465 / 2024-09-17 17:13:00

Synergic variations of the South Pacific deep circulation and carbon reservoir during the late Pliocene intensification of the Northern Hemisphere Glaciation

South Pacific,,Deep Circulation,,Fish teeth Nd isotope,iNHG

The intensification of Northern Hemisphere Glaciation at ~2.7 Ma ago, known as the significant iNHG event during the late Pliocene, has marked a prominent transition of Earth’s climate from uni-polar to bi-polar ice sheets. Here, we present proxy records from the deep sea sediments of Ocean Drilling Program Site 1123 in the South Pacific (41°47´S，171°30´W，water depth 3290 m), covering a time span of approximately 3.5 to 1.7 million years, aiming to reveal the South Pacific deep circulation and ocean carbon reservoir changes during this period. After careful rinsing of the sediment samples, we selected benthic foraminifer C. Wuellerstorfi and G. Mundulus for B/Ca ratio and fish tooth εNd isotope analysis, with a temporal resolution of approximately 3 to 10 thousand years, totaling 112 sediment samples.

Our results show that the eNd of ODP Site 1123 rapidly shifted towards negative values by approximately 2 units during the iNHG period. Meanwhile, the deep-water △[CO₃^2-] reconstructed using the B/Ca ratio of benthic foraminifera exhibited a positive shift at around 2.9 Ma and then a negative shift at around 2.7 Ma. We interpret that the positive shift in △[CO₃^2-] at ODP Site 1123 at ~2.9 Ma might be caused by the weakening of the Antarctic Circumpolar Current (ACC), which reduced the upwelling in the Southern Ocean and thereby caused a shift of the major source of the water mass bathing this site from the north with more negative carbonate ion concentrations to that in the south with more positive carbonate ion concentrations. The enhanced northward expansion of southern sourced water in the deep Pacific Ocean could result in a decrease in the Pacific carbon reservoir. We further hypothesize that the enhanced input of sub-Antarctic dust during the iNHG period had great potential for increasing the iron fertilization effect, thereby strengthening the biological pump in the sub-Antarctic region. This process had resulted in in an increase in the carbon storage and a more negative carbonate ion concentration in the southern sourced water flowing into the Pacific Ocean, ultimately causing the negative shift in △[CO₃^2-] at ODP Site 1123 and enhancing carbon storage in the Pacific Ocean. Our results demonstrate that the Pacific Ocean had played a great role in the decline of the atmospheric pCO₂ and finally contributed to the final formation of the iNHG event.