2500-year Late Holocene vegetation and hydrologic changes from a cave guano-clay sequence in the Danube Gorge, SW Romania

Abstract:

This multi-analytical study on a guano/clay sequence in Gaura cu Muscă Cave (GMC), supported by five ¹⁴C AMS dates, brings new insights to both vegetation dynamics and paleo-hydroclimate in a region of archeological and anthropological significance. GMC is located at the entrance to the Danube Gorge in SW Romania. The cave is short (254 m), consisting of a main gallery (along which an underground stream flows) that is bypassed by the Bat’s Gallery. GMC is a shelter for bats throughout the year and a hibernaculum for five species. Nowadays, the two major guano deposits belong to Myotis capaccinii; one is located underneath the largest colony in the Bat’s Chamber, whilst the other one is much smaller and is found in the Bat’s Gallery. From this latter location the investigated clay/guano profile was recovered from underneath a blanket of 2 to 3mm thick fresh guano. We provide sedimentological, geochemical, mineral magnetic, guano and charcoal δ¹³C data, and pollen-based evidence of fluctuating hydro-climate conditions from ~1230 BC to ~AD 1240. Between 1230 BC and AD 1000 the climate was wetter than present prompting the flooding of the cave, preventing bats from roosting and leading to slow-rate clay accumulation. The pollen diagram for this interval confirms a human presence and agriculture activities in the region. The second half of the Medieval Warm Period (MWP; after AD 1000) the climate became drier, which hydrologically translates in the cave being air-filled. At this time a maternity colony existed in the Bat’s Gallery, under which guano accumulated. One extremely wet event occurred ~AD 1170, when Fe/Mn and Ti/Zr ratios show the highest values and coincident with a substantial increase of sediment load in the underground stream. The mineral magnetic characteristics for the second part of the MWP indicate partial input of surface-sourced sediments reflecting the agricultural development and forest clearance in the area. Pollen and microcharcoal studies confirm that the overall vegetation cover and human land use have changed little in this region since medieval times. The recession in overall herbaceous pollen and increase in Artemisia and Tilia pollen at the top of the profile (after AD 1200) could be attributed to the demise of the MWP and the transition to Little Ice Age.