Impacts of Pliocene-Pleistocene global climatic events on tropical deep-sea biota : investigation based on IODP Expedition 363 Western Pacific warm pool

Abstract

Located in the equatorial western Pacific Ocean, the Western Pacific Warm Pool (WPWP) is an important place in the global climatic system. At the same time, it is the place of the highest marine biodiversity, as it largely overlaps with the Coral Triangle biodiversity hotspot. Past climatic and oceanographic changes could have affected the ecosystems and biodiversity of this most biodiverse marine region on Earth. However, the biotic responses to Neogene climatic changes remain poorly understood and even the Modern biodiversity of this region is not well understood in most taxonomic groups. Here, in this project, we investigated Neogene paleobiological changes in the WPWP and Coral Triangle region using the IODP Expedition 363 materials and microfossil Ostracoda (Crustacea) as a model system to better understand the biodiversity and biotic response to climatic and oceanographic changes in this region. The main points are summarized below:

(1) Potentially undescribed or unrecorded shallow and deep-sea species indicated that the ostracode biodiversity in the Coral Triangle was underestimated in previous studies and that deep-water benthic habitats in the Coral Triangle region, which had been poorly investigated regarding taxonomic and diversity studies of ostracodes and other organisms, could also potentially be a biodiversity hotspot.

(2) As the channel transporting vapor and heat from the Pacific Ocean to the Indian Ocean, the Indonesian Throughflow (ITF) started to restrict at ~10 Ma and recovered at ~4 Ma. In the northwestern Australian margin, we found a three-staged taxonomical composition shift in deep-sea ostracodes during the restriction and recovery of the ITF, including the ITF restriction initial phase (~10 to 7 Ma), ITF restriction core phase (7 to 4 Ma), and Post ITF restriction phase (after 4 Ma). Zabythocypris, Argilloecia are identified as “non-restriction” taxa. Legitimocythere is regarded as "transitional restriction” taxa. Henryhowella, Poseidonamicus, and Anebocythereis are identified as "core restriction” taxa. After the restriction of ITF, Australia shifted from the Humid Interval to the Arid Interval, and the northwest dust pathway was established. The dust from Australia strongly affected the abundance of deep-sea ostracodes in the tropical eastern Pacific Ocean, as it increased the surface productivity (hence the food input to the deep sea) by promoting the photosynthesis of the surface phytoplankton.

(3) In the equatorial western Pacific north of Papua New Guinea, from the late Pliocene to the early Pleistocene, the dominant oxygen-low north-sourced North Pacific Component Water (NPCW, especially the deeper layer NPDW) was alternated by the high-oxygen south-originated Southern Component Water (SCW), mainly the lower CDW (LCDW). This led to the upslope migration of low oxygen-preferred Krithe to shallower water (from ~3500 m to ~1300 m) and the dominance of high oxygen-preferred trachyleberidids in deep water (~3500 m) in the Pleistocene.

(4) During the transition of the river mouth region of the Sepik River from an epicontinental sea to a shallower environment in the middle Pleistocene at ~ 370 ka, new coastal zones were formed between the old coastline and the drilling site U1485 of 1145 m water depth. The newly formed coastal regions created a stepper slope, which enhanced the downslope transportation of dead shells of shallow-water species to deeper areas and led to the appearance of shallow-water taxa in the study site from the early-middle Pleistocene.

(5) Our palaeobiological study improves our understanding of the Neogene dynamics of deep-sea ecological systems in this climatologically and biologically important region. We found temperature and surface productivity were the major drivers of the deep-sea ecosystem in the region, implying that future changes in these properties are projected to affect this biodiverse deep-sea ecosystem.