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

   https://doi.org/10.3389/feart.2021.634482  

   Jaqueto, Plinio; Trindade, Ricardo I.; Feinberg, Joshua M.; Carmo, Janine; Novello, Valdir F.; Stríkis, Nicolás M.; Cruz, Francisco W.; Shimizu, Marília H.; Karmann, Ivo (April 2021, Frontiers in Earth Science)

   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. 

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2. Stalagmite paleomagnetic record of a quiet mid-to-late Holocene field activity in central South America

   https://doi.org/10.1038/s41467-022-28972-8  

   Jaqueto, Plinio; Trindade, Ricardo I.; Terra-Nova, Filipe; Feinberg, Joshua M.; Novello, Valdir F.; Stríkis, Nicolás M.; Schroedl, Peter; Azevedo, Vitor; Strauss, Beck E.; Cruz, Francisco W.; et al (December 2022, Nature Communications)

   Abstract Speleothems can provide high-quality continuous records of the direction and relative paleointensity of the geomagnetic field, combining high precision dating (with U-Th method) and rapid lock-in of their detrital magnetic particles during calcite precipitation. Paleomagnetic results for a mid-to-late Holocene stalagmite from Dona Benedita Cave in central Brazil encompass ~1900 years (3410 BP to 5310 BP, constrained by 12 U-Th ages) of paleomagnetic record from 58 samples (resolution of ~33 years). This dataset reveals angular variations of less than 0.06° yr −1 and a relatively steady paleointensity record (after calibration with geomagnetic field model) contrasting with the fast variations observed in younger speleothems from the same region under influence of the South Atlantic Anomaly. These results point to a quiescent period of the geomagnetic field during the mid-to-late Holocene in the area now comprised by the South Atlantic Anomaly, suggesting an intermittent or an absent behavior at the multi-millennial timescale. 

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3. Uranium-Thorium ages and stable oxygen isotope data on speleothems that grew during Marine Isotope Stages (MIS) 11, 9, and 5e, Northwest Territories, Canada, 2024

   https://doi.org/10.18739/A2SJ19S7M  

   Batchelor, Cameron; McGee, David; Shakun, Jeremy; Woodhead, Jon; Jost, Adam; Arnold, Sarah; Horne, Greg; Kinsley, Christopher; Freudenburg-Puricelli, Markey (January 2024, NSF Arctic Data Center)

   High-resolution records from past interglacial climates help constrain future responses to global warming, yet are rare. This dataset contains seasonally-resolved climate records from subarctic-Canada using micron-scale measurements of oxygen isotopes (δ18O) in speleothems with apparent annual growth bands from three interglacial periods – Marine Isotope Stages 11 (409-376 ka), 9 (336-305 ka) and MIS 5e (123-118 ka). Our study highlights the potential for high-latitude speleothems to yield detailed isotopic records of Northern Hemisphere interglacial climates beyond the reach of Greenland ice cores and offers a framework for interpreting them. Table S1 contains the Uranium-Thorium dates for six speleothems, or more specifically, flowstones, from a cave in Northwest Territories (NWT), Canada. It also contains constructed age models for each sample. Then, we applied a two-tiered methodological approach to reconstruct past subarctic climate. First, we produce an ultra-high-resolution δ18O record that, although not continuous, spans thousands of years for portions of these interglacials. This record was created using Secondary Ion Mass Spectrometry (SIMS) to measure δ18O approximately every 35-micrometer (µm) down each sample’s growth axis. This data is shown in Table S2. Second, we used Confocal Laser Fluorescence Microscopy (CLFM) to identify several fluorescent annual bands in each speleothem, which we then targeted for additional SIMS measurements. This data is shown in Table S3. Though these subarctic speleothems are small in size (most are less than 10 centimeter (cm) in length), the application of both CLFM and SIMS on these samples demonstrate their potential for providing ultra-high-resolution records of high-latitude Northern Hemisphere terrestrial climate outside of Greenland and provide insights into interpretive frameworks for future cold-region speleothem δ18O records. 

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4. From Bulk Rock Magnetism to Individual Inclusions: A Detailed Magnetic Characterization of Refractory Inclusions in Chondrites for Paleomagnetic Analysis

   Bristol, Katie E; Borlina, Caue S; Feinberg, Joshua M (December 2025, American Geophysical Union Fall Meeting Abstracts)

   Magnetic fields in the protoplanetary disk likely play a central role in mass and angular momentum transport, protosolar accretion, and planetary system evolution. Because direct observations are limited, paleomagnetic records from meteorites and their constituents can provide key constraints on this early magnetic environment. Refractory inclusions, including calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), are the oldest solar system solids and are strong candidates for preserving such records. Prior measurements of CAIs in the CO chondrite DOM 08006 demonstrated that they can retain high-fidelity magnetization. To build on this, we performed comprehensive rock magnetic characterization to assess whether inclusions in other chondrite groups are also capable of preserving reliable magnetic records. We analyzed CO (ALHA 77307), CR (GRA 95229), and L (QUE 97008) chondrites, performing fusion crust baked contact tests, ARM paleointensity experiments, fidelity tests, hysteresis analyses, FORCs, and low-temperature magnetic measurements. All samples carry a low-coercivity (10-20 mT) overprint and a distinct higher coercivity component. FORC diagrams reveal tri-lobed patterns characteristic of isolated vortex-state grains. Coercivity unmixing suggests two to three components, consistent with magnetite, kamacite, and minor sulfide or tetrataenite phases. In all three samples, magnetite carries the majority (71-86%) of the signal. ARM paleointensity experiments show that the high-coercivity components were imparted by very low fields, implying weak or absent magnetic fields during parent-body aqueous alteration. These results indicate that the higher coercivity components preserve primary remanence, but the samples contain multiple carriers, representing a mixture of ideal and non-ideal recorders. To isolate the paleomagnetic record from primary kamacite, we are now conducting microscopy and elemental analyses to locate and classify refractory inclusions by type, mineralogy, and size while documenting any textural evidence of melting, alteration, or replacement. These data will guide the selection of inclusions most likely to retain high-fidelity primary magnetization for future high-resolution paleomagnetic study. 

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5. A new MIS 5 to MIS 2 speleothem record from Sandkraal Cave on the South African Cape south coast

   https://doi.org/10.1017/qua.2024.3  

   Braun, Kerstin; Bar-Matthews, Miryam; Ayalon, Avner; Matthews, Alan; Zilberman, Tami; Zolotova, Natalya; Cowling, Richard M.; Karkanas, Panagiotis; Cawthra, Hayley C.; Fisher, Erich C.; et al (March 2024, Quaternary Research)

   Abstract We present new stable oxygen and carbon isotope composite records (δ¹⁸O, δ¹³C) of speleothems from Sandkraal Cave 1 (SK1) on the South African south coast for the time interval between 104 and 18 ka (with a hiatus between 48 and 41 ka). Statistical comparisons using kernel-based correlation analyses and semblance analyses based on continuous wavelet transforms inform the relationships of the new speleothem records to other proxies and their changes through time. Between 105 and ~70 ka, changes of speleothem δ¹⁸O values at SK1 are likely related to rainfall seasonality. Variations of δ¹³C values are associated with changes of vegetation density, prior carbonate precipitation (PCP), CO₂degassing in the cave, and possibly variations of the abundance of C₃and C₄grasses in the vegetation. The relationships of δ¹⁸O with other proxies shift between ~70 and 48 ka (Marine Isotope Stages 4–3) so that both stable isotope records now reflect CO₂degassing, evaporation, and PCP. Similar relationships also continue after the hiatus for the deposition phase between 42 and 18 ka. Our findings support modeling results suggesting drier conditions in the study area when the Southern Hemisphere westerlies are shifted north and the paleo–Agulhas Plain is exposed. 

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