B33B-0253 1340h
Molecular evidence for high dispersal distribution of planktonic foraminifera between Pacific and Atlantic Oceans
Planktonic foraminifera, marine zooplankton with calcareous tests, are widely dispersed in all oceans. Because the diversity of these foraminifera is controlled by the environmental components (sea temperature, salinity, etc.), distinct species assemblages occur in several provinces from tropical to subpolar area dependent on circulation patterns of the ocean surface water. Most of the Recent species originated in the Miocene to Pliocene. . However, the worldOs ocean has been separated by the physiographic changes during the Neogene, yet the species recognized morphologically occur worldwide. In particular, the warm surface waters of the Atlantic and Pacific have been isolated from one another for at least 3.5 my and maybe longer. In this study, we present a molecular phylogeny inferred from 18S partial small subunit ribosomal DNA sequences using the 10 morphospecies obtained from the western Pacific Ocean. The phylogenetic analyses indicate a polyphyletic origin and two major clades; one is composed of the Candenidae and Globorotaliidae with certain benthic foraminifera and another by the Globigerinidae. The sequences were compared with sequences from the same species from the Atlantic Ocean, eastern Pacific Ocean and Mediterranean Sea shown; the genetic distance was estimated about 67 OTU. The partial 18S SSUrDNA of the 10 morphological taxa have almost same sequences, (less than 1% difference). No regionally separated populations are apparent within the same taxa, in particular, the Globorotaliidae and {\it Globigerinoides sacculifer}. However, 5 taxa have two or three cryptic species, respectively. These cryptic species cannot be identified by the morphological differences except for {\it Orbulina universa}. Thus, planktonic foraminifera have high dispersal distribution and have maintained their sequences in their huge population. On the other hand, the cryptic species of 5 species diverged after the Pliocene. As they belong to a group of shallow dwelling plankton, they likely adapted to the high variability of the oceanic condition after the Neogene in the course of their genetic changes.
B33B-0254 1340h
A Molecular Approach to the Study of Green Algal Evolution and Early Terrestrial Ecosystems
The biological nature of pre-land plant terrestrial ecosystems remains an enigmatic chapter of the history of life on earth due to lack of fossil evidence. Molecular phylogenies have shown that Charophycean green algae are the closest relatives of the bryophytes, which have been hypothesized to be the earliest divergent land plants. However, there is no fossil evidence to support this relationship nor is there a reliable fossil record of the earliest land plants. Microfossils representing the earliest land plants appear to have a bryophytes affinity based on limited morphological comparisons but this remains controversial. We are applying a biomolecular approach to study both green algal evolution and its relation to bryophytes using the resistant biopolymer algaenan and phytosterols as biological markers. Algaenan has been shown to have high preservation potential and may be the primary component of enigmatic microfossils assumed to be of algal origin. Algaenan and the green algal sterols, stigmasterol and sitosterol, may also be the precursors of n-alkanes and the hydrocarbon stigmastane that are major components of many Neoproterozoic bitumens. The biological nature and phylogenetic distribution of algaenan is still not well understood. Here we explore the presence and structure of algaenans in terrestrial green algae and bryophytes in relation to their phylogenetic distributions.
B33B-0255 1340h
The Urbilateria: Insights From Comparative Developmental Biology Into The Cambrian Radiation
A critical issue in understanding the patterns of morphological and developmental evolution during the Cambrian radiation of animal body plans is the morphological complexity of the last common ancestor of the two great bilaterian clades, the protostomes and deuterostomes. Often known as the Urbilateria or the protostome-deuterostome ancestor (PDA) this node in metazoan phylogeny distinguishes the sponges and cnidarians from the bilaterian clades. Highly conserved developmental genes and complexes suggests that the PDA was a relatively complex organism, with dorsal-ventral and anterior-posterior patterning, eyes, appendages, heart, central nervous system, segmentation and other relatively complex morphological features. If this view of the PDA is correct, it has important implications for understanding the Cambrian radiation, for such a complex animal is likely to leave traces in the sedimentary record even if it lacked a durable skeleton. However, this interpretation of conserved developmental sequences may overestimate the extent to which homologies of developmental sequences implies morphological conservation. In other words, does the presence of the Nkx 2.5 gene in the PDA necessitate the presence of a heart, or simple the appearance of contractile muscles? If the latter, then the PDA may have been a much less complex organism. This latter view of developmental evolution suggests the PDA possessed a developmental toolkit, but that the specific implementation of these tools in specific body plans occurred after the PDA, within specific clades. Moreover, this view also strongly suggests that changes in the physical environment, or in ecological relationships, was primarily responsible for the evolutionary innovations seen in the Cambrian, rather than developmental innovations.
B33B-0256 1340h
Molecular Fossils as Time Indicators for the Evolution of Diatoms.
Bacillariophyta (diatoms) are one of the most abundant divisions of phytoplankton, and contribute to almost 50% of the primary productivity of today's oceans. However, their ecological dominance is relatively young and little is known about the exact pace of their rapid evolution. DNA analyses on diatoms and the use of molecular clock calculations can help to reconstruct their evolution, but this molecular clock rate needs to be calibrated against the fossil record to determine the mutation rate. Until now, diatom silica skeletons have been used for reconstructing the evolution of diatoms, but their use is limited due to destruction by diagenesis. Molecular fossils may prove to be more useful for time reconstruction. To search for suitable compounds, we have analyzed both the lipid composition and 18S rRNA sequences of ca. 100 marine diatoms. This revealed that some specific phylogenetic clusters within the diatoms produce specific organic compounds, so-called diatom biomarkers. One group of diatom biomarkers are the C$_{25}$ highly branched isoprenoid (HBI) alkenes (1,2). HBI biosynthesis evolved independently at least twice in the diatoms. The first group of HBI producers consists of the centric diatoms of the genus {\it Rhizosolenia}, the second group comprises pennate diatoms of the genera {\it Haslea}, {\it Navicula} and {\it Pleurosigma}. Based on the constructed phylogenetic tree it is likely that the HBI biosynthesis evolved first in the older group of centric diatoms (i.e. the {\it Rhizosolenia} genus). The fossil record was studied to determine the geological occurrence of C$_{25}$ HBI alkenes, and this data set shows that HBI biosynthesis evolved ca. 91.5 My ago, so we can date the evolution of the genus {\it Rizosolenia} to ca. 91.5 My. With this information, we can now accurately predict the mutation rate of the 18S rDNA gene to 1% per 14.8 My for {\it Rhizosolenia}, which is substantially faster than the 1% per 18-26 My reported previously for diatoms in general. Another specific biomarker is 24-norsterol. Its value as an age diagnostic biomarker was already reported (3), but the source of this sterol was still unknown although a diatomaceous source was assumed. We have now found this sterol in the diatom species {\it Thalassiosira aff. Antarctica}. In combination with the knowledge that the 24-norsterol production increased substantially during the Cretaceous this may provide a tool to predict the mutation rate of the Thalassiosirales. Our data show that molecular paleontology can assist in obtaining more reliable estimates of the molecular clock rate and thus be an important tool in reconstructing the evolution of diatoms. References: 1. J.K. Volkman et al., Org.Geochem. 21, 407-413 (1994). 2. J.S. Sinninghe Damste et al., Science 304, 584-587 (2004). 3. A.G. Holba et al., Org.Geochem. 29, 1269-1283 (1998).
B33B-0257 1340h
Microbial Distributions Through Pore Water Geochemical Gradients: How Well do Contemporary Organism Genomes Track Their Environment?
In order to better understand a marine sediment ecosystem, we have measured pore water profiles and microbial community structure through the upper 30 cm of laminated muds collected from 900 m water depth in San Pedro Basin The sediments at this site are hemipelagic, derived from erosion from the Southern California land mass and from biogenic production in the San Pedro Channel water column. These sediments accumulate at a rate of 0.25 mm/year under a water column which has <20 uM oxygen, hence the sediments are not bioturbated. Under these conditions, a biogeochemical zonation should develop with respect to oxidant supply and reactant production. Oxygen is absent within pore waters below 0.5 mm. Nitrate concentration is unusual, peaking at 50-100 uM at depths of 2-5 cm-we suspect that nitrate is injected into these sediments through the microbial community, as diffusion-reaction models cannot explain this nitrate maximum. Although ammonium concentrations increase monotonically with depth, the profile of ammonium 15N values show isotopic enrichment of about 3 to 4 % at 6 -15 cm, relative to the values directly below and above. This enrichment suggests ammonium oxidation occurring at this depth. Hence, within a range of 15 cm of a `simple' hemipelagic sediment column, the biogeochemical variability is extraordinary. Within this context we have used molecular tools to compare the microbial community structure with the biogeochemical structure and identified the abundant members of this community. As molecular tools will be applied to address questions about paleoenvironments and earth history, we stress the importance of understanding complexities within modern sediments.
B33B-0258 1340h
The Macroevolutionary Interplay Between Planktic Larvae and Benthic Predators
Many marine invertebrates have a complex life cycle whereby the egg, rather than developing directly to the juvenile stage, develops instead into a intermediate larval form which may spend weeks to months feeding in the plankton before it becomes competent to undergo metamorphosis into the benthic juvenile. Because the selective advantages provided to the animal by having a planktotrophic larval stage are largely unknown, the reasons behind their origin and subsequent maintenance over geological time are not well understood. Using both a molecular clock and the fossil record, I show that four primitively non-feeding larval forms evolved sometime between the late Ediacaran and Early Cambrian, and feeding larvae appear to have evolved from non-feeding ancestors sometime between the Late Cambrian and Middle Ordovician in at least five, if not eight, of eight different clades analyzed. Thus, the initial exploitation of the predator-free pelagos by larvae was achieved independently multiple times by the end of the Early Cambrian, most likely driven by benthic predation pressures upon developing eggs and embryos. Then, because the evolution of larval planktotrophy from lecithotrophic ancestors correlates with the dramatic rise in the generic number of benthic suspension feeders in the Early Ordovician, it seems likely that benthic suspension feeding selected for fecundity, and thus indirectly for planktotrophy, in multiple lineages of marine invertebrates by the end of the Middle Ordovician.
B33B-0259 1340h
Genes Necessary for Bacterial Magnetite Biomineralization Identified by Transposon Mutagenesis
Magnetic bacteria synthesize nanoscale crystals of magnetite in intracellular, membrane-bounded organelles (magnetosomes). These crystals are preserved in the fossil record at least as far back as the late Neoproterozoic and have been tentatively identified in much older rocks {\it (1)}. This fossil record may provide deep time calibration points for molecular evolution studies once the genes involved in biologically controlled magnetic mineralization (BCMM) are known. Further, a genetic and biochemical understanding of BCMM will give insight into the depositional environment and biogeochemical cycles in which magnetic bacteria play a role. The BCMM process is not well understood, though proteins have been identified from the magnetosome membrane and genetic manipulation and biochemical characterization of these proteins are underway. Most of the proteins currently thought to be involved are encoded within the {\it mam} cluster, a large cluster of genes whose products localize to the magnetosome membrane and are conserved among magnetic bacteria {\it (2)}. In an effort to identify all of the genes necessary for bacterial BCMM, we undertook a transposon mutagenesis of {\it Magnetospirillum magneticum} AMB-1. Non-magnetic mutants (MNMs) were identified by growth in liquid culture followed by a magnetic assay. The insertion site of the transposon was identified two ways. First MNMs were screened with a PCR assay to determine if the transposon had inserted into the {\it mam} cluster. Second, the transposon was rescued from the mutant DNA and cloned for sequencing. The majority insertion sites are located within the {\it mam} cluster. Insertion sites also occur in operons which have not previously been suspected to be involved in magnetite biomineralization. None of the insertion sites have occurred within genes reported from previous transposon mutagenesis studies of AMB-1 {\it (3, 4)}. Two of the non-{\it mam} cluster insertion sites occur in operons containing genes conserved particularly between MS-1 and MC-1. We are undertaking a complementation strategy to demonstrate the necessity of these novel genes in BCMM as well as characterizing the phenotypes of the mutants. 1. S. B. R. Chang, J. F. Stolz, J. L. Kirschvink, S. M. Awramik, Precambrian Res. 43, 305-315 (1989). 2. K. Grünberg, C. Wawer, B. M. Tebo, D. Schüler, Appl. Environ. Microbiol. 67, 4573-4582 (2001). 3. A. T. Wahyudi, H. Takeyama, T. Matsunaga, Appl. Biochem. Biotechnol. 91-3, 147-154 (2001). 4. T. Matsunaga, C. Nakamura, J. G. Burgess, K. Sode, J. Bacteriol. 174, 2748-2753 (1992).
B33B-0260 1340h
Biogeochemical Processes in Late Archean Marine Biosphere Revealed by Isotopic and Molecular Records
The presence of shallow-marine oxygen oases and associated aerobic ecosystems in an otherwise anoxic and anaerobic world has been proposed by researchers to explain the anomalous 40 permil spread in organic-carbon isotope values during the late Archean. To test this hypothesis, we studied isotopic, molecular, and lithologic records of 2.7-2.5 Ga rocks of different depositional facies from the Hamersley Province, Western Australia. Kerogen carbon-isotopic compositions indicate that extreme 13C-depletion (more than -45 permil) was associated with shallow-marine-carbonate environments at 2.72 Ga and with deepwater environments thereafter. Moreover, kerogen-carbon-isotope values associated with carbonate environments became enriched by more than 10 permil over 100-150 Ma. These observations suggest that microbial processes responsible for extreme 13C-depletion became less significant in shallow carbonate environments, but remained important in deeper settings. Molecular biomarker ratios determined for associated bitumens: 1) strongly correlate to kerogen carbon-isotope values and other biomarker ratios, and, 2) show relationships with depositional facies and dolomite abundance giving credence to a syngenetic relationship with host rocks. The biomarker data confirm aerobic methanotrophs in the Late Archean biosphere, but not in strong association with extreme 13C-depletion. Biomarker patterns reflect a greater association of aerobic respiration and oxygenic photosynthesis in shallow carbonate environments compared to deeper settings. Collectively, the data record dramatic changes in carbon cycling associated with environmental partitioning of microbial processes and ecosystems over 100-150 Ma. Most likely, this represents increased bioavailability of strong electron acceptors with the expansion of oxidant-rich oases prior to rise in atmospheric oxygen.
B33B-0261 1340h
Phylogenomic Methods to Guide Paleontological Searches for the Early Cyanobacteria
Phylogenomic methods can help paleontologists target their searches for early microbial microfossils and potentially help them better interpret the early fossil record. In this study, the deep-branching relationships in the cyanobacteria were resolved using whole genome sequences, multiple genes for taxa lacking genomes, and intein presence/absence in the DnaE protein. Once a framework tree was produced, characters were mapped onto the tree. Characters included morphology (unicellular vs. filamentous), habitat (marine vs. freshwater), metabolism (use of sulfide as electron donor, nitrogen fixation), presence/absence of complex morphological traits (akinetes, heterocysts, hormogonia), salt tolerance, and thermal tolerance. It was found that the earliest cyanobacteria were unicellular coccoids, with cell diameters < 2 microns, that lived in freshwater environments. This suggests that paleontologists should focus their searches for the earliest cyanobacteria to freshwater deposits (lakes, streams) and to small diameter coccoids (not mats, not filaments). The earliest "cyanobacterial" microfossils (Eosynechococcus and Eoentophysalis) are large-diameter coccoids found in shallow marine platform carbonates. Because these cells have large diameters, if they were cyanobacteria one would also expect to see their sister taxa in the fossil record (i.e., large-diameter filamentous forms with sheaths, also akinetes). Because these are not found until 2.0 Ga (and akinetes until 1.5 Ga), this suggests that these earliest microfossils are not cyanobacteria. There are several instances in the cyanobacterial tree where ancestors with low salt tolerance gave rise to lineages that grow in brackish, marine, and/or hypersaline environments. This suggests that either the cyanobacteria first originated on continents and later colonized more saline environments, or that the cyanobacteria first originated in shallow "seas" that were not very saline but gradually became more saline by about 2.0 Ga. Because the continents were likely harsh environments (due to lack of an ozone layer and increased chemical and physical weathering), and because there is geochemical evidence of chemical stratification of the early oceans, the latter hypothesis (that the early shallow seas were not very saline) deserves further attention.
B33B-0262 1340h
Targeted genomic discovery of biosynthetic pathways: Anaerobic synthesis of hopanoids by {\it Geobacter sulfurreducens}
The biomarker concept requires that preservable molecules (molecular fossils) carry specific taxonomic and/or metabolic information. Initially, an empirical approach was used to discover which compounds are produced by certain taxa. These observations provided the basis for the interpretation of biomarkers in modern environments and the geologic record. Now, with the rapid sequencing of hundreds of microbial genomes, a more focused genomic approach can be taken to test phylogenetic patterns and hypotheses. To deduce whether specific compounds are indeed taxonomic (and metabolic) markers, candidate organisms can be selected for study on the basis of genes that encode proteins fundamental to the synthesis of certain biomarkers. Hopanoids, a class of pentacyclic triterpenoid lipid biomarkers, provide an illustrative example. For the past twenty years, biomarker studies have worked under the assumption that hopanoids are only produced by aerobic organisms. But the discovery of isotopically-depleted hopanoids in environments of anaerobic methane oxidation suggests that some hopanoids are produced anaerobically. To test these ideas we searched publicly-available genomic databases using squalene-hopene cyclase (a fundamental enzyme responsible for hopanoid biosynthesis) sequences from known hopanoid producers to find a candidate organism potentially capable of anaerobic hopanoid biosynthesis. Here we present evidence from a pure culture that {\it Geobacter sulfurreducens}, a bacterium common in anoxic environments, has the appropriate genes for hopanoid biosynthesis and produces a wide variety of complex hopanoids under strictly anaerobic conditions.
B33B-0263 1340h
Diethers enriched in $^{13}$C suggest carbon-limitation at the Lost City Hydrothermal Field
Active and inactive carbonate vent structures from the Lost City Hydrothermal Field (LCHF) contain up to 0.6% organic carbon including diverse lipids. Values of $\delta$$^{13}$C for total organic carbon (TOC) range from -18.7$\permil$ vs. VPDB at the active, high-temperature vent known as "The Beehive" ($90\deg$C), to -3.1$\permil$ at Marker 7 (active, $70\deg$C). Samples with relatively high levels of $^{13}$C also contained high amounts of isoprenoidal and nonisoprenoidal diethers. Samples more depleted in $^{13}$C lacked or contained low amounts of these diethers. The correlation between high $^{13}$C and abundant diethers is supported by compound-specific isotopic analyses. Archaeal and bacterial diethers are enriched in $^{13}$C relative to photosynthetically derived marine carbon. The biomarkers {\it sn}-2 hydroxyarchaeol, {\it sn}-3 hydroxyarchaeol, and dihydroxyarchaeol - considered diagnostic for methane-cycling archaea - had $\delta$ values ranging from -8.5 to +4.8$\permil$. Phylogenetic data confirms the presence at these vents of a single group of methanogens, related to the Methanosarcinales (Schrenk et al., 2004). Diethers with non-isoprenoidal alkyl chains are also present, are of presumed bacterial origin, and may indicated the presence of sulfate-reducing bacteria. Values of $\delta$ for these compounds range from -7.3 to +1.0$\permil$. At the Beehive vent, diether lipids are absent and the TOC is depleted in $^{13}$C. Coexistence of isotopically similar hydroxyarchaeols and nonisoprenoidal glycerol diethers is typical of marine, cold-seep environments at which concentrations of H$_{2}$ are low and methane is oxidized anaerobically. At the LCHF, however, concentrations of H$_{2}$ in pore waters reach 15 mM (Proskurowski et al., 2003). This H$_{2}$, produced by serpentinization reactions, drives production (rather than oxidation) of methane. Simultaneously, sulfate-reducing bacteria can flourish as carbon-fixing autotrophs. Under such conditions, carbon may be the limiting substrate, its nearly complete consumption accounting for the enrichment of $^{13}$C in biomass. Proskurowski, G., Lilley, M.D., Olsen, E.J., and Larson, B. (2003) Preliminary geochemistry of volatile species from active vents at the Lost City Hydrothermal Field. Eos, Trans. American Geophysical Union 84(46): Fall Meeting Suppl. Abstract \#B12A-0770 Shrenk, M.O., Kelley, D.S., Bolton, S.A., and Baross, J.A. (2004) Low archaeal diversity linked to sub-seafloor geochemical processes at the Lost City Hydrothermal Field, Mid-Atlantic Ridge. Environmental Microbiology 6(10) 1086-95.
B33B-0264 1340h
Biomarker changes across the Toarcian (early Jurassic) ocean anoxic event
The Early Toarcian oceanic anoxic event (in the Jurassic, about 183 million years ago) is marked by the global distribution of black shales rich in organic carbon. It is recorded in the geochemical record with high concentrations of organic carbon and isotopic excursions in carbonate carbon, organic carbon and nitrogen. Although there are many hypotheses regarding this anoxic event, its causes and consequences are still not well understood. Here we investigate the evolution of molecular fossils, or biomarkers, across the Toarcian ocean anoxic event in order to elucidate the dynamics of interactions within the carbon cycle during this time. The biomarkers of thirteen samples which span the bulk organic carbon isotopic excursion are studied in detail. To infer the interaction between the primary and secondary reservoirs of oceanic organic carbon, we have analysed time series of isoprenoid (pristane and phytane) and n-alkane (n-C$_{17}$ and n-C$_{18}$) isotopic compositions. In addition to the isotopic analyses, we trace the evolution of distributions of all hydrocarbon biomarkers. Samples are from the high-resolution and well-studied Hawsker Bottoms section. Preliminary results show that the isotopes of pristane and phytane and the n-alkanes do not trace the negative excursion of bulk $\delta^{13}$C$_{org}$. In addition, there is a change in isotopic ordering between the n-alkanes and the isoprenoids.
B33B-0265 1340h
Testing the Molecular Clock Using the Best Fossil Record: Case Studies from the Planktic Foraminifera
Criticism of molecular clock studies often centres on inadequate calibration and a perceived lack of correlation between reproductive isolation and recognisable morphological evolution. Since many major groups (e.g. birds, mammals, reptiles) have a poor fossil record, it is often difficult to test and refute these limitations. Planktic foraminifera represent an exception to this rule. Deep-sea sediments are super-abundant in foraminifera, and large numbers of specimens and occurrences are easily garnered from Ocean Drilling Programme cores. Planktic foraminifera therefore represent an ideal model group with which to test and refine molecular clock studies. Since the 1990 genetic sequences (principally 18S r-RNA) have been extracted from living planktic foraminifera, and a large genetic library has developed. Our study attempts to contextualise and test molecular data, particularly molecular clock dates, utilising material from two ODP cores (Site 926A (Atlantic) and 806 (Pacific), to examine the evolutionary history of two sibling-species complexes (Globigerinella siphonifera and Globigerinoides ruber, both common shallow-water species and both of considerable palaeoceanographic utility). Recent genetic studies have suggested that these two super-species in fact consist of a number of isolated forms, with contrasting ecologies and longevities, which in Recent and sub-Recent sediments can be distinguished either on the basis of pore ultrastructure (Gl. siphonifera) or test colouration (Gs. ruber). In both cases, molecular clock estimates are indicative of ancient (7-11 Ma) intra-species cryptic divergences, which seem to be considerably older than fossil dates. In particular, the calculated molecular split between the two forms of Gs. ruber (white and pink) of around 11 Ma is considerably discordant with the fossil date of around 0.7 Ma. At first glance, this may appear to be a classic case of molecular over-estimation, often a feature of clock models, especially where, as in the foraminifera, substitution rates may vary widely. However, there is good reason to suspect that fossil range of the derived pink form may have been artificially truncated by diagenetic degradation of the meta-stable test pigmentation. The deep molecular splits for Gl. siphonifera (around 7 Ma for the two main morphologically distinguishable sub-types), whilst not so obviously at odds with the fossil record, still belie the very small amount of morphological evolution observed within the plexus. We have used morphometric methods on a large (over 2000 pooled specimens) dataset in an effort to independently test the molecular clock, using SEM-based measurement of pore metrics (for Gl. siphonifera) and a multivariate analysis of whole-test characteristics (for Gs. ruber). Comparison of results for the two species suggests interesting patterns; whilst the two cryptic sub-types of Gl. siphonifera seemingly can be traced through time and seem to respond to external oceanographic forcing, the sub-types of Gs. ruber appear to be truly cryptic, and cannot be distinguished in the fossil record beyond 0.7 Ma. This raises two important points; firstly, the molecular clock (at least for foraminifera) bears considerable scrutiny, appears to be relatively robust to substitution bias and is seemingly broadly in accordance with morphological data; and secondly, the relationship between form and function in planktic foraminifera appears to be ill-defined, raising important questions for functional morphology.
B33B-0266 1340h
Absolute Dating of Desert Varnish Using Portable X-Ray Fluorescence: Calibration and Testing
Desert varnish, also called rock varnish, is a thin biogenic layer of Mn-oxides, Fe-oxides, and clays that coats rock surfaces in arid and semi-arid regions. The mass of these metals in the varnish registers cumulative biologic activity over time and presents a possible dating mechanism, subject to appropriate assumptions and restrictions. We have used a portable x-ray fluorescence (PXRF) unit to measure Mn and Fe in numerous desert varnishes, both in the field and laboratory; the anticipated relationship between age and mass emerges from these data. Our attempts to refine the PXRF technique for absolute dating of desert varnish are confounded by the limited number of "dated" varnishes available to calibrate and test the method. Although there is no current method to directly ascertain the age of desert varnish, our search for "dated" varnishes has yielded three suitable types of test materials: (1) The ages of young basalt flows dated by various K/Ar radiometric techniques represent the maximum age of varnish developed on those surfaces. Such rocks are useful in the time range of perhaps 250,000 to 10,000 years; surface spalling with loss of varnish presents an upper time limit and difficulty in dating Holocene basalts presents a lower limit. Basalt flows typically provide horizontal surfaces that are ideal for PXRF measurements because, as a biogenic process, varnish development even at a single site varies with solar orientation. (2) Petroglyphs are the rock art that native peoples produced by pecking away varnish to expose fresh rock. This process restarts varnish development and the pecked surface gradually repatinates over time. At some locales, certain figures, symbols, and stylistic elements can be associated with an archaeological culture of known antiquity and duration, thus providing an age range for such glyphs. In the desert Southwest and Great Basin of the United States, appropriate glyphs are known from the present to at least 7000 years BP. Many of these petroglyphs are developed on vertical surfaces, which complicates dating both within and between sites. (3) The ages of the development of a number of geomorphic surfaces have been constrained by stratigraphic correlation and by dating of basalts, Pleistocene lake shorelines, and organic material. Such ages may in turn be associated with the initial exposure of fresh surfaces on cobbles and boulders on these geomorphic surfaces. We have compared the Mn and Fe accumulation in desert varnish from a variety of sites where the age of the fresh surface is constrained by one of the three approaches described above. Results thus far suggest that the current PXRF technique can provide absolute ages useful in situations where a significant uncertainty range is tolerable. Obvious sources of error in calibration and deployment of the PXRF technique that need to be addressed include errors in the dates of materials in the training sets, time lag between exposure of rock surface and varnish development, variations in surface orientation and micro-climate, and climate change over the course of varnish development.
B33B-0267 1340h
Laboratory Studies of Survival Limits of Bacteria During Shock Compression: Application to Impacts on the Early Earth
Shock recovery experiments on suspensions of 10$^{6}$ mm$^{-3}$ {\it E. coli} bacteria contained in water-based medium, within stainless steel containers, are used to simulate the impact environment of bacteria residing in water-filled cracks in rocks. Early Earth life is likely to have existed in such environments. Some 10$^{-2}$ to 10$^{-4}$ of the bacteria population survived initial (800 ns duration) shock pressures in water of 219 and 260 MPa. TEM images of shock recovered bacteria indicate cell wall indentations and rupture, possibly induced by inward invasion of medium into the cell wall. Notably cell wall rupture occurs dynamically at $\sim$0.1 times the static pressures {\it E.coli} have been demonstrated (Sharma et al., 2002) to survive and may be caused by Rayleigh-Taylor instabilities. We infer the invading fluid pressure may exceed the tensile strength of the cell wall. We assume the overpressures are limited to the initial shock pressure in water. Parameters for the Grady & Lipkin (1980) model of tensile failure versus time-scale (strain rate) are fit to present data, assuming that at low strain rates, overpressures exceeding cell Turgor pressure require $\sim$10$^{3}$ sec. This model, if validated by experiments at other timescales, may permit using short loading duration laboratory data to infer response of organisms to lower shock overpressures for the longer times (10$^{0}$ to 10$^{3}$ s) of planetary impacts. An Ahrens & O'Keefe (1987) shock attenuation model is then applied for Earth impactors. This model suggests that Earth impactors of radius 1.5 km induce shocks within water-filled cracks in rock to dynamic pressure such that stresses exceeding the survivability threshold of {\it E. coli} bacteria, to radii of 1.7-2.6$\times$10$^{2}$ km. In contrast, a giant (1500 km radius) impactor produces a non survival zone for {\it E. coli} that encompasses the entire Earth.
http://www.gps.caltech.edu/~mjwillis/research/bugbuster.html