Astronomical tunings of the Oligocene–Miocene transition from Pacific Ocean Site U1334 and implications for the carbon cycle

Helen M Beddow,Lucas J Lourens,Appy Sluijs,Douglas S Wilson,Bridget S Wade,Diederik Liebrand,Frits J Hilgen

doi:10.5194/cp-14-255-2018

Abstract

Abstract. Astronomical tuning of sediment sequences requires both unambiguous cycle pattern recognition in climate proxy records and astronomical solutions, as well as independent information about the phase relationship between these two. Here we present two different astronomically tuned age models for the Oligocene–Miocene transition (OMT) from Integrated Ocean Drilling Program Site U1334 (equatorial Pacific Ocean) to assess the effect tuning has on astronomically calibrated ages and the geologic timescale. These alternative age models (roughly from ∼ 22 to ∼ 24 Ma) are based on different tunings between proxy records and eccentricity: the first age model is based on an aligning CaCO3 weight (wt%) to Earth's orbital eccentricity, and the second age model is based on a direct age calibration of benthic foraminiferal stable carbon isotope ratios (δ13C) to eccentricity. To independently test which tuned age model and associated tuning assumptions are in best agreement with independent ages based on tectonic plate-pair spreading rates, we assign the tuned ages to magnetostratigraphic reversals identified in deep-marine magnetic anomaly profiles. Subsequently, we compute tectonic plate-pair spreading rates based on the tuned ages. The resultant alternative spreading-rate histories indicate that the CaCO3 tuned age model is most consistent with a conservative assumption of constant, or linearly changing, spreading rates. The CaCO3 tuned age model thus provides robust ages and durations for polarity chrons C6Bn.1n–C7n.1r, which are not based on astronomical tuning in the latest iteration of the geologic timescale. Furthermore, it provides independent evidence that the relatively large (several 10 000 years) time lags documented in the benthic foraminiferal isotope records relative to orbital eccentricity constitute a real feature of the Oligocene–Miocene climate system and carbon cycle. The age constraints from Site U1334 thus indicate that the delayed responses of the Oligocene–Miocene climate–cryosphere system and (marine) carbon cycle resulted from highly non-linear feedbacks to astronomical forcing.

Highlights

Tuned age models are important in studies of Cenozoic climate change because they shed light on cause and effect relationships between insolation forcing and the linear and non-linear responses of Earth’s climate system (e.g. Hilgen et al, 2012; Vandenberghe et al, 2012; Westerhold et al, 2017)
Beddow et al.: Astronomical tunings of the Oligocene–Miocene transition (i) determining the absolute timing of events, (ii) the understanding of leads and lags in the climate system, and (iii) the exact astronomical frequencies that are present in climate proxy records after tuning
We explore the application of CaCO3 content and benthic foraminiferal δ13C records as tuning signals for the Oligocene–Miocene transition (OMT) record at Site U1334 in the eastern equatorial Pacific

Summary

Introduction

Tuned age models are important in studies of Cenozoic climate change because they shed light on cause and effect relationships between insolation forcing and the linear and non-linear responses of Earth’s climate system (e.g. Hilgen et al, 2012; Vandenberghe et al, 2012; Westerhold et al, 2017). As more Cenozoic paleoclimate records are generated that use astronomical tuning as the main highprecision dating tool, it is important to understand the assumptions and limitations inherent in this age calibration method, in particular with respect to assumptions related to phase relationships between tuning signal and target curves (i.e. climate proxy records and astronomical solutions, respectively). Beddow et al.: Astronomical tunings of the Oligocene–Miocene transition (i) determining the absolute timing of events, (ii) the understanding of leads and lags in the climate system, and (iii) the exact astronomical frequencies that are present in climate proxy records after tuning

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