John A. Robbins

Records of lead deposition in Lake Michigan sediments since 1800

The distribution of stable lead has been determined in several Lake Michigan cores that have been dated using the /sup 210/Pb technique. A mathematical model, based on records of national and regional fuel use, is developed that accounts for the effect of aquatic residence time, processes of sedimentation, compaction, postdepositional redistribution of sediment solids, and the effect of finite core sectioning. The model gives an excellent quantitative description of the data, indicating that the lead profiles reflect the history of lead input from the burning of coal and leaded gasoline since about 1830 and 1920, respectively. The model is applied to obtain the sedimentation and lead deposition rates from stable lead distributions in undated sediment cores measured previously by others. The 1972 anthropogenic lead flux into the southern basin of the lake is determined to be 1.3 ..mu..g cm/sup -2/ yr/sup -1/ in comparison to the natural (precultural) flux of approximately 0.16 ..mu..g cm/sup -2/ yr/sup -1/. The anthropogenic lead deposited in the southern basin in 1972 is estimated to be approximately 240 metric tons as compared with an estimate of approximately 230 metric tons from air pollution fallout originating in the Chicago-Gary urban-industrial area. The measurement of both leadmorexa0» and lead-210 fluxes in the same core may allow contributions of atmospheric lead to be distinguished from terrigenous pollution inputs. (auth)«xa0less

Historical atmospheric mercury emissions and depositions in North America compared to mercury accumulations in sedimentary records

Abstract Gold and silver production in North America (included United States, Canada and Mexico) released a large amount of mercury to the atmosphere until well into this century when mercury (Hg) amalgamation was replaced by cyanide concentration. Since then, emissions from industries have been the dominant anthropogenic sources of atmospheric Hg in North America as a whole. Past Hg emissions from gold and silver extractions in North America during the 1800s do not show a clear evidence of atmospheric deposition occurred at the coring sites considered in this study. Estimated atmospheric emissions of Hg in North America peaked in 1879 (at about 1708 t yr −1 ) and 1920 (at about 940 t yr −1 ), primarily due to Hg emissions from gold and silver mining. After the Great Economic Depression (1929) Hg emissions peaked again in the 1947 (274 t yr −1 ), in 1970 (325 t yr −1 ) and in 1989 (330 t yr −1 ) as result of increased Hg emissions from industrial sources, though improvements in the emissions control technology in United States and Canada have been substantial. Estimates of total atmospheric deposition fluxes of Hg to water and terrestrial receptors were in the range of 14.3–19.8 μg m −2 yr −1 in North America as a whole, and averaged 135 μg m −2 yr −1 (global background + local emissions) in the Great Lakes. These values were in good agreement with recent estimates reported in literature. The comparison of atmospheric Hg deposition fluxes with Hg accumulation rates in sediment cores suggests that atmospheric deposition was the major source of Hg entering the lakes system at coring sites, however, important contributions to Lake Ontario sediment cores sites from 1940 to 1970 were likely originated from local point sources (i.e. direct discharges).

Records of nutrient-enhanced coastal ocean productivity in sediments from the Louisiana continental shelf

Shelf sediments from near the mouth of the Mississippi River were collected and analyzed to examine whether records of the consequences of anthropogenic nutrient loading are preserved. Cores representing approximately 100 yr of accumulation have increasing concentrations of organic matter over this period, indicating increased accumulation of organic carbon, rapid early diagenesis, or a combination of these processes. Stable carbon isotopes and organic tracers show that virtually all of this increase is of marine origin. Evidence from two cores near the river mouth, one within the region of chronic seasonal hypoxia and one nearby but outside the hypoxic region, indicate that changes consistent with increased productivity began by approximately the mid-1950s when the inorganic carbon in benthic forams rapidly became isotopically lighter at both stations. Beginning in the mid-1960s, the accumulation of organic matter, organic δ13C, and δ15N all show large changes in a direction consistent with increased productivity. This last period coincides with a doubling of the load of nutrients from the Mississippi River, which levelled off in the mid-1980s. These data support the hypothesis that anthropogenic nutrient loading has had a significant impact on the Louisiana shelf.

Response of atmospheric lead to decreased use of lead in gasoline.

Wet-only precipitation collected in urban Minneapolis, MN, and rural, northcentral Minnesota (Marcell) has been analyzed for Pb since the late 1970s. Annual volume-weighted mean concentrations of Pb in precipitation have decreased from 29 to 4.3 ..mu..g/L at the urban site and from 5.7 to 1.5 ..mu..g/L at the rural site in the years 1979-1983. Annual Pb fluxes in precipitation have decreased from 1979 to 1983: 2000 to 370 (ng/cm/sup 2/)/year at the urban site and 430 to 100 (ng/cm/sup 2/)/year at the rural site. Decreases in atmospheric Pb fluxes are closely correlated with decreases in Pb used in gasoline in Minnesota and nationally and with a recently constructed atmospheric Pb source function for the Great Lakes region. 49 references, 3 figures, 3 tables.

Early Eutrophication in the Lower Great Lakes

New Evidence from Biogenic Silica in Sediments New evidence from studies of biogenic silica and diatoms in sediment cores indicates that eutrophication in the lower Great Lakes resulted from nutrient enrichment associated with early settlement and forest clearance. Diatom production peaked from 1820 to 1850 in Lake Ontario, at about 1880 in Lake Erie, but not until 1970 in Lake Michigan. This is the first reported sediment record of the silica-depletion sequence for the Great Lakes.

The partitioning of 7beryllium in fresh water

Abstract Field observations and experimental measurements of the partitioning coefficient ( K d ) of 7 Be in fresh water show that it varies inversely with the solids concentration at typical environmental values (up to 30 mg/1). This behavior is similar to that of many other metals and organic pollutants, which means that 7 Be may be useful as a tracer of the movement of these substances in the water column. However, the wide range in the percentage of 7 Be adsorbed by solids over this range of concentrations (over 50%) means that in order to use 7 Be either to measure total sedimentation rates or to trace lateral sediment movement it will be necessary to monitor changes in sediment concentration over the area and time period of interest. The wide scatter in our data at both high and low solids concentrations suggests that other factors also affect K d . Until these factors are identified, application of our results to other systems will be risky. At high (greater than 100 mg/1) solids concentrations over 90% of 7 Be is associated with the solid phase, so it may be a useful tracer of reworking rates in bottom sediments. 7 Be has a slightly greater affinity for the solid phase in fresh water than in seawater.

Development of recurrent coastal plume in Lake Michigan observed for first time

NOAA CoastWatch satellite imagery from early 1996 captured the initiation, development, and decay of a recurrent coastal plume in southern Lake Michigan (Figure 1). For the past 4 years intermittent satellite coverage has revealed a late winter-early spring plume in the lake, a feature also observed by Mortimer [1988]. In 1996, clear weather conditions allowed researchers to observe the plumes development for the first time and they also collected water samples from helicopter and a small boat.

137Cs and 210Pb transport and geochronologies in urbanized reservoirs with rapidly increasing sedimentation rates

Sedimentation rates have been measured in three reservoirs in northeastern Ohio, U.S.A., by means of 137Cs and 210Pb geochronologies, volumetric surveys and varve counting. These various methods, while only partially overlapping for each reservoir, show dramatic (three-fold) increases in rates of sediment accumulation in each system between about 1940 and 1977. Mass sedimentation rates are very nearly proportional to the size of the population in the region and possess a doubling time of roughly 19 yr. In these systems with changing sedimentation rates, the preferred model for use with 210Pb geochronologies is one which assumes a constant activity of material added to surface sediments. In systems possessing large watershed)/(reservoir area) ratios, increasing erosion is evidently accompanied by a proportionate increase in the erosion of excess 210Pb. High near-surface activities of 137Cs are due to system integration effects with time constants the order of 10 yr. to a few decades. Total accumulation of fallout 137Cs and excess 210Pb far exceed direct atmospheric loadings, thus indicating the importance of watershed contributions and implying annual retention of the radionuclides in the reservoirs of between ∼15% and ∼80%. In Lake Rockwell, sedimentary fluxes of Zn, Pb and Cu have increased with time. The flux of Cu in particular has increased very markedly since 1970, and concentrations are high in surface materials due in large part to addition of CuSO4, an algicide, to the water. Because of increased rates of sedimentation, the remaining useful life of Lake Rockwell has decreased from 203 to 67 yr., while the remaining useful life of Mayfair Lake is now less than 5 yr.

Seasonal cycling of trace elements 137Cs, 7Be, and 239+240Pu in Lake Michigan

Trace elements, fallout 137Cs (t½=30.2 years) and cosmogenic 7Be (t½=53.4 days) were measured in trap samples collected from two sites in southern Lake Michigan at selected times during the period from 1982 through 1987. Concentrations of 137Cs in trap material were virtually constant from top to bottom during the period when the lake was vertically well-mixed. Fluxes of 137Cs were more than two orders of magnitude higher than atmospheric loading. With the development of the thermocline, concentrations of the isotope diminished to undetectable levels at a rate indicating a 1-month residence time of resuspended components in the epilimnion. Declines in mass flux (10 to 0.2 g m−2 d−1) and trace element concentrations (some by 30 x) also occurred at this time. Epilimnetic concentration of 7Be increased from a prestratification value of about 30 dpm/g to a maximum of about 120 dpm/g in July. During calcite formation in August, the concentration rapidly declined mainly as a result of enhanced particle settling rates. Throughout the period of stratification, a maximum in concentration and flux of 7Be persisted in trap material between 40 and 60 m depth but only about 15% of the atmospheric flux was transferred to the benthic boundary region below 60 m. In near-bottom trap materials, the 137Cs concentration was reduced at the onset of stratification by addition of radiocesium-deficient authigenic materials. By November, concentrations of 137Cs had returned to prestratification levels through remineralization of these materials. A decrease of 22% in the seasonally invariant ratio of 137Cs/ 40K between 1982 and 1986 indicated a decay-corrected removal time of about 20 years, which is consistent with the rate of decrease of Pu concentrations in the well-mixed water column measured over more than a decade. In benthic trap samples, 7Be concentration decreased exponentially throughout the stratified period, with a time constant comparable to the half-life of the isotope. The resuspendable sediment pool, evidently isolated from atmospheric loading during stratification, was “recharged” with 7Be following late fall overturn. Box and advection-diffusion models describing the storage and vertical distribution of 7Be during the winter-mixing period successfully account for observations, implying an inverse relation between water depth and winter recharging of the resuspendable pool. A plutonium-calibrated box model for the seasonal cycle of tracers in surface waters describes the main features of the variation of Pu isotopes, 137Cs and 7Be.

Particle Transport, Nutrient Cycling, and Algal Community Structure Associated with a Major Winter-Spring Sediment Resuspension Event in Southern Lake Michigan

Abstract Over the past decade, intermittent satellite imagery revealed the presence of an extensive plume of resuspended sediments in late winter-early spring with a clear offshore projection coinciding with the region of maximum sediment accumulation in the lake. The large scale of the plume implied that this process was important in sediment, and associated constituent, cycling and transport, but it had never been sampled due to severe conditions. The onset of the 1996 event coincided with a major March storm. Within a few days the plume was approximately 10 km wide and over 300 km in length, implying that the source of the reflective materials was widely distributed. An estimate of the total mass of resuspended sediment, 12 days after the storm, was similar to the annual external load of (sand-free) particulate material to the southern basin. The high turbidity plume persisted for over a month, progressing northward along the eastern shore with a major offshore transport feature. Sediment traps within this feature recorded a major mass flux event. The plume was sampled on two occasions and was found to contain 5 to 10 times as much suspended matter as open-lake locations outside the visible plume. Total particulate phosphorus was high within the plume making this episodic process important in sedimentwater exchange. The diatom community structure within the plume was significantly different from outside the plume and was characteristic of more eutrophic waters. Abundance of non-diatom phytoplankton and microbial food web organisms were highest at the plume edge. The episodic nature of this process makes it difficult to sample, but the scale makes it important in designing monitoring programs and massbalance modeling efforts.