Paleoclimate “Proxies” Expose the Geological History
The era of instrumental weather measurements spans only a tiny fraction (<150 years) of Earth’s climate history and thus provides an inadequate perspective of its climate variability and mechanisms. To reconstruct past climate variability for times prior to the era of instrumental measurements, marine geologists have to rely on indirect evidence – on information provided by “proxies”, which stand as surrogates for particular climate variables. The analysis of particular marine sediment properties (“proxies”) offers the potential to reconstruct an array of parameters (e.g., temperature, sea ice cover, global ice volume, nutrients, marine biological productivity, etc.), which responded to or modulated regional and global changes in the climate system.
The main carriers of paleoclimate information, especially for quantitative purposes, are the preserved organic and inorganic remains of microfossils: foraminifers (calcareous zooplankton and zoobenthos), coccolithophores (calcareous algae), diatoms (siliceous algae) as well as radiolarians and silicoflagellates (siliceous zooplankton). For instance, temporal changes in foraminifer and diatom assemblages, Mg/Ca ratios measured on foraminiferal tests, and the alkenone unsaturation index determined at the organic remains of coccolithophores are well-established “proxies” for quantitative reconstructions of upper ocean paleotemperatures. The following table provides an overview of “proxies“ determined by our marine geologists to assess the spatial and temporal variability of particular components within the climate system. A major target of our section is to extend the archive of past climate information, derived from multiple proxies, which can be integrated into computer models to infer and to understand the many different states of the climate system that have existed in the past.
As the pace of progress in paleoclimatology largely depends on “proxy” research, another challenge is to improve our work on well-established “proxies” and to identify new “proxies” for climate variables for which only inadequate information exist (e.g. salinity, carbonate ion concentration). New laboratory facilities have just been installed, which allow for pioneering work, including Si, C, N, and O isotope measurements on siliceous microfossil hardparts (Opal Isotope Laboratory). Such innovations will greatly increase the library of isotopic tools for paleoclimate reconstructions. Non-destructive elemental analyses at sediment records are performed at our new XRF Core Scanner. Such analyses are also emerging as a valuable approach in studies of sedimentary archives.
Overview of major “proxies“ determined within the Section “Marine Geology and Paleontology” to assess the spatial and temporal variability of particular components within the climate system.
|Proxy||Climate relevant variables|
|Diatom population dynamics (transfer function technique)||Sea surface temperature, sea ice extent, changes in oceanic surface circulation|
|Radiolarian population dynamics||Subsurface water mass characteristics, circulation changes|
|δ18O of species-specific diatoms||Sea surface temperature, salinity, ice volume, freshwater discharge|
|δ30Si of diatom silica and the δ15N and δ13C of diatom frustule organic matter||Tracking changes in nutrient utilization (especially in the Southern Ocean) and in Si/N utilization ratios|
|δ18O of species-specific planktonic and benthic foraminifers||Sea surface temperature and salinity related to species-specific habitat depths, upper ocean stratification, global ice volume, freshwater discharge|
|Mg/Ca of species-specific planktonic foraminifers (in work)||Upper ocean temperature, related to species-specific habitat depths|
|Alkenone unsaturation index||Sea surface temperature|
|Benthic foraminiferal assemblage composition||Primary productivity, continental shelf, slope, and deep-sea indicators, bottom current activity|
|δ13C of species-specific planktonic and benthic foraminifers||Marine nutrient concentration related to species-specific habitat depths, changes in deep water circulation, organic carbon flux to the ocean floor|
|Carbonate content, percent silt of carbonate fraction, foraminiferal dissolution index||Productivity, carbonate dissolution, lysocline depth, alkalinity|
|Biogenic opal and organic carbon contents and accumulation rates||Sea surface productivity|
|δ13C of bulk organic matter and biomarkers||Organic-carbon sources|
|Biomarker composition||Identification of organic-carbon sources (marine vs. terrigenous); sea-surface productivity; river discharge|
|δD of specific biomarkers||Sea-surface salinity|
|C/N ratio, Rock-Eval- parameters||Assessing the deposition of marine versus terrestrial organic matter|
|Grain size distribution of terrigenous siliciclastic sediments||River discharge, eolian transport (wind strength and direction), ocean current speeds|
|Magnetic susceptibility||Terrigenous sediment input|
|Element concentrations of sediments (XRF-Scanner)||Origin or transportation route of terrigenous sediments, weathering and erosion processes, productivity, bottom water ventilation|
|Clay mineral assemblages||Identification of source areas and transport pathways of terrigenous matter, weathering and erosion processes, shift of climate zones, ocean current systems|
|Content and petrography of ice rafted debris||Variations in global ice volume, origin of floating ice bergs, ice berg drifts|