V Johnson Sea-Link II; BC, box core; OC, organic carbon; IC, inorganic carbon; m, meter; %, percent; ppm, parts per 

million]

Sample ID Latitude Longitude Water 

depth (m)

Cr Cu La Li Mo Ni Sr V Y Zn IC OC

ppm ppm Ppm ppm ppm ppm ppm ppm ppm ppm % %

End members

F. 1: JSL 3348-3 13°31.00' S 76°43.06' W 183 170 81 12 27 63 150 220 260 13 110 0.21 18

F. 2: JSL 3351-1 13°29.87' S 77°02.07' W 823 460 24 1 14 1 26 42 69 1 47 0.08 1.2

F. 3: JSL 3356-1 13°29.77' S 76°51.98' W 386 89 17 10 11 6 28 2,000 56 21 38 1.70 2.2

F. 4: JSL 3345-1 13°31.36' S 76°19.67' W 75 54 83 19 46 37 30 230 160 14 150 0.12 3.5

Glaucony pellets

BC50 13°30.00' S 77°03.00' W 898 480 30 2 16 1 30 43 66 1 59 0.06 1.1

BC43 13°30.65' S 77°01.83' W 781 510 23 2 15 1 27 34 79 3 42 0.05 1.0

Crusts and nodules

D-527999 13°39.79' S 76°47.80' W 256 130 35 12 6 3 27 2,100 50 19 31

D-528002 13°29.72' S 76°47.89' W 255 130 13 11 11 8 50 1,700 61 11 30

D-528003 13°29.55' S 76°47.79' W 255 110 20 18 12 1 25 2,000 48 27 45

D-528004 13°30.47' S 76°55.42' W 528 140 28 13 11 8 58 2,000 210 19 41

D-528006 13°30.72' S 76°54.63' W 488 150 16 12 9 5 46 2,000 80 17 34

D-528000 13°29.55' S 76°47.79' W 255 37 16 17 24 17 8 500 70 11 43

D-528001 13°29.55' S 76°47.79' W 255 72 17 12 21 16 37 870 120 9 44

D-528005 13°30.72' S 76°54.63' W 488 260 18 16 17 1 17 650 67 17 29

End members:

Factor 1 end member (OC-metals) JSL Dive 3348, submersible push core 3

Factor 2 end member (glaucony) JSL Dive 3351, submersible push core 1

Factor 3 end member (phosphorite) JSL Dive 3356, submersible push core 1

Factor 4 end member (detrital) JSL Dive 3345, submersible push core 1

Glaucony: deck-deployed box cores 50 and 43

Crusts and nodules:

D-527999 mature phosphate nodule from JSL Dive 3350

D-528002 mature phosphate crust from JSL Dive 3352

D-528003 bioturbated phosphate crust from JSL Dive 3352

D-528004 bioturbated phosphate crust from JSL Dive 3354

D-528006 mature phosphate nodule from JSL Dive 3355

D-528000 dolomitic crust from JSL Dive 3352

D-528001 dolomitic, phosphatic crust from JSL Dive 3352

D-528005 dolomitic, phosphatic crust from JSL Dive 3355
22

Factor 3 sediments on the 13.5°S transect are phosphatic, similar to those of factor 2 

sediments on the 12°S transect with high concentrations of Ca, P, Sr, Y, and IC (tables 1 and 2).

Yttrium and other rare-earth elements usually are enriched in CFA (Byrne and others, 1996). The 

phosphate-rich factor 3 sediments on the 13.5°S transect are found on the upper slope between 

250 and 550 m (fig. 8C), which is slightly shallower than the phosphate-rich sediments that 

occur between 350 and 750 m on the 12°S transect (fig. 7B). Factor 3 accounted for 8.5 percent

of the variance in the scaled data.

The relations between Ca, P, and IC for samples from both transects show that calcium 

resides in both CFA and calcite (mostly from benthic forminifera), and allow us to determine the 

concentrations of CFA and calcite independently. On the 12°S transect the linear Ca-P 

relationship (% P=0.44%Ca–1.2, r=0.970; fig. 10A) is essentially the same as for the 13.5°S

transect (% P=0.48%Ca–1.03, r=0.994; fig. 10B). Samples of phosphorite crusts collected by

submersible from both transects contain about 28% Ca and 12% P (P/Ca=0.43; for example,

sample D-527999), identical to the concentrations in the factor 3 end-member sample (table 3).

The P/Ca ratio of 0.43 is close to the P/Ca slope ratios from the two regression equations (0.44 

and 0.48, fig. 10). Using a P/Ca ratio of 0.43, and assuming that all of the P was in CFA, we 

calculated percent CFA-Ca for each sample as percent total-P divided by 0.43. We then 

calculated the percentage of calcite-Ca in each sample by subtracting percent CFA-Ca from 

percent total-Ca. Finally, percent CaCO3-calcite (shown in fig. 6) was calculated by dividing

calcite-Ca by 0.4, the fraction of Ca in CaCO3. Using these relations, an end-member 

phosphorite contains 12% P (table 3; fig. 10), and the amount of phosphorite in any sample can 

be calculated. The average amount of phosphorite in all samples on the12°S transect is 25

percent (range 1–100 percent), and in all samples on the 13.5°S transect is 15 percent (range 1–

100 percent).

Factor 4 sediments on the 13.5°S transect, like factor 3 sediments on the 12°S transect, 

are composed mainly of detrital clastic material only weakly diluted by CFA, glaucony, or 

organic matter. Elements that were used to group factor 4 sediments are Ti, Th, Al, Ce, Li, Mn, 

Ga, Nd, Sc, Ba, Nb, Zn, La, Co, and Cu in order of decreasing correlation coefficients with 

factor 4 loadings (table 2). Sediments with highest loadings for factor 4 occur on the inner shelf 

at about 100 m (fig. 8D), largely in association with OC-rich factor 1 sediments. Factor 4 only 

accounts for 3.4 percent of the variance in the scaled data.

One other consideration is an estimate of the contribution of biogenic Si (opal) to Peru 

margin sediments. We have data for % Si by XRF for most of the surface-sediment samples from 

the 13.5°S transect that allow us to estimate the biogenic Si concentration using the following 

methodology. First, the detrital (aluminosilicate) Si concentration was calculated assuming that 

the Si/Al in the detrital fraction on the Peru margin was the same as that in average continental 

crust (3.57; Wedepohl, 1995). Therefore, % detrital Si = 3.57Al (fig. 11A). “Excess” Si was then

calculated by subtracting % detrital Si from % total Si (fig. 11B). Excess Si consists of Si from

glaucony in sediments from water depths deeper than 400 m and from biosiliceous (diatom) 

debris in sediments from shallower depths. Sediments from water depths shallower than 400 m 

contain no glaucony, so the excess Si for these samples is predominantly biogenic Si. Highest 

glaucony Si values occur in samples from depths deeper than 400 m, reaching a maximum for 

samples from depths deeper than 750 m (fig. 11B).
23

Figure 10. Plots of percent total calcium (Ca) versus percent total phosphorus (P) for all samples along (A) 

the 12°S transect and (B) the 13.5°S transect. Regression equations were calculated for samples with 

>2 percent total phosphorus.
24

Figure 11. Plot of (A) percent aluminum (Al) versus percent total silicon (Si) and (B) water depth versus 

calculated percent excess Si for the 13.5°S transect. See text for methods of calculations of detrital 

and excess Si concentrations. In sediments at depths shallower than about 400 meters, most of the 

excess Si is in biogenic debris (diatoms), and in sediments at depths deeper than 400 meters, most of 

the excess Si is in glaucony.
25

Discussion

Geochemical Indicators of Bioproductivity

Organic Carbon Concentration and Preservation

The source of organic matter to the seafloor on the Peru margin is almost entirely marine 

material that results from high primary productivity (>200–400 gram C per square meter per year 

[gC/m2

/yr]; Suess and others, 1987) associated with the Peru Current upwelling system. The 

dominant marine source is confirmed for sediment samples analyzed in this study by isotopic 

values of OC (δ13C) ranging from -20 to -21.5 ppm relative to the Vienna Pee Dee Belemnite 

marine carbonate standard (VPDB) and by Rock-Eval pyrolysis (Arthur and others, 1998).

The hydrogen index (HI, in milligrams hydrocarbons per gram OC [mgHC/gOC]) 

determined by the Rock-Eval pyrolysis technique is a rapid screening tool for the determination 

of the hydrocarbon source-rock potential of sedimentary rocks (Tissot and Welte, 1984; Peters, 

1986). Pyrolysis can also be used to determine the hydrogen richness of protokerogen in modern 

sediments (for example, Liebezeit and Wiesner, 1990; Calvert and others, 1992; Pedersen and 

others, 1992; Arthur and others, 1994; Dean and Gardner, 1998; Ganeshram and others, 1999).

In sedimentary rocks, low values of HI (<100) usually indicate the presence of either 

highly oxidized organic matter of planktic origin or primarily terrestrially derived higher plant 

material (Peters, 1986). Consistently high HI values of >400 are found mainly in rocks that were 

deposited in environments interpreted to have been anoxic (for example, presence of fine 

laminations) and where there is no significant reworked material (for example, Pratt, 1984).

Values of HI in sediments from both transects are mostly between 200 and 400 (fig. 6B and E).

Mixing of terrestrial organic matter (HI of <100) and well-preserved algal organic matter (HI of 

>400) in various proportions is unlikely for the Peru margin samples given the low influx of 

detrital material. Also, there is no correlation between values of HI and values of δ13C (Arthur 

and others, 1998). Therefore, variations in values of HI in Peru margin surface sediments is 

likely due to variations in the degree of degradation of marine organic matter, particularly for the 

low-OC, low-HI samples below a water depth of 750 m on the 13.5°S transect (fig. 6E).

Concentrations of DO were <5 μM in water depths to 500 m, and no macrobenthic

organisms were observed between depths of 120 and 350 m (Arthur and others, 1994). Despite 

the low DO concentrations and absence of infaunal organisms, most cores did not contain fine 

laminations. Sediment transport and reworking by strong bottom currents must exert a major 

control on OC preservation on the Peru margin, as previously suggested by Suess and others 

(1987). We measured currents as high as 30 cm/s at depths between 175 and 300 m within the 

core of the OMZ on both transects (fig. 3). The strongest southward-flowing (poleward) current 

is the Peru Undercurrent (Brockmann and others, 1980) that has a well-defined core at depths of

50–300 m. The mean poleward velocity of the Peru Undercurrent at 10°S is 10 cm/s, but it 

commonly attains sustained velocities of >25 cm/s over periods of 2 days to 2 weeks 

(Brockmann and others, 1980). Therefore, on much of the Peru upper slope, constant advection 

of DO in low concentrations by bottom waters, resuspension of surficial organic matter, activity 

of organisms, and transport to and from more oxygenated zones contributes to greater oxidation 

and poorer preservation of organic matter than might be expected in a strong OMZ. This upper 

slope region also is characterized by fields of megaripples stabilized by bacterial mats, which is 

further evidence of a constant current flow.
26

Phosphorus Distribution and Phosphorite Formation

As we discussed above, the sediments on the Peru margin are rich in phosphate, but most 

of that phosphate presently resides in CFA, either as unconsolidated sediments or in phosphorite 

nodules and crusts. Extensive pore-water and solid-phase studies of the upper 20–40 cm of box 

cores from the Peru shelf by Froelich and others (1988) demonstrate that CFA is precipitating

today in the upper few centimeters of organic-rich, gelatinous sediments.

One of the most surprising results of submersible observations and coring on the Peru 

margin was that soft sediment is currently being cemented by CFA on the upper slope within the 

OMZ (protocrusts). Previous investigations concluded that nucleation of CFA occurred on preexisting hard substrates or by growth of phosphorite nodules and pellets within sediments (Glenn 

and Arthur, 1988). However, this precipitation must have occurred at or just below the sedimentwater interface based on the young ages (a few thousand years) of the various CFA morphotypes 

(Burnett and Veeh, 1977; Burnett and others, 1980, 1982, 1983, and 1988; Kim and Burnett, 

1986, 1988). Although phosphorite pellets were abundant in muds recovered on a previous

expedition to the same area (1982 R/V Robert Conrad; Baker and Burnett, 1988; Burnett and 

others, 1988), the widespread occurrences of phosphorite nodules, crusts, pavements, and 

phosphatic biogenic material (for example, whale bones) are the most prominent features 

observed on the 1992 R/V Seward Johnson expedition.

Our investigations of the authigenic crusts show that the crusts start by early diagenetic 

cementation of soft surface sediment (protocrusts) in water depths of 200–700 m, with the 

highest concentrations of CFA occurring between 400 and 500 m on the 12°S transect (fig. 7B), 

and between 300 and 60 m on the 13.5°S transect (fig. 8C). The surficial origin of these crusts is 

supported by Burnett and others (2000), who found that CFA cementation of 1- to 3-cm-thick 

layers was initiated in the upper few centimeters of sediments in box cores. They concluded from 

radionuclide and carbon-isotope analyses that the CFA layers grew upwards towards the 

sediment-water interface. Their study suggests that precipitation of CFA occurs near the 

sediment-water interface and that the protocrusts grow upward over thousands of years.

Froelich and others (1988), who studied samples from the same area collected on the 

1982 R/V Robert Conrad cruise, suggested that formation of CFA may be at least a two-step 

process that takes place near the sediment-water interface, first by precipitation of amorphous 

calcium phosphate then by addition of carbonate and fluorine in porewaters to form CFA. Rapid 

decreases in dissolved phosphate just below high values near the sediment-water interface 

suggested to them that phosphogenesis is currently occurring on the Peru margin at or just below 

the sediment-water interface. Froelich and others (1988) concluded that the phosphate flux that 

they observed across the sediment-water interface on the Peru margin was too great to be 

supported by decomposition of diatoms alone. Like Suess (1981) and deVries and Pearcy (1982),

they concluded that fish debris (hydroxyapatite, HAP) is a significant biogenic source of 

phosphorous for phosphogenesis. Some HAP is converted directly to CFA, but most dissolves 

before it reaches the seafloor (Froelich and others, 1982). Another important source of phosphate

and iron might be the “iron-redox pump” of Froelich and others (1979, 1988), which is discussed

further below. Schuffert and others (1998), in their study of a different phosphogenic locality,

called on fish debris as a source of P, but they favored an iron-redox pump as the primary source 

of P. The predominantly young age (Plio-Pleistocene) of the phosphorites recovered on the Peru 

margin during Ocean Drilling Program (ODP) Leg 112 led Garrison and Kastner (1990) to 

conclude that major phosphogenesis during the last two million years on the Peru margin 

occurred in response to intensified oceanic circulation that accompanied Pleistocene glaciation.
27

They also suggested that the main source of phosphorus was from fish debris and not 

decomposition of organic matter.

Geochemical Indicators of Redox Conditions

Organics and Metals

The Q-mode factor analyses showed that the detrital clastic component on the Peru 

margin is a major component but is commonly diluted by organic matter and authigenic 

components. The detrital fraction can be estimated by the Al concentration of the sediments 

because Al is contributed entirely from that fraction. The average concentration of Al in surface 

sediments is 3.5 percent on the 12°S transect and 3.7 percent on the 13.5°S transect. The Al 

concentration in average upper continental crust is 7.7 percent (Wedepohl, 1995). Assuming that 

all of the Al in Peru margin sediments is in detrital clastic debris with an average composition 

close to that of upper continental crust, then the average contribution of detrital clastic debris is 

slightly <50 percent on both transects. The detrital contributions of other elements can be 

estimated by assuming that the element-to-aluminum ratio in detrital clastic debris in Peru 

margin sediments is the same as in average upper continental crust. The difference between the 

total concentration and the detrital concentrations for any element is the excess concentration 

that must be assigned to a hydrothermal or seawater source (Piper, 1994). Because there is no

evidence for hydrothermal activity on the Peru margin, we assume that the excess amount of any 

element must have a seawater source, either in biogenic debris or in a precipitated endogenic or 

authigenic phase. The detrital and excess concentrations of Cd, Mo, Cu, Zn, Ni, and V (the 

metals with highest correlations with the organic-metals factor loadings in tables 1 and 2) are 

given in table 4.

Table 4. Detrital and excess concentrations of selected trace elements in surface sediments.

[ppm, parts per million; % Al, percent aluminum]

Element Upper 

crust1

(ppm)

Ratio to

% Al

in crust

(ppm)

12°S transect 13.5°S transect 

Total Detrital Excess Total Detrital Excess

(ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Cadmium 0.10 0.013 26.2 0.05 26.2 25.3 0.05 25.2

Copper 14 1.82 41.8 6.3 35.4 50.7 6.8 43.9

Zinc 52 6.75 80 23 57 91 25 66

Nickel 19 2.47 61.2 8.6 52.7 61.4 9.2 52.2

Vanadium 53 6.88 133 24 109 150 26 124

Molybdenum 1.4 0.18 31.6 0.6 31.0 42.4 0.7 41.7

Aluminum 77,000 35,000 37,000

1 Wedepohl, 1995.

It is clear from table 4 that a seawater (excess) source is required for all six trace 

elements; only Zn and V have significant detrital contributions. Because of high rain rates of 

biogenic debris on continental margins under upwelling regimes, these areas represent major 

sinks for many trace elements in the oceans today (for example, Collier and Edmond, 1984). This

should be particularly true of the Peru margin because of the unusually high rate of primary 

productivity. However, because of large fluctuations in redox and sulfate-reducing conditions 

often occurring in the water as well as in the sediments, precipitation of redox-sensitive elements 

should also be expected.
28

In Cariaco Basin sediments, 96.7 percent of Mo and 31.8 percent of V were derived from 

bottom water and (or) sediment pore water by adsorption and precipitation reactions (Piper and 

Dean, 2002). It is not possible to determine if this precipitation occurred in the water column

(endogenic) or within the sediments (authigenic), but molecular diffusion of these elements from 

bottom waters into the sediments and precipitation at the sediment-water interface is most likely 

(Crusius and others, 1996; Helz and others, 1996). Most of the nondetrital (excess) metal

concentrations in Peru margin sediments likely came from marine plankton. Some additions, 

particularly of Mo and V, may have resulted by sulfide precipitation in sulfidic bottom and (or) 

pore waters.

Origin of Glaucony Pellets

Sediments recovered from water depths >750 m (to at least 1,067 m) consist almost 

entirely of dark green glaucony pellets. A size analysis of one typical sample of the glaucony 

pellets from a water depth of 781 m (BC43; table 3) showed that the median grain size is 0.5–1.0 

millimeters (mm) (range of 63 micrometers [μm] to 2 mm). Most of the minor finer-grained 

material consists of quartz and feldspar as determined by X-ray diffraction; minor sand-sized 

dolomite and CFA also are present. Most pellets are ellipsoidal in outline with surfaces covered 

with networks of septarian cracks filled with a tan material. The term pellet is used here to 

describe the shape of the grains with no particular implication as to origin. When the pellets are 

crushed, the platy habit of the clay minerals that make up the pellets is apparent.

According to Odin and Letolle (1980), glaucony can form in place from any mineral

“framework” where the requisite ions (Si, Fe, K, and Mg) are “assembled;” it is not necessary to 

start with a related clay mineral (for example, iron smectite). The end product is an aggregate of 

clay minerals with a pelletoid shape. Because K and Mg are most likely derived from seawater to 

alter the original mineral framework, it is necessary for that framework to remain near the 

sediment-water interface in contact with seawater for a considerable time or glaucony will not 

form (Nishimura, 1994). On the other hand, Si and Fe can be supplied by dissolution of diatom

opal and by Fe reduction in sediments beneath the site of formation. Therefore, the sedimentation 

rate in glaucony-forming environments must be extremely slow and there can be little diluting 

clastic material and relatively little agitation. According to Odin (1988), cracked, green pellets

with a K2O content of approximately 6 percent (such as the pellets we collected on the Peru 

margin; table 3) require on the order of 104

years to form; more mature pellets with K2O contents 

of >8 percent require even more time (105 to 106 years).

The close association of glaucony with phosphorite (Odin and Letolle, 1980) suggests

that the environment must have had a delicate balance between slightly oxidizing conditions for 

glaucony formation and slightly reducing conditions in order to mobilize iron and phosphate. Of 

the four areas that were investigated by Glenn and Arthur (1988) on the Peru margin between 

8°S and 15°S, glaucony grains were most abundant in sediments from the most oxidized area at 

about 8°S. There, the glaucony grains were randomly disseminated in CFA cements that filled 

burrows. This intimate association of CFA with glaucony grains also was reported by Burnett 

(1980). Glenn and Arthur (1988) also observed that pyrite commonly fills fractures in and 

replaces parts of glaucony grains. They suggested that glauconitization occurred first, followed 

by phosphatization, and then replacement by pyrite.

Pore-water profiles measured by Froelich and others (1988) from the same areas of the

Peru margin investigated by Glenn and Arthur (1988) suggest that the upper 10 cm of sediment 

is a zone of mixed suboxic to anoxic diagenesis. They proposed the “iron-redox pump” to 
29

explain the close association of glaucony and phosphorite under varying redox conditions.

According to this mechanism, ferric oxyhydroxide from detrital clastic material is reduced at or 

just below the sediment-water interface releasing dissolved iron and adsorbed phosphate. Under 

conditions of sulfate reduction, some of the dissolved iron forms pyrite. Under slightly oxidizing 

conditions, dissolved iron, together with Si, K, and Mg, alters a pre-existing carbonate and (or) 

detrital-clastic framework mineral to form glaucony. Froelich and others (1988) observed that

the concentration of dissolved iron was below detection in the bottom water, was at a maximum 

at the sediment-water interface, decreased rapidly with depth below the sediment-water interface, 

and was below detection by the first traces of hydrogen sulfide. Some dissolved iron escapes 

above the sediment-water interface where it is oxidized to form iron oxyhydroxides with 

adsorbed phosphate providing more material for the iron-redox pump (Froelich and others, 

1979).

The iron-redox pump scenario might explain the apparent occurrence of a small 

proportion (<10 percent) of glaucony detected by factor analysis in relatively OC-rich muds at 

depths of about 100 m (fig. 8B). However, the relative purity of glaucony on the Peru slope and 

the fact that the glaucony band is geographically distinct from the main phosphorite band suggest 

that a strong iron-redox pump did not exist. Suits and Arthur (2000) studied sulfur cycling in this 

area of the Peru margin using cores obtained on the 1992 R/V Seward Johnson expedition and 

concluded that glaucony probably could not form in OC-rich muds on the outer shelf or upper 

slope because the onset of bacterial sulfate reduction was very near the sediment-water interface, 

effectively precipitating all reactive iron as pyrite. Suits and Arthur (2000) concluded that iron

would be available for glaucony formation only near the base of the OMZ where OC fluxes are 

low and sulfate-reduction in near-surface pore waters does not occur.

Geochemical Indicators of Current Effects

Phosphorite

The formation of thick and extensive phosphorite crusts requires that bottom currents 

periodically sweep the seafloor clean of fine-grained detritus above nascent protocrusts. Such 

currents were observed from and measured on the submersible (fig. 3). This winnowing and 

reworking produces lag deposits and the potential for numerous unconformities. From the 

submersible, we frequently observed surface lag deposits composed mainly of benthic 

foraminifera. However, growth of phosphorite crusts requires a continuous or at least intermittent 

source of dissolved phosphate (Froelich and others, 1988; Schuffert and others, 1998). Our study

of crusts suggests that phosphorite crusts begin as layers of sediment up to several centimeters 

thick just below the sediment-water interface that are weakly cemented by CFA, and less 

commonly by dolomite; we have called these weakly cemented layers protocrusts (fig. 4A; fig.

12, Stage 2). The early-formed protocrusts that we observed are only 0.5- to 1.0-cm thick but are

sufficiently cohesive to crop out on the seafloor when undercut by the southward-flowing Peru 

undercurrent. We observed several areas where fragments of protocrust up to several centimeters 

in diameter have broken off of the eroded edge of protocrust outcrops, and covered the sediment 

on the adjacent seafloor. These protocrust fragments are further cemented by CFA to produce 

observed phosphorite breccias and (or) grow into phosphorite nodules. Nodules eventually 

coalesce to form phosphorite crusts and more extensive pavements (figs. 4B and C; fig. 12, 

stages 3 and 4).
30

Figure 12. Hypothetical sequence of 

phosphorite crust formation on the Peru 

margin. Arrow with current velocity 

indicates active currents sweeping 

bottom in all but Stage 5. Otherwise, 

arrows indicate direction of diffusion of 

phosphate (PO4) in the sediments.

Stage 1 represents the initial condition 

of uncemented sediment. In Stage 2, 

the darker shading at the sediment 

surface illustrates the formation of a 

layer of friable, weakly cemented 

sediment (protocrust). In Stages 1 and 

2, vertical branching burrows of 

polychaete worms occur at the 

sediment-water interface. Dark shading 

in Stage 3 indicates a hardened 

phosphorite crust (D-type phosphorite of 

Garrison and Kastner, 1990). The 

protocrust is broken up by organisms 

then, in Stage 4, is exposed, cemented, 

and encrusted by epibionts and broken 

by large crabs. Burial of mature, 

multiple-generation D-type crust occurs 

in Stage 5 as currents slacken and 

sediment accumulates on top (lighter 

shading).
31

Bottom currents keep the friable protocrusts exposed on the seafloor. With time, the 

protocrusts become more strongly cemented, but throughout cementation, the protocrusts and 

well-cemented crusts retain abundant holes formed in the original soft sediment by burrowing 

polychaete worms (figs. 4A and B; fig. 12, Stage 3). Eventually, even the polychaete burrows 

become filled with cement, and the resulting phosphorite crust occurs in the form of hard nodules 

or slabs with a black manganese oxide coating (fig. 4B). Ions for cements and oxide coatings are 

supplied from the underlying suboxic sediments that contain up to 10% OC and have high 

measured benthic flux rates of phosphate, iron, manganese, and other ions out of the sediment.

We also observed slabs that had been broken into fragments and turned over by strong bottom 

currents and (or) organisms (fig. 4C; fig. 12, Stage 4). Such fragments must be cemented 

together to form observed phosphorite “conglomerate” layers and pavements. We commonly 

observed nodules and broken fragments of phosphorite crust that were cemented onto the upper 

and lower surfaces of a pavement. The development of the dense, “mature” crusts must require 

multiple episodes of shallow burial and exhumation (Glenn and Arthur, 1988; Garrison and 

Kastner, 1990). These mature crusts usually show many generations of breaking and 

recementation (fig. 12, Stage 4) to form phosphorite pavements up to 10 cm thick composed of 

nodules and fragments and extending continuously for at least several tens of square kilometers 

as observed during our submersible dives. Phosphorite pavements commonly overlie somewhat 

indurated sediment, and in areas of slack currents, are covered by a blanket several centimeters 

thick of flocculent sediment. The pavement usually cannot be penetrated with a gravity corer or 

box corer. On the 12°S transect, we recovered samples from continuous phosphorite crusts 

growing on bedrock of unknown age exposed on the top and face of a cliff (probably a fault 

scarp) at water depths between 600 and 800 m and associated with diffuse “cold seep” venting 

(crab and clam congregations around bubbling methane). We suspect that the phosphorite 

pavement is frequently swept clean by strong bottom currents. Shifts in the strength and location 

of bottom currents and the OMZ contribute to the jumbled pattern of sedimentary assemblages 

and accumulation rates that are typical of the Peru margin (Reimers and Suess, 1983).

Below the most intense part of the OMZ (below about 450 m), the upper surfaces of 

phosphorite pavements usually are covered with a dense population of epibionts whose hardparts 

often become cemented into the pavement. On a dive on the 13.5°S transect, at a water depth of 

595 m, we recovered a 3-cm-thick slab of dolomite-cemented sediment overlying glaucony with 

phosphorite nodules and fragments cemented on top of it. Original burrows and pore spaces 

between cemented phosphorite fragments are filled with sediment and cemented, helping to 

strengthen the pavement. Boring organisms continually riddle the surface of the pavement with 

holes, but these holes also are eventually filled with sediment and cement. Although we did not 

recover any phosphorite pavements or crusts buried by more than a few tens of centimeters of 

sediment, mature crusts (D-type phosphorite) have been recovered in ODP cores separated by up 

to several tens of meters of sediment (fig. 1; Garrison and Kastner, 1990). We suggest that the

phosphorite crusts and pavements on the Peru margin can only become buried and incorporated 

into the geologic record once bottom currents slacken sufficiently to allow sediment to 

accumulate (fig. 12, Stage 5).

Glaucony

The widespread deposits of glaucony sand at depths below about 750 m on the slope at 

13.5°S also indicate a bottom-current influence. The concentration of sand-sized glaucony pellets 

in the upper 10 to 35 cm of box cores recovered from these depths indicates that fine-grained 
32

material must have been winnowed from the deposit because it seems unlikely that relatively 

pure glaucony can precipitate in place without accumulation of fine-grained sediment. The 

glaucony sands directly overlie more clay-rich glaucony-bearing sediments in a number of the 

box cores recovered from 750 to 1,067 m on this transect, and as discussed above, the glaucony 

mica is presumed to have formed within those underlying clay-rich strata. Observations and 

measurements made during submersible dives, however, would suggest that strong currents are 

not currently active at these depths because the sediment surface is characterized by abundant 

tracks, trails, and mounds produced by burrowing organisms. Polychaete worms were also very 

abundant on the sediment surface. It is possible either that the activity of organisms is sufficient 

to rework and suspend fine-grained sediment that can then be transported by weaker bottom 

currents, or that currents in the recent past were stronger at this depth and concentrated the 

glaucony pellets as a lag deposit and are now sufficiently fast enough to prevent deposition of 

fine-grained particles.

Summary of Controls on Geochemical Facies Patterns

On the two transects studied, the outer shelf and upper slope are characterized by (1) mud 

or calcareous-phosphatic sand partly stabilized by bacterial mats; (2) benthic-foraminifera-rich, 

thick, organic flocs overlying indurated muds or crusts; (3) extensive authigenic (phosphorite 

and dolomite) mineral crusts and phosphatic biogenic debris; and (4) glaucony pellet sands. 

These four facies types are arrayed roughly in successive broad bands with increasing depth on 

the margin (fig. 5). The primary controls on facies distribution on the Peru margin are redox 

conditions and sediment transport and reworking by strong bottom currents. The concentration 

and preservation of organic matter on the Peru margin are strongly related to location and 

intensity of bottom currents as has previously been suggested by Suess and others (1987) and

further documented by Arthur and others (1998).

Gaps in sampling for geochemical analyses that can be noted in the OC depth profiles 

(figs. 6, 7, and 8) result from hard bottoms (mostly phosphorite crusts and nodule horizons) that 

could not be penetrated by push cores or box cores. Such a gap occurs in the 200- to 300-m depth 

interval on the 13.5°S transect where the currents are strongest (fig. 3B) and CFA cementation is 

greatest (fig. 8C). Sediments in the current-swept regions commonly are enriched in silt- and 

sand-sized phosphatic grains and benthic foraminifera. From the submersible, observers 

commonly noted high particulate concentrations in the near-bottom region, as well as traction 

transport of organic flocs and benthic foraminifera at the sediment-water interface at depths 

characterized by strong currents. The high concentrations of CaCO3 from 100 m to about 400 m 

on both transects (fig. 6) result from high benthic foraminifera production in OC-rich sediments 

accompanied by winnowing of fines and concentration of foraminifera tests.

Therefore, constant advection of DO in low concentrations by bottom waters, 

resuspension of surficial organic matter, activity of organisms, and transport to and from more 

oxygenated zones contributes to greater oxidation of organic matter than might be expected 

under a strong OMZ (Arthur and others, 1998). The winnowing and transport of finer grained

material results in lag deposits consisting of benthic foraminifera, terrigenous grains, and 

phosphatic sand over much of the upper slope that contain poorly preserved organic matter as 

indicated by low pyrolysis hydrogen indices (figs. 6B and E; Arthur and others, 1998). This

process also concentrates fine-grained, OC-rich sediments in certain sheltered regions of the 

upper slope and transports that material to deeper regions of the slope (Pak and others, 1980; 

Suess and others, 1987). Similar processes reportedly characterize the Oman margin where low
33

values of HI (220–320) and high C/N ratios in organic matter were attributed by Pedersen and 

others (1992) to winnowing (“hydrodynamic factors”) and attendant reworking of sediment that

lead to progressive degradation of organic matter (see also Cowie and others, 1999). On the

northwestern Mexican margin off Mazatlan, Ganeshram and others (1999) also found that values

of HI were higher in sediments on the slope (>300 m) than on the shelf (<300 m) where current 

winnowing resulted in greater degradation of organic matter.

Redox patterns are controlled by the initial flux of labile organic matter, which in turn is 

influenced by a complex function of surface productivity, water depth, current velocity, and 

water-column DO. The glaucony facies on the 13.5°S transect, for example, occurs below the 

lower boundary of the OMZ where marginally oxic conditions and low settling fluxes of OC 

create the proper conditions for glaucony genesis and concentration by progressive winnowing of 

fine-grained material. Formation of abundant CFA, on the other hand, requires a relatively 

continuous source of dissolved phosphate, probably preferentially released during degradation of 

organic matter supplied at high rates to the sediment-water interface by settling and 

concentration by traction currents. The same currents must periodically strengthen to sweep the 

bottom clean of fine-grained material in order to produce concentrations of phosphorite pellets 

(Baturin, 1982; Burnett and others, 1988; Glenn and Arthur, 1988) and expose phosphorite

crusts, which form over longer periods of time.

Conclusions

The sedimentary geochemical facies patterns on the Peru outer shelf and upper slope are 

primarily controlled by patterns of redox conditions and strength of bottom currents, both of 

which change through time. Redox has long been stressed as a control on sediment compositions 

in the Peru OMZ, but the importance of deep currents has not generally been recognized.

Redox patterns are a function of availability of labile organic matter, which is primarily 

related to the interplay between biologic productivity, water depth, and current winnowing.

Outer shelf environments near the top of the OMZ (depth <100 m) are characterized by relatively 

high sedimentation rates of fine-grained material, moderate OC contents, and low oxygen 

conditions within the sediments. Not surprisingly, these conditions give rise to relatively high 

rates of sulfate reduction just below the sediment-water interface, supporting a sulfide-oxidizing 

biota (for example, filamentous Thioploca bacterial mats) at the interface and leading to 

formation of pyrite within the sediments. Relatively minor amounts of phosphorite and glaucony 

form in these environments, and the sediments are somewhat enriched in biogenic opal from 

diatom debris. The presence of Thioploca helps to stabilize the sediments under moderate current 

velocities.

Precipitation of CFA primarily occurs where concentrations of labile organic matter are 

highest—within the core of the OMZ. As argued in previous studies, phosphate for CFA 

formation is supplied by preferential release of organic P during organic matter decomposition, 

by dissolution of fish debris, by the operation of an iron-redox phosphate pump, and perhaps in 

association with activity of sulfide-oxidizing bacteria. Thus, most CFA is probably formed near 

the sediment-water interface. The band of phosphate-rich sediments predominantly at depths of 

150–350 m extends at least from 12°S to 13.5°S, covering at least 1,500 square kilometers.

Below the base of the OMZ (depth >700 m), at least on the 13.5°S transect, the flux of 

labile OC is greatly diminished and conditions are sufficiently oxidizing at and near the 

sediment-water interface to support a benthic macrofauna. These organisms stir and mildly 

oxygenate the sediment, preventing major sulfate reduction in shallow pore waters. Glaucony 
34

pellets form in this environment, which is characterized by marginally oxic conditions at the 

sediment-water interface and enhanced silica availability from dissolution of biogenic opal. 

Glaucony sands were recovered only on the 13.5°S transect below a depth of 700 m, but their 

broader extent is suggested by Logvinenko and Romankevich (1973).

Although redox conditions determine the assemblage of authigenic minerals that 

precipitate and their geochemical properties, bottom currents are a major determinant of the 

relative concentration of authigenic phases in sediments on the Peru margin at present. The 

relatively high-velocity currents are most important as agents of winnowing and transport of 

fine-grained material such as clays and organic matter. Thus, the widespread phosphatic sand 

and phosphorite crusts at mid-depth in the OMZ and the glaucony sands below the lower 

boundary of the OMZ originate because of winnowing of clay and organic matter and resulting 

concentration of these relatively dense authigenic phases.

The predominantly authigenic origin of the sediments on the Peru margin and the 

importance of currents on slope sedimentation have important implications for sediment 

accumulation on other continental margins with strong OMZs and high organic productivity, 

such as Baja Balifornia (Mexico), Morocco, Oman, and southwest Africa.

Acknowledgments

We gratefully acknowledge the captain and crew of the R/V Seward Johnson and the 

pilots and crew of the DS/V Johnson Sealink II for their outstanding support during the 1992 

Peru expedition (SJ 1092). We thank R.A. Jahnke and C.R. Glenn for discussion and 

collaboration on aspects of Peru margin geochemistry. We also thank K.-C. Emeis, A.E.S. 

Kemp, J.R. Herring, D.Z. Piper, J. Dickens, G. Filippelli, M. Underwood, and J.A. Addison for 

helpful reviews of various drafts of the manuscript. This research was supported by the National 

Science Foundation (NSF OCE 9014801 to Arthur) and by the U.S. Geological Survey Global 

Change and Climate History Program (Dean).

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