Constraints on the ice composition of carbonaceous chondrites from their magnetic mineralogy

Carbonaceous chondrites experienced varying degrees of aqueous alteration on their parent asteroids, which influenced their mineralogies, textures, and bulk chemical and isotopic compositions. Although this alteration was a crucial event in the history of these meteorites, their various alteration pathways are not well understood. One phase that formed during this alteration was magnetite, and its morphology and abundance vary between and within chondrite groups, providing a means of investigating chondrite aqueous alteration. We measured bulk magnetic properties and first-order reversal curve (FORC) diagrams of CM, CI, CO, and ungrouped C2 chondrites to identify the morphology and size range of magnetite present in these meteorites. We identify two predominant pathways of aqueous alteration among these meteorites that can be distinguished by the resultant morphology of magnetite. In WIS 91600, Tagish Lake, and CI chondrites, magnetite forms predominantly from Fe-sulfides as framboids and stacked plaquettes. In CM and CO chondrites, <0.1 μm single-domain (SD) magnetite and 0.1–5 μm vortex (V) state magnetite formed predominantly via the direct replacement of metal and Fe-sulfides. After ruling out differences in temperature, water:rock ratios, terrestrial weathering effects, and starting mineralogy, we hypothesise that the primary factor controlling the pathway of aqueous alteration was the composition of the ice accreted into each chondrite group’s parent body. Nebula condensation sequences predict that the most feasible method of appreciably evolving ice concentrations was the condensation of ammonia, which will have formed a more alkaline hydrous fluid upon melting, leading to fundamentally different conditions that may have caused the formation of different magnetite morphologies. As such, we suggest that WIS 91600, Tagish Lake, and the CI chondrites accreted past the ammonia ice line, supporting a more distal or younger accretion of their parent asteroids.

Magnetic Mineralogy of Speleothems From Tropical-Subtropical Sites of South America

Fe-bearing minerals are a tiny fraction of the composition of speleothems. They have their origin in the karst system or are transported from the drainage basin into the cave. Recent studies on the magnetism of speleothems focused on the variations of their magnetic mineralogy in specific time intervals and are usually limited to a single sample. In this study, we describe a database of environmental magnetism parameters built from 22 stalagmites from different caves located in Brazil (South America) at different latitudes, comprising different climates and biomes. The magnetic signal observed in these stalagmites is dominated by low-coercivity minerals (∼20 mT) whose magnetic properties resemble those of the magnetite formed in pedogenic environments. Also, a comparison with few samples from soils and the carbonate from cave’s walls shows a good agreement of the magnetic properties of speleothems with those of soil samples, reinforcing previous suggestions that in (sub-)tropical regimes, the dominant magnetic phase in speleothems is associated with the soil above the cave. Spearman’s rank correlation points to a positive strong correlation between magnetic concentration parameters (mass-normalized magnetic susceptibility, natural remanent magnetization, anhysteretic remanent magnetization, and isothermal remanent magnetization). This implies that ultrafine ferrimagnetic minerals are the dominant phase in these (sub-)tropical karst systems, which extend across a diverse range of biomes. Although the samples are concentrated in the savannah biome (Cerrado) (∼70%), comparison with other biomes shows a higher concentration of magnetic minerals in speleothem underlying savannahs and lower concentration in those underlying moist broadleaf forests (Atlantic and Amazon biome) and dry forests (Caatinga). Thus, rainfall, biome, and epikarst dynamics play an important role in the concentration of magnetic minerals in speleothems in (sub-)tropical sites and indicate they can be an important target for paleoenvironmental research in cave systems.

PM2.5 Magnetic Properties in Relation to Urban Combustion Sources in Southern West Africa

The physico-chemical characteristics of particulate matter (PM) in African cities remain poorly known due to scarcity of observation networks. Magnetic parameters of PM are robust proxies for the emissions of Fe-bearing particles. This study reports the first magnetic investigation of PM2.5 (PM with aerodynamic size below 2.5 μm) in Africa performed on weekly PM2.5 filters collected in Abidjan (Ivory Coast) and Cotonou (Benin) between 2015 and 2017. The magnetic mineralogy is dominated by magnetite-like low coercivity minerals. Mass normalized SIRM are 1.65 × 10−2 A m2 kg−1 and 2.28 × 10−2 A m2 kg−1 for Abidjan and Cotonou respectively. Hard coercivity material (S-ratio = 0.96 and MDF = 33 mT) is observed during the dry dusty season. Wood burning emits less iron oxides by PM2.5 mass when compared to traffic sources. PM2.5 magnetic granulometry has a narrow range regardless of the site or season. The excellent correlation between the site-averaged element carbon concentrations and SIRM suggests that PM2.5 magnetic parameters are linked to primary particulate emission from combustion sources.

Magnetic properties of PM in indoor/outdoor domestic environments

&lt;p&gt;The finer fraction of the particulate matter (PM) is the most harmful health wise, as it has more capacity to reach deeper parts of the respiratory system. Among other constituents, PM also contains iron oxides, allowing for the use of magnetic methods in its investigation as proxies for the whole of PM. Those methods present advantages in comparison to traditional ones, being quick, cost effective and sensible to investigate iron oxides among PM.&amp;#160;&lt;/p&gt;&lt;p&gt;To better understand the risks related to PM exposition in the domestic context, the assessment of magnetic parameters may be used in outdoor and indoor environments, giving us information on the concentration of iron oxides (and consequently, PM) and its dispersion from one environment to the other.&amp;#160;&lt;/p&gt;&lt;p&gt;We developed a citizen sciences experiment in the city of Toulouse, France. Tree barks were used as bio-collectors. Garlands composed of tree bark pieces were distributed to the population in May-2019, and placed in both indoors and outdoors of flats and homes to capture PM. They were retrieved after one year. Measurement of magnetic susceptibility, ARM, SIRM, S -ratio and estimation of superparamagnetic concentration were performed. A total of 86 bio-collectors kits were successfully analyzed. The preliminary results indicate a higher concentration of iron oxides outdoors, with a mean difference between outdoor and indoor measurements of 6.58x10&lt;sup&gt;-9&lt;/sup&gt;m&lt;sup&gt;3&lt;/sup&gt;/kg and 1.38x10&lt;sup&gt;-5&lt;/sup&gt;Am&lt;sup&gt;2&lt;/sup&gt;/kg in susceptibility and SIRM respectively. The concentration of the SP fraction also follows this trend of higher outdoor values. The magnetic mineralogy is mostly dominated by low coercivity magnetite-like carriers.&lt;/p&gt;&lt;div&gt; &lt;div&gt; &lt;div&gt;&amp;#160;&lt;/div&gt; &lt;/div&gt; &lt;div&gt; &lt;div&gt;&amp;#160;&lt;/div&gt; &lt;/div&gt; &lt;/div&gt;

The Paraná magmatism (Brazil) as the villain regarding the remagnetization of the Paleozoic sediments in the basin – half-truth!

&lt;p&gt;The sedimentation in the Paran&amp;#225; Basin started during the Ordovician and ended with the Early Cretaceous magmatic event. Thick lava pilescovered the entire basin, and voluminous dikes and sills occur in the sedimentary sequences, mainly in the northeastern border of the basin. Despite the thermal effect, some Paleozoic sedimentary formations preserved their primary magnetization. The paleomagnetic results from the glacial Aquidauana Formation, an equivalent to the Itarar&amp;#233; group in the north-western portion of the basin, indicated that the magnetization is compatible with the Middle-Late Permian age assigned in literature. A detailed investigation of the magnetic mineralogy and the magnetization of other Paleozoic sedimentary rocks led to the conclusion that the intrusive rocks were more effective than the lavas in disturbing the primary magnetization, especially in the low-clay content rocks. The secondary magnetizations identified in the different areas of the basin are not always compatible with the Early Cretaceous magnetization imprinted by the Paran&amp;#225; magmatism. This component prevails in the northeastern area, while a Permo-Triassic magnetization was identified in other areas. The results obtained so far are coherent with the geomagnetic reversal scale for the considered time interval, and the paleomagnetic poles agree with the APWP for South America.&lt;/p&gt;

Magnetic Susceptibility Properties of Loess From the Willendorf Archaeological Site: Implications for the Syn/Post-Depositional Interpretation of Magnetic Fabric

At the Willendorf site Upper Paleolithic archeological layers associated to early Aurignacian cultures were found. The environmental conditions of the associated society, potentially co-existing with Neanderthal groups, is still not fully understood. Here, we report on environmental magnetic analyses including anisotropy of the magnetic susceptibility (AMS) carried out on loessic aeolian sediments at the Willendorf site. Data on lineation, foliation and the degree of anisotropy were used to assess depositional and post-depositional magnetic fabric properties and to deduce site-specific environmental processes. Overall, the loess is of aeolian origin and shows magnetic enhancement and magnetic fabric properties similar to those of other European loess geoarchives, but the magnetic mineralogy differs from many ‘dry’ loess sites, insofar as it shows a higher susceptibility during heating. We infer an enhanced neoformation of magnetite during heating due to the presence of organic matter. While at face value the AMS properties are indicative of pure aeolian loess consistent with previous studies, imbrication suggests post-depositional slope movement toward the Danube, which obscures inferences on palaeo-wind direction. It is well possible that these post-depositional magnetic fabric alterations occur at other localities with similar geomorphological settings.

High-coercivity magnetic minerals in archaeological baked clay and bricks

SUMMARY The thorough understanding of magnetic mineralogy is a prerequisite of any successful palaeomagnetic or archaeomagnetic study. Magnetic minerals in archaeological ceramics and baked clay may be inherited from the parent material or, more frequently, formed during the firing process. The resulting magnetic mineralogy may be complex, including ferrimagnetic phases not commonly encountered in rocks. Towards this end, we carried out a detailed rock magnetic study on a representative collection of archaeological ceramics (baked clay from combustion structures and bricks) from Bulgaria and Russia. Experiments included measurement of isothermal remanence acquisition and demagnetization as a function of temperature between 20 and &gt;600 °C. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 and 300 K have been carried out. All studied samples contain a magnetically soft mineral identified as maghemite probably substituted by Ti, Mn and/or Al. Stoichiometric magnetite has never been observed, as evidenced by the absence of the Verwey phase transition. In addition, one or two magnetically hard mineral phases have been detected, differing sharply in their respective unblocking temperatures. One of these unblocking between 540 and 620 °C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140 and 240 °C, and its magnetic properties correspond to an enigmatic high coercivity, stable, low-unblocking temperature (HCSLT) phase reported earlier. In a few samples, high- and low unblocking temperature, magnetically hard phases appear to coexist; in the others, the HCSLT phase is the only magnetically hard mineral present.

Misinterpreting proxy data for paleoclimate signals: A reply to Srivastava and Jovane, 2020

Srivastava and Jovane (2020) have made several comments on our assessment of proxy data and challenged the outcome of Shukla et al. (2020) based mainly on interpretation of environmental magnetic parameters. We respond to their criticisms and re-evaluate our paper, remove ambiguities and validate our conclusions through additional proxies (grain-size and geochemistry). We welcome their comments and do not entirely rule out their interpretation for magnetic mineralogy. We highlight the importance of proxy validation for high-energy environments like Chorabari lake. However, single proxy data correlation is likely to produce biased results with no relevant meaning. The objective of our study was to understand complexities in the glacial-climate system by reconstructing late-Holocene climate variations using the glacial lake sediment records from the Mandakini River Basin, Central Himalaya, India. We presented the complexities in Shukla et al. (2020), and this was also highlighted by Srivastava and Jovane (2020). In response, we provide additional justification of proxy response and substantiate our results with present-day estimates from the Chorabari glacier valley. We disagree with the thesis put forward by Srivastava and Jovane (2020) in their conclusion as they overemphasize the interpretation of a single proxy. We maintain that the investigation of present-day glacial settings is an important precursor of paleoclimatic data interpretation and that this supports our conclusions. We will try to incorporate the important suggestions of Srivastava and Jovne (2020) relating to the interpretation of magnetic data in future work.