Evaluating growth patterns and element-to-calcium ratios of "Arctica islandica" (Bivalvia) shells as environmental proxies
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Abstract
Long-term, coherent, multi-regional records of environmental parameters are needed to quantify the rapidity, persistence and geographic extent of climatic phenomena and/or (extreme) weather events. The aragonitic shell of the long-lived marine bivalve mollusk Arctica islandica records past environmental conditions in the climate-sensitive, extra-tropical North Atlantic Ocean over decades to centuries at an annual or even higher resolution. Due to synchronous shell growth, individual growth records can be stacked together to form so-called composite chronologies. These chronologies provide uninterrupted annual to sub-annual environmental proxy data over several centuries or even millennia. Although previous studies demonstrated that A. islandica shells are reliable paleoclimate archives, there are still research questions. The present study addresses the following issues: (1) Are shells from shallow water specimens also suitable to construct stacked chronologies? (2) Can chronology construction be facilitated and increment-based crossdating be tested by means of geochemical properties? (3) Are environmental factors driving the growth and trace element-to-calcium ratios of A. islandica shells?
In the first manuscript of this thesis, it has been demonstrated that the shell growth patterns of A. islandica from shallow (ca. 9 – 23 m), unpolluted waters off northeastern Iceland (1) grew synchronously over discrete time intervals and (2) shell growth responded to the local oceanographic conditions. (3) The degree to which the environmental signals are expressed in the chronology (i.e. the degree of growth synchrony between the specimens) is high, when the environmental year-to-year variability reaches a critical level, the strength of which needs to be determined in future studies. The second and third manuscript explored the trace element-to-calcium ratios of A. islandica shells by means of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in line scan mode. The second article focused on shell barium-to-calcium (Ba/Ca) ratios of specimens from four distinct habitats (Iceland, Faroe Islands, Isle of Man, Gulf of Maine), showing that (4) shell Ba/Ca ratios were highly synchronous among coeval specimens from the same habitat and (5) may be related to marked increases in oceanic primary productivity. (6) The synchrony in shell Ba/Ca time-series was not influenced by changes in increment widths (as long as the sampling resolution is adequate, e.g., increment widths are not below the laser spot-size). Therefore, (7) shell Ba/Ca time-series can be used to double-check the temporal alignment of contemporaneous shells from the same locality. The last article examined additional means for assessing the reliability of shell element-to-calcium ratios as proxies of environmental parameters. To do so, the reproducibility of trace element time-series was tested within specimens and between coeval shells from the same site. (8) Na/Ca time-series of this species were largely irreproducible and therefore likely contained no environmental information. (9) Sr/Ca and Mg/Ca time-series were well-reproducible within specimens and were synchronous among coeval A. islandica as long as shell growth patterns were similar. Thus, these ratios may function as growth line tracers (when studied with the techniques applied in the present article). (10) The degree to which Mn/Ca time-series were reproducible within each specimen were potentially lowered, because the Mn/Ca ratios of the shells were close to detection limits. Still, Mn/Ca time-series were running synchronous among coeval specimens from the same site over certain time intervals, which may indicate that shell Mn/Ca ratios are potentially influenced by environmental parameters. (11) Shell Ba/Ca ratios were the most reproducible both within and between specimens and thus probably contain a strong environmental signal.