INVESTIGATING THE MIDDLE MIOCENE CLIMATIC TRANSITION & CLIMATIC OPTIMUM (16-13 MA) ON ZEALANDIA: ISOTOPIC & MG/CA RATIO RECORDS FROM IODP SITE U1510

Moderately high-resolution (18 ky) isotopic & Mg/Ca ratio records (16-13 Ma) were developed using benthic foraminifers from IODP Site U1510 to explore changes on the mainly submerged continent of Zealandia during the Middle Miocene Climatic Transition (MMCT) & Climatic Optimum (MMCO). Site U1510 is located on the southern Lord Howe Rise (36°19.74ʹS, 164°33.52ʹE) in 1,238 meters of water depths. The Middle Miocene sediments at Site U1510 were recovered between 70 and 138 meters below sea floor. Taken together, the shallow water depths and burial depths should result in minimal diagenesis and dissolution of the foraminiferal tests. Excellent recovery of the sediments (100.3%) occurred for the entire Middle Miocene strata at Site U1510.

Carbon isotope maxima (i.e., CM events) were identified in the δ¹³C records as well as two δ¹⁸O events (Mi3a and Mi3). Mid-Miocene bottom water temperatures (BWT) derived from benthic foraminiferal Mg/Ca ranged from 8 to 14˚C, which are significantly higher than modern temperatures (~5˚C) of intermediate-depth waters across Zealandia. This is consistent with elevated temperatures recorded in other sites of the Indian Ocean. We posit that the high BWTs registered by the Mg/Ca ratio data in this and other studies located in the Indian Ocean could be ascribed to a warm saline intermediate water mass originating out of the Indian Ocean.

Across the MMCT (13.8 Ma),δ¹⁸O values increased by 1.1‰, which is consistent with previous isotopic studies across this transition. BWT shows a decrease of ~2°C during the transition, suggesting that ~0.6‰ increase in δ¹⁸O of seawater can be attributed to an ice volume increase equivalent to a >60-meter sea-level fall. In addition, a second BWT cooling event occurs at ~13.2 Ma both at Site U1510 as well as Indian Ocean intermediate-water-depth sites.

Calculating glacial land ice based on the δ¹⁸O values and BWT results in nearly a fully glaciated East Antarctic continent or larger during many times of this study interval. While these results are similar to other δ¹⁸O & Mg/Ca ratio studies, they are in conflict with other studies that suggest significantly reduced ice volume during the MMCO concomitant with high atmospheric CO₂.