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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.

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. Plain Language Summary: As sediments slowly accumulate on the ocean floor past variations in magnetic field intensity and direction are recorded. While we have a relatively good record of intensity variations over the last 2–3 million years, we know little about earlier time intervals. Here, we reconstruct past magnetic (or paleomagnetic) variations over parts of the last 4.5 million years using a sediment core from the Caribbean Sea collected by the Integrated Ocean Drilling Program. A deterioration in the quality of the paleomagnetic signal occurred about 2.1 million years ago, however, prior to that time, the sediments preserve a high fidelity paleomagnetic record at a temporal resolution higher than any previously obtained. Good agreement to existing data from 2.1 to 3 million years ago builds confidence in the quality of our new observations. Although we find no evidence for a previously observed change in magnetic field intensity at around 4 million years ago, differences in mean inclination during normal as compared to reverse polarity intervals are observed, suggesting field asymmetries during this time interval. The unparalleled quality and length of our new record means we can now begin to define characteristic geomagnetic field behavior during times rarely observed. Key Points: Rock and paleomagnetic filtering provide a more robust estimate of relative paleointensity (RPI) that is used to refine the Integrated Ocean Drilling Program Site (IODP) U1396 δ18O chronostratigraphyIODP Site U1396 provides the longest, highest‐resolution, independently dated record of RPI to establish characteristic Pliocene behaviorPlio‐Pleistocene inclination anomaly contains a strong polarity bias and RPI asymmetry may be a characteristic feature of the Gauss chron [ABSTRACT FROM AUTHOR]

For understanding the fundamentals of the geodynamo such as the relation between paleointensity and polarity length and time‐averaged field (TAF) structure, continuous records of relative paleointensity (RPI) and inclination anomaly (ΔI) are desired; however, available records older than ~3 Ma are still very limited in time and space. We conducted a paleomagnetic study of the Integrated Ocean Drilling Program Site U1335 sediments in the eastern equatorial Pacific to obtain continuous RPI and ΔI records since ~8 Ma. Slow deposition, ~8.4 m/Myr or less, limits the resolution of the records but did allow for determination of long‐term variations. Rock‐magnetic measurements showed that biogenic magnetite dominates the magnetic mineral assemblages, and the proportion of biogenic to terrigenous magnetic minerals increases prior to ~4 Ma. The average paleointensity between ~4 and 8 Ma is approximately 30% lower than that from 0 to ~4 Ma. The apparent reduction of RPI at ~4 Ma reaches approximately ~50%, but ~20% of this is estimated to be artificial, induced by the increase in the proportion of biogenic magnetite. No relation between paleointensity and polarity length is recognized for the last ~8 Myr. The magnitude of ΔI is slightly larger during reversed polarity chrons (4.43° ± 1.47°) than normal polarity chrons (−0.69° ± 2.98°) over the last ~5 Myr, which agrees with the available TAF models of this time span. Prior to ~6 Ma, the sign of ΔI during the normal chrons might have switched to positive, and ΔI during reversed chrons might have been slightly larger than that after ~5 Ma. Key Points: Continuous relative paleointensity (RPI) and inclination anomaly records over the last ~8 Myr are presented for the first timeThe average RPI prior to ~4 Ma is ~50% lower than that after, in which ~30% is of geomagnetic origin and ~20% of rock magnetic artifactAn inclination anomaly shows polarity asymmetry from 0 to 5 Ma, which is consistent with the available time‐averaged field models [ABSTRACT FROM AUTHOR]

Improvements in the spatial and temporal coverage of paleomagnetic data are essential to better evaluate paleofield behaviour over the past 10 Myr, especially due to data scarcity at low latitudes in the South American region. Here, we provide new Pleistocene-Holocene (0–2 Ma age interval) paleodirectional data from three volcanic systems (Doña Juana Volcanic Complex, Galeras Volcanic Complex and Morasurco Volcano) in southwestern Colombia between latitudes 1.2 and 1.4°N. A total of 38 paleodirectional sites were studied using progressive alternating field and thermal demagnetization treatments. After excluding transitional data, we obtain thirty site-mean directions for analysis of paleosecular variation (PSV) and the time-averaged field (TAF) in the study area. The mean direction (Dec = 351.2°, Inc = −3.4°, α 95 = 6.2°, k = 20.0) and the paleomagnetic pole (Plat = 80.7°N, Plon = 173.1°E, A 95 = 5.2°, K = 29.1) of these sites are not statistically compatible with the expected geocentric axial dipole (GAD) field direction and geographic north pole, respectively. Virtual geomagnetic pole dispersion (S B) for our filtered dataset (S B (2 Ma) = 15.2 12.0 17.6°) and the Brunhes chron (S B (Bru) = 16.0 11.6 19.1°) are consistent at the 95% confidence level with South American studies at equatorial latitudes and recent PSV models for the 0–10 Ma and Brunhes intervals. Likewise, the corresponding inclination anomaly (Δ I) for two age groups Δ I 2 Ma = − 5.9 −12.1 0.3° and Δ I Bru = − 5.3 −13.7 3.1° suggests large deviations relative to the GAD model, in accordance with predictions from zonal TAF models. The high VGP dispersion could be linked to strong longitudinal variability of the magnetic equator position over South America. This feature reflects the presence of significant non-dipole field components in this region that have been detected in geomagnetic field models for the most recent centuries and millennia, probably associated with the presence of the South Atlantic Magnetic Anomaly in the South American region. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: We reconstruct the earth magnetic field in the Brunhes epoch at the Banda Sea by studying the paleomagnetic data from core MD012380, collected during the International Marine Global Change Study (IMAGES) VII Cruise in 2001. Magnetic analysis is carried out for whole core with a sampling spacing of 1cm by using u-channel. Magnetic susceptibility (χ), nature remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) are measured in our paleomagnetic experiment. Results show the low latitude geomagnetic field variation at the Banda Sea during the last ∼820kyr. Except for the Brunhes/Matuyama boundary (BMB), there is no clear signal of reverse events in paleo- inclination and paleo-declination patterns. However, the synthetic paleointensity curve displays the asymmetrical saw-tooth pattern that can be used for determining reverse events, and shows a maximum intensity drop at the BMB. The characteristics of paleointensity provide a useful tool to identify reverse signals and improve the difficulties from only using inclination and declination patterns, especially at low latitude. With the help of paleointensity, inclination and declination, we have identified five reverse events. Furthermore if we consider the secular variation effect, we think that the strong negative inclination observed in our study may be the zonal time-averaged field with paleo secular variation, rather than non-dipole effect within the Brunhes epoch. [Copyright &y& Elsevier]