M.E. Weber *, N.G. Pisias. Oregon State University, College of Oceanic and Atmospheric Sciences, 104 Ocean Adm Bld, Corvallis, OR , USA

Transcription

1 Earth and Planetary Science Letters 174 (1999) 59^73 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 , 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.: ; Fax: ; mweber@unb.ca 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, X / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved. PII: S X(99)

2 60 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 biogenic 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.

3 M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^ 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 (SO- 106), 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 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. [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, N 18 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 N 18 O (77, 184, and 243KL). The N 18 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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5 M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73 63 nate peaks are similar in all cores. Thus, the time scale di erence between insolation-tuned and N 18 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 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 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.

6 64 M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73 Table 1 Age models for Peru Basin sediments Time (ka) 71KL 77KL 164KL 184KL 243KL 286KL ] Table 1 Age models for Peru Basin sediments Time (ka) 71KL 77KL 164KL 184KL 243KL 286KL 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

7 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

8 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 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-

9 M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73 67 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/cm 2 /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 mg/cm 2 /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/cm 2 /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 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. 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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11 M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73 69 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 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.

12 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/cm 2 /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 mg/cm 2 /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-

13 M.E. Weber, N.G. Pisias / Earth and Planetary Science Letters 174 (1999) 59^73 71 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/cm 2 /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/cm 2 /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. 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

14 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/cm 2 /kyr) in the heart of the equatorial upwelling system with sharp boundaries at 6³N and 4³S, where MARs decrease to mg/cm 2 /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/cm 2 /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/cm 2 /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. 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] References [1] L.A. Mayer, Extraction of high-resolution carbonate data for palaeoclimate reconstruction, Nature 352 (1991) 148^ 150. [2] T.K. Hagelberg, N.G. Pisias, L.A. Mayer, N.J. Shackleton, A.C. Mix, Spatial and temporal variability of late Neogene equatorial Paci c carbonate: Leg 138, Proc. ODP Sci. Res. 138 (1995) 321^336. [3] M.E. Weber, F. Niessen, G. Kuhn, M. Wiedicke, Calibration and application of marine sedimentary physical properties using a Multi-Sensor Core Logger, Mar. Geol. 136 (1997) 151^172. [4] M.E. Weber, Estimation of biogenic carbonate and opal by continuous non-destructive measurements in deep-sea sediments from the eastern equatorial Paci c, Deep-Sea Res. 45 (1998) 1955^1975. [5] S.E. Harris, A.C. Mix, T. King, Biogenic and terrigenous sedimentation at Ceara Rise, western tropical Atlantic, supports Pliocene-Pleistocene deep-water linkage between hemispheres, Proc. ODP Sci. Res. 154 (1997) 331^345. [6] C.G. Adelseck Jr., The late Pleistocene record of productivity uctuations in the eastern equatorial Paci c Ocean, Geology 6 (1978) 388^391. [7] J.W. Farrell, W.L. Prell, Climatic change and CaCO 3 preservation: an 800,000 year bathymetric reconstruction from the central equatorial Paci c Ocean, Paleoceanography 4 (1989) 447^466. [8] J.W. Farrell, W.L. Prell, Paci c CaCO 3 preservation and N 18 O since 4 Ma: paleoceanic and paleoclimatic implications, Paleoceanography 6 (1991) 485^498. [9] J. Dymond, Geochemistry of Nazca plate surface sediments: an evaluation of hydrothermal, biogenic, detrital, and hydrogenous sources, in: D. Kulm, J. Dymond, E.J. Dasch and D.M. Hussong (Eds.), Nazca Plate: Crustal Formation and Andean Convergence, Geol. Soc. Am. Mem. 154, 1981, pp. 133^173. [10] J.M. Chuey, D.K. Rea, N.G. Pisias, Late Pleistocene paleoclimatology of the central equatorial Paci c: a quantitative record of eolian and carbonate deposition, Quat. Res. 28 (1987) 323^339. [11] D.W. Murray, J.W. Farrell, V. McKenna, Biogenic sedimentation at Site 847, eastern equatorial Paci c Ocean, during the past 3 m.y., Proc. ODP Sci. Res. 138 (1995) 429^459. [12] A.C. Mix, S.E. Harris, T.R. Janecek, Estimating lithology from nonintrusive re ectance spectra: Leg 138, Proc. ODP Sci. Res. 138 (1995) 413^427. [13] N.G. Pisias, L.A. Mayer, T.R. Janecek, A. Palmer-Julson, T.H. von Andel, Proc. ODP Sci. Res. 138 (1995) 960 pp. [14] M.E. Weber, M. Wiedicke, V. Riech, H. Erlenkeuser, Carbonate preservation history in the Peru Basin paleoceanographic implications, Paleoceanography 10 (1995) 775^800. [15] K. Moran, Notice to users of GRAPE data, Joides J. 19 (3) (1993) 6. [16] T.D. Herbert, L.A. Mayer, Long climatic time series from sediment physical property measurements, J. Sediment. Petrol. 61 (1991) 1089^1108. [17] N.J. Shackleton, S. Crowhurst, T. Hagelberg, N.G. Pisias,

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