Cristian Necula

Late Miocene- early Pleistocene paleoclimate history of the Chinese Loess Plateau revealed by remanence unmixing

Previous studies show that the Quaternary loess sequence and the late Miocene-Pliocene red clay sequence on the Chinese Loess Plateau (CLP) share similar magnetic properties despite their being deposited under different climate conditions. To solve this paradox, we apply a novel remanence unmixing technique and demonstrate that loess and red clay on the central CLP contain a similar low-coercivity pedogenic component, but their high-coercivity components differ dramatically, reflecting different oxidation and temperature conditions. We infer that temperatures on the Chinese Loess Plateau cooled from the late Miocene to the Quaternary, in a manner similar to sea surface temperature records for the same time interval. This coherency between marine and terrestrial records argues for a CO2 forcing on long-term paleoclimatic variations. Key Points Backfield remanence data of Chinese loess and red clay consist of two components Loess and red clay have similar pedogenic but different detrital components The Loess Plateau experienced a cooling trend from the late Miocene to the Quaternary

Dominant 100,000-year precipitation cyclicity in a late Miocene lake from northeast Tibet

Paleoclimatic records reveal >6–million year earlier onset of dominant 100–thousand year eccentricity cycles. East Asian summer monsoon (EASM) precipitation received by northern China over the past 800 thousand years (ky) is characterized by dominant 100-ky periodicity, mainly attributed to CO2 and Northern Hemisphere insolation–driven ice sheet forcing. We established an EASM record in the Late Miocene from lacustrine sediments in the Qaidam Basin, northern China, which appears to exhibit a dominant 100-ky periodicity similar to the EASM records during the Late Quaternary. Because evidence suggests that partial or ephemeral ice existed in the Northern Hemisphere during the Late Miocene, we attribute the 100-ky cycles to CO2 and Southern Hemisphere insolation–driven Antarctic ice sheet forcing. This indicates a >6–million year earlier onset of the dominant 100-ky Asian monsoon and, likely, glacial and CO2 cycles and may indicate dominant forcing of Northern Hemisphere climate by CO2 and Southern Hemisphere ice sheets in a warm world.

Unmixing hysteresis loops of the late Miocene-early Pleistocene loess-red clay sequence

Magnetic paleoclimatic records often represent mixed environmental signals. Unmixing these signals may improve our understanding of the paleoenvironmental information contained within these records, but such a task is challenging. Here we report an example of numerical unmixing of magnetic hysteresis data obtained from Chinese loess and red clay sequences. We find that the mixed magnetic assemblages of the loess and red clay sediments both contain a component characterized by a narrow hysteresis loop, the abundance of which is positively correlated with magnetic susceptibility. This component has grain sizes close to the superparamagnetic/stable single domain boundary and is attributed to pedogenic activity. Furthermore, a wasp-waisted component is found in both the loess and red clay, however, the wasp-waisted form is more constricted in the red clay. We attribute this component to a mixture of detrital ferrimagnetic grains with pedogenic hematite. The abundance of this component decreases from the base to the top of the red clay, a pattern we attribute to decreased hematite production over the Chinese Loess Plateau (CLP) due to long-term climate cooling. This work demonstrates the potential of hysteresis loop unmixing to recover quantitative paleoclimatic information carried by both low and high coercivity magnetic minerals.

The Lower Danube Loess, New Age Constraints from Luminescence Dating, Magnetic Proxies and Isochronous Tephra Markers

Loess and loess-derivative deposits currently form some of the most ubiquitous sedimentary landforms in Europe, including important parts of Romania. Loess-palaeosol sequences (LPS) are continental archives of Quaternary paleoclimates since these deposits are a direct product of geomorphic processes driven by past climate variability. Understanding these processes needs data input provided by absolute chronologies. Loess is generally considered an ideal material for the application of luminescence dating. In this chapter, we present a review of the latest methodological advances in constraining the chronology of several key Romanian LPS alongside the dating of several loess-alluvial deposits that harbour tephra layers. Luminescence chronologies as well as time-depth models based on magnetic susceptibility variations assign the topmost loess layer to the last glacial cycle that comprises, according to the north European stratigraphic terminology, the Weichselian glaciation (Marine Isotope Stage (MIS) 4 and MIS 2 phases). The uppermost palaeosol is assigned to MIS5 especially to the Eemian interglacial (MIS5e), an equivalent to the north European stratigraphy, and to the Riss–Wurm interglacial in the Alpine stratigraphy. As proved by the high-resolution chronologies, the sedimentation rates of loess varied during the last glacial both within a specific loess section, as well as between different loess sections, the major controlling factor being the topographic context. The luminescence chronologies discussed here improved and expanded the long-held stratigraphy of Romanian loess constructed decades ago by using relative methods, suggesting that a re-evaluation of the regional chronostratigraphic inferences in a high-resolution absolute dating approach has to be conducted.

SAR-OSL dating of Late Pleistocene loess in Southern Romania using fine and coarse-quartz

Loess deposits cover significant areas in Europe, extending from NW-France and Belgium through to central Europe, the Ukraine and Western Russia. The loess palaeosol sequences of the Carpathian Basin-Lower Danube region (Romania, Serbia and Bulgaria) are thought to represent the most continuous and high resolution archives of regional climate and environmental change during the Late and Middle Pleistocene in SE Europe and a link between similar deposits in central Europe and Eurasia. However, in comparison to other loess sequences elsewhere in Western, Central and Eastern Europe, the deposits in Romania have been much less extensively studied. Luminescence dating is, at present, the only method that allows establishing an absolute chronology for loess deposits by virtue of its ability to directly date the moment of sediment deposition. Moreover, the aeolian nature of loess ensures that the luminescence signal is completely reset prior to deposition, a prime requisite for luminescence dating. Thus, loess sediments are ideal materials for developing, testing and applying luminescence techniques. This approach is essential for securely linking loess records from Romania in a chronologically reliable regional framework and to extend this information to other sites from central and eastern European loess belt, in order to understand past paleoenvironmental dynamics at both regional and continental scales.

Palaeomagnetism of the Upper Miocene- Lower Pliocene lavas from the East Carpathians: contribution to the paleosecular variation of geomagnetic field

Investigations of the paleosecular variation of the geomagnetic field on geological timescales depend on globally distributed data sets from lava flows. We report new paleomagnetic results from lava flows of the East Carpathian Mountains (23.6°E, 46.4°N) erupted between 4 and 6 Ma. The average virtual geomagnetic pole position (76 sites) includes the North Geographic Pole and the dispersion of virtual geomagnetic poles is in general agreement with the data of the Time Averaged geomagnetic Field Initiative. Based on this study and previous results from the East Carpathians obtained from 0.04–4 Ma old lava flows, we show that high value of dispersion are characteristic only for 1.5–2.8 Ma old lava flows. High values of dispersion during the Matuyama chron are also reported around 50°N, in the global paleosecular variation data set. More data are needed at a global level to determine if these high dispersions reflect the behaviour of the geomagnetic field or an artefact of inadequate number of sites. This study of the East Carpathians volcanic rocks brings new data from southeastern Europe and which can contribute to the databases for time averaged field and paleosecular variation from lavas in the last 6 Ma.

An Assessment of the Direct Radiative Forcing of the PM10-Study Case

Today the interest in aerosols is high mainly because of their effect on human health and their role in climate change. They have also a determining effect on visibility and contribute to the soiling of monuments. Observations and model calculations show that the increase in the atmospheric aerosol burden is delaying the global warming expected from the increase in greenhouse gasses (1). The aim of present paper is to analyze the direct effect of the aerosol and to compute the radiative forcing of the aerosol in different sites and meteorological conditions. The calculation of PM 10 levels and radiative forcing must necessarily be based on a description of the emissions of the individual chemical species and how they transform and mix in the atmosphere. We performed a statistical analysis of the emission of the aerosol for one year, amplitude of the concentrations and the temporal variability as is described in Section 1. The optical parameters and the equations used to compute the radiative forcing are presented in Section 2. Discussion of the results and a few concluding remarks are made in Section 3