Millennial-scale surface water mass changes between the North Atlantic subpolar and subtropical gyre since the last glacial-interglacial cycle

Abstract

This study reconstructed the movement of surface water masses and oceanographic fronts from the Integrated Ocean Drilling Program (IODP) sites U1314 and U1313 in the central North Atlantic Ocean to explore the dynamics of the North Atlantic subpolar gyre (SPG) to subtropical gyre (STG) during the last glacial-interglacial cycle. Records were generated by X-ray fluorescence scanning and water masses were identified by the planktonic foraminiferal census, ice-rafted detritus (IRD) counting, and foraminiferal δ¹⁸O. Data suggest that the STG expanded northward during the MIS 5e peak and early Holocene. As a result, the subtropical water masses mainly affected the STG, while North Atlantic transitional waters (NATW) primarily impacted the SPG. Neogloboquadrina pachyderma increased slightly during MIS 5b and 5d at the STG Site U1313. In contrast, six millennial-scale cold events, in which the N. pachyderma ranges from 50 to 100 % in the eastern SPG Site U1314, suggest a high-frequency variability of subpolar to polar water masses during MIS 5d-5a. In the early phase of MIS 3, the impact of the NATW extended to Site U1314, while the subtropical waters seasonally impacted the STG Site U1313. The entire oceanographic fronts (i.e., PF, AF, SAF, and AzF) migrated southward during the mid-MIS 3, with the polar waters invading the SPG and the subpolar waters invading the STG. The higher (~46 %) North Atlantic transition species at the STG Site U1313 and the high (80–90 %) polar species at the SPG Site U1314 reflect significant hydrographic differences between the STG and SPG during the Last Glacial Maximum. The Polar/Arctic waters influenced the SPG, while the warm NATW influenced the STG, most likely due to the southward movement of the PF to mid-latitude. The subarctic front moved south of Site U1313, and the AF moved south of Site U1314 during the Heinrich events at Site U1313 and their equivalent IRD events at Site U1314. This study demonstrates dynamic changes in the surface water masses between the subpolar and subtropical gyres of the North Atlantic for the first time during the last glacial cycle.