Lipid biomarker analyses of sedimentary organic matter from a marine Triassic-Jurassic (T-J) section at Queen Charlotte
Islands, British Columbia reveal significant bacterial activity and microbial community changes that coincide with faunal
extinctions across the T-J boundary. Bacterial activity is indicated by the 25-norhopane biodegradation index
(25-norhopanes / 25-norhopanes+regular hopanes). Microbial community changes is revealed by variations in relative
abundance of 2-methylhopane which is mainly generated from cyanobacteria. The 2-methylhopane index (2-methyl
hopane/ C30 hopane + C29 25-norhopane) increases above the radiolarian based T-J boundary, and coincides with changes
in the 25-norhopane index. The data reveal a complex microbial event involving both autotrophic and heteorotrophic
bacteria responding to variations in allochthonous organic matter and nutrient supply.
The earliest evidence for amino acids on Earth is in Precambrian sedimentary rocks with varied metamorphic histories.
Igneous rocks rarely contain such compounds, exceptions being those introduced via the migration of fluids into
fractures subsequent to crystallization. Martian meteorites are excellent examples of ancient igneous rocks that
apparently contain amino acids associated with minerals precipitated in rock fractures. The challenge has been to
determine whether the organic compounds present in ancient terrestrial and extraterrestrial materials are indigenous and,
if so, are representative of past life or pre-biotic synthesis. A summary of what is known to date about amino acids in
ancient terrestrial and extraterrestrial materials is presented. Alternative approaches for distinguishing their origin(s) are
A common class of organic compound in low petrographic type meteorites is the sulfur-containing thiophenes. The presence of this compound class in organic-rich meteorites which have experienced substantial levels of aqueous alteration is relatively unexplored. Early reports of these compounds attributed them to artefacts brought about by reactions between elemental sulfur and organic matter during high temperature extraction and analysis steps. Subsequent investigations confirmed their indigeneity, yet their environment of formation remained unconstrained. Here we present data which suggests that thiophenes are parent body alteration products that reflect the role of liquid water on asteroids in the early solar system.
Recent missions to Mars raise the possibility of surface sedimentary sequences that may contain the organic remains of past or present Martian biota. Irrespective of the mechanism of any biological processes on Mars, it seems reasonable to presume that they will involve the transfer and reaction of carbon-bearing molecules. In this case, following the example of terrestrial life forms such as plants and bacteria, it is almost certain that these processes will be accompanied by changes in 12C/13C ratios (which are themselves the result of kinetic isotope effects imparted during the embedded chemical/physical processes). Thus, just as carbon in biological organic matter on Earth is enriched in the lighter carbon isotope relative to mantle (juvenile) carbon, the logical consequence of Martian life is a stable carbon isotopic gradient from the top of the mantle to the surface sedimentary rocks. Stepped combustion-isotope ratio mass spectrometry is a proven technique for measuring the isotopic composition of ambient carbon trapped in crystals during magma solidification. Data from SNC meteorites extracted from different depths on Mars are not inconsistent with a biologically-produced carbon isotope gradient in the Martian crust and provide directions for future research and exploration.
Surface and atmospheric conditions make it unlikely that life as we know it presently exists elsewhere in our solar system. However, this does not preclude the possibility of ancient, extraterrestrial life, which could have originated from Earth or have been introduced to Earth. Since the oldest known sediments on Earth contain evidence for life, it is not possible to determine what the Earth's chemical composition was like prior to life's origin. This evidence can only be sought from meteorites and planetary materials that were formed during the early stages of solar system formation approximately 4.5 to 4.0 GΑa. Criteria are presented that can be used to determine if amino acids in these ancient materials are evidence of ancient life or if they were formed by non-biological processes.
Desert varnish is a black, manganese-rich rock coating that is widespread on Earth. The mechanism underlying its formation, however, has remained unresolved. We present here new data and an associated model for how desert varnish forms, which substantively challenges previously accepted models. We tested both inorganic processes (e.g. clays and oxides cementing coatings) and microbial methods of formation. Techniques used in this preliminary study include SEM-EDAX with backscatter, HRTEM of focused ion beam prepared (FIB) wafers and several other methods including XRPD, Raman spectroscopy, XPS and Tof-SIMS. The only hypothesis capable of explaining a high water content, the presence of organic compounds, an amorphous silica phase (opal-A) and lesser quantities of clays than previously reported, is a mechanism involving the mobilization and redistribution of silica. The discovery of silica in desert varnish suggests labile organics are preserved by interaction with condensing silicic acid. Organisms are not needed for desert varnish formation but Bacteria, Archaea, Eukarya, and other organic compounds are passively incorporated and preserved as organominerals. The rock coatings thus provide useful records of past environments on Earth and possibly other planets. Additionally this model also helps to explain the origin of key varnish and rock glaze features, including their hardness, the nature of the "glue" that binds heterogeneous components together, its layered botryoidal morphology, and its slow rate of formation.
Desert varnish and silica rock coatings have perplexed investigators since Humboldt and Darwin. They are found in arid regions and deserts on Earth but the mechanism of their formation remains challenging (see Perry et al. this volume). One method of researching this is to investigate natural coatings, but another way is to attempt to produce coatings in vitro. Sugars, amino acids, and silicic acid, as well as other organic and (bio)organic compounds add to the complexity of naturally forming rock coatings. In the lab we reduced the complexity of the natural components and produced hard, silica coatings on basaltic chips obtained from the Mojave Desert. Sodium silicate solution was poured over the rocks and continuously exposed to heat and/or UV light. Upon evaporation the solutions were replenished. Experiments were performed at various pH's. The micro-deposits formed were analyzed using optical, SEM-EDAX, and electron microprobe. The coatings formed are similar in hardness and composition to silica glazes found on basalts in Hawaii as well as natural desert varnish found in US southwest deserts. Thermodynamic mechanisms are presented showing the theoretical mechanisms for overcoming energy barriers that allow amorphous silica to condense into hard coatings. This is the first time synthetic silica glazes that resemble natural coatings in hardness and chemical composition have been successfully reproduced in the laboratory, and helps to support an inorganic mechanism of formation of desert varnish as well as manganese-deficient silica glazes.
Silica, amino acids, and DNA were recently discovered in desert varnish. In this work we experimentally test the proposed role of silicic acid and bio-chemicals in the formation of desert varnish and other rock coatings. We have developed a protocol in which the rocks were treated with a mixture of silicic acid, sugars, amino acids, metals and clays, under the influence of heat and UV light. This protocol reflects the proposed mechanism of the polymerization of silicic acid with the biooganic materials, and the laboratory model for the natural conditions under which the desert varnish is formed. Our experiments produced coatings with a hardness and morphology that resemble the natural ones. These results provide a support for the role of silicic acid in the formation of rock coatings. Since the hard silica-based coatings preserve organic compounds in them, they may serve as a biosignature for life, here or possibly on Mars.
Desert varnish coatings are found on rock surfaces throughout arid regions of the world. Rock varnishes may exist on Mars, as suggested by some observations on both Viking and Mars Pathfinder landing sites. There has long been a debate as to whether varnish coatings are microbially mediated or deposited by inorganic processes. Dozens of bacteria have been cultured from the surface of varnish coatings and recently the molecular ecology of varnish coatings have been characterized using 16S rRNA techniques. Colonies of micro colonial fungus are associated with varnish coatings but it is unclear whether bacteria or fungus are directly involved in varnish formation. Another alternative is the incorporation of microbial components into varnish coatings either by complexation with metals or in association with clays. For instance polysaccharides found in bacterial cell walls contain linear polymers of sugars that may be preserved in arid conditions when complexed with usual varnish components such as calcium, aluminum, silicon, iron and manganese. Understanding the organic components of desert varnish may help to resolve the question of the mechanism of formation of rock coatings, biomineralization processes and bacterial fossils and how to detect past microbially activity on planets.
The search for life on Mars is an important goal of NASA and other space agencies. It is not known if chemical evolution on Mars produced the same or similar types of life as on Earth. If not, what would non-Earth biosignatures look like? If life has left its footprint on Mars, what chemical signatures can we recognize, and how can we prevent missing novel life signatures? Alternatively, chemical evolution on Mars may have produced complex chemical systems, which, however, did not lead to life. How can such systems be identified? We use as a model a complex inorganic-organic-biotic system on Earth, commonly called desert or rock varnish, which has been known to Darwin, and which is now also indicated on Mars. We describe unique complex chemical markers that are preserved in rock varnish on Earth. An intricate interaction between minerals, metals, and organic compounds is responsible for their preservation. We suggest some important types of organic compounds to look for in the Martian varnish, should it exist.
The Triassic-Jurassic (TJ) mass extinction (~200 mya) event is one of the most severe in geologic history. It is also one of the most poorly understood. Few geologic sections containing the TJ boundary interval have been identified globally, and most of those are poorly preserved; the paucity of suitable stratigraphic sections has prevented corroborative geochemical studies of this interval. Recently, fullerene molecules (C60 to C200) have been shown to be present in the mass extinction boundary intervals of the Permian-Triassic (PT) event (~251.4 mya), as well as the well-known “dinosaur” extinction event of the Cretaceous-Tertiary (KT) (~65 mya). The presence of fullerenes in both these extinction intervals has been used to invoke an extraterrestrial impact cause for the extinctions. Preliminary results of laser desorption mass spectrometry (LDMS) of selected samples from the Kennecott Point TJ boundary section, Queen Charlotte Islands, British Columbia, suggest that fullerenes (C60 to ~C200) are present in the section, stratigraphically above the extinction interval (as defined by paleontological and isotopic data), but not actually within the interval itself. The presence of fullerenes may not be diagnostic of an impact event.