Implications For The Biosphere

In figure 4.4 the ways in which raised rates of sediment accumulation/subsidence could have affected the hydrosphere, lithosphere, biosphere, and fossil record across the Precambrian-Cambrian transition are summarized. Submergence of shallow shelves inevitably led to an expansion of habitat area and, as we have argued, also caused phosphorus- and silica-rich waters to invade platform interiors. It may be argued that these environmental changes had a major ecologic impact upon the biota, encouraging blooms of eutrophic plankton, which in turn may have favored the development of a wide range of suspension feeders and the migration of pandemic phosphatic and siliceous taxa (figure 4.4). The reciprocal uplift of hinterland margins, indicated by the strontium isotope curve and by the thick succession of siliciclastic sediments, may well have delivered yet more phosphorus and iron into the oceans, thereby sustaining or raising its productivity (Derry et al. 1994).

This evidence for drowning of platforms also helps to explain some peculiar aspects of Cambrian fossil preservation. The development of secondary phosphati-zation of thin CaCO3 or organic-walled skeletons during the latest Ediacarian to Atdabanian (e.g., Brasier 1980) is closely related to the timing of subsidence of carbonate platforms (figures 4.1-4.3). In Mongolia, for example, Cambrian-type siliceous sponge spicules and phosphatized early skeletal fossils first appear in the latest Ediacarian (Brasier et al. 1996, 1997) (figure 4.1). In India and North China, flood-

Figure 4.4 Model showing the inferred influence of global tectonic changes upon subsidence, sediment accumulation rate, sea level, nutrients, fossil preservation, and the adaptive radiation of the Cambrian fauna.

Figure 4.4 Model showing the inferred influence of global tectonic changes upon subsidence, sediment accumulation rate, sea level, nutrients, fossil preservation, and the adaptive radiation of the Cambrian fauna.

ing of the carbonate platforms brought phosphatic sediments with early skeletal fossils that were a little younger (figure 4.1).

In clastic sediments, the first main indications of the Cambrian radiation are given by trace fossils. Here, one of the main puzzles has been the preservation of deep-water Nereites ichnofacies in shallow waters during the Cambrian (e.g., Crimes 1994). At

Figure 4.5 The paradox of deeper-water Nere-ites ichnofacies traces (assemblages 1, 2, and 3) in shallow water subtrilobitic Cambrian facies, which may be related to rapid rates of sediment accumulation. 1 = graphoglyptids (e.g.,

Palaeodictyon isp.), 2 = Helminthopsis and Helminthoida isp., 3 = Taphrhelminthopsis isp., 4 = other ichnogenera. Thickness and ichno-taxa from sources in Crimes (1989).

Figure 4.5 The paradox of deeper-water Nere-ites ichnofacies traces (assemblages 1, 2, and 3) in shallow water subtrilobitic Cambrian facies, which may be related to rapid rates of sediment accumulation. 1 = graphoglyptids (e.g.,

Palaeodictyon isp.), 2 = Helminthopsis and Helminthoida isp., 3 = Taphrhelminthopsis isp., 4 = other ichnogenera. Thickness and ichno-taxa from sources in Crimes (1989).

higher levels in the stratigraphic column, the distribution of these grazing traces has been related to the incidence of event sands, such as turbidites, which cast and preserve the delicate top tier of the ichnofauna (Bromley 1990). A review of the literature suggests that these grazing traces tend to be best represented in subtrilobitic Cambrian successions that are relatively thick (figure 4.5). Hence, the paradox of deeper-water Nereites ichnofacies traces in shallow-water sediments may well have been enhanced by conditions of rapid deposition, which led to the preservation of a greater number of sand-mud interfaces.

A further paradox of Cambrian fossil preservation concerns the restriction of Burgess Shale-type Lagerst├Ątten to the Early and Middle Cambrian (Conway Morris 1992) (figure 4.1), despite the presence of suitable, anoxic, and poorly bioturbated facies at other times. Butterfield (1995, 1996) has suggested that this paradox could be explained by the restricted temporal distribution of volcanogenic clay minerals with antienzymatic and/or stabilizing effects. Here, we wonder whether Burgess Shale-type preservation was enabled by frequent pulses of fine-grained sedimentation along rapidly subsiding margins, leading to rapid burial and early diagenesis.

One of the most dramatic effects of sea floor subsidence on the Cambrian fossil record was arguably that of sudden "explosive phases" in diversification. The explo sion in diversity at the base of the Tommotian certainly coincides with a rapid change in 87Sr/86Sr and lies above a major karstic surface (Brasier et al. 1996). In southwestern Mongolia, where rejuvenation of subsidence began in the latest Ediacarian, there is no clear Tommotian explosion in diversity (Brasier et al. 1996). This "Tommotian explosion" can therefore be regarded as an artefact brought about by missing time followed by abrupt facies changes (Lindsay et al. 1996), together caused by a rejuvenation of subsidence along the margin of the Mongolian arc.

Was this article helpful?

0 0
Swine Influenza

Swine Influenza

SWINE INFLUENZA frightening you? CONCERNED about the health implications? Coughs and Sneezes Spread Diseases! Stop The Swine Flu from Spreading. Follow the advice to keep your family and friends safe from this virus and not become another victim. These simple cost free guidelines will help you to protect yourself from the swine flu.

Get My Free Ebook


Post a comment