Page 51 - Introduction to Paleobiology and The Fossil Record

38 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

to elliptical; 100 kyr cycle), obliquity (wobble
of the Earth’s axis; 41 kyr cycle) and preces- Geological time scale: a common language
sion (change in direction of the Earth’s axis If we are to understand global events and
relative to the sun; 23 kyr cycle). Throughout rates of global processes, geologists must talk
the stratigraphic record there are many suc- the same language when we correlate and
cessions of rhythmically alternating litholo- date rocks (Box 2.5). Rapid developments in
gies, for example limestones and marls stratigraphy during the last few years (Grad-
(calcareous shales), that may have been con- stein & Ogg 2004) have prompted publica-
trolled by Milankovitch processes. Apart from tion of GTS2004, an updated geological time
their obvious value for correlation, such scale (Gradstein et al. 2004). Over 50 of the
rhythms probably also effected changes in 90 Phanerozoic boundaries are now properly

community compositions and structures defined in stratotype sections (GSSPs) and the
together with the extinction and origination new scale uses a spectrum of new stratigraphic
of taxa. methods, such as orbital tuning, together with
Some of the most extensive and remarkable more advanced radiometric dating techniques
decimeter-scale rhythms, probably controlled and new statistical tools (Fig. 2.13). Although
by precession cycles, have been detected in the traditional stratigraphic methods form the
Upper Cretaceous chalk facies, where indi- basis of the geological column and our under-
vidual couplets can be tracked from southern standing of the order of key biological events,
England to the Caucasus, a distance of some the prospect of precisely defi ned radiometric
3000 km. A cyclostratigraphic framework dates makes it possible to determine the rates
can be related to well-established ammonite, of many types of biological process. Not all
inoceramid bivalve and foraminiferan biozones the recommendations have met with universal
together with carbon isotope excursions, approval, and they are only recommenda-
providing a high-resolution and composite tions. For example, GTS2004 removed
stratigraphy (Fig. 2.12b). The dark marly the Tertiary and Quaternary epochs from the
sediments may have been deposited during chronostratigraphic column without the
precession minima at eccentricity maxima approval of the IUGS; but these terms are
during intervals of cool, wet climates (Gale widely used and deeply embedded in the lit-
et al. 1999). erature and are thus unlikely to disappear

Box 2.4 Sequences and fossils

There are eight brachiopod-dominated biofacies recognized across an onshore–offshore gradient in
the Upper Ordovician rocks of Kentucky (Holland & Patzkowsky 2004). These assemblages were
not discrete but rather formed part of a depth-related gradient, and the relative abundance of species
varied through time. The development of these faunas across this part of the Appalachian Basin can
be charted within sequence-stratigraphic frameworks. Figure 2.11 is a plot of the DCA (detrended
correspondence analysis) axis 1 against the litho- and sequence stratigraphy of one of the key sec-
tions, the Frankfort composite section. The DCA axis is a proxy for taxa that were grouped together
in the shallowest-water environments. Thus within the highstand system tracts, values for this axis
are lower than those for the transgressive and system tracts and at the maximum fl ooding surface,
where deeper-water taxa dominate. The upsection faunal changes show that the distribution of taxa
was controlled by ecological factors dependent on sediment supply and sea-level changes, which in

turn built the sequence stratigraphic architecture. Marked fluctuations in the faunas occurred during
net regressive and transgressive events, emphasizing the depth-dependence of these assemblages.
The data used in this study are available at http://www.blackwellpublishing.com/paleobiology/.