Wolfgang Kuhnt
Institute of Geosciences, Christian-Albrechts-University Kiel, Olshausenstr. 40, 24118 Kiel, Germany
The environmental impact of the K/T boundary event on the deep sea benthic ecosystem is still a matter of vigorous discussion. A main reason for this controvery may be that it has become almost canonical knowledge that numerous species persisted across the K/T boundary with little or no change and that Late Cretaceous deep marine benthic organisms differed only slightly from those in the Paleocene. While it is true that there were few genuine extinctions among deep-sea taxa, the abrupt environmental changes at the K/T boundary completely disrupted the deep sea benthic community structure. This has been demonstrated in an exemplary fashion by studies of deep sea benthic foraminiferal assemblages from the Caravaca section.
Modern analogues of mass mortality and recolonisation processes in vast areas of the deep sea were virtually unknown before the June 1991 Mount Pinatubo eruption. This eruption resulted in an ash layer that covered about 30% of the deep South China Sea (more than 400 000 km2) and blanketed the seafloor close to Luzon by as much as 10 cm of volcanic material. Tephra settling rates through the water column were unexpectedly high (more than 1670 m/day) and led to mass mortality of the benthic biota over a vast area of the eastern South China Sea. Thus, the ash layer of the Mt. Pinatubo eruption provided a natural laboratory for a large scale recolonisation experiment in the deep sea. For the first time it became possible to assess the rates of recovery of deep marine ecosystems after a catastrophic event and to draw a parallel with the environmental impact of the K/T boundary event on deep sea communities. Census counts of the living benthic foraminiferal fauna on the ash layer during five consecutive cruises in the years 1994 to 1999 revealed the establishment of a distinct recolonisation fauna, that has many common features with benthic foraminiferal assemblages recovered within and above the boundary clay layer in various deep water K/T boundary sections including Caravaca. The species succession in a first stage of the recolonisation process (1991-1996) in the South China Sea was controlled by differences in species-specific dispersal capabilities and dominated by infaunal agglutinated forms. Approximately five years after the eruption a new fauna, adapted to the ash substrate, was established that exhibited significantly higher diversity than the initial recolonisation fauna, and mainly occupied the shallow infaunal and epifaunal suspension feeding habitat niches. Changes within these assemblages in a later stage of the recolonisation process (1996-1999) are mainly attributable to the seasonal availability of food resources, increasing competition and predator-pressure, and the opening of new habitat niches through macrofaunal bioturbators.
The first recolonisation assemblages after the Mount Pinatubo ash fall exhibit surprising similarities to benthic communities following the K/T boundary event, i.e. the comparatively low abundance and diversity, the absence of suspension feeding forms, the dominance of mobile infaunal forms and the high proportion of agglutinated foraminifers. The first forms to recolonise the deep sea after both catastrophic events were opportunistic forms with high capabilities to colonize newly available niches. These forms are obviously particularly adapted for rapid dispersal, and by analogy with the Recent, may be typical r-selected forms that are able to react rapidly to environmental changes by changing their population structure. However, major differences in the timing and duration of the overall faunal change are evident. In the K/T sections a comparatively stable new benthic foraminiferal assemblage, including suspension feeders, only became established several thousand years after the event. The delayed recovery of the marine food chain and subsequent recolonisation of the deep sea by survivors from shallower seas probably played an important role in the longterm recovery of the benthic community structure.