Post-supereruption recovery at Toba Caldera

Large calderas, or supervolcanoes, are sites of the most catastrophic and hazardous events on Earth, yet the temporal details of post-supereruption activity, or resurgence, remain largely unknown, limiting our ability to understand how supervolcanoes work and address their hazards. Toba Caldera, Indonesia, caused the greatest volcanic catastrophe of the last 100 kyr, climactically erupting ~74 ka. Since the supereruption, Toba has been in a state of resurgence but its magmatic and uplift history has remained unclear. Here we reveal that new 14 C, zircon U-Th crystallization and (U-Th)/He ages show resurgence commenced at 69.7±4.5 ka and continued until at least ~2.7 ka, progressing westward across the caldera, as reflected by post-caldera effusive lava eruptions and uplifted lake sediment. The major stratovolcano north of Toba, Sinabung, shows strong geochemical kinship with Toba, and zircons from recent eruption products suggest Toba's climactic magma reservoir extends beneath Sinabung and is being tapped during eruptions

The Chironomidae of Gróthúsvatn, Sandoy, Faroe Islands: climatic and lake phosphorus reconstructions, and the impact of human settlement

Chironomids were examined as part of a multiproxy palaeolimnological study of Gróthúsvatn, Sandoy, Faroe Islands. A total of 37 taxa was found in the top 1 m of the core. Chironomid-inferred total phosphorus and temperature reconstructions indicate that after landnám (the Norse settlement period) total phosphorus levels rose slightly, then peaked at a temperature minimum (interpreted as the ‘Little Ice Age’). Total phosphorus levels subsequently fell to only slightly above pre-settlement levels at the sediment surface. Little indication of increased erosion in the catchment after landnám was found, and it is likely that the impacts of human settlement on Gróthúsvatn and its catchment were slight. We therefore propose that the temperature decrease caused the increase in the lake’s productivity

Human impact on an island ecosystem: pollen data from Sandoy, Faroe Islands

Aim: To investigate the form and dynamics of ecosystems on an isolated island in the North Atlantic before human settlement in the first millennium ad, and the effects of human activities thereafter. Location: The island of Sandoy, Faroes (6150¢ N, 645¢ W). Methods: Two sequences of lake sediments and one of peat were studied using pollen analysis and sedimentological techniques. Age models were constructed on the basis of radiocarbon dating and, in one case, tephrochronology. The data were analysed statistically and compared with existing data from the region. Results: The pollen data indicate that early Holocene vegetation consisted of fell-field communities probably growing on raw, skeletal soils. These communities gave way to grass- and sedge-dominated communities, which in turn were largely replaced by dwarf shrub-dominated blanket mire communities well before the first arrival of humans. There is evidence for episodic soil erosion, particularly in the uplands. Changes in the records attributable to human impact are minor in comparison with many other situations in the North Atlantic margins, and with certain published sequences from elsewhere in the Faroes. They include: (1) the appearance of cereal pollen and charcoal, (2) an expansion of ruderal taxa, (3) a decline in certain taxa, notably Juniperus communis and Filipendula ulmaria, and (4) a renewed increase in rates of upland soil erosion. The reliability of palaeoecological inferences drawn from these sites, and more generally from sites in similar unforested situations, is discussed. Main conclusions: The subdued amplitude of palynological and sedimentological responses to settlement at these sites can be explained partly in terms of their location and partly in terms of the sensitivity of different parts of the ecosystem to human activities. This study is important in establishing that the imposition of people on the pristine environment of Sandoy, while far from negligible, especially in the immediate vicinity of early farms and at high altitudes, had relatively little ecological impact in many parts of the landscape

Environmental impacts of the Norse settlement: palaeoenvironmental data from Mývatnssveit, northern Iceland

The first stratigraphically continuous pollen profile spanning the Norse and Medieval periods from the archaeologically-rich My´vatnssveit region of northern Iceland is presented. Detailed analyses were made of the tephra, sediment characteristics, pollen and chironomids of a 3 kyr sediment sequence from Helluvaðstjo¨ rn, a small, shallow lake. The pollen data show a steady decline in the percentage abundance of tree birch (Betula pubescens) pollen between the Norse settlement (landna´m, c. AD 870) and c. AD 1300, a pattern that contrasts with the abrupt fall in birch pollen percentages immediately following the Norse colonization at almost all previously studied sites in Iceland. Some lines of evidence suggest that the gradual birch decline could be a result of reworking of soil pollen, but independent evidence suggests that this may not necessarily be the case. The pollen record indicates that birch woodland was replaced by acidophilic taxa (notably Empetrum nigrum and Sphagnum), again contrasting with the more usual pattern of Poaceae expansion seen in post-landna´m pollen diagrams from mires close to farm sites. Chironomid and Pediastrum accumulation data show that the limnic environment became more productive immediately after landna´m, probably because of anthropogenic disturbance. An increase in sedimentation rate after landna´m appears initially to have been caused by increased lake productivity, while reworked inorganic soil materials became a significant contributor to the sediments after c. AD 1200. The data suggest that the impact of settlement on terrestrial vegetation may have been more variable than previously thought, while freshwater ecosystems experienced significant and rapid change

Human impact on freshwater environments in Norse and early medieval Iceland

Preliminary data from fish bone assemblages preserved in middens at Norse and early medieval farm sites in Mývatnssveit, northern Iceland, suggest that the pattern of fish exploitation changed during the first few centuries following the settlement. Freshwater taxa become less common in deposits at some sites between the 9th and 12th centuries AD, replaced by saltwater species such as cod or by domestic mammals. Within the freshwater fish taxa, the proportion of lake (e.g. arctic charr) to river fish (e.g. brown trout) tends to increase over the same period. One possible interpretation of these patterns is that fish stocks in rivers and, perhaps to a lesser extent, lakes declined in response to degradation of freshwater environments following the settlement. Here we examine palaeolimnological data which suggest that increased sedimentation rates, increased nutrient status, and increased algal and chironomid production in some lakes occurred in concert with clearance of woodland and destabilisation of soils following landnám. These changes are unlikely to have had a negative impact on the food supply for freshwater fish, so an alternative explanation must be sought for the patterns in the archaeofaunal data