Lipid biomarker investigation of the origin and diagenetic state of sub-arctic terrestrial organic matter presently exported into the northern Bothnian Bay

Abstract

Predicted climate warming and observed increases in river discharge in the vulnerable Arctic region can lead to alterations in the flux and composition of terrestrial organic matter (terrOM) transported into high latitude coastal waters. A benchmarking of the current sources, transport and degradation dynamics of sub-Arctic terrOM exported into the northernmost Baltic Sea was detailed with lipid biomarkers and bulk geochemical proxies along a transect extending from the unregulated Kalix River to the central Bothnian Bay. Bulk δ 13C and molecular biomarkers suggested a predominance of terrOM throughout both the estuary and the open bay. Terrestrial biomarkers were abundantly present in surface sediments with high molecular weight (HMW) n-alkanes, n-alkanoic acids and n-alkanols between 190 and 960 μg/gOC and β-sitosterol between 120 and 1500 μg/gOC, which is comparable to Russian Arctic estuaries. A relatively large contribution of the n-alkane C 23 and C 25 homologs (as shown by P aq values of 0.57–0.62) indicated a high contribution of Sphagnum-derived OM to the surface sediments. Loss of functionality of lipid biomarkers (e.g., decreasing ratios of HMW n-alkanoic acids to HMW n-alkanes, and stanols to sterols) between surface water particles and surface sediments, and in the offshore direction for both surface water particles and surface sediments, suggested ongoing and rapid degradation of terrOM throughout the coastal system. Comparison of terrestrial biomarker signatures (e.g., n-alkane carbon preference index and C 23 / (C 23 + C 29) values) and bulk δ 13C in terrOM released from Kalix River and the Russian Arctic Rivers shows that Kalix River extends the east–west trends of many molecular-level terrOM source indicators and delivers a Sphagnum-impacted terrestrial signal that resembles that of the River Ob.