Geomorphological and sedimentary evidence for late Pleistocene to Holocene hydrological change along the Río Mamoré, Bolivian Amazon

Aerial photography and satellite imagery reveal manifold geomorphological evidence of a dynamic evolution of past and present rivers in the Bolivian Amazon. Comparison of oxbow lake and meander scar dimensions along an inactive meander belt of the Río Mamoré (Llanos de Moxos, north-eastern Bolivia) and its modern counterpart suggests significant regional paleohydrological variability. We used these features as geomorphological and sedimentary archives to enhance our understanding of longer-term variations of the fluvial system. Late Pleistocene to Holocene hydrological changes of the Río Mamoré are inferred from: (i) the analysis of satellite imagery, (ii) discharge estimates from meander morphology, (iii) stratigraphic, and (iv) chronological information based on luminescence and radiocarbon dating. The combined data from three oxbows indicate that the now abandoned meander belt – the paleo-Mamoré – continued to be active at least until ∼5 ka, and likely even postdating 3 ka. An up to threefold increase in discharge is estimated for the modern Río Mamoré versus the paleo-Mamoré. The altered runoff regime may have triggered an avulsive shift towards the currently active Río Mamoré. The preceding increase in discharge in turn, was possibly related to a shift in climatic conditions, which changed markedly between the mid- and late Holocene in tropical South America. In addition, it may have been the indirect result of capturing the avulsive Río Grande system to the east of the Río Mamoré. Alternative explanations for the differences in dimensions of the paleo versus the modern Río Mamoré, i.e. contemporaneous activity of both rivers or alteration of site factors such as the channel/floodplain relationship, are considered to be unlikely.

Lake-level rise in the late Pleistocene and active subaquatic volcanism since the Holocene in Lake Kivu; East African Rift

The history of Lake Kivu is strongly linked to the activity of the Virunga volcanoes. Subaerial and subaquatic volcanoes, in addition to lake-level changes, shape the subaquatic morphologic and structural features in Lake Kivu's Main Basin. Previous studies revealed that volcanic eruptions blocked the former outlet of the lake to the north in the late Pleistocene, leading to a substantial rise in the lake level and subsequently the present- day thermohaline stratification. Additional studies have speculated that volcanic and seismic activities threaten to trigger a catastrophic release of the large amount of gases dissolved in the lake. The current study presents a bathymetric mapping and seismic profiling survey that covers the volcanically active area of the Main Basin at a resolution that is unprecedented for Lake Kivu. New geomorphologic features identified on the lake floor can accurately describe related lake-floor processes for the first time. The late Pleistocene lowstand is observed at 425 m depth, and volcanic cones, tuff rings, and lava flows observed above this level indicate both subaerial and subaquatic volcanic activities during the Holocene. The geomorphologic analysis yields new implications on the geologic processes that have shaped Lake Kivu's basin, and the presence of young volcanic features can be linked to the possibility of a lake overturn.

Geomorphology and evolution of the late Pleistocene to Holocene fluvial system in the south-eastern Llanos de Moxos, Bolivian Amazon

In the Bolivian Amazon several paleochannel generations are preserved. Their wide spectrum of morphologies clearly provides crucial information on the type and magnitude of geomorphic and hydrological changes within the drainage network of the Andean foreland. Therefore, in this study we mapped geomorphological characteristics of paleochannels, and applied radiocarbon and optically stimulated luminescence dating. Seven paleochannel generations are identified. Significant changes in sinuosity, channel widths and river pattern are observed for the successive paleochannel generations. Our results clearly reflect at least three different geomorphic and hydrological periods in the evolution of the fluvial system since the late Pleistocene. Changes in discharge and sediment load may be controlled by combinations of two interrelated mechanisms: (i) spatial changes and re-organizations of the drainage network in the upper catchment, and/or (ii) climate changes with their associated local to catchment-scale modifications in vegetation cover, and changes in discharge, inundation frequencies and magnitudes, which have likely affected the evolution of the fluvial system in the Llanos de Moxos. In summary, our study has revealed the enormous potential which geomorphic mapping and analysis combined with luminescence based chronologies hold for the reconstruction of the late Pleistocene to recent fluvial system in a large portion of Amazonia.

Late Glacial and Holocene environmental history of the Pirin Mountains (SW Bulgaria): a paleolimnological study of Lake Dalgoto (2310 m)

Diatoms, Cladocera, and chironomids preserved in the sediments of Lake Dalgoto were studied to reconstruct the history of the lake ecosystem in the context of the vegetation history as represented by the pollen stratigraphy. Younger Dryas silty sediments at the base of the core are characterized by low diversity of aquatic organisms. The transition to the Holocene is indicated by a sharp change from silt to clay-gyttja. The migration and expansion of trees at lower elevations between 10200 and 8500 14C-yr BP, along with higher diversities and concentrations of aquatic organisms and the decreased proportion of north-alpine diatoms, point to rapidly rising summer temperatures. After 6500 14C-yr BP the expansion of Pinus mugo in the catchment coincides with signs of natural eutrophication as recorded by an increase of planktonic diatoms. In the late Holocene (4000–0 14C-yr BP) Pinus peuce and Abies are reduced and Picea expands. Cereal grains and disturbance indicators suggest late-Holocene human modification of the vegetation.

A Late-glacial and Holocene record of climatic change from a Swiss peat humification profile

Colorimetric measurements of alkaline extracts from two Swiss peat cores have provided a complete 14500-year-long record of peat humification, a proxy of effective precipitation. Peat from the cold Younger Dryas (11050–9550 cal. bc) was well preserved despite low levels of precipitation. A particularly dry period, peaking at c. 7100 cal. bc, is indicated by well-decomposed peat. Peat from c. 6750–4250 cal. bc shows a low degree of decomposition, indicating a wet bog surface despite relatively warm temperatures and therefore indicating high levels of precipitation. A sharp transition to higher levels of decomposition c. 4450–3750 cal. bc indicates a major transition to a drier bog surface. Subsequently, peat humification generally decreases towards the end of the deeper profile (c. cal. ad 1050), indicating a gradual return to wetter conditions. This gradual decrease is punctuated by periods of particularly low humification which appear to be due to shifts to higher levels of effective precipitation from c. 2500 to 1350 cal. bc, c. 1050 to 550 cal. bc, centered around 150 cal. bc, and from c. cal. ad 550 onwards. Anthropogenic influences appear to have affected peat humification at the site at least since the Middle Ages. This study indicates that humification in colder regions/time periods could be more affected by temperature than precipitation and vice versa.

Late Glacial and Holocene vegetational changes on the Ulagan high-mountain plateau, Altai Mountains, southern Siberia

Three well-dated pollen diagrams from 1985 m, 2050 m, and at the tree line at 2150 m asl show the vegetational succession in the central Altai Mountains since 16 cal ka BP. Pioneer vegetation after deglaciation was recorded first at the lowest site. Subsequently, dense dry steppe vegetation developed coincident with the change from silt to organic sediments at the two lower sites, but silt lasted longer at the highest site, indicating the persistence of bare ground there. Forests of Pinus sibirica, Pinus sylvestris, Picea obovata, Larix sibirica, Abies sibirica, and Betula pendula started to develop about 12 cal ka BP with the change to a warmer and wetter climate at the beginning of the Holocene. Results indicate that the timberline did not rise above the highest site. Mesophilous dark-coniferous forests were fully developed by 9.5 cal ka BP. The role of Abies and Picea decreased by about 7.5 cal ka BP suggesting cooler climate, after which the forests changed little until today. The vegetational development in this portion of the central Altai Mountains is compatible with that described in neighbouring areas of the Altai, southern Siberia, Mongolia, and Kazakhstan.