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Deep‐water of the eastern Mediterranean basins mainly forms in the Adriatic Sea and episodically in the Aegean Sea. However, past and future variability under changing temperature and freshwater inputs remains unclear. We reconstruct seawater εNd from Northern Ionian Sea cores to trace intermediate‐ and deep‐water provenance during the Holocene. High organic matter deposition during S1a and S1b corresponds to radiogenic εNd values (up to −4.9), indicating reduced and shallower Adriatic Deep Water (AdDW) formation, with Ionian Sea increasingly dominated by Aegean deep water (AeDW). The re‐ventilation of deep‐water at ∼8.2 ka is marked by unradiogenic εNd (−6.5), implying active AdDW formation. In contrast, a second re‐ventilation within the S1b at intermediate depth is associated with radiogenic εNd values (−4.5) attributed to the formation of the radiogenic intermediate water from the eastern basin (LIW). These results demonstrate the high sensitivity of Mediterranean deep‐water formation to freshwater forcing and abrupt climate change.

The neodymium isotope signatures (εNd) of the authigenic fraction have been extensively used to reconstruct past seawater εNd and hydrological circulation. Among the various methods, sequential extraction of hydrogenic ferromanganese oxyhydroxides from bulk sediments represents a rapid and straightforward approach that may potentially induce artifacts due to the potential release of non‐seawater‐derived Nd during the extraction procedure. Here we investigated different methods for extracting past seawater Nd isotope compositions from a core collected in the Adriatic Sea whose tephra layers have been previously well documented. We analyzed εNd in planktonic foraminifera samples and in non‐decarbonated sediment leachates obtained with three solutions commonly used in the context of the Mediterranean Sea: (a) 0.02 M hydroxylamine hydrochloride (HH) solution, (b) 1N HCl, and (b) a 25% (v/v) acetic acid (AA). Our results show that (a) the foraminiferal εNd remains unaffected by the diagenesis of tephra content; (b) all three methods indicate significantly more radiogenic εNd values in tephra levels (up to 1.5 εNd unit), which is attributed to tephra dissolution accounting for 2.7% of extracted Nd; (c) of the three leaching methods applied to samples with low tephra content, hydroxylamine hydrochloride (HH) yields εNd values that are more consistent with those obtained on planktonic foraminifera; (d) the εNd values of planktonic foraminifera in core MD90‐917 remained constant indicating that the Adriatic deep water primarily reflects the local Nd isotope composition over the last 20 kyr. Plain Language Summary Neodymium isotope signatures (εNd) from the authigenic fraction of sediments are commonly used to reconstruct past ocean circulation and seawater composition. A common method involves extracting ferromanganese oxyhydroxides from bulk sediments, which is quick and straightforward but may introduce contamination from non‐seawater sources of neodymium during the process. In this study, we tested different methods for extracting εNd from a sediment core in the Adriatic Sea, known for its tephra layers, by comparing data from foraminifera and sediment leachates using three solutions: hydroxylamine hydrochloride (HH), HCl, and acetic acid (AA). We found that: (a) foraminiferal εNd is unaffected by tephra dissolution; (b) all methods showed more radiogenic εNd in tephra layers due to tephra dissolution; (c) the HH solution gave εNd values closest to foraminifera in low‐tephra samples; (d) foraminiferal εNd has remained stable over the last 20,000 years, indicating Adriatic deep water reflects local neodymium isotope composition. Key Points The HCl and AA leaching processes dissolve the tephras and bias the extracted authigenic εNd values εNd of HH leachate closely matches that of foraminifera and past seawater of the Adriatic Sea εNd of the Adriatic Deep Water stay stable during the last 20,000 years due to vertical water mixing and local lithogenic inputs

Located in the foreland domain of the Alpine and Pyrenean mountain belts, the French Massif Central presents enigmatic topographic features—reaching elevations of ∼1700 m above sea level and ∼1000 m of relief—that did not originate from Alpine compressional nor from extensional tectonics. Similar to other Variscan domains in Europe, such as the Bohemian, Rhenish, and Vosges/Black Forest Massifs, a Cenozoic uplift has been postulated, although its timing and quantification remain largely unconstrained. With respect to the other Variscan Massifs, the French Massif Central is wider and higher and shows a more intense late Cenozoic volcanism, suggesting that deep-seated processes have been more intense. In this study, apatite fission-track and (U-Th)/He thermochronometry were applied to investigate the long-term topographic evolution of the Massif Central. Our new thermochronological data come from the eastern flank of the massif, where sampling profiles ran from the high-elevation region down to the Rhône River valley floor with a total elevation profile of 1200 m. Age-elevation relationships, mean track-length distributions, and thermal modeling indicate a two-step cooling history: (1) a first exhumation event, already detected through previously published thermochronology data, with an onset time during the Cretaceous, and (2) a more recent Cenozoic phase that is resolved from our data, with a likely post-Eocene onset. This second erosional event is associated with relief formation and valley incision possibly induced by a long-wavelength domal uplift supported by mantle upwelling.

Interpreting the ages of supergene mineralogical phases in laterite is complex because they consist of polycrystalline mixtures of different phases at the microscopic scale that could be crystalized at different epochs. Among the geochronometers, the (U-Th)/He method on hematite and goethite is more often used, but ages can be difficult to interpret due to phases mixing. To resolve this issue, this study proposes a methodology for performing detailed mineralogical analysis of hematite and goethite single grains prior to their dating using the (U-Th)/He method. Strictly non-destructive mineralogy of single grains is not achievable by classical tools, such as conventional powder XRD (X-ray diffraction; requiring at least some milligrams of powder) or SEM (scanning electron microscopy; that can contaminate the grain by coating or fixing). Therefore, we performed X-ray diffraction patterns of single grains using high-flux X-ray beams from both a rotating anode (XRD_rotat) laboratory diffractometer and a synchrotron beamline (XRD_synch) and compared the results in order to design a method based on XRD_rotat only. For this purpose, two samples from the pisolitic facies of a Brazilian ferruginous duricrust (Alto Paranaíba region, Minas Gerais State, Brazil) were chosen because they presented a usual heterogeneity. Rietveld refinements of the XRD patterns obtained from both XRD_rotat and XRD_synch yielded similar results for the weight percentage ratio of the main phases and mean coherent domain sizes and less similar results for Al substitution rates, thus validating the XRD_rotat approach. No beam damage was observed when increasing X-ray exposure time, neither on XRD patterns nor on (U-Th)/He ages. Hence, sub-millimeter, undisturbed grains can be used to analyze the mineralogy of ferruginous duricrusts by XRD_rotat with a short exposure, and the same grains can subsequently be dated by (U-Th)/He geochronology analysis. The (U-Th)/He dating of pisolitic core and cortex grains also provided meaningful ages: they revealed two evolution phases of the ferruginous duricrust, which occurred at or before the Oligocene for the pisolitic core and middle Miocene for the pisolitic cortex, agreeing with the previous model for the development of pisolites. The mineralogy of single grains selected for dating is helpful for discussing the crystallization ages, and the high-flux XRD approach may be applied to other supergene mineral parageneses used for absolute dating.

Apatite and zircon (U–Th) / He thermochronological data are obtained through a combination of crystal selection, He content measurement by crystal heating with analysis using noble gas mass spectrometry, and measurement of U, Th, and Sm contents by crystal dissolution as well as solution analysis using inductively coupled plasma mass spectrometry (ICP-MS). This contribution documents the methods for helium thermochronology used at the GEOPS laboratory, Paris-Saclay University, between 2007 and the present that allow apatite and zircon (U–Th) / He data to be obtained with precision. More specifically, we show that the He content can be determined with precision (at 5 %) and accuracy using a calibration of the He sensitivity based on the Durango apatite, and its use also appears crucial to check for He and U–Th–Sm analytical problems. The Durango apatite used as a standard is therefore a suitable mineral to perform precise He calibration and yields (U–Th) / He ages of 31.1 ± 1.4 Ma with an analytical error of less than 5 % (1σ). The (U–Th) / He ages for the Fish Canyon Tuff zircon standard yield a dispersion of about 9 % (1σ) with a mean age of 27.0 ± 2.6 Ma, which is comparable to other laboratories. For the long-term quality control of the (U–Th) / He data, attention is paid to evaluating the drift of He sensitivity and blanks through time as well as that of (U–Th) / He ages and Th / U ratios (with Sm / Th when possible), all relying on the use of Durango apatite and Fish Canyon Tuff zircon as standards.

Timing and duration of ore deposit formation are crucial to understanding the mineralization process. To address this, the geochronological (U-Th)/He method, geochemical and H- and O-isotope compositions of pure goethite formed in the Imini karst-hosted Mn district (High Atlas, Morocco) were examined in detail. Two main generations of cavity-filling and fracture-filling goethite are identified, and both precipitated prior to the massive Mn oxide ore. The δD and δ18O values reveal that the mineralizing fluid of cavity and fracture-filling goethite is meteoric-derived but enriched in 18O due to fluid–rock interactions with the host rock dolostone or mixing with O2-rich surface water resident in an open karst system. The cavity-filling goethite precipitated between 95 to 80 Ma, whereas fracture-filling goethite formed between 80 to 50 Ma. Ore deposition occurred discontinuously during the early Atlas doming associated with one or more early compressional events in the Atlas tectonism. The increase in δD values and depletion in U content result from a change in the mineralizing fluid within the karst system. At about 50 Ma, the fluid is notably enriched in U, Cu and trace metals.

Using a multimethod approach, including polarized light microscopy (PLM), scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS), SEM with mineral liberation analyzer (MLA), X-ray diffraction (XRD), and Raman spectroscopy, we examined sub-recent ferruginous crusts in the Xingu River in the Amazon Basin that have formed since the Early Pleistocene (<1.2 Ma), as indicated by (U-Th)/He dating of goethite. Although now preserved as goethite, the size and form of the smallest components of the ironstone (nanorods) and the nature of isomorphic substitution in the goethite point to very early transformation of the original precipitate, an unstable hydrous ferrous oxide (HFO) mineral, into goethite. The fine, multiply undulating laminae of the ironstone contain abundant filamentous microbial molds and casts that together support identification of the crusts as ferruginous microbialites and suggest a role of bioinduction/bioinfluence in ironstone precipitation, although inorganic precipitation is also evident. The Xingu Quaternary ironstones are the first evidence of ferruginous microbialite in a modern freshwater system in South America and may hold clues to the recent history of the Xingu River.

The mining district of Nefza-Sejnane (Tunisia) encloses numerous ores and raw material deposits, all formed in relation with successive Fe-rich fluids of meteoric and/or hydrothermal origins. Here, for the first time in Tunisia, (U-Th)/He ages were obtained on supergene goethite from various localities/deposits of the district highlight direct dating of significant weathering episodes during late Tortonian and late Pleistocene. These weathering events are most likely associated with favorable conditions that combine (i) wet climate displaying sufficient meteoric water/fluid; and (ii) regional exhumation, due to large-scale vertical lithospheric movements enhancing the percolation of fluids. Matched with previous works, these results refine the stratigraphic frame for the polymetallic mineralization and clay deposits in the district, confirming the influence of meteoric fluids circulation during the late Cenozoic. As a consequence of the new (U-Th)/He data, we moreover propose a taxonomic and stratigraphic revision of the well-known mammalian fauna from the Fe-rich Douahria locality, suggesting an early Tortonian age for the fossils, i.e., prior to the first episode of meteoric event in the area.

Apatite fission-track and (U-Th)/He thermochronometry have been applied to investigate the long-term topographic evolution of the French Massif Central. Located in the foreland domain of the Alpine and Pyrenean mountain belts, the French Massif Central presents enigmatic topographic features, reaching 1700 m a.s.l. and 1000 m of relief, that did not originate from Alpine compressional nor from extensional tectonics. The age of the present-day topography, the timing of its formation, and the underlying processes remain debated. Our new thermochronological data come from the eastern flank of the massif, where sampling profiles run from the high-elevation region down to the Rhône river valley floor with a total elevation profile of 1200 m. Age-elevation relationships, mean track-length distributions and thermal modeling indicate a two-steps cooling history: (i) a first exhumation event, already detected through previously-published thermochronology data, with an onset time during Mid-Cretaceous; and interestingly (ii) a more recent Cenozoic phase that is resolved from our data, with a likely post-Eocene onset. This second erosional event is associated with relief formation and valley incision probably induced by a long wavelength domal uplift supported by mantle upwelling. Disclaimer: This is a confidential document and must not be discussed with others, forward in any form, or posted on websites without the express written consent of the Geological Society of America.

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