The efficiency of the biological carbon pump (BCP) is largely regulated by the degradation of particulate organic matter (POM) during vertical transport, yet the mechanisms underlying latitudinal variations in POM degradation remain poorly understood. To address this, we conducted high-resolution sampling (20–2000 m) along a subtropical–subarctic transect at 155°E in the western North Pacific, applying amino acid biomarkers with size-fractionated particle analyses (>51 µm and 1–51 µm) to determine how POM degradation pathways vary with latitude and their implications for the BCP.
Our results revealed marked latitudinal differences in POM degradation. In the oligotrophic North Pacific Subtropical Gyre (NPSG), degradation index (DI) decreased sharply from ~1.5 to −1.0 with depth, accompanied by substantial D-amino acid (D-AA) enrichment and a 42% decline in carbon-normalized yields, indicating microbial remineralization as the primary driver. By contrast, in the Subarctic Boundary (SAB), DI showed only a modest decline (0.06 to −0.45), lower D-AA fractions, and carbon yields >15% at depth, suggesting suppressed microbial alteration. This regional difference was linked to POM sources, with the SAB dominated by diatom-derived material, whose rapid sinking reduced particle residence time in upper waters, limiting microbial reworking.
Size-fractionated analyses and PCA further resolved distinct transformation mechanisms that modulate POM transfer to depth. In the NPSG, compositional separation between large (>51 µm) and small (1–51 µm) fractions indicated different sources and histories, with large particles undergoing rapid microbial degradation during export. In the SAB, the proportion of large particles was substantially higher than in the NPSG throughout the water column, yet declined markedly with depth, especially between the euphotic zone base and 300 m. Compositional overlap between size fractions across these depths pointed to zooplankton-mediated fragmentation as the dominant mechanism, in contrast to the microbial degradation observed in the NPSG.
Together, these findings demonstrate that latitudinal variations in the BCP are shaped by the interplay of particle sources, remineralization pathways, and transformation processes. This study provides mechanistic constraints from molecular and size-fractionated observations, and highlights the need to integrate these factors into models for better prediction of carbon export across oceanic regimes.

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