Mid Miocene Paleoceanography at Western Subtropical Pacific Site 588: Results From Paired Mg/Ca and Oxygen Isotopes of Benthic Foraminifera

The middle-Miocene expansion of the East Antarctic Ice Sheet (16-14 Ma) is well documented in records of foraminiferal δ ¹⁸O values. Here, this time interval is revisited with a study using Mg/Ca ratios paired with δ ¹⁸O values from benthic foraminifera from subtropical western Pacific Site 588A; this approach allows components of seawater δ ¹⁸O (δ ¹⁸O_sw) fluctuation and deep water temperature change to be calculated allowing for more judicious assessment of δ ¹⁸O_sw values, and inferences about ice volume changes. During the interval of mid Miocene ice sheet expansion, our study reveals a longer-term trend of increasing Mg/Ca ratios between 16 -14.5 Ma, followed by a rapid recession to minimum ratios between 14.5 and 14 Ma. After this relatively sharp decline, Mg/Ca ratios remain stable until 12 Ma, whereupon they begin to increase again. Our Mg/Ca data illuminate a ∼2° C warming trend during the majority of the mid Miocene epoch of ice growth (16-14.5 Ma). The warming is followed by more rapid cooling by the same amount the in the bottom water regime at this site. These temperature changes could not have been discerned from the δ ¹⁸O record alone, which is generally interpreted to reflect, in part, a bottom water cooling during this time (e.g., Flower and Kennett, 1993). The mid Miocene warming trend apparent in the Mg/Ca data implies that either the amount of ice growth on Antarctica has been underestimated or that there is a regional water-mass effect on the δ ¹⁸O_sw that is being masked in the benthic foraminiferal δ ¹⁸O record. Forthcoming work focusing on this particular interval of time will help to resolve regional versus global changes in the δ ¹⁸O_sw; Site 588 may be sensitive to variations in regional intermediate water masses perhaps linked to changes in the subtropical front or intermediate water masses intruding from the Indian Ocean.