Late Pleistocene palaeoproductivity patterns during the last climatic cycle in the Guyana Basin as revealed by calcareous nannoplankton

G.-E López-Otálvaro,J.-O Grimalt,J A Flores,E Michel,L Labeyrie,I Cacho,E Cortijo,F J Sierro

doi:10.5194/ee-4-1-2009

Abstract

Variations in the assemblage and abundance of coccoliths reveal changes in oceanic and atmospheric dy- namics in the Guyana Basin over the last climatic cycle, mainly linked to latitudinal variations in the ITCZ (Intertrop- ical Convergence Zone). Records of the N ratio (a palaeo- productivity index of coccolithophores) allowed us to mon- itor nutri-thermocline fluctuations. Nannofossil accumula- tion rates (NAR) vary closely with the N ratio, indicating a strong correlation between these two palaeoproductivity proxies. Decreases in the N ratio and NAR values sug- gest lower palaeoproductivity during glacial substages, in- dicating a deep nutri-thermocline (deep stratification of the mixed layer) as a consequence of the piling up of warm water dragged by the NEC. This setting was favoured by the south- ern shift of the ITCZ and Trade winds which blew perpendic- ular to the Guyana coast. By contrast, increases in the N ra- tio and NAR values revealed higher palaeoproductivity dur- ing interglacial substages, suggesting a shoaling of the nutri- thermocline. This scenario is favoured by a northward dis- placement of the ITCZ with the southeast Trade winds blow- ing alongshore. Additionally, palaeoproductivity changes during substages of Marine Isotope Stage (MIS) 6-5 are of much higher amplitude than those recorded in substages of MIS 4-2 and the early Holocene. Similarities between the palaeoproductivity and the 65 N summer insolation records, suggest a link between the depth of nutri-thermocline, the lat- itudinal migration of the ITCZ and ice-sheet changes in the Northern Hemisphere.

Highlights

Introduction and backgroundCoccolithophorids are photosynthetic planktonic algae living in the photic layer of the ocean (Hibberd, 1976; Brand, 1994; Young, 1994; Winter et al, 1994)
Higher proportions of F. profunda occurred during glacial substages of Marine Isotope Stage (MIS) 5 (80%) but its specific Nannofossil accumulation rates (NAR) values were observed between MIS 5c to 5a (3×1010 liths cm−2 Ka−1)
The NAR record resembles that of the N ratio, suggesting that the NAR is indicative of variations in palaeoproductivity

Summary

Introduction

Coccolithophorids are photosynthetic planktonic algae living in the photic layer of the ocean (Hibberd, 1976; Brand, 1994; Young, 1994; Winter et al, 1994). They play a significant role in carbonate precipitation (Siesser and Winter, 1994; Steinmetz, 1994). The relationship between dwellers from the upper photic zone and those from the lower photic zone has been used to monitor nutri-thermocline depth in the tropical Atlantic (Molfino and McIntyre, 1990a, b), in the Western Tropical Atlantic (Kinkel et al, 2000; Bassinot et al, 1997), as well as in the other parts of the ocean (Okada and Honjo, 1973; Young, 1994; Okada and Wells, 1997; Wells and Okada, 1997; Beaufort et al, 1997, 1999, 2001, 2003; Beaufort and Buchet, 2003; Flores et al, 2000; Liu and Herbert, 2004; Baumann and Freitag, 2004; Lopez-Otalvaro et al, 2008).

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Conclusion