Carbonate accumulation morphology in a soil chronosequence in the southern Pre-Ural, Russia: Significance for Holocene paleoenvironmental reconstruction

Abstract

A chronosequence consisting of paleosols buried under kurgans of the Savromatian (2,500–2,600 yr BP) and the Late Sarmatian Time (1,700–1,800 yr BP), along with the modern Dark Kastanozems was studied in a dry steppe area of the southern Pre-Ural region, Russia. We focus on morphological features of carbonate accumulations, that occur as white soft spots (WSSs) at different scales of soil mass organization, to understand their role in paleoenvironmental reconstruction. The carbonate accumulations in Bk horizons of paleosols buried within the Savromatian period and of the modern Dark Kastanozems are similar in quantity but differ slightly in their diameter. Calcic features in the Savromatian paleosols are diverse and have signs of both dissolution (calcite crystals in voids) and segregation. Only in the Savromatian paleosols, desiccation cracks have broken the WSSs into pieces. We consider highly probable that the Savromatian paleosols formed under slightly drier and more continental climate than present. The Late Sarmatian paleosols are characterized by WSSs in Bk horizons with the largest size and greatest quantity, and their diversity is great enough to divide the Late Sarmatian paleosols into two morphological groups, ‘upper’ and ‘lower’, based on signs of dissolution. The ‘lower’ group shows no signs of dissolution, so we conclude that the WSSs formed in stable dry climatic conditions. Establishment of a more humid climate at the beginning of the Late Sarmatian period produced signs of dissolution preserved in the ‘upper’ group that was buried late in this chronointerval. The WSSs in the modern soils have the clearest signs of dissolution compared with other soils in the chronosequence, which we interpret to indicate that the modern climate is likely the most humid interval of the late Holocene. Detailed investigation of morphological peculiarities of carbonate accumulations on the different scales of soil mass organization in the chronosequences of steppe soils is a valuable source of information on the paleoecological situation of Holocene climate intervals and can be successfully used for that aim.