ProjectB6

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Project B6: Paleomagnetic study of South African Paleozoic and Precambrian formations : ancient ice ages and geodynamics

French pi: J. Besse
South African pi: M. de Wit (with R. Domoney, UWC)

Project Participants

  • France: J. Besse, F. Fluteau, collaboration, Y. Donnadieu (LSCE)
  • South Africa: M de Wit, R. Domoney

Introduction

The climatic history of the Earth is marked by the alternation of hot periods and glacial eras (Fig1). The causes of climatic variability on long timescales are numerous: paleogeographic changes, evolution of atmospheric chemistry, evolution of the solar constant, etc.... Understanding the causes of these climatic changes is thus an essential stake in the current context of global warming and sustainable development.

Fig 1 Main glacial periods (from Hoffman and Shrag)

The purpose of this project is to study the glaciations of Precambrian and EarlyPalaeozoic. South Africa is a good target since most of main glaciations (PermoCarboniferous,Ordovician, Neoproterozoic, 2.2Ga and even a recently discovered 3.3 Ga, de Wit, personal communication) have been recorded. We plan first to focus on the glacial episode at the end of Ordovician, which happens during a greenhouse period.

The Ordovician glaciation resulted in the formation of an ice cap over a broad part of Gondwana. Contrary to the other glaciations, which lasted several tens of million years, this one may not have exceeded a few million years, even less. The causes of this glaciation are far from being elucidated. In addition to its volume and its duration, this glaciation occurs during a period known for its high atmospheric CO2 content. This period is also marked by deep upheavals of the carbon cycle which are marked by d13C anomalies, observed on Baltica and Laurentia). The acquisition of new paleomagnetic poles specifying the drift of this continent during this period is essential. For that, we propose to sample the Cape fold belt glaciogenic neighboring sediments in the Pakhuis and Cedarberg formations.

These data will constitute an essential basis to tackle the numerical modelling of the Late Ordovician glaciation and its validation. This project will also rely upon several numerical models: a coupled ocean-atmosphere GCM: FOAM, a geochemical model: COMBINE and a model of ice-cap: GRIZZLY.

The objectives are to understand the influence of the paleogeographic changes on the climate, the consequences on the carbon cycle and the pCO2, as well as the consequences on the formation of the Gondwanian ice-cap. Finally we propose to compare the glaciations (Precambrian, Late Ordovician, Permo-Carboniferous) occurring within distinct paleogeographic and environmental frameworks in the context of the interactions climate/geodynamic/carbon cycle.

Paleogeography

Figure 2: Polar wander path of Gondwana during the Early Paleozoic.

Palaeogeography is a key element to better understand the climatic evolution during the Early Paleozoic. The Neoproterozoic glacial events occurred in a paleogeography dominated by the amalgamation and disintegration of a supercontinent, Rodinia, sedimentary glacial deposits being observed at low paleolatitudes (Evans 2003), and in a environmental context in which the biologic activity was restricted with respect to Palaeozoic. The Permo-Carboniferous glacial event occurred during a ice-house period, also related to a supercontinent amalgamation, and followed the development of an abundant continental vegetation. The late Ordovician glacial event probably occurred in a fragmented paleogeography, where continental mass dispersed, in a general greenhouse age. The figure 2 shows the South pole position for Gondwana during the Palaeozoic (McElhinny et al., 2003). Upper Cambrian and lower Ordovician poles are relatively well constrained in Western Africa, whereas the rapid Northward drift of Gondwana (more than 6000km in less than 100Ma) is only constrained by two poorly determined paleopoles between 455 and 405 Ma. Whether this rapid drift represents a continuous, plate-related movement, or a local high shift linked to a true polar wander remains unknown. A better knowledge of the location of Gondwana all along the Early Paleozoic is thus required to improve our understanding of the climate changes during the Ordovician. The uncertainties on pole position preclude any reliable modeling of the climatic evolution, as the surface of continent close to the pole is by itself a critical parameter for the inlandsis location. Moreover, the general plate dynamic may also influence the glacial mechanisms. At last, critical parameters to be modelled are the oceanic temperature gradients, the calculation of which requires a precise knowledge of the paleolatitude of the sampling sites.

In the cape Fold belt, Pakhuis and Cedarberg formation describe well the Ordovician glaciations, with the presence of glaciogenic sediments and fossils. Several spots may allow parallel sampling of this important section, already partially sampled by Bachtadse and colleagues (1987), but with an unsufficient number of sites.