Spatial and temporal distribution of biogenic carbonate and opal in

Earth and Planetary Science Letters 174 (1999) 59^73
www.elsevier.com/locate/epsl
Spatial and temporal distribution of biogenic carbonate and
opal in deep-sea sediments from the eastern equatorial Paci¢c:
implications for ocean history since 1.3 Ma
M.E. Weber *, N.G. Pisias
Oregon State University, College of Oceanic and Atmospheric Sciences, 104 Ocean Adm Bld, Corvallis, OR 97331-5503, USA
Received 11 December 1998; accepted 1 October 1999
Abstract
High-resolution records of glacial^interglacial variations in biogenic carbonate, opal, and detritus (derived from
non-destructive core log measurements of density, P-wave velocity and color; r v 0.9) from 15 sediment sites in the
eastern equatorial (sampling resolution is V1 kyr) clear response to eccentricity and precession forcing. For the Peru
Basin, we generate a high-resolution (21 kyr increment) orbitally-based chronology for the last 1.3 Ma. Spectral analysis
indicates that the 100 kyr cycle became dominant at roughly 1.2 Ma, 200^300 kyr earlier than reported for other
paleoclimatic records. The response to orbital forcing is weaker since the Mid-Brunhes Dissolution Event (at 400 ka).
A west^east reconstruction of biogenic sedimentation in the Peru Basin (four cores; 91^85³W) distinguishes equatorial
and coastal upwelling systems in the western and eastern sites, respectively. A north^south reconstruction perpendicular
to the equatorial upwelling system (11 cores, 11³N^8³S) shows high carbonate contents (v 50%) between 6³N and 4³S
and highly variable opal contents between 2³N and 4³S. Carbonate cycles B-6, B-8, B-10, B-12, B-14, M-2, and M-6 are
well developed with B-10 (430 ka) as the most prominent cycle. Carbonate highs during glacials and glacial-interglacial
transitions extended up to 400 km north and south compared to interglacial or interglacial^glacial carbonate lows. Our
reconstruction thus favors glacial^interglacial expansion and contraction of the equatorial upwelling system rather than
shifting north or south. Elevated accumulation rates are documented near the equator from 6³N to 4³S and from 2³N to
4³S for carbonate and opal, respectively. Accumulation rates are higher during glacials and glacial^interglacial
transitions in all cores, whereas increased dissolution is concentrated on Peru Basin sediments close to the carbonate
compensation depth and occurred during interglacials or interglacial^glacial transitions. ß 1999 Elsevier Science B.V.
All rights reserved.
Keywords: carbonate sediments; opal; Paci¢c Ocean; productivity; solution
1. Introduction
* Corresponding author. Present address: Ocean Mapping
Group, Dept. of Geodesy and Geomatics Engineering, University of New Brunswick, Fredericton, N.B. E3B 5A3, P.O.
Box 4400, Canada. Tel.: +1-506-453-4684;
Fax: +1-506-453-4943; E-mail: [email protected]
Biogenic carbonate and opal are important paleoceanographic proxies in the Paci¢c Ocean. The
development of automatic core logging devices
has made it possible to rapidly, non-destructively,
and continuously determine acoustical, physical,
0012-821X / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 4 8 - 4
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M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
Fig. 1. Location map. Cores of this study are from two cruises with R.V. Sonne (SO-79 [14] and SO-106 [16]) and from ODP
Leg 138 [13]. Water depth is contoured. EUC, Equatorial Undercurrent; SEC, South Equatorial Current; CHC, Chile Current;
PC, Peru Current; NEC, North Equatorial Current; NECC, North Equatorial Countercurrent.
and optical properties which provide an indirect
measure for the major biogenic sediment components in sediment cores. Data provided are of
high resolution and quality. Consequently, there
have been signi¢cant e¡orts to estimate the contents of major sediment components from quasicontinuous log data sets (e.g. [1^5]). The advantages of these estimates are that laboratory work
can be reduced and sampling resolution is increased signi¢cantly.
In Paci¢c Ocean sediments, climate variations
of carbonate and opal contain the major information about productivity in surface waters [6] and
dissolution in bottom waters [7,8] whereas the
non-biogenic residual of the two components,
which includes detrital material, clay minerals,
quartz, etc., may provide information about bottom water activity [9] and atmospheric circulation
[10]. In the eastern equatorial Paci¢c (EEP),
changes of wet bulk density and grain density
mainly describe variations in the contents of bio-
genic carbonate and opal [1,2,11]. Furthermore,
changes of color follow carbonate highs and
lows [12].
In this paper, we will apply carbonate and opal
estimates derived from non-destructive measurements of color, density, and velocity by Weber
[4] to two core transects in the EEP in order to
evaluate productivity and dissolution. Here, atmospheric and oceanic circulation lead to an
east^west elongated zone of high surface ocean
productivity near the equator and o¡ Peru [13].
The east^west transect provides new information
about Peru Basin paleoceanography. The north^
south transect crosses all major oceanic fronts of
the equatorial upwelling system (Fig. 1) and allows, for the ¢rst time, the reconstruction of spatial and temporal distribution pattern of biogenic
components on glacial^interglacial time scales, including the response to climate forcing across
both the northern and southern boundaries of
high productivity.
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2. Material and estimation strategy
The sediment cores used in this study were collected during two cruises of R.V. Sonne to the
Peru Basin in 1992 (SO-79, [14]) and in 1996
(SO-106, [4]), and during ODP Leg 138 [13].
Gamma-ray density and P-wave velocity were determined using a Multi-Sensor Core Logger (SO106), and a Minolta Chromatometer (SO-79 and
SO-106) was used to measure sediment color
(lightness). All ODP densities (measured with a
gamma-ray porosity evaluator) were ¢rst reduced
by 6^9% according to an iteration procedure described in Weber et al. [3] to account for the
GRAPE density error reported by Moran [15].
We developed a strategy to estimate contents of
biogenic carbonate and opal as well as the detrital
fraction of the sediment from non-destructive core
log measurements. The estimation procedure, its
limitations as well as potential pitfalls and errors
are described in Weber [4].
All non-destructive data from the Peru Basin
were calibrated to 2600 carbonate and 1100 opal
measurements carried out on discrete samples.
For all ODP sites, we calibrated the GRAPE
data to discrete sample data reported in Pisias et
al. [13]. As a result, biogenic carbonate and opal
can be estimated from core log measurements
with a precision of r = 0.9 to 0.96, i.e. 81^93% of
the density, velocity, and color variance explain
the variance in biogenic carbonate and opal contents. We applied the estimation strategy to 32
cores from the Peru Basin and to 11 sites of
ODP Leg 138. All estimated carbonate, opal,
and detrital contents of the two core transects
reported here are displayed in Fig. 2.
3. Stratigraphy
Physical properties play an increasingly important role in stratigraphic studies of marine sediments (e.g. [16]). High-resolution time scales are
constructed by relating variations of physical
properties to variations of orbital parameters
[17,18]). For the Late Neogene, open marine sediment cycles can be con¢dently calibrated to orbital cycles since the time control is very good (e.g.
61
[19]). Astronomically tuned physical property records are excellent tools for Paci¢c paleoclimatic
reconstructions [17], providing a very high chronostratigraphic resolution.
We developed an orbitally calibrated chronology for Peru Basin sediment cores for the last 1.3
Ma based on £uctuations of estimated carbonate
contents (sampling resolution is V1 kyr). One
problem was to select an appropriate tuning target. Since Arrhenius [20] and Hays et al. [21], we
know that the Paci¢c carbonate cycles provide
excellent tools for stratigraphic correlation. The
basic stratigraphic information for Pleistocene
sediments from the Peru Basin is provided by
cores 77 and 9KL [14] which were dated using
stable oxygen isotope stratigraphy (a moderateresolution composite record of Globigerinoides
sacculifer and Orbulina universa in core 77KL),
magnetostratigraphy, and biostratigraphy. Meanwhile, N18 O stratigraphy also exists for cores 184
and 243KL (data will be published elsewhere).
Accordingly, carbonate lows and highs revealed
a dominant 100 kyr pattern. We used this information to generate an initial low-resolution chronology by correlating all large-amplitude carbonate highs to minima in eccentricity (example
see Fig. 3, lower part). This ¢rst step only served
graphical correlation purposes. The strong coherency of carbonate to orbital precession throughout the Pleistocene [14] favors this orbital frequency as a potential target for a tuning of
carbonate sub-cycles. To be consistent with the
stratigraphy of ODP Leg 138 [17], we have chosen
the mean June insolation at 65³N (data from [22])
which re£ects mostly orbital precession [19]. This
tuning procedure links carbonate maxima to insolation maxima and thereby assumes that there
is no phase shift between forcing and response
[17].
We began the tuning with high-resolution carbonate records (derived from sediment lightness
and density) on those cores having a high-resolution age control from N18 O (77, 184, and 243KL).
The N18 O age control points (age scale see [23]) of
the three cores were used to limit shifting of individual carbonate sub-cycle peaks from one insolation maximum to another under the condition
that the insolation-tuned ages of individual carbo-
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M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
nate peaks are similar in all cores. Thus, the time
scale di¡erence between insolation-tuned and
N18 O chronology is always 91 precession cycle,
usually within a few kyr. Minor di¡erences in
both isotopic and insolation-tuned chronology
may still occur as shown by Farrell et al. [24]
for high-sedimentation rate ODP site 847, but
we did not apply any further correction because
the isotope sample resolution for Peru Basin cores
is lower (5^15 kyr) and the preservation of foraminifera is poorer. We applied the tuning strategy
to all cores of cruises SO-106 and SO-79 used in
this study. In practice, the application of our
strategy consisted of a number of iterations for
each tuning step. In general, it proved to be easier
to work with slightly smoothed carbonate records.
In all cores, the last appearance of Pseudoemiliania lacunosa (NN19/20 boundary) within the isotopic stage 12 [14] and, where available, the paleomagnetic boundaries Brunhes/Matuyama and
the top and the base of the Jaramillo (ages according to Cande and Kent [25]), served as independent stratigraphic control points (see Fig. 2).
For all cores examined in the time domain, the
age depth control points are given in Table 1.
4. Response to orbital forcing
The core records from the Peru Basin provide
the opportunity to examine how the carbonate
system of this section of the Paci¢c responded
to orbital forcing and global climate change.
For this purpose, both a detailed stratigraphy
and a representative signal are basic prerequisites.
Thus, after the tuning, we calculated a stacked
carbonate record for the time period 0^1.3 Ma
63
from cores 71, 77, 184, and 243KL for spectral
analysis (the stratigraphic resolution in core
164KL is too low and core 286KL has a hiatus
in the upper part). Then we examined the response to orbital forcing by analyzing evolutionary spectra, a technique introduced by Mayer et
al. [26]. Using a 300 kyr window, we calculated 34
individual spectra (see arrows in Fig. 4) by shifting the window by 10% of the series length (30
kyr) from one analysis to another. Then we generated an evolutionary spectrum by contouring
levels of equal spectral power in constant intervals
(the higher the energy, the darker the color).
Two frequencies document the response of the
carbonate system in the Peru Basin to orbital variations, the eccentricity cycle centered around 100
kyr which is the dominant cycle, and the precession cycle centered around 21 kyr. Two important
¢ndings should be pointed out. First, carbonate
spectra older than carbonate cycle B-10 (430 ka,
see [14]) show strong coherency to orbital variations, whereas the younger part of the stacked
record has only weak spectral power in both eccentricity and precession band. This change in
cyclicity occurs at the so-called `Mid-Brunhes Dissolution Event' (350^450 ka, [6,27]) which seems
to be a global phenomenon and which may be
indicative of a long-term deviation in the amount
of deep-sea carbonate preservation [28]. In the
Peru Basin, this event is documented as a principal change in sedimentation patterns [14].
The second important ¢nding is that the dominance of the 100 kyr cycle in carbonate variations
starts at roughly 1.2 Ma. Carbonate records from
the Peru Basin covering the period older than that
(lower Matuyama), e.g. 286KL (this study) or
9KL [14] indicate that carbonate has signi¢cantly
6
Fig. 2. Sediment composition in the eastern equatorial Paci¢c for a west^east core transect (top) and two north^south core transects (center and bottom). The three major sediment components biogenic carbonate, biogenic opal, and detritus are estimated
from non-destructive measurements (methodology see [4]) of sediment color (L, lightness component) determined with a Minolta
Chromatometer CR-200, and gamma-ray density (D) determined with a Multi-Sensor Core Logger for Peru Basin cores and with
a gamma-ray porosity evaluator for ODP sites. Measurement increment is 1 cm for Peru Basin sediments and 1^3 cm for ODP
sites [13]. Correlation line refers to carbonate cycle B-10 (0.43 Ma [14]); * refers to the last appearance datum of P. lacunosa
(NN19/20 boundary) within both the isotopic stage 12 and carbonate cycle B-10; B/M is Brunhes/Matuyama magnetic boundary;
J gives duration of the Jaramillo. Cores are shown versus depth for the last 1.3 Ma if not indicated di¡erently. Note that some
depth scales are triple reduced (TR) or triple enlarged to ease comparison. All curves are slightly smoothed using a three-point
Gaussian ¢lter.
EPSL 5291 9-12-99
64
M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
Table 1
Age models for Peru Basin sediments
Table 1
Age models for Peru Basin sediments
Time
(ka)
71KL
77KL
164KL 184KL 243KL 286KL
Time
(ka)
71KL
77KL
0
12
37
59
84
105
117
128
151
176
199
209
220
244
266
293
314
335
355
371
380
388
398
411
428
448
464
486
496
506
532
557
568
579
600
622
651
672
683
694
714
736
751
760
769
788
808
826
836
845
866
886
0
0
0
11
22
37
49
61
84
112
142
183
20
31]
54
27
1005
1032
1102
87
98
114
141
188
209
1084
1146
1189
1218
1258
1137
1185
1222
1249
1273
1302
1317
1328
1300
293
327
358
372
107
121
159
199
226
241
254
300
348
376
419
442
467
1378
1355
1381
1392
114
393
499
907
928
939
959
981
1001
1015
1032
1053
1074
1095
1105
1115
1129
1149
1168
1190
1207
1227
1245
1267
1285
1427
118
206
236
419
434
457
504
508
523
540
553
567
592
607
624
653
683
705
731
755
805
833
858
889
916
946
985
525
538
548
567
590
616
627
637
659
677
690
709
729
745
760
772
792
811
837
877
902
909
922
946
978
994
1006
1017
1044
1065
0
258
280
309
338
369
392
37
39
43
47
48
53
55
59
61
67
73
77
81
411
423
431
440
452
464
486
499
510
530
570
582
596
615
639
695
709
728
748
767
772
787
801
824
857
878
905
924
0
13
29
57
95
114
135
148
184
218
234
245
254
281
299
327
359
379
392
408
438
447
454
464
548
566
598
614
629
643
664
700
726
733
739
757
782
811
827
843
880
908
929
939
948
967
993
0
9
14
25
55
81
103
114
118
122
133
142
153
169
209
216
232
254
272
280
286
292
295
331
338
353
372
400
425
453
1324
1331
164KL 184KL 243KL 286KL
85
92
101
108
111
120
938
948
1011
1030
976
991
1004
1020
1047
1070
1049
1065
1078
1098
1121
1142
1166
1188
1198
1205
1114
1144
1157
1178
1187
1207
1244
1237
1261
467
486
495
513
535
549
558
565
586
599
634
656
676
692
714
742
755
774
788
805
816
Downcore records of carbonate (estimated from non-destructive measurements of sediment color and density) were tuned
to maxima of the mean June insolation at 65³N (data from
[22]) with respect to the isotopic age control points of cores
77KL [14], 184 and 243KL, the Brunhes-Matuyama boundary, the top and the bottom of the Jaramillo (ages according
to [25]) as well as several biodatums (see Fig. 2 and [14])
using the ANALYSERIES software of Paillard et al. [36].
Bold numbers are minima in insolation and carbonate which
were used as additional tuning targets.
less or even no spectral power concentrated in the
eccentricity band prior to 1.2 Ma, whereas the 21
kyr cycle as a response to precession forcing is
present prior to 1.2 Ma. According to Keir and
Berger [29] and Groetsch et al. [30], the Paci¢c
deep-sea carbonate system shows strongly response to the rate of change in sea level, leading
to dissolution maxima during glaciations and to
better preservation during deglaciations. Thus, the
dominance of the 100 kyr cycle since 1.2 Ma in
the Peru Basin is surprising since the 100 kyr
dominance is usually thought to occur later (e.g.
as seen in isotopic records), to develop slowly,
and to govern since roughly 900^800 kyr (e.g.
[31^33]), when ice volume changes became large
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M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
65
Fig. 3. Stratigraphic concept for this study displayed for core 243KL (carbonate contents are estimated from sediment color).
First, the midpoints of carbonate cycles (B and M numbers refer to Brunhes and Matuyama cycles, respectively) are correlated
to minima in eccentricity on a low-resolution time scale (lower curves). Then, maxima of carbonate sub-cycles are tuned to maxima of the mean June insolation at 65³N (data from [22]). Tuning steps were made with respect to isotopic age control points using the ANALYSERIES software of Paillard et al. [36]. Small vertical bars refer to tuning points. Carbonate estimates were
smoothed using a ¢ve-point moving average.
enough to have the dominant in£uence on global
climate (e.g. [23,34]). Nonetheless, at that time the
amplitude of the 100 kyr cycle also increased in
the Peru Basin.
Strong response to orbital precession forcing is
evident until the Mid-Brunhes Event. Seasonal
insolation changes in low latitudes are most affected by variations in orbital precession [35].
The 21 kyr cyclicity thus re£ects the low-latitude
forcing on the carbonate system. Generally, response of carbonate to both eccentricity and precession is consistent with ¢ndings from the western Paci¢c [30]. High-latitude forcing, as would be
expected from the response to the 41 kyr cycle of
obliquity, cannot be observed in the carbonate
record of the Peru Basin (see also [14]), although
this cyclicity is documented for high-productivity
sites to the north and northwest.
5. Spatial and temporal distribution
The study of non-destructive data from Peru
Basin sediment cores shows that the downcore
variations of biogenic sediment components contain a clear response to orbital forcing, which, in
turn, was used to develop a high-resolution chronology in combination with other stratigraphic
evidence. The inferred high-resolution chronostratigraphic control (tie points each 21 kyr) is the
most important prerequisite to examine the spatial and temporal distribution of biogenic sedimentation on glacial-interglacial time scales. Calculation of mass accumulation rates requires
additional knowledge about dry bulk density
and sedimentation rate. Instead of using scattered
empirical correlations to calculate dry bulk densities, we applied an iterative method that is based
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66
M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
Fig. 4. Evolutionary spectra of stacked carbonate record 71, 77, 184, and 243KL for the time period 0^1.3 Ma (methodology see
[26]). Individual sites (sample resolution V1 kyr) were tuned to variations in orbital insolation using the ANALYSERIES software [36], re-sampled in 1 kyr steps, and pre-whitened (0.95) in order to reduce the impact of low-frequency variations. Individual spectra (arrows on the right) are estimated with a 300 kyr window (300 data points; 90 lags; Tuckey window, 80% con¢dence
level) which was o¡set by 10% of the series length (30 kyr) from one analysis to another. Spectra are thus calculated for 0^300,
30^330 kyr, etc. (34 spectra in total). Levels of equal spectral power are contoured in constant intervals. Power scale is arbitrary.
Note dark areas of high spectral power near the 100 kyr eccentricity cycle as well as near the 19 and 23 kyr precession cycles.
on physical relationships, a very precise method
with an error 9 2% (see [3]). Sedimentation rates
are calculated by using the tie points of Table 1
and those of Shackleton et al. [17]. Instead of
calculating linear sedimentation rates between tie
points which produces many spiky and unrealistic
features, we used the smoothed cubic spline function [36] in order to obtain a smoothed continuous sedimentation rate record.
5.1. West^east transect south of the South
Equatorial Current
The sea£oor of the Peru Basin reveals an abyssal hill topography with water depths ranging
mainly between 4000 and 4300 m. The carbonate
compensation depth (CCD) is between 4200 and
4250 m [14]. Accordingly, the sediment composition strongly depends on water depth. In addi-
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M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
tion, possible winnowing of material on small hills
and focusing in troughs alter accumulation rates
[37]. Nevertheless, we can delineate a general sedimentation pattern for a west^east core transect
(91³W to 85³W) south of the South Equatorial
Current (Fig. 2, top).
Sites 71, 77, and 184KL in the western Peru
Basin (Figs. 1 and 2) are located immediately
south of the South Equatorial Current, showing
sedimentation rates of V1 cm/kyr and accumulation rates of V400 mg/cm2 /kyr. The sea£oor is
located just above the CCD and sediment cores
usually show large-amplitude carbonate cycles
with dissolution characteristics during carbonate
lows [14]. Cores 206 and 251KL from the central
Peru Basin are further southeast, away from the
South Equatorial Current and show less in£uence
of surface ocean productivity (sedimentation rates
V0.5 cm/kyr; MARs 9 150 mg/cm2 /kyr) and lower-amplitude carbonate dissolution. Far to the
east, core 254KL has the highest sedimentation
rates (V2.5 cm/kyr) and accumulation rates
(V900 mg/cm2 /kyr). The higher input of biogenic
components is associated with increased productivity due to the intensi¢ed upwelling o¡ Peru.
Furthermore, the facts that (i) average gammaray densities are relatively high despite low carbonate contents, (ii) the relation between carbonate
and opal is as in the western areas, and (iii) the
correlation coe¤cients for carbonate and density
are relatively poor (see also [4]), indicate an additional detrital component (presumably of eolian
origin) in the eastern site close to the South American continent. Thus, downcore variations of the
major sediment components as well as accumulation patterns clearly distinguish di¡erent oceanic
regimes along the four cores from the west^east
transect.
5.2. South^north transect across the equatorial
upwelling system
Steep gradients in sediment composition occur
along two north^south transects at 93³W (8³S to
the equator) and at 110³W (equator to 11³N). We
combined the two transects in order to obtain a
full coverage of the equatorial upwelling system
and its northern and southern boundaries (Fig.
67
5A^C). The combined transect crosses all major
oceanic fronts and connects areas of very low
productivity (e.g. site 164KL, ODP site 854) to
those of high productivity near the equator
(ODP sites 846, 847, 850, and 852). For Peru Basin sediments, we applied the above described orbitally-based chronology ; for all ODP sites, stratigraphic control points were taken from age
models provided by Shackleton et al. [17]. The
temporal resolution of the data sets is roughly
0.7 kyr for ODP sites 846 and 847, 1 kyr for cores
243, 286, and 77KL, and for ODP sites 850 and
852, 2.5 kyr for ODP sites 851 and 854, and 4 kyr
for core 164KL and for ODP site 853.
The general spatial trends can be delineated as
follows. Carbonate contents are high (v 50%)
near the equator between 6³N and 4³S. The
southern boundary is sharp with large-amplitude
£uctuations and lower average contents south of
4³S (Peru Basin). We will refer to this boundary
as the `4³S boundary', although we are aware that
this boundary may be anywhere between 3³S and
5³S (Fig. 5A). Between cores 243KL and 77KL,
large-amplitude £uctuations dominate, whereas
south of approximately 7³S, carbonate contents
are usually very low and seldom exceed 10%
(core 164KL). The northern boundary is more
di¡use with continuously decreasing carbonate
contents between ODP sites 852 and 854. Detrital
contents show the opposite spatial distribution
(Fig. 5C) with v 70% in the far south, 9 20% in
the center, and v 40% north of 8³N. Opal (Fig.
5B) is enriched and highly variable between 4³S
and 2³N. Opal contents are slightly lower in the
Peru Basin (10^15%) with an increase to the
south. Very low opal concentrations are documented between 2³N and 8³N (9 10%), followed
by a slight increase to the north.
There are two reasons for the sharp boundary
of all major sediment components at 4³S. First,
the southern end of the South Equatorial Current
(Fig. 1) apparently lies between ODP site 846 and
core 243KL (and has been there during the entire
reconstruction time of 1.3 Ma), separating the
equatorial upwelling system from the lower-productivity Peru Basin, i.e. carbonate and opal contents are higher relative to detrital contents near
the equator. Second, water depth increases to the
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south whereas the CCD is shoaling in the same
direction (core 164KL is at or slightly below the
present CCD ; cores 77, 286, and 243KL are between the CCD and the lysocline [14]; ODP sites
846 and 847 [2] are at the lysocline) so that increasing carbonate dissolution in the Peru Basin
results in increasing detrital and opal contents relative to carbonate contents to the south.
Boundaries for high opal and carbonate contents di¡er north of the equator (Fig. 5A, B).
Water depth is quite similar along ODP sites
850 through 854 (Fig. 2, bottom) and all cores
are located between the lysocline and the CCD
[2]; i.e. preservation for carbonate should not differ signi¢cantly. Thus, the gradual decrease in carbonate north of 6³N is probably related to decreasing carbonate productivity. The distribution
of opal contents along the entire transect resembles the present-day spatial pattern of silicate contents in near-surface waters of the EEP [39]. Silicate is highest in the upwelling cell o¡ Peru and
along the equator. Accordingly, the eastern part
of the transect (Fig. 5B; core 164KL through
ODP site 847) reveals higher opal contents,
whereas in the western part, opal contents are
much lower (ODP sites 851 through 854 with
the exception of ODP site 850 close to the equator), with a sharp boundary at 2³N.
In all sediment components, strong temporal
variations are documented. Individual carbonate
cycles correlate well (even if absolute contents
may di¡er) across the entire transect. Our carbonate reconstruction shows that the zone of high
carbonate concentration apparently expanded up
to 400 km north and south during glacials and
glacial-interglacial transitions compared to interglacials and interglacial-glacial transitions (Fig.
5A, glacials are indicated at top using dark gray
69
bars). Since carbonate and opal contents are inversely correlated [4], opal contents show the opposite temporal pattern. Together, all major sediment
components
argue
for
recurring
oceanographic and atmospheric circulation in
the EEP upwelling zone. Importantly, the largeamplitude carbonate variations south of the equatorial upwelling system, where carbonate lows are
primarily dissolution driven, ¢nd their counterpart in ODP site 852 north of the equatorial upwelling system, where carbonate concentrations
resemble (at slightly lower amplitude) the temporal pattern of highs and lows detected in Peru
Basin sediments. Therefore, an important conclusion of this study is that the EEP upwelling system contracted and expanded on glacial-interglacial time scales through the last 1.3 Ma rather
than shifted north or south.
Carbonate cycles B-6, B-8, B-10, B-12, B-14,
M-0, M-2, and M-6 show a well-developed latitudinal extension (Fig. 5A). Carbonate cycle B-10
which is an important phenomenon in the paleoceanography of the Peru Basin [14], has the most
signi¢cant imprint on all sediment components
(Fig. 5A^C) and represents the strongest Quaternary productivity and/or preservation signal.
Since carbonate cycle B-10, carbonate highs successively became lower during cycles B-8 and B-6,
resulting in the lowest carbonate and highest opal
and detrital contents since carbonate cycle B-4
(roughly isotopic stage 5; Fig. 5A^C).
Interpretations from concentration pro¢les are
limited because they re£ect the interplay of individual sediment components. In order to evaluate
the distribution of £uxes, we converted the reconstruction along the core transect into MARs (details about the strategy to produce reliable results
are described above). The spatial distribution of
6
Fig. 5. Reconstruction of the spatial and temporal distribution of major sediment components for a north^south (top to bottom)
transect of 11 cores in the eastern equatorial Paci¢c since 1.3 Ma. Age increases from left to right (gray shades at top refer to
glacial isotopic stages). Carbonate (A), opal (B), and detrital contents (C) are contoured in 9, 2.5, and 7% increments, respectively. All contents are derived from non-destructive measurements of sediment color (cores 77 and 164KL) and gamma-ray density (other cores). B and M numbers refer to Brunhes and Matuyama carbonate cycles, respectively. End refers to the core base.
All data were smoothed using a three-point moving average and re-sampled in uniform 3 kyr intervals. Note that biogenic contents are high north of 4³S with di¡ering northern boundaries for carbonate (8³N) and opal (2³N). Note further the huge phenomenon of all records during carbonate cycle B-10 that culminates at roughly 0.43 Ma.
EPSL 5291 9-12-99
70
M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
Fig. 6. Reconstruction of the spatial and temporal distribution of total mass accumulation rates and opal accumulation rates
(both displayed on a logarithmic scale) along the core transect described in Fig. 5 (legend see also therein). Note that periods of
high MAR are concentrated on carbonate maxima (cycles B-2, B-4, B-6, B-8, B-10, B-14, M-0, M-2, M-4, M-6, and M-8).
total MARs (Fig. 6A) shows £uxes of 800^5000
mg/cm2 /kyr for the high-sedimentation rate sites
within the equatorial upwelling system between
4³S and 6³N with sharp northern and southern
boundaries, where total MARs decrease to
9 300 mg/cm2 /kyr. During times of increased total MARs, the equatorial upwelling system expanded up to 100 km to the south and up to
300 km to the north. Carbonate MAR (not
shown) almost exactly mimics the total MAR pattern, especially in the heart of the equatorial upwelling system (ODP sites 846 through 852) where
it comprises 70^75% of the total MAR. The temporal variation indicates (up to 4 times) higher
£uxes during glacials and glacial-interglacial transitions and lower £uxes during interglacials and
interglacial-glacial transitions, i.e. high carbonate
£uxes coincide with high carbonate contents. In-
EPSL 5291 9-12-99
M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
dividual £ux maxima (e.g. during carbonate cycles
B-4, B-6, B-8, B-10, B-14, B-16, M-0, M-2, M-4,
M-6, and M-8) correlate across the transect
although carbonate MARs are 5 to 10 times higher within the equatorial upwelling system. Exceptions may be sites to the far north and south,
where £uxes are extremely low and the low-resolution age models (Table 1 and [17]) do not allow
a detailed reconstruction on glacial-interglacial
time scales.
High opal MARs (v 100 mg/cm2 /kyr) are restricted to a relatively narrow band between
ODP sites 846 and 850 (Fig. 6B). Opal £uxes
were usually increased during times of high carbonate £uxes which clearly points to productivity
as a major trigger. Since common sedimentation
rate variations can be traced through most of the
sites, they are unlikely an artifact of the di¡erent
stratigraphic concepts applied here. Only during
times of very high opal £uxes (usually glacials and
glacial-interglacial transitions), the zone of high
opal productivity expanded up to 100 km to the
south and up to 400 km to the north. MARs of
detrital components (not displayed) have no
strong temporal or spatial pattern. MARs are
80^300 mg/cm2 /kyr with two minima at ODP
site 853 and core 164KL. Therefore, the detrital
MAR does not seem to be able to trace major
oceanic fronts.
The amplitude of carbonate content variations
is low at the equator but high in the Peru Basin.
As mentioned above, Peru Basin sites are located
within the lysocline, have low-sedimentation rates,
and are thus more a¡ected by dissolution. Dissolution is strongest (carbonate contents are lowest)
during interglacials or interglacial-glacial transitions (details see [2,8,14]). Nevertheless, higher
carbonate contents combined with higher £uxes
during glacials or glacial-interglacial transitions
should be indicative for productivity signals as
proposed by many authors (e.g. [7,21,38]), especially for sites which are located well above the
CCD and have higher sedimentation rates (ODP
sites 847 through 853). Thus, the transect across
the EEP upwelling system provides evidence for
both a dominant productivity record for the upwelling system and a productivity and dissolution/
preservation record for the Peru Basin.
71
6. Summary and conclusion
High-resolution records of glacial-interglacial
variations in biogenic carbonate, opal, and detritus (derived from non-destructive core log measurements of density, P-wave velocity and color;
r v 0.9) from 15 sediment sites in the EEP contain
a clear response to eccentricity and precession
forcing during the last 1.3 Ma. This information,
in turn, was used to develop a high-resolution
orbitally-based chronology for Peru Basin sediments for the last 1.3 Ma (with stratigraphic control points at each precession (21 kyr) maximum)
which is necessary for the reconstruction on glacial-interglacial time scales. The response to the
100 kyr cycle became dominant in all cores at
roughly 1.2 Ma which is 200^300 kyr earlier
than reported for other marine paleoclimatic records. In the Peru Basin, the sedimentary response to orbital variation has been weaker since
the Mid-Brunhes Dissolution Event at roughly
400 kyr.
A west^east reconstruction of estimated paleoceanographic proxies along four cores from the
Peru Basin (91³W to 85³W) clearly distinguishes
the in£uence of the equatorial and coastal upwelling systems in the western and eastern sites, respectively. A north^south reconstruction from
11³N to 8³S along 11 cores crosses all major oceanic fronts in the EEP. The spatial reconstruction
shows high biogenic contents (v80%) north of
4³S (apparently the present southern boundary
of the South Equatorial Current) with di¡ering
northern boundaries for carbonate (8³N) and
opal (2³N). Lower and highly variable biogenic
contents (25^80%) are documented for the Peru
Basin. Our reconstruction further shows that the
equatorial upwelling system extended during carbonate highs (glacials and glacial-interglacial transitions) up to 400 km north and south, and con¢ned during carbonate lows (interglacials or
interglacial-glacial transitions) rather than shifted
north or south through time. This pattern is stable and argues for recurring atmospheric and oceanic circulation. The temporal reconstruction
shows well-developed carbonate cycles B-6, B-8,
B-10, B-12, B-14, M-0, M-2, and M-6 with B-10
(430 ka) as the most prominent cycle. Since cycle
EPSL 5291 9-12-99
72
M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73
B-10, carbonate highs successively con¢ned, leading to the lowest carbonate and highest opal and
detrital contents since cycle B-4.
Reconstructed total MARs are high (800^5000
mg/cm2 /kyr) in the heart of the equatorial upwelling system with sharp boundaries at 6³N and 4³S,
where MARs decrease to 9 300 mg/cm2 /kyr. Carbonate mimics this pattern, especially in the upwelling system where carbonate comprises 70^
75% to the total £ux. Opal has high MARs
(100^300 mg/cm2 /kyr) between 2³N and 4³S and
resembles the present-day spatial distribution of
silicate in the surface waters of the EEP. Detrital
MARs (80^300 mg/cm2 /kyr) are only slightly elevated between 6³N and 6³S and do not trace major oceanic fronts.
The high-amplitude variations of carbonate in
sediment cores retrieved close to the CCD (Peru
Basin) argue for better preservation during glacials and glacial-interglacial transitions and enhanced dissolution during interglacials or interglacial-glacial transitions. On the other hand, high
carbonate contents together with high £uxes in
cores well above the CCD point to enhanced productivity during glacials and glacial-interglacial
transitions, especially within the equatorial upwelling system.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Acknowledgements
We are grateful to U. von Stackelberg as the
BGR head of German Peru Basin research activities. We also wish to thank T.D. Herbert, L.A.
Mayer, and two anonymous reviewers for their
suggestions, Alan Mix for his comments, C.
Rolf for magnetic measurements, and M.L. Weber for improving the English. This study was supported by the Deutsche Forschungsgemeinschaft
(DFG ; grant We 2039/1-1) and the Bundesministerium fu«r Bildung, Forschung und Technologie
(BMBF; grant 03 G 0106B).[MK]
[11]
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