Multiple instabilities and modes of glacial rhythmicity in the plio-Pleistocene: a general theory of late Cenozoic climatic change

It has been noted that several distinct modes of glacial oscillation have existed during the past few million years, ranging from low-amplitude, high-frequency oscillations in the early Pliocene, through relatively high amplitude, predominantly near-40 ky period, oscillations in the late Pliocene and early Pleistocene, to the major near-100 ky period oscillations of the late Pleistocene. In addition to other plausible mechanisms suggested previously to explain aspects of this multirhythmic phenomenon, we now illustrate another possible contributor to this type of behavior based on the hypothesis that the slow-response climatic system is bistable and that two kinds of internal instability may be operative along with externally imposed forcing due to earth-orbital (Milankovitch) radiation changes and slow, tectonically-induced changes in atmospheric carbon dioxide. These two instabilities have been discussed previously: one is due to positive feedback in the global carbon cycle leading to near-100 ky free oscillations of the ice sheets, and the other is due to the potential for ice-calving catastrophes associated with bedrock variations that can lead to oscillations of a period near 40 ky, independent of obliquity forcing. Within the framework of a dynamical model containing the possibility for these two instabilities, as well as for stable modes, we show (1) how Milankovitch radiative changes or stochastic forcing influencing ice sheets can induce aperiodic (chaotic) transitions between the possible stable and unstable modes, and more significantly, (2) how progressive, long-term, tectonically-induced, changes in carbon dioxide, acting in concert with earth-orbital radiative variations in high Northern Hemisphere latitudes, can force systematic transitions between the modes. Such systematic changes can result in an ice mass chronology for the past 5 My that is qualitatively similar to the observed record of global ice mass. In essence, we have constructed a minimum dynamical model of the late Cenozoic climatic changes, containing what are believed to be the main physical factors determining these changes: ice mass, bedrock depression, atmospheric carbon dioxide concentration, deep ocean thermohaline state, Milankovitch radiation forcing, and slow tectonically-induced carbon dioxide forcing. This model forms the basis for a coherent theory for the complex climatic events of this long period.     

A 640 kyr geomagnetic and palaeoclimatic record from Lake Baikal sediments

Magnetic remanence vectors for 1737 samples from two ∼100 m cores of Lake Baikal sediments are reported along with complete magnetic susceptibility profiles obtained from a pass-through system. Chronological control is established by means of two independent correlations; first, by matching susceptibility variations to the oceanic oxygen isotope record and second, by matching the relative palaeointensity variations to the SINT-800 global reference curve. These both imply an average deposition rate of 15 cm kyr^–1 and a basal age of ∼640 ka. Spectral analysis reveals the presence of Milankovitch signals at ∼100 kyr (eccentricity), ∼41 kyr (obliquity) and ∼23 and ∼19 kyr (precession). Stable remanence vectors are almost all of normal polarity. The few exceptions comprise brief intervals of low and/or negative inclinations which probably represent geomagnetic excursions. However, these are far less numerous than the high sedimentation rate would lead one to expect. Furthermore, only four of them can be readily matched to the—still poorly understood—global pattern. These are the Laschamp, the Albuquerque, the Iceland Basin and perhaps the West Eifel excursions which occurred at ∼38 000, ∼146 000, at 180 000–190 000 and at 480 000–495 000 yr ago, respectively.     

Metal Speciation in Water of the Flooded Mine “Arsenic” (Karelia,Russia): Equilibrium-Kinetic Modeling with a Focus on the Influence of Humic Substances

Equilibrium-kinetic modeling allows investigating metal behavior in the water–rock-organic matter system with time to evaluate anthropogenic effects on the environment. In the article, the interactions of stagnant mine drainage water of the flooded mine “Arsenic” with ore and gangue minerals were simulated using different organic matter incorporation approaches. If the model is closed to humic substances (no additional organic matter input), most fulvic acids are bound in the Fe fulvate complex. While under the removal of Fe fulvate from the model, the Cu fulvate becomes prevalent, the contribution of the fulvate complexes with Zn, Mg, and Ca also increases. This scenario simulates the organo-mineral complexes behavior well and allows identifying the sequence of metal binding to organic ligands as follows Fe?>?Cu?>?Zn?>?Mg?>?Ca. The second scenario imitates the constant input of organic matter to the model (open system regarding humic substances). The dissolved metal concentrations in the model solution are extremely high in comparison to the mine drainage water. This scenario demonstrates that excessive input of organic matter leads to the accumulation of the metals in a dissolved form and blocks the secondary mineral formation despite the faster dissolution of the primary minerals under a more acidic pH than in the first scenario. However, despite the differences between the model solution and the mine drainage water, this scenario is useful to address specific issues associated with changes in natural and anthropogenic conditions. Both scenarios show the importance of organic matter incorporation to the equilibrium-kinetic models.

     

Density inhomogeneities, isostasy and flexural rigidity of the lithosphere in the Transcaspian region

The 3-D lithospheric-density model for the southeastern part of the Caspian Sea and the Transcaspian area, practically coinciding with the territory of the Turkmen Republic, has been constructed based on geophysical data and in accordance with the principle of isostasy. From the model selected the anomalous density of the subcrustal layer between the Moho discontinuity and the 100-km depth level is found to be — 100 kg/m³ under the Tien-Shan, − 50 kg/m³ under the Kopet-Dag mountain area, + 80 kg/m³ under the central region of the South Caspian basin, −50 kg/m³ under the eastern part of the basin, known as the West Turkmenian depression, and + 45 kg/m³ under the Murgab depression.

Significant disturbances of the local isostasy are determined both in the northern and central areas of the South Caspian basin and also in the area of the Kara-Bogaz swell of the Turan platform and for the Kopet-Dag foredeep. indicating a high level of stresses in the lithosphere. The shape of the Turan plate determined by the seismic profiling is accounted for by elastic deformation resulting from the forces acting on the southern edge of the plate in the area of the Turan plate-Kopet-Dag collision. The elastic thickness of the Turan plate is estimated as 25 ± 5 km. The results obtained seem to confirm the idea that the decomposition of the Turan plate has taken place in the zone of the plates interaction and the decomposed material is situated under the Kopet-Dag ridge.

We propose that the Kara-Bogaz swell is supported by the mantle material upwelling whereas the subsidence of the adjacent part of the South Caspian basin may be due to the downgoing mantle flow i.e., a small convection cell is suggested in that area.