Shock experiments in support of the Lithopanspermia theory: The influence of host rock composition,temperature, and shock pressure on the survival rate of endolithic and epilithic microorganisms

Abstract– Shock recovery experiments were performed with an explosive set‐up in which three types of microorganisms embedded in various types of host rocks were exposed to strong shock waves with pressure pulse lengths of lower than 0.5 μs: spores of the bacterium Bacillus subtilis, Xanthoria elegans lichens, and cells of the cyanobacterium Chroococcidiopsis sp. 029. In these experiments, three fundamental parameters were systematically varied (1) shock pressures ranging from 5 to 50 GPa, (2) preshock ambient temperature of 293, 233 and 193 K, and (3) the type of host rock, including nonporous igneous rocks (gabbro and dunite as analogs for the Martian shergottites and chassignites, respectively), porous sandstone, rock salt (halite), and a clay‐rich mineral mixture as porous analogs for dry and water‐saturated Martian regolith. The results show that the three parameters have a strong influence on the survival rates of the microorganisms. The most favorable conditions for the impact ejection from Mars for microorganisms would be (1) low porosity host rocks, (2) pressures <10–20 GPa, and (3) low ambient temperature of target rocks during impact. All tested microorganisms were capable of surviving to a certain extent impact ejection in different geological materials under distinct conditions.     

Concurrent tectonic and climatic changes recorded in upper Tortonian sediments from the Eastern Mediterranean

The upper Tortonian Metochia marls on the island of Gavdos provide an ideal geological archive to trace variations in Aegean sediment supply as well as changes in the North African monsoon system. A fuzzy-cluster analysis on the multiproxy geochemical and rock magnetic dataset of the astronomically tuned sedimentary succession shows a dramatic shift in the dominance of 'Aegean tectonic' clusters to 'North African climate' clusters. The tectonic signature, traced by the starvation of the Cretan sediment, now enables to date the late Tortonian basin foundering on Crete, related to the tectonic break-up of the Aegean landmass, at c.  8.2 Ma. The synchronous decrease in the North African climate proxies is interpreted to indicate a change in the depositional conditions of the sink rather than a climatic change in the African source. This illustrates that interpretations of climate proxies require a multiproxy approach which also assesses possible contributions of regional tectonism.     

Stabilisation of soil organic matter by interactions with minerals as revealed by mineral dissolution and oxidative degradation

Soil organic matter is known to contain a stable fraction with an old radiocarbon age. Size and stabilisation processes leading to the formation of this old soil carbon pool are still unclear. Our study aims to differentiate old organic matter from young and labile carbon compounds in two acid forest soils (dystric cambisol, haplic podzol). To identify such fractions soil samples were exposed to oxidation with Na₂S₂O₈ and to dissolution by hydrofluoric acid (HF). A negative correlation between ¹⁴C activity and carbon release after dissolution of the mineral matrix by HF indicates a strong association of stabilised carbon compounds with the mineral phase. A negative correlation between the ¹⁴C activity and the relative proportion of carbon resistant to oxidation by Na₂S₂O₈ shows that young carbon is removed preferentially by this treatment. The fraction remaining after oxidation represents a certain stabilised, long residence time carbon pool. This old fraction comprises between 1 and 30% of the total soil organic carbon in the surface horizons, but reaches up to 80% in the sub-surface horizons. Old OC is mainly stabilised by organo-mineral associations with clay minerals and/or iron oxides, whereas intercalation in clay minerals was not found to be important.     

Geochemistry of Peruvian near-surface sediments

Sixteen short sediment cores were recovered from the upper edge (UEO), within (WO) and below (BO) the oxygen minimum zone (OMZ) off Peru during cruise 147 of R/V Sonne. Solids were analyzed for major/trace elements, total organic carbon, total inorganic carbon, total sulfur, the stable sulfur isotope composition (δ³⁴S) of pyrite, and sulfate reduction rates (SRR). Pore waters were analyzed for dissolved sulfate/sulfide and δ³⁴S of sulfate. In all cores highest SRR were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and/or diffusion of sulfate from bottom water. δ³⁴S of dissolved sulfate showed only minor downcore increases. Strong ³²S enrichments in sedimentary pyrite (to −48‰ vs. V-CDT) are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction. Manganese and Co are significantly depleted in Peruvian upwelling sediments most likely due to mobilization from particles settling through the OMZ, whereas release of both elements from reducing sediments only seems to occur in near-coastal sites. Cadmium, Mo and Re are exceptionally enriched in WO sediments (<600 m water depth). High Re and moderate Cd and Mo enrichments are seen in BO sediments (>600 m water depth). Re/Mo ratios indicate anoxic and suboxic conditions for WO and BO sediments, respectively. Cadmium and Mo downcore profiles suggest considerable contribution to UEO/WO sediments by a biodetrital phase, whereas Re presumably accumulates via diffusion across the sediment-water interface to precipitation depth. Uranium is distinctly enriched in WO sediments (due to sulfidic conditions) and in some BO sediments (due to phosphorites). Silver transfer to suboxic BO sediments is likely governed by diatomaceous matter input, whereas in anoxic WO sediments Ag is presumably trapped due to sulfide precipitation. Cadmium, Cu, Zn, Ni, Cr, Ag, and T1 predominantly accumulate via biogenic pre-concentration in plankton remains. Rhenium, Sb, As, V, U and Mo are enriched in accordance with seawater TE availability. Lead and Bi enrichment in UEO surface sediments is likely contributed by anthropogenic activity (mining). Accumulation rates of TOC, Cd, Mo, U, and V from Peruvian and Namibian sediments exceed those from the Oman Margin and Gulf of California due to enhanced preservation off Peru and Namibia.     

Challenges posed by and approaches to the study of seasonal-to-decadal climate variability

The tasks of providing multi-decadal climate projections and seasonal plus sub-seasonal climate predictions are of significant societal interest and pose major scientific challenges. An outline is presented of the challenges posed by, and the approaches adopted to, tracing the possible evolution of the climate system on these various time-scales. First an overview is provided of the nature of the climate system’s natural internal variations and the uncertainty arising from the complexity and non-linearity of the system. Thereafter consideration is given sequentially to the range of extant approaches adopted to study and derive multi-decadal climate projections, seasonal predictions, and significant sub-seasonal weather phenomena. For each of these three time-scales novel results are presented that indicate the nature (and limitations) of the models used to forecast the evolution, and illustrate the techniques adopted to reduce or cope with the forecast uncertainty. In particular, the contributions (i) appear to exemplify that in simple climate models uncertainties in radiative forcing outweigh uncertainties associated with ocean models, (ii) examine forecast skills for a state-of-the-art seasonal prediction system, and (iii) suggest that long-lived weather phenomena can help shape intra-seasonal climate variability. Finally, it is argued, that co-consideration of all these scales can enhance our understanding of the challenges associated with uncertainties in climate prediction.     

Sensitivity of shear zones in orogenic wedges to surface processes and strain softening

We investigate the internal deformation of orogenic wedges growing by frontal accretion with a two-dimensional numerical model. Our models are limited to crustal deformation and assume a horizontal detachment as observed for various natural orogens (e.g. Alaska and Costa Rica). The model wedges develop as a result of convergence of a brittle sediment layer in front of a strong backstop. We find that our reference model develops in-sequence forward-thrusts which propagate upward from the basal detachment. For this reference model we investigate the sensitivity of shear zone activity to surface processes and strain softening. Model results show that diffusive or slope dependent erosion enhances material transport across the wedge and slows down forward propagation of the deformation front. Frictional strain softening focuses deformation into narrow shear zones and enhances displacement along them. This has also been postulated for natural thrusts such as the Glarus thrust in the Swiss Alps and the Moine thrust in the Scottish Caledonides. A second series of models investigates the effects of regularly spaced weak inclusions within the sediment layer which simulate remnants of previous deformation phases. These inclusions facilitate and focus internal deformation, influence the thrust dip and thrust vergence and enable thrust reactivation in the internal part of the wedge. Our results show that inactive thrusts in the internal part of the wedges may be reactivated in models with diffusive surface processes, strain softening or weak inclusions. Thrust reactivation occurs as models seek to maintain their critical taper angle. First order characteristics of our numerical models agree well with natural orogenic wedges and results from other numerical and analogue models.     

Tetraether membrane lipid distributions in water-column particulate matter and sediments: a study of 47 European lakes along a north–south transect

We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in water-column particulate matter and the top 5 cm of sediment from 47 lakes along a transect from southern Italy to the northern part of Scandinavia. Our objective was to investigate the biological sources and potential palaeoenvironmental applications of GDGTs in lacustrine sediments. Both archaea-derived isoprenoid and bacteria-derived branched GDGTs, produced by yet unknown soil bacteria, were identified in all lake sediments. GDGT distributions varied substantially. Crenarchaeotal GDGTs, including the characteristic GDGT crenarchaeol, were found in varying relative concentrations, and were more dominant in lakes from the Alps and some of the lakes from the more southern part of the latitudinal transect. In some lakes, we observed high amounts of the GDGT with no cyclopentane moieties relative to crenarchaeol. As methanogenic Euryarchaeota are known to biosynthesise this GDGT predominantly, these Archaea, rather than Crenarchaeota, may be its dominant biological source. In most of the lakes, high amounts of soil-bacteria-derived, branched GDGTs (>40% of total GDGTs) indicated a substantial contribution from soil erosion. Branched GDGTs dominated, especially in the northern lakes, possibly related to high soil-erosion rates. In many of the lakes, soil input affects the distribution of isoprenoidal GDGTs and prevents the reliable application of the TEX₈₆ temperature proxy for lake water temperature, which is based on in situ crenarchaeotal GDGTs production. In 9 out of the 47 lakes studied, the TEX₈₆ temperature proxy could be used reliably. When we compared the TEX₈₆ correlation with annual and winter lake-surface temperature, respectively, the relationship between TEX₈₆ and winter temperature was slightly stronger. This may indicate the season in which these GDGT-producing organisms have their peak production.     

Ce anomalies in Gough Island lavas — Trace element characteristics of a recycled sediment component

High precision trace element data obtained by inductively coupled plasma mass spectrometry and Sr–Nd isotope analyses are presented for mafic volcanic rocks from Gough Island, South Atlantic. The new data reveal negative Ce anomalies, with Ce/Ce^? values in Gough lavas extending down to ～ 0.92. Ce is only fractionated from other rare earth elements (REE) due to formation of Ce⁴⁺ under oxidizing conditions of near-surface environments while other REE remain trivalent. Ce anomalies in convergent margin magmas have been shown to indicate a contribution of a subducted sediment component. In contrast, Ce anomalies in intra-plate basalts have been attributed to weathering processes, but can be excluded here based on element–element systematics indicating magmatic trends rather than weathering-induced element mobility. Shallow-level contamination by local marine sediments with negative Ce anomaly inherited from seawater can be excluded because Gough lavas with increasingly negative Ce anomalies do not trend towards low Ce/Pb ratios characterizing such sediments. Rather, it is argued that the negative Ce anomalies in Gough Island lavas are consistent with variable amounts of a sediment component in the mantle plume source. Mixtures between estimates of subducting sediment columns with negative Ce anomaly and mantle capable of giving rise to Gough Island magmas without Ce anomalies reproduce the Gough compositional array with the exception of highly fluid-mobile elements. The calculated trace element composition of the deeply recycled sediment in the Gough plume source is depleted in fluid-mobile elements such as Ba and Pb relative to the composition of some present-day subducting sediments. This loss is attributed to the dehydration or flushing of sediment in the subduction factory, consistent with constraints from arc magmas.