Abstract:
The K/P extinction involved the loss of more than 90% of nannoplankton and planktonic foraminiferal species. The recovery of both groups included the appearance of dwarf taxa that existed along with opportunistic disaster taxa such as the nannoplankton Braarudosphaera and Thoracosphaera. Investigation of the ecology of the earliest Danian plankton, especially nannoplankton, is made difficult by their small size, and by the labor-intensiveness of high-resolution assemblage counts. Moreover, there have been no attempts to quantify the flux of the different groups. We use a Malvern Mastersizer, an instrument that optically measures particles between 0.1 and 1000 micron in size to obtain grain size data from the earliest Danian. We have investigated a detailed series of samples taken at intervals between 5 and 20 kyr from Ocean Drilling Program Sites 1212 (Shatsky Rise, Pacific Ocean) and 1262 (Walvis Ridge, South Atlantic Ocean). Orbital time scales at both sites provide precise time control and the paleontological record of the early Danian recovery is expanded and complete at both sites. The dominant earliest Paleocene nannoplankton taxa are highly differentiated in terms of size ranging from miniscule (Neobiscutum and early Prinsius are mostly 1-2 microns), to intermediate (Ericsonia, Cruciplacolithus, 4-8 microns) to very large (whole specimens of Thoracosphaera are >20 microns). Data from both sites clearly show the dominance of disaster forms and the dwarf planktonic foraminiferal taxa (20-40 microns) for 30 kyr after the boundary followed by the progressive appearance of the miniscule Danian nannofloras, then of intermediate nannoplankton taxa at approximately 200 kyr post K/P. Grain size distributions suggest a dramatic increase in the flux of planktonic foraminifera relative to nannoplankton at the K/P boundary followed by sequential, short-term phases of dominance of the two groups. Similarity between the two sites suggests that the evolutionary trends are global as are ecological factors that controlled the relative dominance of nannoplankton and planktonic foraminifera. We speculate that the successive intervals of dominance of the two groups were driven by short-term experimentation with new ecological strategies during the recovery interval.