The interpretation of secular Caspian Sea level records during the Holocene

Abstract

The Caspian Sea (CS) experienced significant changes during the Holocene. The standard deviation for Caspian Sea level (CSL) variations over that interval is estimated as σ = 1.4 m. Based on well-established views, they were climate-induced variations. There are no clear links with the calendar of climatic anomalies, and climate models do not reproduce the changes. Therefore, the question about the origin of “secular” CSL fluctuations remains open. Based on general ideas about the laws of temporal dynamics relating to massive inertial objects, the observed slow changes of the CSL under the semi-steady climate state of the Holocene can be represented as resulting from the accumulation of small anomalies in the water regime, as a kind of “self-developing” system. To test this hypothesis, the model of the water balance of the CS was used. Time scale for the sea fluctuations was estimated as ∼20 years. This model is interpreted as stochastic, and from this perspective, it is a Langevin equation that incorporates the action of precipitation and evaporation as random white noise, so that the whole can be thought of as an analogue of Brownian motion. Under these conditions, the CS is represented by a system undergoing random walk. Modeling results are interpreted from the probabilistic point of view, although the model is deterministically based on the physical law of conservation of water mass. The results showed that the CSL fluctuations under steady state conditions are characterized by σ = 1.1 m, close to the empirical value. “Super-large” anomalies in CSL are not prohibited by the theory, but their development requires a correspondingly long time. However, during long periods of time, background conditions change, and uniformity of the Brownian process becomes disrupted. The origin of large transgressive/regressive stages can be different. For example, the low stand of the Enotaevkian Regression during the LGM was determined by a significant reduction in precipitation over the Volga River catchment and by a corresponding reduction in the volume of river runoff. Hence, based on modeling results, the possibility of “self-development” effects is not prohibited by the theory: there need not be any cause for specific level changes or shifts, merely the expected behavior of red noise processes.