Julio Sepúlveda

Rapid resurgence of marine productivity after the Cretaceous-Paleogene mass extinction.

Algal Rebound The extinction at the Cretaceous-Paleogene boundary 65.5 million years ago represented a sudden and dramatic disruption of global ecosystems. Sepúlveda et al. (p. 129) now show, however, that algae recovered rapidly and that photosynthesis and primary production thus also recovered. The authors tracked algal productivity in the thick boundary layer in Denmark through a series of diagnostic biomarkers and isotopes. Algal productivity dropped abruptly during the extinction event but then recovered within the boundary layer, perhaps as quickly as within 50 years of the impact. Preserved lipids imply that primary production recovered perhaps within 50 years of the end-Cretaceous extinction. The course of the biotic recovery after the impact-related disruption of photosynthesis and mass extinction event at the Cretaceous-Paleogene boundary has been intensely debated. The resurgence of marine primary production in the aftermath remains poorly constrained because of the paucity of fossil records tracing primary producers that lack skeletons. Here we present a high-resolution record of geochemical variation in the remarkably thick Fiskeler (also known as the Fish Clay) boundary layer at Kulstirenden, Denmark. Converging evidence from the stable isotopes of carbon and nitrogen and abundances of algal steranes and bacterial hopanes indicates that algal primary productivity was strongly reduced for only a brief period of possibly less than a century after the impact, followed by a rapid resurgence of carbon fixation and ecological reorganization.

Episodic photic zone euxinia in the northeastern Panthalassic Ocean during the end-Triassic extinction

Severe changes in ocean redox, nutrient cycling, and marine productivity accompanied most Phanerozoic mass extinctions. However, evidence for marine photic zone euxinia (PZE) as a globally important extinction mechanism for the end-Triassic extinction (ETE) is currently lacking. Fossil molecular (biomarker) and nitrogen isotopic records from a sedimentary sequence in western Canada provide the first conclusive evidence of PZE and disrupted biogeochemistry in neritic waters of the Panthalassic Ocean during the end Triassic. Increasing water-column stratification and deoxygenation across the ETE led to PZE in the Early Jurassic, paralleled by a perturbed nitrogen cycle and ecological turnovers among noncalcifying groups, including eukaryotic algae and prokaryotic plankton. If such conditions developed widely in the Panthalassic Ocean, PZE might have been a potent mechanism for the ETE.

Postimpact earliest Paleogene warming shown by fish debris oxygen isotopes (El Kef, Tunisia)

Warming after the big one The Chicxulub impact 65 million years ago, which caused the mass extinction at the Cretaceous-Paleogene boundary, also initiated a long period of strong global warming. Using data from phosphatic microfossils, including fish teeth, scales, and bone, MacLeod et al. estimated global average temperature. Immediately after the asteroid strike, temperatures increased by ∼5°C and remained high for about 100,000 years (see the Perspective by Lécuyer). These results are relevant to current climate projections, because the Chicxulub impact perturbed Earth systems on time scales even shorter than the current rate of change. Science, this issue p. 1467; see also p. 1400 The Chicxulub asteroid impact caused sudden, substantial, protracted global climate warming. Greenhouse warming is a predicted consequence of the Chicxulub impact, but supporting data are sparse. This shortcoming compromises understanding of the impact’s effects, and it has persisted due to an absence of sections that both contain suitable material for traditional carbonate- or organic-based paleothermometry and are complete and expanded enough to resolve changes on short time scales. We address the problem by analyzing the oxygen isotopic composition of fish debris, phosphatic microfossils that are relatively resistant to diagenetic alteration, from the Global Stratotype Section and Point for the Cretaceous/Paleogene boundary at El Kef, Tunisia. We report an ~1 per mil decrease in oxygen isotopic values (~5°C warming) beginning at the boundary and spanning ~300 centimeters of section (~100,000 years). The pattern found matches expectations for impact-initiated greenhouse warming.

Archaeal and bacterial assemblages in the Oxygen Minimum Zone of the upwelling ecosystem off Central Chile as determined by organic biomarkers

Se utilizaron biomarcadores organicos en para investigar la influencia de cambios estacionales en los niveles de oxigenacion y la quimica del agua sobre la distribucion de arqueas y bacterias en la columna de agua y los sedimentos superficiales de la plataforma continental frente a Chile central, un area influenciada por surgencia estacional asociada al desarrollo de una zona de minimo oxigeno. Nuestro interes es establecer si la ocurrencia de arquea y bacteria responde a la oxigenacion y quimica del agua para lo cual analizamos gliceroles dialquil gliceroles tetra-eteres (GDGTs) isoprenoides arqueanos (i) y ramificados bacterianos (r). Nuestros resultados, combinados con datos moleculares de observaciones durante un ano en el mismo lugar y profundidades del sitio de estudio indican la presencia y dominancia del grupo arqueano marino- pelagico Thaumarchaeota. Los cambios observados en la distribucion de iGDGTs podrian explicarse por (i) la presencia de poblaciones de arqueas marinas en la capa de agua sub-oxica, filogeneticamente diferentes a las de aguas superficiales, (ii) cambio en la contribucion relativa de Euryarchaeota con profundidad, y (iii) una relacion entre Thaumarchaeota y factores ambientales distintos a la temperatura. Los GDGTs ramificados fueron mas abundantes en la capa oxica superior durante el periodo de no-surgencia, tal vez influenciado por la alta descarga de rios, mientras que su diversidad fue mas alta en el agua sub-oxica. Nuestros resultados indican una segregacion vertical de los GDGTs isoprenoides y ramificados, con el predominio de biomarcadores arqueanos durante el periodo de baja productividad.