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Transport processes along the river-ocean continuum influence delivery of nutrients, carbon and trace metals from terrestrial systems to the marine environment, impacting coastal primary productivity and water quality. Although trace metal transformations have been studied extensively in the Mississippi River Delta region of the Northern Gulf of Mexico, investigations of manganese (Mn) and the presence of ligand-stabilized, dissolved manganese (Mn(III)-L) and its role in the transformation of trace elements and organic matter during riverine transport and estuarine mixing have not been considered. This study examined the chemical speciation of dissolved and particulate Mn in the water column and sediment porewaters in the Mississippi River and Northern Gulf of Mexico in March of 2021 to explore transformations in Mn speciation along the river-ocean continuum and the impact of different processes on the distribution of Mn. Total dissolved Mn concentrations were highest in the Mississippi River and decreased offshore, while Mn(III)-L contributed most to the dissolved Mn pool in near-shore waters. Porewater profiles indicated that ligand stabilization prevented dissolved Mn(III) reduction below the depth of oxygen penetration and in the presence of equimolar dissolved iron(II). Dissolved Mn(III)-L was enriched in bottom waters at all Northern Gulf of Mexico stations, and diffusive flux modelling of porewater dissolved Mn suggested that reducing sediments were a source of dissolved Mn to the overlying water column in the form of both reduced Mn(II) and Mn(III)-L. A simple box model of the Mn cycle in the Northern Gulf of Mexico indicates that Mn(III)-L is required to balance the Mn budget in this region and is an integral, and previously unconsidered, piece of the Mn cycle in the Northern Gulf of Mexico. The presence of Mn(III)-L in this system likely has an outsized impact on trace element scavenging rates, oxidative capacity, and the carbon cycle that have not been previously appreciated. 

This dataset includes sediment pore water chemistry and porosity measurements from marine sediments offshore of the northern and southern Hikurangi margin, New Zealand. Samples were collected by the R/V Revelle on expedition RR1901/1902 (Principial investigator Evan Solomon; co-chief principal investigators Marta Torres and Robert Harris). Alongside heat flow measurements, push core, gravity core, and piston core samples were collected for porosity measurements and pore water extraction throughout seepage areas and diffuse flow settings spanning the shelf break to the deformation front. This study was designed to evaluate magnitude and distribution of heat flow, fluid flow, and chemical flux at the the investigated sites. This work is supported by NSF awards OCE-1753617, OCE-1753665. 

Vigorous seepage offshore Oregon provides insight into the relationship between margin permeability and megathrust slip behavior.