Integrating IODP-ICDP Drilling On The Mid-Atlantic U.S. Margin With Deep Sea Isotopic Records: Tectonics Or Eustasy?

English Abstract:

Drilling on the mid-Atlantic U.S. “passive” continental margin (New Jersey to Virginia) by ocean (DSDP/ODP/IODP Legs 150, 174A, & Exp. 313) and continental scientific drilling (Legs 150X, 174AX, and the Chesapeake Bay Impact Structure; funding by ICDP, NSF-EAR, NSF-OCE, the USGS, and various state surveys) has provided unprecedented recovery of Upper Cretaceous to Holocene sequences. Ocean drilling has provided a global array of deep sea coreholes allowing application of the d¹⁸O and Mg/Ca proxies for ice volume. Together, margin and deep sea cores allow two approaches for estimating sea-level changes:

1) backstripping of coreholes from the onshore coastal plain and continental shelf, progressively accounting for the effects of compaction, loading, and thermal subsidence; and 2) scaling deep-sea d¹⁸O records using Mg/Ca to remove temperature effects. Comparison of the two methods addresses the long-standing debate about the roles of global average sea-level change (eustasy) and tectonism on the stratigraphic record. Our scaled-isotopic and onshore backstripped sea-level estimates changes are remarkably similar on the Myr scale for the past 34 Myr and testify to the importance of glacioeustasy. The dominant beat in the icehouse world of the past 34 Myr is the 1.2 Myr tilt cycle; forcing for the greenhouse world of the Cretaceous-Eocene appears to be the 2.4 Myr eccentricity cycle. However, there are differences between the onshore and offshore and between New Jersey, Delaware, and Virginia on the 1+ Myr scale that we attribute to the influence of mantle dynamics, including effects of the subducting Farallon slab. The amplitudes of the offsets are consistent with models of mantle dynamic topography that predict observed differences. Such changes in continental elevation explain the patchwork preservation of sequences and regional differences on this passive-aggressive margin; they also complicate estimates of the absolute position of globally averaged sea level, though glacioeustatic changes are well-constrained (better than ±10 m).