BULK COMPOSITION OF SEDIMENTARY ORGANIC MATTER FROM LAMINATED SEDIMENTS IN THE OXYGEN MINIMUM ZONE OFF CENTRAL PERU: PRELIMINARY RESULTS OF THE PALEOPECES PROJECT

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BULK COMPOSITION OF
SEDIMENTARY ORGANIC
MATTER

FROM
LAMINATED SEDIMENTS IN THE OXYGEN MINIMUM ZONE OFF CENTRAL
PERU: PRELIMINARY RESULTS OF THE PALEOPECES PROJECT



Abdelfettah Sifeddine
(1)
, Dimitri Gutierrez
(2)
, Luc Ortlieb
(1)
, Federico Velazco
(2)
, Jorge

Valdes
(3)
,
Didier Keravis

(4)
, Gabriel Vargas
(5)
,Vicente Ferreira

(6)
, Pierre Soler
(7)



(1)

PALEOTROPIQUE, IRD, 32 Avenue Henri Varagnat, 93143 Bondy cedex, Francia

(2)

IMARPE, Callao, CP 22, Callao, Peru

(3)

Instituto de Investigaciones Oceanológicas, U
niversidad de Antofagasta, A. J. Guzman, Antofagasta, Chile

(4)

ISTO, UMR 6113 CNRS/Université d’Orléans, 1A rue de la Férollerie, 47072 Orléans Cedex 2 Francia

(5)

Departamento de Geología, Universidad de Chile, Plaza Ercilla, Santiago, Chile

(7)

CICESE,
C.P. 22860, Ensenada, B.C.N., México

(7)

LODYC, Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, Francia



ORGANIC MATTER AS A
PALEO
-
ENVIRONMENTAL MARKER

Organic matter is an important marker for the reconstruction of marine (and lacustrine)

paleoenvironmental changes. Allochtonous and autochtonous fractions of the organic matter provide
information about the evolution of ecosystems in the sedimentation basin and about physical and
chemical water column conditions. In the marine environment,
the degradation of organic matter
between the sea surface and the bottom sediment is a major process in which up to 90% of the organic
matter is affected, but the diagenetic processes involved do not alter the C/N ratio and the difference in
isotopic compo
sitions of the organic matter derived from algae and terrestrial plants (Meyers and Eddie,
1993; Meyers, 2003). The bulk organic characteristics like TOC and C/N ratio can be used,
respectively, to detect the amount of organic matter and its origin (Meyers
, 2003). The organic matter
originated from aquatic plants is rich in protein and does not exhibit the cellulosic structures which
characterize the terrestrial plants. C/N ratios differ from 4 to 9 in algae to greater than 20 in land
-
derived organic matter
. Rock
-
Eval parameters, described by Espitalié et al. (1977), provide useful
information about the amount and degree of preservation of the organic matter.

This study bears upon the characterization of the organic matter component of recent sediments from

the oxygen minimum zone of the Peruvian continental margin, with an aim to improve the
reconstruction of environmental changes in the last few centuries.


REGIONAL SETTING

The upper continental margin of central Peru is submitted to an intense biogenic
sedimentation
resulting from the high biological production in the surface waters. A major part of the sediments are
characterized by very high contents of organic matter in the surface layers. In addition, a shallow and
intense oxygen minimum zone (OMZ) i
ntercepts the continental margin, while strong gradients in
both bottom
-
water oxygen concentration and organic
-
matter input are observed (Levin et al., 1991;
Levin et al. 2000). The OMZ in the Peruvian upwelling regime intensifies from north to south becau
se
of cumulative oxygen consumption within the poleward
-
flowing undercurrent (Packard et al. 1983;
Codispoti, 1983). Fine sediments and organic carbon content tend to increase from north to south, in a
rough parallelism with the latitudinal trend of the ve
rtical and horizontal extension of the OMZ. Due
to the intensity of the biological pump and the features mentioned above, the central Peruvian
continental margin appears as a region of enhanced sedimentation and burial of organic carbon. It may
represent a

significant carbon sink in the ecosystem. Nevertheless, the export production and the actual
burial efficiency in the central Peruvian margin sediments have not yet been properly assessed.

This work reports organic geochemical measurements made along two

cores collected in the central
Peruvian continental margin, off Callao and Pisco respectively (Figure 1).
The study is part of the
collaborative PALEOPECES project (“
Reconstrucción del Registro Paleoceanográfico de alta
resolución en sedimentos de la zona

de mínimo oxígeno frente a la costa del Perú Central
”).
The
Project aims to reconstruct paloeproductivity, paleoxygenation and climate variations during the last
2000 years in relations with the dynamics of fish pelagic fish populations in a limited area
(11ºS
-

13ºS) of the Humboldt Current System.



Fig.1:
Site location map of the two studied cores.


Based on previous information (
Suess et al. 1990
, ODP Leg 686 (1986) and IMARPE unpublished
data), two

areas were selected in the continental shelf and upper slope off Callao (11º30’S
-
12º15’S)
and Pisco (13º15’S
-
13º45’S), at depths 100 to 400 m. An exploratory cruise was performed on board
the R/V Olaya, between May 20th to 24th, 2003, and a series of sedi
mentary cores were taken. The
morphologic and bathymetric study was carried out with a Simrad EK 500 38 kHz echosounder and
the Echoview software. In front of Callao the shelf is characterized with a low inclination down to 170
m, while off Pisco the trans
ition is smoother from the shelf to the slope and is locatd around a 350 m
depth.

The oxygen minimum zone was identified below 50 m depth, producing an environmental regime
varying from suboxic facies to extreme disoxic and anoxic facies. The sedimentary r
ecords obtained
by an Phleger gravity corer with an internal liner (6.3 cm internal diameter and 63 cm length) show
different kinds of structures: homogeneous, banded, or laminated layers, sometimes with nodules and
cross laminations. The best records of p
redominantly laminated layers were obtained in front of Callao
(253 m depth) and of Pisco (422 m depth). The Pisco core exhibited bioturbation features produced by
macrobenthic fauna (Polychaeta) which did not affect the record quality.


GEOCHEMICAL RESULT
S

The total carbon (TOC) and nitrogen (N) abundance were measured downcore on bulk sediment by
combustion with a Shimatzu CHN analyzer at IRD Geosciences laboratory
-
Bondy (France) and are
expressed as percentages of the weight of the total sediment samples

were measured. The Rock
-
Eval
parameters were performed by Rock
-
Eval 6 at ISTO Geochemical laboratory
-
Orléans (France).

Figure 2 shows the elementary (TOC, C/N, ST) and Rock
-
Eval results of the core collected at Callao.
Along this core, the variation of th
e organic parameters indicates, from bottom to top, between 31 to
23 cm, a decrease of TOC (13 to 7%), ST (1,5 to 1) and C/N (11 to 8), an increase of Hydrogen Index
(300 to 350), and a stability of the Oxygen Index (around 600). Between 23 and 15 cm, the
parameters
scarcely vary. From 15 to 5 cm, they tend to increase, except for the OI, reaching values around 11%
for TOC, 400 for HI, 9 for C/N and 1.2% for ST. In the most recent 5 centimeters, the values decrease
except for HI, which increases up to value
s around 500 at the top.

78.5 °W
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36
Huacho
Chancay
Callao
Pucusana
Cerro Azul
Pisco
Core 2

St. 1

(253 m)

Core 49

St. 31

(
422

m)



Figure2:

Total organic carbon (TOC) concentration, Hydrogen Index (mg HC/g TOC), Oxygen Index
(mg CO2/g TOC), molar C
organic
/N
total

ratios, and

13
C
org

and Total Sulfur (%) along the CAG 0320
-
02
core c
ollected of Callao.


In the core collected at Pisco, all the geochemical parameters show more stable values along the core,
except for TOC content, which shows variation around 8% between 39 and 22 cm, 10% from 22 to 10
cm and tends to increase reaching 14
% at the top (Figure 3). The C/N and ST show stable values
between the bottom to 20 cm and decrease to the top with small variations. Except within a small
interval (10 to 8 cm) the HI and OI values stay stable along the core.


Figure 3:

Total organic carbon (TOC) concentration, Hydrogen Index (mg HC/g TOC), Oxygen Index
(mg CO2/g TOC), molar C
organic
/N
total

ratios, and

13
C
org

and Total Sulfur concentrations along the PIGC
0305 24
-
49 core collected off Pisco.


0
5
10
15
20
25
30
35
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8
9
10
11
12
13
14
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350
400
450
500
0
200
400
600
800
1000
6
7
8
9
10
11
12
0,9
1
1,1
1,2
1,3
1,4
1,5
TOC (%)
IH (mg HC/g COT)
OI (mg CO
2
/g COT)
ST
Depth (cm)
C/N
0
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C/N
0,7
0,8
0,9
1
1,1
1,2
1,3
1,4
1,5
S
TOC (%)
TOC (%)
TOC (%)
TOC (%)
HI (mg HC/g COT)
OI (mg CO
2
/g COT)
CN
ST (%)
Depth (cm)
INTERPRETATION

The

elementary geochemical parameters of the recent laminated sediments, accumulated on the shelf
off Callao and Pisco, in areas influenced by the Oxygen Minimum Zone, indicate that the organic
matter is principally autochthonous (Figure 2 and 3). The values
of Hydrogen Index (HI) vs. Oxygen
Index (OI) values plotted on a pseudo
-
Van Krevelen diagram (Tissot and Welte, 1984) indicate that the
organic matter, accumulated in both regions, is of type II (phytoplanctonic origin), immature and more
oxidized in the C
allao zone (Fig. 4). This suggests a control by the physical and chemical characteristics
of the OMZ in this zone.














Figure 4: Hydrogen Index vs. Oxygen Index for both cores, compared with those of core from ODP
site 686.


These preliminary re
sults on the organic matter parameters need to be compared with other
geochemical, petrographical and biomarkers indicators, presently under study. Clearly, it is only
through a multi
-
proxies approach that reconstructions of the evolution of paleoproductiv
ity and paleo
-
oxygenation conditions will be reached.


Cited references

Codispoti, L.A. 1983 . On nutrient variability and sediments in upwelling regions. In Coastal Upwelling IV/:10a
« Its Sediment Record », E. Suess and J. Thiede Eds, pp 125
-
146.

Espita
lié, J., Laporte, J.L., Madec, M., Marquis, F., Leplat, P., Paulet, J., Boutefeu, A. 1977. Méthode rapide de
caractérisation des roches mères, de leur potentiel pétrolier et de leur degré d’évolution. Revue de l’Institut
Français du Pétrole, 32 (1): 23
-
42
.

Levin, L., C.L. Huggett & K.Wishner, 1991. Control of deep
-
sea benthic community structure by oxygen and
organic
-
matter gradients in the eastern Pacific Ocean. Journal of Marine Research 49: 763

800.

Levin, L., J. Gage, C. Martin & P. Lamont, 2000. Macro
benthic community structure within and beneath the
oxygen minimum zone, NW Arabian Sea. Deep
-
Sea Research II 47: 189

226.

Meyers, P., 2003. Applications of organic geochemistry to palaeolimnological reconstructions: a summary of
examples from the Laurenti
an Great Lakes. Organic Geochemistry 34: 261
-
289.

Packard, T.T., Garfield, P.C. & L.A. Codispoti, 1983. Oxygen consultation and denitrification below the
Peruvian upwelling. In: Coastal Upwelling IV/:10a « Its Sediment Record », E. Suess and J. Thiede Eds
, pp
147
-
174.

Suess, E., R. Von Huene et al. 1990. Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 112,
738 p.

Tissot, B., Welte, D. 1984. Petroleum formation and occurrence. Second ed., Springer Verlag, Berlin. 699 pp.

0
100
200
300
400
500
600
700
800
0
200
400
600
800
1000
Callao
Pisco
ODP Site
686
IO (mg CO2/g C.Org
IH (mg HC/g C.Org)