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.     

Sediment focusing in six small lakes inferred from radionuclide profiles

Sediment focusing in six small lake basins is inferred from sedimentary radionuclide profiles. The activities of¹³⁷Cs and⁹⁺²⁴⁰Pu do not decrease to zero above the sub-surface maximum but tail off gradually. Similar behavior is observed in the sediments of three lakes for¹³⁴Cs and⁶⁰Co, which were added only to the lake. This upward tailing is not caused by bioturbation or erosion from the watershed. Sediment focusing is the likely cause, further substantiated by inventories in deep sediments of⁹⁺²⁴⁰Pu and²¹⁰Pb that are twice as high as expected from atmospheric fallout. Inventories in deep sediments of¹³⁷Cs are in several cases lower than expected from atmospheric fallout, and cannot be explained by hydraulic flushing from the lakes, as this would require unreasonable flushing rates. The low¹³⁷Cs inventories and the sedimentary profiles of the¹³⁷Cs/⁹⁺²⁴⁰Pu ratio are consistent with¹³⁷Cs removal from the water column by boundary scavenging to shallow sediments. The profiles and inventories of all three nuclides could be reproduced with a simple three box model, assuming focusing of shallow sediments (50–60% of total sediment area) to deep sediments with a rate constant of 5–10% yr^–1.This is the seventh of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediment. Dr P. G. Appleby is guest editing this series.     

The sedimentary record of spheroidal carbonaceous fly-ash particles in shallow Danish lakes

Spheroidal carbonaceous particles (SCP) from combustion of oil and coal have been quantified in cores from top sediments of Danish shallow lakes. Chronologies were provided by ²¹⁰Pb-dating supplemented by measurements of other radionuclides (¹³⁷Cs, ¹³⁴Cs, ²⁴¹Am). All cores show indications of sediment mixing but most still retain a characteristic SCP record. Deposition rates of SCP were low until the 1920s, increased strongly after World War II, reached maximum levels around 1970 and decreased thereafter. These results were used to infer a new chronology for a sediment core from a shallow lake, from which a ²¹⁰Pb chronology had previously been published.SCP inventories are positively correlated with ²¹⁰Pb inventories and dry matter accumulated since 1900, suggesting that absolute SCP accumulation rates may be more influenced by sedimentary processes than by atmospheric fluxes. The concentrations of SCP in surface sediments of the six lakes do, however, correlate with regional SO₂ emissions.     

The use of sedimentary algal pigments to infer historic algal communities in Lake Apopka,Florida

The primary producer community of Lake Apopka, a large (125 km²), shallow (mean depth, 1.7 m), polymictic Florida lake, shifted from macrophyte dominance to phytoplankton dominance in the 1940s. Today, frequent wind resuspension of highly organic, unconsolidated sediments supports a meroplanktonic community that is predominantly diatoms, but during calm periods the algal community is dominated by planktonic cyanobacteria. Sedimentary algal pigments (chlorophyll derivatives and carotenoids) and chemical proxies for nutrient enrichment (polyphosphate, total phosphorus and biogenic silica) in three sediment cores were used to investigate historic changes in primary producers. Sediments were separated into three stratigraphic zones using multivariate statistical techniques. Stratigraphic zonation was established in each core although sediment deposition at one site was insufficient to adequately resolve temporal changes. These results show the importance of selecting suitable sites for paleolimnological studies. The oldest zone represents macrophyte-derived sediments, and the two overlying zones represent phytoplankton-derived sediments deposited since the 1940s. Algal pigments in the most recent sediment zone show little degradation, which might be due to the presence of viable meroplankton in the sediment. After the initial primary producer shift from macrophytes to phytoplankton, the lake experienced a short period of cyanobacterial dominance followed by a period of benthic diatom abundance before being replaced by the present algal community consisting of cyanobacteria and meroplanktonic diatoms. Chlorophyll derivatives and carotenoids were highly correlated with total phosphorus. Historic trends inferred from the data include algal and cyanobacterial productivity that increased with increased phosphorus loading. The study demonstrates that valid paleolimnological proxies for historic eutrophication are available in loosely consolidated sediments of shallow, subtropical lakes.     

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.     

Identifying sources of organic matter in sediments of shallow lakes using multiple geochemical variables

We analyzed ²¹⁰Pb-dated sediment cores from four relatively shallow lakes (z_max < 10 m) in the Upper Ocklawaha River Basin, Florida, USA to compare primary producer community structure before and after anthropogenic impacts. We measured physical and chemical sediment variables including density, organic matter (OM), water-soluble phosphorus, polyphosphate (Poly-P), total P (TP), total carbon to total nitrogen mass ratios of OM (TC:TN), biogenic silica (diatoms, sponge spicules), total amorphous silica, and stable carbon and nitrogen isotope ratios of bulk OM. Principal component analysis showed that diatom biogenic silica, TC:TN, Poly-P and TP displayed discernible stratigraphic changes associated with the shift in the primary producer community. We applied k-means cluster analysis to these variables to identify macrophyte-derived, transitional, and phytoplankton-derived sediments. Our approach provides an objective method for identifying sediment sources that may be applied to shallow lakes in other regions. The four study lakes shifted from a macrophyte-dominated state to a transitional state before major anthropogenic disturbances, and became phytoplankton-dominated after ~1950.     

Arsenic contamination of lake sediments in Florida: evidence of herbicide mobility from watershed soils

Organic arsenical herbicides, which include monosodium methylarsonate (MSMA), have been applied to golf courses and lawns throughout Florida, USA, since the 1950s. These products convert rapidly to inorganic forms of arsenic (As) in soils and are mobilized readily. Leachates have been known to contaminate groundwater and surface waters, although past studies have not examined whether use of these products has led to significant As accumulation in lake sediments. We used paleolimnological methods to document the depositional history and inventories of total As in sediments and porewaters of Little Lake Jackson in Florida, which is adjacent to three golf courses. Six sediment cores, four of which were ²¹⁰Pb dated, showed porewater total As concentrations as high as 435 μg l⁻¹, and dry-sediment total As concentrations as high as 148 mg kg⁻¹. Approximately 537 kg of total As is present in >19,000 metric tons of sediment (dry mass), and an additional 18 kg of As is dissolved in 10.8 × 10⁴ m³ of porewaters. Total As content in surface sediments (mean = 47.3 mg kg⁻¹) exceeds the consensus-based sedimentary concentration for probable toxicity effects in freshwater benthic fauna. Surface and subsurface waters flow to the lake from topographically higher areas to the west, where golf courses and residential areas are located. Total As concentrations were elevated highly in monitoring wells and in a stream that flows between the golf courses and lake, but As was below detection limits in wells that were located at the distal perimeter of the golf courses. Subsurface and surface waters exit the lake towards topographically lower areas to the east. Nearly all As in sediments remains bound in the solid phase, indicating that As sedimentary profiles largely reflect depositional history. Sedimentary As concentrations are correlated strongly with aluminum and iron, which suggests that As was scavenged from lake waters during the past. Sedimentary As concentrations increased until the 1980s, then declined somewhat to the present time. Dissolved As was scavenged efficiently from the water column when hypolimnetic waters were oxygenated persistently, but after eutrophication led to a seasonally anoxic hypolimnion in the 1980s, apparently less As was co-precipitated, and more was lost to hydrological outflow. Arsenic accumulation in sediments might be common in areas where As derived from organic arsenical herbicide applications is directed by shallow water tables towards adjacent lakes.     

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.     

210Pb and 137Cs dating methods in lakes: A retrospective study

²¹⁰Pb has been used for more than two decades to provide the geochronology of annually deposited sediments and to construct pollution histories. Evidence from some lakes suggests that this radionuclide may be adequately mobile to compromise dating reliability. This study provides one test of that possibility by comparing recent measurements of ²¹⁰Pb and trace metals to ones carried out more than 20 yrs in the past. ¹³⁷Cs dating is used to confirm sediment accumulation rates in the recent cores. In the three Connecticut, USA, lakes studied, sediment accumulation rates changed abruptly to higher values between 40-50 yrs ago (increasing by factors of 2.2, 2.9, and 3.0). In all three lakes, rates calculated from ²¹⁰Pb distributions both above and below this horizon agreed, within measurement uncertainty, in recent and older cores. Furthermore, when the older data were corrected for 20 yrs of burial, the changes in slope in ²¹⁰Pb distributions occurred at the same depth in each pair of cores. The depth of sharp peaks in concentrations of trace metals also matched. In general, this evidence supports the idea that sediments in these lakes have simply been buried, without significant diagenetic remobilization of ²¹⁰Pb and trace metals . Nevertheless, some important differences were also observed. For two of the three lakes, there was a significant difference in average sediment accumulation rate during the past 33 yrs as calculated from ¹³⁷Cs and ²¹⁰Pb in the recent cores. Most potential causes for this difference can be ruled out, and it appears that one of the two nuclides is remobilized compared to the other. There were also significant differences in the total inventories of both ²¹⁰Pb and trace metals (both up to 2 ×) between recent and older cores in some cases. This may be due to dissimilar sediment focusing, since it is not known for certain whether the new cores were collected at exactly the same sites as in the past.     

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.     

Sediment records of phosphorus-driven shifts to phytoplanktondominance in shallow Florida lakes

Primary producer community structure (PPCS) in shallow lakes isinfluenced by phosphorus (P) load and water column P concentration.Theoretically PPCS may shift between phytoplankton and macrophyte states withintermediate P loading, but phytoplankton dominate when P loading exceeds acritical threshold. We analyzed sediment cores from five shallow, eutrophiclakes (size range: 0.6 to 125 km²) that arephytoplankton dominated to determine whether the development of the currentstate was associated stratigraphically with an increase in sediment total P(TP) and a shift in PPCS. We used sponge biogenic silica(BSi_Sponges) concentrations and total carbon to total nitrogenratios (TC:TN) as proxies for macrophyte abundance and sediment organic mattersource, respectively. Three stratigraphic groups of sediments were identifiedwith k-means cluster analysis. These samples were grouped by increasing TPconcentrations and decreasing age and identified as macrophyte, transitionaland phytoplankton sediments. Results show that as P loading increased in thelate 19th and early 20th centuries, the lakes producedsediments with an increasing contribution from phytoplankton. Four of our lakesmay represent a subset of shallow lakes because of their large size (30 to 125km²) and relatively rapid historic P enrichment. Inthese Florida lakes, PPCS shifted to phytoplankton dominance with nopaleolimnological record of lake-wide alternating stable states or of lake-widephytoplankton dominance before anthropogenic P enrichment.     

Inputs of dissolved and particulate 226Ra to lakes and implications for 210Pb dating recent sediments

Gamma spectroscopy was used to measure radioisotope (²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs) activities in sediment cores from 20 lakes and a wetland in Florida, USA. Nine profiles display relatively low (<5 dpm g^–1) and constant ²²⁶Ra activities, whereas 12 show high (>5 dpm g^–1) and variable ²²⁶Ra activities. In the latter group, most display up-core increases in activity. Upper sediments from two lakes (Round and Rowell) possess very high (>20 dpm g^–1) ²²⁶Ra activities that exceed total ²¹⁰Pb activities, clearly illustrating disequlibrium between ²²⁶Ra and supported ²¹⁰Pb. Supported ²¹⁰Pb activity is generally thought to come from in situ, ²²⁶Ra-containing detrital mineral particles, and is typically assumed to be in secular equilibrium with ²²⁶Ra activity. Since 1966, Round Lake has been augmented hydrologically with ²²⁶Ra-rich (6.2 dpm L^–1) groundwater pumped from the local deep aquifer. Adsorption of dissolved ²²⁶Ra to recent Round Lake sediments probably accounts for the high measured ²²⁶Ra activities and the pronounced disequilibrium between ²²⁶Ra and supported ²¹⁰Pb in topmost deposits. We suspect that many Florida waterbodies receive some ²²⁶Ra-rich runoff and seepage from groundwater pumped for irrigation, residential use, industrial applications, and mining. This may account for up-core increases in ²²⁶Ra activity measured in sediment cores from some Florida lakes. Significant groundwater pumping began within the last century, and there has been insufficient time for supported ²¹⁰Pb to come into equilibrium with adsorbed ²²⁶Ra in uppermost deposits. Input of ²²⁶Ra-rich groundwater to lakes may occur in any geographic region where local bedrock contains ²³⁸U and its daughters. When dissolved ²²⁶Ra adsorbs to recent sediments, it complicates accurate estimation of supported ²¹⁰Pb activity, and confounds calculation of unsupported ²¹⁰Pb activity that is used in dating models.     

Distribution of <Superscript>137</Superscript>Cs and <Superscript>210</Superscript>Pb 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.     

Post-depositional 137Cs Mobility in the Sediments of Three Shallow Coastal Lagoons, SW England

We present ¹³⁷Cs profiles for three low lying coastal lagoons in Southwest England that show a decline in activity with sediment depth. ¹³⁷Cs inventories are lower than expected by comparison with local reference inventories despite the fact that sampling was undertaken in the deep-water zone of each lake where sediment and ¹³⁷Cs focusing would be expected. At all three locations, lake sediment ⁷Be and unsupported ²¹⁰Pb (²¹⁰Pb_un) inventories are not significantly lower than the local reference inventory. ¹³⁷Cs inventories in the study cores range from 38 to 95% of local reference inventories. The standing water level and mud: water interface at two sites are below maximum tide level and, at all three sites, salinity increases significantly in the water columns between low and high tide and in the pore waters of the underlying sediments. We suggest that the difference in hydrostatic pressure between sea level and standing water levels in the lagoons forces salt water up through the sediment column and that monovalent cations (especially Na⁺ and K⁺) replace ¹³⁷Cs on exchange sites leading to the upward migration and loss of ¹³⁷Cs. Rising sea levels may therefore contribute to remobilisation and release of ¹³⁷Cs to the aquatic environment from the sediments of coastal lagoons.     

Historical rates of sediment and nutrient accumulation in marshes of the Upper St. Johns River Basin, Florida, U.S.A.

We used 210Pb-dated sediment cores from wetlands and Blue Cypress Lake, in the Upper St. Johns River Basin (USJRB), Florida, USA, to measure historical accumulation rates of bulk sediment, total carbon (C), total nitrogen (N), and total phosphorus (P). Marsh cores displayed similar stratigraphies with respect to physical properties and nutrient content. Wetland sediments typically contained > 900 mg organic matter (OM) g^–1 dry mass, > 500 mg C g^–1, and 30–40 mg N g^–1. OM, C, and N concentrations were slightly lower in uppermost sediments of most cores, but displayed no strong stratigraphic trends. Total P concentrations were relatively low in bottommost deposits (0.01–0.11 mg g^–1), but ranged from 0.38–2.67 mg g^–1 in surface sediments. The mean sediment accretion rate in the marsh since ~ 1900, 0.33 ± 0.05 cm yr^–1, was calculated from ten ²¹⁰Pb-dated cores. All sites displayed increases in accumulation rates of bulk sediment, C, N, and P since the early part of the 20th century. These trends are attributed to recent hydrologic modifications in the basin combined with high nutrient loading from agricultural, residential, and urban sources.     

Factors affecting sulfur incorporation into lake sediments: paleoecological implications

This paper discusses the use of S as a paleolimnological tracer of limnetic sulfate concentration. A positive relationship (p<0.05) was found between limnetic sulfate and sediment S concentrations for the Great Lakes, English Lakes, and lakes from the Adirondack and Northern New England regions. There is a positive correlation (p<0.05) between C and S concentration in sediment across all regions studied. The importance of C in affecting S content in sediment was also examined by a series of cores taken at different water depths in Big Moose Lake (Adirondacks). There was a strong relationship between C and S among cores with sediment from deeper water having higher C and S concentrations (r ²=0.99). Sulfur from the shallower cores had greater concentrations of chromium-reducible S (pyrite), while cores from deeper waters had a greater proportion of organic S fractions including C-bonded S and ester sulfates.For assessing historical changes in S accumulation in sediments, enrichment factors were calculated for the PIRLA lakes. Pre-1900 net sediment accumulation rates of S were very similar across all regions. Sulfur enrichment was greatest in Adirondack sediment which had total post-1900 S accumulation of 1.1 to 7.4 times pre-1900 S accumulation. Sediment from Northern New England (NNE) generally had lower S concentration than Adirondack sediments and S enrichment factors ranged from 1.2 to 2.1. Sediment from the Northern Great Lakes States region had similar S concentration and distribution with depth to NNE sediment. In two Northern Florida lakes, sediment showed little variation in S concentration with depth, but in two other lakes from the same region, there was higher S concentration in deeper layers. Lakes which had the greatest enrichment factors also exhibited the most marked changes in C:S ratios. Ratios of C:N showed little variation (10.6 to 26.1) among the PIRLA lakes. A first order model indicated slow decomposition within these organic rich sediments.Elemental concentrations and ratios of sediment from a variety of lakes and reservoirs were complied. Maximum and minimum elemental ratios for all the data were 28 to 8.1 for C:N, 0.81 to 0.11 for C:H, and 675 to 12.5 for C:S, respectively. For the C:S ratios in all regions except the Great Lakes, the maximum ratio was less than 231. Both the maximum and minimum amount of N and H concentration of organic matter is related to biotic processes. The minimum concentration of S is regulated not only by nutrient demands but also by non-assimilatory processes.Sulfur incorporation into sediments is a function of a complex of factors, but limnetic sulfate concentration and organic matter content play a major role in regulating the S content of sediment. Further quantification of S incorporation pathways will aid in the paleolimnological interpretation of sediment S profiles. Such information is also important in assessing how S sediment pools will respond to decreases in limnetic sulfate concentration which may occur with decreases in inputs from acidic deposition.This is the eighth of a series of papers to be published by this journal which is a contribution to the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.     

A multi-proxy trophic state reconstruction for shallow Orange Lake, Florida, USA: possible influence of macrophytes on limnetic nutrient concentrations

We retrieved four sediment cores from shallow, eutrophic, macrophyte-dominated Orange Lake (A = 51.4 km2, zmax <5 m, zmean < 2 m), north-central Florida, USA. The 210Pb-dated profiles were used to evaluate spatial and temporal patterns of bulk sediment and nutrient accumulation in the limnetic zone and to infer historical changes in lake trophic state. Bulk density, organic matter, total carbon, total nitrogen, total phosphorus and non-apatite inorganic phosphorus (NAIP) concentrations displayed stratigraphic similarities among three of four cores, as did accumulation rates of bulk sediment, organic matter and nutrients. Accumulation rates were slower at the fourth site. Nutrients showed generally increasing rates of accumulation since the turn of the century. Percentages of periphytic diatom taxa increased progressively in the cores after ~ 1930. Diatom-inferred limnetic total P trends were similar among profiles. Eutrophic conditions were inferred for the period prior to the turn of the century. The lake was hypereutrophic in the early decades of the 1900s, but inferred limnetic total P values declined after ~ 1930. Declining inferred limnetic total P trends for the last 60--70 years were accompanied by concomitant increases in accumulation rates of total P and NAIP on the lake bottom. Several lines of evidence suggest that after ~ 1930, phosphorus entering Orange Lake was increasingly utilized by submersed macrophytes. Paleolimnological records from Orange Lake highlight the importance of using multiple sediment variables to infer past trophic state and suggest that aquatic macrophytes can play a role in regulating water-column nutrient concentrations in shallow, warm-temperate lakes.     

Radiometric dating of lake sediments from Signy Island (maritime Antarctic): evidence of recent climatic change

Sediment cores from three lakes (Moss, Sombre and Heywood) in the maritime Antarctic (Signy Island, South Orkney Islands) have been successfully dated radiometrically by²¹⁰Pb and¹³⁷Cs. The core inventories of both fallout radionuclides are an order of magnitude higher than that which can be supported by the direct atmospheric flux at this latitude. The elevated values may be explained by fallout onto the catchment during the winter being delivered directly to the lakes during the annual thaw. Two of the lakes (Sombre and Heywood) show marked increases in sediment accumulation afterc. 1950. This appears to be associated with a documented rise in temperature in the South Orkney Islands, which has caused extensive deglaciation at Signy Island.This is the tenth 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.     

Sediment geochemical records of environmental change in Lake Wuliangsu, Yellow River Basin, north China

We present a paleolimnological record from shallow Lake Wuliangsu in the Yellow River Basin, north China, using a short (56 cm) sediment core. Our objective was to investigate environmental changes in this semi-arid region over the past ~150 years. The sediment core was dated using ¹³⁷Cs and ²¹⁰Pb. We examined stratigraphic trends in core lithology, nutrients, stable isotopes (δ¹³C and δ¹⁵N) and trace element concentrations in the Lake Wuliangsu core to discern between natural sediments and those affected by human agency. A lithologic transition from yellow, coarse-grained sediment to grey, fined-grained sediment marked the lake’s formation about 1860. Until ~1950, sediments displayed relatively low and constant heavy metal concentrations, indicating little human influence. In the 1950s, enrichment factors (EFs) increased, reflecting greater impact of human activities. Carbon and nitrogen stable isotopes in organic matter (OM), along with heavy metal concentrations, were used to infer past shifts in trophic state and identify pollutants that came from agriculture, industry and urbanization. In the late 1950s, the first evidence for environmental change is recorded by increases in total organic carbon (TOC), total organic nitrogen (TN), TOC/TN, EFs, δ¹³C and a decrease in δ¹⁵N. After about year 2000, a more rapid increase in trophic status occurred, as indicated by greater total phosphorus (TP), EFs, δ¹⁵N and lower δ¹³C values. Changes in isotope and TOC/TN values in the lake sediments may reflect a shift in lake ecology during this period. The first increase in trophic status during the late 1950s was mainly a result of agricultural development in the catchment. In contrast, the change after ca. AD 2000 was driven largely by urban and industrial development. Agreement between paleolimnologic data from Lake Wuliangsu, and both instrumental and written records, indicates that the lake sediments provide a reliable archive for investigating the formation and environmental history of the lake.     

Catchment disturbance inferred from paleolimnological studies of three contrasted sub-humid environments in Morocco

Paleolimnological techniques for assessing recent drainage basin disturbance are evaluated in three Moroccan lakes with catchments contrasted in terms of land-use and vegetation. Rates of sediment accumulation in the two lakes with agricultural catchments were relatively high (>1.6 cm yr^-1) in the most recent past. Dilution effects prevented core dating by the ²¹⁰Pb method alone and post-1953 chronologies were constructed by combining ²¹⁰Pb and ¹³⁷Cs data. The recent sediment accumulation rate at the currently least disturbed site, where natural Cedrus forest is still abundant, was relatively low (<0.4 cm yr^-1) but has increased since the mid-19th century.Magnetic, geochemical, pollen, and diatom studies of all three lake sediment cores linked with modern field survey data show that soil erosion in the most vegetationally disturbed catchment (Dayat-er-Roumi) has been high throughout the recent past and that intensity peaks are probably associated with wetland drainage operations beginning in the 1940's. At the partially forested site (Dayat Affougah), pre-1950's woodland clearance and other land-use changes are the likely cause of past major soil erosion episodes. The site currently dominated by natural Cedrus forest (Lac Azigza) shows only minor disturbance during the past c. 150 years although a major soil erosion episode occrred in the 17th century.Paleolimnological analysis has clearly demonstrated that major landscape change has occurred at all three sites. However, only at the two sites with catchment cultivation do previously accelerated soil erosion and lake sediment accumulation rates persist to the present. Information essential for formulation of appropriate management plans is presented and the importance of paleolimnology in assessing man-induced lake-catchment disturbance is stressed.