Precessional control on ocean productivity in the Western Pacific Warm Pool for the last 400 kyr: Insight from biogenic magnetite

Abstract

Abstracts The Western Pacific Warm Pool plays a significant role in large‐scale atmospheric circulation and global hydrology. We conducted an environmental magnetic study of two late Pleistocene sediment cores from the western equatorial Pacific Ocean offshore of New Guinea in order to better constrain climatic and oceanographic variability, particularly spatiotemporal ocean productivity variations. Magnetic property measurements and transmission electron microscopy reveal that the magnetic mineral assemblages in the studied sediments are a mixture of biogenic and terrigenous magnetite. Variations in the acid soluble sediment component, interpreted as carbonate content, and the proportion of biogenic to terrigenous magnetite estimated from the ratio of anhysteretic to saturation remanent magnetizations are in‐phase with northern hemisphere summer insolation variations. We interpret that ocean productivity increased during insolation maxima, which induced higher populations of magnetotactic bacteria through a larger nutrient supply to the seafloor. This interpretation assumes that magnetotactic bacterial populations are greatest in sediments just below the seafloor. Precessional frequencies in magnetic mineral concentration variations are suppressed after correction for carbonate dilution, whereas cyclic changes with a ∼100 kyr periodicity remain in carbonate‐free magnetic concentration variations. Glacial/interglacial changes in bottom water currents may have influenced transportation and deposition of magnetic minerals. We demonstrate the usefulness of magnetic proxies for paleoceanographic studies, particularly of biogenic magnetite proxies for estimating paleoproductivity variations.

A Geomagnetic Paleointensity Record of 0.6 to 3.2 Ma From Sediments in the Western Equatorial Pacific and Remanent Magnetization Lock‐In Depth

“…The high k ARM /SIRM ratios also suggest the dominance of biogenic magnetite, consistent with the FORC diagrams. The k ARM /SIRM ratios are much higher than the values (~0.5 × 10 −3 m/A) reported from cores KR1505‐PC2 and PC4 nearshore New Guinea (Yamazaki & Horiuchi, ). This suggests far less supply of terrigenous magnetic minerals for core MR1402‐PC1 than cores KR1505‐PC2 and PC4, which reflects a larger distance of the former from New Guinea.…”

“…An apparent delay might appear in core MD982187 RPIs if the age model of the MD982187 core is wrong, in which magnetic concentration lows are correlated to δ 18 O highs assuming no time lag between the two. However, the synchronous changes of the two have been confirmed for other nearby cores in the West Caroline Basin at least during the Brunhes chron (Yamazaki et al, ; Yamazaki & Horiuchi, ; Yamazaki & Ioka, ). If core MR1402‐PC1 has a decimeter‐scale lock‐in depth like core MD982187, which corresponds to a delay of tens of thousand years, no time lag with the Site U1308 and U1314 RPI records requires meter‐scale lock‐in depths to the North Atlantic cores, considering a factor of about 10 higher sedimentation rates of the North Atlantic cores.…”

“…The dominant magnetofossil morphology in the samples is equant octahedra (~90%), and magnetofossils with other morphologies, such as teardrops and parallelepipeds, are minor. This is in contrast to the magnetofossils in sediments nearshore New Guinea in the western equatorial Pacific (cores KR0515‐PC2 and PC4 in Figure ), where teardrop shapes are common (Yamazaki & Horiuchi, ). The dominance of octahedral magnetofossils was reported from red clay in oxic environments (Yamazaki & Shimono, ).…”

“…The dominance of biogenic magnetite in the magnetic mineral assemblage of the studied core is not inconsistent with the near zero lock‐in depth. Although it is often considered that chemical conditions of the oxic–anoxic transition zone (OATZ) are preferable for habitation of magnetotactic bacteria, abundant occurrence of magnetofossils was reported from the surface oxic layer above the OATZ, even at the sediment–water interface (Yamazaki & Horiuchi, ), and in oxic red clay devoid of the OATZ (Yamazaki & Ioka, ; Yamazaki & Shimono, ). In oxic sediments, magnetotactic bacteria may live in microaerophilic or anaerobic microenvironments around organic particles, and organic particles are most abundant near the sediment–water interface.…”

Influence of Magnetofossils on Paleointensity Estimations Inferred From Principal Component Analyses of First‐Order Reversal Curve Diagrams for Sediments From the Western Equatorial Pacific

“…References: 1: Yamazaki et al. (2008), 2: Yamazaki and Horiuchi (2016), 3: Yamazaki and Oda (2005), 4: Tauxe and Yamazaki (2015), 5: Sakuramoto et al. (2017).…”