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Controls on development and diversity of Early Archean stromatolites
The â3,450-million-year-old Strelley Pool Formation in Western Australia contains a reef-like assembly of laminated sedimentary accretion structures (stromatolites) that have macroscale characteristics suggestive of biological influence. However, direct microscale evidence of biologyânamely, organic microbial remains or biosedimentary fabricsâhas to date eluded discovery in the extensively-recrystallized rocks. Recently-identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes, including some that indicate probable microbial mat formation. Furthermore, we find relict fabrics and organic layers that covary with stromatolite morphology, linking morphologic diversity to changes in sedimentation, seafloor mineral precipitation, and inferred microbial mat development. Thus, the most direct and compelling signatures of life in the Strelley Pool Formation are those observed at the microscopic scale. By examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer aspects of the biological inputs to stromatolite morphogenesis. The persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans
The stratigraphy and sedimentation of the Wallace Formation northwest Montana and northern Idaho
A study of personality characteristics and effectiveness of job performance of USAF non-commissioned officers instructing basic military training
Years of service in the Air Force has served to reveal the need for an increased awareness of the high cost of training personnel. While many factors are involved in this problem, one particularly important facet is the type and quality of leadership, instruction and supervision which basic trainees are s u b jected to in their initial period of adjustment to military life. This inchoate period of adjustment in the Armed Services is a particularly sensitive one for the individual
Geomorphic evolution of a storm-dominated carbonate ramp (c. 549 Ma), Nama Group, Namibia
The well-exposed Hoogland Member (c. 549 Ma) of the northern Nama Group (Kuibis Subgroup), Namibia, represents a storm-dominated carbonate ramp developed in a foreland basin of terminal Proterozoic age. The ramp displays facies gradients involving updip grainstones which pass downdip into broad, spatially extensive tracts of microbial laminites and finely laminated mudstones deposited above and below storm wave base. Trough cross-bedded, coarse grainstones are shown to transit downdip into finer-grained calcarenites, irregular microbial laminites and mottled laminites. Siliciclastic siltstones and shales were deposited further downdip. Platform growth was terminated through smothering by orogen-derived siliciclastic deposits. Ramp morphology was controlled by several different processes which acted across many orders of magnitude (millimetres to kilometres), including in situ growth of mats and reefs, scouring by wave-produced currents, and transport and infilling of coarse-grained carbonates and fine-grained carbonates and clastics. At the smallest scale, ârougheningâ of the sea-floor through heterogeneous trapping and binding by microbial mats was balanced by smoothing of the sea-floor through accumulation of loose sediment to fill the topographic lows within the upward-propagating mat. At the next scale up, parasequence development involved roughening of the sea-floor through shoal growth and grainstone progradation, balanced by sea-floor smoothing through shale infilling of resulting downdip accommodation, as well as the metre-scale topographic depressions within the mosaic of shoal-water facies. At even larger (sequence/platform) scales, roughening of the sea-floor occurred through aggradation and progradation of thick carbonates, balanced by infilling of the foreland basin with orogen-derived siliciclastic sediments. At all scales a net balance was achieved between sea-floor roughening and sea-floor smoothing to maintain a more or less constant ramp profile
Physicochemical properties of concentrated Martian surface waters
Understanding the processes controlling chemical sedimentation is an important step in deciphering paleoclimatic conditions from the rock records preserved on both Earth and Mars. Clear evidence for subaqueous sedimentation at Meridiani Planum, widespread saline mineral deposits in the Valles Marineris region, and the possible role of saline waters in forming recent geomorphologic features all underscore the need to understand the physical properties of highly concentrated solutions on Mars in addition to, and as a function of, their distinct chemistry. Using thermodynamic models predicting saline mineral solubility, we generate likely brine compositions ranging from bicarbonate-dominated to sulfate-dominated and predict their saline mineralogy. For each brine composition, we then estimate a number of thermal, transport, and colligative properties using established models that have been developed for highly concentrated multicomponent electrolyte solutions. The available experimental data and theoretical models that allow estimation of these physicochemical properties encompass, for the most part, much of the anticipated variation in chemistry for likely Martian brines. These estimates allow significant progress in building a detailed analysis of physical sedimentation at the ancient Martian surface and allow more accurate predictions of thermal behavior and the diffusive transport of matter through chemically distinct solutions under comparatively nonstandard conditions
Statistical physics and stromatolite growth: new perspectives on an ancient dilemma
This paper outlines our recent attempts to model the growth and form of
microbialites from the perspective of the statistical physics of evolving
surfaces. Microbialites arise from the environmental interactions of microbial
communities (microbial mats). The mats evolve over time to form internally
laminated organosedimentary structures (stromatolites). Modern day
stromatolites exist in only a few locations, whereas ancient stromatolitic
microbialites were the only form of life for much of the Earth's history. They
existed in a wide variety of growth forms, ranging from almost perfect cones to
branched columnar structures. The coniform structures are central to the heated
debate on the oldest evidence of life. We proposed a biotic model which
considers the relationship between upward growth of a phototropic or
phototactic biofilm and mineral accretion normal to the surface. These
processes are sufficient to account for the growth and form of many ancient
stromatolities. These include domical stromatolites and coniform structures
with thickened apical zones typical of Conophyton. More angular coniform
structures, similar to the stromatolites claimed as the oldest macroscopic
evidence of life, form when the photic effects dominate over mineral accretion.Comment: 8 pages, 3 figures. To be published in Proceedings of StatPhys-Taiwan
2004: Biologically Motivated Statistical Physics and Related Problems, 22-26
June 200
Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars
The Sample Analysis at Mars (SAM) instrument on board the Mars Science Laboratory Curiosity rover is designed to conduct inorganic and organic chemical analyses of the atmosphere and the surface regolith and rocks to help evaluate the past and present habitability potential of Mars at Gale Crater. Central to this task is the development of an inventory of any organic molecules present to elucidate processes associated with their origin, diagenesis, concentration, and long-term preservation. This will guide the future search for biosignatures. Here we report the definitive identification of chlorobenzene (150â300 parts per billion by weight (ppbw)) and C_2 to C_4 dichloroalkanes (up to 70âppbw) with the SAM gas chromatograph mass spectrometer (GCMS) and detection of chlorobenzene in the direct evolved gas analysis (EGA) mode, in multiple portions of the fines from the Cumberland drill hole in the Sheepbed mudstone at Yellowknife Bay. When combined with GCMS and EGA data from multiple scooped and drilled samples, blank runs, and supporting laboratory analog studies, the elevated levels of chlorobenzene and the dichloroalkanes cannot be solely explained by instrument background sources known to be present in SAM. We conclude that these chlorinated hydrocarbons are the reaction products of Martian chlorine and organic carbon derived from Martian sources (e.g., igneous, hydrothermal, atmospheric, or biological) or exogenous sources such as meteorites, comets, or interplanetary dust particles
Preliminary Investigations of Early Proterozoic western River and Burnside River Formations : Evidence For Foredeep Origin of Kilohigok Basin, District of Mackenzie
In the Kilohigok Basin, the Western River and Burnside River formations comprise three successively overlying tectono-stratigraphic sedimentary units of regional extent: a basal shallow water siliciclastic/carbonate platform, overlain by deepwater flysch, in turn overlain by shallow
marine and fluvial molasse. This stratigraphy represents an initial stable shelf (passive margin?) whose outer, southerly edge rapidly subsided contemporaneous with arching and subaerial exposure of its interior. Shelf drowning represents the onset of foredeep subsidence subparallel to the trend of Thelon Tectonic Zone. Arching and subsidence were perpendicular to the tectonic transport direction of intrabasinal nappes. indicating that convergence and uplift a long Thelon Tectonic Zone were
probably responsible for foredeep subsidence within Ki lohigok Basin. Following drowning, the platform was buried by deepwater deposits ( flysch); with progressive uplift and basin filling, the foredeep entered the molasse phase and fluvial sediments prograded towards the foreland. The
foredeep model places constraints on the origin of Thelon Tectonic Zone and provides a more comprehensive understanding of the tectonic evolution of the Slave Province and its relation to the Wopmay Orogen
QnAs with John P. Grotzinger. Interview by Prashant Nair.
In late November 2011, the National Aeronautics and Space Administration
(NASA) plans to launch its robotic explorer to scour Mars for signs of the
planetâs ability to support life. The Mars Science Laboratory (MSL) spacecraft is
scheduled to lift off from Cape Canaveral Air Force Station in Florida, shuttling Curiosity,
an SUV-sized rover with a hefty scientific payload, to the red planetâs surface.
John Grotzinger, a member of the National Academy of Sciences and professor
of geology at the California Institute of Technology, helps oversee the mission.
He became involved in the quest after studying how changes in the Earthâs environment
helped influence animal diversity in some parts of our planet. Here, Grotzinger
discusses the MSL with PNAS
A case for biotic morphogenesis of coniform stromatolites
Mathematical models have recently been used to cast doubt on the biotic
origin of stromatolites. Here by contrast we propose a biotic model for
stromatolite morphogenesis which considers the relationship between upward
growth of a phototropic or phototactic biofilm () and mineral accretion
normal to the surface (). These processes are sufficient to account
for the growth and form of many ancient stromatolities. Domical stromatolites
form when is less than or comparable to . Coniform structures with
thickened apical zones, typical of Conophyton, form when . More
angular coniform structures, similar to the stromatolites claimed as the oldest
macroscopic evidence of life, form when .Comment: 10 pages, 3 figures, to appear in Physica
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