GEOLOGY, March 2007 227 Geology, March 2007; v. 35; no. 3; p. 227–230; doi: 10.1130/G23197A.1; 2 figures. © 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. INTRODUCTION A catastrophic bolide impact, possibly in combination with a period of intense volcanism, is now widely recognized as the trigger of the arguably best-studied global bioevent, the mass extinction at the Cretaceous-Paleogene (K-Pg) boundary. One postulated killing mechanism involves a global collapse of food chains due to the shutdown of photosynthesis by sun-block- ing dust clouds, sulfate aerosols, or soot from fires (Alvarez et al., 1980; Pope, 2002). Such a crisis in primary productivity at the K-Pg boundary is supported by a widespread negative excursion of δ 13 C values and a reduction in the δ 13 C gradient from surface to bottom water (e.g., Hsü et al., 1982; D´Hondt et al., 1998). It has been suggested that surface-water productivity in high-latitude oceans was not reduced as dra- matically as in tropical and subtropical regions (Barrera and Keller, 1994; but see Kiessling and Claeys, 2001). Geographic variations in the severity of environmental perturbations are still insufficiently known, but need to be assessed if the intensity of these perturbations is a function of the distance from the impact site or related to paleolatitude. Independent evidence for a productivity crisis is provided by paleoecological studies demon- strating selective survival of starvation-resistant groups and preferential extinction of animals tied to food chains that were dependent on photo- synthetic organisms (e.g., Rhodes and Thayer, 1991; Sheehan et al., 1996). Here we analyze faunal data that are not well known from South- ern Hemisphere middle paleolatitudes. If the respective mollusk-dominated benthic mid-shelf ecosystem underwent a reduction of marine pri- mary productivity in the aftermath of the K-Pg event, we expect (1) a decline in the overall abundance of benthic organisms; (2) an increase in the relative abundance of groups not relying directly on photosynthesis, such as animals in detritus-based food chains or those with the ability to switch food sources; (3) an increased proportion of individuals with larval stages not dependent on phytoplankton; (4) a reduction in the average body size of individuals within paleocommunities; and (5) a shift in abundance toward individuals with low metabolic rates or inactive lifestyles. Our analysis yields evidence that such changes occurred and that the return to pre-extinction conditions of the various ecologi- cal traits was uncoordinated in time. STUDIED SECTIONS AND BIOSTRATIGRAPHY The studied sections (Bajada del Jagüel, BJG, 38°6′S, 68°23′W, and Opaso, OP, 38°8′S, 68°24′W) are located in the Neuquén Basin, western Argentina. The K-Pg boundary is within a 90-m-thick succession of monotonous, biotur- bated, middle to outer shelf mudstones (Jagüel Formation, Fig. 1), indicative of a fairly continu- ous open marine sedimentation below, or close to, storm wave base. The Jagüel Formation is overlain by shallow-marine bioclastic limestones of Danian age (Roca Formation). The position of the K-Pg boundary is biostratigraphically constrained to the base of a coarse-grained, tuffa- ceous sandstone layer (Scasso et al., 2005). To investigate the lateral persistence of faunal pat- terns, the critical interval around the sandstone layer was sampled in a parallel section ~10 m from BJG (here referred to as BJG II). The moderately well preserved fauna is dominated by bivalves. Subordinate benthic organisms include gastropods, echinoids, azooxanthellate corals, and serpulids. In BJG, age control with calcareous nanno- fossils indicates the presence of the first Ceno- zoic nannofossil zone (NP1 or NNTp1) in the upper half of the sandstone bed (Scasso et al., 2005). Nannofossil subzone NNTp1B is veri- fied in the mudstones overlying the sandstone bed, and NNTp2 is recorded 15 cm above the top of the sandstone. The mudstones below the sandstone are late Maastrichtian in age. In OP, Papú et al. (1999) reported the presence of early (NP1-NP3?, P1a-b) to late Danian (NP4, P1c) biozones, but earliest Danian planktic forami- niferal zones P0 and possibly also Pα (sensu Berggren et al., 1995), representing ~100 k.y. (Arenillas et al., 2004), appear to be missing. Furthermore, the absence of impact tracers such as shocked quartz in and immediately above the sandstone suggests that the sections are strati- graphically slightly incomplete (Scasso et al., 2005). In OP the part of the section above the K-Pg boundary sandstone represents ~3 m.y. METHODS AND RATIONALE Macrofossils were bulk sampled in defined intervals of homogenous lithology (20–50 cm near the boundary, as much as 1 m farther away from it), and identified in the lab as precisely as possible (usually at species level). We stan- dardized absolute abundance of each taxon in each interval by considering sampling intensity (time of collecting, number of collectors). Using *E-mail: martin.aberhan@museum.hu-berlin.de. Faunal evidence for reduced productivity and uncoordinated recovery in Southern Hemisphere Cretaceous-Paleogene boundary sections Martin Aberhan* Sven Weidemeyer Museum für Naturkunde, Humboldt-Universität Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany Wolfgang Kiessling Roberto A. Scasso Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Francisco A. Medina Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Argentina ABSTRACT The mass extinction at the Cretaceous-Paleogene boundary is generally explained by a severe crisis in primary productivity, following a catastrophic bolide impact. Consistent with this scenario, Danian mollusk-dominated benthic shelf ecosystems of southern middle paleo- latitudes (Neuquén Basin, Argentina) are characterized by (1) a stratigraphically limited low in macrofossil abundances; (2) an increase in starvation-resistant, nonplanktotrophic deposit feeders and chemosymbionts; (3) a reduction in the average body size of individuals; and (4) individuals with inactive lifestyles being more common than in the late Maastrichtian. Return to pre-extinction conditions of the various synecological attributes occurred over unequal time spans, indicating that recovery was uncoordinated with respect to ecological traits. Global comparison of ecological patterns suggests that reduced food supply (1) was a controlling fac- tor in both hemispheres; (2) affected macrobenthic marine faunas at various distances from the Chicxulub impact site; and (3) was more effective in siliciclastic environments as com- pared to oligotrophic carbonate settings. Keywords: mass extinction, Cretaceous-Paleogene boundary, ecology, productivity, Southern Hemisphere.