Africa was Wetter During the Medieval Warm Period

Hydroclimate in Africa during the Medieval Climate Anomaly

Authors:

Lüning et al

Abstract:

The Medieval Climate Anomaly (MCA) is a recognized period of distinct pre-industrial climate change, with a core period of 1000–1200 CE. The field of palaeoclimatology has made major progress over the past 15 years during which a great number of high- and medium-resolution case studies were published, reconstructing climate change of the past millennia. In many parts of the world, regional data coverage has now reached a point which allows compiling palaeoclimate maps for well-defined time intervals. Here we present hydroclimatic trend maps for the MCA in Africa based on 99 published study locations. Key hydroclimatic proxy curves are visualized and compared in a series of 16 correlation panels. Proxy types are described and possible issues discussed. Based on the combined MCA dataset, temporal and spatial trends are interpreted and mapped out. Three areas have been identified in Africa in which rainfall seems to have increased during the MCA, namely Tunisia, western Sahel and the majority of southern Africa. At the same time, a reduction in precipitation occurred in the rest of Africa, comprising of NW and NE Africa, West Africa, Eastern Africa and the Winter Rainfall Zone of South Africa. MCA hydroclimate change in Africa appears to have been associated with characteristic phases of ocean cycles, as also supported by modern climate observations. Aridity in Morocco typically coincides with the positive phase of the North Atlantic Oscillation (NAO), whilst increased rainfall in the western Sahel is often coupled to the positive phase of the Atlantic Multidecadal Oscillation (AMO). Reduction in rainfall in the region Gulf of Aden/southern Red Sea to Eastern Africa could be linked to a negative Indian Ocean Dipole (IOD) or a derived long-term equivalent Indian Ocean cycle parameter. The Intertropical Convergence Zone (ITCZ) appears to have been shifted pole-wards during the MCA, for both the January and July positions. MCA hydroclimate mapping revealed major data gaps in the Sahara, South Sudan, Somalia, Central African Republic, Democratic Republic of Congo, Angola, northern Mozambique, Zambia and Zimbabwe. Special efforts are needed to fill these gaps, e.g. through a dedicated structured research program in which new multiproxy datasets are created, based on the learnings from previous African MCA studies.

Was Early Martian Climate Cold but Warmed Periodically by Volcanism & Impacts?

Climate Cycling on Early Mars Caused by the Carbonate-Silicate Cycle

Authors:

Batalha et al

Abstract:

For decades, scientists have tried to explain the evidence for fluvial activity on early Mars, but a consensus has yet to emerge regarding the mechanism for producing it. One hypothesis suggests early Mars was warmed by a thick greenhouse atmosphere. Another suggests that early Mars was generally cold but was warmed occasionally by impacts or by episodes of enhanced volcanism. These latter hypotheses struggle to produce the amounts of rainfall needed to form the martian valleys, but are consistent with inferred low rates of weathering compared to Earth. Here, we provide a geophysical mechanism that could have induced cycles of glaciation and deglaciation on early Mars. Our model produces dramatic climate cycles with extended periods of glaciation punctuated by warm periods lasting up to 10 Myr, much longer than those generated in other episodic warming models. The cycles occur because stellar insolation was low, and because CO2 outgassing is not able to keep pace with CO2 consumption by silicate weathering followed by deposition of carbonates. While CO2 by itself is not able to deglaciate early Mars in our model, we assume that the greenhouse effect is enhanced by substantial amounts of H2 outgassed from Mars' reduced crust and mantle. Our hypothesis can be tested by future Mars exploration that better establishes the time scale for valley formation.

Were Titan's Channels Carved by Ethane & Ammonia?

Formation mechanisms of channels on Titan through dissolution by ammonium sulfate and erosion by liquid ammonia and ethane

Authors:

Gilliam et al

Abstract:

Data obtained from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), Imaging Science Subsystem (ISS), and Synthetic Aperture Radar (SAR) instruments have revealed an array of fluvial channels on Titan's surface, often several hundreds of kilometers in length. The paucity of impact craters on Titan's surface suggests a formation by fluvial erosion into the water-ice bedrock. Additionally, at the landing site, the Huygens Probe Descent Imager and Spectral Radiometer (DISR) imaged Earth-like rounded cobbles 0.3–15 cm in diameter composed of water ice, reminiscent of rounded stream clasts on Earth. In this paper we examine different fluvial features on Titan, identified by the Cassini spacecraft, and evaluate the possibilities of channel formation by dissolution of ice by a concentrated solution of ammonium sulfate, and by mechanical erosion by flow of liquid ammonia and liquid ethane. We find that chemical erosion of Titan's channels could be completed in 280 to 1100 years (all units of time in this paper are Terrestrial, not Titanian), much shorter than the period of about 84,000 years that a concentrated (NH4)2SO4-H2O solution could exist as a liquid on the Titan surface. Mechanical erosion of Titan's channels is generally a much slower process, on the order of 102 to 105 years to completion, and is also slower than mechanical erosion of a model river on Earth, averaging 103 to 104 years. The erosional sequence of the channels on Titan may have started after the formation of water-ice on the surface by the process of chemical dissolution by (NH4)2SO4-H2O, overlapping, or followed by, a period of mechanical erosion by liquid NH3. A final stage on the cooling surface of Titan might have been characterized by liquid C2H6 as an agent of mechanical erosion.

Titan has Numerous Methane Filled Canyons

Liquid methane-filled canyons hundreds of meters deep with walls as steep as ski slopes etch the surface of Titan, researchers report in a new study. The new findings provide the first direct evidence of these features on Saturn's largest moon, and could give scientists insights into Titan's origins and similar geologic processes on Earth, according to the study's authors.

New Cassini radar observations of Titan's north pole depict cavernous gorges a little less than a kilome-ter (less than half a mile) wide with walls up to 570 meters (1870 feet) tall -- about 30 meters (98 feet) higher than New York's Freedom Tower. The eight canyons branch off from Vid Flumina, a more than 400-kilometer (249-mile) long river flowing into Titan's second-largest sea, Ligeia Mare. The new data confirm the canyons are filled with flowing methane -- a feature researchers had suspected but not directly observed, according to the study's authors.

The new findings suggest the canyons were likely carved by liquid methane draining into Vid Flumina, a process similar to the carving of river gorges on Earth, according to the study's authors. The new re-search could help scientists better understand these geological processes, they said.

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The Shorelines & Drainage Systems of Mars

Waterfront on the Martian Planitia: Algorithmic emergent catchments on disordered terrain

Authors:

Handmer et al

Abstract:

Under a terraforming scenario, a reactivated hydrological cycle on Mars will result in upwards movement of water due to evaporation and precipitation. If Mars' embryonic fossilized catchments provide inadequate drainage, Mars' limited supplies of water may be absorbed entirely by crater lakes and glaciers, with negative consequences for the terraforming effort. We demonstrate a stable, convergent algorithm for the efficient modeling of water flow over disordered terrain. This model is applied to Mars Orbital Laser Altimeter data and successfully predicts the formation of fossilized waterways and canyons visible only at much higher resolution. This exploratory study suggests that despite its impossibly rugged appearance, ancient water flows have carved channels that provide effective drainage over the majority of Mars' surface. We also provide one possible reconstruction of a terraformed surface water distribution.

Geochemistry and Mineralogy of Western Australian Salt Lake Sediments: Implications for Meridiani Planum on Mars

Geochemistry and Mineralogy of Western Australian Salt Lake Sediments: Implications for Meridiani Planum on Mars

Authors:

Schröder et al

Abstract:

Hypersaline lakes are characteristic for Western Australia and display a rare combination of geochemical and mineralogical properties that make these lakes potential analogues for past conditions on Mars. In our study, we focused on the geochemistry and mineralogy of Lake Orr and Lake Whurr. While both lakes are poor in organic carbon (<1 3.8="" 4.8="" 5.4="" 6.3="" a="" analogue="" and="" another="" area="" as="" at="" be="" between="" blackish="" brownish="" by="" could="" desiccation="" differ="" differences="" dominated="" evaporation="" events="" explain="" extent="" fe="" flooding="" for="" from="" furthermore="" goethite="" has="" hematite="" high="" identification="" identified.="" identified="" implications="" in="" influenced="" interpretation="" iron="" is="" jarosite="" lake="" lakes="" least="" likely="" main="" make="" mars="" meridiani="" mineralogy="" minerals="" not="" observed="" of="" on="" orange="" orr="" p="" paramagnetic="" particular="" ph="" phase="" planum="" processes="" pyrite.="" pyrite="" range="" red="" salinity="" salt="" sampling="" secondary="" sediment="" sediments.="" sediments="" some="" strongly="" such="" suitable="" tentative="" tentatively="" that="" the="" these="" this="" to="" two="" values="" was="" were="" where="" which="" whurr.="" whurr="" zones="">

Evidence of Amazonian era ice Ages From Terra Cimmeria, Mars

Amazonian-aged fluvial system and associated ice-related features in Terra Cimmeria, Mars

Authors:

Adeli et al

Abstract:

The Martian climate throughout the Amazonian is widely believed to have been cold and hyper-arid, very similar to the current conditions. However, ubiquitous evidence of aqueous and glacial activity has been recently reported, including channels that can be tens to hundreds of kilometres long, alluvial and fluvial deposits, ice-rich mantles, and glacial and periglacial landforms. Here we study a ∼340 km-long fluvial system located in the Terra Cimmeria region, in the southern mid-latitudes of Mars. The fluvial system is composed of an upstream catchment system with narrow glaciofluvial valleys and remnants of ice-rich deposits. We observe depositional features including fan-shaped deposits, and erosional features such as scour marks and streamlined islands. At the downstream section of this fluvial system is an outflow channel named Kārūn Valles, which displays a unique braided alluvial fan and terminates on the floor of the Ariadnes Colles basin. Our observations point to surface runoff of ice/snow melt as the water source for this fluvial activity. According to our crater size–frequency distribution analysis the entire fluvial system formed during early to middle Amazonian, between View the MathML source∼1.8−0.2+0.2 Ga to View the MathML source510−40+40 Ma. Hydraulic modelling indicates that the Kārūn Valles and consequently the alluvial fan formation took place in geologically short-term event(s). We conclude that liquid water was present in Terra Cimmeria during the early to middle Amazonian, and that Mars during that time may have undergone several episodic glacial-related events.

Titan's Kraken Mare Should NOT be There

The influence of subsurface flow on lake formation and north polar lake distribution on Titan

Authors:

Horvath et al

Abstract:

Observations of lakes, fluvial dissection of the surface, rapid variations in cloud cover, and lake shoreline changes indicate that Saturn's moon Titan is hydrologically active, with a hydrocarbon-based hydrological cycle dominated by liquid methane. Here we use a numerical model to investigate the Titan hydrological cycle – including surface, subsurface, and atmospheric components – in order to investigate the underlying causes of the observed distribution and sizes of lakes in the north polar region. The hydrocarbon-based hydrological cycle is modeled using a numerical subsurface flow model and analytical runoff scheme, driven by a general circulation model with an active methane-cycle. This model is run on synthetically generated topography that matches the fractal character of the observed topography, without explicit representation of the effects of erosion and deposition. At the scale of individual basins, intermediate to high permeability (10−8–10−6 cm2) aquifers are required to reproduce the observed large stable lakes. However, at the scale of the entire north polar lake district, a high permeability aquifer results in the rapid flushing of methane through the aquifer from high polar latitudes to dry lower polar latitudes, where methane is removed by evaporation, preventing large lakes from forming. In contrast, an intermediate permeability aquifer slows the subsurface flow from high polar latitudes, allowing greater lake areas. The observed distribution of lakes is best matched by either a uniform intermediate permeability aquifer, or a combination of a high permeability cap at high latitudes surrounded by an intermediate permeability aquifer at lower latitudes, as could arise due to karstic processes at the north pole. The stability of Kraken Mare further requires reduction of the evaporation rate over the sea to 1% of the value predicted by the general circulation model, likely as a result of dissolved ethane, nitrogen, or organic solutes, and/or a climatic lake effect. These results reveal that subsurface flow through aquifers plays an important role in Titan's hydrological cycle, and exerts a strong influence over the distribution, size, and volatile budgets of Titan's lakes.

Martian north polar cap summer water cycle

Martian north polar cap summer water cycle

Authors:

Brown et al

Abstract:

A key outstanding question in Martian science is 'are the polar caps gaining or losing mass and what are the implications for past, current and future climate?' To address this question, we use observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of the north polar cap during late summer for multiple Martian years, to monitor the summertime water cycle in order to place quantitative limits on the amount of water ice deposited and sublimed in late summer.

We establish here for the first time the summer cycle of water ice absorption band signatures on the north polar cap. We show that in a key region in the interior of the north polar cap, the absorption band depths grow until Ls=120, when they begin to shrink, until they are obscured at the end of summer by the north polar hood. This behavior is transferable over the entire north polar cap, where in late summer regions 'flip' from being net sublimating into net condensation mode. This transition or 'mode flip' happens earlier for regions closer to the pole, and later for regions close to the periphery of the cap.

The observations and calculations presented herein estimate that on average a water ice layer ~70 microns thick is deposited during the Ls=135-164 period. This is far larger than the results of deposition on the south pole during summer, where an average layer 0.6-6 microns deep has been estimated by Brown et al. (2014).

Does Water Boiling On Mars Explain the Strange Surface Features?

Water boiling under Mars's thin atmosphere could explain some of the planet's puzzling geological features, such as gullies (pictured) and hillside streaks, which some scientists have attributed to liquid water flowing today.

link.

Evidence of Water Discharges From Nili Fossae and Syrtis Major on Mars

Extensive aqueous deposits at the base of the dichotomy boundary in Nilosyrtis Mensae, Mars

Authors:

Bandfield et al

Abstract:

Thermal emission imaging system (THEMIS) and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectral datasets were used to identify high bulk SiO2 and hydrated compositions throughout the Nilosyrtis Mensae region. Four isolated locations were identified across the region showing short wavelength silicate absorptions within the 8–12 μm spectral region, indicating surfaces dominated by high Si phases. Much more extensive exposures of hydrated compositions are present throughout the region, indicated by a spectral absorption near 1.9 μm in CRISM data. Although limited in spatial coverage, detailed spectral observations indicate that the hydrated materials contain Fe/Mg-smectites and hydrated silica along with minor exposures of Mg-carbonates and an unidentified hydrated phase. The high SiO2 and hydrated materials are present in layered sediments near the base of topographic scarps at the hemispheric dichotomy boundary, typically near or within low albedo sand deposits. The source of the high SiO2 and hydrated materials appears to be from groundwater discharge from Nili Fossae and Syrtis Major to the south, where there is evidence for extensive aqueous alteration of the subsurface. Although discontinuous, the exposures of high SiO2 and hydrated materials span a wide area and are present in a similar geomorphological context to previously identified deposits in western Hellas Basin. These regional deposits may reflect aqueous conditions and alteration within the adjacent crust of the martian highlands.

Evidence of a Declining Martian Hydrological Cycle Starting 3.7 Billion Years Ago at the Noachian/Hesperian Boundary

Insights into surface runoff on early Mars from paleolake basin morphology and stratigraphy

Authors:

Goudge et al

Abstract:

We present observations on the morphology and stratigraphy of more than 400 paleolake basins on Mars. We show that there are two distinct classes of Martian paleolake basins: (1) paleolakes fed by regionally integrated valley networks (N = 251), and (2) paleolakes fed by isolated inlet valleys not integrated into broader regional drainage systems (N = 174). We conclude that valley network–fed paleolakes primarily formed prior to approximately the Noachian-Hesperian boundary, ca. 3.7 Ga, while isolated inlet valley paleolakes primarily formed later in Martian history. All 174 isolated inlet valley paleolakes are closed-basin lakes; however, there are surprisingly few (31) valley network–fed closed-basin lakes compared to a large number (220) of valley network–fed open-basin lakes. This observation is consistent with declining levels of fluvial activity over time on the Martian surface. Our results imply that during the era of valley network formation, ∼90% of topographic basins breached by an inlet valley had sufficiently high ratios of water influx to losses to fill, overtop, and form an outlet valley. This conclusion provides an important constraint on the balance between surface runoff production and water losses on early Mars that must be satisfied by any model of the early Martian climate and hydrologic cycle.

A Pop Sci Take on Titan's "Bathtub Scum" (evaporites)

It's not everyday that you get to discover something new. But when you do it is a rather strange and quite brilliant feeling. You don't really cry out 'Eureka' (there's usually about a million things going about it your head pointing out how it could be wrong). When you finally conquer the 'wrong' demon and satisfy yourself that you have something new, well then you usually sit back in your chair and smile to yourself. Maybe at a push grab a cup of coffee and a celebratory chocolate bar from the vending machine. That's pretty much how I felt when I worked out the latest crystal structure I've just published, of the 'bath scum' of Titan.

The great thing about this column space is that I can use it to tell you all the back-story behind a paper, how it came about and why I think it's really exciting.

Titan, the largest moon of Saturn, is in many ways pretty similar to Earth. It's the only moon in the solar system with a substantial atmosphere, much thicker than our own. If you don't mind the cold and lack of oxygen, moving about the surface there will feel a bit like walking under water. It's pretty nice when you consider that the atmosphere most other planets and moons will barely shield you from the vacuum of space.

The other similarity with our own home is that Titan has liquid on the surface, vast lakes and seas. It's the only other place we know where you can watch the sun set into a sea (albeit very very slowly). But, where our seas are composed of water, the seas and lakes of Titan are filled with methane and ethane, which are liquid at the frigid temperatures on the surface (around -180°C). And in fact, the Cassini spacecraft and the team of scientists behind it, have gone further than just the discovery of standing liquid - they have shown that Titan has a kind of hydrological cycle.

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The fate of ethane in Titan’s hydrocarbon lakes and seas

The fate of ethane in Titan’s hydrocarbon lakes and seas

Authors:

Mousis et al

Abstract:

Ethane is expected to be the dominant photochemical product on Titan’s surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini’s 2.2 cm radar by Ligeia Mare however suggests that this north polar sea is dominated by methane. In order to explain this apparent ethane deficiency, we explore the possibility that Ligeia Mare is the visible part of an alkanofer that interacted with an underlying clathrate layer and investigate the influence of this interaction on an assumed initial ethane–methane mixture in the liquid phase. We find that progressive liquid entrapment in clathrate allows the surface liquid reservoir to become methane-dominated for any initial ethane mole fraction below 0.75. If interactions between alkanofers and clathrates are common on Titan, this should lead to the emergence of many methane-dominated seas or lakes.

Alluvial Fan Morphology, distribution and formation on Titan

Alluvial Fan Morphology, distribution and formation on Titan

Authors:

Birch et al

Abstract:

Titan is a hydrologically active world, with dozens of alluvial fans that are evidence of sediment transport from high to low elevations. However, the distribution and requirements for the formation of fans on Titan are not well understood. We performed the first global survey of alluvial fans on Titan using Cassini Synthetic Aperture Radar (SAR) data, which cover 61% of Titan’s surface. We identified 82 fans with areas ranging from 28 km2 to 27,000 km2. A significant fraction (∼60%) of the fans are restricted to latitudes of ±50–80°, suggesting that fluvial sediment transport may have been concentrated in the near-polar terrains in the geologically recent past. The density of fans is also found to be correlated with the latitudes predicted to have the highest precipitation rates by Titan Global Circulation Models. In equatorial regions, observable fans are not generally found in proximity to dune fields. Such observations suggest that sediment transport in these areas is dominated by aeolian transport mechanisms, though with some degree of recent equatorial fluvial activity. The fan area-drainage area relationship on Titan is more similar to that on Earth than on Mars, suggesting that the fans on Titan are smaller than what may be expected, and that the transport of bedload sediment is limited. We hypothesize that this has led to the development of a coarse gravel-lag deposit over much of Titan’s surface. Such a model explains both the morphology of the fans and their latitudinal concentration, yielding insight into the sediment transport regimes that operate across Titan today.

Reconstructing the Paleohydrology of a Cretaceous Alaskan Paleopolar Coastal Plain

Reconstructing the paleohydrology of a cretaceous Alaskan paleopolar coastal plain from stable isotopes of bivalves

Authors:

Suarez et al

Abstract:

As global mean annual temperatures continue to rise, modern Arctic climates are changing at an incredibly fast rate. To predict changing climate dynamics, a number of researchers have been focused on characterizing Arctic climate from ancient greenhouse worlds such as the Cretaceous and Eocene. Characterization of these climates requires analysis of a variety of proxy materials. Here we use bivalve shells, identified as Nucula aff. Nucula percrassa Conrad, from the Cretaceous (early Maastrichtian) Prince Creek Formation in combination with isotope data from previous lithologic and isotopic studies to characterize a cool-house Cretaceous climate bounded by greenhouse climates (mid-Cretaceous thermal maximum and Late Paleocene climate). Bivalves were analyzed by powder X-ray diffraction (PXRD), cathodoluminescence (CL), and thin section petrography to determine mineralogy and integrity of shells. PXRD analysis indicates that shells are composed of primary aragonite and not secondary calcite. Thin section and CL images reveal well preserved shell morphology including daily growth lines, and no luminescent patches or coarsening of crystal sizes to indicate diagenetic alteration/recrystallization. Aragonite carbon isotopic composition (δ13Carag) range from + 0.45‰ to − 3.00‰ VPDB and average − 1.58‰. Aragonite oxygen isotopic composition (δ18Oarag) range from − 10.48‰ to − 15.42‰ VPDB and average − 12.83‰. Using temperatures from 4.5 °C to 12.5 °C, the salinity–δ18Owater relationship of a modern Arctic setting from Torres et al. (2011), and the water–aragonite equation of Grossman and Ku (1986) reveal salinity ranges between 8 and 17.7 ppt. Using these salinity ranges and temperatures, N. percrassa precipitated shell aragonite from water that ranged between δ18Owater = − 14.41‰ VSMOW (at 12.5 °C) and − 16.23‰ (at 4.5 °C). Shell growth was in equilibrium with water that represents a coastal, mixed water setting such as an estuary. When combined with previous studies, the mixture of fresh:marine water represents a 50%:50% to 72%:28% mixture and ranged between − 14.41‰ and − 16.23‰ during spring to summer months.

Amazon Region Expected to Have More Frequent, Extreme Droughts and Extreme Rains

A new paper co-authored by WHRC scientists Philip Duffy and Paulo Brando evaluates the accuracy of current climate models and uses them to project future drought and wet periods in the Amazon. They conclude that the whole of the Amazon will confront more hydrological extremes, and that most of the region will experience much more frequent and extensive drought. These changes would have profound implications for forest structure, composition, biomass, and carbon emissions.

According to Dr. Duffy, "Historically, the main source of CO2 emissions from Amazon forests has been direct human action, especially deforestation. However, in the future, climate change may cause large emissions that result from changes in the large-scale environment rather than from direct human action, and hence are much more difficult to control. This study, based on 35 climate models, suggests that future climate change will increase the frequency and geographic extent of meteorological drought in most of Amazon. This may contribute to forest degradation and increased emissions of CO2 to the atmosphere, amplifying global warming."

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Lakes, Fans, Deltas and Streams of the Martian Gale Crater

Comparing Topographic Signatures of Martian and Terrestrial Arroyos

The comparison of topographic long profiles of gullies on Earth to gullies on Mars: A signal of water on Mars

Authors:

Conway et al

Abstract:

The topographic signature of a landform can give important clues as to its formation process. Here, we have used topographic long profiles to study the process of gully formation on Mars. We studied topographic long profiles of gullies on Earth to (1) confirm that previously published generalisations of how long profile shape varies with process also applies at the kilometre-scale of martian gullies, and (2) use as a direct comparison with the martian data. We have compared 24 fluvial and 22 debris flow long profiles of terrestrial gullies derived from laser altimeter and GPS measurements, to 78 long profiles of a range of gullies on Mars derived from a stereo-photogrammetry point-matching technique. We have confirmed that this manual point-matching technique is reliable for the martian data by comparison with full digital elevation models. We used nine different characteristics of the long profiles, including slope and curvature parameters, to perform a canonical discriminant analysis, which allowed us to identify the variables most important for differentiating between fluvial and debris flow gullies on Earth. In agreement with published literature for larger-scale features, we found that terrestrial debris flow gullies tend to be steeper and less concave than fluvial gullies. We have found that gully long profiles on Mars can resemble long profiles of terrestrial gullies formed by either fluvial or debris flow processes, with slightly more affinity to fluvial systems. Gullies on Mars can only be weakly separated from those on Earth: they can be separated from terrestrial fluvial gullies on curvature parameters and from terrestrial debris flow gullies by slope parameters. In addition, we have found that different alcove types identified from planview morphology are also distinctive in terms of their long profile morphology: gullies which incise back into the bedrock are more similar to terrestrial debris flows whereas polar-pit gullies are most similar to terrestrial fluvial gullies. Our findings suggest that the presence of a bedrock alcove promotes debris flow behaviour in gullies on Mars.

Cause, Effect or Coincidence? Hydrological Cycle Changed at Pleistocene Quaternary Megafauna Extinction in Australia

Hydrological transformation coincided with megafaunal extinction in central Australia

Authors:

Cohen et al

Abstract:

Central to the debate over the extinction of many of Australia's last surviving megafauna is the question: Was climate changing significantly when humans arrived and megafauna went extinct? Here we present a new perspective on variations in climate and water resources over the last glacial cycle in arid Australia based on the study of the continent's largest lake basin and its tributaries. By dating paleoshorelines and river deposits in the Lake Eyre basin, we show that major hydrological change caused previously overflowing megalakes to enter a final and catastrophic drying phase at 48 ± 2 ka just as the giant bird, Genyornis newtoni, went extinct (50–45 ka). The disappearance of Genyornis and other megafauna has been previously attributed to "ecosystem collapse" coincident with the spread of fire-wielding humans. Our findings suggest a climate-driven hydrological transformation in the critical window of human arrival and megafaunal extinction, and the results call for a re-evaluation of a human-mediated cause for such extinctions in arid Australia.

The question becomes could people cause the hydrological change?  Fire was used extensively by the Aboriginals and that would have transformed things considerably.  Or did the climate change on its own and drive an extinction 30k years before the rest of the megafaunal extinctions?  Or was the arrival of humanity a coincidence?  or...?