OXYGEN AND CARBON ISOTOPIC DISSECTION OF BACULITES-COMPRESSUS (MOLLUSCA, CEPHALOPODA) FROM THE PIERRE SHALE (UPPER CAMPANIAN) OF SOUTH-DAKOTA - IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTIONS
Jw. Fatherree et al., OXYGEN AND CARBON ISOTOPIC DISSECTION OF BACULITES-COMPRESSUS (MOLLUSCA, CEPHALOPODA) FROM THE PIERRE SHALE (UPPER CAMPANIAN) OF SOUTH-DAKOTA - IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTIONS, Palaios, 13(4), 1998, pp. 376-385
A systematic stable isotopic study of an exceptionally well-preserved
Baculites compressus specimen from the Pierre Shale (Upper Campanian)
of South Dakota was conducted to assess whether the variations recorde
d in the calcareous shells of baculitids can be used as a reliable ind
icator of paleoenvironmental conditions. We sampled the B. compressus
conch along four sampling profiles: (1) an ontogenetic traverse parall
el to growth direction, (2) a virtually isochronous traverse along two
different lirae delineated by growth lines, (3) a septal traverse, an
d (4) a shell-wall traverse through the conch wall. To date, the ontog
enetic sequence is the Longest record produced from baculitids. We int
erpret the isotopic variations determined from the ontogenetic, septal
, and depth-profile traverses to be dominated by an. environmental sig
nal, especially in shell material deposited prior to septal crowding (
i.e., maturity). In contrast, we interpret the isotopic variations det
ermined from the isochronous traverse along the two different lirae to
be altered by ''biological vital effects.'' Our results indicate that
a careful sampling strategy that avoids strongly ornamented areas of
the shell is required ifa ''true'' paleoenvironmental signal is to be
extracted from the calcareous shells of baculitids, and perhaps other
ammonites as well. Our best estimate of seasonal seawater-temperature
variation in, the Late Campanian Western Interior Seaway at 42 degrees
N, based on the oxygen isotopic data from the baculitid specimen, is
10.0 degrees C (range of 29.7 degrees C to 19.7 degrees C).