Paper No. 11
Presentation Time: 4:15 PM
DIACHRONEITY AND DISSOLUTION IN THE MAASTRICHTIAN OF SHATSKY RISE, NW PACIFIC
CLARK, Kendra R., Department of Geosciences, University of Massachusetts Amherst, 611 N. Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003, LECKIE, R. Mark, Department of Geosciences, Univ of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 and DAMERON, Serena, Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003, mleckie@geo.umass.edu
A thorough taxonomic and biostratigraphic study of planktic foraminifera from well-preserved and diverse late Campanian-Maastrichtian (76.5-65.5 Ma) assemblages from ODP Sites 1209 and 1210 on Shatsky Rise, has revealed: 1) significant differences in species ranges compared to detailed studies from the western North Atlantic (Blake Nose) and eastern South Atlantic (Walvis Ridge) indicating that many first and last occurrences appear highly diachronous between ocean basins, and
2) a dissolution event beginning ~66.1 Ma and lasting to the Cretaceous/Paleogene boundary (65.5 Ma) is characterized by chalky, highly fragmented planktic forams, increased dissolution of all larger taxa, abundance of tiny planktic forams, greatly reduced P/B ratio, and a sharp decrease in planktic species richness. The dissolution event is preceded by a transitional interval beginning ~66.7 Ma containing assemblages with highly variable preservation. The transition interval is characterized by a precipitous decline in species richness. Although studies of calcareous nannofossils at Shatsky Rise (Holes 577A, 1209C and 1210B) do not report dissolution of species in the latest Maastrichtian, a decrease in primary productivity evidenced by a decrease of meso- and eutrophic species in the latest Maastrichtian, coincides with a drop in nannofossil species richness at the onset of the “transitional” interval in parallel with the planktic forams.
Population counts (>63 mm) through the dissolution interval reveal assemblages of juvenile biserial, trochospiral, and planispiral taxa; there are no ‘blooms’ of Guembelitra in the uppermost Maastrichtian at Site 1209. These assemblages of tiny planktics may have survived dissolution by way of fecal pellet transport to the seafloor where they are found mixed in an assemblage of calcareous benthic foraminifera. We conclude that the lysocline shoaled in the central North Pacific during latest Maastrichtian time due to changes in deep water circulation. The observed dissolution at Shatsky Rise is one possible or partial explanation for the high diachroneity of species ranges between Pacific and Atlantic sites. Alternatively, the observed diachroneity may reflect site-specific influences of ocean circulation and productivity conditions in the three regions compared in this study.
