Chlorophyll–particle backscattering displacement reveals multiple deep chlorophyll maximum structures in the North Pacific

Abstract. Subsurface chlorophyll-a (Chl) maxima are common features of stratified oceans, but a Chl maximum does not necessarily indicate a maximum in particle-associated optical structure. Here we use Biogeochemical-Argo (BGC-Argo) observations from the North Pacific during 2022–2025 to examine how the previously recognized separation between Chl maxima and particle-associated optical structure is expressed across a large set of vertically resolved in situ profiles. For 10,801 usable paired Chl and particulate backscattering at 700 nm (BBP700) profiles, we diagnosed the depths of the Chl DCM (deep chlorophyll maximum) and BBP700 maximum within the upper 300 dbar and classified profiles by their vertical displacement. Coupled profiles, in which the two maxima occurred within 25 dbar of each other, were the most frequent class and had median Chl DCM and BBP700 maximum depths of 52.5 and 47.5 dbar, respectively. In Chl deeper profiles, the Chl DCM was displaced downward relative to a shallower BBP700 maximum, consistent with a stronger contribution of photoacclimation or pigment adjustment to the Chl maximum. In Chl shallower profiles, a shallow Chl maximum coexisted with a deeper BBP700 maximum, demonstrating that Chl–BBP700 decoupling is not unidirectional. The three displacement classes showed coherent basin-scale spatial organization and distinct nitracline contexts, with the nitracline generally deeper than both optical maxima in Coupled and Chl deeper profiles, whereas Chl shallower profiles tended to show a deeper BBP700 maximum near the nitracline depth range. Together, these findings indicate that North Pacific Chl DCMs do not represent a single uniform vertical bio-optical structure. Instead, the previously recognized separation between Chl maxima and particle-associated optical structure is expressed in mature BGC-Argo observations as coherent vertical, spatial, and nitracline-related structures across the North Pacific.