Organo-mineral interactions stabilize soil organic matter (SOM) by protecting from microbial enzymatic attack. Soil water content affects aggregation, mineral weathering, and microbial respiration, thus influencing the relative importance of SOM stabilization mechanisms. While the response of microbial respiration to momentary changes in water content is well established, it is unclear how microbial activity will impact stabilization mechanisms under different long-term moisture contents.
To understand how long-term soil moisture affects SOM stabilization mechanisms we studied fallow soils from upstate New York situated on a naturally occurring water content gradient. Wetter (but not saturated) soils contained more exchangeable Ca and had more strongly stabilized SOM, resulting in SOM accumulation. But it was not clear whether Ca-driven surface interactions or occlusion in micro-aggregates was more important, and if interactions with Fe and Al played a role in the Ca-poor soils. Also, the role of biotic drivers in SOM stabilization at different water contents was unknown.
We tested which mechanisms governed SOM stabilization by determining C and N contents and natural isotope abundances in particulate and mineral-associated organic matter fractions. We also extracted the C bound to Ca and to reactive Fe+Al phases. Wetter, Ca-rich soils had higher oPOM content, and in the heavy mineral fraction, higher relative concentrations of Ca-bound C, lower C:N values, and more oxidized C forms. In addition, wetter soils had greater microbial biomass. Together, these results showed that high long-term soil moisture increased microbial SOM cycling, and that processed SOM was better stabilized, in agreement with the recent notion that stable SOM consists of processed labile C. Additionally, higher soil moisture augmented the role of Ca in SOM stabilization over that of Al+Fe phases. We then manipulated the exchangeable Ca content and incubated soils with 13C15N labeled plant litter. Ca-amended soils emitted less CO2 while incubated with litter, confirming that Ca is instrumental in SOM stabilization. Tracing the labeled isotopes in the gaseous phase and soil fractions will allow us to gain a clearer understanding of how water content and soil Ca interact to stabilize SOM.
AuScope is an Australian consortium of Earth Science institutes cooperating to develop national research infrastructure. AuScope received federal funding in 2019 to establish the AuScope Geochemistry Laboratory Network (AGN), with the objective of coordinating FAIR-based open data initiatives, support user access to laboratory facilities, and strengthen analytical capability on a national scale.
Activities underway include an assessment of best practices for researchers to register samples using the International Geo Sample Number (IGSN) system in combination with prescribed minima for meta-data collection. Initial activities will focus on testing meta-data schema on high value datasets such as geochronology (SHRIMP U-Pb, Curtin University), geochemistry (Hf-isotopes, Macquarie University) and low-temperature thermochronology analyses (fission track/U-He, University of Melbourne). Collectively, these datasets will lead to a geochemical data repository in the form of an Isotopic Atlas eResearch Platform that is available to the public via the AuScope Discovery Portal. Over time, the repository will aggregate a large volume of publicly funded geochemical data, providing a key resource in quantitatively understanding the evolution of Earth system processes that have shaped the Australian continent and its resources.
The "black chimney" type of hydrothermal vents in the modern deep sea have become a popular research topic in many disciplines. Due to the actual conditions, the research on palaeo-thermal vents in geological history is relatively low. Fortunately, the discovery of hydrothermal vents and bio-fossils from the Chang 7 source rocks of the Yanchang Formation of the Triassic in the Ordos Basin, China, provides the best evidence for deciphering hydrothermal activity during geological history. Here, we report a case study. Through ordinary sheet observation, scanning electron microscopy and electron probe observation, layered grained siliceous rocks, dolomites, and hydrothermal mineral combinations, such as pyrite + dolomite + gypsum and calcite + barite, are found. Their unique petrological characteristics, mineral composition, and structure confirm the existence of palaeo-thermal fluid vents. We further analysed the geochemical characteristics and in situ isotope characteristics. The study found that Cs, U, Th, Pb, Ba and other trace elements of the sample showed positive abnormalities, in which values of U/Th were high; in addition, the enrichment of major elements such as Sr, Mn, and the in situ sulphur isotopes of pyrite reached 7.89%-10.88%. This study of hydrothermal vents over geological history is expected to provide new insights on the life forms of various extreme microorganisms in hydrothermal environments and on their formation of high-quality source rocks.
Microorganisms can modify the composition of their lipid membrane in response to variations in environmental parameters. This is the case for bacterial lipids such as glycerol dialkyl tetraethers (GDGT) and 3-hydroxy fatty acids (3-OH FAs), both used for temperature and pH reconstructions in terrestrial paleoenvironmental studies. However, a major concern with these proxies is that their structure may be influenced by other environmental parameters than temperature or pH. The present study aimed at identifying and quantifying the influence of environmental parameters such as soil moisture, vegetation types and soil types on bacterial GDGTs and 3-OH FAs. These lipids were analyzed in 49 soil samples collected between 200 m and 3,000 m altitude in the French Alps. The soils cover a wide range of temperature (0 °C to 15 °C) and pH (3 to 8) and are representative of the diversity of soils and vegetation encountered along the investigated altitudinal transects. Using this new well-documented and unique dataset, the GDGT-pH correlation was confirmed, but the one between 3-OH FAs and pH was lower than in previous studies. For the temperature, correlations were lower than in previous studies for the GDGTs and absent for the 3-OH FAs. These observations could be explained thanks to different statistical analyses. Redundancy analysis (RDA) showed that pH is the main driver of the variability of 3-OH FAs and GDGTs, explaining 20.5 % and 56 % of the distribution of these bacterial lipids, respectively, followed by the altitude (8 % influence on the distribution of 3-OH FAs, and 11 % on GDGTs) and granulometry (5 % impact on 3-OH FAs and 7.5 % on GDGTs). Taken together, these results highlight the major influence of the vegetation cover and soil types on the distribution of bacterial lipids. Indeed, we quantified and explained for the first time the impact of the different environmental factors (temperature, vegetation, soil type…) on the distribution of bacterial lipids. This novel comprehension of the impacts of environmental parameters will allow to refine the use of proxies based on these compounds. These results pave the way for new types of applications of GDGTs and 3-OH FAs as environmental proxies in paleosoils, peat or lacustrine sediments.
Plinian eruptions are the most hazardous yet enigmatic style of volcanism at basaltic systems. The low viscosity of basaltic magma should preclude its fragmentation; however, there are several recognised examples of basaltic Plinian activity. Historical eruptions of Masaya caldera, Nicaragua; Etna, Italy (122 BC); and Tarawera, New Zealand (1886) have ejected > 1 km3 of material. The Las Sierras-Masaya volcanic complex (Masaya caldera) has produced several basaltic Plinian eruptions, yet currently exhibits low explosive-effusive activity. This volcano has erupted chemically homogeneous magmas over at least the past 6000 years, which suggests that this significant difference in eruptive style is not attributable to a compositional change. Therefore, the cause of increased explosivity at Masaya caldera remains uncertain.
We present new measurements of major, trace and volatile elements in basaltic Plinian eruption products from the Fontana Lapilli (60 ka) and Masaya Triple Layer (2.1 ka) eruptions of the Las Sierras- Masaya volcanic complex. We use our data in rheological and thermometric models to define the pre- and syn-eruptive conditions that favour highly explosive activity. We then combine our petrological data with a numerical conduit model to constrain the pre-eruptive condition of the magma reservoir and simulate the conduit processes, to understand the magmatic conditions that promote fragmentation during magma ascent. The common physico-chemical magmatic conditions that promote basaltic Plinian activity at Masaya are high microlite crystallinity, moderate storage temperatures and a low initial H2O concentration. Our combined approach greatly improves our general understanding of explosive basaltic activity and provides new insight into the effusive-explosive transition of the highly hazardous Las Sierras-Masaya system.
The collision of large bolides with planets with substantial atmospheres, such as Earth and early Mars, results in significant climatic and surface effects. For very large impacts, forming basins >~500 km in diameter, these post-impact effects would be global and include [1]: (1) transient high atmospheric and surface temperatures, (2) deposition of material that was vaporized by the impact event and subsequently condensed (e.g. terrestrial spherule layers), (3) a transient, vigorous hydrologic cycle characterized by rainfall rates sufficient to produce flooding, and (4) surface aqueous alteration, made possible by the hot rainfall and high temperatures. On Mars, the formation of such large basins, including Hellas, Isidis, and Argyre, occurred in the early- to mid-Noachian [2]; while younger, smaller basins would have influenced the climate and surface on a local or regional scale, such intense, global effects would have occurred only during the earliest parts of Mars history. Previous work has qualitatively [1] and quantitatively [in 3D; 3,4] constrained the effects from large basin-scale impacts on Mars, but lacks detailed application to any specific impact.
The fact that these drastic, global effects would occur following each large basin-scale impact [1,3,4] implies that the effects from formation of the youngest of the large basins would be best preserved and closest to the present-day surface. Here, we build upon previous work [1,3,4] by qualitatively and quantitatively exploring the climatic and surface effects from the formation of the youngest large basin, Argyre. We find that: (1) a tens of meters thick, near-globally-distributed, olivine and glass-rich spherule layer should be preserved on or very near the surface, (2) the induced hydrologic cycle would have been characterized by rainfall rates akin to Earth rainforests and would have lasted for decades to centuries, (3) the intense rainfall would have caused flooding, significant erosion, and smoothing of landforms, and (4) hot rainfall and high temperatures would have caused surface aqueous alteration, including partial alteration of the olivine-rich layer to carbonates as well as alteration of basaltic material to Fe/Mg-smectites and Al-phyllosilicates, which would present in a leaching profile.
Implications of these findings include: (1) distinguishing the role of impact-induced aqueous alteration from that associated with normal climate conditions, (2) predictions of areas where the spherule layer and alteration products may be observed, (3) the transition from a basin-scale impact-dominated regime to a basin-free regime in martian climate evolution, and (4) guidelines for exploration and recognition of these impact-related units at rover and sample return scale.
References
[1] Palumbo, Head (2017), Impact cratering as a cause of climate change, surface alteration, and resurfacing during the early history of Mars, MAPS, 5
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