Precessional variability of⁸⁷Sr/⁸⁶Sr in the late Miocene Sorbas Basin: An interdisciplinary study of drivers of interbasin exchange

Abstract

We present the first subprecessional record of seawater 87Sr/86Sr isotope ratios for a marginal Mediterranean subbasin. The sediments contained in this interval (three precessional cycles between 6.60 and 6.55 Ma) are important because they record conditions during the transition to the Messinian Salinity Crisis (MSC; 5.97 to 5.33 Ma), an event for which many details are still poorly understood. The record, derived from planktic foraminifera of the late Miocene Sorbas Basin (SE Spain), shows brief excursions with precessional cyclicity to 87Sr/86Sr ratios higher than coeval ocean 87Sr/86Sr. The hydrologic conditions required to generate the observed record are investigated using box modeling, constrained using a new paleodepth estimate (150 to 250 m) based on benthic foraminiferal assemblages. The box model results highlight the role of climate‐driven interbasin density contrast as a significant driver of, or impediment to, exchange. The results are particularly significant in the context of the MSC, where 87Sr/86Sr excursions have been interpreted purely as a consequence of physical restriction. To replicate the observed temporal patterns of lithological variations and 87Sr/86Sr isotope excursions, the Sorbas Basin “box” must have a mainly positive hydrologic budget, in contrast with the Mediterranean's negative budget during the late Miocene. This result has implications for the assumption of synchronous deposition of specific sedimentary layers (sapropels) between marginal and open Mediterranean settings at subprecessional resolution. A net positive hydrologic budget in marginal Mediterranean subbasins may reconcile observations of freshwater inclusions in gypsum deposits.

Precessional Drivers of Late Miocene Mediterranean Sedimentary Sequences: African Summer Monsoon and Atlantic Winter Storm Tracks

“…In fact, it has already been suggested that tying the midpoint of the sapropelic layers to precession minima may be incorrect for the tuning of some Messinian sections (e.g. Krijgsman et al, ; Perez‐Folgado et al, ), particularly in the well‐exposed marginal basin sections where different lithologies may be time‐equivalent to sapropels formed in the main basin (Modestou et al, ). Although these phase lags do not significantly change Messinian chronology, they do affect our interpretation of the marginal and open‐marine Mediterranean environments.…”

“…This could be achieved with the combination of idealized experiments (e.g. Modestou et al, 2017) and regional models (e.g. Meijer and Tuenter, 2007) informed by global climate simulations such as those used in this study.…”

Precessional Cyclicity of Seawater Pb Isotopes in the Late Miocene Mediterranean

“…The interbedded marls contain benthic organisms and bioturbation, recording the transition between anoxic and well‐ventilated bottom water conditions (Sierro et al, ). Based on benthic foraminiferal assemblages, the Sorbas Basin was approximately 150‐ to 250 m deep during the interval investigated in this study (Modestou et al, ). The cyclical changes in planktic foraminiferal assemblages, and the planktic assemblages themselves, are identical to the ~1,000 m deep sediments exposed at Gavdos, Crete in the Eastern Mediterranean (Kouwenhoven et al, ; Pérez‐Folgado et al, ).…”

“…In the Sorbas Basin, Pérez‐Folgado et al () determined from foraminiferal assemblages that sapropelic layers do not coincide with insolation maxima, but rather are triggered by the transition from insolation minima to insolation maxima, implying they lead insolation maxima. In contrast, Modestou et al () found that Sorbas Basin sediment layers may lag open Mediterranean deposition by ~1 ka based on hydrological modeling of Sr isotopes preserved in foraminifera.…”

“…Fluvial fluxes are also an important component of a basin's hydrologic budget, which could fluctuate with precession and impact the Pb budget. The behavior of other isotope systems, such as Sr or Nd, in the Mediterranean has been successfully modeled (e.g., Cornuault et al, ; Modestou et al, ; Tachikawa et al, ), providing much needed insight into the physical processes responsible for shifting isotope signatures, including changes in fluvial fluxes. However, in the case of Pb, so little is known about natural Pb riverine concentrations in the Mediterranean region that it is difficult to assess the validity of model results for isotopes of this element.…”