Abstract
This study investigates the temporal and spatial trends of allochthonous organic matter (AOM) contributions to coastal sediments from the boreal Glomma River to the outer Oslofjord, southeastern Norway. Seven sediment cores from the area were analysed, ranging from proximal estuarine sites to distal marine sites. Stable isotope analyses of carbon (δ¹³C) and nitrogen (δ¹⁵N), coupled with C/N ratios, revealed clear gradients in AOM contributions, with proximal sites dominated by terrestrial signatures (-28‰ δ¹³C) and distal sites reflecting marine sources (clustering around − 23‰ δ¹³C). Temporal trends showed increasing sediment accumulation rates (SAR) across all sites, with SAR increasing on average by ~ 1,440 g m⁻² yr⁻¹ after the onset of sustained sediment accumulation, which occurred broadly between the 1960s and 1980s. Terrestrial organic carbon accumulation (C org AR) trends followed those of SAR closely, also increasing over the past century and reaching up to 208 gC m⁻² y⁻¹ at the most proximal site and 68 gC m⁻² y⁻¹ at the most distal site. Both trends were driven by intensified land use changes and climate effects such as increased precipitation and prolonged growing seasons. Notably, the δ¹³C profiles indicated a steady decline of δ¹³C over time, with a vertical range of change of up to 1.52‰ at the proximal site, reflecting enhanced terrestrial inputs into marine sediments. This corresponded to an average increase in allochthonous organic carbon of 0.58% per decade across all cores over the past 100 − 150 years. Variability in sediment characteristics across sites highlighted the importance of riverine inputs in proximal sites, where allochthonous organic carbon constituted 58–89% of total organic carbon, contrasting with near-zero fractions observed at distal sites. This work emphasizes the historic increase in AOM input to the studied coastal area. This has many ecological implications, including coastal darkening – an increase in light attenuation by the dissolved terrestrial organic matter in water column, which limits light availability for phototrophs. If this trend of AOM enrichment continues, it could have significant consequences for the Oslofjord ecosystem, including suppression of phototrophs and subsequent cascading effects on the food web and ecosystem productivity.