THE STABLE ISOTOPIC RECORDS OF FOSSILS FROM THE PETERBOROUGH MEMBER, OXFORD CLAY FORMATION (JURASSIC), UK - PALEOENVIRONMENTAL IMPLICATIONS

Carbon and oxygen isotopic compositions of calcareous nekton (aragonit e Kosmoceras and calcite belemnites) and benthos (aragonite nuculacean bivalves and calcite Gryphaea) from the Peterborough Member of the Ox ford Clay Formation vary considerably at any one horizon but show no o bvious trend with time. This suggests that the temperature and isotopi c composition of ambient seawater may have varied considerably over sh ort time intervals but did not change systematically during deposition of the unit. Thus, short-term fluctuations were superimposed on longe r-term 'stability' in this shallow-shelf environment. The overlap in c arbon isotopic compositions between calcareous nekton and benthos sugg est a well-mixed water column. Oxygen isotopic palaeotemperatures sugg est thermal stratification, provided that Kosmoceras (16-28 degrees C) inhabited near-surface waters and belemnites (12-19 degrees C, indist inguishable from the range for benthic bivalves) were nektobenthic. Ox ygen isotopic compositions of phosphate from both shallow- and possibl e deeper-dwelling vertebrates (reptiles, bony fish and sharks) yield t he same range of palaeotemperatures (20-29 degrees C), suggesting that all vertebrate taxa may have lived predominantly in warm, shallow wat ers.