Timescale and paleoceanographic implications of a 3.6 m.y. oxygen isotope record from the northeast Indian Ocean (Ocean Drilling Program Site 758)

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Abstract
Numerous studies have shown that δ18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous δ18O and δ13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991) (0‐2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic δ18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980). The filtered 41‐ and 23‐kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991a, b), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of δ18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0–3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100‐, 41‐ and 23‐kyr cycles, and variation of global ice volume and deepwater temperature during the past 3.6 m.y.