Middle to late Miocene oxygen isotope stratigraphy of ODP site 1085 (SE Atlantic): new constrains on Miocene climate variability and sea-level fluctuations

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Abstract

The middle Miocene δ18O increase represents a fundamental change in earth's climate system due to a major expansion and permanent establishment of the East Antarctic Ice Sheet accompanied by some effect of deepwater cooling. The long-term cooling trend in the middle to late Miocene was superimposed by several punctuated periods of glaciations (Mi-Events) characterized by oxygen isotopic shifts that have been related to the waxing and waning of the Antarctic ice-sheet and bottom water cooling.

Here, we present a high-resolution benthic stable oxygen isotope record from ODP Site 1085 located at the southwestern African continental margin that provides a detailed chronology for the middle to late Miocene (13.9–7.3 Ma) climate transition in the eastern South Atlantic. A composite Fe intensity record obtained by XRF core scanning ODP Sites 1085 and 1087 was used to construct an astronomically calibrated chronology based on orbital tuning. The oxygen isotope data exhibit four distinct δ18O excursions, which have astronomical ages of 13.8, 13.2, 11.7, and 10.4 Ma and correspond to the Mi3, Mi4, Mi5, and Mi6 events. A global climate record was extracted from the oxygen isotopic composition. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The oxygen isotope data support a causal link between sequence boundaries traced from the shelf and glacioeustatic changes due to ice-sheet growth.

Spectral analysis of the benthic δ18O record shows strong power in the 400-kyr and 100-kyr bands documenting a paleoceanographic response to eccentricity-modulated variations in precession. A spectral peak around 180-kyr might be related to the asymmetry of the obliquity cycle indicating that the response of the dominantly unipolar Antarctic ice-sheet to obliquity-induced variations probably controlled the middle to late Miocene climate system. Maxima in the δ18O record, interpreted as glacial periods, correspond to minima in 100-kyr eccentricity cycle and minima in the 174-kyr obliquity modulation. Strong middle to late Miocene glacial events are associated with 400-kyr eccentricity minima and obliquity modulation minima. Thus, fluctuations in the amplitude of obliquity and eccentricity seem to be the driving force for the middle to late Miocene climate variability.

Introduction

The Neogene period comprises a major change in climate state from relatively global warmth of the early Miocene to colder climates at the end of the Pliocene. As inferred from co-varying δ18O values of planktonic and benthic foraminifers, the general cooling trend in the middle to late Miocene was superimposed by several punctuated periods of intensive glaciations (Mi-Events) which supposedly reflect continental ice sheet growth and/or bottom water cooling (Miller et al., 1991, Lear et al., 2000, Turco et al., 2001, Billups and Schrag, 2002). Due to the scarcity of continuous marine records spanning the middle to late Miocene, a period of widespread erosion in the deep sea (Keller and Barron, 1987), the characteristics of and the control on the Miocene glacial events is controversial. Recently, astronomically calibrated ages for the Mi5 and Mi6 event showed (Turco et al., 2001) that these coincide with periods of low-amplitude variations in obliquity related to the ∼1.2 Ma cycle as proposed by Lourens and Hilgen (1997).

ODP Site 1085 is one of the most expanded and complete middle to late Miocene marine records retrieved from the Southwest African continental margin during ODP Leg 175. In this paper we present a unique high-resolution benthic oxygen isotope record spanning the middle to late Miocene (13.8–7.3 Ma). In addition, an astronomically calibrated time scale based on tuning of a composite Iron (Fe) intensity record from ODP Sites 1085 and 1087 has been constructed. For the first time, this high-resolution δ18O record enables us to decipher the driving mechanisms of the middle to late Miocene climate system in detail. Therefore, this study focuses on the characterization and timing of the Mi-events, the orbital control of δ18O variability, and inferences on sea-level fluctuation during the middle to late Miocene.

Section snippets

Material and methods

ODP Holes 1085A and 1087C were drilled in the Cape Basin during Leg 175. ODP Site 1085 is located at the southwestern African continental margin (Fig. 1) (29°22.47′S, 13°59.41′E, 1713 m water depth) off the mouth of the Orange River, a perennial river discharging into the South Atlantic (Wefer et al., 1998). A continuous hemipelagic sedimentary section composed of nannofossils ooze, diluted by various amounts silt and clay, was recovered from Site 1085 reaching down to the middle Miocene (14

XRF data

In Fig. 2, Fe intensities derived from the XRF core logs of hole 1085A and 1087C, respectively, are plotted versus log-depth (mbsf; Tables 1 and 2). Fe intensities vary between 500 and 6000 cps, and show high-frequency cyclicity. The long-term trend in the Fe intensity data is characterized by six periods of higher Fe intensity located at 433–453, 470–481, 528–537, 547–553, 556–561, and 565–570 m log-depth. These periods correspond to the middle to late Miocene carbonate crash events in the

Miocene isotopic events

The middle to late Miocene open-ocean δ18O record in the investigated time period is punctuated by five episodes of increased values known as the Mi3 through Mi7 events (Miller and Feigenson, 1991, Miller et al., 1991, Wright and Miller, 1992, Wright et al., 1992). These oxygen isotopic shifts have been ascribed to a combination of glacioeustatic sea-level lowering and bottom water cooling of 1–2 °C, but are primarily related to the waxing and waning of the Antarctic ice-sheet (Miller et al.,

Conclusion

The high-resolution benthic stable oxygen isotope data of ODP Site 1085 provide new insight into the middle to late Miocene paleoclimatic evolution. A detailed chronology based on orbitally tuning of a high-resolution Fe intensity composite record has been developed. The long-term cooling trend in δ18O record of Site 1085 approximates the general trend in the global δ18O deep-sea composite of Zachos et al. (2001a) resembling the middle to late Miocene buildup of the East Antarctic Ice Sheet.

Acknowledgments

We are indebted to M. Segl and her team for carefully supervising the stable isotope analyses. We thank A. Wuelbers and W Hale for the help at the Bremen Core Repository. This contribution benefits from discussion with C. M. John, K. G. Miller, J. C. Zachos, and L. J. Lourens. T. Kouwenhoven and K. Billups are thanked for their critical review and suggestions for the manuscript. We thank the Ocean Drilling Program for providing samples. This study was funded by the Deutsche

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