Late-Holocene climate dynamics recorded in the peat bogs of Tierra del Fuego, South America

The ombrotrophic peat bogs of Tierra del Fuego are located within the southern westerly wind belt (SWWB), which dominates climate variability in this region. We have reconstructed late-Holocene water-table depths from three peat bogs and aimed to relate these records to shifts in regional climate. Water-table depths were quantified by the analysis of testate amoeba assemblages, and a regional transfer function was used to infer past water-table depths. During the last 2000 years, testate amoeba assemblages have been relatively stable, with a dominance of Difflugia pulex and Difflugia pristis type, and an increase in Assulina muscorum and other Euglyphida at the top of each section. Multivariate analyses show that water-table depth remained the main environmental variable explaining assemblages along the TiA12 core, but reconstructions were not significant for the two other cores. In line with the low variability in assemblages, water tables were relatively stable during the last 2000 years. Slightly wetter conditions were found between ~1400 and 900 cal. BP and a pronounced recent dry shift was reconstructed in all of the three peat profiles. Considering the regional climatic context, this recent shift may have been forced by a decrease in precipitation and warmer conditions linked to an increase in the importance of the SWWB. Nevertheless, we cannot exclude the influence of higher UV-B radiation resulting from the local degradation of the ozone layer since the late 1970s, which may have had an additional effect on the relative presence of A. muscorum in the southern Patagonian region

An alternative approach to transfer functions? Testing the performance of a functional trait-based model for testate amoebae

publisher: Elsevier articletitle: An alternative approach to transfer functions? Testing the performance of a functional trait-based model for testate amoebae journaltitle: Palaeogeography, Palaeoclimatology, Palaeoecology articlelink: http://dx.doi.org/10.1016/j.palaeo.2016.12.005 content_type: article copyright: © 2016 The Authors. Published by Elsevier B.V

Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward

On Quaternary time scales, the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae (notably diatoms, for which Si is an essential nutrient). Large tropical river systems dominate the export of Si from the continents to the oceans. Here, we investigate variations in Si cycling in the upper White Nile basin over the last 15 ka, using sediment cores from Lakes Victoria and Edward. Coupled measurements of stable O and Si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and Si cycling, while the abundances of lipid biomarkers characteristic of terrestrial/emergent higher plants, submerged/floating aquatic macrophytes and freshwater algae document past ecosystem changes. During the late-glacial to mid-Holocene, 15–5.5 ka BP, orbital forcing greatly enhanced monsoon rainfall, forest cover and chemical weathering. Riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower Si-isotope values. Since 5.5 ka BP, increasingly dry climates and more open vegetation, reinforced by the spread of agricultural cropland over the last 3–4 ka, have reduced dissolved silica inputs into the lakes. Centennial-to millennial-scale dry episodes are also evident in the isotopic records and merit further investigation

Impact of pre-Columbian 'geoglyph' builders on Amazonian forests

Over 450 pre-Columbian (pre-AD1492) geometric ditched enclosures ('geoglyphs') occupy ca. 13,000 km2 of Acre state, Brazil, representing a key discovery of Amazonian archaeology. These huge earthworks were concealed for centuries under terra firme (upland interfluvial) rainforest, directly challenging the 'pristine' status of this ecosystem and its perceived vulnerability to human impacts. We reconstruct the environmental context of geoglyph construction and the nature, extent and legacy of associated human impacts. We show that bamboo forest dominated the region for ≥6000 y and that only small, temporary clearings were made to build the geoglyphs; however, construction occurred within anthropogenic forest that had been actively managed for millennia. In the absence of widespread deforestation, exploitation of forest products shaped a largely forested landscape that survived intact until the late 20th century

A late-Quaternary perspective on atmospheric pCO(2), climate, and fire as drivers of C-4-grass abundance

Various environmental factors, including atmospheric CO2 (pCO(2)), regional climate, and fire, have been invoked as primary drivers of long-term variation in C-4 grass abundance. Evaluating these hypotheses has been difficult because available paleorecords often lack information on past C-4 grass abundance or potential environmental drivers. We analyzed carbon isotope ratios (C-13) of individual grains of grass pollen in the sediments of two East African lakes to infer changes in the relative abundance of C-3 vs. C-4 grasses during the past 25000 years. Results were compared with concurrent changes in pCO(2), temperature, moisture balance, and fire activity. Our grass-pollen C-13 analysis reveals a dynamic history of grass-dominated vegetation in equatorial East Africa: C-4 grasses have not consistently dominated lowland areas, and high-elevation grasses have not always been predominantly C-3. On millennial timescales, C-4 grass abundance does not correlate with charcoal influx at either site, suggesting that fire was not a major proximate control of the competitive balance between C-3 and C-4 grasses. Above the present-day treeline on Mt. Kenya, C-4 grass abundance declined from an average of 90% during the glacial period to less than 60% throughout the Holocene, coincident with increases in pCO(2) and temperature, and shifts in moisture balance. In the lowland savanna southeast of Mt. Kilimanjaro, C-4 grass abundance showed no such directional trend, but fluctuated markedly in association with variation in rainfall amount and seasonal-drought severity. These results underscore spatiotemporal variability in the relative influence of pCO(2) and climate on the interplay of C-3 and C-4 grasses and shed light on an emerging conceptual model regarding the expansion of C-4-dominated grasslands in Earth's history. They also suggest that future changes in the C-3/C-4 composition of grass-dominated ecosystems will likely exhibit striking spatiotemporal variability as a result of varying combinations of environmental controls