Sources and effects of short-term environmental changes in Gullmar Fjord, Sweden, inferred from the composition of sedimentary organic matter

Abstract: In this study I will assess sources and effects of short-term environmental changes on the sedimentary organic matter in Gullmar Fjord. To achieve this, a high resolution sediment core (sedimentation rate 0.7 cm yr -1) was retrieved from the deepest part in the fjord and analyzed with multiple geochemical methods, targeting bulk organic matter and biomarker composition. The distribution of n-alkanes did not display any major changes for the terrestrial fraction in the investigated time frame from 1954 until 2014. Marine organic matter was found to be the dominant source of sedimentary organic matter in the fjord, though it is most likely produced remotely in the Skagerrak. Sea surface temperature (SST) reconstructions based on long-chain alkenones do not show any general increase during the studied interval, as observed from the hydrographic data set available for Gullmar Fjord. Temperatures of 14.1 to 21.1°C (mean 16.4 ± 1.6) were reconstructed, so the haptophyte blooms producing the long-chain alkenones occurred most likely during summer to early autumn. However, they would also fit to temperatures and blooming seasons reported for the Skagerrak. Even though Gullmar Fjord is considered to be free from industrial pollution, we found indications which could be interpreted as a slight contamination of the sediments by petrogenic hydrocarbons. Lower oxygen minimum concentrations observed in the bottom water since the 1980s are thought to be caused by changes in the North Atlantic Oscillation (NAO) and were related with a possibly ongoing eutrophication in previous studies. Signs for an ongoing eutrophication in the fjord were not visible in our sedimentary organic matter record, suggesting that most marine organic matter is produced in the Skagerrak, or that there is no ongoing eutrophication in the fjord since the 1950s.