Closing an early Miocene astronomical gap with Southern Ocean δ18O and δ13C records: Implications for sea level change

We present orbital-scale resolution (~10-kyr) benthic foraminiferal δ18O and δ13C records from the Kerguelen Plateau (Ocean Drilling Program [ODP] Sites 751 and 747) from 14.5-20.0 Ma spanning the Miocene climate optimum (MCO; 15-17 Ma). Our records fill a critical gap from ~17-18 Ma, a time when many other deep-sea records are affected by dissolution. We tested the fidelity of published magneto-biostratigraphic age models for these sites by astronomically tuning to the 405-kyr eccentricity cycle. A comparison of spectral estimates between the untuned and tuned records, as well as coherency with Laskar's (2004) eccentricity solution, revealed quasi-100-kyr cyclicity in δ18O and δ13C. There is only a weak signal associated with the 41-kyr obliquity cycle, likely due to the 10-kyr sampling limiting resolution. The δ18O variations point to persistent 405- and quasi-100-kyr modulations of temperature and sea-level changes through the early to middle Miocene as predicted by astronomical solutions, with changing dominance of the 100- and 41-kyr beat. Comparison of δ18O records with early to middle Miocene sequences from the New Jersey shelf, northeast Australian margin, Bahamas, and Maldives suggests that the dominant sea-level period preserved is the 1.2-Myr obliquity cycle, with sequence boundaries associated with δ18O increases or maxima. On the New Jersey margin, higher order sequences reflect the quasi-100-kyr eccentricity cycles as modulated by 405-kyr cycles. We suggest that “nesting” of stratigraphic cycles is a function of: 1) pervasive (though changing) Milankovitch forcing of global mean sea-level change; and 2) preservation that depends on sufficient sediment supply and accommodation.