EXPLORING HEMISPHERIC AND OCEAN BASIN SYMMETRY FROM THE LATE MIOCENE TO THE PRESENT USING A NOVEL SOUTH ATLANTIC OCEAN SURFACE TEMPERATURE RECORD

Understanding the climatic history of the late Miocene epoch is of interest to the scientific community due to its relatively low hypothesized atmospheric carbon dioxide concentrations (pCO₂) yet warm mean global temperatures. A recent study using records from the north Pacific Ocean suggests that the warmth of the late Miocene was due to changes in thermocline structure and ocean circulation patterns, rather than changes in atmospheric pCO₂. However, there currently exists a paucity of late Miocene sea surface temperature (SST) and pCO₂ records to evaluate this hypothesis. To address the shortage of SST records, particularly those from the southern hemisphere, we present a new late Miocene to the present SST record for the south Atlantic Ocean, which is based on alkenone analysis of sediments from Ocean Drilling Program (ODP) site 1088 on the Agulhas Ridge (40°S, 15°E). The ODP site 1088 SST record is structurally consistent with SST records from mid-latitude sites in other ocean basins, showing a broad cooling from the late Miocene to the present. However, the site 1088 record exhibits a slightly smaller magnitude of cooling (~5°C) than other localities (6-10°C). One exception to the close structural coherence of the site 1088 SST record with existing Miocene SST records is a dramatic cooling event around 7 million years ago (Ma), which is concomitant with a major shift in benthic carbon isotopes towards isotopically lower values. Notably, global and site 1088 benthic foraminiferal δ¹⁸O do not register this excursion. However, other significant changes in the climate system occurred at this time, including the expansion of C4 ecosystem and the inception of perennial desert conditions in the Sahara. Our results point to a remarkable, globally consistent spatial and temporal pattern of temperature change over the past 10 million years, with the notable exception of the ~7 Ma cooling event, which may have been caused by a basin-wide shift in Atlantic deep water ventilation and its associated impacts on the carbon cycle. We preliminarily contend that our results suggest a more global mechanism for the evolution of late Miocene ocean temperature than has been proposed based upon recent Pacific-based datasets.