Yr AD Research Articles

Concentration of 10Be in an ice core from the Dome Fuji station, Eastern Antarctica: Preliminary results from 1500 to 1810 yr AD

We present preliminary data of our 10Be analysis of an ice core retrieved from Dome Fuji station, Eastern Antarctica for 1500–1810 yr AD. The concentration of 10Be from 1500-1810 yr AD ranged between 7.0×104 and 13.0×104 atoms g−1 and a prominent peak was observed in the period 1645–1715 yr AD (i.e., the Maunder Minimum period). An increase in the concentration was also observed in the periods before 1540 yr AD and near to 1800 yr AD. A comparison of our 10Be record with the South Pole 10Be record shows a clear similarity in their temporal fluctuations. On the other hand, our record shows ∼2.5 times higher concentration of 10Be than in the South Pole record attributable to the difference in local snow accumulation rates. These observations suggest a direct (uncomplicated) fallout of atmospheric 10Be onto inland Antarctica, which in turn, leads to a detailed 10Be stratigraphy throughout this region. We believe that the 10Be profiles of the ice cores from the Dome Fuji station are therefore likely to be a good proxy indicator of changes in solar activity.

A multiproxy climate record from a raised bog in County Fermanagh, Northern Ireland: a critical examination of the link between bog surface wetness and solar variability

AbstractA proxy climate record from a raised bog in County Fermanagh, Northern Ireland, is presented. The record spans the interval between 2850 cal. yr BC and cal. yr AD 1000 and chronological control is achieved through the use of tephrochronology and 14C dating, including a wiggle‐match on one section of the record. Palaeoclimatic inferences are based on a combination of a testate amoebae‐derived water table reconstruction, peat humification and plant macrofossil analyses. This multiproxy approach enables proxy‐specific effects to be identified. Major wet shifts are registered in the proxies at ca. 1510 cal. yr BC, 750 cal. yr BC and cal. yr AD 470. Smaller magnitude shifts to wetter conditions are also recorded at ca. 380 cal. yr BC, 150 cal. yr BC, cal. yr AD 180, and cal. yr AD 690. It is hypothesised that the wet shifts are not merely local events as they appear to be linked to wider climate deteriorations in northwest Europe. Harmonic analysis of the proxies illustrates statistically significant periodicities of 580, 423–373, 307 and 265 years that may be related to wider Holocene climate cycles. This paper illustrates how the timing of climate changes registered in peat profiles records can be precisely constrained using tephrochronology to examine possible climatic responses to solar forcing. Relying on interpolated chronologies with considerable dating uncertainty must be avoided if the climatic responses to forcing mechanisms are to be fully understood. Copyright © 2007 John Wiley & Sons, Ltd.

Climatic changes vs. catastrophic events in lacustrine systems: A geochemical approach

A geochemical study of oligotrophic Lake Sanabria (NW Iberian Peninsula) allows us to distinguish the geochemical response of a lacustrine system to catastrophic events and climatic events. The main major elements analysed were: K, Na, Mg, Ca, Ti, Al, Fe, Mn, P and S. The organic matter content in the sediment of Lake Sanabria is determined by the rainfall regime of the region. Periods when sediments were enriched in Ti, Mg, K, and Al are indicative of relatively wet paleoenvironmental conditions, whereas intervals when sediments were enriched in S and P (associated with organic matter) are indicative of dry conditions. A factor analysis of the geochemical data shows a progressive temporal displacements from organic rich conditions (relatively high values of S and P) to inorganic rich conditions (relatively high values of Ti, Mg, K, Al, and Na). Maximal inorganic content occurred at the beginning of two discrete sedimentary levels: the LOL2 (cal. 1160–1230 yr AD) and the LOL1 (cal. 1510–1680 yr AD) horizons. The LOL2 and LOL1 horizons constitute two discrete events related to climatic changes during the beginning and end of the Little Ice Age, respectively. In contrast, the geochemical analysis provides evidence that the samples associated with the 1959 catastrophic event that occurred in the catchment area of Lake Sanabria do not follow the long-term geochemical evolution paths of the climate driven system.

A chronology for late prehistoric Madagascar

A database has been assembled with 278 age determinations for Madagascar. Materials 14C dated include pretreated sediments and plant macrofossils from cores and excavations throughout the island, and bones, teeth, or eggshells of most of the extinct megafaunal taxa, including the giant lemurs, hippopotami, and ratites. Additional measurements come from uranium-series dates on speleothems and thermoluminescence dating of pottery. Changes documented include late Pleistocene climatic events and, in the late Holocene, the apparently human-caused transformation of the environment. Multiple lines of evidence point to the earliest human presence at ca. 2300 14C yr BP (350 cal yr BC). A decline in megafauna, inferred from a drastic decrease in spores of the coprophilous fungus Sporormiella spp. in sediments at 1720 ± 40 14C yr BP (230–410 cal yr AD), is followed by large increases in charcoal particles in sediment cores, beginning in the SW part of the island, and spreading to other coasts and the interior over the next millennium. The record of human occupation is initially sparse, but shows large human populations throughout the island by the beginning of the Second Millennium AD. Dating of the “subfossil” megafauna, including pygmy hippos, elephant birds, giant tortoises, and large lemurs, demonstrates that most if not all the extinct taxa were still present on the island when humans arrived. Many taxa overlapped chronologically with humans for a millennium or more. The extinct lemurs Hadropithecus stenognathus, Pachylemur insignis, Mesopropithecus pithecoides, and Daubentonia robusta, and the elephant birds Aepyornis spp. and Mullerornis spp., were still present near the end of the First Millennium AD. Palaeopropithecus ingens, Megaladapis edwardsi, and Archaeolemur sp. (cf. edwardsi) may have survived until the middle of the Second Millennium A.D. One specimen of Hippopotamus of unknown provenance dates to the period of European colonization.

Late-Holocene natural and anthropogenic vegetation changes in the Dongbei Pingyuan (Manchurian Plain), northeastern China

Two peat profiles from the Manchurian Plain in northeastern China, with chronology based on 20 AMS measurements, provide new evidence for natural and anthropogenic environmental changes that occurred in the region during late Holocene. The onset of continuous organic accumulation interpreted as climatically induced shift in hydrological regime was dated at the Muchang site to 4200 (uncal.) BP, and at the Dahuofang site to at least 3560 BP. The start of peat growth, documented in the Muchang profile, was preceded by temporal intensification of fluvial processes around 4400/4300 BP. The evidence of hydrological changes in the Manchurian Plain correlates with the spread of Korean pine in the nearby Changbai mts. area in 4300–4000 BP, and can be associated with the initial stages of a trend towards present-day climatic conditions in the region. Late Holocene pollen sequences from Muchang and Dahuofang revealed the presence of oak and pine forests with the other minor deciduous constituents as Ulmus, Tilia, Carpinus, Acer and Fraxinus. Local distribution of oak was confirmed by plant macrofossil remains (leaf fragments) ascribed to Quercus x hopeiensis. The vegetation cover formed sparse woodland or mosaic of forests and steppe communities with Artemisia, Gramineae and Chenopodiaceae. Fossil pollen data clearly indicates that the spread of grasslands in the western part of Manchurian Plain occurred as a result of human impact. Extensive deforestation accompanied by agricultural practices (buckwheat cultivation) has been dated to 900–1100 cal yr AD and were correlated with development of the state of Liao dynasty (907–1125 AD). Evidence of earlier human involvement in forest destruction has been dated to around 900 cal yr BC. With each progressing anthropogenic deforestations phase, there was increased circulation of eolian dust resulting from landscape opening and soil erosion.

Sea-surface temperature variability in the 16th century at Bermuda inferred from coral records

Records of skeletal δ 18O in monthly and Sr/Ca ratios in half-yearly resolution were obtained from a Bermuda coral ( Diploria labyrinthiformis) for the time period 1520–1603 (±15 yr) AD within the Little Ice Age. Annual and decadal averages of both sea-surface temperature proxies indicate temperature variabilities of 0.5°C (standard deviation) and 0.3°C, respectively. Both numbers are close to recent instrumental observations. Approximately 30% of the interannual time series variance of δ 18O is concentrated in broad bands centered at periods of ∼30, 16, and 7.8 yr, the last two reflecting the influence of the North Atlantic Oscillation. Although this large-scale climate signal is present in the record, there is no correlation with other contemporaneous northern hemisphere proxy data, resulting from spatial differences in climate variability.

Seismogenic faulting in an area of low seismic activity: Paleoseismicity of the El Camp fault (Northeast Spain)

AbstractGiven that earthquakes do not occur only along high slip-rate faults, slow moving seismogenic faults should be characterized in order to minimize seismic hazard uncertainties. Although no historical earthquakes related to the El Camp fault have been documented, earlier regional geological analysis and the presence of a fault scarp provide evidence of its activity. A paleoseismological study on the southern part of the fault was performed in accordance with the following steps: 1) geological and géomorphologie analysis focussing on the detection of evidence for the seismogenic behavior of the fault, 2) near fault analysis to select the best trenching sites, and 3) trenching to establish and characterize the paleoseismic events. Different dating procedures were used in regional and trenching analyses (Thermoluminiscence, U/Th, Radiocarbon, Pollen analysis). The seismogenic nature of the fault was established by the presence of liquefaction features related to the fault, and by the presence of colluvial wedges composed of large angular blocks. We identified a segment boundary to the north of the Porquerola creek and we focussed our attention on the southern segment, which was active after 125000 yr. The slip rate in this southern segment is 0.02 mm/yr. A minimum of three seismic events were detected, from young to old: the last event Z took place some time prior to 1195 yr AD, the penultimate event Y between event X and the Holocene, and, finally, event X occurred after 125000 yr and prior to 60000 yr. The recurrence period is between 25000 and 35000 yr, the elapsed time is estimated to be no longer than 3000 yr; and the maximum estimated earthquake considering both the onshore and the offshore part of the fault is Mw 6.7 +/-0,5.

Diatoms as an aid in identifying late-Holocene tsunami deposits

Diatoms (Bacillariophyta) help identify the onshore deposits of tsunamis from earthquakes on the Cascadia subduction zone along the Pacific coast of Oregon, Washington, and British Columbia, and on faults high in the North American plate in the Puget Sound area of Washington. At the Copalis River, Washington, diatom analyses suggest that a tsunami deposit about 300 calendric years old (300 yr BP) originated from sandy shoals of the lower estuary rather than nearby beaches or coastal dunes. At Cultus Bay and West Point, Washington, well-preserved benthic estuarine diatoms in sand sheets overlying tidal-marsh peat indicate that the deposits came from intertidal or nearshore areas of Puget Sound. On an abruptly uplifted mudflat at the landward end of Hood Canal at Lynch Cove, Washington, tidal-flat diatoms refute the possibility of a terrestrial source for the sand. Diatoms in 300-yr-BP tsunami deposits on the Niawiakum River, Washington, confirm that the sand in these deposits had a marine source, and help to identify the landward extent of tsunami inundation. Diatom assemblages in deposits of the 300 yr BP and AD 1964 tsunamis at Port Alberni, British Columbia, consist of different dominant taxa, but both indicate that the sand units originated from Alberni Inlet. Diatoms add to stratigraphic evidence that tsunamis flooded Bradley Lake, a freshwater lake on the south-central Oregon coast, three times during the past 1700 years. Planktonic marine diatoms only found above 1-70-cm-thick sand layers in otherwise clayey lacustrine sediment imply tsunami inundation.