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 Sites 751 and 747) from 14.5 to 20.0 Ma spanning the Miocene climate optimum (15–17 Ma). Our records fill a critical gap from ~17 to 18 Ma, a time when many other deep-sea records are affected by dissolution. We tested the fidelity of published magnetobiostratigraphic 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 the following: (1) pervasive (though changing) Milankovitch forcing of global mean sea level change and (2) preservation that depends on sufficient sediment supply and accommodation.