Drivers of late Miocene tropical sea surface cooling: a new perspective
from the equatorial Indian Ocean
Abstract
During the late Miocene, global cooling occurred alongside the
establishment of near-modern terrestrial and marine ecosystems.
Significant (3 to 5 °C) sea surface cooling from 7.5 to 5.5 Ma is
recorded by proxies at mid to high latitudes, yet the magnitude of
tropical cooling and the role of atmospheric carbon dioxide (pCO2) in
driving this trend are debated. Here, we present a new
orbital-resolution sea surface temperature (SST) record spanning the
late Miocene to earliest Pliocene (9 to 5 Ma) from the eastern
equatorial Indian Ocean (International Ocean Discovery Program Site
U1443) based on Mg/Ca ratios measured in tests of the planktic
foraminifer Trilobatus trilobus. Our SST record reveals a 3.2 °C
decrease from 7.4 to 5.8 Ma, significantly increasing previous estimates
of late Miocene tropical cooling. Analysis of orbital-scale variability
shows that before the onset of cooling, SST variations were dominated by
precession-band (19-23 kyr) variability, whereas tropical temperature
became highly sensitive to obliquity (41 kyr) after 7.5 Ma, suggesting
an increase in high latitude forcing. We compare a revised global SST
database with new paleoclimate model simulations and show that a pCO2
decrease from 560 ppm to 300 ppm, in the range suggested by pCO2 proxy
records, could explain most of the late Miocene sea surface cooling
observed at Site U1443. Estimation of meridional sea surface temperature
gradients using our new Site U1443 record as representative of tropical
SST evolution reveals a much more modest increase over the late Miocene
than previously suggested, in agreement with modelled gradients.