Ikuko Kitaba

High-resolution record of the Matuyama–Brunhes transition constrains the age of Javanese Homo erectus in the Sangiran dome, Indonesia

A detailed paleomagnetic study conducted in the Sangiran area, Java, has provided a reliable age constraint on hominid fossil-bearing formations. A reverse-to-normal polarity transition marks a 7-m thick section across the Upper Tuff in the Bapang Formation. The transition has three short reversal episodes and is overlain by a thick normal polarity magnetozone that was fission-track dated to the Brunhes chron. This pattern closely resembles another high-resolution Matuyama–Brunhes (MB) transition record in an Osaka Bay marine core. In the Sangiran sediments, four successive transitional polarity fields lie just below the presumed main MB boundary. Their virtual geomagnetic poles cluster in the western South Pacific, partly overlapping the transitional virtual geomagnetic poles from Hawaiian and Canary Islands’ lavas, which have a mean 40Ar/39Ar age of 776 ± 2 ka. Thus, the polarity transition is unambiguously the MB boundary. A revised correlation of tuff layers in the Bapang Formation reveals that the hominid last occurrence and the tektite level in the Sangiran area are nearly coincident, just below the Upper Middle Tuff, which underlies the MB transition. The stratigraphic relationship of the tektite level to the MB transition in the Sangiran area is consistent with deep-sea core data that show that the meteorite impact preceded the MB reversal by about 12 ka. The MB boundary currently defines the uppermost horizon yielding Homo erectus fossils in the Sangiran area.

Midlatitude cooling caused by geomagnetic field minimum during polarity reversal

The climatic effects of cloud formation induced by galactic cosmic rays (CRs) has recently become a topic of much discussion. The CR–cloud connection suggests that variations in geomagnetic field intensity could change climate through modulation of CR flux. This hypothesis, however, is not well-tested using robust geological evidence. Here we present paleoclimate and paleoenvironment records of five interglacial periods that include two geomagnetic polarity reversals. Marine oxygen isotope stages 19 and 31 contain both anomalous cooling intervals during the sea-level highstands and the Matuyama–Brunhes and Lower Jaramillo reversals, respectively. This contrasts strongly with the typical interglacial climate that has the temperature maximum at the sea-level peak. The cooling occurred when the field intensity dropped to <40% of its present value, for which we estimate >40% increase in CR flux. The climate warmed rapidly when field intensity recovered. We suggest that geomagnetic field intensity can influence global climate through the modulation of CR flux.

Geological support for the Umbrella Effect as a link between geomagnetic field and climate

The weakening of the geomagnetic field causes an increase in galactic cosmic ray (GCR) flux. Some researchers argue that enhanced GCR flux might lead to a climatic cooling by increasing low cloud formation, which enhances albedo (umbrella effect). Recent studies have reported geological evidence for a link between weakened geomagnetic field and climatic cooling. However, more work is needed on the mechanism of this link, including whether the umbrella effect is playing a central role. In this research, we present new geological evidence that GCR flux change had a greater impact on continental climate than on oceanic climate. According to pollen data from Osaka Bay, Japan, the decrease in temperature of the Siberian air mass was greater than that of the Pacific air mass during geomagnetic reversals in marine isotope stages (MIS) 19 and 31. Consequently, the summer land-ocean temperature gradient was smaller, and the summer monsoon was weaker. Greater terrestrial cooling indicates that a reduction of insolation is playing a key role in the link between the weakening of the geomagnetic field and climatic cooling. The most likely candidate for the mechanism seems to be the increased albedo of the umbrella effect.

High-Resolution Magneto–Climatostratigraphy of MIS 19 from the Osaka Group, Japan

Marine oxygen isotope stage (MIS) 19 has an abnormal climate where the temperature maximum postdated by about 5 kyr the sea-level peak in mid-latitudes, and includes the Matuyama–Brunhes magnetic polarity transition that persisted for several kyr. The stratigraphy of this stage must be precisely understood as a candidate for the Early–Middle Pleistocene boundary. The marine clay layer correlated with MIS 19 in the Osaka Group is able to provide millennial-scale magneto–climatostratigraphy because of its high accumulation rate of about 60 cm/kyr. A new age model based on a diatom sea-level proxy curve tuned to oxygen isotope stack LR04 reveals that the sedimentation rate is almost uniform. The following dated events provide a useful time scale: (1) Two high sea-level intervals that span 784–778 ka and 768–762 ka, correlated with substages 19.3 and 19.1, respectively. The first interval includes a sea-level peak at 780 ka. (2) The warmest temperature interval spans 777–774 ka. (3) The low-palaeointensity interval (LPI) spans 784–776 ka and includes multiple polarity swings. We confirm that the LPI is a useful time measure, applying to the preliminary palaeomagnetic results from the Chiba section of the Kazusa Group in eastern Japan. The beginning of the warmest temperature interval almost coincides with the end of the LPI at 777–776 ka and may provide an important datum level near the Early–Middle Pleistocene boundary.

Millennial-scale northern Hemisphere Atlantic-Pacific climate teleconnections in the earliest Middle Pleistocene

Suborbital-scale climate variations, possibly caused by solar activity, are observed in the Holocene and last-glacial climates. Recently published bicentennial-resolution paleoceanic environmental records reveal millennial-scale high-amplitude oscillations postdating the last geomagnetic reversal in the Marine Isotope Stage (MIS) 19 interglacial. These oscillations, together with decoupling of post-reversal warming from maximum sea-level highstand in mid-latitudes, are key features for understanding the climate system of MIS 19 and the following Middle Pleistocene. It is unclear whether the oscillations are synchronous, or have the same driver as Holocene cycles. Here we present a high resolution record of western North Pacific submarine anoxia and sea surface bioproductivity from the Chiba Section, central Japan. The record reveals many oxic events in MIS 19, coincident with cold intervals, or with combined cold and sea-level fall events. This allows detailed correlations with paleoceanic records from the mid-latitude North Atlantic and Osaka Bay, southwest Japan. We find that the millennial-scale oscillations are synchronous between East and West hemispheres. In addition, during the two warmest intervals, bioproductivity follows the same pattern of change modulated by bicentennial cycles that are possibly related to solar activity.