Relative Paleointensity Research Articles

Chronology of detrital carbonate events in Baffin Bay reveals different timing but similar average recurrence time of North American-Arctic and Laurentide ice sheet collapse events during MIS 3

Baffin Bay Detrital Carbonate (BBDC) layers represent periods of elevated discharge of terrigenous sediments, icebergs and freshwater originating from the North American Arctic-ice sheet complex. Distinct from Heinrich event layers in their dolomitic composition, these sedimentary deposits are found throughout Baffin Bay. They are considered a signature of large-scale instability of the ice sheets discharging into the Baffin Bay, akin to the Heinrich events of the North Atlantic. However, the precise timing of and potential forcing mechanisms for the North American Arctic-ice sheet instabilities remains elusive. Previous work suggests that BBDC events were not in phase with Heinrich events originating from the Hudson Strait, but this hypothesis could not be tested without rigorous chronological constraints. By combining radiocarbon ages and relative paleointensity (RPI) dating in a central western Baffin Bay marine sedimentary sequence, we were able to derive a new chronology for BBDC events covering a large part of MIS3. Sedimentological and elemental data indicate the occurrence of seven distinct BBDC events over the last 52 ka. These events occur during both glacials (stadials) and interglacials (interstadials) and have no consistent phase relationship with Heinrich events. However, the mean duration and mean recurrence time of BBDC events appears similar to Heinrich events. We conclude that BBDC events reflect repeated ice sheet instability periods, with similar statistical properties but different timing compared to the Heinrich events. This implies that episodic instability was an intrinsic property of the North American Arctic-ice sheet complex as well, but over the studied period of MIS3, instabilities of the Arctic and the Laurentide Ice sheets did not appear to have been linked.

Quaternary Magnetic Stratigraphy of Deep‐Sea Sediments in the Western North Pacific: Influences of Paleomagnetic Recording Efficiency and Lock‐In Delay

AbstractUnderstanding past environmental changes in the western North Pacific is rather limited due to the general paucity of robust chronology for carbonate‐lacking pelagic clay sediments. Here, we present a new magnetostratigraphy of Quaternary deep‐sea sediments in the northern Mariana Basin. A comprehensive set of paleomagnetic, rock magnetic, and microscopic results reveal that paleomagnetic signals are mainly carried by detrital magnetic minerals of Asian eolian dust and volcanic origins. The volcanic materials, most likely from the Mariana Arc, are characterized by low coercivity (∼30 mT) interacting single domain (SD) and pseudo‐SD grains, associated with abundant dendritic magnetic crystals/inclusions. This volcanic component is a suitable remanence carrier but is less efficient in recording geomagnetic intensity than the higher coercivity eolian component. The relative paleointensity record derived with minimum rock magnetic artifacts shows comparable patterns with global templates. At the Matuyama‐Brunhes transition, a ∼2‐cm remanence lock‐in depth offset is identified from beryllium isotope records, and the estimated magnetostratigraphic age delay is small (∼8 kyr). Our results provide an improved understanding of remanence recording processes in carbonate‐poor deep‐sea clays and demonstrate magnetic stratigraphy as a valuable chronology in the western North Pacific region.

Paleoclimate evolution of the North Pacific Ocean during the late Quaternary: Progress and challenges

• Climate systems force the evolution of orbital-scale NPIW formation. • Deciphering the forcing mechanism on dust fluxes to the North Pacific Ocean. • Model-data comparison expected to play an important role in understanding climate changes. High- and low-latitude climatic processes in the North Pacific Ocean are important components of the global climate system. For example, the interplay among North Pacific atmospheric circulation, ocean circulation, and biological productivity affects atmospheric carbon dioxide levels and marine oxygen concentrations. Here we review recent research on the North Pacific paleoclimatic and paleoceanographic evolution during the late Quaternary and its response to external forcings such as orbital insolation, ice-sheet extent, and greenhouse gas concentrations. First, we summarize the principles and application of relative paleointensity as a critical chronological tool in North Pacific paleoclimate research. Second, we illustrate the latest discoveries on the interaction between North Pacific Intermediate Water formation and high-to-low latitude teleconnection processes. Third, recent progress in linking dust fluxes and marine productivity and their global significance for the carbon cycle are presented. Finally, several key scientific problems are highlighted for future research on ocean-atmosphere-climate interactions in the North Pacific, pointing to the importance of combining paleo-records and modeling simulations. Overall, this review also aims to provide a broad insight into possible future changes of ocean-atmosphere circulation in the North Pacific region under a rapidly warming climate.

Paleomagnetic secular variation and revised chronostratigraphy of Bering sea (IODP Ex. 323) deep-sea sediments (MIS 1–4)

This study develops composite full-vector paleomagnetic secular variation (PSV) records from IODP Ex. 323 Sites U1339, U1343, U1344, and U1345 in the Bering Sea (51°N-60°N). These PSV records cover the last 71 ka (Marine Isotope Stages (MIS) 1–4). This is the first time that long PSV records have been recovered from such high latitudes. The records have been dated by oxygen isotope records and paleointensity correlations. The records have an age uncertainty of ∼1000 years. The paleomagnetic data come from shipboard paleomagnetic studies of deep-sea sediments with sediment accumulation rates of 20–40 cm/ky. The sample interval is 5 cm, with a time resolution of ±120–200 years. We have identified 66 correlatable inclination features and 59 declination features in all four sites. The relative paleointensity has been estimated by normalizing the natural remanence (after 20 mT af demagnetization) by magnetic susceptibility. This normalization process is strongly biased by environmental variability and so our relative paleointensity records are limited to correlating variability among the four sites. We can identify 8 key, dated paleointensity highs/lows in our composite records. There are two magnetic field excursions recorded reproducibly at these four sites – the Laschamp Excursion (∼41 ka) and the Norwegian-Greenland Sea Excursion (∼61 ka). Both occur quickly, in 1000 years or less, are associated with the lowest paleointensities, and show no significant evidence for pre- or post-excursion anomalous PSV. Both of these are Class I excursions. There are no excursional directions associated with the time interval of the Mono Lake Excursion (∼34 ka). We have carried out a statistical study using our PSV records to assess 3ky and 9ky average variability over the last 71 ky after removing all excursional directions. The averaged inclinations and declinations have correlatable variability, mostly on ∼104 yr time scales. We see significant variation in averaged angular dispersion that appears to be bimodal. Most time is spent with low angular dispersion (∼10°–12°) with shorter time intervals with significantly higher angular dispersion (∼24–30°). This study suggests that excursions, high angular dispersion, and low paleointensities occur together. This may define a distinctive low-intensity (or low-energy) state to the geomagnetic field.

Late Holocene paleosecular variation and relative paleointensity records from Lagoa dos Patos (southern Brazil)

Lake and lagoon sediments are important recorders of the Earth's magnetic field variations. However, the Southern Hemisphere, particularly the South American continent, contributes only a small fraction of the global paleosecular variation (PSV) and relative paleointensity data, which hinders a better understanding of the global PSV. Moreover, the scarcity of information on the geomagnetic field in South Brazil for the past few millennia impedes, for example, a detailed analysis of the evolution of the South Atlantic Magnetic Anomaly (SAMA), which encompasses the weakest geomagnetic field on the Earth's surface. Here, we present high-resolution paleomagnetic and rock magnetic data of two cores collected at the lagoon of Lagoa dos Patos, Rio Grande do Sul State, Brazil. Sediment cores from Lagoa dos Patos represent the period from ∼4540 to 3320 cal years BP. Rock magnetic results show the remanent magnetization resides in pseudo single-domain (PSD) magnetite and/or titanomagnetite. Magnetization inclinations and declinations were isolated after alternating field demagnetization (AFD) and principal component analysis (PCA). Mean inclinations are −39,6° and − 38,4° for cores PT-04 and PT-06, respectively. Relative paleointensity results are compatible with geomagnetic field models, implying very promising results in the reconstruction of a reference curve for the region. As there is no PSV and relative paleointensity data for this region in this period, this study helps to elucidate the past field and the presence of SAMA in South America.

Pleistocene oceanographic variability in the Ross Sea: A multiproxy approach to age model development and paleoenvironmental analyses

Understanding how ocean circulation has varied around Antarctica in the past is vital because significant volumes of ice are grounded below sea level, and therefore ice sheet variations are strongly coupled with oceanographic variability at the continental shelf margin. This study investigates the 11.75 m sediment core RS15-LC42, retrieved from the Central Basin of the Ross Sea. A magneto-biostratigraphic age model was constructed from a magnetic reversal stratigraphy, diatom biostratigraphy, and the relative paleointensity of the geomagnetic field. The core spans the last 1.34 Myr with a mean sedimentation rate of 0.88 cm/kyr, providing insights into paleoceanographic variability in the Ross Sea throughout the Pleistocene. Winnowed deposits enriched in iceberg rafted debris were formed due to a combination of enhanced glacial calving during glacial retreats, and export of Dense Shelf Water during interglacial and weak glacial periods when the ice-sheet was not grounded in the outer Joides Trough. During periods where the ice sheet grounded on the outer Ross Sea continental shelf, laminated sediments rich in reworked diatoms are interpreted to have formed due to glacial scouring and transport of shelf sediments to the mouth of the Joides Trough, where slope failures and meltwater pulses remobilized this material downslope to the Central Basin. Dense shelf water formation is inferred to be most intense during interglacial periods, with reduced export from the Joides Trough during ice sheet advances. The study does not find any evidence for prolonged ice-sheet grounding in the outer Ross Sea since ~0.3 Ma, however, we conclude that the grounding line reached the outer shelf frequently between 0.3 and 1.2 Ma, supporting higher Antarctic ice volumes during glacial periods in this interval. • High-resolution sediment age modelling of Pleistocene Antarctic sediments • The WAIS has likely not been grounded in the outer Ross Sea since MIS 10. • Bottom water export from the Ross Sea is strongest during interglacial periods.

A summary of paleomagnetic secular variation and excursions for the last 380 ky of the Brunhes normal polarity Chron

This study develops composite high-resolution, full-vector PSV records for the last 380 ka from three regions associated with ODP Leg 172 in the western North Atlantic Ocean. These records identify 207 inclination features and 196 declination features that are common to all three regions. The average sediment accumulation rates are ~25 cm/ky, which produce sampling intervals of ~100–200 years. Each region has been dated by a combination of oxygen isotope stratigraphy and carbonate stratigraphy. We also have developed relative paleointensity records from all three regions that have 34 features in common. The paleointensity features can also be correlated to the PISO-1500 global paleointensity record of Channell et al. (2009) and provide an independent chronostratigraphy. The directions have near Gaussian distributions with inclinations averaging about 6° lower than axial dipole expectation due to the ΔI anomaly. The intensities also have a near Gaussian distribution, but have some more low intensities than expected. There is reproducible evidence for 7 excursions in these records with good waveform information. The excursions last from ~900 to ~8400 years. Five of the excursions are Class I excursions with out-of-phase inclination and declination variability that produces clockwise or counter-clockwise circularity. Two of the excursions (Blake Event, 8α Excursion) are Class II excursions with in-phase inclination and declination variability. Both of these excursions should be considered local reversals due to their very fast (<200 year) in-phase directional movement to reversed polarity directions that hover there before returning quickly to normal polarity directions. We have complete full-vector PSV records that surround each of these excursions. Some excursions occur quickly with no distinctive pre- or post-excursion anomalous directional variability. Other excursions are preceded by 10–20 ky of anomalous directional variability (high angular dispersion). Some also show continuing anomalous directions for 10–20 ky after the excursions. All of these features suggest a close relationship between excursions themselves and the surrounding PSV. Statistical studies of the PSV also show distinctive patterns to the PSV variability. The angular dispersion in 3-ky intervals shows a distinctive bimodal distribution with high (low) values associated with low (high) intensities. The high angular dispersion (and low intensity) are associated with the seven recorded excursions in these regions. There is reproducible evidence for another 5 excursions elsewhere in the world, but not in our records, that are also associated with high angular dispersion intervals. We think this is corroborative evidence for the reality of these excursions. That means that 12 excursions have occurred in the last 380 ka. Overall assessment of the PSV records suggests that important non-linear behavior occurs in the geodynamo as a function of intensity. This behavior is noted in the sharp changes between low/high angular dispersion intervals. It suggests that the 9-ky averaged field is more multipolar and far from axial dipole expectation during low paleointensity. Non-linearity is also seen in the fast polarity flips associated with the two Class II excursions.

Holocene palaeoenvironmental conditions in NE Bulgaria uncovered by mineral magnetic and paleomagnetic records of an alluvial soil

Retrieving well-dated terrestrial archives of Holocene climate change is fundamentally important for building robust climate models and predictions. In this study we obtained records of paleomagnetic direction and relative paleointensity of the Earth's magnetic field from an alluvial soil situated on a flood river terrace close to the well-dated and stratigraphically confined Chalcolithic mound near the village of Koprivetz in NE Bulgaria. Anisotropy of magnetic susceptibility data revealed a coherent imbricated magnetic fabric reflecting the effects of water flow in the lower alluvial clays and the direction of delluvial sedimentation from the slope in the upper soil horizons. Variations of declination, inclination and relative paleointensity along depth of the soil profile were correlated to the archaeomagnetic secular variation curves for Bulgaria, using the age of the earliest archaeological stratigraphic horizon as the main tie point. Based on this correlation, an age model was constructed and temporal variations of environmental magnetic parameters were examined for their suitability as paleoclimate proxies. The ratio of frequency dependent susceptibility to anhysteretic susceptibility (χfd/χARM) is a reasonable proxy for paleoprecipitation due to its close resemblance to paleo-flood records from the Northern and Southern Alps. Events of increased flood intensities during the late Holocene (post – 4700 y BP) are registered through χfd/χARM and reveal good correspondence to the flood records of rivers in Southern Alps and Carpathians. The ratio of isothermal remanent magnetization after 20 mT alternating field demagnetization (IRM20mT) normalized to the full IRM showed good consistency with the GISP2 ice record of temperature variations in Greenland during the Holocene. Six major cold incidents during the last 7000 y BP with the major one at 4500 y BP are evidenced. The obtained results suggest that climate at mid-latitude SE Europe is sensitive to the changes in North Atlantic circulation.

Understanding the Role of Biogenic Magnetite in Geomagnetic Paleointensity Recording: Insights From Ontong Java Plateau Sediments

AbstractMarine sediments can preserve continuous paleomagnetic intensity records. Different magnetic minerals may acquire remanent magnetizations differently, so that compositional variations of magnetic mineral assemblages in sediments may hinder extraction of reliable relative paleointensity (RPI) records. To better understand this issue, we conducted a paleo‐ and rock magnetic study of a sediment core from the Ontong Java Plateau, western equatorial Pacific Ocean. RPI estimated by normalizing natural remanent magnetization (NRM) with anhysteretic remanent magnetization (ARM) decreases downcore with an inverse correlation with the ratio of ARM susceptibility to saturation isothermal remanent magnetization. This relationship indicates that the RPI signal weakens as the proportion of biogenic magnetite increases. The NRM–ARM demagnetization diagrams obtained have concave‐down curvature. These observations indicate that the RPI recording efficiency of the biogenic component is lower than that of the terrigenous component if we assume that the magnetizations of the high‐ and low‐coercivity windows are carried dominantly by biogenic and terrigenous components, respectively. This assumption is supported by first‐order reversal curve measurements, transmission electron microscope observations, low‐temperature measurements, and extraction of silicate‐hosted magnetic inclusions from the sediments. Previous studies have suggested that the RPI recording efficiency of biogenic magnetite is higher than that of the terrigenous component, which disagrees with our results. Different concentrations of silicate‐hosted magnetic inclusions in different sedimentary environments might explain this contradiction. We conclude that biogenic magnetite contributes to RPI records with lower efficiency than unprotected terrigenous magnetic minerals in the studied sediments. Changing biogenic magnetite proportions distort ARM‐normalized RPI estimations.

Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba section. The sediment succession consists of a massive siltstone layer of the Kokumoto Formation, Kazusa Group. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the Byk-E tephra layer from the Chiba section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were collected at intervals of 1–10 cm from the siltstone. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. In addition, magnetization during PThD showed sharp drops around 175 °C, which decreased gradually between 175 °C and ~ 300 °C, and became unstable above ~ 350 °C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175 °C followed by PAFD was conducted. Combined analysis of remagnetization circles enables extraction of primary magnetization with improved reliability. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200 °C, 400 °C, 450 °C and 550 °C. Rock magnetic analyses and electron microscopy indicate that titanomagnetite/magnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified preserving secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested as secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The correlation of relative paleointensity with the Chiba section provides an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra. VGP latitudes are highly consistent with those from the Chiba section based on the age model, which assigns the main directional swing from reversed to normal polarities as 772.8 ± 0.5 ka.Graphical

Eocene relative paleointensity of the geomagnetic field from Integrated Ocean Drilling Program Site U1403 and U1408 sediments in the northwest Atlantic

Published relative paleointensity (RPI) records older than ∼3Ma are extremely limited in time and space. To develop a more robust understanding of RPI variations, we have conducted rock magnetic and paleomagnetic measurements on Eocene marine sediments recovered at Integrated Ocean Drilling Program (IODP) Sites U1403 and U1408 in the northwest Atlantic. A series of rock magnetic measurements indicate that the main remanence carrier is single domain biogenic magnetite. Paleomagnetic measurements yielded RPI records for Chrons C18–C21 and C22n, which correlates to ∼38.4–49.6Ma. The record is compromised in a few short intervals by inhomogeneous rock magnetic properties. RPI minima always occur at chron boundaries and RPI fluctuates between highs and lows within each chron. This record is the first to show that these characteristics persist at least since the onset of Chron C22n at ∼49.3Ma. We conclude that these are intrinsic and fundamental features of the geomagnetic field regardless of the polarity reversal rate. We produce a stacked RPI curve for Chron 18, named PIS-C18, on the basis of the RPI records obtained in this study and those from IODP Sites U1331 and U1332 in the equatorial Pacific that are matched by visual inspection. The PIS-C18 RPI of the stack is generally high with no prominent lows during Chron C18n.2n, whereas it is not as high and has several prominent lows almost equivalent to the RPI minima at the chron boundaries during Chrons C18n.1n and C18r. A histogram of RPI during Chron 18 is slightly skewed to the right, and the ratio of the standard deviation to the mean paleointensity is 0.38. These characteristics are similar to a histogram of the RPI stack for the last 1.5 million years. We interpret this to imply that character of the geodynamo for 104–106 years timescales has been unchanged since the Eocene.

A Relative Paleointensity (RPI)-Calibrated Age Model for the Corinth Syn-rift Sequence at IODP Hole M0079A (Gulf of Corinth, Greece)

The Corinth basin (Greece) is a young continental rift that recorded cyclic basin paleoenvironment variations (i.e., marine to lacustrine) caused by glacio-eustatic sea level fluctuations during its initial connection to the global oceans. The Corinth syn-rift sequence offers therefore a unique opportunity to determine the timing and modality of connection of young rifts to the global oceans, and investigate how sediment supply change during this extremely dynamic stage of their evolution. Here we use magnetostratigraphic and relative paleointensity (RPI) constraints from 885 discrete samples from International Ocean Discovery Program (IODP) Hole M0079A to generate an unprecedented high-resolution (∼15kyr) age model for the youngest part of the Corinth’s offshore syn-rift sequence. Our RPI-calibrated age model spans the last ∼850 kyr and reveals that initial connection of the Corinth basin to the Mediterranean Sea occurred at ∼530 ky, more recently than previously thought and after a short-lived marine incursion at ∼740 kyr. Accumulation rates calculated from our age model indicate two significant changes in sediment supply at 530 (from 74 to 156 cm/kyr on average) and 70 ka (from 156 to 258 cm/kyr on average), interpreted as due to increased local fault activity in the southern margin of the Corinth basin. Sedimentation rates also display a short-term variation indicating a lower sediment supply (98 cm/kyr on average) during interglacial periods and a higher input (156 cm/kyr on average) during glacial periods. We conclude that long-term variations of sediment supply in young rifts connecting to the global oceans are predominantly controlled by local tectonics (i.e., new depocenters formation), while short-term variations may be driven by global climate (i.e., increased erosion aided by fewer vegetation during glacial periods). Grain size of the Corinth syn-rift sequence shows a weak correlation with glacial cycles too (i.e., coarser sediments during interglacials), but we suggest this to be controlled by the hydrodynamics of the basin (i.e., stronger bottom currents when the basin was connected to the Mediterranean Sea during interglacial periods).

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

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.

Paleosecular Variations During the Last Glacial Period From Tengchong Qinghai Lake, Yunnan Province, China

AbstractIn recent years, the evolution of the magnetic flux lobes in terms of location and intensity has been attributed to paleosecular variation of the geomagnetic field, reflecting outer‐core fluid convection close to the core‐mantle boundary. However, most results were based on observational data for the past 400 years supplemented by sparse archeomagnetic studies with data for approximately 3 kyr. In this paper, we construct a relative paleointensity (RPI) record and an inclination record spanning the past ∼90 kyr based on two cores from Tengchong Qinghai Lake (Yunnan Province, China), a closed crater lake that formed in the late Pleistocene. The evolution of flux lobes and the non‐dipole field can be assessed on a much longer time scale. Although the concentration and domain state of magnetic grains in the cores vary significantly, the RPI record is still representative of geomagnetic field features in the region of Yunnan Province, China. The contrasting trends in RPI with the records for eastern China since the late Holocene indicate the southeastward migration of the Siberian flux lobe. During the last glacial period, the RPI records of Tengchong Qinghai Lake and the North Atlantic Ocean show opposite trends, denoting the diverse and strong influence of the two flux lobe systems. The strong reverse flux lobe in the North Atlantic Ocean strengthened the non‐dipole field, while the weak normal flux lobe in Tengchong weakened the non‐dipole field.

A Physical Interpretation of Asymmetric Growth and Decay of the Geomagnetic Dipole Moment

AbstractObservations of relative paleointensity reveal several forms of asymmetry in the time dependence of the virtual axial dipole moment (VADM). Slow decline of the VADM into a reversal is often followed by a more rapid rise back to a quasi‐steady state. Asymmetry is also observed in trends of VADM during times of stable polarity. Trends of increasing VADM over time intervals of a few 10s of kyr are more intense and less frequent than decreasing trends. We examine the origin of this behavior using stochastic models. The usual (Langevin) model can account for asymmetries during reversals, but it cannot reproduce the observed asymmetry in trends during stable polarity. Better agreement is achieved with a different class of stochastic models in which the dipole is generated by a series of impulsive events in time. The timing of each event occurs randomly as a Poisson process and the amplitude is also randomly distributed. Predicted trends replicate the observed asymmetry when the generation events are large and the recurrence time is long (typically longer than 3 kyr). Large and infrequent generation events argue against dipole generation by small‐scale turbulent flow. Instead, the observations favor a mechanism that relies on expulsion of poloidal magnetic field from the core.

Determination of the optimized late Pleistocene chronology of a lacustrine sedimentary core from the Heqing Basin by geomagnetic paleointensity and its paleoclimate significance

Long and continuous lacustrine sedimentary sequences from SW China faithfully recorded the evolution of the Indian summer monsoon (ISM). However, the limited dating range (<50 ka) of 14C dating has restricted high-resolution research on the last glacial cycle.We presented a relative paleointensity (RPI) record from a lacustrine sedimentary core from the Heqing Basin to optimize its AMS 14C chronology. Detailed rock magnetic analyses indicated that SD and vortex state magnetite (and/or maghemite) was the dominant magnetic mineral of lacustrine sediments. Moderate variations in concentration-dependent (SIRM, ARM) and grain-size-dependent (ARM/SIRM) parameters satisfied the criteria for the RPI studies. Consequently, we determined the RPI by NRM20-40 mT/SIRM20-40 mT, where NRM20-40 mT denotes the NRM after demagnetization between 20 and 40 mT. Using 6 AMS 14C dates younger than 40 ka BP as constraints, 7 age control points were determined by correlating the lows in the RPI record with those of the target paleointensity stacks PISO-1500. We established a reliable chronological framework of the upper 11.1 m of the HQ drill core by binomial fitting based on the 7 RPI age control points and the climatic transition termination II. The consistent variations in our RPI curve and other global target paleointensity curves indicated that the optimized chronology was reliable.In our optimized chronology, the visible precession cycle in ARM/SIRM indicated that the ISM was a direct response to solar insolation during the last glacial cycle. The strong ISM suggested by the high content of Mesic conifers and tropical and subtropical trees during MIS 3 shifted to MIS 5a. Thus, the “MIS 3 Mega-paleolake” hypothesis in China, which was mainly deduced by the 14C age model, was challenged.

Iceberg discharge events in the northwest Pacific and related sequence of Kamchatka glaciations over the last 190 kyr

Evidence of meltwater and ice rafted debris from the Tenji and Detroit seamounts in the northwestern corner of the North Pacific are key to understanding ice sheet dynamics across the Kamchatka Region. Marine isotopic stages 6.5–1 were established in this study for the Tenji Seamount sediment core based on accelerator mass spectrometry (AMS) 14C data, benthic foraminifera δ18O stratigraphy, and relative paleointensity excursions. Records of planktic foraminifera δ18O and ice rafted debris (IRD) in sediment cores pertaining to the last 190 kyr were used to outline several periods of surface water freshening and increased accumulation of coarse terrigenous grains induced by iceberg supply and melting in the northwest Pacific. Iceberg discharge events (IDEs) were reconstructed over the last and penultimate glacial periods in the central and eastern parts of the Okhotsk Sea and southwestern part of the Bering Sea using relevant published proxies. Similar reconstructions of IDEs were also performed for two published sediment cores from the Detroit Seamount. Synthesis of planktic foraminifera δ18O and IRD records along with other related proxies from the studied areas concordantly revealed five IDEs in the northwest Pacific and its marginal seas during the last 190 kyr: IDE 1 (42–29 ka), IDE 2 (76–57 ka), IDE 3 (118–110 ka), IDE 4 (152–141 ka), and IDE 5 (179–160 ka). Data showed that IDEs 4–1 occurred at low values of Earth obliquity and under a weak Aleutian Low, facilitating snowfall precipitation and enhancing glacier formation at Kamchatka, which is a priority area for capturing atmospheric moisture transferred into northeast Asia. Compared with the other IDEs, IDEs 2 and 5 were more pronounced and prolonged events over the last 190 kyr. In addition, IDE 2 was also characterised by a catastrophic and short IRD peak that was probably forced by the influx of lake water from the Central Kamchatka Depression. It is likely that IDEs 2 and 1 occurred during phases I (more extensive) and II (less extensive) of the published Kamchatka glaciations during the Late Pleistocene.

Key Signatures of Magnetofossils Elucidated by Mutant Magnetotactic Bacteria and Micromagnetic Calculations.

Magnetotactic bacteria (MTB) produce single‐stranded or multi‐stranded chains of magnetic nanoparticles that contribute to the magnetization of sediments and rocks. Their magnetic fingerprint can be detected in ancient geological samples and serve as a unique biosignature of microbial life. However, some fossilized assemblages bear contradictory signatures pointing to magnetic components that have distinct origin(s). Here, using micromagnetic simulations and mutant MTB producing looped magnetosome chains, we demonstrate that the observed magnetofossil fingerprints are produced by a mixture of single‐stranded and multi‐stranded chains, and that diagenetically induced chain collapse, if occurring, must preserve the strong uniaxial anisotropy of native chains. This anisotropy is the key factor for distinguishing magnetofossils from other populations of natural magnetite particles, including those with similar individual crystal characteristics. Furthermore, the detailed properties of magnetofossil signatures depend on the proportion of equant and elongated magnetosomes, as well as on the relative abundances of single‐stranded and multi‐stranded chains. This work has important paleoclimatic, paleontological, and phylogenetic implications, as it provides reference data to differentiate distinct MTB lineages according to their chain and magnetosome morphologies, which will enable the tracking of the evolution of some of the most ancient biomineralizing organisms in a time‐resolved manner. It also enables a more accurate discrimination of different sources of magnetite particles, which is pivotal for gaining better environmental and relative paleointensity reconstructions from sedimentary records.

Comparison and Renormalization of Holocene Paleointensity Records From Central North America (17°N–51°N, 205°E–295°E)

AbstractThis paper develops a composite absolute paleointensity record for Holocene paleomagnetic secular variation (PSV) from central North America. Twelve full‐vector (inclination, declination, paleointensity) PSV records were assessed in order to build the composite record. Nine of the paleointensity records come from sediment paleomagnetic studies and are considered relative in intensity. Three of the paleointensity records come from absolute paleointensity measurements of archeological materials and lava flows. This paper develops a new method to normalize the sediment relative paleointensity records to the absolute intensity records. The final composite paleointensity record describes intensity variability over a region of Central North America delineated by 35°–48.6°N and 240.4–291.4°E (∼14° × 50°). This composite record shows a distinctive long‐duration (∼104 year) oscillation and a series of millennial‐scale intensity oscillations that are consistent over our study region.

Influence of Magnetofossils on Paleointensity Estimations Inferred From Principal Component Analyses of First‐Order Reversal Curve Diagrams for Sediments From the Western Equatorial Pacific

AbstractRelative abundance of magnetite originated from magnetotactic bacteria (magnetofossils) in sediments may influence relative paleointensity (RPI) estimations of the geomagnetic field, as some studies reported an inverse correlation between RPI and the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), a proxy of the proportion of biogenic to terrigenous magnetic minerals as well as magnetic grain size. This study aims to evaluate the influence of magnetofossils on RPI estimations more selectively using first‐order reversal curve (FORC) diagrams. We studied three cores (KR0515‐PC4, MD982187, and MR1402‐PC1) from the western equatorial Pacific, among which large differences exist in the average natural remanent magnetization intensity normalized by ARM and kARM/SIRM. Principal component analyses (PCAs) were applied to FORC diagrams measured on bulk specimens from the three cores and silicate‐hosted magnetic inclusions extracted from Core MD982187, and three endmembers (EMs) were revealed (EM1: silicate‐hosted magnetic inclusions, EM2: other terrigenous, EM3: biogenic). EM3 proportions vary widely among the three cores. The average RPI decreases with increasing EM3 proportion, which is probably caused by higher ARM acquisition efficiency of magnetofossils due to small magnetostatic interactions. EM3 proportion correlates with kARM/SIRM, which confirms that kARM/SIRM represents the proportion of biogenic to terrigenous magnetic components. Core MR1402‐PC1 has the highest EM3 proportion, and its within‐core variation is small. From FORC‐PCA applied solely to this core, we infer that the configurations of biogenic magnetite chains such as bending and collapse may also influence kARM/SIRM and RPI estimations.