Tephra from historic and prehistoric eruptions of Oraefajøkull and Hekla, Iceland, have been found in Irish peats. Using a series of stratigraphically related high-precision radiocarbon measurements, the date of a known-age eruption has been satisfactorily estimated. Based on the same techniques, the calendrical date of the Hekla 4 eruption has been estimated as 2310 ± 20 BC.
Discrete layers of tephra have been found in both raised and blanket bogs in the north of Ireland. Some of the layers have been identified to an Icelandic source and, in two cases, specific eruptions of known radiocarbon age are suggested. Nine layers spanning the Holocene are replicated in three lowland raised bogs. One of the major layers has also been identified at a number of sites in two separate upland blanket peat systems and in a lake sediment.
Volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and/or asynchronous behaviors in Earth system and archaeological sciences. However, it is generally assumed that only exceptionally powerful events, such as supereruptions (≥450 km 3 of ejecta as dense-rock equivalent; recurrence interval of ~10 5 yr), distribute ash broadly enough to have an impact on human society, or allow us to address geologic, climatic, and cultural questions on an intercontinental scale. Here we use geochemical, age, and morphological evidence to show that the Alaskan White River Ash (eastern lobe; A.D. 833-850) correlates to the "AD860B" ash (A.D. 846-848) found in Greenland and northern Europe. These occurrences represent the distribution of an ash over 7000 km, linking marine, terrestrial, and ice-core records. Our results indicate that tephra from more moderate-size eruptions, with recurrence intervals of ~100 yr, can have substantially greater distributions than previously thought, with direct implications for volcanic dispersal studies, correlation of widely distributed proxy records, and volcanic hazard assessment.
Microscopic studies of late-Quaternary tephras from Iceland yield a detailed tephrochronology in deposits of the British Isles, where the quantities of volcanic glass employed are much less than those used to construct almost any other late-Quatemary tephrochronology. We summarize improved methods of isolating extremely sparse glass shards for light microscope and electron microprobe analysis and provide major element geochemistry for 20 volcanic glass layers detected in Ireland, including those which comprise an outline historic tephrochronology for Ireland. The paper highlights possible limitations of chronology construction when attempting to separate dated tephras whose major element geochemistries are almost identical. Tephras in other parts of the British Isles and Europe are used, with those in Ireland, to demonstrate further applications of A tephrochronology, especially for dating deposits where other dating methods (e.g., radiocarbon) have proved HOLOCENE difficult.RESEARCH REPORT R
Annually resolved fossil records of nitrogen (N) inputs (as sedimentary d 15 N, N content), aquatic production (d 13 C, C content), and algal abundance and gross community composition (pigments, nonsiliceous microfossils) from Lough Neagh, Northern Ireland (NI), were compared with annual records of climatic variability, atmospheric and urban nutrient loading, whole-catchment nutrient budgets, and limnological monitoring data to identify the unique effects of N on the eutrophication of a phosphorus (P)-rich lake during ca. 1933-1995. Cluster analysis revealed two major biostratigraphic zones. Zone I (ca. 1933-1955) was characterized by moderate lake production, as inferred from low concentrations of most fossil pigments and reduced d 15 N signatures but elevated d 13 C values and chlorophyte microfossil concentrations. In contrast, Zone II (ca. 1955II (ca. -1995 exhibited greatly increased contents of 15 N, N, C, and algal pigments, combined with strongly reduced d 13 C ratios and chlorophyte fossil abundance, a pattern consistent with recent severe eutrophication. Overall, microfossils of diazotrophic cyanobacteria were most abundant during the transition period between zones (ca. 1955-1964). Regression analysis revealed that past N influx to the lake (as d 15 N; r 2 5 0.916, p , 0.0001), colonial cyanobacterial abundance (as myxoxanthophyll; r 2 5 0.837, p , 0.0001), and total algal standing crops (as b-carotene; r 2 5 0.388, p , 0.0001) were all strongly correlated to agricultural inputs of N to NI farmland, weakly correlated to P inputs to NI farmland (r 2 d15N 5 0.503, p , 0.0001; r 2 cyanobacteria 5 0.296, p , 0.0001; r 2 total algae 5 0.046, p . 0.05), and uncorrelated to most measures of climatic variability and atmospheric or urban nutrient inputs. Thus, degradation of water quality during the 20th century resulted from excessive loading of diffuse N to the lake from P-rich agricultural lands.
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