Deglacial pattern of circulation and marine productivity in the upwelling region off central-south Chile

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Abstract

A high-resolution sea surface temperature and paleoproductivity reconstruction on a sedimentary record collected at 36°S off central-south Chile (GeoB 7165-1, 36°33′S, 73°40′W, 797 m water depth, core length 750 cm) indicates that paleoceanographic conditions changed abruptly between 18 and 17 ka. Comparative analysis of several cores along the Chilean continental margin (30°–41°S) suggests that the onset and the pattern of deglacial warming was not uniform off central-south Chile due to the progressive southward migration of the Southern Westerlies and local variations in upwelling. Marine productivity augmented rather abruptly at 13–14 ka, well after the oceanographic changes. We suggest that the late deglacial increase in paleoproductivity off central-south Chile reflects the onset of an active upwelling system bringing nutrient-rich, oxygen-poor Equatorial Subsurface Water to the euphotic zone, and a relatively higher nutrient load of the Antarctic Circumpolar Current. During the Last Glacial Maximum, when the Southern Westerlies were located further north, productivity off central-south Chile, in contrast to off northern Chile, was reduced due to direct onshore-blowing winds that prevented coastal upwelling and export production.

Introduction

Presently, 50% of the global ocean export production to the deep sea occurs in upwelling areas and in the coastal seas (Berger et al., 1989). Among these areas, the Humboldt Current System (HCS) in the Eastern South Pacific (ESP), which includes the coastal upwelling ecosystems off Peru and Chile, is one of the most productive marine systems in the world. As ocean productivity might play an important role in modulating the atmospheric CO2 concentration, regions such as the HCS are of great importance for the reconstruction of paleoproductivity and its relationship to climate through the Late Quaternary climatic cycles.

Paleoceanographic changes encompassing the Last Glacial Maximum (LGM) and the Holocene in mid-latitudes off the west coast of South America have been interpreted in relation to the latitudinal migration of the Southern Westerly Wind belt (SWW), and the strength and position of the Southeast Pacific anticyclone (e.g. Lamy et al., 1999, Lamy et al., 2002, Lamy et al., 2007, Kim et al., 2002, Kaiser et al., 2005). Information about the history of productivity off Chile has grown in the last decade, yet mostly confined to the area north of 35°S. Hebbeln et al. (2002) suggested that during the LGM, marine productivity at 33°S was higher than at present due to the northward displacement of the SWW and the Antarctic Circumpolar Current (ACC) as the main nutrient source in this region. Romero et al. (2006) inferred that a northerly position of the SWW during the LGM shut down coastal upwelling and led to low glacial paleoproductivity at 35°S. Mohtadi and Hebbeln (2004) observed that paleoproductivity north of 33°S reached maximum values prior and after the LGM, and tentatively attributed this to increased onshore precipitation in northern South America and the related supply of micronutrients. This interpretation was supported by Dezileau et al. (2004), who demonstrated that on orbital timescales, higher river runoff transported significant amounts of iron-rich terrigenous material from the Andes during precessional maxima thus enhancing biological productivity off northern Chile. In summary, despite the progress in understanding the HCS within the past decade, different local features and lack of suitable cores have led to poorly constrained regional paleoproductivity particularly off central and southern Chile.

With the aim of reconstructing the regional history of oceanic circulation and paleoproductivity off central-south Chile, we performed a multi-proxy investigation including stable isotopic data and faunal composition of planktonic foraminifera, alkenone-derived Sea Surface Temperatures (SST), chlorin and protein concentrations, δ15N of organic matter, organic carbon (Corg), carbonate, biogenic opal, and elemental analyses on a sediment core from the coastal upwelling area off Concepción (~ 36°S, Fig. 1) covering the period between ~ 22 ka and 6 ka. We compare our data with recently published data on other cores along the Chilean continental margin (Kim et al., 2002, Lamy et al., 2004, Lamy et al., 2007, Mohtadi and Hebbeln, 2004, Romero et al., 2006, De Pol-Holz et al., 2007). Our results show that the change from glacial to interglacial conditions had important and distinctive effects on the HCS, and caused different local responses both spatially and temporally.

Section snippets

Background

Along the Chilean coast, there are numerous, well-identified areas of upwelling that together sustain one of the richest pelagic fisheries of the world. Upwelling is rather continuous in the north (18–30°S, e.g. Blanco et al., 2001) and strongly seasonal in the central-south region (35–40°S, e.g. Thomas et al., 2001). Here, the upwelling intensifies during the austral spring–summer period and downwelling occurs in winter due to the prevalence of strong northerly winds. Prevailing

Materials and methods

Gravity core GeoB 7165-1 was collected ~ 60 km offshore Concepción (36°33′S, 73° 40′W, 797 m water depth, core length 750 cm, Fig. 1) during the SO-156 cruise (Hebbeln and cruise participants, 2001).

Samples were analyzed every 5 cm for P and Al by XRF using Philips® PW 2400 XRF spectrometer by means of fused glass beads. ICP-MS (Finnigan MAT Element) was used to analyze Cd in acid digestions. The error of the overall analytical precision/accuracy (checked with replicate analysis of sediment

Results and discussion

The age model of core GeoB 7165-1 is based on four 14C AMS dates and linear interpolation between the age control points (Table 1). Due to low carbonate content in the upper 3 m of the core, AMS 14C dating was not possible in that section. Thus, an additional age control point at 10.77 ka (158 cm core depth) was derived by comparing the available d18O data to the published record from ODP 1233 (Lamy et al., 2004). The resulting age model yielded a good fit with the d18O ice core record of Byrd (

Conclusions

The SST records from the ESP show different deglacial patterns that are related to the southward migration of the SWW both directly and indirectly through changes in the local paleoproductivity. The timing and magnitude of the deglacial warming steps are strongly dependent on the position of the investigated records relative to the position of the SWW, and the related, site-specific changes in upwelling intensity and paleoproductivity. Our multi-proxy approach suggests that subsurface

Acknowledgements

We are thankful to L. Nuñez, A. Avila, and R. Castro at the University of Concepción, and to M. Segl, B. Meyer-Schack, and H. Buschoff at the University of Bremen for laboratory analyses. The manuscript benefited from constructive reviews by A. Mix and two anonymous reviewers. This work was supported by the FONDAP-COPAS Center (Project No. 150100007), and the German Bundesministerium für Bildung und Forschung (Project PUCK). The data presented in this paper are also available in digital format

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