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During the Miocene (23.0–5.3 Ma) North Africa experienced both humid and arid intervals, but the underlying cause of these transitions is unknown. Earth's climate was characterized by a unipolar icehouse with a dynamic Antarctic ice sheet, which may have influenced regional hydrology through atmospheric teleconnections. However, the Miocene also witnessed the restriction of the Mesopotamian Seaway, which may have had significant climatic impacts. The Maltese il-Blata section (Central Mediterranean) comprises Late Oligocene to Early Miocene marine deposits previously used to constrain the timing of the Mesopotamian Seaway restriction using the εNd tracer. The location of this section also makes it sensitive to climatic changes in the North African region, and biogeochemical changes in the central Mediterranean. Here, we present lithological and geochemical records of the il-Blata section. We find a marked shift in lithology and an increase in sedimentation rate coeval with the Early Miocene (∼19–20 Ma) restriction of the Mesopotamian Seaway. Concomitant changes in bulk sediment CaCO3, Sr/Ca, K/Al, Ti/Al, Zr/Al, and Si/Ti support a major humid climate transition and associated intensification of river systems over western North Africa. We propose that these changes in North African hydroclimate reflect either a tipping point effect in a gradually warming global climate, or are the result of the initial restriction of the Mesopotamian Seaway, perhaps through consequent changes in Atlantic Meridional Overturning Circulation and the West African Monsoon. We also suggest the restriction of the Mesopotamian Seaway inhibited phosphorite deposition at low latitudes., peer-reviewed

This work was funded by NERC Grant NE/I006427/1 (CHL and GLF). PNP and GLF were supported by NERC/DEFRA/DECC Grant NE/H017581/1 (U.K. Ocean Acidification Research Programme). MPH was supported by NERC fellowship (NE/K00901X/1). SMS acknowledges the financial support provided by the Welsh Government and Higher Education Funding Council for Wales through the Sêr Cymru National Research Network for Low Carbon, Energy and Environment Returning Fellowship. Over the course of the Neogene, the Earth underwent profound climatic shifts from the sustained warmth of the middle Miocene to the development of Plio-Pleistocene glacial–interglacial cycles. Major perturbations in the global carbon cycle have occurred alongside these shifts, however the lack of long-term carbonate system reconstructions currently limits our understanding of the link between changes in CO2, carbon cycling, and climate over this time interval. Here we reconstruct continuous surface ocean pH, CO2, and surface ocean aragonite saturation state using boron isotopes from the planktonic foraminifer Trilobatus trilobus and we perform a sensitivity analysis of the key variables in our calculations (e.g. δ11Bsw, [Ca]sw, CCD). We show that the choice of δ11Bsw influences both seawater pH and CO2 while [Ca]sw reconstructed dissolved inorganic carbon exerts a significant influence only on CO2. Over the last 22 Myr, the lowest pH levels occurred in the Middle Miocene Climate Optimum (MMCO; 17–14 Myr ago) reaching ∼7.6 ± 0.1 units in all our scenarios. The extended warmth of the MMCO corresponds to mean CO2 and aragonite saturation state levels of 470–630 ppm and 2.7–3.5, respectively. Despite a general correspondence between our CO2 record and climate, all CO2 scenarios show a peak at ∼9 Ma not matched by corresponding changes in climate reconstructions. This may suggest decoupling (i.e. significant CO2 change without a discernible climate response) for a limited interval in the Late Miocene (11.6–8.5 Ma), although further refinement of our understanding of the temporal evolution of the boron isotopic composition of seawater is necessary to fully evaluate the nature of the relationship between CO2 and climate. Nonetheless, from our long-term view it is clear that low-latitude open ocean marine ecosystems are unlikely to have experienced sustained surface pH and saturation levels below 7.7 and 1.7, respectively, during the past 14 million years (66% CI). Publisher PDF

Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high (> 800 ppmv) atmospheric CO2 concentrations. Although a post-hoc intercomparison of Eocene (~50 million years ago, Ma) climate model simulations and geological data has been carried out previously, models of past high-CO2 periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the latest Paleocene and the early Eocene. Together these form the first phase of DeepMIP – the deeptime model intercomparison project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design consists of three core paleo simulations and a set of optional sensitivity studies. The experimental design specifies and provides guidance on boundary conditions associated with palaeogeography, greenhouse gases, orbital configuration, solar constant, land surface parameters, and aerosols. Initial conditions, simulation length, and output variables are also specified. Finally, we explain how the geological datasets, which will be used to evaluate the simulations, will be developed.

Supporting Information forSimulating Miocene warmth: insights from an opportunistic Multi-Model ensemble (MioMIP1) Table S1: New synthesis of global terrestrial MATs generated for the Middle Miocene. Table S2: Miocene SST reconstructions. Sheet 1, contains a basic description of published Miocene ocean surface temperature records. Sites are identified by their site numbers (Fig. 2), which are primarily from the Deep Sea Drilling Program, Ocean Drilling Program, or Integrated Ocean Discovery Program. The column on the far right contains hyperlinks to where the data for each site can be accessed electronically. Different colors in the proxy column are used to facilitate the ability to rapidly distinguish between records that utilize different SST proxies (Uk’37 = black, Mg/Ca = green, TEX86= purple). Note that all records are on their original age models using their original calibration. Sheet 2 lists the Late Miocene, Middle Miocene and MCO mean SST values and uncertainty bounds calculated from these records and used in the model-data comparison. Dataset S1: Updated Miocene boundary conditions used within the new CESM simulationsand described in Text S1. NetCDF file "miocene_topo_pollard_antscape_dolan_0.5x0.5.nc" contains theMiddle Miocene paleogeography and "surfdata_0096x0144_desert_BAM.nc" theland surface datasets. Dataset S2: "MioMIP1.nc" NetCDF file containing the MioMIP1 variables used to make all the figures.

Supporting Information for Simulating Miocene warmth: insights from an opportunistic Multi-Model ensemble (MioMIP1) Table S1: New synthesis of global terrestrial MATs generated for the Middle Miocene. Table S2: Miocene SST reconstructions. Sheet 1, contains a basic description of published Miocene ocean surface temperature records. Sites are identified by their site numbers (Fig. 2), which are primarily from the Deep Sea Drilling Program, Ocean Drilling Program, or Integrated Ocean Discovery Program. The column on the far right contains hyperlinks to where the data for each site can be accessed electronically. Different colors in the proxy column are used to facilitate the ability to rapidly distinguish between records that utilize different SST proxies (Uk’37 = black, Mg/Ca = green, TEX86= purple). Note that all records are on their original age models using their original calibration. Sheet 2 lists the Late Miocene, Middle Miocene and MCO mean SST values and uncertainty bounds calculated from these records and used in the model-data comparison. Dataset S1: Updated Miocene boundary conditions used within the new CESM simulations and described in Text S1. NetCDF file "miocene_topo_pollard_antscape_dolan_0.5x0.5.nc" contains the Middle Miocene paleogeography and "surfdata_0096x0144_desert_BAM.nc" the land surface datasets. Dataset S2: "MioMIP1.nc" NetCDF file containing the MioMIP1 variables used to make all the figures.