My research focuses high-resolution climate and seasonality reconstructions from mollusk shells (mollusk sclerochronology). During growth bivalves record environmental changes in their shells in the form of variable increment widths (= variable growth rates) and variable geochemical properties including stable isotopes and trace elements.
Extremely long-lived bivalves (some reach ontogenetic ages of up to 375 years) are of particular interest for long-term paleoclimate studies, particularly in settings for which other high-resolution proxy records are not available. Based on similar growth patterns, contemporaneous shells with overlapping life spans were strung together to form so-called mean or master chronologies (similar to dendrochronology). Such chronologies reflect the environmental conditions that affected all these specimens. For example, using numerous live-collected specimens of Arctica islandica from the upper mixed layer of the North Atlantic, a 500-yr master chronology for the North Atlantic was constructed. This time-series reflects clear decadal and multi-decadal oscillations in the growth and stable isotope records which can be related to the North Atlantic Oscillation and the North Atlantic SST Tripole Index. Extreme events, such as the culmination of the Little Ice Age, the aftermaths of the global volcanic eruption can clearly be identified in the growth, stable isotope and trace element record of the shells. The carbon isotope composition of shells of this ‘bivalved Methuselah’ is not affected by ontogenetic trends and hence reflects synchronous changes of the carbon isotope ratios of the seawater DIC. The expanded use of fossil fuels since 1960, resulting in a strong shift toward more negative values of carbon isotopes, is recorded in these shells. Aside from the trend toward elevated CO2 levels, shell carbon isotopes indicate strong subseasonal to multidecadal variations of the primary productivity. Shell oxygen isotope data permitted a detailed and quantifiable analysis of the effects of global warming in surface waters and deeper settings. For example, during the last 150 years, both surface and subsurface waters of the North Sea showed a significant increase by about 1°C.
Short-lived bivalves can provide details on subannual environmental changes including paleoseasonality and paleoweather. For example, by analyzing the daily growth pattern, the date of death of can be determined to the nearest week. Such information is of particular interest for archaeologists who are interested in clarifying the seasonal site occupation and subsistence patterns of Indigenous peoples.
Other research foci are on developing new and optimizing existing proxies for environmental change from trace elements and non-traditional stable isotopes of molluskan shells. My research team set up tank experiments in order to identify external and internal factors that govern shell growth. Time-lapse cameras take pictures at 15 second intervals of different bivalve mollusk specimens of Anodonta cygnea. Results suggest the presence of ultradian oscillations at 2 to 4, 20 to 30 and 50 to 70min of shell activity. In addition, shells exhibit clear circadian cycles. All cycles are clearly reflected in the shell growth patterns substantiating the use of mircogrowth structures as precise calendars.