Daniel R. Engstrom

A Synthesis of Progress and Uncertainties in Attributing the Sources of Mercury in Deposition

Abstract A panel of international experts was convened in Madison, Wisconsin, in 2005, as part of the 8th International Conference on Mercury as a Global Pollutant. Our charge was to address the state of science pertinent to source attribution, specifically our key question was: “For a given location, can we ascertain with confidence the relative contributions of local, regional, and global sources, and of natural versus anthropogenic emissions to mercury deposition?” The panel synthesized new research pertinent to this question published over the past decade, with emphasis on four major research topics: long-term anthropogenic change, current emission and deposition trends, chemical transformations and cycling, and modeling and uncertainty. Within each topic, the panel drew a series of conclusions, which are presented in this paper. These conclusions led us to concur that the answer to our question is a “qualified yes,” with the qualification being dependent upon the level of uncertainty one is willing to accept. We agreed that the uncertainty is strongly dependent upon scale and that our question as stated is answerable with greater confidence both very near and very far from major point sources, assuming that the “global pool” is a recognizable “source.” Many regions of interest from an ecosystem-exposure standpoint lie in between, where source attribution carries the greatest degree of uncertainty.

Climate-driven ecosystem succession in the Sahara: The past 6000 years

Desiccation of the Sahara since the middle Holocene has eradicated all but a few natural archives recording its transition from a “green Sahara” to the present hyperarid desert. Our continuous 6000-year paleoenvironmental reconstruction from northern Chad shows progressive drying of the regional terrestrial ecosystem in response to weakening insolation forcing of the African monsoon and abrupt hydrological change in the local aquatic ecosystem controlled by site-specific thresholds. Strong reductions in tropical trees and then Sahelian grassland cover allowed large-scale dust mobilization from 4300 calendar years before the present (cal yr B.P.). Todays desert ecosystem and regional wind regime were established around 2700 cal yr B.P. This gradual rather than abrupt termination of the African Humid Period in the eastern Sahara suggests a relatively weak biogeophysical feedback on climate.

Age and Survivorship of Diapausing Eggs in a Sediment Egg Bank

We determined the densities of diapausing eggs of the copepod Diaptomus sanguineus in sediments from two small freshwater lakes in Rhode Island. Sediment cores, sliced at 1-cm intervals, showed that egg densities ranged between 4 X 10 4 and 8 x 10 4 eggs/m 2 near the sediment surface and declined to very low values at depths of 10-15 cm in both lakes, although eggs were found as deep as 30 cm in the sediment of one lake. Between 10 and 50% of these eggs hatched in short-term laboratory experiments, and actual egg viability is probably higher. 210 Pb-dating revealed relatively constant sedimentation rates in both lakes, and we use this information to estimate egg ages. In one lake, the mean diapausing-egg age is 70.4 yr (median age = 45.9 yr) and the maximum age of eggs we hatched was 332 yr. In the other lake, the mean egg age is 48.9 yr (median age = 35.9 yr) and the maximum age of eggs we hatched was 112 yr. We calculated egg mortality rates by regressing In(egg density) on the age of the sediment from which the eggs were taken to obtain estimates of 1.1 and 1.5% mortality/yr for the two lakes. Diapausing eggs of zooplankton represent a long-lived life history stage of an otherwise short-lived organism. They provide generation overlap that can have substantial significance for both ecological and evolutionary dynamics.

Chemical and biological trends during lake evolution in recently deglaciated terrain.

As newly formed landscapes evolve, physical and biological changes occur that are collectively known as primary succession. Although succession is a fundamental concept in ecology, it is poorly understood in the context of aquatic environments. The prevailing view is that lakes become more enriched in nutrients as they age, leading to increased biological production. Here we report the opposite pattern of lake development, observed from the water chemistry of lakes that formed at various times within the past 10,000 years during glacial retreat at Glacier Bay, Alaska. The lakes have grown more dilute and acidic with time, accumulated dissolved organic carbon and undergone a transient rise in nitrogen concentration, all as a result of successional changes in surrounding vegetation and soils. Similar trends are evident from fossil diatom stratigraphy of lake sediment cores. These results demonstrate a tight hydrologic coupling between terrestrial and aquatic environments during the colonization of newly deglaciated landscapes, and provide a conceptual basis for mechanisms of primary succession in boreal lake ecosystems.

Paleosalinity from trace metals in fossil ostracodes compared with observational records at Devils Lake, North Dakota, USA

Abstract The chemistry of closed-basin lakes responds directly to the hydrologic budget through evaporative concentration of dissolved salts, and the sediments of these basins contain several excellent fossil and geochemical proxies for past changes in water chemistry and salinity. The trace-metal content of ostracode shells, among the most promising of these paleosalinity methods, is further explored in this study of Mg and Sr partitioning in Candona rawsoni, a widespread ostracode of the North American Great Plains. The molar distribution coefficients KD[Mg] and KD[Sr] for this species, K D (Mg/Ca) CaCO 3 −0.004 (Mg 2+ /Ca 2+ )H 2 O =9.68×10 −5 ×T( ° C) K D [Sr]= (Sr/Ca) CaCO 3 (Sr 2+ /Ca 2+ )H 2 Oz =0.406 are determined from laboratory experiments in which C. rawsoni are cultured under controlled temperatures and salinities. Our formulation of KD[Mg] contains a correction term for excess Mg that is incorporated into the shell during early calcification. The accuracy of the distribution coefficients is tested against field collections of live ostracodes from a suite of 12 Dakota lakes. The KD-inferred values for (Mg2+/Ca2+)H2O and (Sr2+/Ca2+)H2O are in close accord with the same ratios measured in the lakes. A comparison of ostracode-reconstructed and historically measured salinities for Devils Lake, North Dakota, shows good agreement within the salinity range preferred by C. rawsoni (1–10%0), although salinity excursions beyond this optimum are poorly represented in the sedimentary record. In most lakes salinity is more readily inferred from (Mg/Ca)H2O than (Sr/Ca)H2O because Sr solubility is a complex function of several carbonate phases. As shown for Devils Lake, (Sr/Ca)H2O may not vary systematically with the salinity of some brines.

DROUGHT CYCLES AND LANDSCAPE RESPONSES TO PAST ARIDITY ON PRAIRIES OF THE NORTHERN GREAT PLAINS, USA

Widespread drought is among the most likely and devastating consequences of future global change. Assessment of drought impacts forecast by atmospheric models requires an understanding of natural drought variability, especially under conditions more arid than today. Using high-resolution lake-sediment records from the northern Great Plains, we show pronounced 100- to 130-yr drought cycles during the arid middle Holocene (8000 calendar yr BP). During drought phases, grass productivity declined, erosion and forbs increased, and fuel limitation reduced fire importance. Intervening humid decades saw grass production rise, with stabilization of soils and renewed fire as fuels became abundant. Although both C3 and C4 grasses declined during droughts, a lasting shift to C3 dominance occurred during a single drought -8200 calendar yr BP. During the more humid Late Holocene (2800 calendar yr BP), climate was less variable and without evident drought cyclicity. Consequently, drought severity during past, and possibly future, arid phases cannot be anticipated from the attenuated climate variability evident during contemporary humid phases. Our study demonstrates that agriculturally important grassland ecosystems respond sensitively to drought variability, uncertainty in which has profound implications for the future of these ecosystems.

A possible younger dryas record in Southeastern Alaska

A stratigraphic record of climatic cooling equal in timing and severity to the Younger Dryas event of the North Atlantic region has been obtained from lacustrine sediments in the Glacier Bay area of southeastern Alaska. Fossil pollen show that a late Wisconsin pine parkland was replaced about 10,800 years ago by shrub- and herb-dominated tundra, which lasted until about 9,800 years ago. This vegetational change is matched by geochemical evidence for loss of organic matter from catchment soils and increased mineral erosion. If this event represents the Younger Dryas, then an explanation for a hemisphere-wide propagation of a North Atlantic climatic perturbation must be sought.

Large Shift in Source of Fine Sediment in the Upper Mississippi River

Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.

Pronounced climatic variations in Alaska during the last two millennia

Paired oxygen-isotopic analyses of abiotic carbonate and benthic-ostracode shells from lake sediments provide a continuous quantitative record of growing-season temperature for the past 2000 years in the northwestern foothills of the Alaska Range. This record reveals three time intervals of comparable warmth: anno Domini (A.D.) 0–300, 850-1200, and post-1800, the latter two of which correspond to the Medieval Climatic Anomaly and climatic amelioration after the end of the Little Ice Age. The Little Ice Age culminated at A.D. 1700, when the climate was ≈1.7°C colder than at present. A marked climatic cooling also occurred around A.D. 600, coinciding with extensive glacial advances in Alaska. Comparisons of this temperature record with ostracode trace-element ratios (Mg/Ca, Sr/Ca) further suggest that colder periods were wetter and vice versa during the past 2000 years.

Geochemistry of ostracode calcite: Part 2. The effects of water chemistry and seasonal temperature variation on Candona rawsoni

Abstract Reconstruction of lake paleochemistry from the isotope and trace-element composition of fossil ostracodes requires an understanding of geochemical variations induced by the seasonality of shell formation. Monthly field collections of live Candona rawsoni from two hyposaline lakes in the eastern Dakotas reveal large variations in σ 18 O values (2–5%o) and Mg/Ca and Sr/Ca ratios (up to 35%) among individual adults within the same assemblage. Because the measured variations in water chemistry were small, the amplitude of ostracode σ 18 O and Mg/Ca variations must result from seasonal temperature variation. Ostracode Sr/Ca ratios, which should be temperature independent, shows strong positive covariance with shell Mg/Ca. These results imply that Sr uptake in ostracode calcite increases with the Mg content of the shell. The partitioning coefficient for Mg, on the other hand, appears to decreases at high values of Mg/Ca in the host water. The geochemical variability in the field collections indicates that C. rawsoni populations are composed of multiple overlapping generations that attain maturity at different times. Many juveniles apparently undergo several molts in mid-summer and persist as penultimate instars (A-1) into the following spring. Geochemical analysis of late-instar juvenile C. rawsoni from lacustrine sedimentary records is likely to provide information on mid-summer conditions. Large numbers of fossil ostracodes should be analyzed from each stratigraphic interval to eliminate seasonal noise from the paleochemistry records of temperate-region lakes.