Amy Myrbo

Radiocarbon dating suitability of aquatic plant macrofossils

Paleolimnological and plant physiological literature were reviewed to determine which types of aquatic plant macrofossils are suitable for radiocarbon dating, with a particular focus on the uptake of reservoir-aged dissolved inorganic carbon (DIC) by emergent plants. Submerged aquatic plants utilize large amounts of DIC and are clearly not suitable for radiocarbon dating. Under certain environmental conditions, some emergent aquatic plants can metabolize DIC in quantities large enough to introduce old-carbon error to radiocarbon dates acquired from their remains (plant macrofossils). Over 300 plant macrofossil images are included in the online resource Tool for Microscopic Identification; http://tmi.laccore.umn.edu) along with guidance on identification and suitability for radiocarbon dating.

Variable ecosystem response to climate change during the Holocene in northern Minnesota, USA

Both terrestrial and aquatic ecosystems should respond to abrupt climate changes such as those that affected the midcontinent of North America during the Holocene. A variety of paleorecords indicate that the onset of prolonged dry conditions in this region occurred as rapidly as 300 yr during the early Holocene, with a subsequent increase in moisture occurring rapidly in the late Holocene. Here, we report a 9500 yr multiproxy sediment record from Deming Lake, Minnesota, USA, that demonstrates only subtle dynamics during rapid climate changes that caused vegetation in the catchment to shift among pine forest, open grassland, and deciduous forest. The most substantial changes in ecosystem properties immediately followed deglaciation of the landscape, formation of the lake, and initial development of pine forests. In contrast, there were only muted responses to pronounced mid-Holocene climate changes that caused vegetation in the catchment to switch from pine forest to open grassland (ca. 8000 yr B.P.), and then deciduous forest (ca. 5400 yr B.P.). The flux of organic and inorganic terrigenous material, the processing of carbon, and catchment erosion changed rapidly during the most recent shift at 3300 yr B.P. to the modern pine forest. These changes coincided with the onset of meromictic conditions that influenced internal lake dynamics. However, the terrestrial influence on the lake sedimentary record gradually diminished over time, indicating a trajectory of increasing catchment stabilization that was relatively impervious to dramatic regional climate changes. The relative complacency of the Deming Lake record during the late Holocene indicates relative resistance to abrupt climate change at later stages of ecosystem development.

A qualitative and quantitative model for climate-driven lake formation on carbonate platforms based on examples from the Bahamian archipelago

Lakes on carbonate platform islands such as the Bahamas display wide variability in morphometry, chemistry, and fauna. These parameters are ultimately driven by climate, sea level, and carbonate accumulation and dissolution. The authors propose a model that integrates climatological, geomorphological, and stratigraphic frameworks to understand processes of carbonate-hosted lake formation and limnological characteristics in modern day environments, with applications to carbonate lake sedimentary records. Fifty-two lakes from San Salvador Island and Eleuthera, Bahamas, were examined for water chemistry, basin morphology, conduit development, conductivity, and major ions. Using non-metric, multi-dimensional scaling ordination methods, the authors derived a model dividing lakes into either constructional or destructional formational modes. Constructional lakes were further divided into pre-highstand and highstand types based on whether their formation occurred during a marine regressive or transgressive phase. Destructional lakes are created continually by dissolution of bedrock at fresh/saline water interfaces and their formation is therefore related to changing climate and sea level. This model shows that lake formation is influenced by the hydrologic balance associated with climatic conditions that drives karst dissolution as well as the deposition of aeolian dune ridges that isolate basins due to sea-level fluctuations. It allows for testing and examining the climatic and hydrologic regime as related to carbonate accumulation and dissolution through time, and for an improved understanding of lake sensitivity and response to climate as preserved in the lacustrine sedimentary record.

Sedimentary and historical context of eutrophication and remediation in urban Lake McCarrons (Roseville, Minnesota)

Abstract Geochemical analysis of the varved (annually laminated) sediments of a small, deep urban lake in east-central Minnesota shows that some water-quality and watershed indicators are approaching prehistoric (pre-1850s) values after an excursion to anomalous levels during the 1920s through 1950s. This high-resolution paleolimnological information (annually resolved for most of the 20th century), including varve thickness measurements on a digital image, carbon stable isotopic composition of organic matter, and sedimentary component quantification, provides historical perspective to lake managers planning remedial measures with reference to the “natural” state of the lake, especially in regard to the perception that water quality has declined only in recent decades (i.e., in the late 20th century). The lake is at present eutrophic and oligomictic, with oxygen-depleted bottom waters enriched in dissolved solids relative to surface waters; an alum treatment has contributed to the problem of persistent stratification at the same time as it has reduced phosphorus and increased transparency in lake surface waters. The sedimentary record shows the lakes strong response to agricultural, recreational, and urban development in the watershed, gradual improvement beginning at the time of sanitary sewer installation in the 1960s, and only a minor additional response to a concerted remediation effort in the mid-1980s to 1990s. Indices of terrestrial inputs and algal productivity show evidence of a lake that was at its most heavily impacted during the mid-20th century, and which has improved in some respects since the 1960s.

The Neotoma Paleoecology Database: A Research Outreach Nexus

Paleoecological data from the Quaternary Period (2.6 million years ago to present) provides an opportunity for educational outreach for the earth and biological sciences. Paleoecology data repositories serve as technical hubs and focal points within their disciplinary communities and so are uniquely situated to help produce teaching modules and engagement resources. The Neotoma Paleoecology Database provides support to educators from primary schools to graduate students. In collaboration with pedagogical experts, the Neotoma Paleoecology Database team has developed teaching modules and model workflows. Early education is centered on discovery; higher-level educational tools focus on illustrating best practices for technical tasks. Collaborations among pedagogic experts, technical experts and data stewards, centered around data resources such as Neotoma, provide an important role within research communities, and an important service to society, supporting best practices, translating current research advances to interested audiences, and communicating the importance of individual research disciplines.

Sulfide Generated by Sulfate Reduction is a Primary Controller of the Occurrence of Wild Rice (Zizania palustris) in Shallow Aquatic Ecosystems

Field observations suggest that surface-water sulfate concentrations control the distribution of wild rice, an aquatic grass (Zizania palustris). However, hydroponic studies show that sulfate is not toxic to wild rice at even unrealistically high concentrations. To determine how sulfate might directly or indirectly affect wild rice, potential wild rice habitat was characterized for 64 chemical and physical variables in over 100 sites spanning a relatively steep climatic and geological gradient in Minnesota. Habitat suitability was assessed by comparing the occurrence of wild rice with the field variables, through binary logistic regression. This analysis demonstrated that sulfide in sediment porewater, generated by the microbial reduction of sulfate that diffuses or advects into the sediment, is the primary control of wild rice occurrence. Water temperature and water transparency independently control the suitability of habitat for wild rice. In addition to generating phytotoxic sulfide, sulfate reduction also supports anaerobic decomposition of organic matter, releasing nutrients that can compound the harm of direct sulfide toxicity. These results are important because they show that increases in sulfate loading to surface water can have multiple negative consequences for ecosystems, even though sulfate itself is relatively benign.

NSF-OEDG Manoomin Science Camp Project: A model for engaging American Indian students in science, technology, engineering, and mathematics

ABSTRACT The Manoomin “wild rice” Science Camp program, a partnership between the University of Minnesota, the Fond du Lac Tribal and Community College, and the Fond du Lac Band of Lake Superior Chippewa is an example of how a community-based participatory research project can become the catalyst for STEM learning for an entire community, providing effective learning opportunities for grades 5–12 and undergraduate students, elementary and secondary school teachers, and scientists from the reservation, tribal college, and university. Focusing the research on a resource (wild rice) that has important economic, cultural and spiritual meaning for a community, we promote place-based education and support the development of strong science/teacher/community partnerships. Key components of this approach are the Circle of Learning, a conceptual framework that emphasizes trust- and relationship-building between researchers, teachers, students, and American Indian community members, and the Seven Elements of STEM Learning, a pedagogical framework derived from an extensive review of the literature on American Indian education that focuses on a holistic approach to learning that emphasizes the whole student.

The Evolution of Sulfide in Shallow Aquatic Ecosystem Sediments: An Analysis of the Roles of Sulfate, Organic Carbon, and Iron and Feedback Constraints Using Structural Equation Modeling

The generation of elevated concentrations of sulfide in sediment pore waters that are toxic to rooted macrophytes is problematic in both marine and freshwaters. In marine waters, biogeochemical conditions that lead to toxic levels of sulfide generally relate to factors that affect oxygen dynamics or the sediment iron concentration. In freshwaters, increases in surface water sulfate have been implicated in decline of Zizania palustris (wild rice), which is important in wetlands across the Great Lakes region of North America. We developed a structural equation (SE) model to elucidate key variables that govern the evolution of sulfide in pore waters in shallow aquatic habitats that are potentially capable of supporting wild rice. The conceptual basis for the model is the hypothesis that dissimilatory sulfate reduction is limited by the availability of both sulfate and total organic carbon (TOC) in the sediment. The conceptual model also assumes that pore water sulfide concentrations are constrained by the availability of pore water iron and that sediment iron supports the supply of dissolved iron to the pore water. A key result from the SE model is that variations in three external variables (sulfate, sediment TOC, and sediment iron) contribute nearly equally to the observed variations in pore water sulfide. As a result, management efforts to mitigate against the toxic effects of pore water sulfide on macrophytes such as wild rice should approach defining a protective sulfate threshold as an exercise tailored to the geochemistry of each site that quantitatively considers the effects of ambient concentrations of sediment Fe and TOC.

Increase in Nutrients, Mercury, and Methylmercury as a Consequence of Elevated Sulfate Reduction to Sulfide in Experimental Wetland Mesocosms

Microbial sulfate reduction (MSR) in both freshwater and marine ecosystems is a pathway for the decomposition of sedimentary organic matter (OM) after oxygen has been consumed. In experimental freshwater wetland mesocosms, sulfate additions allowed MSR to mineralize OM that would not otherwise have been decomposed. The mineralization of OM by MSR increased surface water concentrations of ecologically important constituents of OM: dissolved inorganic carbon, dissolved organic carbon, phosphorus, nitrogen, total mercury, and methylmercury. Increases in surface water concentrations, except for methylmercury, were in proportion to cumulative sulfate reduction, which was estimated by sulfate loss from the surface water into the sediments. Stoichiometric analysis shows that the increases were less than would be predicted from ratios with carbon in sediment, indicating that there are processes that limit P, N, and Hg mobilization to, or retention in, surface water. The highest sulfate treatment produced high levels of sulfide that retarded the methylation of mercury, but simultaneously mobilized sedimentary inorganic mercury into surface water. As a result, the proportion of mercury in the surface water as methylmercury peaked at intermediate porewater sulfide concentrations. The mesocosms have a relatively high ratio of wall and sediment surfaces to the volume of overlying water, perhaps enhancing the removal of nutrients and mercury to periphyton. The presence of wild rice decreased sediment sulfide concentrations by 30%, which was most likely a result of oxygen release from the wild rice roots. An additional consequence of the enhanced MSR was that sulfate additions produced phytotoxic levels of sulfide in sediment porewater.

Kerry Kelts (1947–2001)

Kerry Kelts died on 8 February 2001 at the age of 54 from Hodgkins disease. A professor in the department of geology and geophysics at the University of Minnesota, he was also director of that universitys Limnological Research Center from 1990 to 2000. Kerry had been a member of AGU since 1982 (Ocean Sciences). Kerry was a leader in establishing the utility of lake sedimentary records as archives, not only of past climate, but of past biogeochemical, tectonic, and anthropogenic processes. He educated us on the daunting complexity and individuality of lake systems, and demonstrated that lacustrine sequences can be interpreted as profitably as marine sequences. Kerry believed that terrestrial paleoclimate records are an essential complement to those from the ocean: while the ocean plays a substantial role in driving climate, lakes respond to actual conditions on the continents.