Paleoenvironmental and paleoclimate evolution in an alpine peat bog in southern Iberia: a geochemical approach

Alpine lakes and peat bog from Sierra Nevada mountains in southern of Iberia are excellent paleoarchives since ecosystems are especially sensitive to climate variability and have been unaltered by humans until very recently. The Borreguil de la Virgen is a north-faced peat bog (BdlV; 37° 03′ 15″N, 3° 22′ 40″ W) located at 2945 m above sea level in a depression formed during the glacial retreat. A sedimentary record of 169 cm length has been analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) to obtain major and trace elemental composition, respectively. Different geochemical proxies have been used to reconstruct the paleoenvironmental evolution in the BdlV during the Holocene. Rb/Al, Zr/Al and Ca/Al have been used as detrital proxies while Mo/Al and U/Al have been used has oxygenation proxies. The Rb/Al ratio showed the highest values between 8.5 and 5.5 cal ka BP suggesting enhanced runoff related to enhanced humid conditions. This period is also characterized by the highest sedimentation rates and high values of U/Al ratio. Pervasive high runoff input during this interval was interrupted by two periods of reduced values of Rb/Al ratio between 8 and 7.5 cal ka BP and around 7.2 cal ka BP. More arid and/or colder conditions could have reduced the precipitation, explaining the decrease in runoff. After 5.5 cal ka BP the Rb/Al depicted a decreasing trend, suggesting less humid conditions and an aridification trend. At 5.5 cal ka BP the Zr/Al ratio also increased, which suggests the onset of the Saharan eolian dust export as result of the African Humid Period demise. Between 5.3 and 3.6 cal ka BP a major environmental change occurred involving the drying of the wetland, which promoted increases in the Mo/Al ratio due to the less oxic conditions and in the Ca/Al ratio since the Ca dissolved in water precipitated. This environmental change is followed by a period of low sedimentation rate between 3.6 and 1 cal ka BP which precludes us to make any accurate interpretation. The comparison between elemental ratios from BdlV with other nearby lake and bog records from Sierra Nevada highlight that the peat bogs are more prone to have enrichments in some elements when sedimentary conditions change, thus showing a higher local influence.