1104 / 2024-09-20 12:36:19

Multi-paramter approach to constrain paleoproductivity and phosphatization events using in ferromanganese crust over the past 23 Myrs

Barium,Paleoproductivity,Phosphatization event,Ferromanganese crust,Central North Pacific,Deep ocean circulation

The reconstruction of past marine biological productivity and its interactions with ocean circulation and climate change have been a key challenge in understanding Earth's climate system. Phosphatization events often coincide with periods of high productivity in geological history, cause episodic carbonate fluorapatite (CFA) precipitation in hydrogenetic Fe-Mn crusts, and are thus critical for understanding processes of climate change vs. ocean circulation variations in driving the marine biological carbon cycle. Nevertheless, our knowledge of past changes in export productivity in the North Pacific and its relationship with phosphorus cycles and ocean circulation modes remain limited. Here, we present time-series Ba_Caxs/Mn ratios, where Ba_Caxs is Ba content from the non-carbonate phases, across the phosphatization interface from a Fe-Mn crust sample (CXD31-1) recovered from the central North Pacific to explore the coupling processes governing changes in marine export production and phosphorus cycles. Age model is established with magnetic scanning and ¹⁰Be/⁹Be ratios, showing a stable growth rate of ~2.65 mm/Myr over the last ~23 Ma. Elemental concentrations were measured using an electron probe microanalyzer (EPMA) and a laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), showing a decreasing trend in Ba, Cu, Ni and Zn concentrations and Mn/Fe ratios since Miocene. A strong correlation between Ba, Ba_Caxs/Mn and P is observed in the phophatized crust. Mineralogical structures were investigated using a focused ion beam (FIB), a high-resolution transmission electron microscopy (HRTEM) and a time-of-flight secondary ion mass spectrometry (ToF-SIMS). At the phophatization interface, higher Ba concentrations and Ba_Caxs/Mn ratios are present in the phases of vernadite and CFA, implying Ba is preferentially incorporated during the phosphatization events in these mineral forms. These changes in Ba_Caxs/Mn ratios and concentrations of  Ba, Cu, Ni and Zn are in line with variations in paleoproductivity recorded from nearby sediment cores and are consistent with the oceanic biogeochemistry domains. Conversely, changes in circulation modes have been previously observed during the same time intervals when Ba_Caxs/Mn ratios peaks in both major (~37 Ma and ~25 Ma) and minor (~7.35 Ma and ~2.72 Ma) phosphatization events, suggesting a concurrent change in deep water circulation and P concentrations. We proposed that P accumulation and phosphatization occurred after the cessation of North Pacific Deep Water (NPDW) and before oxygen-rich Antarctic Bottom Water (AABW) intensified. Our study demonstrates that marine paleoproductivity is strongly influenced by oceanic phosphatization events, the latter of which likely occurred in response to major variations in deep water formation.