Vegetational disruption at the Cretaceous/Paleogene boundary in Neuquén, Argentina: evidence from spores and pollen

BARREDA, V.; PALAMARCZUK, S.; CHAMBERLAIN J.A. JR

San Luis

Otro; 10º Reunión Argentina de Sedimentologia. Primer Simposio sobre el límite Cretácico/Terciario en Argentina; 2004

The purpose of this study was to analyze the spores and pollen grains recovered from a 1.10m interval of the Jagüel Formation, (Neuquén Province, Argentina) containing the Cretaceous/Paleogene (K/Plg) boundary. Palamarczuk (1997) and Palamarczuk and Habib (2001) identified the presence of the K/Plg boundary in this interval on the basis of a marked turnover in organic–walled microplankton. Further work on planktonic foraminifera and radiometric dating (Palamarczuk et al., 2002) confirmed the placement of the boundary at the base of a 17cm thick sandy layer composed of more than 90% volcanic constituents.             Numerous studies of northern hemisphere palynomorphs of continental origin suggest that a catastrophic event, such as the impact of a meteorite, was imprinted on the vegetation. A sharp vegetational change, defined in terms of spores and pollen, has been observed to occur at the boundary. This turnover primarily affected angiosperms, and was followed by an increase in fern spore content (fern spike). This spike was first documented by Orth (1981), and was interpreted as the result of a rapid colonization by pioneer fern species after a general demise of angiosperms and other seed-bearing plants.             In the southern hemisphere, the turnover that marks the K/Plg boundary in the marine realm has been recognized at several locations, but there is still a gap in information dealing with the effect of the event on the plant communities. Studies of exposures on Seymour Island, Antarctica, and in Australia, show little change in the palynologic record at the boundary (Askin, 1988; Macphail, 1994). However, the sampling interval used in these studies may not have been small enough for an accurate interpretation of a short term, catastrophic event. Recent studies from New Zealand show a strong disruption of the vegetation at the boundary with an increase in fern spores, reduction of relative abundance of gymnosperms and temporary loss of angiosperm pollen (Vajda et al., 2001; Vajda and Raine, 2003).             In the work reported here, the palynomorph content of 13 closely spaced samples covering a 1.10m interval across the K/Plg boundary is analysed. Five samples were collected below the K/Plg boundary in the Jagüel Formation identified in our previous work; three samples were collected from within the boundary layer; and the remaining five samples covered the initial stages of the Danian. The result of the analysis of the five samples from the uppermost Maastrichtian shows moderate to rich assemblages composed of: 60% of gymnosperm pollen grains (mainly Podocarpaceae and Cheirolepidiaceae); 30% of fern spores (mainly Cyatheaceae) and 10% of angiosperm pollen grains. Close to the boundary layer and within the layer, the absolute number of terrestrial palynomorphs strongly decreases. The same tendency was observed among marine dinoflagellate assemblages (Palamarczuk, 2004). The composition of the assemblages above the boundary shows a dramatic increase in gymnosperm (Classopollis) abundance, reaching up to 90% of the total microfossil population. Immediately above the K/Plg boundary, an increase in the abundance of a highly variable spore, single or in tetrads, of uncertain botanic affinity, was observed. This increase, from 1% below, to 6% above the boundary, was documented throughout the basal Danian interval covered in this study, but peak abundance occurs approximately 2m above the boundary (data from a separate, on-going stratigraphic analysis). Other gymnosperm pollen grains and fern spores were affected by the event, but the group that suffered the greatest losses was the angiosperms. Many families disappear at the boundary (Liliaceae, Ulmaceae, Loranthaceae, Olacaceae), and others become highly reduced in abundance (Proteaceae).             The Neuquén data support the idea of a global decline in vegetation abundance at the K/Plg boundary. According to the impact hypothesis of global mass extinctions, a short-lived, impact-generated dust cloud decimated insect pollinators, and as a consequence, flowering plants with entomophilous pollination suffered major losses (Macphail, 1994). Moreover, dust cloud-induced low light levels could have interfered with photosynthesis. These two effects are thought to have contributed to the temporary decline of the angiosperms. On the other hand, the dominant gymnosperm group (Classopollis pollen) seems to have had anemophilous pollination and to have been adapted to severe conditions. These plants were found to be the dominant group in other species-poor communities (Alvin, 1982).             The changes in palynomorph composition and diversity observed in the Neuquén region across the K/Plg boundary are clearly related to a global event, but further work is needed to fully decouple global from local effects on the environmental conditions prevailing in the Neuquén Basin at the close of the Cretaceous and initiation of the Danian.