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The magnetic susceptibility of Pleistocene paleosols as a Martian paleoenvironment analog
Abstract
This work aims to introduce and test various semiquantitative field methods and environmental magnetic measurements to help prepare future planetary missions on Mars. For analog observations, paleosols of loess successions in various stages of soil development were investigated and were used as models to infer environmental change during environmental change on early Mars. Methods commonly used in terrestrial soils and sediment environments, such as soil development indices and low field and frequency-dependent magnetic susceptibility, are introduced and evaluated as potential proxies to constrain paleoenvironmental conditions and climate change on Mars billions of years ago. These methods include the following: 1) low field magnetic susceptibility, which may aid in the identification of weathered horizons (e.g., palaeosols) and provide insight into the degree of weathering intensity; 2) frequency-dependent magnetic susceptibility can constrain nanoscale magnetic contributions, including some with possible biogenic/bacterial origin, but its applicability to indicate the degree of pedogenesis is limited; and 3) the vertical distribution of low field magnetic susceptibility, i.e., the pattern of magnetic susceptibility curves, seems to work well in the indication of the balance between the sedimentary and pedogenic environment. Analysis of magnetic susceptibility curves may contribute to the identification of Martian paleosols, the characterization of the transition period between the soil-forming and subsequent sedimentary periods (Noachian-Hesperian and Hesperian-Amazonian transitions) and the identification of climate cycles; thus, these may be used as a frame of reference for evaluating paleoclimatic changes on Mars to e.g., the Noachian warm Mars and”Snowball Mars” periods.
The results also suggest that the time dependence of the magnetic enhancement of paleosols seems to be nonlinear compared to the length of the pedogenic period itself, and magnetic susceptibility may work as a relative chronometric parameter, which can help to constrain the duration of pedogenic alteration and soil formation on Mars.
We present the geography of Oxia Planum, the landing site for the ExoMars 2022 mission. This map provides the planetary science community with a framework to understand this, until recently, unexplored area. The map comprises (1) a mosaic of the panchromatic Context Camera (CTX) Digital Elevation Models (DEM) and Ortho Rectified Images (ORI) controlled to the High Resolution Stereo Camera (HRSC) multiorbit Digital Elevation Models (DEM) and (2) a mosaic of Colour and Stereo Surface Imaging System (CaSSIS) synthetic colour data products, registered to the CTX ORI mosaic. We define a grid of exploration quadrangles (quads) and an informal group of geographic regions to describe Oxia Planum. These regions bridge the scale gap between features observed on large areas (∼100s km²) and the local geography (10s km²) relevant to the Rosalind Franklin rover’s operations in Oxia Planum.
The Curiosity rover is exploring Hesperian-aged stratigraphy in Gale crater, Mars, where a transition from clay-bearing units to a layered sulfate-bearing unit has been interpreted to represent a major environmental transition of unknown character. We present the first description of key facies in the sulfate-bearing unit, recently observed in the distance by the rover, and propose a model for changes in depositional environments. Our results indicate a transition from lacustrine mudstones into thick aeolian deposits, topped by a major deflation surface, above which strata show architectures likely diagnostic of a subaqueous environment. This model offers a reference example of a depositional sequence for layered sulfate-bearing strata, which have been identified from orbit in other locations globally. It differs from the idea of a monotonic Hesperian climate change into long-term aridity on Mars and instead implies a period characterized by multiple transitions between sustained drier and wetter climates.
Reconciling the geology of Mars with models of atmospheric evolution remains a major challenge. Martian geology is characterized by past evidence for episodic surface liquid water, and geochemistry indicating a slow and intermittent transition from wetter to drier and more oxidizing surface conditions. Here we present a model that incorporates randomized injection of reducing greenhouse gases and oxidation due to hydrogen escape to investigate the conditions responsible for these diverse observations. We find that Mars could have transitioned repeatedly from reducing (hydrogen-rich) to oxidizing (oxygen-rich) atmospheric conditions in its early history. Our model predicts a generally cold early Mars, with mean annual temperatures below 240 K. If peak reducing-gas release rates and background carbon dioxide levels are high enough, it nonetheless exhibits episodic warm intervals sufficient to degrade crater walls, form valley networks and create other fluvial/lacustrine features. Our model also predicts transient build-up of atmospheric oxygen, which can help explain the occurrence of oxidized mineral species such as manganese oxides at Gale Crater. We suggest that the apparent Noachian–Hesperian transition from phyllosilicate deposition to sulfate deposition around 3.5 billion years ago can be explained as a combined outcome of increasing planetary oxidation, decreasing groundwater availability and a waning bolide impactor flux, which dramatically slowed the remobilization and thermochemical destruction of surface sulfates. Ultimately, rapid and repeated variations in Mars’s early climate and surface chemistry would have presented both challenges and opportunities for any emergent microbial life. Mars’s early climate and surface chemistry varied between a generally cold, oxidizing environment and warmer, more reducing conditions, according to a model of atmospheric evolution driven by stochastic, random injection of greenhouse gases.
The European Space Agency and Roscosmos' ExoMars rover mission, which is planned to land in the Oxia Planum region, will be dedicated to exobiology studies at the surface and subsurface of Mars. Oxia Planum is a clay-bearing site that has preserved evidence of long-term interaction with water during the Noachian era. Fe/Mg-rich phyllosilicates have previously been shown to occur extensively throughout the landing area. Here, we analyze data from the High Resolution Imaging Science Experiment (HiRISE) and from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instruments onboard NASA's Mars Reconnaissance Orbiter and the Colour and Stereo Surface Imaging System (CaSSIS) onboard ESA's Trace Gas Orbiter to characterize, at a high spatial resolution, the morphological and spectral variability of Oxia Planum's surface deposits. Two main types of bedrocks are identified within the clay-bearing, fractured unit observed throughout the landing site: (1) an orange type in HiRISE correlated with the strongest detections of secondary minerals (dominated by Fe/Mg-rich clay minerals) with, in some locations, an additional spectral absorption near 2.5 μm, suggesting the mixture with an additional mineral, plausibly carbonate or another type of clay mineral; (2) a more bluish bedrock associated with weaker detections of secondary minerals, which exhibits at certain locations a ∼1 μm broad absorption feature consistent with olivine. Coanalysis of the same terrains with the recently acquired CaSSIS images confirms the variability in the color and spectral properties of the fractured unit. Of interest for the ExoMars mission, both types of bedrocks are extensively outcropping in the Oxia Planum region, and the one corresponding to the most intense spectral signals of clay minerals (the primary scientific target) is well exposed within the landing area, including near its center.
The European Space Agency (ESA) and Roscosmos ExoMars mission will launch the "Rosalind Franklin" rover in 2022 for a landing on Mars in 2023.The goals of the mission are to search for signs of past and present life on Mars, investigate the water/geochemical environment as a function of depth in the shallow subsurface, and characterize the surface environment. To meet these scientific objectives while minimizing the risk for landing, a 5-year-long landing site selection process was conducted by ESA, during which eight candidate sites were down selected to one: Oxia Planum. Oxia Planum is a 200 km-wide low-relief terrain characterized by hydrous clay-bearing bedrock units located at the southwest margin of Arabia Terra. This region exhibits Noachian-aged terrains. We show in this study that the selected landing site has recorded at least two distinct aqueous environments, both of which occurred during the Noachian: (1) a first phase that led to the deposition and alteration of ∼100 m of layered clay-rich deposits and (2) a second phase of a fluviodeltaic system that postdates the widespread clay-rich layered unit. Rounded isolated buttes that overlie the clay-bearing unit may also be related to aqueous processes. Our study also details the formation of an unaltered mafic-rich dark resistant unit likely of Amazonian age that caps the other units and possibly originated from volcanism. Oxia Planum shows evidence for intense erosion from morphology (inverted features) and crater statistics. Due to these erosional processes, two types of Noachian sedimentary rocks are currently exposed. We also expect rocks at the surface to have been exposed to cosmic bombardment only recently, minimizing organic matter damage.
The geomorphological characteristics of the loess succession at Malá nad Hronom (Slovakia) mean that it provides a valuable opportunity for the investigation of differences in soil formation in various topographic positions. Along with the semiquantitative characterization of the paleosols (on the basis of physical properties, texture, the characteristics of peds, clay films, horizon boundaries), high-resolution field magnetic susceptibility measurements and sampling were carried out along four different sections of the profile. Samples for luminescence dating were also taken, in order to establish the chronostratigraphical position of the paleosols studied. The comparison of various proxies revealed the differences in soil formation in a dynamic aggradational microenvironment for the same paleosol horizons located in various positions along the slope. Contrary to expectation, paleosols developed in local top or slope topographical positions did not display significant differences in e.g. in their degree of development, nor the characteristics of their magnetic susceptibility curves. In the case of paleosols in positions lower down the slope, signs of quasi-permanent sediment input could be recognized as being present as early as during the formation of the soil itself. This sediment input would seem to be surpassed in the case of pedogenesis strengthened by the climate of the last interglacial (marine isotope stage - MIS 5). Pedogenesis seems to be sustained by renewed intense dust accumulation in the Late Pleistocene, in MIS 3, though compared to MIS 5, the climate of MIS 3 did not favor intense pedogenesis. Despite the general belief that loess series formed in plateau positions can preserve terrestrial records without significant erosion, in the case of the Malá nad Hronom loess this is not so. Compared to the sequence affected by erosional events in the local top position, the sequence affected by quasi-continuous sediment input in the lower slope position seems to have preserved the soil horizons intact.
The emerging field of astropedology is the study of ancient soils on Earth and other planetary bodies. Examination of the complex factors that control the preservation of organic matter and other biosignatures in ancient soils is a high priority for current and future missions to Mars. Though previously defined by biological activity, an updated definition of soil as planetary surfaces altered in place by biological, chemical or physical processes was adopted in 2017 by the Soil Science Society of America in response to mounting evidence of pedogenic-like features on Mars. Ancient (4.1–3.7 billion year old [Byr]) phyllosilicate-rich surface environments on Mars show evidence of sustained subaerial weathering of sediments with liquid water at circumneutral pH, which is a soil-forming process. The accumulation of buried, fossilized soils, or paleosols, has been widely observed on Earth, and recent investigations suggest paleosol-like features may be widespread across the surface of Mars. However, the complex array of preservation and degradation factors controlling the fate of biosignatures in paleosols remains unexplored. This paper identifies the dominant factors contributing to the preservation and degradation of organic carbon in paleosols through the geological record on Earth, and offers suggestions for prioritizing locations for in situ biosignature detection and Mars Sample Return across a diverse array of potential paleosols and paleoenvironments of early Mars. A compilation of previously published data and original research spanning a diverse suite of paleosols from the Pleistocene (1 Myr) to the Archean (3.7 Byr) show that redox state is the predominant control for the organic matter content of paleosols. Most notably, the chemically reduced surface horizons (layers) of Archean (2.3 Byr) paleosols have organic matter concentrations ranging from 0.014–0.25%. However, clay mineralogy, amorphous phase abundance, diagenetic alteration and sulfur content are all significant factors that influence the preservation of organic carbon. The surface layers of paleosols that formed under chemically reducing conditions with high amounts of iron/magnesium smectites and amorphous colloids should be considered high priority locations for biosignature investigation within subaerial paleoenvironments on Mars.
Magnetic enhancement of Chinese loess‐paleosol sequences has been used extensively as a proxy for East Asian summer monsoon variations. However, the pedogenic magnetic particles contributing to this magnetic enhancement are difficult to extract, so it is not clear how they formed. In this study, we reveal pedogenic magnetite and hematite using electron microscopy, synchrotron radiation X‐ray diffraction, and rock magnetic methods. First‐order reversal curves indicate that superparamagnetic/single domain/vortex state magnetic properties dominated both loess and paleosol samples. Samples of muscovite and chlorite, which are paramagnetic, have weak spontaneous magnetization. The 1‐ to 10‐μm‐sized fraction of host silicatesis responsible for most of the magnetic enhancement of paleosols. In the paleosol fraction, we found weathered phyllosilicates (muscovite/chlorite), including many elongated submicron to a few microns authigenic magnetite and hematite particles between layers; however, few such interlayer particles were found in phyllosilicates of the loess fraction. The concentration of magnetite/hematite particles within paleosol muscovite/chlorite grains and in aggregates of phyllosilicate fragments is much higher than that of the submicron iron oxides found on silicate surfaces. Interlayer magnetite particles are dominantly prism‐shaped with aspect ratios >~4. The authigenic magnetite must be mainly responsible for the spontaneous magnetization of the muscovites and chlorites and the paleosol magnetic properties. The protective silicates account for the low extraction efficiency and also the near absence of surface oxidation of pedogenic magnetite. Based on our results, we suggest that magnetite/hematite in weathered phyllosilicates contribute significantly to the magnetic enhancement of mature paleosols.
The Mars Science Laboratory Curiosity rover arrived at Mars in August 2012 with a primary goal of characterizing the habitability of ancient and modern environments. Curiosity was sent to Gale crater to study a sequence of ∼3.5 Ga old sedimentary rocks that, based on orbital visible and near- to short-wave infrared reflectance spectra, contain secondary minerals that suggest deposition and/or alteration in liquid water. The sedimentary sequence in the lower slopes of Mount Sharp in Gale crater preserves a dramatic shift on early Mars from a relatively warm and wet climate to a cold and dry climate, based on a transition from smectite-bearing strata to sulfate-bearing strata. The rover is equipped with instruments to examine the sedimentology and identify compositional changes in the stratigraphy. The Chemistry and Mineralogy (CheMin) instrument is one of two internal laboratories on Curiosity and includes a transmission X-ray diffractometer (XRD) and X-ray fluorescence (XRF) spectrometer. CheMin measures loose sediment samples scooped from the surface and drilled rock powders, and the XRD provides quantitative mineralogy to a detection limit of ∼1 wt.% for crystalline phases. Curiosity has traversed >20 km since landing and has primarily been exploring an ancient lake environment fed by streams and groundwater. Of the 19 drilled rock samples analyzed by CheMin as of sol 2300 (January 2019), 15 are from fluvio-lacustrine deposits that comprise the Bradbury and Murray formations. Most of these samples were drilled from units that did not have a clear mineralogical signature from orbit. Results from CheMin demonstrate an astounding diversity in the mineralogy of these rocks that signifies geochemical variations in source rocks, transportation mechanisms, and depositional and diagenetic fluids. Most detrital igneous minerals are basaltic, but the discovery in a few samples of abundant silicate minerals that usually crystallize from evolved magmas on Earth remains enigmatic. Trioctahedral smectite and magnetite at the base of the section may have formed from low-salinity pore waters with a circumneutral pH in lake sediments. A transition to dioctahedral smectite, hematite, and Ca-sulfate going up section suggests a change to more saline and oxidative aqueous conditions in the lake waters themselves and/or in diagenetic fluids. Perhaps one of the biggest mysteries revealed by CheMin is the high abundance of X-ray amorphous materials (15 to 73 wt.%) in all samples drilled or scooped to date. CheMin has analyzed three modern eolian sands, which have helped constrain sediment transport and mineral segregation across the active Bagnold Dune Field. Ancient eolian sandstones drilled from the Stimson formation differ from modern eolian sands in that they contain abundant magnetite but no olivine, suggesting that diagenetic processes led to the alteration of olivine to release Fe(II) and precipitate magnetite. Fracture-associated halos in the Stimson and the Murray formations are evidence for complex aqueous processes long after the streams and lakes vanished from Gale crater. The sedimentology and composition of the rocks analyzed by Curiosity demonstrate that habitable environments persisted intermittently on the surface or in the subsurface of Gale crater for perhaps more than a billion years.
The presence of abundant phyllosilicate minerals in Noachian (>3.7 Ga) rocks on Mars has been taken as evidence that liquid water was stable at or near the surface early in martian history. This study investigates some of these clay-rich strata exposed in crater rim and inverted terrain settings in the Mawrth Vallis region of Mars. In Muara crater the 200-m-thick, clay-rich Mawrth Vallis Group (MVG) is subdivided into five informal units numbered 1 (base) to 5 (top). Unit 1 consists of interbedded sedimentary and volcanic or volcaniclastic units showing weak Fe/Mg-smectite alteration deposited in a range of subaerial depositional settings. Above a major unconformity eroded on Unit 1, the dark-toned sediments of Unit 2 and lower Unit 3 are inferred to represent mainly wind-blown sand. These are widely interlayered with and draped by thin layers of light-toned sediment representing fine suspended-load aeolian silt and clay. These sediments show extensive Fe/Mg-smectite alteration, probably reflecting subaerial weathering. Upper Unit 3 and units 4 and 5 are composed of well-layered, fine-grained sediment dominated by Al-phyllosilicates, kaolinite, and hydrated silica. Deposition occurred in a large lake or arm of a martian sea. In the inverted terrain 100 km to the NE, Unit 4 shows very young slope failures suggesting that the clay-rich sediments today retain a significant component of water ice. The MVG provides evidence for the presence of large, persistent standing bodies of water on early Mars as well as a complex association of flanking shoreline, alluvial, and aeolian systems. Some of the clays, especially the Fe/Mg smectites in upper units 1 and 2 appear to have formed through subaerial weathering whereas the aluminosilicates, kaolinite, and hydrated silica of units 3, 4, and 5 formed mainly through alteration of fine sediment in subaqueous environments.
Please find the open access paper at:
https://doi.org/10.1016/j.gca.2018.11.026
The loess sequences in Tajikistan are an important archive of information about the development of climate and atmospheric circulation in central Asia during the Pleistocene. Here, we present the results of an iron mineralogical study of a loess sequence in Tajikistan to reconstruct paleoclimate evolution during the mid‐ and late Pleistocene. The record indicates that interglacial intervals were relatively humid and glacials were dry. We propose a shift in the character of the interglacial climate of the region to more humid after beginning of MIS 9, however, temperature was relatively stable. The location and intensity of Westerlies has a close link with Asian Summer Monsoon (ASM) circulations. From the beginning of MIS 9, there was a southward movement of the Westerlies circulation, which corresponded to a retreat of the Indian Summer Monsoon (ISM). A similar relationship also existed between the East Asian Summer Monsoon (EASM) circulation and the Westerlies, which is evidenced by the formation of the most weakly developed soil unit in the EASM‐dominated regions during MIS 9, in contrast with the formation of the most strongly developed soil unit in the Westerlies‐dominated regions. As conditions in the areas of loess deposition became more humid, the sedimentary basins which are the dust source areas became progressively more arid. The aridification of the source areas may be the result of increased Northern Hemisphere ice volume and accelerated high mountains and/or plateaus uplift in the surrounding regions.
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. C, H, O, S, N, and P were measured directly as key biogenic elements, and by inference P is assumed to have been available. The environment likely 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.
Based on stable isotope analyses of worldwide reference curves, it has long been apparent that duration, intensity and climatic conditions of Pleistocene interglacial periods were significantly diverse. As a consequence of negligible fresh, detrital material admixture during interglacials, the soil formation intensity and maturity of various kinds of past soils have been holding vital information on the environmental conditions at the time the soils formed. This, in turn, means that several physicochemical properties of soils allow us to reconstruct past climatic regimes. Loess-paleosol sequences in Hungary (Central Europe) provide insight into the cyclic nature of glacial-interglacial variations of the last 1 million years. The paleosoils have been recognized as the product of warmer and moister interglacials, when the (glacial) loess material was altered by chemical weathering and pedogenic processes. The gradual change from oldest red Mediterranean soils via forest and forest-steppe soils to steppe soils represents well the continuous decrease of chemical alteration of interglacial paleosoils determined by environmental factors and duration of soil formation. Pedogene units from MIS-21 to MIS-5 strata were analysed in the course of this study. Major element analyses were carried out to get a proper picture on the paleoenvironmental conditions. Geochemical transfer functions have been applied to derive mean annual precipitation and mean annual temperature. These kinds of quantitative data on past climate and the stratigraphic data allow us to fit our pedostratigraphic units into a global context. The present paper is aimed at providing new information on the various climatic and environmental characteristics of Pleistocene interglacial periods and soil forming processes. © 2015, Reasearch Centre for Astronomy and Earth Sciences Hungarian Academy. All rights reserved.
The Danube River drainage basin is the second largest river catchment in Europe and contains a significant and extensive region of thick loess deposits that preserve a record of a wide variety of recent and past environments. Indeed, the Danube River and tributaries may themselves be responsible for the transportation of large volumes of silt that ultimately drive loess formation in the middle and lower reaches of this large catchment. However, this vast loess province lacks a unified stratigraphic scheme. European loess research started in the late 17th century in the Danube Basin with the work of Count Luigi Ferdinand Marsigli. Since that time numerous investigations provided the basis for the pioneering stratigraphic framework proposed initially by Kukla (1970, 1977) in his correlations of loess with deep-sea sediments. Loess-palaeosol sequences in the middle and lower reaches of the Danube River basin were a key part of this framework and contain some of the longest and most complete continental climate records in Europe, covering more than the last million years. However, the very size of the Danube loess belt and the large number of countries it covers presents a major limiting factor in developing a unified approach that enables continental scale analysis of the deposits. Local loess-palaeosol stratigraphic schemes have been defined separately in different countries and the difficulties in correlating such schemes, which often change significantly with advances in age-dating, have limited the number of basin-wide studies. A unified basin-wide stratigraphic model would greatly alleviate these difficulties and facilitate research into the wider significance of these loess records. Therefore we review the existing stratigraphic schemes and define a new Danube Basin wide loess stratigraphy based around a synthetic type section of the Mošorin and Stari Slankamen sites in Serbia. We present a detailed comparison with the sedimentological and palaeoclimatic records preserved in sediments of the Chinese Loess Plateau, with the oxygen isotope records from deep-sea sediments, and with classic European Pleistocene stratigraphic subdivisions. The hierarchy of Danubian stratigraphic units is determined by climatically controlled environmental shifts, in a similar way to the Chinese loess stratigraphic scheme. A new unified Danube loess stratigraphic model has a number of advantages, including preventing confusion resulting from the use of multiple national schemes, a more transparent basis, and the potential to set Pleistocene palaeoenvironmental changes recorded in the Danube catchment area into a global context. The use of a very simple labelling system based on the well-established Chinese loess scheme facilitates interpretation of palaeoenvironmental information reported from the Danube Basin loess sites in a wider more accessible context that can be readily correlated world-wide. This stratigraphic approach also provides, for the first time, an appropriate framework for the development of an integrated, pan-European and potentially pan-Eurasian loess stratigraphic scheme.
Fluviolacustrine sediments filling Gale Crater on Mars show two levels of former exposure and weathering that provide new insights into late Noachian (3.7 ± 0.3 Ga) paleoenvironments of Mars. Diagnostic features of the two successive paleosols in the Sheepbed member include complex cracking patterns of surface dilation (peds and cutans), a clayey surface (A horizon), deep sand-filled cracks with vertical lamination (sand wedges), and replacive sulfate nodules aggregated into distinct bands (gypsic By horizon) above bedded sandy layers (sedimentary C horizon). Shallow gypsic horizon, periglacial sand wedges, and limited chemical weathering are evidence of a hyperarid frigid paleoclimate, and this alternated with wetter conditions for the lacustrine parent materials in Gale Crater during the late Noachian. Depletion of phosphorus, vesicular structure, and replacive gypsic horizons of these Martian paleosols are features of habitable microbial earth soils on Earth, and encourage further search for definitive evidence of early life on Mars.
[1] The Mars Exploration Rovers each carry a set of Magnetic Properties Experiments designed with the following objectives in mind: (1) to identify the magnetic mineral(s) in the dust, soil and rocks on Mars, (2) to establish if the magnetic material is present in the form of nanosized (d < 10 nm) superparamagnetic crystallites embedded in the micrometer sized airborne dust particles, and (3) to establish if the magnets are culling a subset of strongly magnetic particles or if essentially all particles of the airborne dust are sufficiently magnetic to be attracted by the magnets. To accomplish these goals, the Mars Exploration Rovers each carry a set of permanent magnets of several different strengths and sizes. Each magnet has its own specific objective. The dust collected from the atmosphere by the Capture magnet and the Filter magnet (placed on the front of each rover) will be studied by the Mössbauer spectrometer and the Alpha Particle X-ray Spectrometer, both of which are instruments located on the rover's Instrument Deployment Device. The captured dust particles will also be imaged by the Pancam and Microscopic Imager. The Sweep magnet will be imaged by Pancam and is placed near the Pancam calibration target. The four magnets in the Rock Abrasion Tool (RAT) are designed to capture magnetic particles originating from the grinding of Martian surface rocks. The magnetic particles captured by the RAT magnets will be imaged by Pancam.
Mawrth Vallis contains one of the largest exposures of phyllosilicates on Mars. Nontronite, montmorillonite, kaolinite, and hydrated silica have been identified throughout the region using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). In addition, saponite has been identified in one observation within a crater. These individual minerals are identified and distinguished by features at 1.38–1.42, ∼1.91, and 2.17–2.41 μm. There are two main phyllosilicate units in the Mawrth Vallis region. The lowermost unit is nontronite bearing, unconformably overlain by an Al-phyllosilicate unit containing montmorillonite plus hydrated silica, with a thin layer of kaolinite plus hydrated silica at the top of the unit. These two units are draped by a spectrally unremarkable capping unit. Smectites generally form in neutral to alkaline environments, while kaolinite and hydrated silica typically form in slightly acidic conditions; thus, the observed phyllosilicates may reflect a change in aqueous chemistry. Spectra retrieved near the boundary between the nontronite and Al-phyllosilicate units exhibit a strong positive slope from 1 to 2 μm, likely from a ferrous component within the rock. This ferrous component indicates either rapid deposition in an oxidizing environment or reducing conditions. Formation of each of the phyllosilicate minerals identified requires liquid water, thus indicating a regional wet period in the Noachian when these units formed. The two main phyllosilicate units may be extensive layers of altered volcanic ash. Other potential formational processes include sediment deposition into a marine or lacustrine basin or pedogenesis.
The magnetic method is firmly established as an investigative procedure in applied geophysics, and it is increasingly recognised that an understanding of the elements of magnetic petrophysics assists in optimising field data interpretations. These notes expand on the rock magnetism data previously presented by Emerson (1979) and Clark (1983); the data presented are based on magnetic property studies in the CSIRO Division of Exploration Geoscience and the Department of Geology and Geophysics University of Sydney and on published studies and compilations. The systematic collection of petrophysical data by many workers over the last decade has greatly expanded the size and scope of available data. Works listed in the Bibliography should be consulted for further information.
Saturation magnetization and saturation remanence of 139 loess and paleosol samples from Tajikistan, China and Hungary increase linearly with low field susceptibility in all sections investigated. Thus magnetic enhancement is caused by increasing quantities of ferrimagnetic minerals. The observed variations of high field susceptibility chi(h) are attributed to variations in the amount of clay minerals as chi(h) is linearly correlated with the < 2 mu m grain size mineral fraction. The magnetic enhancement process is compared with a theoretical mixing model based on two magnetic mineral assemblages with different but invariable coercivities. The two-component model provides a good approximation to the data from Hungary and the magnetically enhanced paleosol samples in Tajikistan and China. Discrepancies between model calculation and data arise in the behaviour of the loess samples in Tajikistan. Here a three-component model improves the agreement between data and mixing calculation. The magnetic enhancement in the loess in Tajikistan (and China) is different from the one in the paleosols and;the link between paleoclimate and susceptibility is not as strightforward as in the Hungarian loess-paleosol sequence.
Ancient (4.1–3.7-billion-year-old) layered sedimentary rocks on Mars are rich in clay minerals which formed from aqueous alteration of the Martian surface. Many of these sedimentary rocks appear to be composed of vertical sequences of Fe/Mg clay minerals overlain by Al clay minerals that resemble paleosols (ancient, buried soils) from Earth. The types and properties of minerals in paleosols can be used to constrain the environmental conditions during formation to better understand weathering and diagenesis on Mars. This work examines the mineralogy and diagenetic alteration of volcaniclastic paleosols from the Eocene-Oligocene (43–28 Ma) Clarno and John Day Formations in eastern Oregon as a Mars-analog site. Here, paleosols rich in Al phyllosilicates and amorphous colloids overlie paleosols with Fe/Mg smectites that altogether span a sequence of ~ 500 individual profiles across hundreds of meters of vertical stratigraphy. Samples collected from three of these paleosol profiles were analyzed with visible/near-infrared (VNIR) spectroscopy, X-ray diffraction (XRD), and evolved gas analysis (EGA) configured to operate like the SAM-EGA instrument onboard Curiosity Mars Rover. Strongly crystalline Al/Fe dioctahedral phyllosilicates (montmorillonite and nontronite) were the major phases identified in all samples with all methods. Minor phases included the zeolite mineral clinoptilolite, as well as andesine, cristobalite, opal-CT and gypsum. Evolved H2O was detected in all samples and was consistent with adsorbed water and the dehydroxylation of a dioctahedral phyllosilicate, and differences in H2O evolutions between montmorillonite and nontronite were readily observable. Detections of hematite and zeolites suggested paleosols were affected by burial reddening and zeolitization, but absence of illite and chlorite suggest that potash metasomatism and other, more severe diagenetic alterations had not occurred. The high clay mineral content of the observed paleosols (up to 95 wt%) may have minimized diagenetic alteration over geological time scales. Martian paleosols rich in Al and Fe smectites may have also resisted severe diagenetic alteration, which is favorable for future in-situ examination. Results from this work can help differentiate paleosols and weathering profiles from other types of sedimentary rocks in the geological record of Mars.
The continental accumulation of dust during the Quaternary led to the formation of widespread loess deposits in southern Tajikistan. In this area, the accumulation of loess is commonly associated with the occurrence of dust storms and the widespread distribution of loess provides evidence of dust storms becoming more frequent in arid Central Asia at least since the Early Pleistocene. Southern Tajikistan represents one of the largest loess deposits in Central Asia. We conducted a thorough study on the magnetostratigraphy, grain size, and magnetic susceptibility of the Chashmanigar section to reconstruct the stratigraphy of loess deposits and paleoclimate of Tajikistan. Based on our new data, the lower boundary of the basal ages of the Olduvai and Reunion subchrons were established for the studied Chashmanigar section. Rock magnetic analyses showed that the predominant ferrimagnetic minerals are large pseudo-single domain grains of magnetite, including limited maghemite. Standard demagnetization techniques yielded a characteristic component of natural remanent magnetization, which was used to obtain a well-defined magnetostratigraphy. In southern Tajikistan, paleosols consistently exhibit finer grain size distribution and higher magnetic susceptibility than loess horizons, suggesting that the environment of the Chashmanigar section was colder, drier, and dustier during glacial periods than during interglacial periods. Through correlation with astronomically tuned oxygen isotope records, sophisticated dating of the loess-paleosol sequence at Chashmanigar could be achieved, and the global significance of the recorded paleoclimatic variations could be revealed. The resulting grain size, magnetic susceptibility, and correlation with astronomically tuned oxygen isotope clearly provide information about the climatic pattern during the Early Pleistocene.
Anticipated human missions to Mars require a methodical understanding of the unconsolidated bulk sediment that mantles its surface, given its role as an accessible resource for water and as a probable substrate for food production. However, classifying martian sediment as soil has been pursued in an ad-hoc fashion, despite emerging evidence from in situ missions for current and paleo-pedological processes. Here we find that in situ sediment at Gusev, Meridiani and Gale are consistent with pedogenesis related to comminuted basalts mixing with older phyllosilicates – perhaps of pluvial origin -- and sulfates. Furthermore, a notable presence of hydrated amorphous phases indicates significant chemical weathering that mirrors pedogenesis at extreme environments on Earth. Effects of radiation and reactive oxygen species are also reminiscent of such soils at Atacama and Mojave. Some related phases, like perchlorates and Fe-sulfates, may sustain brine-driven weathering in modern martian soils. Meanwhile, chemical diversity across in situ and regional soils suggests many different soil types and processes. But the two main soil classification systems –the World Reference Base for Soil Resources (WRB) and the U.S. Soil Taxonomy – only inadequately account for such variability. While WRB provides more process insight, it needs refinement to represent variability of martian soils even at the first level of categorical detail. That will provide a necessary reference for future missions when identifying optimal pedological protocols to systematically survey martian soil. Updating Earth-based soil classification systems for this purpose will also advance soil taxonomy as a research field.
After the successful landing of the Mars Science Laboratory rover, both NASA and ESA initiated a selection process for potential landing sites for the Mars2020 and ExoMars missions, respectively. Two ellipses located in the Mawrth Vallis region were proposed and evaluated during a series of meetings (three for Mars2020 mission and five for ExoMars). We describe here the regional context of the two proposed ellipses as well as the framework of the objectives of these two missions. Key science targets of the ellipses and their astrobiological interests are reported. This work confirms that the proposed ellipses contain multiple past martian wet environments of a subaerial, subsurface, and/or subaqueous character, in which to probe the past climate of Mars; build a broad picture of possible past habitable environments; evaluate their exobiological potentials; and search for biosignatures in well-preserved rocks. A mission scenario covering several key investigations during the nominal mission of each rover is also presented, as well as descriptions of how the site fulfills the science requirements and expectations of in situ martian exploration. These serve as a basis for potential future exploration of the Mawrth Vallis region with new missions and describe opportunities for human exploration of Mars in terms of resources and science discoveries.
Three well-developed paleosols from the Paks loess succession (Hungary), a key profile of the European Loess Belt, were sampled and studied by rock magnetic methods, such as low-field magnetic susceptibility and hysteresis measurements. The studied paleosols formed in MIS19, MIS15 and MIS11 and represent key periods in the Middle Pleistocene, namely, the Middle Pleistocene transition (MPT) (MIS19), and frame the mid-Brunhes transition (MIS15 and 11).
The results of the low-field magnetic susceptibility measurements, a popular interglacial intensity proxy, showed different patterns and values. The different characteristics of the susceptibility curve indicate different degrees of pedogenesis and therefore different soil-forming palaeoenvironments. The comparison of the rock magnetic (hysteresis) parameters supports the results of the susceptibility measurements and revealed some components of two characteristic palaeoclimate types: i.) a humid and warm MIS19 (pre-)MPT interglacial environment with very intense weathering and pedogenesis and ii.) a moderate climate with seasonal precipitation in a post-MPT interglacial environment during MIS15 and MIS11 characterized by intense but different types of pedogenesis compared to MIS19. This result, compared with other loess successions in the Middle and Lower Danube Basin and the western part of the East European Plain, helps to improve the climate model regarding the changing of the characteristic palaeoenvironment from a sub-Mediterranean (in the south) and temperate and humid climate (with forests, in the north) towards a cooler grassland/forest steppe-ruled environment in the region.
On the basis of a key model of the appearance of hematite and goethite in some Chinese and European loess successions, paleosols generally contain higher proportions of hematite formed by pedogenic processes during warm and humid interglacials, while loess contains sedimentary goethite deposited during colder, glacial periods. Rock magnetic measurements were conducted on samples from Paks (Hungary) loess, revealing an anomalous case. Sediments were found to contain higher amounts of hematite and/or goethite, while the well‐developed paleosols were found to be lacking in hematite but had significant amounts of magnetite and maghemite. This observation demonstrates that the character of pedogenic magnetic mineral alteration and neoformation differs from the hematite/goethite model; the model is therefore not applicable to the Paks succession, possibly due to differences in the regional paleoclimate and pedogenic environment. The results indicate the existence of a further model, in which hematite formation is not significant in the course of pedogenesis.
The Navua Valles are comprised of a system of channels and valleys on the inner Northeastern rim of Hellas Basin, which is a 1500-km-long sloping terrain. Drainage systems and regional geology in this unique setting were not previously mapped in detail. We mapped this region using CTX (6 m/px) as the base map and assessed surface unit ages resulting from our crater counting. We found that the timing of the deposit-forming episodes in this region during the Hesperian and Early to Middle Amazonian largely correlated to active phases of the Hadriacus Mons volcanic center. We found evidence for several episodes of fluvial activity Hesperian to the Amazonian with declining intensity, and transitioning to ice-dominated processes. The channels in the Navua Valles region erode into deposits dating from the Noachian to Early Amazonian, including the Noachian highlands, Noachian to early Amazonian crater ejecta, and likely volcanic plains formed from the Hesperian to the Hesperian–Amazonian transition. Channels directly originating from Hadriacus Mons are younger, while precipitation-fed channels at larger distance from the volcanic center are older, indicating different triggers for fluvial activity. Crater counting results indicate that almost all channel floors were at least partially resurfaced during the Amazonian and that several channel deposits formed during the last 0.5 Gyr. Water pathways likely included surface channels, lakes, and subsurface flow. The Navua Valles channel system is discontinuous, and the number of terminal deposits (sink locations) is almost as high as the number of channel sources, which is unusual for valley networks elsewhere on Mars. Interior channels formed only in the major Navua channels, they are even more fragmented than their parent channels, but occur along their entire length. Channels and valley systems within the Navua Valles are potential targets for in situ astrobiological studies, as they could have provided potential habitats at least periodically, from the Late Hesperian to the Late Amazonian.
Significance
In the famous Chinese Loess Plateau (CLP), weakly magnetic, windblown dust layers alternate with variably magnetic fossil soils, recording monsoonal variations through the last ∼3 My. The soils contain strongly magnetic iron oxides, formed in situ, the mineralogy and paleoclimatic significance of which are controversial. Reduction of iron to form Fe ²⁺ -bearing magnetite has been linked to soil wetting/drying. Conversely, oxidation of iron to form Fe ³⁺ -bearing maghemite, which ages into Al-substituted hematite, has been linked to paleotemperature. This study uses structural fingerprinting and electron energy loss spectroscopy to resolve this debate, proving that magnetite is the dominant soil-formed magnet. The magnetic variations of the CLP paleosols thus record changes in monsoonal rainfall, providing a key time series for testing of general circulation climate models.
The Dorsa Argentea Formation (DAF), a set of geomorphologic units covering ∼1.5 million square kilometers in the south circumpolar region of Mars, has been interpreted as the remnants of a large south polar ice sheet that formed near the Noachian-Hesperian boundary and receded in the early Hesperian. Determining the extent and thermal regime of the DAF ice sheet, as well as the mechanism and timing of its recession, can therefore provide insight into the ancient martian climate and the timing of the transition from a presumably thicker CO2 atmosphere to the present climate. We used the Laboratoire de Météorologie Dynamique (LMD) early Mars global climate model (GCM) and the University of Maine Ice Sheet Model (UMISM) glacial flow model to constrain climates allowing development of a south polar ice sheet of DAF-like size and shape. In addition, we modeled basal melting of this ice sheet in amounts and locations consistent with observed glaciofluvial landforms. A large, asymmetric region of ice stability surrounding the south pole is a robust feature of GCM simulations with spin-axis obliquity of 15° or 25° and a 600–1000 mb CO2 atmosphere. The shape results from the large-scale south polar topography of Mars and the strong dependence of surface temperature on altitude under a thicker atmosphere. Of the scenarios considered in this study, the extent of the modeled DAF ice sheet in UMISM simulations most closely matches that of the DAF when the surface water ice inventory of Mars is a ∼137 m global equivalent layer (GEL) and spin-axis obliquity is 15°. In climates warmed only by CO2, significant basal melting does not occur except when the ice inventory is larger than plausible estimates for early Mars. In this case, the extent of the south polar ice sheet is also much larger than that of the DAF, and basal melting is more widespread than observed landforms indicate. When an idealized greenhouse gas warms the surface by at least 20°C near the poles relative to CO2 alone, the stable extent of the ice sheet is less than that of the DAF units, but widespread basal melting occurs, with maxima in the locations where eskers are currently observed. We therefore conclude that warming by a gas other than CO2 alone was necessary to enable the construction of glaciofluvial landforms in the DAF. Previously published crater exposure ages of eskers in the DAF indicate that eskers were being exposed as activity was ceasing in the equatorial valley networks, suggesting that the warming that allowed basal melting at the edges of the DAF ice sheet were broadly contemporaneous with those in which the valley networks were carved. Finally, elevated Tharsis topography is required to produce an ice sheet with the shape of the DAF. Thus, our results are not consistent with the DAF (and the valley networks) forming before the emplacement of Tharsis, as recently suggested.
Recent investigations of the Semlac loess section in the south-eastern Carpathian Basin, which is situated at an undercut slope position on the right bank of the Mureş River in its lower reaches (Banat region, western Romanian), are presented and discussed. Dating back to marine isotope stage (MIS) 10, the more than 10 m thick loess sequence includes four fossil soil-complexes developed in homogenous and relatively fine silty loess. Because of the good preservation of the sediment, Semlac is regarded as a key section for the Carpathian Basin, which offers possibilities to a) improve the understanding of the type and composition (loess homogeneity and pedogenic alteration) of the lowland loess sequences in the Carpathian Basin also beyond the last interglacial palaoesol complex, b) to reconstruct the temporal evolution of the local loess-palaoesol successions, c) gain better insight into the regional paleoenvironments of the last 300 ka and d) to compare the loess of the region to loess-sequences in adjacent areas and to dust proxy data in the northern hemisphere.
Whether during past climate stages or into a progressively warming world, changes in precipitation constitute a key component of climatic change. Quantitative proxies for palaeo-precipitation are relatively rare. The magnetic properties of the windblown loess units and interbedded palaeosols of the famous Chinese Loess Plateau provide key palaeo-precipitation data for this populous, monsoon-dominated region. The loess/palaeosol sediments record rainfall totals, directly complementing the oxygen isotope records of Chinese speleothems. These isotopic records predominantly reflect moisture source, and hence large-scale atmospheric circulation changes. The two major Asian monsoon systems appear to display antiphase behaviour. Dominance of the Indian summer monsoon system seems associated with minimum precession/maximum northern hemisphere summer heating; dominance of the East Asian summer monsoons with maximum precession. At ∼2.8 Ma, more intense development of the East Asian winter monsoon initiated major increases in dust deposition rates, and formation of relatively unweathered loess layers. Glacial-stage loess units then interleaved with interglacial/interstadial-stage palaeosols throughout the Quaternary period. Decoupling of the loess/palaeosol rainfall records from the Chinese cave records of moisture source shows that the Indian and East Asian monsoon winds were continuously driven by precessional forcing while summer monsoonal rainfall was greatly suppressed during cool, glacial stages. The timing of these East Asian climatic transitions, the subsequent intensification of northern hemisphere glaciations, and the association between monsoon circulation changes and North Atlantic temperatures, indicates a possibly leading global role for these monsoonal changes via alterations in the poleward distribution of heat and moisture.
Palaeosols are ancient soils formed in sedimentary successions between events of sedimentation, erosion and volcanic activity. Soil formation is regulated by circumstances of climate, vegetation, topographic relief, parent material and time. These factors are quantified by nomopedology, in the form of climofunctions, chronofunctions and other relationships useful for interpreting conditions of the past from palaeosols. In deep time, palaeosols reveal the timing and extent of the Great Oxidation Event of 2.4 Ga. There is also circumstantial evidence for life in palaeosols back to 3.5 Ga on Earth and 3.7 Ga on Mars. These are the oldest known intact profiles, but pieces of palaeosols some 4.56 Ga in age may be represented by carbonaceous chondrite meteorites. Astropedology is the study of very ancient palaeosols and meteorites relevant to the origin of life and different planetary soil systems. Complex chemical assembly, metal catalysis of organic compounds, and the course of hydrolytic reactions as a kind of planetary metabolism make soils an attractive theoretical site for the origin of life. Because dilute solutions tend to an equilibrium that undoes organosynthetic reactions, life is more likely to have arisen on a soil planet like Mars than a water planet like Earth.
The growing inventory of post-Noachian fluvial valleys may represent a late, widespread episode of aqueous activity on Mars, contrary to the paradigm that fluvial activity largely ceased around the Noachian-Hesperian boundary. Fresh shallow valleys (FSVs) are widespread from ~30-45° in both hemispheres with a high concentration in northern Arabia Terra. Valleys in northern Arabia Terra characteristically start abruptly on steeper slopes and terminate in topographic depressions at elevations corresponding to model-predicted lake levels. Longer valley systems flowed into and out of chains of paleolakes. Minimum discharges based on the dimensions of the incised channel assuming medium to coarse sand-size grains ranges from 10s to 100 s m3 s-1, respectively, consistent with formation via snowmelt from surface or sub-ice flows. Hydrologic calculations indicate the valleys likely formed in hundreds of years or less, and crater statistics constrain the timing of fluvial activity to between the Hesperian and mid Amazonian. Several craters with channels extending radially outward supports evidence for overflow of interior crater lakes possibly fed by groundwater. Most FSVs occur away from young impact craters which makes an association with impact processes improbable. The widespread occurrence of FSVs along with their similar morphology and shared modest state of degradation is consistent with most forming during a global interval of favorable climate, perhaps contemporaneous with alluvial fan formation in equatorial and mid-latitudes. Evidence for a snowmelt based hydrology and considerable depths of water on the landscape in Arabia supports a cold, wet and possibly habitable environment late in Martian history.
The nature of the early Martian climate is one of the major unanswered questions of planetary science. Key challenges remain, but a new wave of orbital and in situ observations and improvements in climate modeling have led to significant advances over the last decade. Multiple lines of geologic evidence now point to an episodically warm surface during the late Noachian and early Hesperian periods 3-4 Ga. The low solar flux received by Mars in its first billion years and inefficiency of plausible greenhouse gases such as CO2 means that the steady-state early Martian climate was likely cold. A denser CO2 atmosphere would have caused adiabatic cooling of the surface and hence migration of water ice to the higher altitude equatorial and southern regions of the planet. Transient warming caused melting of snow and ice deposits and a temporarily active hydrological cycle, leading to erosion of the valley networks and other fluvial features. Precise details of the warming mechanisms remain unclear, but impacts, volcanism and orbital forcing all likely played an important role. The lack of evidence for glaciation across much of Mars' ancient terrain suggests the late Noachian surface water inventory was not sufficient to sustain a northern ocean. While mainly inhospitable on the surface, early Mars may nonetheless have presented significant opportunities for the development of microbial life.
Rock-magnetic studies of the Belovo key loess-paleosol section (52.4° N, 83.4° E) have shown that the loess accumulation and soil genesis on the Ob' Plateau were affected by a specific regional paleoclimate. During the periods of loess formation corresponding to periods of cooling, the main factor responsible for the increased magnetic properties of loess was the force of wind transporting products of disintegration and weathering of rocks with abundant magnetic material. In the periods of warming, the wind attenuation led to a decreased supply of clastic and magnetic material and to weaker magnetic properties of paleosols to follow the "Alaskan" model. During the interglacials, the climatic conditions were warm and wet enough to favor the enrichment of the rocks of soil complexes in fine-grained magnetic fraction according to the "Chinese" type. Thus, the magnetic properties of the loesses and paleosols of southern West Siberia display the superposition of two known types of record of paleoclimatic fluctuations.
The Mawrth Vallis region is covered by some of the largest phyllosilicate-rich outcrops on Mars, making it a unique window into the past history of Mars in terms of water alteration, potential habitability and the search for past life. A landing ellipse had been proposed for the Curiosity rover. This area has been extensively observed by HiRISE and CRISM data, offering the possibility to produce geologic, structural and topographic maps at very high resolution. These observations provide an unprecedented detailed context of the rocks at Mawrth Vallis, in terms of deposition, alteration, erosion and mechanical constraints. Our analyses demonstrate the presence of a variety of alteration environments on the surface and readily accessible to a rover, the presence of flowing water at the surface postdating the formation of the clay-rich units, and evidence for probable circulation of fluids in the rocks at different depths. These rocks undergo continuous erosion, creating fresh outcrops where potential biomarkers may have been preserved. The diversity of aqueous environments over geological time coupled to excellent preservation properties make the area a very strong candidate for future robotic investigation on Mars, like the NASA Mars 2020 mission.
We present the results of a study of some petromineralogical, chemical,
and spectral properties of Etnean palagonitic soils. These soils exhibit
highly variable mineralogy although they are considered among the best
candidates as Martian soil analogs. Five specimens of Etnean volcanic
soil were collected at different altitudes. Four of these are weathered
and reworked pyroclasts from the historic eruptive activity; the fifth
represents an ancient paleosoil (5420-2370 years B.P.). The chemical
composition is hawaiitic. Petrography and mineralogy of grain mounts
show they mainly consist of glass particles (black and red-orange)
containing microphenocrystals of plagioclase, clinopyroxene, olivine,
and Ti-magnetite, in order of abundance. One sample also contains rare
amphiboles. IR (2.3-25 μm) transmittance and UV-visible-near IR
(0.4-2.6 μm) diffuse reflectance spectra were obtained and compared
with those of Martian dust.
The Paks loess-paleosol sequence is one of the most important terrestrial records of Middle and Late Pleistocene environmental changes in East Central Europe, spanning the last ca. 0.8 Ma. While geochemical proxies demonstrate a general decreasing chemical weathering trend over the last 0.8 Ma in the Carpathian Basin, mineralogy and derived indices reflect intensifying physical erosion. In theory, the observed chemical weathering trend can be accounted for both by enhanced input of relatively unweathered material and by climate deterioration during the Quaternary, as the proxies such as CIA are not capable of distinguishing between pre- and post-depositional weathering. Enhanced physical erosion of the source areas, driven by tectonism, and resulting increased sedimentation of fresh mineral dust at the depositional site are demonstrated by increasing dolomite, illite and chlorite contents and sme/ill, sme/(ill + chl) ratios from older to younger sediments in the profile, together with increasing thickness of loess layers towards the youngest part of the sequence. At the same time, constant smectite contents (30–40%) in paleosols appear to disprove progressive aridization of interglacials through time and suggest that the duration of pedogenesis played an important role in determining soil types. Further, the increasing proportion of inherited phyllosilicates (illite and chlorite) would, in theory, raise the possibility that the decreasing values of chemical weathering indices are just artifacts of enhanced physical erosion and resulting increased dust deposition by a dilution effect. The above findings highlight the fact that the general view on chemical weathering is oversimplistic, as its ‘equation’ includes two basic variables, tectonism and time beyond climate and the interplay of these equally important factors will eventually determine its final value. To get a better grasp of these processes needs further data (more age control in loess profiles, data on uplift in and around sedimentary basins) and more sophisticated proxies, as the mineralogical data presented here can be considered only semiquantitative.
Regarding the provenance of sediments in the Paks profile, geochemical data demonstrate that felsic rocks dominated the source areas and there have been only very little variations in provenance over the last ca. 0.8 Ma. Significant contributions from mafic/ultramafic rocks to the sediments can be ruled out as revealed by lower abundances of ferromagnesian trace elements. The appearance of amphiboles and high dolomite contents suggest that loess material was at least partly sourced from local rocks and geochemical data reveal a genetic link between floodplain sediments and loess deposits.
In Hungary, many loess/palaeosol sequences have been found to be discontinuous. In order to allow for correlations with other Quaternary records, reliable chronologies are needed. We therefore apply post-IR infrared (IR) stimulated luminescence (post-IR IRSL; pIRIR290) dating to the uppermost 20 m of the loess sequence at Paks. The pIRIR290 ages are compared with blue quartz OSL ages to test for potential age overestimation due to poor signal re-setting, and the observed good agreement is taken to imply that the more difficult to bleach pIRIR290 signal was reset prior to deposition. Our pIRIR290 based chronology reveals that most of the Late Pleistocene loess was deposited during marine isotope stage (MIS) 3 and during the Last Glacial Maximum (LGM). This is in disagreement with formerly published ages. The discrepancy can most likely be explained by anomalous fading (resulting in an age underestimate); this conclusion is supported by our uncorrected ‘standard’ IRSL ages. We further confirm that the Basaharc Double soil complex can be correlated with MIS 7; the underlying loess and soils cannot be dated accurately because the pIRIR290 signal approaches saturation.
Early Mars (>3 Ga) underwent aqueous alteration as evidenced by fluvial/lacustrine morphologies and the recent discovery of widespread hydrous clays. Despite compelling evidence for diverse and localized aqueous environments, the possibility for sustained liquid water globally on the martian surface and over geological timescales is still highly debated. Instead, a durably cold and dry Mars scenario is often proposed. By studying specific Fe/Mg and Al-rich clay stratigraphies on Mars by means of a planetary scale orbital investigation, we present new evidence that Mars experienced an early era (>3.7 Ga) of widespread aqueous alteration consistent with surface weathering.
Most of the geomorphic changes on Mars occurred during the Noachian
Period, when the rates of impact crater degradation and valley network
incision were highest. Fluvial erosion around the Noachian/Hesperian
transition is better constrained than the longer-term landscape
evolution throughout the Noachian Period, when the highland intercrater
geomorphic surfaces developed. We interpret highland resurfacing events
and processes using a new global geologic map of Mars (at 1:20,000,000
scale), a crater data set that is complete down to 1 km in diameter, and
Mars Orbiter Laser Altimeter topography. The Early Noachian highland
(eNh) unit is nearly saturated with craters of 32-128 km diameter, the
Middle Noachian highland (mNh) unit has a resurfacing age of ~4 Ga, and
the Late Noachian highland unit (lNh) includes younger composite
surfaces of basin fill and partially buried cratered terrain. These
units have statistically distinct ages, and their distribution varies
with elevation. The eNh unit is concentrated in the high-standing Hellas
basin annulus and in highland terrain that was thinly mantled by basin
ejecta near 180° longitude. The mNh unit includes most of Arabia
Terra, the Argyre vicinity, highland plateau areas between eNh outcrops,
and the Thaumasia range. The lNh unit mostly occurs within highland
basins. Crater depth/diameter ratios do not vary strongly between the
eNh and mNh units, although crater losses to Noachian resurfacing appear
greater in lower lying areas. Noachian resurfacing was spatially
non-uniform, long-lived, and gravity-driven, more consistent with
arid-zone fluvial and aeolian erosion and volcanism than with air fall
mantling or mass wasting.
Fluvial landforms of the Hesperian-Amazonian transition are of
particular interest because they may represent the last widespread
episode of aqueous activity. This activity took place during a time
probably characterized by a thin cold atmosphere.
Mathematical models of the frequency-dependent susceptibility in rocks,
soils and environmental materials have been adapted to measurements
performed with multiple operating frequencies (465, 976, 3904, 4650, 15
616, 100 000 and 250 000 Hz) on the basis of log-normal volume
distribution of magnetic particles. The XFD parameter
depends, in addition to the amount of SP particles, also on the
operating frequencies, whose values should be therefore also presented.
The model curves of the XFD parameter versus arithmetical
mean (μ) of the logarithms of grain volume are roughly bell-like
shaped. The width and peak position of these curves is controlled by
mean and standard deviation of the logarithmic volume distribution.
Magnetic susceptibility contributions from paramagnetic minerals, and
from ferrimagnetic particles not belonging to a unimodal SP/SD volume
distribution, tend to decrease the XFD parameter. Therefore,
low XFD values do not therefore necessarily indicate low
amount of SP particles, but can also be indicative of the presence of
the paramagnetic fraction. A new parameter XR is introduced
based on susceptibility measurements at three operating frequencies; it
is insensitive to dia- and paramagnetic fractions and helps us to
differentiate between wide and narrow size distributions of
ferromagnetic particles. A new XFB parameter is introduced
that originates through normalizing the XFD parameter by the
difference of natural logarithms of operating frequencies and related to
the decade difference between the frequencies. It is convenient for
comparison of the Bartington MS-2 Susceptibility Meter data with the
MFK1-FA Kappabridge data.
The loess-paleosol sequences of China, Siberia, Alaska and many other regions, along with lake sediments and glaciers, provide the only accurate paleoclimatic terrestrial records for intervals of thousands to hundreds of thousand years. The frequency dependence (FD) of magnetic susceptibility (MS) in such sequences has become the leading parameter for analyzing climatic change and Milankovitch (astronomical) periodicity in Siberian sequences; it is always higher in soil horizons than in loess. The enhanced FD parameter in soils is associated with ferromagnetic minerals, mostly magnetite, produced during pedogenesis. The MS and FD parameters of 670 samples from five sections in Siberia are reported here. Inter-section correlation is used to produce a combined FD time series for the studied sections. Chronological control is established by absolute dating and stratigraphic correlation. Spectral analysis of the FD time series reveals the presence of Milankovitch signals at ˜100 kyr (eccentricity), ˜40 kyr (obliquity) and ˜23 kyr (precession) and demonstrates that Siberian loess-paleosol sequences are excellent continental recorders of long-term paleoclimatic changes. This suggests that the FD parameter can potentially be used more widely for evaluation of climate periodicity in loess/paleosol sequences in other parts of the world.
The present study discusses the findings of detailed stratigraphical, sedimentological, geochemical and paleoecological investigations implemented as part of a 1990 campaign to Pécsi’s studied profile at Hungary’s important Danubian loess profile at Basaharc. In older studies, there were major discrepancies between the old lithostratigraphic classifications and the numerical ages. According to the findings of this study, the Basaharc Double Paleosol Complex, described formerly from the lower brickyard as a stratotype of Middle Würmian, is an isochronous heterotype of the Mende Upper Paleosol Complex from Profile #1 due to reworking. However, the lower member of the Mende Upper Paleosol Complex must have formed not at the boundary of the Upper and Middle Würmian as originally presumed, but during the Riss-Würmian interglacial. Conversely, the upper member of this same complex developed during an interglacial between the Upper and Middle Würmian, implying the presence of a huge erosional hiatus between the two members of the paleosol complex. The genetics of the individual paleosol layers needs a re-evaluation as well. The majority of the soils formerly conceived to be of steppe and forest steppe origin developed among closed woodland conditions.
Multivariate techniques were applied to 11 chemical and physical properties of soil samples collected along a prograded beach chronosequence, located on the west coast of Vancouver Island, British Columbia. The samples were taken from three horizons at each of seven sites. The 21 samples were considered as independent observations. Application of principal component analysis with a normal varimax rotation gave highly interpretable results. The first component was readily identified with podzolic pedogenic processes, whereas the second component appeared to relate closely to sea spray input. A nonlinear optimization procedure was used to fit the first principal component to an empirical equation incorporating a logistic term for time and an exponential term for depth. A correlation coefficient value of 0.99 was obtained. The notion of a pedogenic damping depth analagous to a thermal damping depth is suggested.