Synchronous mid-Miocene upper and deep oceanic δ13C changes in the east equatorial Pacific linked to ocean cooling and ice sheet expansion.

We present mid-Miocene (∼12.0–16.0 Ma) high-resolution (∼3.8 kyr) deep thermocline planktonic foraminiferal δ 18 O and δ 13 C records at IODP Site U1337 from the eastern equatorial Pacific. The benthic and planktonic δ 18 O records of Site U1337 have a similar long-term cooling trend, but display obvious amplitude differences of glacial/interglacial cycles after 13.9 Ma. Planktonic δ 18 O cycles are usually larger than 1.0‰, nearly 2 times those of the benthic δ 18 O. The post-13.9 Ma change is probably caused by intensified upwelling along with ocean cooling, but may include upper ocean circulation changes. Both the benthic and planktonic δ 13 C records at Site U1337 reveal marked 400-kyr carbon isotope cycles during the MMCO (Middle Miocene Climate Optimum) with a trend toward lower δ 13 C from 16.0 Ma to 12.0 Ma. The similarity in response between surface and deep carbon isotopes indicates that the mid-Miocene carbon excursions in the east equatorial Pacific involved the whole ocean basin water column and thus are a global signal. Box model simulations reveal that the long eccentricity (400 kyr) paced carbon inputs from weathering changed the burial ratio of carbonates to organic carbon and helped to produce the significant 400-kyr cycles in oceanic δ 13 C. The increased weathering of carbonate, silicate and kerogen rocks are the major factors controlling the long-term decrease in oceanic δ 13 C after 16.0 Ma. [ABSTRACT FROM AUTHOR]