Searching for the elusive “A” horizon in buried paleosols

ABSTRACT

Presence of an A horizon in paleosols is important for taxonomic classification, interpreting pedogenic processes, and reconstructing paleoenvironments. However, identifying the A horizon in buried contexts is challenging because of potential erosion during burial and post-burial diagenesis. Thus, 745 master horizons from 299 training pedons were analyzed to develop indicators for identifying buried A horizons and non-A horizons. Indicators of A horizons include < 12% organic carbon or < 24% LOI, ped diameters ≤ 1 cm, platy peds, or > 2% root traces in sandy substrates. Non-A horizon indicators represent properties of the O, E, transitional (e.g., AB), B, or C horizons. Together, the indicators yield the highest proportion of true positives for classification of known A horizons and true negatives (TN) for classification of known non-A horizons in the database (∼ 80 to 90% success rate). Confusion-matrix statistics show that the classification performance of the indicators are optimized when considering sensitivity, specificity, precision, and accuracy. In addition, there were no significant differences (z-test) in the performance of the indicators between the training pedons and 30 validation pedons. Testing the indicators on buried Quaternary paleosol sequences demonstrates that most A horizons survive the burial process in alluvial settings, giving confidence in their potential preservation in lithified paleosols of deeply buried contexts. Applying the indicators to selected Quaternary and lithified Miocene through Permian buried paleosols with previously assigned master horizons altered some pedogenic and climate interpretations. Examples are given of problematic buried paleosols for further testing of the indicators.