Interpretation of210Pb profiles and verification of the CRS dating model in PIRLA project lake sediment cores

This paper reports results and analysis of²¹⁰Pb-activity measurements in 51 lake-sediment cores from 32 lakes in the four PIRLA (Paleoecological Investigations of Recent Lake Acidification) project regions (Adirondack Mountains [New York], Northern New England, Northern Florida, and the Northern Great Lakes States). General application of the Constant Rate of Supply (Constant Flux) model for²¹⁰Pb dating is valid for lakes in the PIRLA study, although application of the model is equivocal in a few lakes.²¹⁰Pb inventories and profiles are replicable among closely spaced cores within a lake. Specific²¹⁰Pb activity in surface sediments is negatively correlated with bulk sediment accumulation rate in seepage lakes, but not in drainage lakes. Drainage lakes with lower pH have lower unsupported²¹⁰Pb inventories in sediments, but the relationship does not occur in seepage lakes.²¹⁰Pb profiles in only seven of the cores, all from either the Adirondacks or the northern Great Lakes states, exhibit exponential decay curves. Deviations from an exponential profile include a flattening of the profile in the top few cm or excursions of one or a few measurements away from an exponential curve.²¹⁰Pb dates typically agree with other chronostratigraphic markers, most of which are subject to greater uncertainty. Several hypotheses, including sediment mixing, hydrologic regime, sediment focusing, and acidification, are proposed to explain variation of²¹⁰Pb distribution among lakes and regions. Hydrologic factors exert control on unsupported²¹⁰Pb inventories in PIRLA lakes, and there is a strong focusing effect in drainage lakes but a weak focusing effect in seepage lakes.This is the third of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

Resolving the chronology of recent lake sediments: an example from Devils Lake,North Dakota

Devils Lake is a closed, saline lake in North Dakota; it is typical of lake basins in the Great Plains Region, where windy conditions and fluctuating water-levels disturb sediment and confound chronostratigraphy. Pollen analysis and ²¹⁰Pb dating of two cores collected from bathymetrically contrasting embayments demonstrate (1) how certain agriculture-related pollen types differ in their value as chronostratigraphic markers, (2) how pollen and ²¹⁰Pb stratigraphies can be reconciled to determine the approximate depth of sediment mixing, and (3) the importance of coring-site selection, especially in lakes with unstable sedimentary conditions.     

How reliable is the210Pb dating method? Old and new results from Switzerland

We present a historical overview of applications of²¹⁰Pb dating in Switzerland with a special emphasis on the work performed at the University of Bern. It is demonstrated that the average specific activity of²¹⁰Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several²¹⁰Pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by¹³⁷Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout).A mean sedimentation rate of about 0.14 g cm^–2 y^–1 is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the²¹⁰Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm^–2 y^–1. In other parts of the lake a significant deficit in the inventory of²¹⁰Pb was found in the sediments. This could be due to a chemical redissolution of²¹⁰Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (135 m depth) the flux of²¹⁰Pb was about twice the atmospheric input. This excess is not caused by allochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of²¹⁰Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved²¹⁰Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and²¹⁰Pb in sediments of Lake Zurich was found.This is the first of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

Low-background gamma counting: applications for210Pb dating of sediments

Sediment cores from three lakes were dated with²¹⁰Pb using a constant rate of supply (CRS) model. We used low-background gamma counting to measure naturally occurring levels of²¹⁰Pb,²²⁶Ra, and¹³⁷Cs in sediment samples because sample preparation is simple and non-destructive,²²⁶Ra activity provides a direct measure of supported²¹⁰Pb activity for each sample analyzed, and¹³⁷Cs activity may provide an independent age marker for the 1962–1963 peak in atmospheric fallout of this radionuclide. In one core supported²¹⁰Pb activity was estimated equally well from²²⁶Ra activity of each sampling interval or from the mean total²¹⁰Pb activity of constant activity samples at depth. Supported²¹⁰Pb activity was constant with depth in this core. In a short freeze core, determining²²⁶Ra activity of every sample proved advantageous in estimating supported²¹⁰Pb activity because supported²¹⁰Pb activity could be estimated from²¹⁰Pb measurements only at the deepest sampling interval. Supported²¹⁰Pb activity estimated from²²⁶Ra activity also yielded more precise estimates of highly variable sedimentation rates. In the third core²²⁶Ra activity exceeded²¹⁰Pb activity at the top of the core and varied 20 fold with depth. This high input of²²⁶Ra in disequilibrium with²¹⁰Pb is attributed to recent erosion of radium-bearing materials in the drainage basin. These data invalidate the assumption that supported²¹⁰Pb activity is constant in sediment cores and can be estimated from the mean total²¹⁰Pb activity at depths where²¹⁰Pb activity is constant. We recommend using gamma counting or another independent assay of²²⁶Ra to validate the assumption of constant supported²¹⁰Pb activity in sediment cores if there is reason to expect that²²⁶Ra activity varies with depth.This is the fourth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr. P.G. Appleby is guest editing this series.     

Distribution coefficients of210Pb and210Po in laboratory and natural aquatic systems

We have measured the distribution coefficient (Kd) of²¹⁰Po and²¹⁰Pb in laboratory systems and in natural freshwater systems. In the laboratory systems, an inverse relationship was observed between the particle concentration of sand or lake sediment, and the distribution coefficients of²¹⁰Po and²¹⁰Pb. The slope of the log-linearK _d vs particle concentration relation is consistent with existingK _d-particle concentration theories. These laboratory observations are consistent with similar measurements in two lakes. TheK _d values of Po and Pb for the bottom sediment-pore water system with a high particle concentration were 10 to 100 times lower than those for dilute concentrations of particles suspended in the lake water. TheK _d of²¹⁰Pb in the sediments was >10⁴ so that the diffusive transport of²¹⁰Pb has only a small influence on the interpretation of²¹⁰Pb concentration-depth profiles and the²¹⁰Pb dating of these sediments.This is the second of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing.     

Evaluation of210Pb dating in lake sediments using stable Pb,Ambrosia pollen,and137Cs

We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of²¹⁰Pb derived dates was determined using common sediment event markers: the stable Pb rise, the¹³⁷Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on²¹⁰Pb derived dates. When comparing the concordance of two²¹⁰Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between¹³⁷Cs and²¹⁰Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to immobilize Cs.This is the ninth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments, Dr. P. G. Appleby is guest editing this series.     

A test of the integrity of metal records in sediment cores based on the documented history of metal contamination in Lac Dufault (Québec, Canada)

There is an important volume of published information on Lac Dufault (Québec) which describes the history of metal inputs over 70 years and the changes that occurred in the lake as a result of this contamination. We used this abundant source of chronological markers to test the hypothesis that lake sediments can provide true historical records of trace metal loading from metal mining. Sediment cores were obtained from the deepest zone of the lake (19 m). The sediments were dated using ²¹⁰Pb and ¹³⁷Cs and they were analyzed for total elemental concentrations (Ca, Cd, Cu, Fe, P, Pb, S, Zn). Metal profiles in the sediment core preserved the distinct signatures of different mine exploitations documented in the lake watershed. In particular, the core recorded: the beginning of industrialization in 1926; increasing sedimentation rates associated with perturbations in the lake watershed; the maximum of sediment [Cu] and the contemporary exploitation of ore bodies rich in Cu; the maximum of sediment [Fe] and the contemporary production of pyrite by a mining operation; the low sediment [Cu] and [Zn] in the 1950s and the low contemporary production of these metals by mining operations; the maximum of sediment [Cd] and the contemporary production of Cd by a subsequent mining operation. Anomalies in the distribution of ²¹⁴Pb activities in sediments reflected the intensity of acid mine drainage (AMD). There is good evidence that the lake resisted acidification from this AMD for the last thirty years. Overall, our results support the thesis that profiles of sedimentary Cd, Cu, Zn, Pb and Fe levels reflect the past history of metal input to Lac Dufault.     

The transport and mass balance of fallout radionuclides in Blelham Tarn, Cumbria (UK)

Although lake sediment archives are widely used for reconstructing historical records of atmospherically delivered pollutants, the quantitative relationship between fallout levels and their record in the sediments is complex and not well known. The original input signal from the atmosphere can be significantly distorted by mediating transport processes in the catchment, through the water column and within the sediments themselves. Since these processes also affect the fallout radionuclide ²¹⁰Pb commonly used to date sediments, a better understanding of their impact is also important to improving the accuracy and reliability of sediment dating. Blelham Tarn has been the subject of a number of palaeolimnological investigations using radiometric dating techniques since the early 1970s. More recently it was the site of a major study carried out within the EU Transuranics project concerning the long-term fate of fallout radionuclides in catchment/lake systems. This paper reviews the radiometric data from this study and uses the results to determine mass balances for fallout ²¹⁰Pb, ¹³⁷Cs and ^239+240Pu in Blelham Tarn, and their spatial distribution over the bed of the lake. Atmospheric fluxes were determined by measuring concentrations in rainwater and cumulative inventories in soil cores from non-eroding sites. Sediment records in a grid of 16 cores were used to determine the spatial distribution over the bed of the lake, and net inputs from the catchment. Mass balance calculations indicate that c. 47% of ²¹⁰Pb in the sediments derives from erosive inputs from the catchment. For ^239+240Pu the figure rises to 61%. Reduced amounts of ¹³⁷Cs in the sediments are attributed to greater losses of this radionuclide from the water column via the outflow due to its greater solubility. Inputs of radionuclides from the catchment are concentrated near one of the major input streams. Away from this part of the lake the sediment record is dominated by direct atmospheric fallout, though the detailed pattern is influenced by sediment focussing. A one parameter catchment/lake transport model is developed that incorporates the assumption that transport rates will decline with time as fallout on the catchment diffuses into the soil and becomes less available for removal. Values of the transport parameter were calculated for ²¹⁰Pb and ^239+240Pu and found to be comparable. The results suggest that it will take c. 11000 years to remove 50% of ^239+240Pu from the catchment to the lake.     

A chronological assessment of Lake Okeechobee (Florida) sediments using multiple dating markers

Reconstructing recent limnological history often relies on lead-210 dating to accurately ascribe a chronology to a sediment profile. In Lake Okeechobee, Florida, a large, shallow subtropical lake that may experience severe mixing, multiple dating methods are required to confirm that conformable sedimentation has been preserved and that the assumptions of the ²¹⁰Pb method are satisfied. This study uses stratigraphic profiles of heavy metals, ¹³⁷Cs, PCBs and pollen as independent dating markers to validate the sediment chronology as determined by ²¹⁰Pb for three cores from the central mud zone of the lake. Unsupported ²¹⁰Pb and most dating markers show distinct concentration/depth profiles, suggesting that the sediments have not been severely mixed for at least the last 75 years. Onset and maximum activity of the radioisotope ¹³⁷Cs in the cores coincides with the ²¹⁰Pb-dated interval of 1945–1970. This agrees well with the known timing of atmospheric deposition of ¹³⁷Cs that resulted from above-ground nuclear testing during late 1940s until 1963. Sediment core profiles of atmospherically deposited metals such as Zn and Pb, which reflect regional increases during industrialization and decreases after regulation in the 1970s, exhibit expected concentration increases and peaks coinciding within 5–15 years of the predicted ²¹⁰Pb dates. Uranium, a contaminant in some phosphate fertilizers, shows large concentration increases at core depths dated to be about 1950 by ²¹⁰Pb, matching the intensification of agriculture after WWII. PCBs, which are expected to peak in the early 1970s, were measured in one core, and the observed peak corresponds to a ²¹⁰Pb date of about 1960. Pollen makers were unable to verify specific events, but increases in disturbance taxa and declines in native types correspond generally with the expected dates assigned by ²¹⁰Pb dating. Conformity between the ²¹⁰Pb defined dates and independent markers of < ±15 years confirm that Lake Okeechobee sediments do preserve a sequential and reliable stratigraphic history of the lake, useful for reconstructing past limnological conditions.     

Late holocene paleoclimatic and paleobiologic records from sediments of Devils Lake,North Dakota

An 84 cm sediment core collected from the center of Devils Lake, North Dakota, was analyzed at 1-cm intervals for,²¹⁰Pb,¹³⁷Cs, sediment conductivity, the concentrations of, biogenic silica, total organic carbon, carbon to nitrogen ratio, and the carbon and nitrogen isotopic composition of the organic fraction. Variations in²¹⁰Pb activities in the upper 20 centimeters indicate that sediment accumulations rates in Devils Lake are not constant, and that accumulation rates were highest during periods of high lake level. The mean sedimentation accumulation rate was calculated as 0.24 cm^–1 yr. The¹³⁷Cs profile is characterized by near-surface maximum concentrations, possibly the result of redistribution of¹³⁷Cs during salinity excursions.Biogenic silica is strongly correlated to lake level in Devils Lake. Periods of low lake level (characterized by high sediment conductivity) correspond to low biogenic silica concentrations. The trends in biogenic silica are attributed to variations in diatom productivity in the lake and to variations in sediment accumulation rates. Based on biogenic silica content and the composition of organic matter in the sediment (total organic carbon, carbon:nitrogen ratio and the ¹³C and ¹⁵N composition of total organic matter), paleobiologic conditions of Devils Lake during low lake stands were characterized by, (1) decreased primary productivity, (2) decreased input of detrital organic matter, and (3) increased nitrogen availability.During the 350 years of sediment accumulation represented by the 84-cm sediment core Devils Lake has experienced two periods of sustained high lake level; one between about 130 and 170 years ago (1820 to 1860 A.D.) and the second between 270 and 310 years ago (1680 to 1720 A.D.). Devils Lake experienced a period of intense drying about 260 years ago (1720 A.D.).     

Power function for interpolating dates in recent sediment

Many paleolimnological studies deal with short cores; the upper 20 to 30 centimeters of lake sediment are often poorly compacted and it may be perilous to interpolate dates using straight lines. A power function (AGE=a+b*DEPTH^c) provides a good approach for interpolating dates where only a few dated horizons are available in a core. This may be particularly useful where multiple short cores are taken from a single site, and dated by correlation of key horizons with a master core which is ²¹⁰Pb dated. This approach assumes a constant rate of dry sediment deposition.     

Sediment focusing and210Pb dating: a new approach

In this paper we test the utility of the mud deposition boundary depth (mud DBD) theory (Rowanet al. 1992) as a means of maximizing sampling efficiency in paleolimnological investigations, particularly those that apply to²¹⁰Pb dating. The mud DBD is defined by the relationship between near bottom wave velocity and particle threshold velocity, with wave and particle threshold theory simplified to terms of exposure and depth. Mud DBD theory can be used to define the depositional zone in lakes, and within the depositional zone defined by the mud DBD: 1) there is a high probability of obtaining a representative core, 2) variation in mass sediment accumulation rate (MSAR) is not correlated with water depth, and 3) variation in MSAR is considerably reduced from the whole lake average. This suggests that mud DBD theory can account for the effects of sediment focusing, and that the mud DBD defined depositional zone is the zone to which fine-grained sediments are focused. Finally, we have shown that to optimize sampling effort, 5 to 10 cores within the depositional zone are necessary for a reasonably precise estimate of the mean mass sediment accumulation rate. In addition, the use of mud DBD theory prior to sampling can dramatically reduce the cost associated with analyzing large numbers of cores for²¹⁰Pb.This is the fifth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of take sediments. Dr P. G. Appleby is guest editing this series.     

江苏王港盐沼的现代沉积速率

江苏海岸湿地总面积超过5000km^2，但围垦活动的加剧使盐沼湿地面积迅速减少；为了达到海岸防护、保滩促淤的目的，先后引种了大米草和互花米草。通过采集江苏王港潮滩盐沼湿地9处柱状样和挖取探槽剖面，以及对沉积剖面样品进行^210Pb和^137Cs测年分析，探讨了^210Pb的本底及影响^210Pb富集的因素，估算了王港盐沼湿地的现代沉积速率。王港潮滩盐沼沉积物对^210Pb的吸附作用较弱；受风暴潮、生物扰动、物源变化及实验误差等因素的影响，在^210Pbex剖面上出现了数据异常点，将这些数据点剔除后计算得到的沉积速率为33cm yr^-1，^137Cs测年显示，该地区1963年以来的平均沉积速率3．1cm yr^-1，与^210Pb法及前人研究结果相一致。根据一个典型剖面的^137Cs测年数据分析，王港潮滩盐沼的沉积过程根据地貌特征的不同可分3个阶段，大米草覆盖阶段滩面高程迅速增加，互花米草的生长提高了滩面淤积速率。     

Long-term rates of sediment accumulation in the Tonle Sap,Cambodia: a threat to ecosystem health?

The Tonle Sap is an invaluable resource for the people of Cambodia, and is a globally significant ecological site. Much of the literature published on the environmental health and management of the lake suggests that its sustainability is threatened by accelerated rates of infilling related to unfettered land-use in the catchment. However, the evidence for supposedly increased sedimentation rates is confused and contradictory, and there have been very few studies that have attempted to actually measure the rate at which sediment is accumulating on the bed of the lake. This research measures long-term sedimentation rates in the Tonle Sap in an attempt to provide an empirically derived baseline against which more recent and allegedly accelerated sedimentation rates can be compared. Radiometric dating techniques (¹⁴C, ²¹⁰Pb, ¹³⁷Cs) reveal that sediment has been accumulating in the lake at an average long-term rate of less than 1 mm per year. There is a clear and consistent decline in the rate of sediment accumulation in the lake basin over time, with the highest rates recorded in the middle to early Holocene epoch. It is recommended that research be directed toward measuring changes in the morphology of the lake margin over time, rather than focus on changes in the bathymetry of the lake basins themselves, which are here shown to be effectively stable in terms of sediment accumulation.     

Anthropogenic impacts recorded in recent sediments from Otisco Lake,New York,USA

Geochemical analysis of ²¹⁰Pb-dated sediment cores from Otisco Lake (New York) combined with analysis of recent land cover change in the watershed revealed the history of land use change, lake management, and industrial pollution since European settlement in the northeastern US. Clearance of forestland for agriculture characterized the early settlement era that was marked in the Otisco Lake sediments by a decline in organic carbon (OC) and OC:N ratios, which indicate a change in the sources of organic matter to the lake. Agricultural land use reached its greatest areal extent in the Otisco watershed around 1900, followed by field abandonment and reforestation over the last century. In the 1920s sediment accumulation rates began to increase coincident with residential development along the lake shore. Nitrogen and organic carbon, which are transported from the watershed, show increased fluxes to the lake in response to this change. Deposition of inorganic carbon increased markedly over a short period from the 1940s to the 1980s, which is consistent with enhanced mobilization of watershed calcium by increased acidic deposition, followed by alkalinization of the lake waters and calcite precipitation. An increase in copper content in the sediments reflects application, since 1942, of the algicide copper sulfate to the lake waters to control algal blooms. This CuSO₄ marker confirmed the accuracy of the ²¹⁰Pb chronology. Atmospheric mercury (Hg) fluxes to the lake were affected by increased sediment transport from the watershed. The maximum Hg peak in the sediment record, however, was dated to the early 1970s and coincides with maximum Hg emissions to the atmosphere in the Great Lakes region.     

Historical trace metal loading to a large river recorded in the sediments of Lake St. Croix,USA

The global cycling of anthropogenic trace metals intensified during the twentieth century, impacting aquatic systems throughout the world. There are, however, few quantitative records showing the history of this contamination in large rivers. Here we present a well-dated sedimentary record of trace metal accumulation in Lake St. Croix, a natural riverine lake on the St. Croix River (Minnesota and Wisconsin, USA), revealing the history of heavy metal inputs to the river over the past 200 years. Concentrations of Hg, Pb, Ag, Cd, Cr and Zn and stable Pb isotopes were measured in eight ²¹⁰Pb-dated sediment cores collected from profundal depositional areas throughout the lake. Time trends of trace metal concentrations and accumulation rates differed greatly between the upper lake (above Valley Creek) and the lower lake, reflecting contrasting sediment sources along the flow axis of the lake. For most of the study period (1800–2000 AD), sediment deposited throughout the lake derived almost exclusively from the suspended sediment load carried by the main-stem river into the lake. From 1910 through 1970, however, large inputs of eroded soils and stream channel sediments from side-valley tributaries resulted in greatly increased sediment and trace metal accumulation in the lower lake. Anthropogenic accumulation rates of Hg, Pb, Cd, Zn, and Ag in the upper lake correlate well with those from Square Lake, a small, relatively undisturbed nearby lake that has received trace metal inputs almost exclusively via atmospheric deposition. The similarity of these records suggests that atmospheric deposition was primarily responsible for trace metal accumulation trends in upper Lake St. Croix. Trace metal accumulation in the lower lake was also strongly influenced by atmospherically derived inputs, but metal contributions from native soils were important, as well, during the period of elevated sediment inputs from side-valley tributaries. Concentrations and accumulation rates of trace metals in both upper and lower lake sediments have decreased substantially since the 1970s due to decreased atmospheric inputs and sediment loadings, but accumulation rates remain well above pre-settlement values. Metal inputs to Lake St. Croix have been far lower than those to nearby Lake Pepin, located on the Mississippi River downstream of the Minneapolis-St. Paul metropolitan area, but there nevertheless remains a clear record of anthropogenic impact on the relatively pristine St. Croix River.     

Bryozoan statoblasts in the recent sediments of Douglas Lake, Michigan

Statoblast valves produced by the freshwater bryozoan Plumatella nitens were recovered in three sediment cores from Douglas Lake, Michigan. Douglas Lake is a multi-depression lake of glacial origin. The region was heavily logged from 1880 to 1920. Sediment cores were taken from three of the seven depressions, and dated using 210Pb isotopes and pollen. Sedimentation rates were very low in the Grapevine Point core as compared to the other two cores. Concentrations of statoblasts ranged from three to 140 ml-1 of wet sediment. Profiles of statoblast concentrations and accumulation rates indicate a sharp decline in Plumatella populations corresponding to the time the Douglas Lake watershed was being clear-cut. It appears that logging and the resulting increase in erosion had adverse effects on bryozoan populations, and possibly on the entire littoral zone of the lake. High sediment loads to the lake could have caused mortality in Plumatella by interfering with the feeding of these animals, or by decreasing numbers of macrophytes which are frequently used as substrate. Populations of Plumatella have not recovered to pre-disturbance levels.     

Biological and Geochemical Record of Anthropogenic Impacts in Recent Sediments from Lake Pupuke, Auckland City, New Zealand

Frozen sediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high resolution decadal to annual scale record of changing lake paleoenvironments and geochemistry that reflects changing landuse and hydrology in the catchment over the past c. 190 years. A reliable chronology is available from AMS ¹⁴C and ²¹⁰Pb dating of the sediments, with the timing of the older part of the record supported by the first appearance of pollen of introduced Pinus and Plantago lanceolata associated with European settlement of Auckland in the early 1840s. Diatom stratigraphy, sediment elemental and carbon isotope geochemistry reflect changes in sediment sources and lake conditions commensurate with European development of the Lake Pupuke catchment, in particular enhanced algal productivity controlled by the influx of nutrients after c. 1920 AD. Attempts to prevent nuisance algal blooms in 1933, 1934 and 1939 using CuSO₄ addition produced Cu spikes in the sediment that allowed confirmation of the accuracy of the ²¹⁰Pb chronology. Hence, the elemental and isotopic composition of the Lake Pupuke sediments reflect the timing of significant anthropogenic activities, rather than climatic variations, that have occurred within the watershed over the past c. 190 years. The comparison of records of land use change in the catchment with the multi-proxy record of changes in the sediments usually allowed unambiguous identification of the signatures of change and their causes.     

Distribution of 137Cs and 210Pb in sediments of tidal flats in north Jiangsu Province

Seven cores were collected from different sediment zones of tidal flats at Xinyanggang in north Jiangsu province in August 2007. Sediment grain-size distribution and radioisotopes of 137Cs and 210Pb analysis were carried out for these cores. Sediment rates of the cores and radioisotopes distribution in surface sediment in different zones of the tidal flat were calculated from the 137Cs and 210Pb activities in sediments cores. The results indicated that each tidal zone had experienced different evolution phases, hydrological dynamics in the tidal flats made the grain-size of the surface sediment change gradually. 137Cs and 210Pb activities on the superficial layer of the cores varied spatially and the reason was discussed. On tidal flats, the fluctuation of 137Cs and 210Pb activities in the cores reflected the special sedimentary characteristics. Vegetation affects the grain-size distribution and the vertical profiles of 137Cs and 210Pb in the upper depths. 137Cs and 210Pb chronology got the comparable average sediment rates on the tidal flat. The characteristics of 137Cs and 210Pb in the cores reflected various depositional dynamical environments in different tidal zones and gave information on the different evolvement phases of the tidal zones. Based on the information of grain-size distribution, texture of the cores, sediment rates and topography, the evolution lines of the tidal flat were reconstructed.     

A diatom record of recent environmental change in Lake Duluti, northern Tanzania

Lake Duluti is a small, topographically closed crater lake located on the flanks of Mt Meru, northern Tanzania. Analyses of diatoms in three short sediment cores and four modern samples from Lake Duluti were used to infer past environmental changes. ²¹⁰Pb and ¹³⁷Cs activity profiles combined with AMS ¹⁴C dates provide the chronological framework. Weak agreement between the ²¹⁰Pb and ¹⁴C records, together with dating uncertainty, precludes construction of precise age models. The modern diatom flora, from plankton and three periphytic habitats, is dominated by Aulacoseira ambigua (Grunow) Simonsen, Gomphonema parvulum (Kützing) Grunow and Nitzschia amphibia Grunow. All three cores display similar stratigraphic succession, but the relative ratio of habitats represented by the diatoms varies substantially between cores. Diatoms indicate that the oldest part of the record is characterized by relatively low lake level and swampy vegetation. In the late nineteenth or early twentieth century there was a rapid lake level rise and the swamp turned into an open-water lake. High lake levels have prevailed since that time.