Sustained wet-dry cycling on early Mars.

The presence of perennially wet surface environments on early Mars is well documented1,2, but little is known about short-term episodicity in the early hydroclimate3. Post-depositional processes driven by such short-term fluctuations may produce distinct structures, yet these are rarely preserved in the sedimentary record4. Incomplete geological constraints have led global models of the early Mars water cycle and climate to produce diverging results5,6. Here we report observations by the Curiosity rover at Gale Crater indicating that high-frequency wet-dry cycling occurred in early Martian surface environments. We observe exhumed centimetric polygonal ridges with sulfate enrichments, joined at Y-junctions, that record cracks formed in fresh mud owing to repeated wet-dry cycles of regular intensity. Instead of sporadic hydrological activity induced by impacts or volcanoes5, our findings point to a sustained, cyclic, possibly seasonal, climate on early Mars. Furthermore, as wet-dry cycling can promote prebiotic polymerization7,8, the Gale evaporitic basin may have been particularly conducive to these processes. The observed polygonal patterns are physically and temporally associated with the transition from smectite clays to sulfate-bearing strata, a globally distributed mineral transition1. This indicates that the Noachian-Hesperian transition (3.8-3.6 billion years ago) may have sustained an Earth-like climate regime and surface environments favourable to prebiotic evolution.

Deposition, exhumation, and paleoclimate of an ancient lake deposit, Gale crater, Mars.

The landforms of northern Gale crater on Mars expose thick sequences of sedimentary rocks. Based on images obtained by the Curiosity rover, we interpret these outcrops as evidence for past fluvial, deltaic, and lacustrine environments. Degradation of the crater wall and rim probably supplied these sediments, which advanced inward from the wall, infilling both the crater and an internal lake basin to a thickness of at least 75 meters. This intracrater lake system probably existed intermittently for thousands to millions of years, implying a relatively wet climate that supplied moisture to the crater rim and transported sediment via streams into the lake basin. The deposits in Gale crater were then exhumed, probably by wind-driven erosion, creating Aeolis Mons (Mount Sharp).

A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale crater, Mars.

The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.

Diagenetic origin of quartz silt in mudstones and implications for silica cycling.

Mudstone--the most abundant sedimentary rock type, composed primarily of clay- or silt-sized particles--contains most of the quartz found in sedimentary rocks. These quartz grains, which are chemically and mechanically resistant and therefore preserve their characteristics well, have long been considered to be derived from the continental crust. Here we analyse quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe. Our results indicate that up to 100% of the quartz silt in our samples does not originate from the continental crust. Instead, it appears to have precipitated early in diagenesis in algal cysts and other pore spaces, with silica derived from the dissolution of opaline skeletons of planktonic organisms, such as radiolaria and diatoms. Transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 Myr ago and molecular evidence for a much earlier appearance between 266 or even 500 Myr ago. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated--rather than detrital--quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.

[Socioprofessional consequences of Crohn disease]. / Répercussions socio-professionnelles de la maladie de Crohn.

To evaluate evolutivity and social repercussions of Crohn's disease, the same gastroenterologist interviewed 151 out of 197 patients with Crohn's disease (77.5 percent) who had been hospitalized from 1964 to 1986 in our unit. The other 46 could not be traced or refused to participate. There was no difference regarding age, sex, duration of illness, pattern of lesions and operations between the two groups. At the time of observation, mean follow-up was 7.8 years and two thirds of the patients had inactive disease. During the previous year, the course of the disease was quiescent in 50 percent of all patients; this rate increased with duration of follow-up and time from surgery. A constant rate of stable course was observed (20 percent) irrespective of duration of disease and time of surgery. The interviews were compared with those obtained from an age and sex-matched group of 70 previously healthy subjects admitted to the hospital for less than one month. The education and socioeconomic levels were higher in patients with Crohn's disease than in controls, in spite of the fact that 25 percent of patients whose disease begun during scholarship complained of difficulties at school, and nearly 20 percent of all patients were partially or totally unable to work. Sports and cultural activities were the same in Crohn's disease and in the control group. Tobacco intake was slightly higher at time of diagnosis but it was equal in both groups at the time of the interview. Non allopathic advice or treatment were sought by 48 percent of Crohn's disease patients (7 percent in controls).(ABSTRACT TRUNCATED AT 250 WORDS)

[Vital capacity restitution curve: prognostic value in chronic decompensated respiratory insufficiency]. / La courbe de restitution de la capacité vitale: intérêt pronostique au cours des insuffisances respiratoires chroniques décompensées.

From a series of 50 patients with acute decompensation of chronic obstructive lung disease (38 of whom were treated by mechanical ventilation), the authors demonstrate the prognostic value of an easily obtained parameter of respiratory function: the vital capacity restitution curve (VCRC). From daily measurements of vital capacity, beginning on the day of admission, a graph is constructed which shows an initial period of increase in the degree of restitution, followed by stabilization of the values. An analysis of the various parameters embodied in this graph provides information about the prognosis. Such graphs can be divided into 3 zones of prognostic value: a favourable zone, an intermediate zone (mediocre survival with or without mechanical ventilation) and an unfavourable zone (death during the acute phase). Although a favourable prognosis can be made after 4 days of observation and almost always by the 10th day, an unfavourable prognosis cannot be made before the 21st day.

Ultradian oscillations in plasma renin activity: their relationships to meals and sleep stages.

The 24-h pattern of PRA was studied in 6 supine normal subjects, and the relationship between sleep stages and PRA oscillations was analyzed using 18 nighttime profiles and the concomitant polygraphic recordings of sleep. Blood was collected at 10-min intervals. The slow trends obtained by adjusting a third degree polynomial to the 24-h data were not reproducible among individuals, and no circadian pattern was detected. Sustained oscillations in PRA occurred throughout the day. Spectral analysis revealed that PRA oscillated at a regular periodicity of about 100 min during the night. This periodicity was modified during the daytime by meal intake, which induced PRA peaks with large interindividual variations in size. A close relationship was found between the nocturnal PRA oscillations and the alternance of rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep invariably coincided with increasing or peaking PRA levels. REM sleep occurred as PRA was declining or at nadirs. More precisely, increases in PRA marked the transition from REM sleep to stage II, whereas stages III and IV usually occurred when PRA was highest. This relationship between the periodic nocturnal oscillations in PRA and the alternance of the REM-non-REM cycles may translate a similar oscillatory process in the central nervous system or may be linked to hemodynamic changes during sleep that might be partly controlled by the renin-angiotensin system.

Characteristics of rapid eye movement production during REM sleep in man: organization and rhythmicity.

The purpose of this study was to verify statistically the existence of certain characteristics of rapid eye movement (REM) production during REM sleep: organization and rhythmicity. REM data were collected intermittently, over a period of 20 consecutive nights, from 6 normal adult subjects of both sexes. Results concerning the organization of REM production revealed the presence of 3 evolving slow trends: quadratic, linear and quasi-sinusoidal. The occurrence of a given trend type was not related to subject, night, time of night or length of REM sleep phase being analysed. Results of the analysis showed that it is possible for a series of REMs to contain one, two or more statistically significant rhythmic periods, or no significant rhythmic period. The rhythmic periods that were detected ranged from 1 to 21 min, and no predominant periods were present. As in the case of the slow trends, the occurrence of a given rhythmic period was not dependent on subject, night, time of night or length of the REM sleep phase.

K-complexes and sleep spindles before transient activation during sleep.

Six subjects spent three consecutive nights in the sleep laboratory. Activation phases (PATs), spontaneous K-complexes, and sleep spindles were visually detected in sleep stages 2 and 3 for nights 2 and 3. The K-complex rate was significantly greater in the 10 sec prior to the PATs than at any other time spent in stage 2 or 3. K-complexes associated with sheep spindles occurred significantly less frequently during the epochs just preceding the PATs. In all subjects, there was a sharp increase of sleep spindles associated with K-complexes when PATs did not follow within 10 sec. These results suggest that spontaneous K-complexes and sleep spindles act antagonistically with respect to the occurrence of PATs. These two phasic events are significantly related to regulating the probability of occurrence of PATs in sleep stages 2 and 3; K-complexes may reflect an organismic state leading towards PAT, whereas sleep spindles may inhibit the occurrence of PAT.