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1. The Amplitude and Timescales of 0–15 ka Paleomagnetic Secular Variation in the Northern North Atlantic

   https://doi.org/10.1029/2023JB026891  

   Reilly, Brendan T.; Stoner, Joseph S.; Ólafsdóttir, Sædís; Jennings, Anne; Hatfield, Robert; Kristjánsdóttir, Gréta Björk; Geirsdóttir, Áslaug (June 2023, Journal of Geophysical Research: Solid Earth)

   Abstract We investigate the amplitude and frequency of directional geomagnetic change since 15 ka in the Northern North Atlantic (∼67°N) using five “ultra‐high” resolution continental shelf sediment cores deposited at rates greater than 1 m/kyr. The ages of these cores are constrained by 103 radiocarbon dates with reservoir ages assessed through tephra correlation to terrestrial archives. Our study aims to address many of the uncertainties that are common in sedimentary paleomagnetic studies, including signal attenuation in low to moderate resolution archives and difficulty to demonstrate reproducibility in higher resolution archives. The “ultra‐high” accumulation rates of our cores reduce “lock‐in” and smoothing uncertainties associated with magnetic acquisition processes. Abundant radiocarbon dates along with an objective alignment algorithm provide a test of signal reproducibility at sub‐millennial timescales. The paleomagnetic secular variation (PSV) signal, evaluated as individual records and as a new stack (GREENICE15k), validates prior results, but provides stronger geochronological constraints, demonstrates a reproducible PSV signal and amplitude, and extends through the abrupt Bølling–Allerød and Younger Dryas climate transitions of the latest Pleistocene. While broadly consistent with time‐varying spherical harmonic models and varve dated records from Northern Europe, we demonstrate greater variance and higher amplitudes—particularly at sub‐millennial timescales. This robust variability on centennial timescales is rarely observed or discussed, but is likely important to our understanding of some of the most intriguing aspects of the geodynamo. 

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2. Relative Paleointensity Record of Integrated Ocean Drilling Program Site U1396 in the Caribbean Sea: Geomagnetic and Chronostratigraphic Observations in the Pliocene

   https://doi.org/10.1029/2021GC009677  

   Hatfield, R. G.; Stoner, J. S.; Fraass, A. J. (June 2021, Geochemistry, Geophysics, Geosystems)

   Abstract Continuous sedimentary records of paleomagnetic directional variability and relative paleointensity (RPI) provide valuable information on the evolution of the geodynamo while also facilitating stratigraphic correlation and age control. While the Quaternary RPI record has received much attention, Pliocene records are relatively rare. Here, a u‐channel paleomagnetic study from Integrated Ocean Drilling Program (IODP) Site U1396 in the Caribbean Sea refines the shipboard‐derived polarity stratigraphy and generates an RPI and directional record extending back 4.5 Ma. Rock magnetic data reveal changes in magnetic coercivity around 2.1 Ma that influences the quality of the paleomagnetic record; while the older record is well‐resolved and passes RPI quality criteria, much of the younger section does not. To facilitate the development of the RPI record, spike noise associated with discrete intervals of volcanogenic sediments are filtered from the data set. The resulting record passes RPI reliability criteria between 0–0.6 Ma and 2.1–4.5 Ma and represents the highest resolution RPI record extending into the early Pliocene. We use this record to refine the existing benthic δ¹⁸O chronology and open the door to high‐resolution RPI chronostratigraphies during the Pliocene. Although we find no evidence for a previously observed increase in magnetic field intensity after ∼4 million years ago, we do observe an asymmetrical form to RPI in the normal polarity intervals of the Gauss chron. We also find a polarity bias in inclination that cannot be simply explained by a drill string overprint, suggesting polarity‐driven field asymmetries that are particularly pronounced during the Pliocene. 

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3. Paleomagnetism and age of the Leucite Hills Volcanic complex, Wyoming: Implications for eruptive history, landscape evolution, and the geomagnetic instability timescale (GITS)

   https://doi.org/10.1016/j.jvolgeores.2023.107942  

   Welsh, JT; Feinberg, JM; Schneider, E; Pares, JM; Jicha, BR; Singer, BS; Carroll, AR (December 2023, Journal of Volcanology and Geothermal Research)

   The Leucite Hills Volcanic Field, southwest Wyoming comprises two dozen volcanic features including necks, flows, dikes, and plugs. It has been the focus of many petrologic studies as its volcanic and shallow intrusive rocks are one of the only surficial manifestations of ultrapotassic lamproite. We build on paleomagnetic findings of Sheriff and Shive (1980) by providing further paleomagnetic data from the Boars Tusk dike and Black Rock flows. We also characterize the magnetic mineral assemblage of these lamproites. Principal component analysis of alternating field (AF) and thermal demagnetization data indicate that the dike and breccias of Boars Tusk record a reversed magnetic polarity and the Black Rock lava records a normal polarity, both consistent with previous findings. This recording is typically carried by minerals with coercivities >15 mT and susceptibility measurements indicate magnetite, maghemite, and titanomagnetite as likely magnetic carriers. AF and thermal demagnetization experiments evince secondary magnetizations held by lower coercivity grains, likely caused by lightning strikes. 40Ar/39Ar incremental heating experiments from Boars Tusk and Black Rock give plateau ages of ∼ 2500 ka and ∼ 900 ka, respectively. Recent advances in the chronology of geomagnetic field reversals and excursions during the Quaternary permit integration of the Boars Tusk dike into the lower Matuyama chron, whereas the Black Rock lavas most probably record the Kamikatsura excursion. Notably, Black Rock records high inclinations that suggest the short-lived excursion achieved a full geomagnetic reversal, something not observed at other localities recording the Kamikatsura excursion. The Leucite Hills offer further opportunities to refine the Quaternary geomagnetic instability time scale (GITS), and to improve understanding of the eruptive and geomorphic evolution of this unusual volcanism. 

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4. A high resolution relative paleointensity record from the Gerlache-Boyd paleo-ice stream region, northern Antarctic Peninsula

   https://doi.org/10.1016/j.yqres.2006.01.006  

   Willmott, Verónica; Domack, Eugene W; Canals, Miquel; Brachfeld, Stefanie (July 2006, Quaternary Research)

   Abstract Herein we document and interpret an absolute chronological dating attempt using geomagnetic paleointensity data from a post-glacial sediment drape on the western Antarctic Peninsula continental shelf. Our results demonstrate that absolute dating can be established in Holocene Antarctic shelf sediments that lack suitable material for radiocarbon dating. Two jumbo piston cores of 10-m length were collected in the Western Bransfield Basin. The cores preserve a strong, stable remanent magnetization and meet the magnetic mineral assemblage criteria recommended for reliable paleointensity analyses. The relative paleomagnetic intensity records were tuned to published absolute and relative paleomagnetic stacks, which yielded a record of the last ∼8500 years for the post-glacial drape. Four tephra layers associated with documented eruptions of nearby Deception Island have been dated at 3.31, 3.73, 4.44, and 6.86 ± 0.07 ka using the geomagnetic paleointensity method. This study establishes the dual role of geomagnetic paleointensity and tephrochronology in marine sediments across both sides of the northern Antarctic Peninsula. 

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5. A precisely-dated, composite stalagmite record of Indian summer monsoon variability from Siddha Baba cave, central Nepal, for the last 2700 yr

   Denniston, R.F.; Sharafuddin, E.K.; Bransel, L.R.; Creech, P.D.; Hughes, E.K.; Wanamaker, A.D.; Asmerom, Y.; Polyak, V.J.; Ummenhofer, C.C.; Thatcher, D.L.; et al (January 2022, Transactions American Geophysical Union)

   The Indian summer monsoon (ISM), which today supplies ~75% of annual precipitation to South Asia, has been reconstructed across previous centuries using a variety of hydroclimate-sensitive proxies. In some of these cases, ISM variability far exceeds that observed in the century-and-a-half-long instrumental record. Understanding the origins of these events is best addressed by developing a wide-ranging, multi-proxy network of high-resolution ISM reconstructions. In Nepal, ISM variability has been examined through tree rings, glacial ice, and lake sediments, but no stalagmite isotopic records of ISM rainfall have yet been published. Here we present a sub-decadally-resolved, precisely-dated, composite aragonite stalagmite record of ISM variability from Siddha Baba cave, central Nepal, for the last 2.7 kyr. A rainwater sampling program near the cave site, and a published study from Kathmandu (Adhikari et al., 2020), 150 km to the southeast, reveal that rainfall amount explains little of the observed variance in d18O values. Local hydroclimate is thus reconstructed from stalagmite 13C values, which we interpret as reflecting prior aragonite precipitation driven by changes in effective precipitation above the cave. ISM variability is apparent across a number of time scales, including centennial periods of reduced or enhanced rainfall coincident with societally-relevant precipitation regimes identified at other sites across South Asia. These include the Neo-Assyrian drought in the eastern Mediterranean and Middle East (2.7-2.5 kyr BP; Kathayat et al., 2019), the Mauria Empire (2.1-1.9 kyr BP), and the Guge Kingdom (0.9-0.3 kyr BP) pluvials in India and Tibet (Kathayat et al., 2017). A secular shift toward drier conditions since 0.5 kyr BP in the Siddha Baba record tracks the 18O records from Dasuopu glacier, Nepal Himalaya, and Sahiya cave, North India. Numerous multidecadal oscillations are also evident, including markedly wetter conditions during the 18th century, in the late Little Ice Age, apparent in the Dasuopu and Sahiya records. References Adhikari et al. (2020) Tellus B: Chem. Phys. Meteor., 72, 1-17. Kathayat et al. (2017) Sc. Adv., 7, e1701296. Kathayat et al. (2019) Sci. Adv., 5, eaax6656. 

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