This dataset provides geochemical measurements for sediments and particulates in water across a land to sea gradient in the Romaine River Watershed, Quebec. Specifically, δ13C (carbon isotopic composition), C:N (carbon to nitrogen ratio), OC% (organic carbon percentage), THg (total mercury), and lignin phenols were measured on 76 sediment samples, when possible (e.g., not all proxies were run at the same resolution). A combination of δ13C (carbon isotopic composition), C:N (carbon to nitrogen ratio), and/or OC% (organic carbon percentage) were measured on 24 particulate samples from fresh or marine water. These observations were made in order to trace particulate organic matter along a transect from land to sea, in order to asess if certain quantitites or types of organic matter were retained on land versus flushed out to the nearby coast. The data was collected during the day over the course of a week across two summer seasons (August 2021, June 2022, August 2022) for the terrestrial samples. Marine samples were taken during the day and night onboard the R/V Coriolis II across two weeks in August 2022. The temporal coverage for the water samples taken spans months/yearly changes. The temporal coverage for the sediments covers millennia. The data collection of terrestrial samples was done within the Romaine watershed, Quebec, Canada. The data collection of marine samples was done within the mouth of the Romaine river and further towards the East along the North Shore of Quebec, Canada ending near Pakuashipi, Quebec, Canada. Rivers within boreal regions are ecologically and culturally important, and are often utilized for energy development through hydropower. Improving our understanding of the transfer and storage of carbon and mercury from land to the sea remains critical. Few studies tracking the movement of carbon and other contaminants/nutrients are currently published along the coast on the North Shore of Quebec. For the first time within the dammed Romaine river watershed, we present geochemical measurements from freshwater sediment, lake water, river water, estuary water, and estuary sediment in a transect from land to coast to assess storage and transport of carbon and mercury. Importantly, we add a molecular (lignin biomarkers) lens that is not always considered within land to ocean aquatic continuum studies. For the terrestrial samples (in lakes), the data was collected using small boats (kayak, outboard motor), gravity coring. The marine samples were taken within traditional oceanographic sampling methods onboard the R/V Coriolis II within the Inclusion Mission 2022.