Development and application of the mollusc Arctica islandica as a paleoceanographic tool for the North Atlantic ocean
Until now there has been no tool comparable to corals for reconstructing long term high-resolution geochemical time-series for the colder, higher-latitude oceans. In this thesis, the long-lived (+100 years) boreal mollusc (Bivalvia) Arctica islandica is shown to be practical for this purpose in the northern North Atlantic Ocean. The evidence, compiled here, demonstrates that the carbonate shell of this species faithfully records the ambient dissolved inorganic carbon's (DIC) radiocarbon (Δ14C) concentration and accurately reflects the ambient temperature in its stable oxygen isotope (δ180) composition. However, the stable carbon isotope (δ13C) composition of the A. islandica shell likely is not a good recorder of ambient DIC δl3C, and likely responds to physiological controls. Four Δ14C time histories are reconstructed from the annual bands of A. islandica shells for the higher-latitudes of the northern North Atlantic Ocean (from 41°N to 70°N). These ocean records show significant spatial and temporal differences in the evolution of the radiocarbon signal between the subpolar and subtropical regions and between eastern and western regions of the northern North Atlantic, which are attributed to regional differences in mixed-layer depth and the presence of deepwater sources. A 109-year interannual record of bottom temperatures for a location near the former Nantucket Lightship position has been reconstructed for the period 1875-1983 from the overlapped stable oxygen isotope composition of four A. islandica shells. This record's annual temperature anomalies (variation from the running ten-year mean) show significant positive correlation with regional bottom, sea surface and air temperature anomalies. However, the shell-derived bottom temperature record describes a century-long cooling (~1°C) in contrast to a century-long warming of regional sea surface temperatures of equal magnitude, indicating a long term divergence between surface and bottom conditions. It is suggested that this contrast may be owed to a reduction in vertical mixing and increased seasonal stratification of shelf waters. This thesis fulfills the prophesy laid out nearly two decades ago by Thompson and Jones [1977] that Arctica islandica could someday be used to reconstruct past ocean history as "the tree of the North Atlantic".
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Massachusetts Institute of Technology and Woods Hole Oceanographic Institution