Yr In Duration Research Articles

Punctuated anagenesis and the importance of stratigraphy to paleobiology

The depositional history of Upper Miocene through Recent sediments from DSDP Site 214 (Ninetyeast Ridge, Indian Ocean) is reexamined. Samples of the Globorotalia tumida planktic foraminiferal lineage, originally obtained from these sediments by Malmgren et al. (1983), serve as the empirical basis for the recognition of punctuated anagenesis as a distinct mode of phenotypic evolution and have been the subject of numerous additional investigations. However, conclusions reached by previous authors depend strictly on the validity of the original chronostratigraphic interpretation of these sediments. Graphic correlation analysis of first- and last-appearance datum levels for a total of 41 planktic foraminiferal, radiolarian, and calcareous nannoplankton taxa provides evidence for a more complex depositional history at this deep-sea site than originally believed. Based on a conservative model of variation in the pattern of sediment accumulation rates, the lowermost portion of the studied section (6.5-4.3 Ma) represents an interval of temporally condensed sediment accumulation (1.88 cm/1,000 yr) followed by an interval (4.3-2.8 Ma) of temporally expanded sediment accumulation (3.97 cm/1,000 yr). This interval, in turn, is followed by a depositional hiatus or an extremely condensed interval, at least 800,000 yr in duration, which is followed by another relatively condensed (1.36 cm/1,000 yr) interval from 2.0 Ma-Recent. Although this chronostratigraphic reinterpretation deviates substantially from the original, which recognized Site 214 as being both temporally continuous and exhibiting a constant sediment accumulation rate from the Upper Miocene through the Upper Pliocene, it is more consistent with expectations based on Neogene eustatic sea-level fluctuations and global surveys of Neogene hiatus distributions. Age assignments for samples of the Gr. tumida lineage based on the revised chronostratigraphic model reverse some findings of previous investigators with respect to the distinctiveness of phenotypic evolutionary rates characterizing the transition from Gr. plesiotumida to Gr. tumida. Finally, a brief survey of similar marine invertebrate lineage studies shows that changes in the rate of phenotypic evolution often appear to coincide with major physical changes in the paleoceanographic environment. Such correspondences may be due, at least in part, to the effect of these environmental changes on sediment accumulation rates. Paleobiologists who seek to understand patterns of phenotypic change over time must remove the effects of variations in sediment accumulation rates from their data before evolutionary hypothesis testing and remain aware of the limitations imposed on their interpretations by the uncertain nature of chronostratigraphic inference.

Cycle stratigraphy as a paleogeographic tool: Point Lookout Sandstone, southeastern San Juan basin, New Mexico

The Point Lookout Sandstone in the southeastern part of the San Juan basin was deposited as a moderate- to low-energy mainland or barrier beach which separated an extensive tidal creek and tidal distributary system from open-marine environments. Fluvial processes dominated locally along this northeastward-prograding coastline, resulting in development of dip-aligned, deltaic sandstone bodies. This regressive sheet sandstone is composed of numerous small-scale, asymmetric transgressive-regressive (T-R) cycles, three to five of which are intersected in a typical vertical section. Recognition and correlation of individual T-R cycles (<100,000 yr in duration) facilitate division of the Point Lookout Sandstone into genetic “shoreline blocks” and allow construction of detailed paleogeographic maps of shoreline position. Using this technique, shoreline irregularities and subtle differences between wave- and fluvial-dominated portions of the strandline are recognizable. Variability in strandline orientation and energy regime strongly influenced the accumulation of potential reservoir sandstone bodies. This technique is thus a useful predictive tool in energy exploration.

Measurement strategies for estimating long-term average wind speeds

The uncertainty and bias in estimates of long-term average wind speeds inherent in continuous and intermittent measurement strategies are examined by simulating the application of the strategies to 40 data sets. Continuous strategies have the smaller uncertainties for fixed duration measurement programs, but intermittent strategies make more efficient use of instruments and have smaller uncertainties for a fixed amount of instrument use. Continuous strategies tend to give biased estimates of the long-term annual mean speed unless an integral number of years' data is collected or the measurement program exceeds 3 yr in duration. Intermittent strategies with 3 or more month-long measurement periods/yr do not show any tendency toward bias.

Cometary Versus Asteroidal Origin of Chondritic Meteorites

Much of what we know about the early history of the solar system has been learned from the study of meteorites. This results from the fact, demonstrated by isotopic age measurements, that all of the various classes of stone and iron meteorites were formed 4.6 X 109 yr ago within a short period of time, probably less than 100 million yr in duration. This is also the age of Earth and the Moon and may be presumed to be the time of formation of the solid bodies in the solar system. Measurements of the products of the decay of the extinct radioactive isotopes 129Xe and 244Pu show, furthermore, that the formation of these solid bodies occurred within 100 million yr of the time of separation of the solar nebula from interstellar matter. Except for physical fragmentation into smaller bodies, the chemical and mineralogical composition of most meteorites has been essentially unaltered since this time during the formation interval of the solar system.