Investigating MIS3 Sea Level Using Sulu Sea Surface and Thermocline Reconstructions

Generating accurate records of past global mean sea level (GMSL) and global ice volume is critical to understanding how they have responded to past climate variability. Marine Isotope Stage (MIS) 3 (60-26 ka) GMSL has been a subject of recent debate. Estimates of global ice volume from foraminiferal δ¹⁸O generally suggest that GMSL was far lower during MIS 3 than during MIS 5a-5d. More direct coral-based reconstructions of MIS 3 GMSL are inconclusive, with maximum MIS 3 ranging from ~20-50 m below maximum MIS 5a-5d GMSL. More recent shore line and ice margin reconstructions suggest MIS 3 GMSL was much higher and global ice volume was much lower than previously thought and that both were approximately equivalent to conditions that existed during MIS 5a-5d. In the far western tropical Pacific, previously published surface-dwelling foraminifera Globigerinoides ruber δ¹⁸O results from Sulu Sea sediment cores have been proposed to accurately reflect variations in GMSL and demonstrate nearly equivalent δ¹⁸O during MIS 3 and MIS 5a-5d. While the Sulu Sea results appear to support more recent GMSL and ice margin reconstructions, the Sulu Sea is a marginal sea located in the heart of the Indo-Australian Monsoon Region and thus, as a surface-based record, it is susceptible to changes in local temperature and salinity obscuring the GMSL signal. Here we present a new high-resolution δ¹⁸O record from Sulu Sea core MD97-2141 spanning MIS 3 to MIS 5d using the thermocline dwelling foraminifera Globorotalia tumida. As Sulu Sea thermocline and surface conditions are controlled by different mechanisms, we are able to deconvolve MIS 3 local influences on the G. ruber δ¹⁸O record. Our new results suggest the difference between MIS 3 and MIS 5a-5d in G. tumida δ¹⁸O is slightly larger than in G. ruber δ¹⁸O, but notably smaller than in most other foraminiferal based estimates of GMSL. We also present new G. tumida Mg/Ca data from the same core spanning the same interval and extend the existing G. ruber Mg/Ca record through MIS 5d. These Mg/Ca records allow us to deconvolve the local temperature and salinity signals in the Sulu Sea surface and thermocline δ¹⁸O records. Ultimately, our new Sulu Sea surface and thermocline stable isotope and trace metal records provide novel insight into MIS 3 and MIS 5a-5d sea level and Indo-Australian Monsoon variability.