129-43 Textural Variability of Terra Rossa Soils in Croatia

Abstract:

Terra rossa are reddish, clay-rich soils found blanketing limestone and dolomitic bedrock substrates throughout the Mediterranean. These soils vary in thickness from centimeters to meters and infill karst and fractures to several meters, but their provenance and origin remain debated. Previous research attributes terra rossa formation solely to the weathering of insoluble residue left by limestone dissolution, but more recent work posits that insoluble residue alone is insufficient to account for the formation of terra rossa and suggests a polygenetic origin entailing a combination of allochthonous dust influx and chemical weathering. Grain size analysis of these terra rossa can provide insights into their genesis, but the soil’s clay-rich nature presents challenges in obtaining accurate grain-size distributions. 

We utilized a laser particle-size analyzer (LPSA) on two terra rossa soil profiles collected in the Radovin and Bosilijevo regions of Croatia. The profile from Radovin is of unknown age and field data suggests increasing clay content with depth and significant sand throughout the profile. The profile at Bosilijevo is of Quaternary age and situated in a karstic fissure of Mesozoic limestone. Field data suggests a silty loam that increases in clay content with depth. Two pretreatment methods were tested prior to LPSA analysis. Method 1 removed carbonates with 1.7M buffered acetic acid and then organics with 30% hydrogen peroxide. Method 2 only applied the hydrogen peroxide treatment. Following pretreatment, dispersant was added and samples were sonicated immediately prior to LPSA analysis. For both methods, particle size distributions measured by the LPSA did not corroborate the increase in clay content observed in the field suggesting incomplete disaggregation of clay minerals during pretreatment. Work is ongoing to determine the most effective methods to facilitate laser particle size analysis so that differences in the soil forming processes and/or nature of allochthonous sediment input to these terra rossa can be assessed.

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