Demonstrating the biogenicity of presumptive microfossils in the geological record often requires supporting chemical signatures, including isotopic signatures. Understanding the mechanisms that promote the preservation of microbial biosignatures associated with microfossils is fundamental to unravelling the palaeomicrobiological history of the material. Organomineralization of microorganisms is likely to represent the first stages of microbial fossilisation and has been hypothesised to prevent the autolytic degradation of microbial cell envelope structures. In the present study, two distinct fossilisation textures(permineralised microfossils and iron oxide encrusted cell envelopes)identified throughout iron-rich rock samples were analysed using nanoscale secondary ion mass spectrometry(NanoSIMS). In this system, aluminium is enriched around the permineralised microfossils, while iron is enriched within the intracellularly, within distinct cell envelopes. Remarkably,while cell wall structures are indicated, carbon and nitrogen biosignatures are not preserved with permineralised microfossils. Therefore, the enrichment of aluminium, delineating these microfossils appears to have been critical to their structural preservation in this iron-rich environment. In contrast,NanoSIMS analysis of mineral encrusted cell envelopes reveals that preserved carbon and nitrogen biosignatures are associated with the cell envelope structures of these microfossils. Interestingly, iron is depleted in regions where carbon and nitrogen are preserved. In contrast aluminium appears to be slightly enriched in regions associated with remnant cell envelope structures. The correlation of aluminium with carbon and nitrogen biosignatures suggests the complexation of aluminium with preserved cell envelope structures before or immediately after cell death may have inactivated autolytic activity preventing the rapid breakdown of these organic, macromolecular structures.Combined, these results highlight that aluminium may play an important role in the preservation of microorganisms within the rock record. 相似文献
With sea levels projected to rise as a result of climate change, it is imperative to understand not only long-term average trends, but also the spatial and temporal patterns of extreme sea level. In this study, we use a comprehensive set of 30 tide gauges spanning 1954–2014 to characterize the spatial and temporal variations of extreme sea level around the low-lying and densely populated margins of the South China Sea. We also explore the long-term evolution of extreme sea level by applying a dynamic linear model for the generalized extreme value distribution (DLM-GEV), which can be used for assessing the changes in extreme sea levels with time. Our results show that the sea-level maxima distributions range from ~?90 to 400 cm and occur seasonally across the South China Sea. In general, the sea-level maxima at northern tide gauges are approximately 25–30% higher than those in the south and are highest in summer as tropical cyclone-induced surges dominate the northern signal. In contrast, the smaller signal in the south is dominated by monsoonal winds in the winter. The trends of extreme high percentiles of sea-level values are broadly consistent with the changes in mean sea level. The DLM-GEV model characterizes the interannual variability of extreme sea level, and hence, the 50-year return levels at most tide gauges. We find small but statistically significant correlations between extreme sea level and both the Pacific Decadal Oscillation and El Niño/Southern Oscillation. Our study provides new insight into the dynamic relationships between extreme sea level, mean sea level and the tidal cycle in the South China Sea, which can contribute to preparing for coastal risks at multi-decadal timescales.
Olivine offers huge, largely untapped, potential for improving our understanding of magmatic and metasomatic processes. In particular, a wealth of information is contained in rare earth element (REE) mass fractions, which are well studied in other minerals. However, REE data for olivine are scarce, reflecting the difficulty associated with determining mass fractions in the low ng g?1 range and with controlling the effects of LREE contamination. We report an analytical procedure for measuring REEs in olivine using laser ablation quadrupole‐ICP‐MS that achieved limits of determination (LOD) at sub‐ng g?1 levels and biases of ~ 5–10%. Empirical partition coefficients (D values) calculated using the new olivine compositions agree with experimental values, indicating that the measured REEs are structurally bound in the olivine crystal lattice, rather than residing in micro‐inclusions. We conducted an initial survey of REE contents of olivine from mantle, metamorphic, magmatic and meteorite samples. REE mass fractions vary from 0.1 to double‐digit ng g?1 levels. Heavy REEs vary from low mass fractions in meteoritic samples, through variably enriched peridotitic olivine to high mass fractions in magmatic olivines, with fayalitic olivines showing the highest levels. The variable enrichment in HREEs demonstrates that olivine REE patterns have petrological utility. 相似文献
The extraction of P-T histories from metamorphic rocks provides a valuable dataset for the elucidation of the tectonic mechanisms for orogeny.
While continued re-equilibration frequently obliterates early information, garnet zonation and inclusion assemblages can often
surmount this problem. The task is more difficult in high variance assemblages or if inclusions are not preserved, but one
approach is to use pseudosections that are specific to the bulk composition of a given rock. In the latter case, the compositions
and abundances of all the minerals are fixed at a given P-T point such that, if the effective bulk composition is known, the garnet composition alone can be used to reconstruct the
history. Here, we explore this approach using examples from the Zanskar Himalaya, NW India. Pseudosections have been calculated
for four pelitic to semipelitic rocks from the Zanskar Himalaya and have been contoured for garnet composition. The calculations
adequately model the mineral assemblages in the rocks and predict the presence of chlorite in the early assemblage where chlorite
is found as inclusions within garnet. Moreover, the pseudosections successfully model the garnet core compositions, with all
three independent compositional contours overlapping at a single pressure and temperature. This occurs at ∼550 °C and at pressures
varying from 3–7 kbar for the four rocks studied. We have been less successful, however, at modelling garnet compositions
beyond the cores because fractionation of the effective bulk composition is caused by garnet growth itself. However, in this
case, a combination of the␣pseudosection and conventional thermobarometry using␣Fe-Ti inclusions and matrix phases allows
us to reconstruct␣the entire P-T history. The resulting P-T paths record burial of 3–5 kbar without significant temperature increase followed by isobaric heating of 50–100 °C. This
evolution is consistent with Himalayan collision in the early Tertiary but a combination of the P-T data presented here and published geochronological data suggests renewed thrusting south of the suture zone in the Oligocene.
In addition, the data demonstrate that no extra heat source is required to cause melting of the Himalayan crust in the Miocene.
While melting could have occurred both by dehydration during decompression or in the presence of a fluid, the lack of garnet
resorption does suggest decompression was rapid and followed quickly by cooling. This scenario favours melting by decompression.
Received: 17 July 1997 / Accepted: 6 April 1998 相似文献
In the shallow water of Orbetello lagoon, macroalgae harvesting boats produce sediment disturbance. To evaluate the effect of this, during 2001-2002, a trial study was carried out in the lagoon in order to: verify seasonal and diurnal trends in nutrients and estimate the quantity of resuspended organic sediment. An unbalanced and balanced ANOVA (one and two way) analysis was applied. The disturbance did not produce strong and lasting eutrophication effects. In the seasonal survey, dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) showed significant decreases in disturbed areas at the end of the trial, while the control area showed a constant, significant increase. Dissolved organic nitrogen (DON) and phosphorus (DOP) significantly increased everywhere. In disturbed areas, sediment redox (Eh((NHE))) increased and porosity values decreased, contrary to the control area. Total organic carbon (TOC) remained unchanged in disturbed areas, but increased in the control area, where the C:P ration increased. The fall-out of sedimentary material resuspended by boat action for as far as 50 m from the boat route, was 189 g(dw)m(-2) made up mainly of organic matter. This disturbance could be the cause of change in vegetation in the lagoon. 相似文献