Masayuki Torii

Geochemical characterization of the Luochuan loess-paleosol sequence, China, and paleoclimatic implications

Abstract Chemical (major and trace element) and isotopic (Sr, Nd) analyses of the Luochuan loess-paleosol sequence in China were performed in order to examine the following problems: (1) potential source heterogeneity; (2) element behavior during pedogenesis; (3) paleoclimatic implications; and (4) the use of loess data to determine the average composition of the upper continental crust. New results clearly indicate that Rb, Sr, U and Ce abundances are severely depleted in paleosols relative to their “parental” loess. The behavior of these elements is strongly controlled by the breakdown of primary minerals such as carbonate (Sr) or by alteration processes in highly oxidizing environments (Ce and U). The pattern of element variations through the upper section of the Luochuan sequence mimics the magnetic susceptibility signal both in position and in intensity, thus reinforcing the pedogenesic origin of the enhanced magnetic susceptibility in paleosols. All loess samples display highly uniform REE patterns (except for Ce) characterized by the upper continental crust (UCC) ratios: ( La Yb ) N ≈ 10 and Eu Eu ★ ≈ 0.66. The paleosol REE patterns are similar to those of the loess, with the same LREE/HREE fractionation, but they also show distinct and variable negative Ce anomalies. Nd and Sr isotopic compositions are rather uniform in both loess and paleosols and do not vary with stratigraphic position (ϵNd = −10.5 to −9.2; 87 Sr 86 Sr = 0.715 to 0.719). The isotopic homogeneity in the Luochuan sequence strongly suggests a uniform source region during the entire period of deposition since ∼ 800 ka ago to the present. Several conclusions may be drawn from the present study: 1. (1) No geochemical distinction can be made between loess layers L1 to L7. The sources and the dust storm trajectories must have been essendally the same for the last 800 ka. 2. (2) Some elements, (e.g., Ca, Rb, Sr, U, Ce) are strongly fractionated by pedogenesis between loess and paleosols. Ce mobility in soils is clearly demonstrated but the process responsible for this leaching is still poorly understood. 3. (3) Systematic variations of element abundances and ratios between loess and paleosols can be used as chemical indicators for pedogenesic intensity and so for paleoclimatic change. These chemical indicators serve a function similar to that of oxygen isotopes in deep-sea sediments (pelagic foraminifera), or that of magnetic susceptibility in loess sequences. They are different recorders of paleoclimatic change. 4. (4) The striking uniformity of REE patterns and La Th ratios in the Luochuan loess and paleosols, as well as in loess worldwide, is an excellent starting point for estimating the average composition of the upper continental crust.

Asian summer monsoon instability during the past 60,000 years : magnetic susceptibility and pedogenic evidence from the western Chinese Loess Plateau

Abstract The 28 m high-resolution Shajinping loess section in Lanzhou on the western Chinese Loess Plateau records a 60 ka, millennial summer monsoon variation. The record shows that Asian summer monsoons have rapid episodic pulse enhancements spanning only ca. 1–2 ka in high-frequency domain and having sub-Milankovitch cycles of progressive weakening in low frequency domain in the last glaciation. Soil formation seems to occur with a surprisingly fast response to these summer monsoon enhancements, resulting in weakly or moderately developed paleosol sequences. Both the pattern and timing of the summer monsoon enhancements show that they can be correlated to most major warm (Dansgaard–Oeschger) episodes and long-term cooling (Bond) cycles of the North Atlantic climatic records, indicating a possible teleconnection between tropic oceanic air masses and the North Atlantic climatic system. But differences exist for the transition of MIS 2/3 and the Holocene, where extraordinarily heavy dust-input events and fairly variable climatic fluctuations occur for the former and latter, respectively. A westerlies-swing model is proposed to interpret this link.

Astronomically calibrated ages for geomagnetic reversals within the Matuyama chron

We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine δ18O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama/Brunhes boundary, 781 ± 3 ka (slightly above δ18O Stage 19.3); top of the Santa Rosa polarity interval, 920 ± 2 ka (Stage 23/24); base of the Santa Rosa polarity interval, 925 ± 1 ka (Stage 24); top of the Jaramillo subchron, 988 ± 3 ka (Stage 27); base of the Jaramillo subchron, 1072 ± 2 ka (Stage 31); top of the Cobb Mountain subchron, 1173 ± 4 ka (Stage 35/36); base of the Cobb Mountain subchron, 1185 ± 5 ka (Stage 36); top of the Olduvai subchron, 1778 ± 3 ka (Stage 63/64); base of the Olduvai subchron, 1945 ± 4 ka (Stage 71/72); top of the Réunion II subchron, 2118 ± 3 ka (Stage 80/81); and base of the Réunion II subchron, 2133 ± 5 ka (Stage 81). This astronomically calibrated chronology independently confirms the ages of major reversals in recently published astronomically calibrated polarity timescales for the Matuyama chron. It also provides the first astronomically calibrated dates for the lower and upper reversals associated with the Cobb Mountain and Santa Rosa polarity intervals, respectively.

Magnetic discrimination of pyrrhotite‐ and greigite‐bearing sediment samples

By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite- and greigite-bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite-bearing samples showed unambiguous TB temperature spectra, greigite-bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero-field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation.

Inconsistent magnetic polarities between greigite- and pyrrhotite/magnetite-bearing marine sediments from the Tsailiao-chi section, southwestern Taiwan

Abstract To establish a magnetobiostratigraphy for a 620-m-thick middle to late Pleistocene mudstone sequence in the Lower Gutingkeng Formation of the Tsailiao-chi (TLC) section in southwestern Taiwan, we conducted paleomagnetic and mineral magnetic measurements, together with sediment granulometry and calcareous nannofossil identification. Paleomagnetic samples from 65 sites revealed two types of thermal demagnetization (25–400°C) behavior: (1) single-component stable characteristic remanence in magnetite- and pyrrhotite-dominated samples (Type S), and (2) abrupt changes in polarity when samples with significant greigite concentrations were heated above 320–340°C (Type C). The characteristic polarities derived from Type S samples and from magnetite-dominated Type C samples (obtained above 340°C) are consistent with those determined from nannofossil biostratigraphy. This implies that the NRM carried by magnetite and pyrrhotite is reliable. The essentially antiparallel remanence components in Type C samples below 340°C are attributed to greigite. The almost antiparallel direction could have resulted from delayed formation of greigite, but in this case, the different direction of this component must have resulted from variable remanence lock-in times. Alternatively, the opposite polarities may result from self-reversal, which warrants further investigation. Pyrrhotite and greigite may have both formed authigenically, but there is no clear explanation for the observed differences in direction.

Paleomagnetism of the Early Miocene Kani Group in southwest Japan and its implication for the opening of the Japan Sea

Paleomagnetic directions of a volcanic and sedimentary sequence of the Kani Basin, ranging from 22 to 15 Ma, show a clockwise declination shift of about 50{degree}. This result implies that detectable rotation of Southwest Japan did not occur in this period, but after 15 Ma. The initial opening of the Japan Sea before 15 Ma might have caused a southeastward translation of Southwest Japan, which was then followed by clockwise rotation between 16 and 14 Ma.

Variable shape of magnetic hysteresis loops in the Chinese loess-paleosol sequence

Shape of magnetic hysteresis loops of the Chinese loess-paleosol sequence is variable with low-field susceptibility and is weakly constricted in samples with intermediate susceptibility (~1.0 × 10−6 m3/kg). The analyses of the hysteresis loops show that both low- (ferrimagnetic) and high-coercivity (antiferromagnetic) components are present and the ferrimagnetic component dominate the magnetic characteristics. The ratio of ferrimagnetic over antiferromagnetic minerals (S ratio) and the superparamagnetic fraction increase with increasing susceptibility. Neither simple two-component mixtures of ferrimagnetic and antiferromagnetic minerals nor of single-domain and superparamagnetic grains fully explain the constricted hysteresis loops. We interpret the variation of the loop shape with susceptibility in the following way. When the ratio of the ferrimagnetic to antiferromagnetic contribution is relatively low (low susceptibility), the broad loop is controlled by lithogenic ferrimagnetic and antiferromagnetic minerals. For samples with intermediate susceptibility values, constricted shape originates from an addition of a broad loop from the lithogenic fraction and a narrow loop from a pedogenic fraction with high superparamagnetic content. Then with further susceptibility increase, the constricted shape almost disappears and the loop is dominated by the pedogenic fraction. The variation of hysteresis loop shape with susceptibility can be a useful indicator of the degree of pedogenesis for loess-paleosol samples.

Stratigraphy of multiple piston-core sediments for the last 30,000 years from Lake Biwa, Japan

Analysis of seven piston cores obtained from three sites in Lake Biwa provide a detailed stratigraphy of the lake sediments for the last 30 kyrs. Description of lithology and measurement of magnetic susceptibility reveal occurrence of volcanic ashes at ten horizons, that were visually observed or microscopically found from clayey sediments. Most ash layers are observed in multiple cores from the three sites and also in earlier cores, establishing correlation and age assignment of the core sediments. This result demonstrates usefulness of magnetic susceptibility data in finding dispersed volcanic ashes. The age-depth curves deduced from published tephra ages show that significant difference in sedimentation rates occurs in Lake Biwa. The central part of the depression is characterized by homogeneous sediments deposited with a high sedimentation rate. The 250-m-thick lacustrine clay in this sub-basin has great potential for high-resolution study of the past environment and paleomagnetism for the last 0.5 Myr.

Environmental magnetic record and paleosecular variation data for the last 40 kyrs from the Lake Biwa sediments, Central Japan

We have conducted paleomagnetic and environmental magnetic analysis of a sediment piston core recovered from Lake Biwa, central Japan. Tephrochronology and AMS radiocarbon dating showed that this core covers the time period since about 40 kyr BP. The variation of paleomagnetic direction shows a good agreement with the PSV record for the last 10 kyrs from the deeper water site (BIWA SV-3; Ali et al., 1999), although the amplitudes are subdued probably due to the relatively lower accumulation rate at the shallower site. Inclination lows of the pre-Holocene interval are correlated to PSV records reported from the marine sediments off Shikoku and in the Japan Sea. In addition, the variation of magnetic mineral concentration reflects environmental changes during the last glacial period. It is suggested that the flux of fine-grained magnetite, probably associated with greater precipitation, was increased during interstadial periods. The variation of anhysteretic remanent magnetization is likely correlated to the Dansgaard-Oeschger (D-O) cycles recorded in Greenland ice cores. An apparent swing of the PSV curve is recognized at about 27 ka, but evidence for the Mono Lake excursion at 32 ka around the D-O events 6 and 7 is unclear. Combination of the detailed paleomagnetic record and the sub-Milankovitch climate cycles thus provides better resolution for understanding geomagnetic secular variation and polarity excursions in space and time.

Diagenetic effect on magnetic properties of marine core sediments from the southern Okhotsk Sea

It is well known that magnetic records of marine sediments are affected to a certain degree by early diagenesis, the extent and mode of which are controlled by depositional environments. In order to investigate variations in the effects of early diagenesis, we have analyzed 11 gravity cores estimated to be Holocene in age that constitute a depth transect off the southern coast of the Okhotsk Sea. The rock magnetic parameters have peaks produced by the presence of volcanic ashes, basaltic granules and/or relatively coarser-grained layers. The concentration-dependent parameters display characteristic trends of a rapid decrease within the homogeneous silty clay zone in the middle parts of the cores, assuming that the total amount of magnetic minerals decreases through early diagenesis at this interval. This decrease is found in all of the cores from the transect (from both near-shore and offshore sites). In near-shore sites where larger volumes of organic materials are supplied compared to offshore sites, the depth of the decrease in magnetic minerals is relatively shallow. These results imply that the presence of organic materials accelerates the pyritization through the bacterial reduction of ferric ion, thereby causing the decreases in magnetic minerals to become shallower.