Cm-2 Year-1 Research Articles

Historic land use and sedimentation in two urban reservoirs, Occoquan Reservoir and Lake Manassas, Virginia, USA.

Human population growth and subsequent land use intensification are closely linked to contemporary increases in sediment and associated contaminants fluxes to fluvial systems, lakes, reservoirs, and coastal zones worldwide. In most urban areas, reservoirs that are the main source of fresh water supply, if not effectively managed, suffer from water quality decline and loss of capacity associated with accelerated siltation. This study analyzes watershed soil losses and sediment accumulation rates in two reservoirs in the Occoquan river basin, a sub-watershed of the Chesapeake Bay in the suburbs of the greater Washington, DC area. Lake Manassas is located in the upper reaches of the basin, characterized by mixed land use and cover of mostly forest, residential areas, and agriculture, whereas Occoquan Reservoir is located in the more urbanized lower reach of the basin in the heavily populated suburban zone south of Washington, DC. Five sediment cores from each lake were used in 210Pb-based sediment accumulation rates analysis, and GIS-based Revised Soil Loss Equation (RUSLE) model and a sediment delivery ratio (SDR) were used to evaluate basin soil losses and sediment fluxes to the fluvial systems. 210Pb sediment accumulation rate estimates in Occoquan Reservoir range from 0.26 g cm-2 year-1 in the upper reaches to 0.37 g cm-2 year-1 in the lower reaches. Lake Manassas also had comparable accumulation values ranging from 0.22 to 0.40 g cm-2 year-1. RUSLE/SDR estimated watershed sediment fluxes were 0.26 Mg ha-1 year-1 (Mg-mega gram) in the upper watershed, which is significantly higher than 0.07 Mg ha-1 year-1 estimates for the lower reaches of the watershed. The variability in the reservoirs' sediment accumulation rates and basin soil losses reflects the variability of land use and cover, basin slopes, and erosion mitigation efforts within the watershed. The lower reaches, though more urbanized, have well-developed storm drain systems limiting run-off related soil losses. The well-managed riparian zones surrounding both reservoirs also limit sediment fluxes, hence the relatively low sediment accumulation rates. Although surficial sediment sources seem to be well managed, some of these efforts might be associated with the uptick in intrinsic sediment sources, leading to localized high sediment accumulation in the mouth of tributaries draining the high-intensity urban areas of the basin.

First data on the climate variability in the vicinity of Vostok Station (central Antarctica) over the past 2,000 years based on the study of a snow-firn core

Central Antarctica is characterized by a crucial lack of information on climate variability during the late Holocene. The study of firn cores obtained from the central part of the East Antarctic Ice Sheet is essential for the reconstruction of the paleoclimatic conditions at the continental and regional scales over the past 2,000 years (the Late Holocene). Based on glaciological and isotopic data from the shallow VK16 core drilled in the vicinity of Vostok Station over the periods of the 62th (2016-2017) and 63th (2017-2018) summer seasons of RAE, climatic characteristics have been reconstructed for the period 216 BC - 2010 AD. Studies of the VK16 core were carried out in two stages: firn density and electrical conductivity measurement (ECM), as well as sampling, was performed in the glaciological laboratory of the 5G drilling complex (Vostok Station) soon after the firn recovery, while the isotopic composition of the core samples was measured by a Picarro L-2120i laser analyzer in the Climate and Environmental Research Laboratory of AARI. In the ECM profile of the core, we discovered 14 absolute age markers (layers containing the products of known volcanic eruptions). These markers have allowed us to develop a robust chronostratigraphic scale for this core. We have shown that the main feature of the Late Holocene climate in this part of Antarctica is that the near-surface air temperature remained essentially constant throughout the whole time period under consideration. At the same time, the snow accumulation rate varied significantly around a mean value of 1.83 g cm-2 year-1, while the last 200 years were characterized by the highest snow accumulation rate, equal to 2.08 g cm-2 year-1. In this paper, we describe methods for studying firn cores, which can be useful for further research, and present first preliminary data on the climate variability in the vicinity of Vostok Station during the late Holocene.

Methane emissions reduce the radiative cooling effect of a subtropical estuarine mangrove wetland by half.

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide (CO2 ) and mitigating climate change has received increasing attention in recent years. While recent studies have shown that methane (CH4 ) emissions can potentially offset the carbon burial rates in low-salinity coastal wetlands, there is hitherto a paucity of direct and year-round measurements of ecosystem-scale CH4 flux (FCH4 ) from mangrove ecosystems. In this study, we examined the temporal variations and biophysical drivers of ecosystem-scale FCH4 in a subtropical estuarine mangrove wetland based on 3years of eddy covariance measurements. Our results showed that daily mangrove FCH4 reached a peak of over 0.1gCH4 -Cm-2 day-1 during the summertime owing to a combination of high temperature and low salinity, while the wintertime FCH4 was negligible. In this mangrove, the mean annual CH4 emission was 11.7±0.4gCH4 -Cm-2 year-1 while the annual net ecosystem CO2 exchange ranged between -891 and -690gCO2 -Cm-2 year-1 , indicating a net cooling effect on climate over decadal to centurial timescales. Meanwhile, we showed that mangrove FCH4 could offset the negative radiative forcing caused by CO2 uptake by 52% and 24% over a time horizon of 20 and 100years, respectively, based on the corresponding sustained-flux global warming potentials. Moreover, we found that 87% and 69% of the total variance of daily FCH4 could be explained by the random forest machine learning algorithm and traditional linear regression model, respectively, with soil temperature and salinity being the most dominant controls. This study was the first of its kind to characterize ecosystem-scale FCH4 in a mangrove wetland with long-term eddy covariance measurements. Our findings implied that future environmental changes such as climate warming and increasing river discharge might increase CH4 emissions and hence reduce the net radiative cooling effect of estuarine mangrove forests.

Correction to: Chronic prescribed burning alters nutrient deposition and sediment stoichiometry in a lake ecosystem.

The publication contained a dating model that was based on AD/BC dates instead of years before present (YBP) dates for the three C14 AMS values. As a result, dates reported as YBP should be reported as BC. While all of the dates for the prescribed burning period are correct given that they were based on the 210Pb model, all dates reported as YBP should read BC. Specific changes to the manuscript are as follows: The abstract should read, "throughout the last 8000 years." The final paragraph in the introduction should read, "from the mid-Holocene (~ 6000 BC) to present." The end of the first paragraph in the Results section should read, "the sediment core represented the past ~ 8000 YBP and the core sections below the 210Pb record averaged sedimentation rates of 1.73 ± 2.1 mg cm-2 year-1." All dates presented as Years Before Present or YPB for the remainder of the manuscript should be reported as BC. Added to Acknowledgements: "The authors would like to thank Sally Horn and Matt Boehm with help with the age model and dating." Table1 has been updated with the "Calibrated Age" column reflecting the correct dates in YBP notation. Figure2 has been updated to reflect the BC to YBP changes in the calibrated AMS C14 dates. Both panels have been changed to include the older dates.

ROPES Reveals Past Land Cover and PPEs From Single Pollen Records

Quantitative reconstructions of past vegetation cover commonly require pollen productivity estimates (PPEs). PPEs are calibrated in extensive and rather cumbersome surface-sample studies, and are so far only available for selected regions. Moreover, it may be questioned whether present-day pollen-landcover relationships are valid for palaeo-situations. We here introduce the ROPES approach that simultaneously derives PPEs and mean plant abundances from single pollen records. ROPES requires pollen counts and pollen accumulation rates (PARs, grains cm-2 year-1). Pollen counts are used to reconstruct plant abundances following the REVEALS approach. The principle of ROPES is that changes in plant abundance are linearly represented in observed PAR values. For example, if the PAR of pine doubles, so should the REVEALS reconstructed abundance of pine. Consequently, if a REVEALS reconstruction is ‘correct’ (i.e. ‘correct’ PPEs are used) the ratio ‘PAR over REVEALS’ is constant for each taxon along all samples of a record. With incorrect PPEs, the ratio will instead vary. ROPES starts from random (likely incorrect) PPEs, but then adjusts them using an optimization algorithm with the aim to minimize variation in the ‘PAR over REVEALS’ ratio across the record. ROPES thus simultaneously calculates mean plant abundances and PPEs. We illustrate the approach with test applications on nine synthetic pollen records. The results show that good performance of ROPES requires data sets with high underlying variation, many samples and low noise in the PAR data. ROPES can deliver first landcover reconstructions in regions for which PPEs are not yet available. The PPEs provided by ROPES may then allow for further REVEALS-based reconstructions. Similarly, ROPES can provide insight in pollen productivity during distinct periods of the past such as the Lateglacial. We see a potential to study spatial and temporal variation in pollen productivity for example in relation to site parameters, climate and land use. It may even be possible to detect expansion of non-pollen producing areas in a landscape. Overall, ROPES will help produce more accurate landcover reconstructions and expand reconstructions into new study regions and non-analogue situations of the past. ROPES will be available within the R package DISQOVER.

Polybrominated Diphenyl Ethers in the Sediments of the Great Lakes. 3. Lakes Ontario and Erie

Sediment cores were taken in 2002 in Lakes Ontario and Erie at four locations. A total of 48 sediment samples were characterized, dated using 210Pb, and analyzed for 10 congeners of polybrominated diphenyl ethers (PBDEs) including BDE209 as well as 39 congeners of polychlorinated biphenyls (PCBs). The surficial concentrations of nine tri- through hepta-BDE congeners (sigma9PBDE) are 4.85 and 6.33 ng g(-1), at sampling sites ON40 and ON30 in Lake Ontario, and 1.83 and 1.95 ng g(-1) at ER37 and ER09 in Lake Erie, respectively, based on dry sediment weight. The surficial BDE209 concentrations are 242 and 211 ng g(-1) at ON40 and ON30 and 50 and 55 ng g(-1) at ER37 and ER09. The sigma(9-) PBDEs fluxes to the sediment around 2002 are 147 and 195 pg cm(-2) year(-1) at ON40 and ON30 and 136 and 314 pg cm(-2) year(-1) at ER37 and ER09, respectively. The fluxes of BDE209 are 6.5 and 7.3 ng cm(-2) year(-1) at ON30 and ON40 and 3.7 and 8.9 ng cm(-2) year(-1) at ER37 and ER09, respectively. Dramatic increases in PBDE concentrations and fluxes upward toward the sediment surface and the present time are evident at both locations in Lake Ontario, while PCBs concentrations peak in the middle of sediment cores around the dated time of 1970s and 1960s. For both locations of Lake Erie, the increasing trends of both PBDEs and PCBs from the bottom to the surficial segments were distorted by sediment mixing. BDE209 is the most abundant congener among PBDEs in the sediments, constituting about 96 and 91% of the total PBDEs on mass basis in Lakes Ontario and Erie, respectively.

Seasonal and interannual mobility of arsenic in a lake impacted by metal mining.

Detailed examination of the water column, sediments, and interstitial waters was conducted in Balmer Lake, Ontario, Canada, in 1993-1994 and 1999 in order to assess the seasonal and interannual controls governing the behavior of As. High-resolution profiles of dissolved (<0.45 microm) Fe, Mn, SO4(2-), and sigmaH2S across the sediment-water interface indicate the presence of reducing conditions in close proximity to the benthic boundary during ice-free periods, which are characterized by fully oxygenated bottom waters. Dissolved As is remobilized as As(III) in suboxic sediment horizons via the redox-controlled dissolution of Fe (and perhaps Mn) oxide phases. During 1993-1994, As fluxes to the water column were relatively low (2-15 microg cm(-2) year(-1)) and contributed between 2 and 18% of the water column inventory. Dissolved As in the lake waters was derived primarily from external mining-related loadings during this period. Between 1993 and 1999, external loadings of As to Balmer Lake decreased while [As]aq within the lake increased, suggesting an increase in the proportion of sediment-derived As. Indeed, benthic dissolved As fluxes in 1999 ranged from 179 to 380 microg cm(-2) year(-1), representing approximately 33-60% of the water column burden. The relatively recent importance of sedimentary arsenic sources is suggested to reflect changes to sediment redox conditions associated with a postulated increase in lake primary productivity. Ironically, the increased contribution of dissolved arsenic to the water column appears to have resulted from an otherwise improvement in water quality. Reduced loadings of Cu, Zn, and Ni to the lake since 1994 appear to have allowed increased phytoplankton production that has stimulated arsenic release.

Pollen influx values measured in different sedimentary environments and their palaeoecological implications

Aspects which need to be considered when calculating pollen influx values in three sedimentary environments (pollen traps, peats and lake sediments) are reviewed. Examples of influx values (grains cm-2 year-1) in northern Fennoscandia for two arboreal taxa Betula and Pinus, calculated from these three different environments are presented. Such long term average pollen influx values can be used to indicate the presence/absence and density of the species within the vicinity of the sampling site. Contrary to expectations, conditions do exist in which numerically comparable values are obtained from all three environments. Using these pollen influx values to interpret forest and woodland history in tree-line situations, from both peats and lake sediments, a higher degree of precision can be obtained than through the more classical pollen percentage method alone.

Recent Metal Pollution in Agassiz Ice Cap

Metal data for Pb, Zn, Al, Cd, and Tl show seasonal variations of high levels during the winter−early spring period and low levels during the summer−fall period. In terms of Pb magnitudes and seasonal variations, our data of the late 1980s and early 1990s (unleaded gasoline era) appear compatible with those of Murozumi et al. (Murozumi, M.; Chow, T. J.; Patterson, C. C. Geochim. Cosmochim. Acta 1969, 33, 1247−1294.) of the 1950s and 1965 (leaded gasoline era). This is probably due to the close proximity and similar elevation of the two areas (Agassiz Ice Cap, Canadian Arctic and Camp Century, northwest Greenland), which are likely subjected to the same polluted air masses. Despite the implementation of unleaded gasoline since the early 1970s, our data show that the Agassiz Ice Cap still received a significant amount of lead during the late 1980s and early 1990s; this is in contrast to Summit, Greenland, which saw a very drastic decrease of the element during similar periods. This is because of the different locations and altitudes, as well as different sources, mainly Eurasian for Agassiz versus mainly U.S. for Summit. Lead was determined by laser-induced fluorescence spectrometry via the direct injection of microliter sample sizes. Its fallout flux was estimated to be 1.2 ng cm-2 year-1. The lead concentration in the surface snow appears to follow the order of Agassiz and northwest Greenland > central Greenland > Antarctica.

Neutron damage of chemical vapour deposition diamond

Many of the components of particle detectors for the next generation of high energy experiments need to be extremely radiation-hard if they are to continue to function for more than a few months. For example, in the first few year's operation of the large hadron collider, we expect about 1013 neutrons cm-2 year-1 1 cm from the beam pipe, with other particles causing about ten times as much additional damage. Diamond is being considered as both a detector material and perhaps as a structural component. We report on tests of chemical vapour deposition diamond films irradiated by 6*1014 neutrons cm-2 of energy below 10 keV, plus (2.0+or-0.5)*1014 neutrons cm-2 of energy above 10 keV peaking at about 1 MeV. The diamonds were metallized with titanium-gold contacts-strips on the front and a continuous electrode on the back. The leakage current (already extremely small) decreased by factors of about 50 as a result of the irradiation, both measured between adjacent strips and through the thickness of the samples. The detection efficiency of charged particles improved by 30-40% after irradiation. In addition, natural diamonds irradiated by 4*1014 neutrons cm-2 showed very little optical absorption in the GR1 and ND1 bands (neutral and negative vacancy) corresponding to a concentration of vacancies of 4(+or-2)*1014 cm-3.

Industrial Mercury in Combination with Natural Pb210 as Time-dependent Tracers of Sedimentation and Mercury Removal from Haifa Bay, Israel

The input rate of a particle reactive pollutant, mercury, from a chloralkali plant, which was known in detail, was used in combination with Pb210 to determine the sedimentation and mixing rates in several locations in the area of a factory outfall. Sediment profiles were modelled using a numerical solution to the diagenetic equation. The calculated sedimentation rate of ∼0·5 cm year-1 was close to values based on estimates of regional sand transport. Surface mixing rates of 7-8 cm2 year-1 with an exponential decrease with depth are similar to rates estimated in other near-shore areas. An atmospheric flux of excess Pb210 to the area of 0·41 dpm cm-2 year-1 was estimated from the inventory in the sediment from two nearby fishponds. The model predicted that despite the input of 0·5 cm year-1 of clean sand into the area, the level of mercury in surface sediments would remain ∼0·3 ppm above background (precontamination) levels for more than 50 years after input controls were introduced due to particle mixing. Unlike most previous studies on the discharge of mercury into the near-shore environment, this receiving area is one of relatively coarse sandy sediment and particle poor oceanic water. The majority of the mercury in the sediment was not attached to fine-grained particles. Only 10% of the mercury supplied in the effluent, was found in the sediments within a 150 km2area adjacent to the outfall. It was hypothesized that there were insufficient fine-grained particles to remove dissolved mercury from the water column. Thus the principal pathway of even particle reactive pollutants to the biosphere in this coastal area may be via the planktonic food-chain, with the benthic food-web being important only locally.

Aspects of the limnology of Yeppen Yeppen Lagoon, central Queensland

Aspects of the morphometry and physical and chemical limnology of Yeppen Yeppen Lagoon, in tropical Australia, have been investigated. The lagoon is a channel billabong lying in the old bed of the Fitzroy River. It has a relatively small, shallow and elongated basin. Seasonal variations in water temperature, light regime, oxygen concentration, pH and conductivity suggest that the lagoon exhibits a warm monomictic pattern of thermal stratification rather than a continuous warm polymictic one. The annual heat budget was 3294 calories cm-2 year-1. Maximum work of the wind was 238.8 g-cm cm-2, and maximum stability was 34.5 g-cm cm-2. Despite the apparently low stability of stratification, the lagoon remained thermally stratified for much of the year. Analysis of wind-distributed heat suggested that slow mixing was taking place even during periods of relatively high stability, although this mixing was insufficient to reoxygenate the hypolimnion, which remained anoxic for much of the year. Yeppen Yeppen Lagoon is likely to prove eutrophic, and it is suggested that primary productivity will be high because a large volume of the lagoon's water is well lit and a large sediment surface area is in contact with the epilimnion. Notes on the biota of Yeppen Yeppen Lagoon are also given.

A Whole-Basin Diatom Accumulation Rate for a Small Eutrophic Lake in Northern Ireland and its Palaeoecological Implications

(1) Twelve cores from a lake affected by creamery effluent disposal (Lough Augher, Co. Tyrone, N. Ireland) were analysed for diatom stratigraphy. Cores were dated by biostratigraphic correlation to a 2 0Pb-dated master core. Diatom accumulation rates were estimated for individual cores, and for the average of all cores. (2) The plankton diatom succession caused by the creamery effluent disposal represents a simple eutrophication sequence, and agrees well with that hypothesized by culture studies of diatom responses to changing Si: P ratios. (3) Relative frequency stratigraphies in deep-water cores are clearly repeatable. However, diatom accumulation rates for individual cores are variable, and no single core is strictly representative of the mean accumulation rate for the whole lake, because of a complex pattern of diatom deposition. (4) A mean accumulation rate was calculated for the whole lake. The maximum, approximately 18 x 106 cells cm-2 year1, occurs between 1940 and 1957, suggesting that this was the most productive diatom period. In contrast, maximum Stephanodiscusparvus percentages, a diatom commonly assumed to indicate lake eutrophy, occur between 1964 and 1974. This later period relates, perhaps, to maximum whole-lake productivity with greater contributions from non-siliceous algae. (5) Percentage data and accumulation rates in individual cores under-estimate the importance of the periphyton; their mean accumulation rate c. 1900-80 for the whole lake (10-12 x 106 cells cm-2 year1) is greater than that of the plankton (1-5-6 x 106 cells cm-2 year-1). This has significant palaeoecological implications for the assessment of diatom responses to eutrophication. (6) The contrasting palaeoecological interpretations of the different data types are discussed. The different conclusions are reconcilable in terms of the variable nutrient requirements and sources of the plankton and periphyton.

Modelling of 210Pb behaviour in the catchment and sediment of Lake Tali Karng, Victoria, and estimation of recent sedimentation rates.

The technique of 210Pb dating of sediments over time scales of up to 120 years is described and illustrated by a study of Lake Tali Karng. Results from the study of a sediment core from the deepest part of the lake shows that the flux of unsupported 210Pb to the sediment is about 145 mBq cm-2 year-1. Over the last 100 years, about 75 cm of sediment has accumulated at an average rate of 150-200 mg cm-2 year-1. Calculations using two models to convert 210Pb activity-depth profiles to age-depth profiles indicate that the sedimentation rate has increased slightly over the last 100 years. It is estimated that about 14% of the 210Pb falling in the catchment reaches the lake sediment, but depending on the assumptions made the possible range is 7-60%.

Oxygen-Isotope Measurements on 1700 ± 300 Year Old Antarctic Ice

Abstract Values of δ18OSMOW measured on ice from 250 m depth at old “Byrd station”. Antarctica, averaged about A monotonie trend in values through the 16 cm long core piece suggested that part of an annual layer was represented. Snow accumulation in excess of 14.4 g cm-2 year-1 ca 1 700 years B.P. is indicated.

Oxygen-Isotope Measurements on 1700 ± 300 Year Old Antarctic Ice

AbstractValues of δ18OSMOW measured on ice from 250 m depth at old “Byrd station”. Antarctica, averaged about A monotonie trend in values through the 16 cm long core piece suggested that part of an annual layer was represented. Snow accumulation in excess of 14.4 g cm-2 year-1 ca 1 700 years B.P. is indicated.