The agglutinated foraminifera from the SW Pacific bathyal sediments of the last 550 kyr: relationship with the deposition of tephra layers

The agglutinated foraminiferal content from the last 550 kyr record of the IMAGES core MD 97-2114 (Chatham Rise, New Zealand) was analysed in order to detect the possible linkage existing between the composition of the grains forming the agglutinated tests and the deposition of tephras. The core was collected east of New Zealand, about 680 km from the active Taupo Volcanic Zone (TVZ) located on the North Island, thus it contains numerous macro- and microscopic tephra layers. Scanning Electron Microscopy and Energy Dispersive Spectroscopy analyses were carried out on entire agglutinated foraminifera as well as on sectioned specimens, sampled around and within the tephra layers. The analyses show that the studied foraminifera built structurally complex tests picking and selecting mineral and biogenic particles on the basis of their availability and abundance in the substratum, as well as their composition, size and shape. In most of the studied species, belonging to the Order Textulariida, the composition of the agglutinated grains does not change when the deposition of the tephra layer strongly enriched the substratum in volcanic glass shards. Only the species Karreriella novangliae changed significantly its grain composition, mostly selecting volcanic glass fragments to cover the test surface. Nevertheless, the tephra deposition seems to influence the wall microstructure of the agglutinated tests. Textulariid specimens coming from the volcanoclastic layers have a thinner wall which is also characterised by a less abundant calcareous matrix with respect to the specimens sampled above or below the tephra layer. We hypothesise that the volcanic ash deposition probably interfered with the normal agglutinating process by causing the development of more aggressive waters at the sea floor which, in turn, could have induced carbonate dissolution. Our observations therefore suggest that the sediment type of the substratum is not the only controlling factor in the construction of the agglutinated foraminiferal test and grain selection, which appears to be species-dependent.