Radiogenic isotopes in the Mediterranean Sea: water mass exchange and precessional variability during the Messinian

During the late Miocene, exchange between the Mediterranean Sea and Atlantic Ocean changed dramatically, culminating in the Messinian Salinity Crisis (MSC). Understanding Mediterranean-Atlantic exchange at that time could answer the enigmatic question of how so much salt built up within the Mediterranean, while furthering the development of a framework for future studies attempting to understand how changes may have impacted global thermohaline circulation. Due to their association with specific water masses at different scales, radiogenic Sr, Pb, and Nd isotope records were generated from various archives contained within marine deposits to endeavour to understand better late Miocene Mediterranean-Atlantic exchange. The archives used include foraminiferal calcite (Sr), fish teeth and bone (Nd), dispersed authigenic ferromanganese oxyhydroxides (Nd, Pb), and a ferromanganese crust (Pb). The primary focus is on sediments preserved at one end of the Betic corridor, a gateway that once connected the Mediterranean to the Atlantic through southern Spain, although other locations are investigated. The Betic gateway terminated within several marginal sub-basins before entering the Western Mediterranean; one of these is the Sorbas Basin, a well-studied location whose sediments have been astronomically tuned at high temporal resolution, providing the necessary age control for sub-precessional resolution records. Since the climatic history of the Mediterranean is strongly controlled by precessional changes in regional climate, the aim was to produce records at high (sub-precessional) temporal resolution, to be able to observe clearly any precessional cyclicity driven by regional climate which could be superimposed over longer trends. This goal was achieved for all records except the ferromanganese crust record. The 87Sr/86Sr isotope record (Ch. 3) shows precessional frequency excursions away from the global seawater curve. As precessional frequency oscillations are unexpected for this setting, a numerical box model was used to determine the mechanisms causing the excursions. To enable parameterisation of model variables, regional Sr characteristics, data from general circulation model HadCM3L, and new benthic foraminiferal assemblage data are employed. The model results imply that the Sorbas Basin likely had a positive hydrologic budget in the late Miocene, very different to that of today. Moreover, the model indicates that the mechanism controlling the Sr isotope ratio of Sorbas Basin seawater was not restriction, but a lack of density-driven exchange with the Mediterranean. Beyond improving our understanding of how marginal Mediterranean sub-basins may evolve different isotope signatures, these results have implications for astronomical tuning and stratigraphy in the region, findings which are crucial considering the geological and climatic history of the late Miocene Mediterranean is based entirely on marginal deposits. An improved estimate for the Nd isotope signature of late Miocene Mediterranean Outflow (MO) was determined by comparing Nd isotope signatures preserved in the deeper Alborán Sea at ODP Site 978 with literature data as well as the signature preserved in the Sorbas Basin (Ch. 4; -9.34 to -9.92 ± 0.37 εNd(t)). It was also inferred that it is unlikely that Nd isotopes can be used reliably to track changes in circulation within the shallow settings characteristic of the Mediterranean-Atlantic connections; this is significant in light of a recent publication documenting corridor closure using Nd isotopes. Both conclusions will prove useful for future studies attempting to understand changes in Mediterranean-Atlantic exchange. Excursions to high values, with precessional frequency, are also observed in the radiogenic Pb isotope record for the Sorbas Basin (Ch. 5). Widening the scope to include locations further away from the gateways, records were produced for late Miocene sections on Sicily and Northern Italy, and similar precessional frequency cyclicity was observed in the Pb isotope records for these sites as well. Comparing these records to proxies for Saharan dust and available whole rock data indicates that, while further analysis is necessary to draw strong conclusions, enhanced dust production during insolation minima may be driving the observed signal. These records also have implications for astronomical tuning; peaks in Pb isotope records driven by Saharan dust may be easier to connect directly to the insolation cycle, providing improved astronomical tuning points. Finally, a Pb isotope record derived using in-situ laser ablation performed on ferromanganese crust 3514-6 from the Lion Seamount, located west of Gibraltar within the MO plume, has provided evidence that plume depth shifted during the Pliocene. The record also suggests that Pb isotopes may not be a suitable proxy for changes in late Miocene Mediterranean-Atlantic exchange, since the Pb isotope signatures of regional water masses are too similar. To develop this record, the first published instance of laser ablation derived 230Thexcess measurements are combined with 10Be dating

Southern Ocean bottom-water cooling and ice sheet expansion during the middle Miocene climate transition

The middle Miocene climate transition (MMCT), around 14 Ma, was associated with a significant climatic shift, but the mechanisms triggering the event remain enigmatic. We present a clumped isotope (Δ47) bottom-water temperature (BWT) record from 16.0 to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean and compare it to existing BWT records from different latitudes. We show that BWTs in the Southern Ocean reached 8–10 ∘C during the Miocene climatic optimum. These high BWT values indicate considerably warmer bottom-water conditions than today. Nonetheless, bottom-water δ18O (calculated from foraminiferal δ18O and Δ47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset further demonstrates that BWTs at Site 747 were variable with an overall cooling trend across the MMCT. Notably, a cooling of around 3–5 ∘C preceded the stepped main increase in benthic δ18O, interpreted as global ice volume expansion, and appears to have been followed by a transient bottom-water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom-water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Feedbacks required for substantial ice growth and/or tectonic processes may have contributed to the observed decoupling of global ice volume and Southern Ocean BWT.publishedVersio

Coupled Southern Ocean cooling and Antarctic ice sheet expansion during the middle Miocene

The middle Miocene climate transition (~14 million years ago) was characterized by a dramatic increase in the volume of the Antarctic ice sheet. The driving mechanism of this transition remains under discussion, with hypotheses including circulation changes, declining carbon dioxide in the atmosphere and orbital forcing. Southern Ocean records of planktic foraminiferal Mg/Ca have previously been interpreted to indicate a cooling of 6–7 °C and a decrease in salinity that preceded Antarctic cryosphere expansion by up to ~300,000 years. This interpretation has led to the hypothesis that changes in meridional heat and vapour transport along with an early thermal isolation of Antarctica from extrapolar climates played a fundamental role in triggering ice growth. Here we revisit the middle Miocene Southern Ocean temperature evolution using clumped isotope and lipid biomarker temperature proxies. Our records indicate that the Southern Ocean cooling and the associated salinity decrease occurred in phase with the expansion of the Antarctic ice sheet. We demonstrate that the timing and magnitude of the Southern Ocean temperature change seen in previous reconstructions can be explained if we consider pH as an additional, non-thermal, control on foraminiferal Mg/Ca ratios. Therefore, our new dataset challenges the view of a thermal isolation of Antarctica preceding ice sheet expansion, and suggests a strong coupling between Southern Ocean conditions and Antarctic ice volume in times of declining atmospheric carbon dioxide.acceptedVersio

Precessional variability of 87 Sr/86 Sr 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 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

Clumped isotope and Δ17O measurements of carbonates in CM carbonaceous chondrites: new insights into parent body thermal and fluid evolution

The CM carbonaceous chondrites are key archives for understanding the earliest history of the solar system. Their C-complex asteroid parent body(ies) underwent aqueous alteration, among the products of which are carbonate minerals that can faithfully record the conditions of their formation. In this study we report carbon, triple oxygen and clumped isotope compositions of carbonates in six CM chondrites which span a range in degrees of aqueous alteration (Allan Hills 83100, Cold Bokkeveld, LaPaz Icefield 031166, Lonewolf Nunataks 94101, Murchison, Scott Glacier 06043). Δ¹⁷O values range from −2.6 to −1.0 ‰ (±0.1), and where calcite and dolomite co-exist their Δ¹⁷O differ by 0.6 permil, suggesting precipitation from distinct fluids. Calculated crystallization temperatures range from 5 to 51 °C for calcite (typically ± 10 °C) and 75 to 101(±15) °C for dolomite. The δ¹⁸ᴼVSMOW of the aqueous fluids from which they formed ranges from −6.6 to 2.3 ‰, with no relationship to the δ¹³C of carbonates. As the population of carbonates in any one CM chondrite can include multiple generations of grains that formed at different conditions, these values represent the mode of the temperature of carbonate formation for each meteorite. We observe that in the more altered meteorites carbonate Δ¹⁷O values are lower and formation temperatures are higher. These correlations are consistent with aqueous alteration of the CM chondrites being a prograde reaction whereby the hotter fluids had undergone greater isotope exchange with the anhydrous matrix. Our data are broadly consistent with the closed system model for water/rock interaction, but carbonate mineral formation in the latter stages of aqueous alteration may be linked to fluid movement via fractures

Tracing the salt crystallization front in limestone using the DRMS

Salt crystallization is widely recognised as one of the most damaging factors affecting stone monuments and buildings. The conservation of stone heritage suffering from weathering due to the presence of salts can be very expensive and laborious. Furthermore, the assessment of salt-laden buildings alone is often challenging and costly in its own right. This paper focuses on the use of a micro-destructive technique, the Drilling Resistance Measurement System (DRMS), to trace the salt crystallization front in natural building stone subjected to continuous partial immersion in a salt solution. The results provide strong evidence of the efficacy of micro-drilling to “map” the crystallization front. This is facilitated by increased resistances recorded during drilling in areas where pore clogging due to salt crystallization occurs. The experimental results are important for future research aiming to validate models relating rising damp to salt crystallization. The combination of DRMS and continuous partial immersion in salt solution laboratory tests may also be used to investigate the effect of water repellents and coatings/finishes on salt transport and crystallization in porous materials. Last but not least, the potential of the portable DRMS to trace the salt crystallization front in limestone may be used to detect cryptoflorescence in situ, on stone monuments, before it becomes damaging

Micro-destructive mapping of the salt crystallization front in limestone

Salt crystallization is widely recognized as one of the most damaging factors affecting stone monuments and buildings. Conservation of structures suffering from weathering due to salt crystallization can be immensely expensive and time-consuming. The assessment of salt-laden buildings alone is often challenging and costly in its own right. In this paper, the Drilling Resistance Measurement System (DRMS) and the scratch tool are evaluated for their ability to map salt crystallization in natural building stone. The laboratory results indicate that the two aforementioned micro-destructive techniques are indeed capable of detecting the location of the salt crystallization front in limestone impregnated with sodium sulfate. This is facilitated due to the increased resistances recorded during drilling and scratching in areas where pore clogging due to salt crystallization exists. The DRMS was further successfully employed in-situ on masonry exhibiting the effects of salt decay. The successful application of both micro-destructive techniques in the laboratory, and of the DRMS alone in-situ, suggests that these may potentially be used to detect subflorescence before it becomes damaging

Information archived on the Web Titre et Résumé Utilisation de nanoparticules d'hydroxyde de calcium pour la restauration de la pierre calcaire : imbibition et adhérence

Abstract This paper describes the effects on Portland limestone of colloidal calcium hydroxide nanoparticles ("nanolimes") dispersed in ethanol, available commercially under the trade name CaLoSiL. The effects of this conservation treatment on water uptake by the stone are discussed along with pore filling by the treatment (because deterioration processes are driven by water flux, understanding the impact on water movement and pore filling is essential). A two-layer model was applied to describe the imbibition behaviour exhibited by our samples. Depth penetration of the nanoparticles was investigated through scratching and drilling techniques, which are novel methods for quantifying consolidation. In addition, the efficacy of a nanolime-based repair mortar as an adhesive for repairing detached surface features was assessed through lap shear strength of repaired stone. CaLoSiL penetration depth into unweathered Portland limestone was found to be < 1 mm. Drilling resistance tests predicted little alteration in compressive strength of stone treated with CaLoSiL. Shear strength of the repair mortar was found to increase with time, probably due to carbonation. While the force per unit area generated by the weight of the re-attached fragment is less than the initial strength of the mortar-stone adhesive bond, the use of this adhesive is favourable

Middle Miocene bottom water carbonate clumped isotope temperatures, ODP Hole 120-747A, Kerguelen Plateau

This table includes temperatures calculated from carbonate clumped isotope measurements performed on middle Miocene benthic foraminifera from Ocean Drilling Program (ODP) Hole 120-747A, Kerguelen Plateau, and is an accompaniment to the manuscript "Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition", Climate of the Past, submitted. A complete set of individual measurement data, including sample labels, depth and raw mass spectrometer data, can be found on the EarthChem database (Leutert et al., 2021 doi:10.26022/IEDA/111808)