Ramil Wright

Mass Mortality and Its Environmental and Evolutionary Consequences

The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 102 or 103 years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the oceans planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.

Atlas of Benthic Shelf Foraminifera of the Southwest Atlantic

Area of Study.- Water masses.- Cape Horn current.- Malvin current.- Coastal Argentine waters.- Area influenced by the Rio de la Plata.- Coastal waters of Uruguay and Southern Brazil.- Benthic foraminifera provinces.- North Patagonian subprovince.- South Patagonian subprovince.- Malvin subprovince.- Summary of Benthic Foraminiferal Investigations in the Area.- Principal Factors Influencing the Benthic Foraminiferal Distribution.- Salinity.- Euhaline foraminifera.- Hypohaline foraminifera.- Fresh water foraminifera.- Salt marsh foraminifera.- Temperature.- Depth.- Water mass.- Systematics and Distribution.- Euhaline species.- Hypohaline and fresh water species.- References.- Plates.- Maps.

Late cretaceous—cenozoic magnetostratigraphic and biostratigraphic correlations of the South Atlantic Ocean: DSDP Leg 73

Abstract DSDP Leg 73 sediment cores allow direct calibrations of magnetostratigraphy and biostratigraphy for much of the lates Cretaceous to Cenozoic in the mid-latitude South Atlantic Ocean. A complete record of the Cenozoic was not obtained, however, because strong dissolution, poor core recovery and intense core disturbance have masked the biostratigraphy or magnetostratigraphy over some intervals of all recovered sections. DSDP Leg 73 results show the following correlations: Early/middle Miocene in Chron 16 Oligocene/Miocene within Subchron C6N Eocene/Oligocene within Subchron C13R Middle/late Eocene top of Chron C17 Early/late Paleocene top of Subchron C27N Cretaceous/Tertiary within Subchron C29R

DSDP Leg 73: Contributions to Paleogene stratigraphy in nomenclature, chronology and sedimentation rates

Abstract DSDP Leg 73 was successful in determining magnetostratigraphic—biostratigraphic correlations throughout much of the Paleogene. This paper treats three aspects of the data analysis. The first section treats the development of a chron nomenclature which facilitates the precise correlation of arbitrary events with respect to the geomagnetic polarity history. The second section analyzes the accuracy of radiometric dates for the Paleogene. The conclusion is that despite the recent advances in radiochronology ‘South Atlantic Standard’ remains the most convenient and probably the most reliable chronological standard. The final section studies the correlation in sedimentation rates between the Umbrian and South Atlantic sites. The conclusion is that sedimentation rate changes determined from magnetostratigraphy provide a high-resolution source of paleoenvironmental information. Strong correlations are noted between sites and with respect to other paleo-environmental studies involving oxygen isotope ratios, biogeography and CCD fluctuations within the Paleogene marine sediments.

Numerical ages of Cenozoic biostratigraphic datum levels: Results of South Atlantic Leg 73 drilling

Six sites were drilled in the South Atlantic during the Leg 73 cruise of the Deep Sea Drilling Project (DSDP). Hydraulic piston coring at five of the six sites obtained a nearly complete sequence of undisturbed Cenozoic samples. The magnetostratigraphy at those sites was investigated by close sampling representing time intervals of about 10 4 yr. Most of the Cenozoic nannofossil and many of the foraminiferal zonal boundaries were accurately determined and magnetostratigraphically calibrated at those five Leg 73 boreholes. Their numerical ages have been computed assuming a linear spreading rate and a radiometric age of 66.5 m.y. for the Cretaceous-Tertiary boundary. Alternative magnetostratigraphic ages were obtained with the adoption of a 63.5-m.y. age for the Cenozoic. Our data confirm previous determinations of the Pleistocene-Pliocene boundary at 1.8 (1.7) m.y. B.P. and of the Pliocene-Miocene boundary at 5.1 (5.0) m.y. B.P. The Miocene-Oligocene boundary is placed within chron C-6 C, with a magnetostratigraphic age of 23.8–24.0 (22.7–22.9) m.y. The Oligocene-Eocene age is also very precisely located within chron C-13 R, with a magnetostratigraphic age of 37.1–37.2 (35.5–35.6) m.y. The Eocene-Paleocene boundary should be located within an uncored interval of chron C-24, with a magnetostratigraphic age of 59.0 (55.4) ± 0.2 m.y. A general accord of the magnetostratigraphic ages from Leg 73 sites and the radiometric ages published in the literature for the various zonal boundaries validates the assumption of a linear sea-floor–spreading rate during the Cenozoic.

Benthic foraminiferal repopulation of the Mediterranean after the messinian (late miocene) event

Abstract The Pliocene repopulation of the Mediterranean after a 0.5 m.y. episode of bottom sterility during the Messinian salinity crisis was a remarkably rapid event. Deep-water benthic foraminiferal faunas appeared very soon after the initiation of Pliocene sedimentation and reached pre-Messinian levels of population structure within 0.5 m.y. after the onset of the Pliocene. The primary source of oceanic water during the Pliocene was the Atlantic but there is weak evidence that some of the Pliocene fauna may have come from the Indo-Pacific area. Migration patterns across the Mediterranean were primarily from west to east reflecting the Atlantic source. There are, however, some east to west migrational routes. These may be the result of faunas originating in the eastern Mediterranean and radiating outward, or of an eastern source of oceanic water and faunas. The most likely time for a oceanic influx from the east was mid-Pliocene (biostratigraphic zones MPl-3 and MPl-4). Barriers stood between the Pliocene basins and caused some faunal endemism which is detectable by quantitative comparisons of the basinal faunas.

Area of Study

This study is based on those benthic foraminifera of the southwest Atlantic which occupy the broad continental shelf off the Argentine, Uruguayan and south Brazilian coasts.

The Systematic Position and Importance of the Foraminifera

The foraminifera are members of the Order Foraminiferida, Subclass Rhizopoda, Class Sarcodina, Phylum Protozoa. They constitute about 2.5% of all animals known from the Cambrian to the Recent.

Collection of Material

Although this book is dedicated principally to Recent foraminifera, we wish to treat briefly the methods of collection and preparation of fossil tests. Many of the methods and techniques of collection and preparation are the same for both fossil and Recent material. Most marine biologists and geologists who study the living foraminifera of some region draw their taxonomic conclusions on the basis of the test rather than on the soft parts of the organism. The paleontologist draws his conclusions from the same information.

Principal Factors Influencing the Benthic Foraminiferal Distribution

The specific ecology of benthic foraminifera is treated in Boltovskoy & Wright (1976) and data on environmental parameters from the area can be found in the various publications cited in the bibliography. Consequently we are going to discuss here only some general aspects of ecology and environmental relationships of the foraminifera with special reference to conditions in the southwest Atlantic shelf.