Dependence of Near-Surface Magnetic Susceptibility on Dust Accumulation Rate and Precipitation on the Chinese Loess Plateau

Magnetic susceptibility (MS) of surface sediment varies systematically across the Loess Plateau in central China, decreasing exponentially from >200×10⁻⁸ m³/kg at the northern margin of the Qinling Shan to ≤30×10⁻⁸ m³/kg near the southern margin of the Mu Us Desert. MS correlates highly with loess median grain size (r²=0.79), which decreases south-southeastward across the plateau. It also correlates with mean annual temperature (MAT) and mean annual precipitation (MAP) (r²=0.58 and 0.60, respectively), and with their product MAT×MAP (r²=0.83), which is considered a measure of potential pedogenic activity. Because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel, it is difficult to determine their relative influence on MS. A simple MS model, based on the observed spatial variation in loess thickness, permits quantitative assessment of the effect of the dust accumulation rate on the MS signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals. The model suggests that 84% of the loess MS variance is dictated by the diluting effect of dust and 10–11% is associated with meteorological factors, primarily precipitation. The observed and modeled relationships support hypotheses that attribute variations in MS in the loess-paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils, both of which are controlled by monsoon climate.     

Composition and Flux of Holocene Sediments on the Eastern Laptev Sea Shelf, Arctic Siberia

A 467-cm-long core from the inner shelf of the eastern Laptev Sea provides a depositional history since 9400 cal yr. B.P. The history involves temporal changes in the fluvial runoff as well as postglacial sea-level rise and southward retreat of the coastline. Although the core contains marine fossils back to 8900 cal yr B.P., abundant plant debris in a sandy facies low in the core shows that a river influenced the study site until 8100 cal yr B.P. As sea level rose and the distance to the coast increased, this riverine influence diminished gradually and the sediment type changed, by 7400 cal yr B.P., from sandy silt to clayey silt. Although total sediment input decreased in a step-like fashion from 7600 to 4000 cal yr B.P., this interval had the highest average sedimentation rates and the greatest fluxes in most sedimentary components. While this maximum probably resulted from middle Holocene climate warming, the low input of sand to the site after 7400 cal yr B.P. probably resulted from further southward retreat of the coastline and river mouth. Since about 4000 cal yr B.P., total sediment flux has remained rather constant in this part of the Laptev Sea shelf due to a gradual stabilization of the depositional regime after completion of the Holocene sea-level rise.     

Extensive Early and Middle Wisconsin Glaciation on the Western Olympic Peninsula, Washington, and the Variability of Pacific Moisture Delivery to the Northwestern United States

Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,000 ¹⁴C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,000 ¹⁴C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,300 ¹⁴C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,300 ¹⁴C yr B.P.; the Twin Creeks 1 advance, 19,100–18,300 ¹⁴C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.     

Late Pleistocene Glaciations in the Northwestern Sierra Nevada, California

Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 ¹⁰Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 ¹⁰Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.     

Late Pleistocene Environments of the Central Ukraine

The Vyazivok loess sequence from the Dnieper Plain, Ukraine, documents regional environmental changes during the late Pleistocene and Holocene. Pedological and palynological analyses and low-field magnetic susceptibility document changes from dense temperate forest during the last interglacial maximum to open, harsh, loess–steppe during the latest Pleistocene. The Vyazivok section overlies hillwash derived from a lower Pleistocene terrace and consists of two stratified soil complexes (Kaydaky and Pryluky; marine isotope stage [MIS] 5 equivalent) separated by a layer of eolian dust (Tyasmyn silt). The lower soils in both complexes formed within forest. These soils are overlain by the Uday (MIS4) and Bug (MIS2) loess units, which are separated by boreal soils of the Vytachiv (MIS3) complex. The coldest conditions within the record occurred in the youngest loess. Holocene soils cap the Bug loess. The Vyazivok section shows remarkable similarities with other classical loess sequences in western Europe, the Czech Republic, and Austria. The Kaydaky, Pryluky, and Vytachiv deposits, correlate with the PKIII, PKII, and PKI soil complexes, respectively, of the Czech Republic. The Tyasmyn and Prylyky silt layers correspond to marker horizons from central Europe.     

Late Quaternary Upwelling Intensity and East Asian Monsoon Forcing in the South China Sea

Foraminifera from two cores off eastern Vietnam and the northwestern Philippines, where modern summer and winter monsoon-driven upwelling occurs in the South China Sea, respectively, were analyzed to evaluate the changes in paleoproductivity and upper water structure over the last 220,000 yr. We observed enhanced organic carbon flux and a shoaled thermocline when upwelling intensified off eastern Vietnam during interglacial ages and off the northwestern Philippines during glacial ages. This indicates that the East Asian summer monsoon increased while the winter monsoon decreased during interglacial ages. Particularly, the upwelling reached a maximum off eastern Vietnam during late marine isotopic stage (MIS) 5 and off the northwestern Philippines during MIS 2, implying that the summer monsoon decreased gradually since MIS 5 while the winter monsoon displayed an opposite trend. The variations in upwelling proxies exhibit a distinct cyclicity with frequencies near 41,000 yr and 23,000 yr off eastern Vietnam, in contrast to a strong frequency peak near 100,000 yr off the northwestern Philippines. We suggest that the East Asian summer monsoon has been forced by changes in solar insolation associated with precession and obliquity, while ice-volume forcing is probably a primary factor in determining the strength and timing of the East Asian winter monsoon but with less important insolation forcing.     

Changes in the Thermal Climate in Northwestern Europe during Marine Oxygen Isotope Stage 3, Estimated from Fossil Insect assemblages

More than 30 fossil coleopteran (beetle) assemblages have been recorded from oxygen isotope stage 3 in northern Europe, comprising several hundred identified species. Using the mutual climatic range method for quantifying palaeotemperatures, these assemblages show that the climates of the times can be divided into two distinct interstadial types. One, a short phase of temperate/oceanic climate occurred between 43,000 and 42,000 ¹⁴C yr B.P. when conditions were not much different from those of the present day. In spite of the temperate climate, trees did not colonize northwestern Europe at this time. Two, both before and after this temperate interlude there were phases of cold/continental climate during which summers were too cold for trees to grow and winters were of Siberian intensity. It is possible that periods of even colder conditions intervened from time to time between the interstadial but, because of the intensity of the cold, biological systems were so inhibited that no fossil evidence for them can be found in northern Europe. Evidence for them must be sought in the fossil records from the warmer parts of southern Europe or in the physical evidence in the Greenland ice cores.     

A High-Resolution Sea-Surface Temperature Record from the Tropical South China Sea (16,500–3000 yr B.P.)

The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, ¹⁴C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (U^k′₃₇) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the U^k′₃₇ SST estimates show an average late glacial–interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial–interglacial SST change, respectively. Both the U^k′₃₇ and the FP-12E SST estimates, as well as the planktonic foraminiferal δ¹⁸O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the U^k′₃₇ SST estimates show a cooling of ca. 0.2–0.6°C compared to the Bølling–Allerød period. These U^k′₃₇ SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period.     

Small-Mammal Data on Early and Middle Holocene Climates and Biotic Communities in the Bonneville Basin, USA

Archaeological investigations in Camels Back Cave, western Utah, recovered a series of small-mammal bone assemblages from stratified deposits dating between ca. 12,000 and 500 ¹⁴C yr B.P. The cave's early Holocene fauna includes a number of species adapted to montane or mesic habitats containing grasses and/or sagebrush (e.g., Lepus townsendii, Marmota flaviventris, Reithrodontomys megalotis, and Brachylagus idahoensis) which suggest that the region was relatively cool and moist until after 8800 ¹⁴C yr B.P. Between ca. 8600 and 8100 ¹⁴C yr B.P. these mammals became locally extinct, taxonomic diversity declined, and there was an increase in species well-adapted to xeric, low-elevation habitats, including ground squirrels, Lepus californicus and Neotoma lepida. The early small-mammal record from Camels Back Cave is similar to the 11,300–6000 ¹⁴C yr B.P. mammalian sequence from Homestead Cave, northwestern Utah, and provides corroborative data on Bonneville Basin paleoenvironments and mammalian responses to middle Holocene desertification.     

Chronology of Inland Eolian Dunes on the Coastal Plain of Georgia, USA

Most inland eolian dunes associated with rivers on the Georgia Coastal Plain probably date to glacial periods. Direct dating of the dune sand by optically stimulated luminescence dating, combined with limiting ages from radiocarbon dating, shows that dunes formed during isotope stage 2, stage 3, the transition between stages 4 and 5, and isotope stage 6 or earlier. Most of the dates indicate dune activity between 30,000 and 15,000 years ago. Holocene activity included limited and local reworking of the crests of some thick dunes along streams that flow from the Piedmont.     

Comparison of the Climate during Marine Isotope Stage 9 and 11 Inferred from a Speleothem Isotope Record from Northern Norway

A stalagmite from northern Norway is dated with 12 thermal ionization mass spectrometry U-Th dates, and at least four separate growth periods are identified that correspond with marine isotope stages 9, 11, 13, and probably 15. The calcite is tested for isotopic equilibrium with the Hendy test. Oxygen isotope measurements on 231 subsamples on a vertical transect are used as a paleotemperature proxy. The detailed isotopic record from MIS 9 show apparent similarities to a Holocene record from the same cave, both in the climatic evolution and the overall temperatures: both show temperature oscillations changing from high-frequency, low-amplitude cycles in the beginning of the interglacial period to lower frequency, higher amplitude cycles in the later part of the interglacial period. The isotope record from MIS 11 shows a distinct isotopic event toward heavier values. The isotopic record together with the porous, humus-rich calcite are interpreted as indicating a warmer than present interglacial period with several episodes of heavy rainfall.     

Environments of the Russian Plain during the Middle Valdai Briansk Interstade (33,000–24,000 yr B.P.) Indicated by Fossil Mammals and Plants

Abundant mammal (45 localities) and plant (52 sections) fossil data collected from the Russian Plain deposits of the Middle Valdai Briansk Interstade indicate a period of noticeable warming. These materials were jointly analyzed. The data were organized in the database software PARADOX and then moved to the GIS program ARC/INFO. Mathematical methods as well as traditional research methods were used for the analysis. The reconstructed biogeographical provinces illustrate the specific environmental and climatic conditions of the Briansk Interstade. Analogues of modern natural zones did not exist at this time on the Russian Plain. The Briansk landscapes reflect the moderate-cool climate of this period, when the majority of northern subarctic plant and mammal species extended their ranges considerably to the south to the Central Russian Plain. At the same time, steppe species penetrated farther north and west. This combination shows that the continuous forest zone was destroyed during the Briansk Inter-stade. Five biogeographical provinces have been reconstructed from the north to the south for the Russian Plain during the Briansk warming on the basis of joint analysis of mammalian and botanical data.     

Determination of Cl Production Rates Derived from the Well-Dated Deglaciation Surfaces of Whidbey and Fidalgo Islands, Washington

The ³⁶Cl dating method is increasingly being used to determine the surface-exposure history of Quaternary landforms. Production rates for the ³⁶Cl isotopic system, a critical component of the dating method, have now been refined using the well-constrained radiocarbon-based deglaciation history of Whidbey and Fidalgo Islands, Washington. The calculated total production rates due to calcium and potassium are 91±5 atoms ³⁶Cl (g Ca)⁻¹ yr⁻¹ and are 228±18 atoms ³⁶Cl (g K)⁻¹ yr⁻¹, respectively. The calculated ground-level secondary neutron production rate in air, P_f(0), inferred from thermal neutron absorption by ³⁵Cl is 762±28 neutrons (g air)⁻¹ yr⁻¹ for samples with low water content (1–2 wt.%). Neutron absorption by serpentinized harzburgite samples of the same exposure age, having higher water content (8–12 wt.%), is 40% greater relative to that for dry samples. These data suggest that existing models do not adequately describe thermalization and capture of neutrons for hydrous rock samples. Calculated ³⁶Cl ages of samples collected from the surfaces of a well-dated dacite flow (10,600–12,800 cal yr B.P.) and three disparate deglaciated localities are consistent with close limiting calibrated ¹⁴C ages, thereby supporting the validity of our ³⁶Cl production rates integrated over the last 15,500 cal yr between latitudes of 46.5° and 51°N. Although our production rates are internally consistent and yield reasonable exposure ages for other localities, there nevertheless are significant differences between these production rates and those of other investigators.     

Rates of Deglaciation during the Last Glaciation and Holocene in the Cordillera Vilcanota-Quelccaya Ice Cap Region, Southeastern Perú

Moraine chronology is combined with digital topography to model deglacial rates of paleoglacier volumes in both the Huancané Valley on the west side of the Quelccaya Ice Cap and the Upismayo Valley on the northwest side of the Cordillera Vilcanota. The fastest rates of deglaciation (39×10⁻⁵ to 114×10⁻⁵ km³ yr⁻¹ and 112×10⁻⁵ to 247×10⁻⁵ km³ yr⁻¹ for each valley, respectively) were calculated for the most recent paleoglaciers, corresponding to the last few centuries. These results are consistent with observations in the Venezuelan Andes showing high rates of deglaciation since the Little Ice Age. These rates also fall within the range of 20th century rates of deglaciation measured on the Quelccaya Ice Cap (29×10⁻⁵ to 220×10⁻⁵ km³ yr⁻¹, Brecher and Thompson, 1993; Thompson, 2000). These results imply that rates of deglaciation may fluctuate significantly over time and that high rates of deglaciation may not be exclusive to the late 20th century. Equilibrium line altitude (ELA) depressions for the ice volumes of the last glaciation modeled here were computed as 230 m for the Quelccaya Ice Cap and 170 m for the Cordillera Vilcanota. Maximum ELA depressions are lower than previously published: <500 m for the Cordillera Vilcanota and <400 m for the Quelccaya Ice Cap. These lower values could imply a topographic control over paleoglacier extent.     

Fire and Vegetation History from the Coastal Rain Forest of the Western Oregon Coast Range

High-resolution charcoal and pollen analyses were used to reconstruct a 4600-yr-long history of fire and vegetation near Taylor Lake in the wettest forests of coastal Oregon. Today, fires in these forests are rare because the season of ignition does not coincide with months of dry fuels. From ca. 4600 to 2700 cal yr B.P. fire episodes occurred at intervals of 140±30 yr while forest vegetation was dominated by disturbance-adapted taxa such as Alnus rubra. From ca. 2700 cal yr B.P. to the present, fire episodes have become less common, occurring at intervals of 240±30 yr, and fire-sensitive forest taxa, such as Tsuga heterophylla and Picea sitchensis, have become more prominent. Fire occurrence during the mid-Holocene was similar to that of the more xeric forests in the eastern Coast Range and suggests that summer drought was widespread. After ca. 2700 cal yr B.P., a decrease in fire episode frequency suggests that cooler conditions and possibly increased summer fog allowed the establishment of present-day Picea sitchensis forests within the watershed. These results provide evidence that fire has been an important disturbance agent in the Coast Range of Oregon, and variations in fire frequency and climate have led to the establishment of present-day forests.     

Fossil Rotifers and the Early Colonization of an Antarctic Lake

Early Holocene sediments from a continental Antarctic lake (Ace Lake, Vestfold Hills, East Antarctica) contained abundant fossil rotifers of the genus Notholca. The fossil is similar to specimens of Notholca sp. present in modern-day Ace Lake and other fresh and brackish lakes of the Vestfold Hills. Cyanobacteria and protists (chrysophyte cysts, dinoflagellate cysts, and rhizopod tests) were also recovered from the core samples. These sediments were deposited early in the freshwater phase of Ace Lake, soon after deglaciation of the area. The occurrence of this trophically diverse assemblage of organisms at an early stage in the evolution of the lake suggests either that they were part of an endemic Antarctic flora and fauna which pre-dated the last glacial maximum and survived in glacial refugia or that efficient intercontinental dispersal had occurred.     

The Structure and Morphotectonics of the Gesher Benot Ya'aqov Area, Northern Dead Sea Rift, Israel

Geological, geomorphological, and morphotectonic studies at the Gesher Benot Ya'aqov Acheulean site, Northern Jordan Valley, provide: 1) a unique opportunity to establish the ecological background of hominid behavior during the early stages of widespread human occupation; 2) crucial data for understanding the paleo-environment and the various processes that affected the area near the Gesher Benot Ya'aqov site; and 3) a basis for further comprehensive research in the region.Quaternary tectonic activity near the Acheulean archeological site has produced a complex local setting that controls the surrounding landscape. Tectonic activity formed the Benot Ya'aqov embayment, which attracted hominids and a variety of other fauna. Sediments of the Benot Ya'akov formation deposited in the embayment facilitated excellent preservation of the remnants of Acheulean hominid activities and of abundant floral and faunal remains. The formation was subsequently affected by faulting and folding. Local uplift ended the deposition of lacustrine sediments in the embayment of Hula Valley Basin and caused erosion and vertical incision of the Jordan River that resulted in the capture of the Rosh Pinna River and the exposure of the Benot Ya'akov formation. These deposits are found in different structural positions along the north–south oriented morphotectonic embayment. The outcrops contain numerous archeological sites, giving the study area an unusually high potential for future hominid discoveries.     

Late Quaternary Spring-Fed Deposits of the Grand Canyon and Their Implication for Deep Lava-Dammed Lakes

One of the most intriguing episodes in the Quaternary evolution of the Grand Canyon of the Colorado River, Arizona, was the development of vast lakes that are thought to have backed up behind lava erupted into the gorge. Stratigraphic evidence for these deep lava-dammed lakes is expectedly sparse. Possible lacustrine deposits at six areas in the eastern canyon yielded no compelling evidence for sediment deposited in a deep lake. At two of the sites the sediment was associated with late Quaternary spring-fed pools and marshes. Water-lain silt and sand at lower Havasu Creek was deposited 3000 cal yr ago. The deposit contains an ostracode assemblage similar to that living in the modern travertine-dammed pools adjacent to the outcrop. The second deposit, at Lees Ferry, formed in a spring-fed marsh 43,000 cal yr ago, as determined by ¹⁴C and amino acid geochronology. It contains abundant ostracode and mollusk fossils, the richest assemblages reported from the Grand Canyon to date. Our interpretation of these sediments as spring-fed deposits, and their relative youth, provides an alternative to the conventional view that deposits like these were formed in deep lava-dammed lakes that filled the Grand Canyon.     

Long Plio-Pleistocene Terrestrial Record of Climate Change and Mammal Turnover in Southern Spain

Cyclostratigraphic analysis of the Pliocene Zújar section (Guadix-Baza Basin, southeastern Spain) has enabled the recognition of a number of climatically forced cycles reflecting alternating dry and wet periods. Peaks of aridity are recorded at ca. 3.95, 3.55, 3.2, 2.8, and 1.8 myr B.P. The first dry period at about 4.0 myr B.P. corresponds to the early Ruscinian Mammal age, while the second arid interval at about 3.6 myr B.P. corresponds to the establishment of the Mediterranean double seasonality. The significant mammal turnover between the late Ruscinian and early Villanyian stages is placed between chron 2An.2n and the very base of chron 2An.1n, coincident with the dry phase recognized at about 3.2 myr B.P. The fourth aridity maximum at 2.8 myr B.P. roughly coincides with the Equus event in western Europe and is probably related to the beginning of the glacial–interglacial dynamics in the Northern Hemisphere. Finally, the last dry peak at about 1.8 myr B.P. is probably related to the set of mammalian events characterizing the transition from the late Pliocene faunas to those of the early Pleistocene.     

Unstable Climate Oscillations during the Late Holocene in the Eastern Bransfield Basin, Antarctic Peninsula

Core A9-EB2 from the eastern Bransfield Basin, Antarctic Peninsula, consists of pelagic (diatom ooze-clay couplets and bioturbated diatom ooze) and hemipelagic (bioturbated mud) sediments interbedded with turbidites (homogeneous mud and silt–clay couplets). The cyclic and laminated nature of these pelagic sediments represents alternation between the deposition of diatom-rich biogenic sediments and of terrigenous sediments. Sediment properties and geochemical data explain the contrasting lamination, with light layers being finer-grained and relatively rich in total organic carbon and biogenic silica content. Also, the high-resolution magnetic susceptibility (MS) variations highlight distinct features: high MS values coincide with clastic-rich sections and low MS values correspond to biogenic sections. The chronology developed for core A9-EB2 accounts for anomalous ages associated with turbidites and shows a linear sedimentation rate of approximately 87 cm/10³ yr, which is supported by an accumulation rate of 80 cm/10³ yr calculated from ²¹⁰Pb activity. The late Holocene records clearly identify Neoglacial events of the Little Ice Age (LIA) and Medieval Warm Period (MWP). Other unexplained climatic events comparable in duration and amplitude to the LIA and MWP events also appear in the MS record, suggesting intrinsically unstable climatic conditions during the late Holocene in the Bransfield Basin of Antarctic Peninsula.