Geology

The isotopic compositions of oxygen and hydrogen in ice cores are invaluable tools for the reconstruction of past climate variations. Used alone, they give insights into the variations of the local temperature, whereas taken together they can provide information on the climatic conditions at the point of origin of the moisture. However, recent analyses of snow from shallow pits indicate that the climatic signal can become erased in very low accumulation regions, due to local processes of snow reworking. The signal-to-noise ratio decreases and the climatic signal can then only be retrieved using stacks of several snow pits. Obviously, the signal is not completely lost at this stage, otherwise it would be impossible to extract valuable climate information from ice cores as has been done, for instance, for the last glaciation. To better understand how the climatic signal is passed from the precipitation to the snow, we present here results from varied snow samples from East Antarctica....

The oldest ice core records are obtained from the East Antarctic plateau. Water stable isotopes records are key for reconstructions of past climatic conditions both over the ice sheet and at the evaporation source. The accuracy of such climate reconstructions crucially depends on the knowledge of all the processes affecting the water vapour, precipitation and snow isotopic composition. Atmospheric fractionation processes are well understood and can be integrated in Rayleigh distillation and complex isotope enabled climate models. However, a comprehensive quantitative understanding of processes potentially altering the snow isotopic composition after the deposition is still missing, especially for exchanges between vapour and snow. In low accumulation sites such as found on the East Antarctic Plateau, these poorly constrained processes are especially likely to play a significant role. This limits the interpretation of isotopic composition from ice core records, specifically at short ...

La civilisation égyptienne s'est organisée et développée pendant la période prédynastique (7300 à 5000 années avant le présent c'est-à-dire 1950, ou années « BP »), parallèlement à un changement climatique important : la fin de la Période Humide Africaine. Le déclin des précipitations associé à ce changement climatique affecterait d'abord l'Éthiopie (peut-être dès 8000 BP), puis l'ensemble des régions tropicales et sub-tropicales entre 6000 et 5000 BP. Plusieurs sources documentent un pic d'aridité vers 5200 BP (Thompson et al., 2002 ; Bar-Matthews et al., 1999). Celui-ci pourrait avoir contribué au regroupement des populations sahariennes dans la vallée du Nil, et donc à l'essor de la civilisation égyptienne. Après cet événement, le climat est généralement aride mais instable, les pics d'aridité (notamment à 4000 BP) alternant avec des périodes aux conditions plus humides. Enfin, à 1500 BP, les précipitations se stabiliseraient à un niveau particulièrement bas. Par conséquent, les fluctuations climatiques auraient également pu affecter l'Égypte au cours de la période dynastique (5000 à 2000 BP). Ici, des mesures de rapports d'isotopes stables de l'oxygène (18 O/ 16 O) du phosphate d'os et d'émail de momies égyptiennes ont été réalisées afin de suivre l'évolution du climat entre 5500 et 1500 BP. Les valeurs de δ 18 O dans les tissus minéralisés reflètent la composition isotopique de l'eau de boisson des Égyptiens, c'està-dire de l'eau du Nil. Les valeurs du δ 18 O de l'eau du Nil ont été calculées et montrent un enrichissement en isotopes lourd à la fin de l'intervalle considéré. Ceci traduit une réduction de la quantité de précipitations et/ou une augmentation de température dans les régions des sources du fleuve, en Éthiopie et sur le Plateau des Lacs Équatoriaux. Ces résultats confirment que l'aridification s'est poursuivie pendant la période dynastique conduisant ainsi à s'interroger sur son effet sur la prospérité égyptienne.

To evaluate the impact of vapor diffusion on isotopic composition variations in snow pits and then in ice cores, we introduced water isotopes in the detailed snowpack model Crocus. At each step and for each snow layer, (1) the initial isotopic composition of vapor is taken at equilibrium with the solid phase, (2) a kinetic fractionation is applied during transport, and (3) vapor is condensed or snow is sublimated to compensate for deviation to vapor pressure at saturation. We study the different effects of temperature gradient, compaction, wind compaction, and precipitation on the final vertical isotopic profiles. We also run complete simulations of vapor diffusion along isotopic gradients and of vapor diffusion driven by temperature gradients at GRIP, Greenland and at Dome C, Antarctica over periods of 1 or 10 years. The vapor diffusion tends to smooth the original seasonal signal, with an attenuation of 7 to 12 % of the original signal over 10 years at GRIP. This is smaller than the observed attenuation in ice cores, indicating that the model attenuation due to diffusion is underestimated or that other processes, such as ventilation, influence attenuation. At Dome C, the attenuation is stronger (18 %), probably because of the lower accumulation and stronger δ 18 O gradients.

The oldest ice core records are obtained from the East Antarctic Plateau. Water isotopes are key proxies to reconstructing past climatic conditions over the ice sheet and at the evaporation source. The accuracy of climate reconstructions depends on knowledge of all processes affecting water vapour, precipitation and snow isotopic compositions. Fractionation processes are well understood and can be integrated in trajectory-based Rayleigh distillation and isotopeenabled climate models. However, a quantitative understanding of processes potentially altering snow isotopic composition after deposition is still missing. In low-accumulation sites, such as those found in East Antarctica, these poorly constrained processes are likely to play a significant role and limit the interpretability of an ice core's isotopic composition. By combining observations of isotopic composition in vapour, precipitation, surface snow and buried snow from Dome C, a deep ice core site on the East Antarctic Plateau, we found indications of a seasonal impact of metamorphism on the surface snow isotopic signal when compared to the initial precipitation. Particularly in summer, exchanges of water molecules between vapour and snow are driven by the diurnal sublimation-condensation cycles. Overall, we observe in between precipitation events modification of the surface snow isotopic composition. Using high-resolution water isotopic composition profiles from snow pits at five Antarctic sites with different accumulation rates, we identified common patterns which cannot be attributed to the seasonal variability of precipitation. These differences in the precipitation, surface snow and buried snow isotopic composition provide evidence of post-deposition processes affecting ice core records in low-accumulation areas. 1 Introduction Ice is a natural archive of past climate variations. Its physicochemical composition, including trapped air bubbles, is used as paleoclimate proxies (Jouzel and Masson-Delmotte, 2010, and references therein). The water isotopes in ice cores collected over large ice sheets permit reconstructions of past temperatures as far back as the last glacial period in West

Advances in laser spectroscopic techniques now enable high-frequency in situ measurements of the water vapour isotope composition on platforms such as ship and aircraft. However, the isotope compositions and the d-excess measured by this technique can be substantially affected by water concentrations, especially at low humidity. This measurement artefact is known as the "humidity dependency", and is commonly corrected for uniformly across all isotope compositions. Here we systematically investigate the behaviour of the humidity dependency on three commercial cavity ring-down spectrometers for different water standards within a humidity range of 500 to 23'000 ppmv. We find that the isotope compositions of measured water do have a substantial impact on the shape of the humidity dependency at low humidity (below 4'000 ppmv). This isotope composition dependency can, for example, create an offset of ±0.5 and ±6 ‰ for δ 18 O and δD at ∼2'000 ppmv in the case of a Picarro L2130-i, resulting in an offset of 2-3 ‰ for the d-excess. We show that the combined isotope composition-humidity dependency can be expressed by a surface function, which allows to build a correction framework. The impact of this correction is illustrated for a case of aircraft measurements and ship-based measurements in cold environments. A systematic assessment of the robustness of our findings for three laser spectrometers shows overall constant behaviour with time (for periods ranging from one month to two years), independent of the method of characterisation (discrete liquid injection and continuous vapour streaming), dry gas supply (N 2 and synthetic air), or the sequence of humidity levels. Based on our analysis, we recommend a procedure for the characterisation and subsequent correction of the combined isotope composition-humidity dependency for this type of laser spectrometers when performing water vapour measurements in cold and dry conditions. 1 Introduction Stable water isotopes (hydrogen and oxygen) are natural tracers in the atmosphere and hydrosphere, and have long been used to improve our understanding of the hydrological cycle and climate processes (Dansgaard, 1953, 1954; Gat, 1996). Advances in laser spectroscopic techniques now allow high-frequency in situ measurements of isotope compositions of water vapour in the atmosphere (Kerstel, 2004; Kerstel and Gianfrani, 2008). One commercially available instrument of this type is the Cavity Ring-Down Spectrometer (CRDS) manufactured by Picarro Inc., USA. The measurement principle of CRDS is based upon 1

To evaluate the impact of vapor diffusion onto isotopic composition variations in the snow pits and then in ice cores, we introduced water isotopes in the detailed snowpack model Crocus. The isotopes routine is run with a 1 s resolution. At each step and for each snow layer, 1) the initial isotopic composition of vapor is taken at equilibrium with solid phase, 2) kinetic fractionation is applied during transport, and 3) condensation is realized. We study the different effects of temperature gradient, compaction, wind compaction and precipitation on the final vertical isotopic profiles. We also run complete simulations of vapor and isotopic diffusion at GRIP, Greenland and at Dome C, Antarctica over periods of 1 or 10 years. The vapor diffusion tends to smooth the original seasonal signal, with an attenuation of 9.5% of the original signal over 10 years at GRIP. This is smaller than the observed attenuation in ice cores, indicating that the model underestimates attenuation due to diffusion or that other processes, such as ventilation, also contribute to the observed attenuation. At Dome C, the attenuation is stronger (14 %), probably because of the lower accumulation and stronger δ 18 O gradients. Because vapor diffusion is not the only process responsible of the signal attenuation, it would be useful to implement in the model ventilation of the snowpack and exchanges with the atmosphere to evaluate their contribution.

Earthquakes or explosive eruptions generate tsunami, which are at the origin of thick and chaotic coastal sediments. These commonly fossiliferous deposits are formed instantaneously at the historical or geological timescale and therefore have the potential to provide snapshot records of past climates. In Crete, near the city of Palaikastro, crops out a 1- to 9-m-thick sedimentary layer deposited by a huge tsunami that has been previously estimated to be about 9 m high. The presence of volcanic ash, the geometry, the archeological and faunal contents of the sedimentary deposit along with radiocarbon dating converge for interpreting this tsunamite as coeval with the Minoan Santorini (Thera) eruption ≈3350 yr BP. During its drawback, the tsunami deposited rocky blocks and a muddy matrix containing mollusc shells dredged from the seabed as well as cattle skeletal remains and various artifacts belonging to the contemporaneous Minoan civilization. While the oxygen isotope compositions of ...

Recent advances in laser spectroscopy enable high-frequency in situ measurements of the isotope composition of water vapour. At low water vapour mixing ratios, however, the measured stable water isotope composition can be substantially affected by a measurement artefact known as the mixing ratio dependency, which is commonly considered independent of the isotope composition. Here we systematically investigate how the mixing ratio dependency, in a range from 500 to 23 000 ppmv of three commercial cavity ring-down spectrometers, is affected by the isotope composition of water vapour. We find that the isotope composition of water vapour has a substantial and systematic impact on the mixing ratio dependency for all three analysers, particularly at mixing ratios below 4000 ppmv. This isotope composition dependency can create a deviation of ± 0.5 ‰ and ±6.0 ‰ for δ 18 O and δD, respectively, at ∼ 2000 ppmv, resulting in about 2 ‰-3 ‰ deviation for the d-excess. An assessment of the robustness of our findings shows that the overall behaviour is reproducible over up to 2 years for different dry gas supplies, while being independent of the method for generating the water vapour and being the first order of the evaluation sequence. We propose replacing the univariate mixing ratio dependency corrections with a new, combined isotope composition-mixing ratio dependency correction. Using aircraft-and ship-based measurements in an Arctic environment, we illustrate a relevant application of the correction. Based on our findings, we suggest that the dependency on the isotope composition may be primarily related to spectroscopy. Repeatedly characterising the combined isotope composition-mixing ratio dependency of laser spectrometers when performing water vapour measurements at high elevations, on aircraft, or in polar regions appears critical to enable reliable data interpretation in dry environments.

Present-day ecosystems host a huge variety of organisms that interact and transfer mass and energy via a cascade of trophic levels. When and how this complex machinery was established remains largely unknown. Although exceptionally preserved biotas clearly show that Early Cambrian animals had already acquired functionalities that enabled them to exploit a wide range of food resources, there is scant direct evidence concerning their diet and exact trophic relationships. Here I describe the gut contents of Ottoia prolifica, an abundant priapulid worm from the middle Cambrian (Stage 5) Burgess Shale biota. I identify the undigested exoskeletal remains of a wide range of small invertebrates that lived at or near the water sediment interface such as hyolithids, brachiopods, different types of arthropods, polychaetes and wiwaxiids. This set of direct fossil evidence allows the first detailed reconstruction of the diet of a 505-million-year-old animal. Ottoia was a dietary generalist and had no strict feeding regime. It fed on both living individuals and decaying organic matter present in its habitat. The feeding behavior of Ottoia was remarkably simple, reduced to the transit of food through an eversible pharynx and a tubular gut with limited physical breakdown and no storage. The recognition of generalist feeding strategies, exemplified by Ottoia, reveals key-aspects of modern-style trophic complexity in the immediate aftermath of the Cambrian explosion. It also shows that the middle Cambrian ecosystem was already too complex to be understood in terms of simple linear dynamics and unique pathways.

Reexamination of the type material of the solutan Plasiacystis mobilis from the Šárka Formation (Darriwilian) of Bohemia (Czech Republic) shows that its original reconstruction is a composite, with a mixture of elements belonging to two different solutans, a rhenopyrgid edrioasteroid, and a mitrate stylophoran. The “beaver-tailed” solutan P. mobilis is redescribed, and probable affinities with both Castericystis and minervaecystids are suggested. A second solutan is also described, but left in open nomenclature (solutan indet., gen. et sp. nov.). Possible biological and ecological implications of the presence of an isorophid edrioasteroid (?Agelacrinites bohemicus) attached to the theca of this new solutan are discussed. Other fossil remains formerly assigned to P. mobilis are also described (rhenopyrgid indet. and Mitrocystites mitra ). The name “homoiostele” is proposed for the stem-like appendage of solutans. Soluta is reaffirmed as a valid class name (instead of Homoiostelea).

The preservation of digestive structures of trilobites is extremely rare. Here we describe two new examples of trilobites
from the Upper Ordovician Letná Formation (Prague Basin, Czech Republic), which display remains of the digestive
system. The first specimen, assigned to
Selenopeltis buchi
(Barrande, 1846), exhibits cavities under the posterior part of
the glabella and the axis of most thoracic segments. These cavities are interpreted as remains of metamerically paired di
-
gestive caeca and constitute the first example of preserved digestive structures in the order Odontopleurida. The second
specimen belongs to
Birmanites ingens
(Barrande, 1852) and displays a tube-like structure, filled with a finely-grained
material, that runs under the axial lobe of the entire trunk. We interpret this structure as a gut infilling similar to that re
-
peatedly observed in the Moroccan
Basilicus calzadai
. These specimens confirm that the depositional environment of
the Letná Formation was locally favourable to soft-tissue preservation. They also further document the presence of two
different types of digestive systems in trilobites. The possibility that different processes might have been involved in the
preservation of different parts of the trilobite gut is discussed, and several criteria to differentiate genuine gut remains
from scavenger burrows are proposed.

A new edrioblastoid Cambroblastus guolensis is described from the Furongian (late 8 Cambrian) Sandu Formation (South China). Surprisingly it preserves a complete 9 stem and a distal holdfast suggesting edrioblastoids were hard substrate attachers 10 living in soft bottom quite environments. 11 12 13 Edrioblastoids are an extremely rare grade of Lower Palaeozoic echinoderms known 14 only from a limited number of species ranging from the late Cambrian to the Late 15

Aglaspidids represent an obscure group of lower Palaeozoic arthropods with a patchy biogeographic distribution. Before the recent description of a representative from Tasmania, these arthropods were exclusively known from Laurentia during the late Cambrian. Here we describe a new species, Aglaspella sanduensis sp. nov., from the Furongian of China, confirming that aglaspidids sensu stricto were already widely distributed worldwide by the late Cambrian; this demonstrates that some aglaspidids had great dispersal capabilities. A new diagnosis of the genus Aglaspella is proposed and the species formerly known as Aglaspella eatoni is assigned to a new taxon, Hesselbonia gen. nov.

The Guzhangian Weeks Formation preserves a diverse, yet virtually unstudied, non-trilobite arthropod fauna. Here we describe Tremaglaspis vanroyi sp. nov., the oldest representative of an enigmatic group of extinct arthropods, the Aglaspidida. Tremaglaspis was previously known from the Lower Ordovician and its morphology was regarded as particularly derived within the clade. Its occurrence in the Cambrian of Utah suggests that much of the early evolutionary history of the Aglaspidida remains unknown. A review of the environmental settings of previous aglaspidid findings suggests that these arthropods preferentially inhabited shallow-water environments, which may partially explain their limited fossil record.(Received March 05 2013)(Accepted April 19 2013)

Enrolment was a major defensive strategy for trilobites that significantly contributed to their evolutionary success. The ability to enrol also helped to constrain the morphological evolution of trilobites, which in part was driven by the need to improve this capability to encapsulate the soft parts of the body within the mineralized dorsal exoskeleton. Here, we describe a unique example of gut content fossilization in an enrolled trilobite from the Cambrian of China, and we propose a taphonomic scenario that considers the possible implication of enrolment in this exceptional preservation. A micro-facies analysis indicates that the specimen was entombed during an obrution event and remained intact due to limited infaunal activity. The encapsulation of the body did not prevent the decay of soft tissues, but it permitted the delicate gut content to be protected during diagenesis. In isolating the decaying soft tissues from the external environment, enrolment might also have favoured the establishment of microenvironmental conditions conducive to the precipitation of pyrite framboids. Within the gut, the formation of such crystals may have led to the consolidation of the ingested material. These results suggest that fossilized gut contents might be quite common in enrolled trilobites. Textural and compositional analyses reveal that the gut content is similar to the sediment surrounding the fossil except for the presence of pyrite framboids that indicate higher initial organic matter content. The complete enrolment of the body argues against an accidental ingestion of this material or a diagenetic origin for it. Accordingly, detritus feeding habits are inferred for this ptychopariid trilobite.

Isoxys is a cosmopolitan bivalved arthropod genus known almost exclusively from Cambrian Konservat-Lagerstätten. Despite its wide geographical distribution in such sites of exceptional preservation, little was known of its soft-part anatomy until recently when remains of eyes and raptorial frontal appendages were discovered. This absence has precluded determination of affinities. The new discovery of soft parts led to two important hypotheses: (1) that Isoxys was related to the ‘great-appendage’ arthropods and (2) that its contained species were not congeneric. Neither has been tested using a detailed cladistics analysis. The morphology of Isoxys is re-evaluated and coded into an extensive cladistics analysis. Our results indicate that Isoxys was indeed a monophyletic genus with all representatives united by the presence of an expansive dorsal shield with prominent antero- and posterolateral cardinal spines. It also indicates that Isoxys occupies a crucial role in arthropod evolution, resolving at the base of Arthropoda. The ‘great appendages’ of Isoxys are interpreted as innovating from either the protocerebral or deutocerebral somite and are therefore not homologous to those of other ‘great-appendage’ arthropods, which are interpreted as originating from the tritocerebral somite of the head.