Paleoenvironmental Records and Early Diagenesis of Marl Lake Sediments: A Case Study from Lough Carra, Western Ireland
What: The 2012 Lough Carra, Ireland project will investigate Holocene climate, aquatic productivity, and pollution records by collecting and analyzing lacustrine carbonate sediments. In addition, we will verify the fidelity of the marl delta 13C record by conducting a series of pore water incubation experiments. The summer program will consist of fieldwork in Ireland and laboratory analyses at Amherst College and Wesleyan University.
When: July 11-August 8
Where: 10-day field session in Lough Carra, Ireland then ~2.6 weeks at Amherst College and Wesleyan University.
Who: Anna Martini (Amherst College) and Tim Ku (Wesleyan University) with 3 students
Project Overview and Goals
This project involves a return for the PI to a marl lake that has within it a record of sedimentation that spans the Holocene. Previous expeditions were limited to short push cores, but the 2012 trip will include long piston cores from the deepest section of the lake, expanding both the quality and duration of the sedimentary record. In addition, on-site use of membrane-inlet mass spectrometry (MIMS) technology allows for a far more detailed understanding of biogeochemical processes that may complicate sedimentary paleoenvironmental interpretations.
Carbonate sediments from lacustrine environments have provided long term records of climate and land-use changes through the use of geochemical and biological proxies such as stable isotopic data, trace metal concentrations, faunal assemblages and pollen analyses. More specifically, the oxygen isotope composition of carbonate minerals precipitated via biological mediation is assumed to be in near isotopic equilibrium with delta18O of lake water and the delta 13C of the DIC pool. In lakes with good drainage (~relatively short residence times) the delta18O marl values reflect the isotopic composition of the meteoric precipitation, which is directly related to mean annual temperature. The marl delta 13C values reflect lake water delta 13C values, which, in turn, are controlled by lake productivity, inflowing water delta 13C values, and exchange with the atmosphere. Sediment concentrations of Hg, Pb, Cu, Cr, and Zn record anthropogenic activities and preserved invertebrate and pollen assemblages reveal vegetation and water quality changes. Overall, carbonate lake deposits can provide excellent long-term paleoenvironmental records, but careful interpretations of these proxies is necessary since post-depositional processes may significantly alter the signal and result in misleading conclusions.
- The Paleoclimate Record of Lough Carra from the Last Glacial Maximum (LGM) to the Present: Using delta 13C, delta18O, and trace element chemistries (Ca/Mg) of carbonate marl sediments we will build a record of climate change for the Lough Carra region. Sediment ages and accumulation rates will be determined from 14C, 210Pb, pollen analyses, and radiogenic isotope measurements on carbonate minerals, organic matter, or tephra deposits. One main goal will be to better constrain the timing of deposition. Keck students will use the University of Massachusetts Stable Isotope facility for carbon and oxygen isotope analyses. Trace elemental composition of carbonate for climate signals and identification of ash falls will be analyzed using the ICP-OES at Amherst College.
- The Paleoproductivity Record of Lough Carra. A Holocene record of paleoproductivity will be constructed by combining combining organic and inorganic carbon concentrations, C/N ratios, and organic matter delta 13C and delta15N measurements with bulk sediment accumulation rates (g/cm2/yr) based on bulk densities and linear sedimentation rates. The paleoproductivity record is crucial to the interpretation of the delta13CCaCO3 record. Recent sedimentation will be used to document cultural eutrophication and older sediments will supply materials for delta13C calculations, thus determining any changes in lake water delta13CDIC. Sedimentation rates and age models will be constructed from 210Pb, 14C, and/or U-Th measurements of suitable materials.
- Metal Loading History of Lough Carra. Anthropogenic metal pollution has increased dramatically since the Industrial Revolution. Some metals, such as Pb and Hg, have experienced a rise in global atmospheric deposition since the dawn of metallurgy. A record of Roman mining is recorded in a marl lake in the Aran Islands, western Ireland (Schettler and Romer, 2006). Pollution histories of Cd, Hg, and Pb have been found in ombrotrophic peat bogs near Galway and show distinct peak concentrations over the course of industrialization since the mid-1800s (Coggins et al., 2006). Metal concentrations in lake sediments will aid in determining sediment chronologies as well as narrate the environmental legacy of the “anthropocene”. Students will compile a sediment metal data set by analyzing marl sediments by ICP-OES, XRF, and direct mercury combustion techniques.
- Carbonate Diagenesis of Marl Lake Sediments. Calcite saturation states and pore water measurements will allow us to evaluate potentially rapid carbonate recrystallization processes that may shift delta13CCaCO3 values in Lough Carra sediments. Sediment pore waters will be analyzed for pH, DIC concentration, delta13CDIC, Ca+2/Cl–, PCO2, and PCH4. Solutes concentrations will be determined by standard analytical techniques, delta13CDIC aliquots will be sampled in Exetainers and analyzed at the UC-Davis Stable Isotope Facility, and gas partial pressures will be determined in whole cores by membrane-inlet mass spectrometry (MIMS; Lloyd et al., 2002). In addition, closed-system sediment incubation experiments will allow us to track carbonate dissolution / precipitation and delta13CDIC during early diagenesis. To help identify specific organic carbon and carbonate precipitation mechanisms, 13C-enriched organic matter or DIC will be added to a series of incubations and tracked by MIMS.
Students will be collecting lake waters and sediments on boats or coring platforms. All students are required to be proficient at swimming. Previous coring experience and analytical instrumentation experience is preferred. Lodging in Ireland will consist of a rustic bed-and-breakfast and lodging at Amherst or Wesleyan will be college housing.
Students should have some combination of hydrogeology, geochemistry, and/or sedimentology coursework. Students with interests in isotope geochemistry or with strong chemistry backgrounds are encouraged to apply.
- Coggins, A.M., Jennings, S.G., and Ebinghaus, R., 2006. Accumulation rates of the heavy metals lead, mercury and cadmium in ombrotrophic peatlands in the west of Ireland: Atmospheric Environment, v. 40, p. 260-278.
- Lloyd, D., Thomas, K. L., Cowie, G., Tammam, J. D., and Williams, A. G., 2002. Direct interface of chemistry to microbiological systems: membrane inlet mass spectrometry: Journal of Microbiological Methods, v. 48, p. 289-302.
- Schettler, G., and Romer, R.L., 2006. Atmospheric Pb-pollution by pre-medieval mining detected in the sediments of the brackish karst lake An Loch Mór, western Ireland: Applied Geochemistry, v. 21, p. 58-82.