Late Miocene to earliest Pliocene stable isotope record and sedimentology of ODP Site 114-704

Carbonate stratigraphy and stable isotopic ratios of benthic and planktonic foraminifers were used to study paleoceanographic changes that occurred during the late Miocene to earliest Pliocene in the subantarctic South Atlantic, between 9.8 and 4.5 Ma in ODP Hole 704B on the Meteor Rise (47°S, 7°E; 2532 m water depth).
During the late Miocene, between 9.8 and 6.4 Ma, carbonate content was high with little variability (generally 84.5% ± 10%), with sustained productivity dominated by foraminifers and calcareous nannoplankton in surface waters north of the Subantarctic Front. Decreased carbonate (40%), along with first significant occurrence of biogenic opal, occurred between 8.45 and 8.2 Ma. The first signals of increased cooling occurred between 8.8 and 8.0 Ma.
The interval from 6.3 to 4.5 Ma represents low carbonate values with high variability (61.7% ± 17%), suggesting markedly fluctuating conditions in the production and/or dissolution of carbonate. The onset of this interval in Hole 704B is marked by a decrease in carbonate values and a well-defined 0.85 per mil decrease in d13C values of both planktonic and benthic foraminifers between 6.4 and 6.0 Ma, correlated to the Chron C3AR (upper reversed of Chron 6) "carbon shift." The interval of the carbon shift (6.4 to 6.0 Ma) is characterized by decreasing d18O values, anomalously low d18O minima in planktonic foraminifers, and intervals dominated by temperate and low-latitude diatom and silicoflagellate assemblages, suggesting warm interglacial conditions with brief events of extreme warming or low salinity in the subantarctic South Atlantic. An anomalously low planktonic d18O minimum at 6.15 Ma in Hole 704B is correlated with the deposition of the laminated organic-rich Neobrunia ooze found in Hole 701 during this leg and the Ethmodiscus ooze drilled at DSDP Site 520 on Leg 73. Suboxic bottom water may have formed in response to a meltwater lid that temporarily halted the production of Antarctic Bottom Water during the rapid deglaciation events that were occurring in West Antarctica.
The benthic d18O record displays a strong glacial interval between 5.8 and 5.4 Ma, which is coincident with the time of a major increase in an upwelling diatom assemblage between 6.0 and 5.4 Ma. The earliest Gilbert Chron (5.35 to 4.77 Ma) was marked by an intense carbonate dissolution event and low surface productivity. This dissolution event, recognized globally, ended abruptly near the base of the Thvera Subchron at 4.8 Ma when carbonate values increased to 75.3% ± 5.2%. The low carbonate content and highly variable surface water conditions between 6.3 and 4.8 Ma are time equivalent to the Messinian Stage in the Mediterranean, suggesting a possible link between the Messinian salinity crisis and the Southern Ocean during the latest Miocene.