Global Compilation of Marine Varve Records
Arndt Schimmelmann, Indiana University, Bloomington, Earth and Atmospheric Sciences, Bloomington, IN, United States, Carina Lange, Universidad de Concepción, Departamento de Oceanografía, Investigador Asociado COPAS Sur-Austral & Centro FONDAP-IDEAL, Concepción, Chile; University of Concepcion, Departamento de Oceanografía, COPAS, Concepcion, Chile, Juergen Schieber, Indiana University Bloomington, Bloomington, IN, United States, Pierre Francus, Institut National de la Recherche Scientifique, Centre Eau Terre Environnement, Quebec City, QC, Canada, Antti Ojala, Geological Survey of Finland, Espoo, Finland and Bernd Zolitschka, Univ. of Bremen, Bremen, Germany
Abstract:
Marine varves contain highly resolved records of geochemical and other paleoceanographic and paleoenvironmental proxies with annual to seasonal resolution. We present a global compilation of marine varved sedimentary records throughout the Holocene and Quaternary covering more than 50 sites worldwide. Marine varve deposition and preservation typically depend on environmental and sedimentological principles, such as a sufficiently high sedimentation rate, severe depletion of dissolved oxygen in bottom water to exclude bioturbation by macrobenthos, and a seasonally varying sedimentary input to yield a recognizable rhythmic varve pattern. Additional oceanographic factors may include the strength and depth range of the Oxygen Minimum Zone (OMZ) and regional anthropogenic eutrophication. Modern to Quaternary marine varves are not only found in those parts of the open ocean that comply with these principles, but also in fjords, embayments and estuaries with thermohaline density stratification, and nearshore ‘saline lakes’ with strong hydrologic connections to ocean water. Marine varves have also been postulated in pre-Quaternary rocks. In the case of non-evaporitic laminations in fine-grained ancient marine rocks, laminations may not be varves but instead may have multiple alternative origins such as event beds or formation via bottom currents that transported and sorted silt-sized particles, clay floccules, and organic-mineral aggregates in the form of migrating bedload ripples.
Modern marine ecosystems on continental shelves and slopes, in coastal zones and in estuaries are susceptible to stress by various factors that may result in oxygen-depletion in bottom waters. Sensitive laminated sites may play the important role of a ‘canary in the coal mine’ where monitoring the character and geographical extent of laminations/varves serves as a diagnostic tool to judge environmental trends. Analyses of modern varve records will gain importance for simultaneously providing high-resolution and longer-term perspectives. Especially in regions with limited resources or at remote sites, the comparatively low cost of high-resolution sediment analyses for environmental monitoring is an essential advantage over continuous monitoring of oceanographic conditions in the water column.
