Altai megafloods – the temporal context

Abstract

Pleistocene floods, from ice-dammed lakes in the Russian Altai Mountains, are among the largest freshwater megafloods in Earth history. Improved dating techniques have led to new dates constraining the timing of flood erosional and depositional events. In addition to a variety of interpretations and explanations of event histories, a proliferation of dates may increase confusion with respect to the characteristics and chronology of the flood events.

In a succinct review, the main ice-dammed lakes features, the outburst floods and their repeated formation are presented. Among other indicators, interbedded units of lacustrine deposits and outburst flood sediments in Inya River valley demonstrate conclusively that there was a series of outburst floods with significant evidence for at least three high magnitude flood events.

Prior study focussed on the interpretation of: lake sediments, shorelines, giant bars, gravel dunes; dated by radiocarbon, luminescence and cosmogenic nuclide assay. The literature presents sometimes contradictory findings with respect to flood chronology. These different interpretations, evidence and arguments, are reviewed for plausibility and consistency. This consideration includes methodical and technical dating problems: limited bleaching for luminescence techniques; low purity for cosmogenic nuclide dating; contamination of samples dated by radiocarbon assay.

At the current stage of knowledge, the main period of repeated high magnitude outburst floods occurred between 28 ka and 15 ka BP. Indicators of earlier events are less distinct. Possible younger smaller floods can be identified after 15 ka BP, but it is difficult to relate these to catastrophic lake-draining. Rather, they may relate to non-catastrophic draining of residual small lakes within the larger basins. The termination of the “lake period” took place before 9.9 ± 0.3 ka cal BP in the Kuray Basin, and in the Chuya Basin before 8.2 ± 0.2 ka cal BP. Indicators are lacking for any drainage events after 7.7 ± 0.6 ka as indicated by OSL ages of aeolian deposits at low elevations in the Katun valley.

Relating the outburst floods to the temporal and spatial dynamics of the impounding glacier is challenging as, with regard to Pleistocene glaciation, several conceptual models have been established supported by field evidence that, in part, can be contradictory. During the period of the high magnitude outburst floods the extension/thickness of the impounding valley glaciers were sufficient to form deep glacial lakes around the time of the last glacial maximum (LGM); yet the timing of the LGM remains controversial. Transferring glacial chronologies from mountain areas adjacent to the Russian Altai Mountains to the environment of the megafloods does not provide a clearer picture as there is evidence for maximum stages at MIS 4 and MIS 3 - in addition to recent postulations of a LGM in MIS 2 for the Russian Altai Mountains. The Russian Altai Mountains are located in a climatic transitional zone across which different LGM stages could be developed due to different moisture availability and temperature chronologies. The effects of spatially-variable rain-shadows influencing the supply of moisture from the regional prevailing winds and local moisture sources from expanded ice-dammed lakes complicates the picture and require further investigations.

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