Drilling the Cretaceous/Paleogene boundary in NJ:

Testing the relationship of geochemical anomalies to event beds

Most scientists view the K/P boundary as “case closed,” with the cause of the mass extinction linked directly to the Chicxulub impact; however, there are at least two datasets that appear to contradict this link.  The first is the record from the Continental Dynamics Program corehole at Brazos, TX (Keller et al., 2006, 2007).  Though this interpretation promises to be controversial, careful work is required to evaluate the nature of the boundary at Brazos and to test the postulated succession of multiple spherule horizons, a sea-level event, and the mass extinction event (Keller et al., 2006, 2007).  The second is the K/P boundary in New Jersey where outcrop studies have revealed a remarkable ammonite and molluscan fauna at the top of the Cretaceous above an Ir anomaly (Landman et al., 2006).  Though the anomaly in outcrop is modest (~0.5 ppb) and could be attributed to remobilization of Ir, additional studies are needed to test the relationship of the Ir anomaly to the mass extinctions.  Further constraints can be placed on the timing of geochemical signals by measuring Os isotopes, which will be sensitive to impact sources even when the Ir signal is weak, but will also reflect longer term changes in seawater Os isotopes associated with Deccan Trap weathering substantially before the K/P boundary (Fig. 1).  Separation of Ir and Os isotope signals at the boundary would be a clear signal of differential mobilization of Ir and Os during sediment diagenesis.  We propose here a relatively inexpensive coring expedition to intensely sample the K/P boundary in New Jersey with multiple samplings in the 3 areas (Figs. 8-10) where the boundary is best developed in subcrop.  Drilling in this case is needed because the outcrops are either poorly or no longer exposed and suffer from weathering.  Though drilling can never yield the 3-dimensionality possible in outcrops, our transects will allow us to sample the spherule horizon(s) and to determine the relationships of the various event beds (spherules, Pinna, clay clast, sequence boundary) with various geochemical anomalies (Ir, Sr-isotopes, carbon isotopes, Os isotopes).  We have assembled a team with the expertise in biostratigraphy (Olsson), sequence and litho- stratigraphy (Miller, Sugarman, Browning), geochemistry (including collaborations with J. Wright, R. Sherrell, M. Feigenson), and benthic foraminiferal paleobathymetry (Browning, Olsson, Miller) that will reveal the timing of the event beds relative geochemical anomalies.

 

This research is supported by a grant from the NSF Sedimentary Geology Program.