Publication Date:
2016-10-14
Description:
Planktonic foraminifera are free-floating protozoa, which react sensitively on their immediate environment. The geographical distribution of planktonic foraminifera is, above all, related to water temperature. On the other hand, the vertical distribution is much more differentiated and governed by an interaction of physical, biological and chemical parameters. Chemical entities are also imprinting foraminiferal shells during their ontogenesis and thus, make the foraminifera a signal carrier for environmental and paleoceanographical proxies. Therefore, I focused on biogeographical studies and on chemical aspects to assess planktonic foraminiferal habitats and to improve our understanding of the role of planktonic foraminifera as environmental signal carriers. The present study focuses on three regions of the Atlantic Ocean, the seas to the west of Ireland, Cape Verde and central tropical Atlantic.
A comparison of faunal composition in intermediate water of the North Atlantic Ocean did not reveal differences between recent faunal composition to subfossil assemblages from surface sediments, but the species proportions were different. This contrast was mainly driven by the subtropical species Globorotalia hirsuta, which was frequent in 2004 but rare in surface sediment samples and in earlier plankton collections obtained from the southern Porcupine Seabight during the 1990s. Furthermore, the study revealed the influence of phytoplankton blooms on foraminiferal weight and thus, a strong contribution of G. hirsuta, Neogloboquadrina incompta and Globigerina bulloides to the foraminiferal carbonate flux.
In the tropical Atlantic, the fine-scale vertical distribution coupled with the size distribution of planktonic foramininferal species revealed a distinctly different standing stock between the central Atlantic and the Cape Verde region. The most regular pattern was observed in the surface dwelling Globigerinoides sacculifer, which had a steady abundance over the entire sampling period in both regions. In contrast, Globorotalia menardii showed highly variable abundances at the different locations but their depth range was similar. A comparison of the faunal assemblages with historical data revealed significant changes in foraminiferal assemblage and abundances that took place during the past 80 years. Notably, a distinct, historical decline in the abundance of Globigerinoides ruber white was recognized. The observation was confirmed by core-top assemblages where this species was frequent. The faunal changes seem to be linked to a general increase of sea surface temperature of approximately 2°C in this area.
The trace element analyses of several planktonic foraminiferal species revealed highly variable concentrations within each specimen, species and even individual chambers. Though the barium and boron content of our samples are much higher than reported in other studies, the values support assumptions that non-spinose species incooperate more barium than spinose species, and symbiont-bearing foraminifera show higher boron concentrations than symbiontbarren species. The calculation of the calcification temperatures of G. sacculifer following
established paleotemperature equations are 2.6°C and 3.1°C higher than the temperatures that were actually measured. The difference in G. menardii was only 0.5°C however. The data demonstrated that measured seawater oxygen isotopes, ambient temperature, and oxygen isotopes from the precipitated shell carbonate infer temperature relationships with a much higher accuracy than previous approaches using derivated parameteres.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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