Radiocarbon Age Data Research Articles

Identifying the sources and timing of ancient and medieval atmospheric lead pollution in England using a peat profile from Lindow bog, Manchester.

A peat core from Lindow bog near Manchester, England, was precisely cut into 2 cm slices to provide a high-resolution reconstruction of atmospheric Pb deposition. Radiocarbon and (210)Pb age dates show that the peat core represents the period ca. 2000 BC to AD 1800. Eleven radiocarbon age dates of bulk peat samples reveal a linear age-depth relationship with an average temporal resolution of 18.5 years per cm, or 37 years per sample. Using the Pb/Ti ratio to calculate the rates of anthropogenic, atmospheric Pb deposition, the profile reveals Pb contamination first appearing in peat samples dating from ca. 900 BC which clearly pre-date Roman mining activities. Using TIMS, MC-ICP-MS, and SF-ICP-MS to measure the isotopic composition of Pb, the (208)Pb/(206)Pb and (206)Pb/(207)Pb data indicate that English ores were the predominant sources during the pre-Roman, Roman, and Medieval Periods. The study shows that detailed studies of peat profiles from ombrotrophic bogs, using appropriate preparatory and analytical methods, can provide new insight into the timing, intensity, and predominant sources of atmospheric Pb contamination, even in samples dating from ancient times.

The chronology of anthropogenic, atmospheric Pb deposition recorded by peat cores in three minerogenic peat deposits from Switzerland

Peat cores were collected from three minerogenic peatlands in Switzerland: La Tourbière des Genevez (TGE), in Canton Jura; Gola di Lago (GDL), in Canton Ticino; and Mauntschas (MAU), in Canton Grisons. Chemical analyses of the pore waters and the peats document the increasingly minerogenic character with depth. In particular, the Sr concentration profile in the peats reflects the growing importance of mineral sediment dissolution with increasing distance below the peat surface. Despite this circumstance, Pb concentrations are greatest in the surface layers, and decrease with depth. Thus, dissolution of the basal sediments at these sites appears to be an unimportant source of Pb, compared with atmospheric deposition. Twenty-one new radiocarbon age dates for the three cores provide a chronology of Pb enrichment, and these are consistent with the record of atmospheric Pb deposition recorded by the ombrotrophic peat bog ‘Etang de la Gruère’ (EGR). Sediment samples from TGE and EGR were reacted at pH 5 with ammonium acetate to dissolve the carbonate fraction, filtered, and the residues analyzed using XRF. These analyses show that much of the Ca, Mg, Sr and Mn is lost when the carbonate fraction is dissolved, but this process does not measurably affect Pb. This finding suggests that carbonate dissolution dominates the weathering of the sediments underlying these peat deposits, but this fraction does not contain significant concentrations of Pb. Minerogenic peat deposits such as those described here, therefore, can serve as reliable archives of atmospheric Pb deposition, provided that mineral dissolution in the underlying sediments does not contribute measurably to the Pb inventory of the peat profile.

Recurrence rates of large earthquakes in the South Carolina Coastal Plain based on paleoliquefaction data

We present a reanalysis of results of 15 years of paleoliquefaction investigations in the South Carolina Coastal Plain. All earlier radiocarbon age data and locations of organic material collected by various investigators were reviewed and recalibrated to obtain a uniform data set. The calibrated dates and the spatial extent of the sandblows having similar dates were used to estimate ages and magnitudes of prehistoric earthquake episodes. The results of this analysis suggest seven episodes (episodes A‐G) of prehistoric liquefaction in the past 6000 years and two possible scenarios for their occurrence. In the first scenario, three seismic sources exist within the Coastal Plain of South Carolina; at Charleston (A, B, E, and G) with magnitudes M 7+, Georgetown (C and F), and Bluffton (D) with magnitudes M∼6. In the second scenario, episodes C and D are combined into one episode, episode C′. In this scenario all earthquakes occurred at Charleston and with M 7+. Episodes A and B seem to be more representative of the earthquake cycle and suggest a recurrence time of 500–600 years for M 7+ earthquakes at Charleston. The recurrence times and magnitudes for episodes C and D are estimated at ≥2000 years and ∼6.0, respectively. The older episodes are less frequent, a fact that may be attributable to times of low ground water table. Before ∼6000 years B.P., the ground water table was too low to permit observable liquefaction features to develop at the surface.

Estimate of recharge from radiocarbon dating of groundwater and numerical flow and transport modeling

This paper combines radiocarbon age and hydraulic data to estimate recharge to a regional groundwater aquifer. The 14C ages of groundwater are first corrected for the effects of chemical reactions through geochemical modeling. Recharge rates to the aquifer are then calibrated to observed 14C ages using a linked numerical 14C transport and flow model while hydraulic conductivity values are proportionally adjusted to match observed heads. The methodology is general but has particular applications to semiarid and arid regions where the need for reliable recharge estimates is greatest but the task is the most difficult. The ability of this method for estimating recharge on the scale of thousands of years makes it a valuable tool for studies of global environmental changes and performance assessment of radioactive waste repositories. A case study of a regional aquifer in northeastern Arizona shows the recharge estimates are within the bounds determined by chloride mass balance but are significantly higher than previous estimates derived from the Maxey‐Eakin method.

Radiocarbon-dated Holocene pollen and ostracod sequences from barrage tufa- dammed fluvial systems in the White Peak, Derbyshire, UK

Sedimentary evidence, in the form of pollen, ostracod and radiocarbon-age data from three barrage tufa-dammed systems in the White Peak area of Derbyshire, is used to reconstruct the region's environmental history from 10000 to 4000BP. The data highlight the useful role that such fluvial systems can play in environmental reconstructions, through their ability to act as sumps for a wide range of proxy indicators, in a region where environmental change during the Holocene period has been relatively poorly documented. Pollen data suggest that the period of climatic amelioration following the last (Devensian) glaciation facilitated the spread of thermophilous woodland taxa on the White Peak, although there are some indications that the first immigrant trees were not established until a relatively late date. Forest clearance commencing during the mid-Holocene, and possibly as early as the late Mesolithic, was most probably to provide land for grazing, and was virtually complete by 4000BP. Ostracod data reveal fluctuations in discharge of the River Lathkill draining the White Peak, particularly lower flow rates after 7400 BP, which are difficult to explain simply in terms of climate change.

Evidence for the Two Creeks interstade in the Lake Huron basin

New radiocarbon age dates for the Cheboygan bryophyte bed in northern lower Michigan indicate that the bed was not deposited during the Mackinaw interstade, as was previously proposed, but is correlative to the Two Creeks forest bed deposited during the Two Creeks interstade approximately 11 850 BP. Furthermore, the till overlying the bryophyte bed does not represent continuous deposition by ice throughout the Two Creeks interstade, as proposed by others, but represents deposition during the Greatlakean stade. A major implication resulting from the reassignment of the age of the Cheboygan bryophyte bed is that the Straits of Mackinac could have been ice free during the Two Creeks interstade and that during that time the Kirkfield phase of glacial Lake Algonquin may have extended into the Lake Michigan basin.

Sea level control of stacked late Quaternary coastal sequences, central Great Barrier Reef

ABSTRACTLithofacies analysis, pollen assemblages and radiocarbon age dates of 20 stratigraphic drill holes are used to develop an evolutionary history for late Quaternary sedimentation in two coastal embayments landward of the central Great Barrier Reef. Different physiographic settings of the embayments result in two contrasting styles of sedimentary sequence: (a) an exposed, moderate energy, beach barrier‐lagoon system (Wyvuri Embayment) and (b) a protected, low energy, muddy inlet fill sequence (Mutchero Inlet). Despite sharp contrast in sequence style, similar depositional cycles occur in both embayments in response to late Quaternary sea level fluctuations including: (1) a last interglacial highstand (+2 m; c. 125 000 yr bp) beach barrier (Wyvuri); (2) an early to mid‐Holocene (8000–6100 yr bp) transgressive beach barrier‐lagoon (Wyvuri) and estuarine infill (Mutchero); and (3) mid‐Holocene to present highstand beach barrier (Wyvuri) and estuarine (Mutchero) progradation. Preservation of such cycles in the stratigraphic record would produce a series of vertically stacked and offset linear barrier sands surrounded by lagoonal mud and fine grained shoreface sediment juxtaposed to muddy, estuarine infills.Sea level elevations are well recorded by the upward transition from Rhizophora‐dominated intertidal mangrove mud to freshwater swamp (clearly identified by pollen analysis) and by the basal contacts of beach barrier sediments which sharply overlie the upper shoreface. Transgressive sedimentation is interrupted in both embayments by a constructional beach barrier (Wyvuri) and abbreviated progradation (Mutchero) corresponding to a — 5 m pause in relative sea level rise at c. 6800 yr bp. Sea level control of fine scale coastal sedimentation patterns is beginning to be widely recognized and provides an accurate analogue for stacked ancient sequences.

Timing of the last highstand of Lake Lahontan

Radiocarbon and uranium-series ages of a variety of materials from the Lahontan basin indicate that the last highstand lake occurred between 14 500 and 13 000 yr B.P. Although few in number, existing radiocarbon and uranium-series age data also indicate that lakes in the western Lahontan subbasins were small or moderate in size between 30 000 and 25 000 yr B.P. Existing data do not support the conclusions of Bradbury et al. (1989) who did not find evidence of a 14 000±yr B.P. highstand lake in the sediments of the Walker Lake subbasin. These data also do not support the existence of a highstand lake in the Walker Lake subbasin between 30 000 and 25 000 yr B.P.

Mud redeposition and problems of assessing microfossil, isotopic and radiocarbon data in the Mediterranean

A literature review of Mediterranean margin and basin stratigraphy indicates that most sampling for microfossil, isotopic and radiocarbon age data is performed on finegrained sediments. Comprehensive petrologic analyses of suites of cores collected in different Mediterranean slope—basin plain systems indicate that at least two-thirds of the muds of Pliocene and Quaternary age were emplaced by downslope-directed mass flow and related sediment gravity flows, such as mud-carrying turbidity currents, rather than by suspension settling (hemipelagic “rain”) mechanisms. Not surprisingly microfossil and isotopic studies undertaken without detailed petrologic examination of sampled mud sections have led to some spurious interpretations. Sedimentological surveys in most of the Mediterranean sectors indicate that episodic redepositional events have disturbed the sedimentary record sufficiently to produce discontinuity of strata, complicate regional litho- and biostratigraphic correlations and seriously confuse paleoecological assessment. Radiocarbon dates also remain suspect unless it is demonstrated that the dated organic matter, usually microfossils, is not of downslope displaced origin but, rather, primarily by suspension settling to the seafloor. It is essential, therefore, to test for the reworking factor as a consequence of important local influences, such as physiographic configuration and accessibility to sediment sources and dispersal paths, as well as transport mechanisms. Reliability of interpretations is enhanced by closely coupling biostratigraphic with sedimentological analyses.

Author's reply

Morphological analysis of the Fortore River coastal plain and the Lesina Lake coastal barrier integrated with radiocarbon age data indicates that the evolution of the coastal landscape has been strongly affected by a number of strong earthquakes and related tsunamis which occurred during the last 3000 years. The first seismic event struck this coastal area in the V century BC. It produced strong erosion of the Fortore River coastal plain and significant emersion of Punta delle Pietre Nere, as well as the large tsunami responsible for the development of the Sant'Andrea washover fan. The second event occurred in 493 AD; it induced severe erosion of the Fortore River coastal plain and triggered the large tsunami that hit the Lesina Lake coastal barrier, producing the Foce Cauto washover fan. Then later in 1627, an earthquake was responsible for the further coseismic uplift of Punta delle Pietre Nere, the subsidence of Lesina village area and the development of a tsunami which produced two washover fans.Morphological analysis points out that seismic events strong enough to control the morphological evolution of local coastal landscapes show a statistical return period of about 1000 years. These major events produced important coseismic vertical movements and large tsunamis. However, the correct identification of the tectonic structure responsible for the generation of these strong earthquakes is still an unsolved problem.

Chemical evolution of groundwater in a drainage basin of Holocene age, east-central Alberta, Canada

Chemical evolution of groundwater in a small drainage basin of glacial origin (10,250 yr. B.P., based on radiocarbon age dating of gyttja from a closed saline lake in the basin) was studied in order to understand the generation of salts in surface-mined areas on the interior plains of Alberta. The basin was considered to be a natural analogue of a surface-disturbed area because of the large volumes of rock that had been redistributed by glaciers with the resulting change in topography and drainage. The distributions of hydraulic head, total dissolved solids (TDS), and environmental isotopes essentially reflect the superimposition of groundwater flow systems associated with the post-glacial topography upon a regional bedrock flow system of older but undertermined age. In the glacial drift aquifers and aquitards (sands and till), the groundwater composition was typically Ca-Mg-bicarbonate type at depths less than 30 m, but at depths of 30–100 m, the composition was Na-bicarbonate-sulfate type. In the deeper bedrock aquifers (> 100 m), Nabicarbonate-sulfate and Na-bicarbonate-chloride types were present. TDS was as low as 400 mg/l in the shallow drift aquifer, generally constant at ∼1000 mg/l in the deep drift and shallow bedrock aquifer, and over 1700 mg/l in the deep bedrock aquifer system. Chemical evolution of groundwater in the area appears to be dominated by two depth zones having different types of water-rock interaction. In the shallow drift zone, the dissolution of soil CO 2 in infiltrating groundwater, oxidation of organic carbon, sulfur and pyrite result in the formation of carbonic and sulfuric acids that attack carbonate and silicate minerals. On the basis of X-ray diffraction analysis, these minerals were calcite, dolomite, plagioclase feldspar, and smectite clays. However, in the deep regional bedrock aquifer, conditions are reducing (presence of methane), groundwater is alkaline (pH 8.6–10.3), and the Na-bicarbonate-chloride composition of groundwater is believed to result from the hydrolysis of volcanic glass or feldspar crystals of oligoclase-andesine composition under conditions of very slow leaching of reaction products and low partial pressure of CO 2. Under such conditions, calcite and possibly Ca-zeolite are sinks for Ca ion, but Na can accumulate in the pore water. As a result of groundwater movement induced by the postglacial hummocky topography, water from the drift aquifers mixes with water from the deep bedrock aquifers in the groundwater discharge area, yielding a range of intermediate compositions that may be explained by dilution and calcite precipitation, using the MIX2 chemical equilibrium model. Chemical diffusion was shown to be of negligible importance in comparison with mechanical dispersion to explain the mixing effect.