We investigated the response of late Paleocene-middle Eocene (~60-37.5 Ma) benthic foraminiferal assemblages to long-term climate change and hyperthermal events including the Paleocene-Eocene Thermal Maximum (PETM) at Ocean Drilling Program (ODP) Site 865 on Allison Guyot, a seamount in the Mid-Pacific Mountains. Seamounts are isolated deep-sea environments where enhanced current systems interrupt bentho-pelagic coupling, and fossil assemblages from such settings have been little evaluated. Assemblages at Site 865 are diverse and dominated by cylindrical calcareous taxa with complex apertures, an extinct group which probably lived infaunally. Dominance of an infaunal morphogroup is unexpected in a highly oligotrophic setting, but these forms may have been shallow infaunal suspension feeders, which were ecologically successful on the current-swept seamount. The magnitude of the PETM extinction at Site 865 was similar to other sites globally, but lower diversity postextinction faunas at this location were affected by ocean acidification as well as changes in current regime, which might have led to increased nutrient supply through trophic focusing. A minor hyperthermal saw less severe effects of changes in current regime, with no evidence for carbonate dissolution. Although the relative abundance of infaunal benthic foraminifera has been used as a proxy for surface productivity through bentho-pelagic coupling, we argue that this proxy can be used only in the absence of changes in carbonate saturation and current-driven biophysical linking.

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The early Eocene greenhouse world was marked by multiple transient hyperthermal events. The most extreme was the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), linked to the extinction of the globally recognised deep-sea benthic foraminiferal Velasco fauna, which led to the development of early Eocene assemblages. This turnover has been studied at high resolution, but faunal development into the later early Eocene is poorly documented. There is no widely accepted early Eocene equivalent of the Late Cretaceous-Paleocene Velasco fauna, mainly due to the use of different taxonomic concepts. We compiled Ypresian benthic foraminiferal data from 17 middle bathyal-lower abyssal ocean drilling sites in the Pacific, Atlantic and Indian Oceans, in order to characterise early Eocene deep-sea faunas by comparing assemblages across space, paleodepth and time. Nuttallides truempyi, Oridorsalis umbonatus, Bulimina trinitatensis, the Bulimina simplex group, the Anomalinoides spissiformis group, pleurostomellids, uniserial lagenids, stilostomellids and lenticulinids were ubiquitous during the early Eocene (lower-middle Ypresian). Aragonia aragonensis, the Globocassidulina subglobosa group, the Cibicidoides eocaenus group and polymorphinids became ubiquitous during the middle Ypresian. The most abundant early Ypresian taxa were tolerant to stressed or disturbed environments, either by opportunistic behavior (Quadrimorphina profunda, Tappanina selmensis, Siphogenerinoides brevispinosa) and/or the ability to calcify in carbonate-corrosive waters (N. truempyi). Nuttallides truempyi, T. selmensis and other buliminids (Bolivinoides cf. decoratus group, Bulimina virginiana) were markedly abundant during the middle Ypresian. Contrary to the long-lived, highly diverse and equitable Velasco fauna, common and abundant taxa reflect highly perturbed assemblages through the earliest Ypresian, with lower diversity and equitability following the PETM extinction. In contrast, the middle Ypresian assemblages may indicate a recovering fauna, though to some extent persistently disturbed by the lower-amplitude Eocene hyperthermals (e.g., Eocene Thermal Maximum 2 and 3). We propose the name ‘Walvis Ridge fauna’ for future reference to these Ypresian deep-sea benthic foraminiferal assemblages.

ResumenUn análisis estratigráfico efectuado en el arroyo San Dionisio, aportó datos sobre la evolución del margen occidental de la cuenca San José del Cabo, Baja California Sur. Esta cuenca forma parte de la Provincia Extensional del Golfo de California y su evolución inició en el Mioceno tardío. La cuenca, con estructura de medio graben, está limitada en su margen oeste por la falla lístrica San José del Cabo. En el arroyo San Dionisio están expuestas la falla marginal y tres formaciones estratigráficas divididas informalmente en dos unidades (inferior y superior), con orientación perpendicular a la margen de la cuenca y separadas por una discordancia angular. La unidad inferior está integrada por las formaciones Trinidad y Los Barriles, mientras que la unidad superior está constituida por la Formación El Chorro. En la parte oeste del arroyo, junto a la falla San José del Cabo y debido a la rotación a lo largo de la misma, aflora solamente la Formación El Chorro, la cual está compuesta por conglomerado y arenisca conglomerádica depositados en un ambiente de abanico aluvial. En afloramientos localizados más al este, arenisca conglomerádica y conglomerado de la Formación Los Barriles, depositados también en un ambiente de abanico aluvial, subyacen a la Formación El Chorro. Los sedimentos de ambas formaciones provienen del basamento expuesto en la sierra La Victoria. Aproximadamente 7 km al este de la falla marginal, se encuentra una interdigitación entre las formaciones Los Barriles y Trinidad; ésta última conformada por arenisca gruesa, arenisca arcillosa y lutita, de ambiente marino somero. La interdigitación muestra la contemporaneidad de ambas formaciones, probablemente en el Plioceno, además de que pone en evidencia la existencia de una zona costera con abanicos deltaicos durante el depósito de los sedimentos. En este abanico deltaico no se desarrollaron clinoformas, probablemente debido a la falta de espacio en una posición alejada de la falla marginal. La discordancia angular entre las secuencias superior e inferior marca un evento de reorganización de la cuenca; como consecuencia, se erosionó una parte de la unidad inferior en el Plioceno o inicios del Pleistoceno. Posteriormente, la rotación a lo largo de la falla confinó los depósitos aluviales de la Formación El Chorro a la parte occidental de la cuenca. Los depósitos más jóvenes de la Formación El Chorro cubren a la falla San José del Cabo y además muestran su disposición original, lo que sugiere la escasa o nula actividad de la misma desde finales del Pleistoceno.

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The Paleocene-Eocene Thermal Maximum (PETM) event at ,55.5 Ma, is characterized by globally elevated temperatures, a negative d 13 C excursion and major biotic changes on land and in the oceans, including the major extinction of deepsea benthic foraminifera.

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