EOCENE TO MIDDLE MIOCENE SEA-LEVEL CHANGES DERIVED FROM BACKSTRIPPING NEW JERSEY ONSHORE-OFFSHORE SEQUENCE AND DEEP-SEA OXYGEN ISOTOPIC AND MG/CA RECORDS

We provide million year-scale eustatic estimates for the late middle Eocene to middle Miocene using two approaches: 1) backstripping of sequence from New Jersey onshore (Eocene-Miocene) and shelf (early-middle Miocene) coreholes; and 2) scaling deep-sea δ¹⁸O records using Mg/Ca to constrain long-term (>2 Myr scale) temperature changes. We compiled a high-resolution (kyr-scale) composite (spliced) deep sea benthic foraminiferal δ¹⁸O record, including new Miocene data from Sites 747 and 751. We scale this to sea level by removing long-term (>2 Myr scale) temperature changes using Mg/Ca records (Cramer et al., 2011) and bracketing the ice-volume versus temperature contribution on the Myr scale as 50-50 and 80-20%. Backstripping shows that Myr sea-level changes are typically 25 m, with the largest <75 m. Comparison of backstripped and isotope-based estimates requires that only about 50% of the Myr scale variability was due to ice volume. Our scaled-isotopic and backstripped eustatic estimates show: 1) small sea-level changes (<20 m) in the Eocene other than a large 30-40 m lowering at ca. 42 Ma, to ice free just prior to the earliest Oligocene Oi1 event; 2) a remarkably large lowering (~75 m) at Oi1 time (33.8 Ma) reflecting the development of a continental-scale Antarctic Ice Sheet (AIS) that was larger in areas than the modern; as discussed previously (Katz et al., 2009; Pusz et al., 2011), a higher amplitude computed using isotopes (nearly 100 m) is partly an artifact of not fully accounting for a deep sea cooling of 1.5°C; 3) eustatic changes in the Oligocene to middle Miocene of 40-50 m for large Oi- and Mi-events and 20-40 for smaller events primarily driven by growth and decay of the entire East AIS; 4) in a series of three ice-growth events (14.6, 13.7, and 13.0 Ma), a permanent EAIS developed, resulting in lower amplitude (<25 m) Myr-scale sea-level changes during the remainder of the Neogene. The Eocene-Oligocene transition and Oi1 mark a fundamental change in the cryosphere, though Antarctic ice sheets predate this event.