Search for: All records

We investigate the origin and fate of lithogenic sediments using magnetic mineral assemblages in Barilari Bay, west Antarctic Peninsula (AP) from sediment cores recovered during the Larsen Ice Shelf System, Antarctica (LARISSA) NBP10‐01 cruise. To quantify and reconstruct Holocene changes in covarying magnetic mineral assemblages, we adopt an unsupervised mathematical unmixing strategy and apply it to measurements of magnetic susceptibility as a function of increasing temperature. Comparisons of the unmixed end‐members with magnetic observations of northwestern AP bedrock and the spatial distribution of magnetic mineral assemblages within the fjord, allow us to identify source regions, including signatures for “inner bay,” “outer bay,” and “northwestern AP” sources. We find strong evidence that supports the establishment of a late Holocene ice shelf in the fjord coeval with the Little Ice Age. Additionally, we present new evidence for late Holocene sensitivity to conditions akin to positive mean Southern Annual Mode states for western AP glaciers at their advanced Neoglacial positions.

 

We document the presence, composition, and number density (TND) of titanomagnetite nanolites and ultra‐nanolites in aphyric rhyolitic pumice, obsidian, and vesicular obsidian from the 1060 CE Glass Mountain volcanic eruption of Medicine Lake Volcano, California, using magnetic methods. Curie temperatures indicate compositions of Fe2.40Ti0.60O4 to Fe3O4. Rock‐magnetic parameters sensitive to domain state, which is dependent on grain volume, indicate a range of particle sizes spanning superparamagnetic (<50–80 nm) to multidomain (>10 μm) particles. Cylindrical cores drilled from the centers of individual pumice clasts display anisotropy of magnetic susceptibility with prolate fabrics, with the highest degree of anisotropy coinciding with the highest vesicularity. Fabrics within a pumice clast require particle alignment within a fluid, and are interpreted to result from the upward transport of magma driven by vesiculation, ensuing bubble growth, and shearing in the conduit. Titanomagnetite number density (TND) is calculated from titanomagnetite volume fraction, which is determined from ferromagnetic susceptibility. TND estimates for monospecific assemblages of 1,000 nm–10 nm cubes predict 10^12 to 10^20 m^−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 10^18 to 10^19  m^−3. These ranges agree well with TND determinations of 10^18 to 10^20  m^−3 made by McCartney et al. (2024), and are several orders of magnitude larger than the number density of bubbles in these materials. These observations are consistent with the hypothesis that titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation. 

Nucleation of H2O vapor bubbles in magma requires surpassing a chemical supersaturation threshold via decompression. The threshold is minimized in the presence of a nucleation substrate (heterogeneous nucleation, <50 MPa), and maximized when no nucleation substrate is present (homogeneous nucleation, >100 MPa). The existence of explosively erupted aphyric rhyolite magma staged from shallow (<100 MPa) depths represents an apparent paradox that hints at the presence of a cryptic nucleation substrate. In a pair of studies focusing on Glass Mountain eruptive units from Medicine Lake, California, we characterize titanomagnetite nanolites and ultrananolites in pumice, obsidian, and vesicular obsidian (Brachfeld et al., 2024,https://doi.org/10.1029/2023GC011336), calculate titanomagnetite crystal number densities, and compare titanomagnetite abundance with the physical properties of pumice to evaluate hypotheses on the timing of titanomagnetite crystallization. Titanomagnetite crystals with grain sizes of approximately 3–33 nm are identified in pumice samples from the thermal unblocking of low‐temperature thermoremanent magnetization. The titanomagnetite number densities for pumice are 10^18 to 10^20 m^−3, comparable to number densities in pumice and obsidian obtained from room temperature methods (Brachfeld et al., 2024,https://doi.org/10.1029/2023GC011336'>https://doi.org/10.1029/2023GC011336). This range exceeds reported bubble number densities (BND) within the pumice from the same eruptive units (average BND ∼4 × 10^14 m^−3). The similar abundances of nm‐scale titanomagnetite crystals in the effusive and explosive products of the same eruption, together with the lack of correlation between pumice permeability and titanomagnetite content, are consistent with titanomagnetite formation having preceded the bubble formation. Results suggest sub‐micron titanomagnetite crystals are responsible for heterogeneous bubble nucleation in this nominally aphyric rhyolite magma. 

Two basalts with compositions relevant to the crusts of Mars and Earth were synthesized at igneous temperatures and held at 650°C for 21 to 257 days under quartz‐fayalite‐magnetitefO₂buffer conditions. The run products are germane to slowly cooled igneous intrusions, which might be a significant volumetric fraction of the Martian crust and carriers of magnetic anomalies in the Southern Highlands. Both basalts acquired intense thermoremanent magnetizations and intense but easily demagnetized anhysteretic remanent magnetizations carried by homogeneous multidomain titanomagnetite. Hypothetical intrusions on Mars composed of these materials would be capable of acquiring intense remanences sufficient to generate the observed anomalies. However, the remanence would be easily demagnetized by impact events after the cessation of the Mars geodynamo. Coercivity enhancement by pressure or formation of single domain regions via exsolution within the multidomain grains is necessary for long‐term retention of a remanence carried exclusively by multidomain titanomagnetite grains.

 

Glacial-marine sediments from the Antarctic continental margin provide a record of depositional environment, oceanographic variability and ice dynamics that is tapped with scientific ocean drilling. This study focuses on Ocean Drilling Program Core 693A-2R, a 9.7 m sediment core retrieved from near the continental margin of the Archean Grunehogna Craton in Dronning Maud Land (DML), East Antarctica. The results contribute to a better understanding of ice-shelf behavior in DML during the mid-Pleistocene transition (MPT), a well-known transition from 40-kyr to 100-kyr cycle periods. The age model, constructed based on Sr isotope stratigraphy and geomagnetic reversals, indicates that the core spans 1.20 to 0.65 Ma. The dynamic behavior of DML ice shelves with periodic iceberg calving is revealed by the glacial–interglacial variation in sedimentation patterns, with interglacials characterized by higher concentrations of ice-rafted debris (IRD) associated with enhanced paleo- productivity than glacial intervals. The responses of DML ice shelves to warm climates are represented by a prolonged interglacial period at 1.0–1.1 Ma (MIS 31–27) and significant interglacial expressions during MIS 19 and 17. The 40Ar/39Ar ages of individual ice-rafted hornblende grains are compared with the on-land geology of DML and neighboring regions to determine the provenances of IRD. Specifically, 40Ar/39Ar results record pri- marily late Neoproterozoic to Cambrian ages (600–400 Ma) with a predominant peak of 520–480 Ma. This Pan- African/Ross orogeny signature is very common in East Antarctica but is not found in the most proximal margin of the Grunehogna Craton, and is instead associated with the region of DML several hundred kilometers east of the deposition site. This indicates that significant discharges of icebergs occurred in the remote DML, which were then transported by the westward-flowing Antarctic Coastal Current to deposit IRD at the studied site during the MPT. This study establishes a confirmed MPT sedimentary sequence off DML, against which future MPT proxy records from the Weddell Sea embayment and other sectors in Antarctica can be compared and correlated, and provides a basis for more detailed analyses of the response of DML ice sheet to Pleistocene climate variations. 

We evaluate the relationship between the intensity of remanent magnetization andfO₂in natural and synthetic Mars meteorites. The olivine‐phyric shergottite meteorite Yamato 980459 (Y‐980459) and a sulfur‐free synthetic analog (Y‐98*) of identical major element composition were analyzed to explore the rock magnetic and remanence properties of a basalt crystallized from a primitive melt, and to explore the role of magmatic and alteration environmentfO₂on Mars crustal anomalies. The reducing conditions under which Y‐980459 is estimated to have formed (QFM‐2.5; Shearer et al. 2006) were replicated during the synthesis of Y‐98*. Y‐980459 contains pyrrhotite and chromite. Chromite is the only magnetic phase in Y‐98*. The remanence‐carrying capacity of Y‐980459 is comparable to other shergottites that formed in thefO₂range of QFM‐3 to QFM‐1. The remanence‐carrying capacity of these lowfO₂basalts is 1–2 orders of magnitude too weak to account for the intense crustal anomalies observed in Mars's southern cratered highlands. Moderately oxidizing conditions of >QFM‐1, which are more commonly observed in nakhlites and Noachian breccias, are key to generating either a primary igneous assemblage or secondary alteration assemblage capable of acquiring an intense remanent magnetization, regardless of the basalt character or thermal history. This suggests that if igneous rocks are responsible for the intensely magnetized crust, these oxidizing conditions must have existed in the magmatic plumbing systems of early Mars or must have existed in the crust during secondary processes that led to acquisition of a chemical remanent magnetization.