Reconstructing Latitudinal Shifts of the Southern Westerlies from Marine

PEP I Newsletter 2000–2

Reconstructing Latitudinal Shifts of the Southern Westerlies from Marine Sediment Studies along the Chilean Continental Margin Compared to the North Atlantic region, little attention has been paid so far to the We can now provide a consistent picture high resolution paleoclimate studies marine paleoclimate record present in of the history of shifts of the Southern from mid-latitudes of the Southern continental margin sediments along the Westerlies ranging from millenial time- Hemisphere are still rare despite the west coast of southern South America. scale Milankovitch precession-driven fact that this part of the world plays a In 1995 a coring expedition by the insolation changes during the last major role in understanding the inter- University of Bremen using the German 120,000 years, to decadal time-scales hemispheric pattern of climate change RV Sonne retrieved sediment cores and within the Holocene. both during the last glacial cycle and the surface samples from the Chilean conti- Off the Chilean Norte Chico (ca. 27°S) Holocene. A key region to study past cli- nental slope between ca. 25°S and 45°S. (Fig. 1), we ﬁ nd strong evidence for pre- mates of the Southern Hemisphere mid- Several studies on both the paleoceano- cession-controlled shifts of the Southern latitudes is undoubtedly southern South graphic history of the Peru Chile Current Westerlies (Lamy et al., 1998a) resulting America. This fact has lead to consider- (Hebbeln et al., in press; Marchant et al., in more humid intervals in the Andes of able effort by the terrestrial paleoclimate 1999) and the paleoclimates of adjacent this region roughly every 20,000 years community, e.g. within the PAGES PEP I Chile (e.g., Lamy et al., 1998a) have been (Fig. 2a). This record suggests more transect (Markgraf et al., 2000). However, completed or are in in preparation. humid conditions during the Last Gla-

W The present climate of Chile is char- cial Maximum (LGM) coinciding with 70° 73°W acterized by a strong latitudinal gradi- a precession maximum, i.e. a summer ent of rainfall extending from hyper- insolation maximum in the Southern arid conditions in the Atacama desert of Hemisphere. More humid LGM climates northern Chile through semiarid Medi- have also been recently reconstructed for Copiapo GeoB 3375-1 terranean type conditions in central the Bolivian (Thompson et al., 1998) and 4-10 cm/kyr Chile to extremely high rainfall in the Peruvian Altiplano (Seltzer et al., 2000). mountains of southern Chile. This cli- However, in the Altiplano region precip- mate zonation is principally controlled itation is clearly dominated by tropical 30°S by the position of the southern west- summer rain, whereas north of the Atac- Coquimbo erly storm tracks (Southern Westerlies). ama Desert, winter rain is rare today. a It has been known since the early stud- In addition there are no indications for i l e

h ies by Scholl et al. (1970) that the amount tropical moisture inﬂ ux crossing the

GIK 17748-2 C t i n 7-70 cm/kyr of terrigenous sediment supplied to the South American arid diagonal zone in n Chilean continental margin is related to the past. Therefore, the humid intervals e Valparaiso the strong increase in onshore precipi- in the Chilean Norte Chico are clearly GeoB 3302-1 5-40 cm/kyr r g tation from north to south. Therefore, caused by increased winter rain due to A m 0 0 sedimentological analyses of the terrig- a more northward location of the South- 0 5 35°S - enous fraction of marine sediments, sup- ern Westerlies. We estimate a northward plied by rivers and wind to the ocean in shift of ca. 5° latitude for precession this area, provide a unique tool to study maxima including the LGM. Besides continental climate in the past. Based on showing dominantly precession driven Concepcion sedimentological background obtained rainfall changes over the Andes, the by the analysis of surface samples (Heb- records also reveal millennial-scale beln et al., 2000; Klump et al., submitted; changes in weathering intensity over Lamy et al., 1998b), terrigenous signals the Chilean Coastal Range most likely in marine sediments, e.g., source areas, induced by changes in coastal fog occur-

Peru-Chile Current system 40° S weathering conditions, and mode of sed- rences (Lamy et al., in press). As the fre- iment input, can clearly be related to the quency and intensity of the so-called Pto. Montt climatic zonation and allow its recon- ´Camanchacas` along the Chilean coast GeoB 3313-1 struction in the past. Due to the presence are today also related to the position of 80-230 cm/kyr hyper-arid of planktic foraminifera, Chilean conti- the Southern Westerlies, our record indi- arid, rare winter rain nental margin sequences provide good cates rapid latitudinal shifts of this wind semiarid w ith w inter rain potential for dating, both by 14C AMS belt throughout at least the last 80,000 humid temperate 18 m no dry season and δ O isotope stratigraphic meth- years with a similar pattern to rapid cli- 0

0 5 humid cool-temperate with - heavy precipitation ods, which are often more accurate than mate changes recorded in Greenland ice 45° S dating of terrestrial sequences. cores and North Atlantic sediments. Figure 1: Location of sediment cores along the Due to the large differences in the Further south, off central Chile (ca. Chilean continental margin in relation to the amount of supplied terrigenous mate- 33°S), higher sedimentation rates (Fig. 1) present climate zonation. Sedimentation rates rial (see sedimentation rates in Fig. 1), allow reconstruction of the climate evo- and thus the time resolution of the paleoclimate our sediment cores allow past climate lution of the last ca. 30,000 years in more records increase strongly to the south. reconstructions on different time-scales. detail (Lamy et al., 1999). Here we also

8 Newsletter 2000–2 PEP I

Figure 2: Iron content (Fe/Al ratios measured by ICP-AES spectroscopy, Fe intensity measured Calendar age (kyr) with a CORTEX X-ray ﬂ uorescence scanner) as 020406080100120 a proxy for changes in the terrigenous sediment N a GeoB 3375-1 supply related to rainfall changes induced by lat- (27°S)

itudinal shifts of the Southern Westerlies. Off humid 0.50 northern Chile (27°S) more humid intervals are characterized by the input of iron-rich material from the Andes. Off central and southern Chile (33°S and 41°S) the Andean material is diluted

0.45 Westerlies by iron-poor material derived from the Coastal Fe/Al ratio Range during more humid phases. arid S 0.40 ﬁ nd clear evidence for more humid con- 0.04 ditions during the LGM. The deglacia- tion is characterized by a trend towards 0.00 a more arid climate culminating during

the middle Holocene (8000–4000 cal yr -0.04 Precession-index BP). The late Holocene was marked by variable climate with generally more humid conditions (Fig. 2b). 0102030 This Holocene long-term trend can also be found off southern Chile (41°S) b N (Fig. 1) where extremely high sedimen- 0.55

tation rates allow a reconstruction of lati- humid tudinal shifts of the Southern Westerlies at decadal time-scales (Fig. 2c). Most of the multi-centennial to millennial scale GeoB 3302-1 0.60 (33°S) latitudinal shifts appear to be related to Westerlies Fe/Al Fe/Al ratio climate changes in the North Atlantic region (e.g., Bianchi & McCave, 1999; GIK 17748-2 arid

(33°S) S Bond et al., 1997) whereas climate vari- 0.65 ability on centennial to decadal time- scales might partly be related to the El Niño-Southern-Oscillation which sig- niﬁ cantly affects the interannual rainfall variability in mid-latitude Chile GeoB 3313-1 N (41°S) (Ruttland & Fuenzalida, 1991). 5

Future research will focus on fur- humid ther high resolution studies off central and southern Chile and a comparison 6 of the continental paleoclimate record and the paleoceanographic changes in Westerlies the Peru-Chile current system. A new 7 Fe Fe intensity (1000 cps) S coring expedition in spring 2001 prom- arid ises to retrieve further sediment cores c 8 especially from the southern Chilean 02 4 68 continental margin with its extremely Calendar age (kyr) high sedimentation rates.

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