LATE CRETACEOUS - EARLY PALEOCENE BIVALVE STABLE ISOTOPE RECORD FROM SEYMOUR ISLAND, ANTARCTICA

The K/T boundary sequence on Seymour Island, Antarctica consists of a relatively thick, continuous section of mid-shelf siliciclastic sediment containing a well-preserved macrofauna.  The K/T boundary itself is characterized by a glauconite sandstone associated with a fish mortality horizon closely overlying an iridium anomaly.  Previous studies of this stratigraphic section have primarily focused on the paleontology and temporal nature of the extinction.  Here we present stable isotope (d¹⁸O, d¹³C) and elemental (Ca, Mg, Sr, Fe, Mn) analyses of bivalve carbonate that span this Late Cretaceous – early Paleocene interval, representing a duration of ~2.5 m.y. 

Four aragonite bivalve genera have been analyzed to produce a record of climate.  This record consists of 360 individual analyses where each specimen is represented by three analyses to provide an estimate for bulk shell composition.  Fe and Mn concentrations were measured to evaluate diagenesis and were elevated with respect to biogenic marine aragonite in only a few samples.  Cross-plots of d¹⁸O vs. Fe+Mn and d¹³C vs. Fe+Mn suggest that carbonate in equilibrium with diagenetic fluids has a d¹⁸O value of ~1.2‰ and ~1.0‰ for d¹³C. 

The Maastrichtian bivalve d¹⁸O record parallels the trend observed at southern high latitude deep sea localities (Sites 690 and 511) and has similar absolute values.  Bivalve d¹⁸O values increase until ~66.5 m.y.a. and then steadily decrease until the K/T transition.  Bivalve d¹³C values remain relatively invariant during the ~2 m.y. prior to the K/T boundary.  Near the boundary, there is an apparent negative d¹³C excursion (recorded in one shell) immediately followed by a 0.5‰ decrease in d¹⁸O, which corresponds to a warming of 2.5°C.  Both the d¹⁸O and d¹³C records rebound to pre-excursion values after ~200 k.y. 

Interestingly, two bivalve genera that persist across the K/T transition, Cucullaea and Lahillia, appear to be offset from each other with respect to both d¹³C and d¹⁸O except at one horizon near the K/T boundary where these stable isotope records converge.  This offset may be due to different seasons of shell growth where convergence of these records is the result of a collapse in seasonality.  This hypothesis will be tested using high-resolution sampling to evaluate seasonal amplitudes of d¹³C and d¹⁸O in these bivalves through this interval.