Precessional Cyclicity of Seawater Pb Isotopes in the Late Miocene Mediterranean

M Gutjahr,R M Ellam,R Flecker,M Van Der Schee,S E Modestou

doi:10.1029/2018pa003538

Abstract

AbstractAstronomical tuning in the Mediterranean region is primarily based on organically‐mediated proxies, such as cyclicity of organic rich layers or changes in foraminiferal assemblages. Both during and post deposition, organic proxies can be affected by complex processes not immediately related to the changes in precession (insolation) they are assumed to reflect. Here we present an isotopic proxy which exhibits precessional cyclicity yet is inorganic. Seawater lead (Pb) isotope records over four precessional cycles between 6.6 and 6.5 Ma, from bulk sediment leachates of three Messinian, circum‐Mediterranean marginal locations, show variations consistent with precessional cyclicity. During insolation minima, the Pb isotope signatures from all three sites converge to similar values, suggesting a regional process is affecting all three locations at that time. Data from the marginal sites are compared with new data from ODP Site 978 and published data from a variety of geological archives from the Mediterranean region to determine the mechanism(s) causing the observed variability. While the comparisons are not fully conclusive, the timing of events suggest that increased dust production from North Africa during insolation minima is the most likely control. This hypothesis implies that authigenic marine Pb isotope records have the potential to provide a reliable inorganic tie point for Mediterranean cyclostratigraphy where sub‐precessional resolution is required. An inorganic tie point could also provide the means to resolve long‐standing problems in Mediterranean stratigraphy on precessional and sub‐precessional timescales which have been obscured due to post‐depositional changes (e.g., sapropel burn‐down) or suboptimal ecological conditions (e.g., the Messinian Salinity Crisis).

Highlights

The near landlocked nature of the Mediterranean Sea reduces the buffering effect of the Atlantic, rendering this basin sensitive to regional climatic changes (Gladstone et al, 2007)
This hypothesis implies that authigenic marine Pb isotope records have the potential to provide a reliable inorganic tie point for Mediterranean cyclostratigraphy where sub‐precessional resolution is required
The stratigraphy at each site is based on similar principles used to astronomically tune the deep basin Plio‐Pleistocene sections of the Mediterranean, assuming sapropelic layers coincide with precession minima (Lourens et al, 1996)

Summary

Introduction

The near landlocked nature of the Mediterranean Sea reduces the buffering effect of the Atlantic, rendering this basin sensitive to regional climatic changes (Gladstone et al, 2007). Mediterranean marine sediments reflect a dominantly precessional (approximately 20 ka) cyclicity This has been documented for both marginal and open Mediterranean basin locations since at least 13.5 Ma, in terms of visible differences in sediment characteristics (Hüsing et al, 2009; Lourens et al, 1996; Murat, 1999), changes in faunal assemblages (Blanc‐Valleron et al, 2002; Negri et al, 1999; Raffi et al, 2003; Sierro et al, 2003), and other geochemical signatures (see Rohling et al, 2015 for a review). More generally, the strong precessional cyclicity in Mediterranean sediments, have enabled high‐resolution (precessional) astronomical tuning in the Mediterranean, vastly improving the precision of the Neogene to present geologic timescale and improving chronology even beyond the Neogene (Hilgen et al, 1995, 2012; Kuiper et al, 2008; Lourens et al, 2004)

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Discussion

Conclusion