Jd. Marshall et al., GLOBAL CARBON ISOTOPIC EVENTS ASSOCIATED WITH MASS EXTINCTION AND GLACIATION IN THE LATE ORDOVICIAN, Palaeogeography, palaeoclimatology, palaeoecology, 132(1-4), 1997, pp. 195-210
Mass extinctions and glacioeustatic sea-level changes in the lower par
t of the Hirnantian (final stage of the Ashgill) are accompanied by sh
ifts in marine stable-isotope compositions. Previously published stabl
e-isotope changes have been used to identify the onset and demise of t
he Gondwana glaciation and to suggest relationships between biotic cha
nges and carbon cycling within the oceans. However, the existing isoto
pic data set had limitations because it was derived from Ordovician lo
w-latitude settings and from carbonates ol organic carbon in separate
areas, We report new data from Ordovician high-latitude carbonates and
demonstrate parallel shifts in organic and carbonate delta(13)C from
Baltica. Brachiopod shells from a high-palaeolatitude, periglacial set
ting in Argentina have elevated delta(13)C values similar to those des
cribed previously from low-latitude sites, The new data demonstrate th
at the positive Hirnantian delta(13)C excursion, previously only recog
nised from low-palaeolatitude areas, was widespread and probably globa
l in extent, The poor preservation state of the brachiopods unfortunat
ely prevented the determination of a reliable oxygen isotopic value fr
om the same material. Preliminary carbon isotopic data from thermally
immature or nic matter from Estonia provide the first indication of a
synchronous shift in organic and inorganic delta(13)C in sediments fro
m the same basin. This work provides new data of critical importance f
or constraining models of end-Ordovician palaeoceanography and climate
change. (C) 1997 Elsevier Science B.V.