Jan Bloemendal

Grain-size distribution function of polymodal sediments in hydraulic and aeolian environments, and numerical partitioning of the sedimentary components

Most continental sediments are polymodal, composed of overlapping components of which the grain size generally obeys some type of natural distribution. The grain-size components and their function types can be determined from frequency and cumulative curve plots in order to define the function formula of the grain-size distribution. The function parameters can be estimated by fitting a defined function formula to the measured grain-size data of the sample, which simultaneously achieves numerical partitioning of the sedimentary components. Genetic analysis of grain-size components of hydraulic and aeolian sediments demonstrates the following environmental implications: Fluvial sediment is composed of isolated saltation and suspension components. The sediments in closed lake basins are dominated by a suspension silt-clay component with a small proportion of saltation sand. The fine sand component makes up the majority of desert sand, overlapping with a small proportion of fine dust. Aeolian loess is composed of two overlapping components: a short suspension-time silt component and a long suspension-time fine component. Aeolian material in the North Pacific deep-sea sediments is dominated by long suspension-time fine dust. The fine component in aeolian sediments shows a consistent grain-size distribution and genetic connection from the desert sand, loess of northern China to the North Pacific Ocean, which is mainly transported by westerly winds and is dispersed in the atmosphere, forming a background dust

High-resolution multi-proxy climate records from Chinese loess: evidence for rapid climatic changes over the last 75 kyr

The loess/paleosol sequence in loess layer L1 (Malan Loess) is investigated in three regions of the Western Chinese Loess Plateau. Nine pedogenic layers are found in L1 and three proxy climate indices, magnetic susceptibility (MS), grain size (GS) and CaCO3 content, are measured at intervals of 0.2 kyr in order to recover records of monsoon climate variations. Time series of MS, GS and CaCO3 content document the high resolution history of summer and winter monsoon climate variations over the last 75 kyr. The records show a high degree of similarity to the warm interstadials recorded in ice cores from Greenland and the Antarctic, and with Heinrich events in the North Atlantic, during the last glacial cycle. Bond cycles are also recorded by the Chinese loess records. Overall, our results indicate that numerous rapid changes in climate occurred in China during the last glacial cycle, but that the range of climate variations was smaller than recorded in Greenland.

Environmental Applications of Magnetic Measurements

A wide range of examples of the application of magnetic measurements to environmental studies illustrate the advantages of magnetic techniques over conventional methods. Magnetic measurements, in both the field and the laboratory, are particularly useful for reconnaissance work because of their spee and flexibility, Quantification as well as simple diagnosis of the transformation and movement of magnetic minerals within and between the atmosphere, lithosphere, and hydrosphere is practical. Techniques of investigating intrinsic and mineral magnetic properties, in addition to paleomagnetic remanence, are described in subjects as diverse as meteorology, hydrology, sedimentology, geophysics, and ecology.

Quantitative estimates of palaeoprecipitation at Xifeng, in the Loess Plateau of China

Abstract Using10Be, recent studies (Heller et al., 1993; Beer et al., 1993) have tried to reconstruct palaeoclimatic records by separating the secondary (pedogenic) and primary (aeolian; detrital) contributions to the magnetic susceptibility of Chinese loess. Here we use a technique, which monitors the temperature dependence of magnetic remanence, to estimate the magnetisation and relative contribution of the superparamagnetic (SP), single-domain (SD) and multidomain (MD) ferrimagnetic components in the Xifeng loess section. On the assumption that the SP ferrimagnetic component is dominantly of pedogenic origin, the magnetisation of this component has also been determined for samples taken from the topmost palaeosol (S0), at six sites over the Loess Plateau, and directly correlated with present day precipitation at these sites. This correlation is then used to estimate quantitatively palaeoprecipitation for the topmost 16 m of the Xifeng section, which covers about the last 140,000 years. The data indicate a considerably reduced precipitation during the glacial, loess-forming periods and a significantly higher level of precipitation than today during the last interglacial period.

Bimodal grain-size distribution of Chinese loess, and its palaeoclimatic implications

Grain-size analysis indicates that Chinese loess generally shows a bimodal distribution with a coarse and a fine component. The coarse component, comprising the main part of the loess, has pronounced kurtosis and is well sorted, which is interpreted to be the product of dust storms generated by low-altitude northwesterly winds. Its grain-size reflects the strength of the low-altitude circulation in the dust seasons of the year, and its percentage provides an indicator of the source area aridity and the frequency of dust storms. Conversely, the fine component has a wide grain-size range and is poorly sorted. Sedimentary illustrations based on the grain-size distribution characteristics of bulk samples and of detrital quartz suggest that the fine component probably represents the background dust load of the atmosphere and is mainly transported by high-altitude westerly airstreams. Its grain-size provides an estimate of the westerly air stream intensity. The coarse and fine components of a loess sample can be mathematically separated by fitting a designated mathematical distribution function to the measured grain-size data, and this procedure constitutes an approach for reconstructing the palaeowind system of Northern China

East Asian summer monsoon precipitation variability since the last deglaciation

The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the “present” is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7–7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8–5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5–8.5 ka, and by two centennial-scale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change.

Delayed build-up of Arctic ice sheets during 400,000-year minima in insolation variability

Knowledge of the past variability of climate at high northern latitudes during astronomical analogues of the present interglacial may help to inform our understanding of future climate change. Unfortunately, long-term continuous records of ice-sheet variability in the Northern Hemisphere only are scarce because records of benthic 18O content represent an integrated signal of changes in ice volume in both polar regions. However, variations in Northern Hemisphere ice sheets influence the Siberian High (an atmospheric pressure system), so variations in the East Asian winter monsoon (EAWM)—as recorded in the aeolian dust deposits on the Chinese Loess Plateau—can serve as a useful proxy of Arctic climate variability before the ice-core record begins. Here we present an EAWM proxy record using grain-size variations in two parallel loess sections representative of sequences across the whole of the Chinese Loess Plateau over the past 900,000 years. The results show that during periods of low eccentricity and precessional variability at approximately 400,000-year intervals, the grain-size-inferred intensity of the EAWM remains weak for up to 20,000 years after the end of the interglacial episode of high summer monsoon activity and strong pedogenesis. In contrast, there is a rapid increase in the EAWM after the end of most other interglacials. We conclude that, for both the 400,000-year interglacials, the weak EAWM winds maintain a mild, non-glacial climate at high northern latitudes for much longer than expected from the conventional loess and marine oxygen isotope records. During these times, the less-severe summer insolation minima at 65° N (ref. 4) would have suppressed ice and snow accumulation, leading to a weak Siberian High and, consequently, weak EAWM winds.

Correlation of the magnetic susceptibility stratigraphy of Chinese loess and the marine oxygen isotope record : chronological and palaeoclimatic implications

Abstract The thick deposits of wind-blown silt of the Loess Plateau in central China constitute arguably the most important terrestrial record of the late Pliocene and Pleistocene climate. Magnetic susceptibility stratigraphy has proven extremely useful for quantifying the palaeoclimatic record of Chinese loess, with intervals of high magnetic susceptibility corresponding to warm, humid interglacials and intervals of low susceptibility to cooler, drier glacials. Several workers have noted that the magnetic susceptibility stratigraphy strongly resembles the marine oxygen isotope record, for which a high resolution chronology is available from tuning to the frequencies of Earth orbital parameters. Attempts have been made to correlate the two records statistically and we use this approach to generate a new magnetic susceptibility time series for the central Chinese Loess Plateau. Our results suggest that the two records are sufficiently similar over the interval ca. 0–1.5 My BP for this approach to yield a useful age model for loess accumulation. Spectral analysis of the resulting magnetic susceptibility time series clearly shows that the growth of 100 ky variance, which occurs in the marine oxygen isotope record after about 1 My BP, also occurs synchronously in the magnetic susceptibility record, reinforcing recent suggestions of a strong linkage between the high latitude northern hemisphere and the Asian climate. We also show that a significant improvement in the strength of the loess magnetic susceptibility-marine oxygen isotope correlation occurs after about 1.5 My BP, which corresponds to a significant increase in the rate of sediment accumulation across the Loess Plateau.

The organic matter content of street dust in Liverpool, UK, and its association with dust magnetic properties

Abstract The organic matter content of street dust in Liverpool, UK, measured by means of the loss-on-ignition method, is 4.0% on average (standard deviation 1.3%). The magnetic measurements of the street dust samples indicate that the dominant magnetic component is multidomain grains of ferrimagnetic minerals and that superparamagnetic and stable single-domain ferrimagnetic grains and paramagnetic, diamagnetic, and canted antiferromagnetic minerals are present in small magnetic concentrations. Our study demonstrates good linear correlations between the organic matter content and some magnetic mineral concentration-related parameters: low-frequency susceptibility, frequency-dependent susceptibility, susceptibility of anhysteretic remanent magnetisation, and high-field susceptibility. Among them, frequency-dependent susceptibility shows the strongest correlation, suggesting that soil may be an important source of dust organic material. Finally, it is suggested that simple, rapid, and non-destructive magnetic measurements may be used as proxies for the organic matter content in street dust.

Terrestrial impact of abrupt changes in the North Atlantic thermohaline circulation: Early Holocene, UK

Abrupt cooling events are features of Holocene climate and may recur in the future. We use lake records from Hawes Water, NW England, to quantify the impact of two prominent early Holocene climatic events. Subdecadal oxygen isotope records from sedimentary carbonate (18δOc), dated using thermal ionization mass spectrometry (TIMS) U-series analyses, provide evidence for abrupt cold events, lasting ∼50 and ∼150 yr at 9350 and 8380 yr ago, which correlate with the 9.3 ka and 8.2 ka events recognized in Greenland ice cores. At Hawes Water, mean July air temperatures, inferred from chirono-mid assemblages, decreased by ∼1.6 °C during each event. Calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation (18δOp) by ∼1.3‰; this is interpreted as a direct downstream response to cooling and freshening of northeast Atlantic surface water by melting ice sheets. Intermediate in magnitude between events observed in Greenland and central Europe, the effects are consistent with a partial shutdown of the North Atlantic thermohaline circulation.