Paleoproductivity in the southern Peru–Chile Current through the last 33 000 yr

doi:10.1016/S0025-3227(02)00331-6

Marine Geology

Volume 186, Issues 3–4, 10 July 2002, Pages 487-504

https://doi.org/10.1016/S0025-3227(02)00331-6 Get rights and content

Abstract

Based on a multiparameter approach including organic carbon, biogenic opal, carbonate and the species composition of planktic foraminifera, this paper provides the first qualitative assessment of the history of paleoproductivity on glacial–interglacial time scales in the southern Peru–Chile Current (PCC) off Chile, which belongs to the least studied parts of the world ocean. During the Last Glacial Maximum (LGM) highest relative paleoproductivity of the last 33 kyr, indicated by high accumulation rates of organic carbon, biogenic opal and carbonate, has been found, contrasted by lowest values during the early and Middle Holocene. This shift from high to low productivity is accompanied by a major change in the species composition of planktic foraminifera from a dominance of Neogloboquadrina pachyderma (sin.) to the dominance of N. pachyderma (dex.). The temporal pattern of paleoproductivity off Chile is very similar to the history of the continental paleoclimate in the region, which is supposed to be driven by the position of the Southern Westerlies. This observation points to a functional relationship between the position of the Southern Westerlies and the history of paleoproductivity off Chile. Assuming that atmospheric and oceanographic circulation are closely linked, a northward displacement of the Antarctic Circumpolar Current during the LGM, in line with the northward movement of the Southern Westerlies, would bring the main nutrient source closer to our core sites resulting in increased productivity. However, based on the available data, it is not clear if the higher productivity in the southern PCC during the LGM reflects generally higher productivity during this time or only a regional displacement of the main productivity centers.

Introduction

The pattern of changing atmospheric contents of the major greenhouse gas carbon dioxide between low glacial and high interglacial values, which runs parallel to cold and warm periods of the Earth’s climate in the Late Quaternary (Barnola et al., 1987), is assumed to at least partly result from variations in the productivity of the oceans (Broecker, 1982). Increased productivity by marine algae results in an increased drawdown of atmospheric carbon dioxide into the ocean by photosynthetic fixation in organic matter and the subsequent deposition of parts of it in the ocean sediments (Berger et al., 1989). However, marine productivity is rather uneven distributed over the world ocean, as ∼50% of it takes place on only 15% of the world ocean area (Berger et al., 1989), namely in the high productivity regions associated with the equatorial and subpolar divergence zones and the eastern boundary currents. Thus, major changes in the paleoproductivity of the world ocean, able to significantly affect the atmospheric carbon dioxide content, are most likely related to these high productivity regions.

For some of these areas assessments of the paleoproductivity on glacial–interglacial time scales exist, although the conclusions drawn are not always unequivocal. For some equatorial and coastal upwelling areas the available data point to higher productivity during glacials (Berger and Herguera, 1992, Lyle, 1988, McIntyre et al., 1989, Rühlemann et al., 1999, Sarnthein and Winn, 1990, Schneider et al., 1996). From the Antarctic increased (Keir, 1990) as well as decreased (Mortlock et al., 1991) glacial productivities have been reported. These observations are now interpreted to reflect merely a latitudinal shift of the high productivity zone rather than significant quantitative changes in productivity (Mackensen et al., 1994).

Among the eastern boundary currents the Peru–Chile Current (PCC) stands out with an extension over 40° of latitude. With its persistent coastal upwelling it belongs to the most productive marine environments (Berger et al., 1987) supporting an intensive pelagic fishery (Alheit and Bernal, 1993). However, about its southern part extending over 20° of latitude along the Chilean coast, only little information based on marine sediments is available focusing mostly on terrigenous sediments (Lamy et al., 1998, Lamy et al., 1999, Lamy et al., 2000). Only one analysis of planktic foraminifera covering the last 13 kyr reveals some information about the paleoproductivity of the region (Marchant et al., 1999). Here we provide the first estimate of paleoproductivity variations in the southern PCC on glacial–interglacial time scales based on a multiparameter approach using accumulation rates of various biogenic sediment components and the species composition of planktic foraminifera.

Section snippets

Regional setting

The regional oceanography of the Southeast Pacific has recently been described by Shaffer et al. (1995) and by Strub et al. (1998). The circulation pattern is dominated by the northward flowing PCC (Subantarctic Surface Water; Fig. 1), which originates between 40 and 45°S where the Antarctic Circumpolar Current (ACC) approaches the South American continent (Boltovskoy, 1976). The northward deflection of the ACC mainly forms the PCC, which stretches all along the South American west coast before

Methods

The two gravity cores were sampled at 5-cm intervals. One sample set was ground for geochemical analyses, while another sample set was washed over a 63-μm sieve to separate the coarse fraction for foraminifera analyses. The dry bulk density of the sediments was measured by weighing a fixed sample volume before and after freeze-drying.

Chronology

The stratigraphy of core 17748-2 is based on six ¹⁴C AMS dates and linear interpolation between the age control points (converted into calendar years, see Table 1) (Lamy et al., 1999, Marchant et al., 1999). The sediment surface with a modern age is set to −13 cm because comparison with samples of a boxcorer indicates that the uppermost 13 cm of the core is missing (Stoffers and Shipboard Scientific Party, 1992). Additionally, three turbiditic layers (9–16 cm, 47–54 cm, and 160–177 cm) are

Results

The bulk sedimentary data, i.e. the contents of organic carbon, carbonate and biogenic opal, display some similarities in the two cores. The CaCO₃ contents range between 2.1 and 17.7 wt% (Fig. 2). Between 3000 and 11 000 cal yr BP it is continuously above 12 wt%. Throughout most of the remaining time the carbonate content is <8 wt%. Comparatively low carbonate contents between 13 000 and 15 000 cal yr BP in core 17748-2 probably reflect dilution of the carbonate signal by redeposited sediments

Reworking of sediments

As indicated by a small, but steady contribution of redeposited benthic foraminifera, some redeposition of sediments derived from upslope seems to be a continuous process in the Valparaiso Basin (Marchant et al., 1999). Although their absolute number keeps more or less constant throughout the core (<2 ind/gr), their relative amount is increased in the lower part of core 17748-2, probably reflecting a dilution of the accumulation of autochthonous benthic foraminifera. Based on this observation

Conclusions

The data presented here clearly indicate a significantly higher productivity in the southern PCC during the LGM compared to the Holocene. However, based on the unequivocal data from the Antarctic region (Keir, 1990, Mackensen et al., 1994, Mortlock et al., 1991) and based on our understanding of the functioning of the upwelling/productivity system in the southern PCC (Hebbeln et al., 2000a), we do not draw this conclusion at the moment. The data might reflect only a northward shift of the main

Acknowledgments

We thank M. Segl and B. Meyer-Schack, who performed the stable isotope measurements, and B. Bohling, who run the opal analyses. We are also indebted to W.H. Berger, F. Lamy and R. Schneider for helpful comments on an earlier version of the manuscript. This study was supported by the German Bundesministerium für Bildung und Forschung through funding of the project ‘CHIPAL’ (SO-102). M.M. acknowledges financial support by the Deutscher Akademischer Austauschdienst (DAAD).

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Paleoproductivity in the southern Peru–Chile Current through the last 33 000 yr

Abstract

Introduction

Section snippets

Regional setting

Methods

Chronology

Results

Reworking of sediments

Conclusions

Acknowledgments

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Mar. Geol.

Deep-Sea Res. II

Quat. Res.

Mar. Micropaleontol.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Quat. Res.

Deep-Sea Res.

Mar. Geol.

Deep-Sea Res.

Nucl. Instrum. Methods Phys. Res.

Deep-Sea Res.

Correction of accelerator mass spectrometry 14C ages measured in planctonic foraminifera: Paleoceanographic implications

Paleoceanography

Vostok ice core provides 160 000-year record of atmospheric CO2

Nature

Oxygen isotopes and sea level

Nature

Revised carbonate-water isotopic temperature scale

Bull. Geol. Soc. Am.

Correction of accelerator mass spectrometry ¹⁴C ages measured in planctonic foraminifera: Paleoceanographic implications

Vostok ice core provides 160 000-year record of atmospheric CO₂