Chemical analyses of surficial covers associated with alluvial fans in the western state of Bahia, Brazil

This work aimed to present partial results of chemical analyses conducted on surficial cover samples collected in alluvial fans located in the lower course of the Fêmeas River, in the Western region of Bahia, Brazil. This region is characterized by a sub-humid climate with two well-defined seasons: a rainy season (November to April) and a dry season (May to October). The hypothesis of this study considers that the deposits exhibit erosive-depositional discontinuities with different values of potassium (K) and sodium (Na), which could indicate the influence of complex morphodynamic processes throughout the Holocene. Therefore, the amounts of K and Na in different layers identified in the field and confirmed through laboratory textural analyses were measured. The analysis of these two cations allows for the possibility to infer a relative geochronology considering different exposures of the layers to weathering. The samples were collected at three sites on the left bank of the Rio das Fêmeas (LQME4; LQME5; LQME6). In LQME4, the results showed high potassium and sodium content at 20-30 cm (K=358 mg/dm³, Na=36.73 mg/dm³), 30-50 cm (K=347 mg/dm³, Na=36.73 mg/dm³), and 50-70 cm (K=307 mg/dm³, Na=53.38), except for the more superficial layer at 0-20 cm (K=8 mg/dm³, Na=16.08). This result suggests that the deeper layers underwent less weathering, while the low values of K and Na in the surface layer are possibly due to leaching from the runoff. With two layers, LQME5 presented high K and low Na values in the deeper layer of 20-60 cm (K=228 mg/dm³, Na=4.95 mg/dm³) contrasting with the more superficial layer at 0-20 cm (K=111 mg/dm³, Na=13.88 mg/dm³), where the K value decreases considerably while Na increases. The values in the most superficial layer are correlated with the modern sub-humid period that has resulted in lower runoff activity and, therefore, influencing mineral dissolution as indicated by the low Na value. The deeper layer suggests more humid periods characterized by lower K and higher Na values, indicating a more humid period resulting in more efficient mineral dissolution. In LQME6, six layers were analysed: 0-10 cm (K=195 mg/dm³, Na=11.07), 10-50 cm (K=147 mg/dm³, Na=11.49), 50-60 cm (K=69 mg/dm³, Na=13.72 mg/dm³), 60-75 cm (K=42 mg/dm³, Na=13.19), 75-85 cm (K=85 mg/dm³, Na=12.90 mg/dm³), 85-140 cm (K=87 mg/dm³, Na=16.30). The first two layers presented the highest K value in contrast to the other layers showing low values. LQME6 exhibits several discontinuities in K values, indicating that the upper layers underwent less weathering than the deepest ones. This research is being funded by the Bahia State Research Support Foundation (Grant 4341/2022).