Rachel Flecker

The Early Eocene equable climate problem: can perturbations of climate model parameters identify possible solutions?

Geological data for the Early Eocene (56–47.8 Ma) indicate extensive global warming, with very warm temperatures at both poles. However, despite numerous attempts to simulate this warmth, there are remarkable data–model differences in the prediction of these polar surface temperatures, resulting in the so-called ‘equable climate problem’. In this paper, for the first time an ensemble with a perturbed climate-sensitive model parameters approach has been applied to modelling the Early Eocene climate. We performed more than 100 simulations with perturbed physics parameters, and identified two simulations that have an optimal fit with the proxy data. We have simulated the warmth of the Early Eocene at 560 ppmv CO2, which is a much lower CO2 level than many other models. We investigate the changes in atmospheric circulation, cloud properties and ocean circulation that are common to these simulations and how they differ from the remaining simulations in order to understand what mechanisms contribute to the polar warming. The parameter set from one of the optimal Early Eocene simulations also produces a favourable fit for the last glacial maximum boundary climate and outperforms the control parameter set for the present day. Although this does not ‘prove’ that this model is correct, it is very encouraging that there is a parameter set that creates a climate model able to simulate well very different palaeoclimates and the present-day climate. Interestingly, to achieve the great warmth of the Early Eocene this version of the model does not have a strong future climate change Charney climate sensitivity. It produces a Charney climate sensitivity of 2.7°C, whereas the mean value of the 18 models in the IPCC Fourth Assessment Report (AR4) is 3.26°C±0.69°C. Thus, this value is within the range and below the mean of the models included in the AR4.

Mediterranean outflow pump: An alternative mechanism for the Lago-mare and the end of the Messinian Salinity Crisis

The final stage of the Messinian salinity crisis (MSC) was characterized by brackish-water “Lago-mare” conditions in the intermediate and marginal basins of the Mediterranean Sea. The presence of Paratethyan (former Black Sea) fauna in these deposits has fueled long-lasting controversies over the connectivity between the Mediterranean and Paratethys and contemporary sea-level drops in both basins. Here, we use the results of sub-precessional climate simulations to calculate the freshwater budget of the Mediterranean and Paratethys in the Messinian. We show that, during the MSC, the freshwater budget of Paratethys was positive, while the Mediterranean was negative. Using these numerical constraints, we propose a Mediterranean outflow pump as an alternative scenario for the two most dramatic hydrological changes in the MSC: (1) the Halite–Lago-mare transition and (2) the Pliocene reestablishment of marine conditions. Following the maximum MSC lowstand during halite formation, progressive Mediterranean sea-level rise resulting from African river runoff and overspill from both the Atlantic and Paratethys eventually reached the level of the Paratethys sill. A density contrast at this gateway caused dense Mediterranean waters to flow into the Paratethys, driving a compensatory return flow. This “pump” mechanism significantly enhanced Paratethyan inflow to the Mediterranean, creating suitable conditions for the Lago-mare fauna to migrate and thrive. When the Mediterranean sea level finally reached the height of the Gibraltar sill, Mediterranean outflow restarted there and enhanced exchange with the Atlantic Ocean. During this reorganization of the circulation, brackish and hypersaline waters were pumped out of the Mediterranean, and open-marine conditions were reestablished without major flooding of the basin at the Miocene-Pliocene boundary.

The impacts of Tibetan uplift on palaeoclimate proxies

Abstract Several palaeoclimate proxy records have been interpreted as representing the direct effects of Tibetan uplift on climate, and particularly the intensity of the Asian summer monsoon. However, there are other possible causes for the transitions and changes which have been observed, such as varying greenhouse gas concentrations, nodes or extremes in orbital forcing, and changing continental configurations. In this study we model the direct effects of Tibetan uplift on sea surface temperatures (SSTs), vegetation, and river discharge. We investigate whether these climatic effects of topographic uplift are likely to be detectable in proxy records, and also whether the proxies could be used to distinguish between different paradigms for the history of plateau uplift. We find that the SSTs in the western Pacific, South China Sea and Indian Ocean are generally insensitive to Tibetan uplift; however, vegetation in the region of the plateau itself, and river discharge from the Yangtze, Pearl, and in particular the Ganges/Brahmaputra, could provide a good test of our understanding of Tibetan uplift history.

Miocene to Pleistocene osmium isotopic records of the Mediterranean sediments

In the late Miocene the Mediterranean Sea experienced a salinity crisis and thick sequences of evaporites precipitated across the deep and marginal basins. In this study we report Os isotopic records from Deep Sea Drilling Project and Ocean Drilling Project cores in the Mediterranean: the Balearic Sea (Site 372), the Tyrrhenian Sea (Site 654), the Ionian Basin (Site 374), and the Florence Rise (Sites 375–376), as well as Integrated Ocean Drilling Project Site U1387 in Gulf of Cadiz, North Atlantic. Pliocene-Pleistocene sediments at all sites show 187Os/188Os values close to that of the coeval ocean water, indicating that the Mediterranean was connected to the North Atlantic. Evaporitic sediments deposited during the latest Miocene, however, have 187Os/188Os values significantly lower than coeval ocean water values. The offset of the Mediterranean evaporite 187Os/188Os is attributed to limited exchange with the North Atlantic during the Messinian salinity crisis. The source of unradiogenic Os is likely to be weathering of ultramafic rocks (ophiolites) cropping out in the Mediterraneans drainage basins. Based on a box model we estimated the amount of unradiogenic Os and the Atlantic-Mediterranean exchange rate to explain this offset. Os isotopic ratios of the pre-evaporite sediments in the western Mediterranean are almost identical to that of the coeval ocean water. In contrast, equivalent sediments from the Florence Rise have significantly lower 187Os/188Os values. The offset in the Os isotopic ratio on the Florence Rise is attributed either to limited water exchange between eastern and western Mediterranean or to local effects associated with exhumation of the Troodos ophiolites (Cyprus).

Injected Sand Sills in a Strike-slip Fault Zone: A Case Study from the Pil'sk Suite (Miocene), Southeast Schmidt Peninsula, Sakhalin

This chapter describes sandstone sills intruded into the middle Miocene Pilsk Suite, which are well exposed along a continuously exposed 8-km (5-mi) section on the southeast coast of Schmidt Peninsula, Sakhalin (Russian Far East). This suite forms part of a relatively thin Neogene succession (probably no more than 1100 m thick [3608 ft]), which unconformably overlies Cretaceous deposits. The succession was deposited on a topographic high, which marks the Mesozoic outer arc high and is now deformed within a large-scale dextral strike-slip fault system, part of the Eurasia–North America plate boundary. The Pilsk Suite is dominated by organic-rich biosiliceous rocks (opoka, porcelanite, and rare chert). Within this siliceous succession are abundant brown, well-cemented sandstones, subparallel to bedding, with erosional top and bottom contacts. Their changing stratigraphic position makes correlation across distances greater than about 10 m (33 ft) difficult. They are interpreted as having an injected origin. Trains of upright to gently inclined folds deform the pre-late Miocene succession, including the brown sandstones. The vergence of these structures, and the transport direction of associated thrust faults, is generally toward the southwest. The injected sandstone sills were clearly cemented prior to brittle deformation. Deformation is concentrated into high-strain zones spaced approximately 1 km (0.6 mi) apart, which are probably part of a large flower structure. Almost all the sandstone sills are found within these zones, adding several tens of meters to the stratigraphic thickness. Given the relatively thin post-middle Miocene succession, it is likely that this injection resulted in coeval deformation of the sediment surface.

Precessional variability of 87Sr/86Sr in the late Miocene Sorbas Basin: An interdisciplinary study of drivers of interbasin exchange

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 Mediterraneans 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.

Data on lithofacies, sedimentology and palaeontology of South Rifian Corridor sections (Morocco)

We provide lithological, sedimentological and micropalaeontological descriptions of 39 sections and boreholes crossing the upper Miocene deposits of the Rifian Corridor. These deposits represent the sedimentary remnants of the marine gateway that connected the Atlantic to the Mediterranean in the late Miocene. Results from these 39 sites were adopted to reconstruct the palaeogeographic evolution of the gateway presented in the associated research article (Capella et al., 2018) [1]. For each outcrop we present a synthesis of field observations, lithofacies, key sedimentological features, planktic and benthic assemblages.

ABSTRACT: Structural Control of Deltas: Are Deltaic Sediments Predictable?

Deltaic sediments are commonly described in terms of dominant control: fluvial processes, tidal currents or waves. Intermediate types are recognised in this classification, but most sedimentological deltaic models are expressed in passive (autocyclic) ways. Deltas are easily fitted into a sequence stratigraphic framework, with sea level (‘global’, or at least ‘regional’) as the main external control. Control of prodelta and slope sedimentation by relatively shallow syndepositional deformation (slumping and diapirism) is also recognised. However, more deep-seated tectonic processes must be taken into account; evidence from active margins shows that even very large deltas are susceptible to external (allocyclic) control by contemporaneous structural deformation. In the Miocene-Pliocene delta of the Amur on Sakhalin, sand distribution is counterintuitive, with dispersal patterns controlled by transtensional deformation. In the cases of the Pliocene-Recent delta of the Colorado River (North America) and the Cretaceous delta in the southern part of the Antarctic Peninsula the dominant control was basin shape and pre-existing topography. In all three cases, the deltaic sediments appear abruptly and are clearly compositionally different from pre-existing sediments, suggesting profound tectonic control. Similar processes can be inferred for the delta of the Orinoco and for many of the deltas on the island of Borneo. The field and review evidence that we present here shows that the traditional triangular classification is not adequate to capture the full range of delta architectures possible. We propose a new classification including the possibility of syndepositional structural control.