VARVES Annually laminated lake sediments • Varve Formation • Varve Preservation • Varve Analysis • Sample Preparation • Analysis Techniques • Paleoclimatic Information
Why Bother? newsbusters.org Paleoclimate data provides a baseline record of climatic variability in which we can evaluate current climatic regimes and to understand how climate might change in the future.
What Causes Varve formation? • Annual cycles depositing variable sediments= couplets users.utu.fi
Seasonally Variable Components Mechanical Varves • Variation in clastic sedimentation from seasonal runoff • In Northern environments runoff seasonality is associated with snow melt • In Tropical environments runoff seasonality controlled by Monsoonal circulation or other factors controlling the rainy season • In arid environments clastic layers might represent flood events rather than annual sedimentation because of a lack in rainfall
Biological Varves Variation in seasonal biological activity • Diatom varves
Carbonate varves • Carbonate layers precipitate out of the water column caused by decreased dissolved CO2 • results from increased water temperature • or increased photosynthesis
Evaporites • Annual changes in lake level or lake chemistry causing precipitation of minerals
Combinations of annual processes • Varves from Eastern Finland, • light layer = spring flood mineral layer, • dark layer=organic summer-winter layer. users.utu.fi
Varve Preservation • Absence of Bioturbation • Reduce the impact of digging animals • Lack of or strong seasonality in benthic O2 • Stratification of the water column • No mixing to stir sediments www.sci.sdsu.edu science.jpl.nasa.gov
Basin DynamicsWhere do we find the above conditions? • Protected, Deep, Small surface area • Large and very deep with stratification of the water column • Small lakes that the sedimentation rate exceeds the rate of turbation Lake Malawi is one of the world's largest and deepest lakes
Varve Lake AnalysisHow do we extract paleoclimatic information from laminated sediments? • Get core • Split core • Generate age structure • Subsample core • Analysis users.utu.fi
Split core marine.rutgers.edu
Develop Age Structure • Count layers • Lead 210 • Radiocarbon • Others?
Preparation • Slice out a slab from the core half • Freeze dry frozen samples. • Use acetone to remove water from wet Samples. • Then use resin to remove the acetone and set sample into a resin puck.
Varve Thickness • In clastic varves, varve thickness can be used to reconstruct past hydroclimatic regimes • Caution: varve thickness can vary over decadal to century time scales--changing sediment discharge from rivers caused by channel migration or land use change • In biogenic varves, varve thickness can be used as a proxy for biological productivity--inferences in temp etc. • Evaporites:
Pollen Analysis • Potential for high resolution analysis of vegetation change remf.dartmouth.edu www.biologie.uni-hamburg.de
Digital image analysis Take a digital image of the core Graph the color density Calibrate this to a climatic variable if you find a match en.gtk.fi
Scanning Electron Microscopy (SEM) Back-scatter electron imagery Useful for resolving physical and biological components of the annual cycle of sedimentation Potential for identifying inter-annual microlaminationsSeasonal faunal successions in laminae less than 100µ thick
Scanning XRF Analysis XRF=X-ray Florescence Primary x-ray source the electrons in the elements. The sample then emits (florescence) x-rays with wave lengths characteristic of the elements in the sample. Elements from Na-U Shananon dissertation 2006 XRF allows fro the identification of changes in elemental abundance on 20-100µ resolution Measurements can be made on clean wet surfaces or resin imbedded pucks
What can we learn from all this? • Grain size can provide insight into past runoff intensity • Organic matter can be used to determine strength and length of the primary production season • Diatom species can be used to reconstruct past temperature regimes • Pollen can be used to reconstruct past vegetation regimes
Calibrate varve record to instrumental record Shananon dissertation 2006 Shananon dissertation 2006