Production, composition and preservation of phytoplankton on the northwest shelf of Australia through the Pliocene-Pleistocene period

Abstract

In this study, we applied steroid records at IODP Site U1461 to reconstruct long-term variations of production and composition of phytoplankton, and the microbial degradation on phytoplankton-derived organic carbon (OC) on the northwest shelf of Australia through the Pliocene-Pleistocene period. We aim to probe whether and how these behaviors are impacted by strengths of the Indonesian Throughflow (ITF) and the Leeuwin Current (LC), sea level variation, and continental material input. The results indicate that production of phytoplankton generally increased during 6–1.2 Ma, and decreased after ∼1.2 Ma, due to combination effect from intensified upwelling strength by pronounced Pliocene-Pleistocene cooling and low-salinity, tropical nutrient-deficient surface water input by the ITF and LC. The results also demonstrate increasing contributions of diatoms at ∼5.2 Ma, relatively higher contributions of dinoflagellates during 3.8–1.7 Ma, and another increasing contribution of diatoms after ∼1.7 Ma. We suggest that stronger ITF during ∼6.0–3.8 Ma and stronger LC during ∼1.2–0 Ma both induced higher contributions of diatoms to total phytoplankton, while increased terrestrial soil input in the mid-Pleistocene might be the potential reason why the timing of increasing contribution of diatoms (∼1.7 Ma) is slightly ahead of the intensified LC condition (∼1.2 Ma). Compared with the Pliocene, the microbial degradation on phytoplankton-derived organic carbon was more active in the mid-late Pleistocene, which could be related to terrestrial soil input, subsurface currents, and sea level variation. With synchronously lowered bioproductivity, phytoplankton-derived organic carbon burial capacity on the northwestern shelf of Australia lowered since ∼1.7 Ma.