Quaternary Research

Using the concept of “orbital tuning”, a continuous, high-resolution deep-sea chronostratigraphy has been developed spanning the last 300,000 yr. The chronology is developed using a stacked oxygen-isotope stratigraphy and four different orbital tuning approaches, each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data. Each approach yields a separate chronology. The error measured by the standard deviation about the average of these four results (which represents the “best” chronology) has an average magnitude of only 2500 yr. This small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used. Excellent convergence between chronologies developed using each of five different paleoclimatological indicators (from a single core) is also obtained. The resultant chronology is also insensitive to the specific indicator used. The error associated with each tuning approach is estimated independently and propagated through to the average result. The resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr, in excellent agreement with the 2500-yr estimate. Transfer of the final chronology to the stacked record leads to an estimated error of ±1500 yr. Thus the final chronology has an average error of ±5000 yr.

Particle-size measurements of some typical loess-soil samples taken in different localities of the Chinese Loess Plateau demonstrate that the grain size ratio of <2 μm/>10 μm (%) can be used as an indicator of variations in intensity of the East Asian winter monsoon winds. Grain-size curves of the Baoji and Weinan sections show that this proxy indicator is very sensitive to loess-soil alterations. Analytical results also suggest that during soil-forming periods, eolian dust accumulation was still substantial and, hence, loess deposition can be regarded as a nearly continuous process during the Quaternary period. In this study we compared the Baoji grain-size time series with the SPECMAP marine isotope record with the objective of elucidating the dynamic linkage between changes in global ice volume and the winter monsoon circulation. Both records show good agreement at both time and frequency domains. In particular, the winter monsoon variations are also dominated by a 100,000 yr period over the past 800,000 yr. It is thus inferred that direct local insolation forcing could be less important in driving the East Asian winter monsoon variability, and, alternatively, variations in glacial-age boundary conditions may have played a key role in modulating and pacing its strength and timing.

Radiocarbon-dated macrofossils are used to document Holocene treeline history across northern Russia (including Siberia). Boreal forest development in this region commenced by 10,000 yr B.P. Over most of Russia, forest advanced to or near the current arctic coastline between 9000 and 7000 yr B.P. and retreated to its present position by between 4000 and 3000 yr B.P. Forest establishment and retreat was roughly synchronous across most of northern Russia. Treeline advance on the Kola Peninsula, however, appears to have occurred later than in other regions. During the period of maximum forest extension, the mean July temperatures along the northern coastline of Russia may have been 2.5° to 7.0°C warmer than modern. The development of forest and expansion of treeline likely reflects a number of complimentary environmental conditions, including heightened summer insolation, the demise of Eurasian ice sheets, reduced sea-ice cover, greater continentality with eustatically lower sea level, and extreme Arctic penetration of warm North Atlantic waters. The late Holocene retreat of Eurasian treeline coincides with declining summer insolation, cooling arctic waters, and neoglaciation.

The reef-crest coral Acropora palmata from late Pleistocene reefs on Barbados has recorded the same global variations in oxygen isotopes as planktonic and benthonic foraminifera. Although the record of oxygen isotopes in Acropora palmata is discontinuous, it offers several advantages over the isotope records from deep-sea sediments: (1) the coral grows at water depths of less than 5 m; (2) the samples are unmixed; (3) specimens may be sampled from various elevations of paleo-sea level; and (4) aragonitic corals are suitable for ²³⁰Th/²³⁴U and He/U dating techniques. The latter advantage means that direct dating of the marine oxygen isotope record is possible. Oxygen isotope stage 5e corresponds to Barbados III, dated at 125,000 ± 6000 yr BP. Petrographic and geochemical evidence from five boreholes drilled into the south coast of Barbados indicates a major eustatic lowering (greater than 100 m below present sea level) occurred between 180,000 and 125,000 yr BP. The age and isotopic data suggest correlation of this change in sea level to Emiliani's oxygen isotope stage 6. Acropora palmata deposited at various elevations of sea level during oxygen isotope stage 6 vary by 0.11 ‰ δ¹⁸O for each 10 m of change in sea level. We further hypothesize a minimum drop of 2°C in the average temperature occurred during the regressive phase of oxygen isotope stage 6. These data indicate that temperature lowering of surface water near Barbados lagged behind a major glacial buildup during this time period. Using the δ¹⁸O vs sea level calibration herein derived, we estimate the relative height of sea stands responsible for Barbados coral reef terraces in the time range 80,000 to 220,000 yr BP.

Lake Lisan, the late Pleistocene precursor of the Dead Sea, existed from ∼70,000 to 15,000 yr B.P. It evolved through frequent water-level fluctuations, which reflected the regional hydrological and climatic conditions. We determined the water level of the lake for the time interval ∼55,000–15,000 cal yr B.P. by mapping offshore, nearshore, and fan-delta sediments; by application of sequence stratigraphy methods; and by dating with radiocarbon and U-series methods. During the studied time interval the lake-level fluctuated between ∼340 and 160 m below mean sea level (msl). Between 55,000 and 30,000 cal yr B.P. the lake evolved through short-term fluctuations around 280–290 m below msl, punctuated (at 48,000–43,000 cal yr B.P.) by a drop event to at least 340 m below msl. At ∼27,000 cal yr B.P. the lake began to rise sharply, reaching its maximum elevation of about 164 m below msl between 26,000 and 23,000 cal yr B.P., then it began dropping and reached 300 m below msl at ∼15,000 cal yr B.P. During the Holocene the lake, corresponding to the present Dead Sea, stabilized at ca. 400 m below msl with minor fluctuations. The hypsometric curve of the basin indicates that large changes in lake area are expected at above 403 and 385 m below msl. At these elevations the lake level is buffered. Lake Lisan was always higher than 380 m below msl, indicating a significantly large water contribution to the basin. The long and repetitious periods of stabilization at 280–290 m below msl during Lake Lisan time indicate hydrological control combined with the existence of a physical sill at this elevation. Crossing this sill could not have been achieved without a dramatic increase in the total water input to the lake, as occurred during the fast and intense lake rise from ∼280 to 160 m below msl at ∼27,000 cal yr B.P.

The origins of the magnetic susceptibility variations of the Chinese loess and paleosols are explored by scanning and transmission electron microscopy of magnetic extracts, and by magnetic modeling of magnetic hysteresis data, to provide quantified estimates of the major magnetic components. Microscopy identifies several distinct size and shape characteristics in the magnetic carriers. Lithogenic magnetites, intact and abraded, dominate the coarse-grained magnetic fraction. The smallest of the coarse grains is ∼ 2 μm. The remaining magnetic materal is ultrafine in size, with two types of magnetite particles present. Type A particles strongly resemble soil magnetites produced by inorganic precipitation. Type B particles, which occur rarely, are probably bacterial in origin. Quantitative modeling of these magnetic assemblages shows that over 90% of the susceptibility variations is accounted for by the superparamagnetic magnetite component. Compared to the loess units, the paleosols are richer in magnetite, particularly of superparamagnetic size, and have a threefold higher ratio of magnetite to hematite. We identify pedogenic formation of magnetite as the major contributor to the loess magnetic record. Matching this record against other paleoclimatic records, we find an extremely high correlation with the standard ¹⁸O record. The Chinese loess sequences record a very high resolution magnetic stratigraphy directly related to changing climate.

Millennium-old alerce trees (Fitzroya cupressoides (Mol.) Johnst.) have been used to develop a 1120-year reconstruction of the summer temperature departures for the Andes of northern Patagonia in Argentina. Four main climatic episodes can be distinguished in this proxy paleoclimatic record. The first, a cold and moist interval from A.D. 900 to 1070, was followed by a warm-dry period from A.D. 1080 to 1250 correlative with the Medieval warm epoch of Europe. Afterward, a long, cold-moist period followed from A.D. 1270 to 1670, peaking around A.D. 1340 and 1650. These cold maxima are contemporaneous with two principal Little Ice Age events registered in the Northern Hemisphere. Warmer conditions then resumed between A.D. 1720 and 1790. These episodes are supported by glaciological and palynological data in Patagonia. Following a cold period in the early 1800s, tree-ring indices have oscillated around the long-term mean, except for a warmer period from A.D. 1850 to 1890. Correlations between the Rio Alerce reconstruction and the regional weather stations indicate that the tree-ring variations are correlated with a homogeneous summer weather pattern covering Patagonia east of the Andes from 38° to 50°S.

Lacustrine sediments from southeastern Arabia reveal variations in lake level corresponding to changes in the strength and duration of Indian Ocean Monsoon (IOM) summer rainfall and winter cyclonic rainfall. The late glacial/Holocene transition of the region was characterised by the development of mega-linear dunes. These dunes became stabilised and vegetated during the early Holocene and interdunal lakes formed in response to the incursion of the IOM at approximately 8500 cal yr BP with the development of C3 dominated savanna grasslands. The IOM weakened ca. 6000 cal yr BP with the onset of regional aridity, aeolian sedimentation and dune reactivation and accretion. Despite this reduction in precipitation, the lake was maintained by winter dominated rainfall. There was a shift to drier adapted C4 grasslands across the dune field. Lake sediment geochemical analyses record precipitation minima at 8200, 5000 and 4200 cal yr BP that coincide with Bond events in the North Atlantic. A number of these events correspond with changes in cultural periods, suggesting that climate was a key mechanism affecting human occupation and exploitation of this region.

The time series of volcanically produced sulfate from the GISP2 ice core is used to develop a continuous record of explosive volcanism over the past 110,000 yr. We identified ∼850 volcanic signals (700 of these from 110,000 to 9000 yr ago) with sulfate concentrations greater than that associated with historical eruptions from either equatorial or mid-latitude regions that are known to have perturbed global or Northern Hemisphere climate, respectively. This number is a minimum because decreasing sampling resolution with depth, source volcano location, variable circulation patterns at the time of the eruption, and post-depositional modification of the signal can result in an incomplete record. The largest and most abundant volcanic signals over the past 110,000 yr, even after accounting for lower sampling resolution in the earlier part of the record, occur between 17,000 and 6000 yr ago, during and following the last deglaciation. A second period of enhanced volcanism occurs 35,000–22,000 yr ago, leading up to and during the last glacial maximum. These findings further support a possible climate-forcing component in volcanism. Increased volcanism often occurs during stadial/interstadial transitions within the last glaciation, but this is not consistent over the entire cycle. Ages for some of the largest known eruptions 100,000–9000 yr ago closely correspond to individual sulfate peaks or groups of peaks in our record.

Lake records from northern Eurasia show regionally coherent patterns of changes during the late Quaternary. Lakes peripheral to the Scandinavian ice sheet were lower than those today but lakes in the Mediterranean zone were high at the glacial maximum, reflecting the dominance of glacial anticyclonic conditions in northern Europe and a southward shift of the Westerlies. The influence of the glacial anticyclonic circulation attenuated through the late glacial period, and the Westerlies gradually shifted northward, such that drier conditions south of the ice sheet were confined to a progressively narrower zone and the Mediterranean became drier. The early Holocene shows a gradual shift to conditions wetter than present in central Asia, associated with the expanded Asian monsoon, and in the Mediterranean, in response to local, monsoon-type circulation. There is no evidence of mid-continental aridity in northern Eurasia during the mid-Holocene. In contrast, the circum-Baltic region was drier, reflecting the increased incidence of blocking anticyclones centered on Scandinavia in summer. There is a gradual transition to modern conditions after ca. 5000 yr B.P. Although these broad-scale patterns are interrupted by shorter term fluctuations, the long-term trends in lake behavior show a clear response to changes in insolation and glaciation.