New geochemical techniques now make it possible to precisely determine the water depths at which plankton used to live. This opens new opportunities for palaeoceanographic research.
Marine geologists often use the fossil remains of calcareous marine zooplankton (planktic foraminifera) to obtain information on the past ocean. The chemistry of the tiny shells of these organisms provides important information on, for example, the ocean's temperature, density stratification, ice volume or may be used to reconstruct past changes in biological productivity or CO2. It is often assumed that species grow their calcite shells at a fixed level in the water column.
Using plankton tows, Iris Wilke (Geosciences Department, University of Bremen) and Frank Peeters (Vrije Universiteit, Amsterdam) have collected living planktic foraminifera at various locations in the Atlantic and Indian Oceans. Their results, presented on a poster at the EGU 2006, show that the shell geochemistry does not reflect a single depth level or a fixed temperature. They use an oxygen isotope mass-balance model to precisely quantify the depth habitat of ten species of modern planktic foraminifera which are frequently used in palaeo-oceanographic studies. The model fits to field data clearly indicate species-specific and species-characteristic depth integrated growth patterns.
Wilke and Peeters, show that some species grow their calcite at very shallow levels in the water column (i.e. within the surface mixed layer), while other species may continue to grow in water layers down to hundreds of metres below the sea surface, much deeper than previously assumed. Considering the chemisty of shells of different species that have grown over different parts of the water column, the authors suggest that their findings may be used to potentially unravel the density structure of the past water column and hence density driven circulation.
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