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1.
《Quaternary International》1999,53(1):103-110
The magnetic enhancement of paleosol is observed in the Chinese loess. The origin of this magnetic enhancement is still very uncertain. It is, however, a key problem in correctly understanding the paleoclimatic significance of changes of magnetic susceptibility and in transferring the magnetic signals to paleoclimatic parameters. Two main models have been proposed to explain the mechanism of magnetic enhancement in paleosols: a depositional model and a pedogenic model. Together with composition and concentration, grain size distribution of magnetic minerals also plays an important role to the magnetic enhancement of paleosols. Systematic susceptibility measurements were carried out on the samples of the upper part (S0 to top of L2) of three loess sections from Jixian, Xifeng, and Xining, along an east–west transect in the loess plateau, China. The samples with the highest value of magnetic susceptibility in S1 and the lowest value in L2 of each section were selected as the representatives. These representatives were separated into different grain size fractions based on Stokes’ law for coarse grains and by centrifuge for fine grains. Measurements of magnetic susceptibility and mass have been carried out on these fractions. Results show that for the loesses magnetic susceptibility changes little in the fractions with different grain size and for the paleosols it increases with decreasing grain size. The magnitude of changes is bigger in the east (Jixian and Xifeng) than that in the west (Xining). The fraction with the finest particle size in paleosols does not show very high magnetic susceptibility values. A new approach is used to estimate the contribution of each fraction to total magnetic susceptibility. The contribution comes mainly from coarse grains (>10 μm) for loess samples. It reaches about 90%. The main contribution comes, however, from the particles with medium size (10–0.2 μm). The very fine grained particle (<0.3 μm), which is considered to be with the pedogenic origin, contributes little to the bulk magnetic susceptibility, no more than 3% because of their very little amount. This approach provides a sounder basis for the study of the origin of the magnetic susceptibility enhancement in paleosols and of the paleoclimatic significance of magnetic susceptibility of loess and paleosols.  相似文献   

2.
A rock magnetic investigation was carried out on a sedimentary core taken from the distal portion of the Bengal Fan in order to reconstruct the South Asian monsoon variability during the past 800 kyr. The 10.2 m long piston core MR0503-PC3, recovered at a water depth of 4400 m, consists of clay to silty clay with minor amounts of nannofossils. An age model for the MR0503-PC3 core is established by correlating a relative paleointensity record of the core [Suganuma Y., Yamazaki, T., Kanamatsu, T., Hokanishi, N., 2008. Relative paleointensity record during the last 800 kyr from the equatorial Indian Ocean: implication for relationship between inclination and intensity variations. Geochemistry, Geophysics, Geosystems. 9, Q02011. doi:10.1029/2007GC001723.] to the global paleointensity stack “Sint-800” [Guyodo, Y., Valet, J.P., 1999. Global changes in intensity of the Earth's magnetic field during the past 800 kyr. Nature. 399, 249–252.]. The age model is consistent with the published ages of tephra layers intercalated in the core, and shows continuous sedimentation during the past 800 kyr.Temporal variations in rock magnetic proxies for the magnetic concentration (ARM, IRM, and HIRM), the grain size (Mrs/Ms), and the composition (S?0.3T and S?0.1T) show that the amount of fine-grained magnetite increased during interglacial stages, and then gradually decreased toward the following glacial maxima. This indicates that the supply of fine-grained magnetite probably originated from areal expansion and/or increased pedogenic activity in the Ganges and Brahmaputra Rivers catchment. Increases during warmer periods suggest intensification of the South Asian summer monsoon during interglacial stages. During marine isotope stages (MIS) 15–11, enhancement of fine-grained magnetite and increased hematite and maghemite contributions are observed. These suggest a significant intensification of the South Asian summer monsoon during this period. Our record and other paleoclimatic reconstructions mainly from the low and mid-latitudes suggest that a major climatic event possibly occurred prior to the mid-Brunhes event (MBE), but the timing is not synchronous.  相似文献   

3.
An abrupt climatic change during the MIS 5a/4 transition is evident in the loess records of China (S1/L1). Proxies including geochemical elements, grain size, soil color, magnetic susceptibility and carbonate (CaCO3) content indicate a warming interval, which lasted approximately 3 ka, during the MIS 5a/4 transition in both the Wangguan and Shagou loess sections, located in Sanmenxia (Henan Province) and Wuwei (Gansu Province), respectively. Both the winter and summer monsoon proxies demonstrate that this warming interval occurred at the same time in both sections (nearly 70.5–73.6 ka BP), with maximum warming from 71.4 to 72.0 ka BP. This study suggests a universal abrupt warming interval in the East Asia monsoon region at this time. Comparisons with marine, terrestrial and ice-core records indicate this event was very likely an abrupt global warming interval during the last glacial–interglacial transition.  相似文献   

4.
Rock magnetic properties have been measured in a loess–paleosol sequence in the Gorina Quarry, which previously have been suggested to contain records of the Brunhes and part of the Matuyama polarity chrons. Lowest susceptibility values (below 50 × 10−8 m3/kg) are generally related to intensive weathered horizons, whereas highest values are obtained in loess layers (250 × 10−8 m3/kg) as a result of the greater ferromagnetic content in the parent material. The frequency-dependent part of susceptibility ranges between 0.5% and 6.8%; the higher value was obtained in B horizons of paleosols, which can be attributed to superparamagnetic contributions. Hysteresis loops indicate that the differences in ferrimagnetic and paramagnetic content in the sequence reflect the degree of pedogenesis. The same conclusion can be drawn with isothermal remanent magnetization. This point is relevant for determining past climatic changes because the wind-blown titanomagnetites from Cordillera de Los Andes during glacial periods were altered during interglacial periods. The mode of pedogenesis appears not only to control such alterations but also to produce other magnetic minerals.  相似文献   

5.
A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess–paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200–300°C and 620–680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity (RPI) studies. RPI records have been constructed using natural remanent magnetization (NRM) intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility (NRM300/χ). The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM300/χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic (climatic) influence, so it might be unsuitable for a reliable paleointensity study.  相似文献   

6.
A magnetic investigation of a loess/paleosol sequences record in Ili area   总被引：1，自引：0，他引：1  
The pedogenic ferrimagnetic minerals have been considered to be the cause of magnetic susceptibility enhancement in loess deposits distributing in the Chinese Loess Plateau and Central Europe, while “wind intensity” mechanism is proposed to be responsible for the magnetic susceptibility enhancement of loess in Alaska and Siberia. However, the magnetic enhancement mechanism is still open for loess in Ili valley, Xinjiang Uygur Autonomous Region, China. To understand this, we conducted a rock magnetic investigation on Axike section that is located in Ili valley. Results show that transitional stage from magnetic (χlf) trough to magnetic (χlf) peak corresponds to soil units. The PSD and MD-grained magnetite dominate the magnetic properties of AXK sequences, and the main factor affecting magnetic properties is the concentration of ferrimagnetic fraction. For the “pedogenesis enhancement” and “wind intensity” model, it seems hard to explain the magnetic susceptibility enhancement in this area. For the concentration of fine-grained magnetite in magnetic mineral shows positive relationship with the intensity of pedogenesis, the magnetic parameters of loess deposit in Ili valley can be used to recover paleoclimatic variations.  相似文献   

7.
The Xiashu Loess, in comparison to the well-studied loess sequences in the Chinese Loess Plateau (CLP), provides a good opportunity for studying East Asian monsoon variations from a southern China perspective. Here we present a study of the iron oxide mineralogy of the Xiashu Loess using integrated geochemical and diffuse reflectance spectroscopy (DRS) measurements as well as magnetic data. Our results show that the free iron oxide (Fed) to total iron (Fet) ratio (Fed/Fet), hematite (Hm) to goethite (Gt) ratio (Hm/Gt) and saturation isothermal remanent magnetization (SIRM) to magnetic susceptibility (χ) ratio (SIRM/χ) all indicate particularly strong summer monsoons during the formation of paleosols PS5 and PS4 (equivalent to Marine Isotope Stage 13 and 11, respectively). However, magnetic susceptibility and Fed/Fet are not consistently reliable indicators of summer monsoon intensity for the whole section. Our results indicate that a multi-proxy approach can give a more reliable summer monsoon intensity reconstruction. The summer monsoon shows a cooling trend and a declining of precipitation from 0.5 to ～0.3 Ma, after which it becomes warmer and wetter towards the top of paleosol PS1 (equivalent to MIS 5). However, PS1 was formed under a relatively cooler temperature and wetter soil conditions in comparison to PS5 and PS4. Such supra-orbital variations in the East Asian summer monsoon superimposed on the effects of glacial–interglacial cycles in southern China are also reflected in the 0.4–0.5 Ma cycle of marine carbon isotopes in the global ocean, possibly indicating a strong link between terrestrial weathering and the global carbon cycle.  相似文献   

8.
《Quaternary Science Reviews》2003,22(2-4):225-244
The Thebes Section in unglaciated southwestern Illinois contains a well preserved ∼500 kyr loess–paleosol sequence with four loesses and three interglacial soils. Various magnetic, mineralogical, and elemental properties were analyzed and compared over the thickness of soil sola. These proxies for soil development intensity have the following trend: Yarmouth Geosol>Sangamon Geosol>modern soil. Quartz/plagioclase, Zr/Sr, and TiO2/Na2O ratios were most sensitive to weathering. Frequency dependent magnetic susceptibility and anhysteretic remanent magnetization, greatest in A horizons, also correspond well with soil development intensity. Neoformed mixed-layered kaolinite/expandables, suggestive of a warm/humid climate, were detected in the Sangamon and Yarmouth soil sola. Clay illuviation in soils was among the least sensitive indicators of soil development. Differences in properties among interglacial soils are interpreted to primarily reflect soil development duration, with climatic effects being secondary. Assuming logarithmic decreases in weathering rates, the observed weathering in the Sangamon Geosol is consistent with 50 kyr of interglacial weathering (Oxygen Isotope Stage 5) compared to 10 kyr for the modern soil (Oxygen Isotope Stage 1). We propose that the Yarmouth Geosol in the central Midwest formed over 180 kyr of interglacial weathering (including oxygen isotope stages 7, 9, and 11).  相似文献   

9.
This study tests if burnt soils and sediments can provide reliable records of geomagnetic field strength at the time of burning by carrying out an experiment to reproduce the prehistoric use of fire on a clayish soil substratum. Rock magnetic experiments showed that in the upper 0–1 cm of the central part of the burnt surface, remanence is a thermoremanent magnetization carried by single-domain magnetite and that samples are thermally stable. Fourteen specimens from that area were subjected to paleointensity experiments with the Coe method (1967). An intensity of 42.9 ± 5.7 μT was estimated below 440°C, whereas at higher temperatures magneto-mineralogical alterations were observed. Corresponding successful microwave intensity determinations from two specimens gave a mean value of 47.6 μT. Both results are in reasonable agreement with the expected field value of 45.2 μT. Burnt soils of archeological fires thus have the potential to record accurately the paleofield strength and may be useful targets for archeointensity investigations. Coincident results obtained from two different paleointensity determination methods support this conclusion.  相似文献   

10.
《Quaternary Research》2014,81(3):433-444
A high-resolution rock magnetic investigation was performed on the Chaona Quaternary loess/paleosol sequences in the Central Chinese Loess Plateau. Based on a newly developed independent unturned time scale and magnetic records, we reconstructed the history of the East Asia monsoons during the last 3 Ma and explored the middle Pleistocene climate transition (MPT). Rock magnetic results show that the loess layers are characterized by relatively high coercivity and remanent coercivity, lower magnetic susceptibility (MS), and that the paleosol layers are characterized by relatively high MS, saturation magnetization and remanent saturation magnetization. Spectrum analyses indicate that there are various periods in addition to orbital periodicities. According to the onset and stable appearance of 100 kyr period, we consider that the MPT recorded in this section began at ~ 1.26 Ma and was completed by ~ 0.53 Ma, which differs from previous investigations based on orbitally tuned time scales. The forcing mechanism for the MPT was more complicated than just the orbital forcing. We conclude that the rapid uplift of the Tibetan Plateau may have played an important role in the shift of periodicities during the middle Pleistocene.  相似文献   

11.
《Quaternary Science Reviews》2007,26(5-6):759-772
Quantitative reconstruction of the climatic history of the Chinese Loess Plateau is important for understanding present and past environment and climate changes in the Northern Hemisphere. Here, we reconstructed mean annual temperature (MAT) and mean annual precipitation (MAP) trends during the last 136 ka based on the analysis of phytoliths from the Weinan loess section (34°24′N, 109°30′E) near the southern part of the Loess Plateau in northern China. The reconstructions have been carried out using a Chinese phytolith–climate calibration model based on weighted averaging partial least-squares regression. A series of cold and dry events, as indicated by the reconstructed MAT and MAP, are documented in the loess during the last glacial periods, which can be temporally correlated with the North Atlantic Heinrich events. Our MAT and MAP estimations show that the coldest and/or driest period occurred at the upper part of L2 unit (Late MIS 6), where MAT dropped to ca 4.4 °C and MAP to ca 100 mm. Two other prominent cold-dry periods occurred at lower Ll-5 (ca 77–62 ka) and L1-1 (ca 23–10.5 ka) where the MAT and MAP decreased to about 6.1–6.5 °C and 150–370 mm, respectively, ca 6.6–6.2 °C and 400–200 mm lower than today. However, the highest MAT (average 14.6 °C, max. 18.1 °C) and MAP (average 757 mm, max. 1000 mm) occurred at Sl interval (MIS 5). During the interstadial of L1-4–L1-2 (MIS 3) and during the Holocene warm-wet period, the MAT was about 1–2 °C and MAP 100–150 mm higher than today in the Weinan region. The well-dated MAT and MAP reconstructions from the Chinese Loess Plateau presented in this paper are the first quantitatively reconstructed proxy record of climatic changes at the glacial–interglacial timescale that is based on phytolith data. This study also reveals a causal link between climatic instability in the Atlantic Ocean and climate variability in the Chinese Loess Plateau.  相似文献   

12.
Irene Zembo 《Sedimentary Geology》2010,223(3-4):206-234
The sedimentary record of the Val d'Agri basin is of great importance for understanding the Quaternary tectonic activity and climatic variability in the Southern Apennines. Changes in tectonic controls, sediment supply and climatic input have been identified. The interval from ～ 56 to ～ 43 ka was associated with asymmetric subsidence restricted to the north-eastern actively faulted margin of the basin and development of axial braided river and transverse alluvial fan systems. Short-lasting Mediterranean-type pedogenesis between ～ 43 and ～ 32 ka (MIS Stage 3) coexisted with progradation–aggradation of the southern alluvial fan deposits and southwards tilting of the basin floor. Aggradation ended with consumption of accommodation space after 32 ka. During a subsequent stage of decline of vegetation cover, possibly as a consequence of climatic cooling (probably MIS Stage 2), active progradation of alluvial fans occurred. Breakthrough of the basin threshold and entrenchment of the drainage network must therefore be attributed to a latest Pleistocene to Holocene age. The first stages of basin opening and fill, predating ～ 56 ka have only been inferred by stratigraphic considerations: the earliest lacustrine sedimentation should be middle Pleistocene or older in age. The following south-eastward basin widening allowed progradation of alluvial fan systems, which completely filled the lacustrine area (tentatively late middle Pleistocene). Pedogenesis in “Mediterranean-like” climate conditions caused the final development of a highly mature fersiallitic paleosol at the top of the fan surfaces, in areas of morpho-tectonic stability, plausibly during MIS Stage 5. The study results demonstrate the potential of applying a multidisciplinary approach in an intermontane continental settings marked by a relative rapid and constant tectonic subsidence and a high rate of sediment supply during the Pleistocene glacial–interglacial cycles.  相似文献   

13.
The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been studied for mineralogical and chemical composition to assess the range of their physical and chemical variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous concentration. Significant variation in composition has been documented with alteration. The most magnetic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 × 10?6 m3 kg?1, indicates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of the ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with an undesirably high Nb2O5 (0.28%), Cr2O3 (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara is compared with that from the nearby Manavalakurichi deposit of similar geological setting and provenance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO2 (56.31–66.45%) contents highlight the advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe3+/Fe2+ ratio compared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/leucoxene.  相似文献   

14.
Multivariate statistical analysis has been used for detailed examination of the relationship between the magnetic properties of Xuzhou urban topsoil, for example concentration-dependent properties (mass magnetic susceptibility (χ), susceptibility of anhysteretic remanent magnetization (χ ARM), saturation isothermal remanent magnetization (SIRM), soft remanent magnetization (SOFT), and frequency-dependent magnetic susceptibility (χ FD)) and feature-dependent properties (S −100 mT ratio, SIRM/χ ratio and F 300 mT ratio), and the concentrations of metals (Ti, Fe, Cr, Al, Ga, Pb, Sc, Ba, Li, Cd, Be, Co, Cu, Mn, Ni, V, Zn, Mo, Pt, Pd, Au, As, Sb, Se, Hg, Bi, Ag, and Sn), S, and Br in the soil. The results show that SIRM/χ ratios correlate best with the heavy metals (Hg, Cr, Sb, As, and Bi) which are mainly derived from coal-combustion emissions whereas χ FD correlates best with the metals (Al, Ti, V, Be, Co, Ga, Mn, and Li) which principally originate from soil parents. Concentration-dependent magnetic properties (χ ARM, χ, SIRM, and SOFT) correlate well with elements (Se, Pb, Cu, Zn, Fe, Ag, Sc, Ba, Mo, Br, S, Cd, Ni, etc.) which are mainly derived from road-traffic emissions. For the same chemical element, χ ARM, SIRM, and SOFT values are frequently better correlated than χ values, and χ ARM values are the best indicators of the concentrations of these elements associated with traffic emissions in this study area. In addition, S −100 mT ratios significantly correlate positively with Se, Sc, Pb, Cu, Zn, Mo, and S whereas F 300 mT ratios only correlate positively with Pt and negatively with Fe. These results confirm the suitability of different magnetic properties for characterizing the concentrations of heavy metals, S, and Br in Xuzhou urban topsoil.  相似文献   

15.
Hydrogen isotopic composition of n-alkanes was measured in sediments from an excavated profile of the Early Cretaceous Yixian Formation in Liaoning Province, NE China, aiming to assess the significance of the δD value of n-alkanes in ancient lacustrine sediments as the indicator for determining the source inputs of organic matters and paleoclimatic conditions. The δD values of n-alkanes are in the range of − 250‰ to − 85‰ and display an obvious three-stage variation pattern through the profile, which is consistent with the distribution of the dominated n-alkanes and the profile of their δ13C values. The δD and δ13C values of n-alkanes suggest that short-chain n-alkanes are primarily derived from photosynthetic bacteria and algae; n-C29 and n-C31 are mainly originated from terrestrial higher plants; n-C28 and n-C30 may be derived from the same precursor but via the different biological mechanism of hydrogen isotopic fractionation; while the source inputs of medium-chain n-alkanes are more complicated, with n-C23 being derived from some specific algae or biosynthesized by various aquatic organisms. The paleoclimatic conditions are reconstructed via two approaches. The reconstructed hydrogen isotopic values of lake water and meteoric water (expressed as δDLW and δDMW, respectively) were at the intervals of − 51.8‰ to 17.0‰ and − 118.1‰ to − 43.5‰, respectively, indicating a general climate transition from semi-arid to arid. The calculated ΔδDLW-MW values vary from 37.0‰ to 89.1‰ and display a similar but a significant large-scale variation trend with the ΔδDC23 ‐ long (− 28.8‰ to 85.0‰; long represents long-chain n-alkanes) and ΔδDmid-long (− 15.4‰ to 43.4‰; mid represents medium-chain n-alkanes) values. The discrepancy may be attributed to the source input overlap for n-alkanes and the uncertainties of εwater/lipid values. The coupling of ΔδDC23 ‐ long, ΔδDmid-long and ΔδDLW-MW values with the paleoclimatic evidence indicates that the δD values of n-alkanes could be more sensitive to the change of paleoclimatic conditions.  相似文献   

16.
This is a summary of new oxygen isotope record of diatoms from Lake Kotokel sediments, with implications for responses of the lake system and its environment to global change over the past 46 kyr. Fossil diatoms in all samples are free from visible contamination signatures and contain no more than 2.5% Al2O3, which ensures reliable reconstructions. The 518O values in diatoms vary between + 23.7 and + 31.2%c over the record. The results present mainly diatom assemblages of summer blooming periods, except for the time span between 36 and 32 kyr, when the isotopic signal records mainly a shift from summer to spring blooming conditions. Possible water temperature changes only partly explain the changes in the isotopic record. The observed isotopic patterns are produced mainly by isotope changes in lake water in response to variations in air temperature, hydrology, and atmospheric circulation in the region. During Marine Isotope Stage (MIS) 2 (Last Glacial maximum), high 518Odiatom resulted from rapid evaporation and low fluvial inputs. The high 518Odiatom values of about + 29 to + 30%c during the first half of MIS 1 (Holocene interglacial) suggest an increased share of summer rainfalls associated with southern/southeastern air transport. The 518Odiatom decrease to + 24%c during the second half of MIS 1 is due to the overall hemispheric cooling and increased moisture supply to the area by the Atlantic air masses. The record of Lake Kotokel sediments provides an example of complex interplay among several climatic/environmental controls of 518Odiatom during the Late Pleistocene and the Holocene.  相似文献   

17.
Variations in long chain alkenone-based sea surface temperature (SST) from a piston core (M04-PC1A) collected from the Korea Plateau in the East Sea (Sea of Japan) were investigated to understand paleoceanographic variations over the last 300,000 years. By combining sedimentological and geochemical proxies (the lithological marker of crudely laminated mud, alkenone SST, foraminiferal oxygen isotope values, and 14C age determination) and by comparison with previous works, we examined paleoceanographic variations back to Marine Isotope Stage (MIS) 8, approximately 300,000 years B.P. In particular, analysis of alkenones suggests that SSTs were about 8 °C and 11 °C lower during MIS 8 and MIS 6 than that in the present-day SST, respectively. Furthermore, SST was estimated to be 5 °C lower during the Last Glacial Maximum. These significant SST differences among MIS 8, 6, and 2 may be attributable to not only the formation of distinctive water masses, but also to differential alkenone synthesis under different environmental conditions. These results suggest that SSTs in the East Sea during the last three glacial periods (MIS 8, 6, and 2) were different, but rather were closely linked with regional oceanographic conditions overlapped with sensitive responses to the intensity of the East Asian monsoon. Surface-water freshening was a local paleoceanographic consequence that was imprinted in the core during MIS 2 and MIS 6, and potentially during MIS 4. Furthermore, alkenone-based SST data suggested that surface water circulation and biological productivity were strongly associated with the inflow of the Tsushima Warm Current during interglacial periods.  相似文献   

18.
Western tropical Pacific sea surface temperatures and Pacific Deep Water temperatures during Marine Isotope Stage 3 have been reconstructed from the δ18O and Mg/Ca of planktonic and benthic foraminifera from Marion Dufresne core MD98-2181. This 36 m marine core was collected at 6.3°N from a water depth of 2114 m. With sediment accumulation rates of up to 80 cm/ky, it provides a decadally resolved history of ocean variability during the Last Glacial period. Surface temperatures and salinities at this site varied in close association with millennial-scale atmospheric temperature swings at high northern latitudes as reflected in the GISP2 ice core. At times of colder atmospheric temperatures over Greenland, the western Pacific was more saline and summer season SSTs were ～2 °C colder. These millennial-scale changes within the tropics are attributed to a southward displacement of the summer season ITCZ in response to steeper meridional temperature gradients within the Pacific. The benthic δ18O record from MD98-2181 documents upper Pacific Deep Water temperature and salinity variability. Benthic δ18O variations of 0.3–0.5‰ during MIS 3 indicate deep waters within the Pacific were varying by ～1–1.5 °C, with the possibility that some of the variability was due to changing salinity and minor glacial–eustatic changes. The observed deep-water variability correlates to changes in Antarctic surface temperatures and thus reflects changes in Southern Ocean temperatures at the site of Pacific Deep Water formation. The combined planktonic and benthic records from MD98-2181 thus provide a northern and southern hemispheric climate record of anti-phased variability during MIS 3 as has been inferred previously from ice core records. Furthermore, the deep sea temperature excursions appear to have led millennial variations in atmospheric CO2 as recorded in the EDML ice core by ～1 kyr.  相似文献   

19.
《Quaternary Science Reviews》2003,22(18-19):1859-1878
The Chinese Loess Plateau (CLP) contains an extensive record of aeolian deposition through multiple glacial–interglacial cycles. Independent chronologies based on pedostratigraphy, magnetic susceptibility, radiocarbon and luminescence dating were developed for 79 sites and used to estimate aeolian mass accumulation rates (MARs) for marine isotope stages 1–5. The regional median value of MAR for Stage 2 is 310 g/m2/yr compared to an estimate of 65 g/m2/yr for Stage 5. Estimated MARs from individual sites for Stage 2 are approximately 4.3 times greater than MARs for Stage 5 and 2.1 times greater than for Stage 1. MAR values at individual sites are consistently highest in the northwest and lowest in the southwest of the CLP during all marine isotope stages. MARs estimated on sections through loess terraces are consistently higher than MAR estimates at other sites, indicating that local recycling of loess material from exposed river valley deposits has been significant throughout the last 130 kyr. Although the spatial and temporal patterns in MAR are robust, there are uncertainties about the magnitude of these changes due to (a) lack of bulk density measurements and uncertainties in the chronologies for individual sites, (b) site and chronological biases in the sampling used to derive regional estimates, and (c) the unquantified nature of human impact on accumulation rates during the late Holocene. Nevertheless, the records from the CLP pose a number of challenges which could be addressed by numerical models of the palaeo-dust cycle.  相似文献   

20.
《Quaternary Science Reviews》2007,26(1-2):142-154
We present chironomid-based temperature reconstructions from lake sediments deposited between ca 26,600 cal yr BP and 24,500 cal yr BP from Lyndon Stream, South Island, New Zealand. Summer (February mean) temperatures averaged 1 °C cooler, with a maximum inferred cooling of 3.7 °C. These estimates corroborate macrofossil and beetle-based temperature inferences from the same site and suggest climate amelioration (an interstadial) at this time. Other records from the New Zealand region also show a large degree of variability during the late Otiran glacial sequence (34,000–18,000 cal yr BP) including a phase of warming at the MIS 2/3 transition and a maximum cooling that did not occur until the global LGM (ca 20,000 cal yr BP).The very moderate cooling identified here at the MIS 2/3 transition confirms and enhances the long-standing discrepancy in New Zealand records between pollen and other proxies. Low abundances (<20%) of canopy tree pollen in records from late MIS 3 to the end of MIS 2 cannot be explained by the minor (<5 °C) cooling inferred from this and other studies unless other environmental parameters are considered. Further work is required to address this critical issue.  相似文献   

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