Calibration of Cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic Atlantic Ocean — a stratigraphic tool for carbonate-poor Quaternary sediments

Abstract

We calibrated the Cycladophora davisiana abundances versus oxygen isotope stratigraphy back to 220 ka for the subantarctic Atlantic Ocean. The relative abundances of C. davisiana and δ¹⁸O measurements of benthic and planktic foraminifera have been determined in two sediment cores. Oxygen isotope stratigraphy has been used to date the C. davisiana records and to assign SPECMAP ages to the C. davisiana events. Comparisons with an existing calibration from the subantarctic Indian Ocean show, that the C. davisiana events ‘b₂, c₁, c₂, d, e₁, e₂, e₃, f, h, i₁ and i₂’ occur synchronous within the errors of the oxygen isotope stratigraphy in the Indian and the Atlantic sectors of the Southern Ocean. Larger deviations occur only for events ‘b₁’ and ‘g’. Furthermore, the long-term fluctuations in C. davisiana abundances have been studied in a sediment core covering the last 700 kyr. Based on biostratigraphic extinction levels, ages for early Brunhes C. davisiana events have been estimated. Major C. davisiana abundance maxima occur approximately every 100 ka in conjunction with glacial/interglacial cycles over the entire record.

Introduction

Broad areas of the Southern Ocean sea floor are covered with carbonate poor or carbonate barren siliceous oozes. The lack or absence of carbonate shells in these sediments precludes the application of ‘standard dating methods’ such as oxygen isotope stratigraphy or ¹⁴C dating. Hays et al., 1976a, Hays et al., 1976b developed a stratigraphy based on fluctuations in the abundances of the radiolarian species Cycladophora davisiana Ehrenberg relative to the entire radiolarian assemblage for carbonate barren Southern Ocean sediments. Hays et al., 1976a, Hays et al., 1976b recognized the general correlation of high C. davisiana abundances with glacial times and low C. davisiana abundances with interglacial times in late Quaternary sediments. The late Quaternary maxima and minima in the C. davisiana abundances have been named with lower case letters ‘a–o’ and calibrated versus oxygen isotope stages, back to oxygen isotope stage 13 for the subantarctic Indian Ocean and back to oxygen isotope stage 2 for the subantarctic Atlantic Ocean. The correlation of oxygen isotope stage 2 and C. davisiana abundance peak ‘b’ allowed the inclusion of carbonate barren sediment samples from the Southern Ocean into the Last Glacial Maximum study of the CLIMAP project (CLIMAP, 1981). Based on correlations between an oxygen isotope dated C. davisiana record from the subantarctic Indian Ocean and an oxygen isotope dated C. davisiana record from the North Atlantic Ocean, Morley and Hays (1979) were able to show, that the abundance fluctuations of C. davisiana occur synchronous in the high latitudes of both hemispheres.

An explanation for the glacial/interglacial abundance variations of C. davisiana has been searched in the modern ecology of the species. C. davisiana is a cosmopolitan and deep-dwelling radiolaria (Petrushevskaya, 1968, Petrushevskaya, 1972, Morley and Hays, 1979). Plankton data from the North Atlantic show, that C. davisiana prefers depths below 500 m in the open ocean (Bjørklund and Ciesielski, 1994). In similar depths (400–1000 m) occurrences of living C. davisiana have been reported from the Southern Ocean (Abelmann and Gowing, 1997). In modern Southern Ocean surface sediment samples C. davisiana rarely exceeds 5% (Hays et al., 1976b, Morley, 1989). There is no modern analog for the high glacial abundances (about 20–30%) of C. davisiana within the Southern Ocean (Hays et al., 1976b). The abundances in the Southern Ocean surface sediment samples exhibit no correlation with longitude, latitude or temperature gradients (Hays et al., 1976b). Obviously C. davisiana is not controlled by sea surface temperatures (Hays et al., 1976b) and has been excluded from all Southern Ocean radiolarian based transfer functions (Lozano and Hays, 1976; Hays et al., 1976a, Morley, 1979, Morley, 1989, Abelmann et al., 1999). In the modern World Ocean, sediments containing high C. davisiana abundances (up to 49%) have only been found in the Sea of Okhotsk, a marginal sea in the northwest Pacific (Morley and Hays, 1983). Thus the oceanographic conditions of the Sea of Okhotsk, with a low salinity surface layer accompanied by a strong temperature minimum near its base, have been proposed as an analog for the glacial Southern Ocean (Morley and Hays, 1983, Bjørklund and Ciesielski, 1994).

Since the early works of Hays et al., 1976a, Hays et al., 1976b, Morley and Hays, 1979C. davisiana has been established as a valuable stratigraphic tool not only in the Southern Ocean (CLIMAP, 1981, Cooke and Hays, 1982, Labeyrie et al., 1986, Abelmann and Gersonde, 1988, Howard and Prell, 1992, Pudsey and Howe, 1998), but also in the Northwest Pacific Ocean (Morley et al., 1982, Morley et al., 1995), and the North Atlantic Ocean (Ciesielski and Bjørklund, 1995). Despite its importance for correlating and dating carbonate poor sediments, there are very few records in which C. davisiana events have been cross-correlated with oxygen isotope stages. In this work we present a first calibration of C. davisiana events versus oxygen isotope stages over the last 220 kyr for the subAntarctic Atlantic Ocean. Our results show, that C. davisiana events ‘b₂, c₁, c₂, d, e₁, e₂, e₃, f, h, i₁ and i₂’ occur synchronous in the subantarctic Atlantic Ocean and the subantarctic Indian Ocean within the errors of the oxygen isotope stratigraphy. Larger deviations occur only for the C. davisiana maximum ‘b₁’ and the broad minimum ‘g’. Furthermore, we present a continuous C. davisiana record covering the last 700 kyr, which exhibits the long-term fluctuations in the C. davisiana abundances. It is well known from late Brunhes sediments, that major C. davisiana abundance maxima appear approximately every 100 ka. Our record shows, that this trend can also be observed in early Brunhes sediments.