Holocene environmental changes disclosed from anoxic fjord sediments by biomarkers and their radiocarbon content

Abstract

The power and validity of compound-specific radiocarbon dating was evaluated using sediments from Saanich Inlet, Canada, in age ranging from recent to 5000 yr BP. Compounds characteristic of higher plants, phytoplankton and archaea, were isolated by preparative GC and semi-preparative HPLC. For preparative HPLC a new method was designed, to be able to rapidly isolate relatively polar and high molecular weight biomarkers. After radiocarbon analysis, compound 14C ages were calibrated to produce calendar ages, for comparison with the sediment ages and the age of the total organic carbon (TOC). Phytoplanktonic and archaeal lipids showed on average age differences with the sediment of ~800 years, consistent with published DIC reservoir ages. Because the sediment ages were known, specific reservoir ages could be determined and compared with model reservoir ages. Planktonic biomarkers showed considerable variation, while crenarchaeol, from ubiquitously occurring marine archaea, showed minor variations in reservoir age. This indicates that compound specific 14C dating of crenarchaeol proves to be a promising new dating method for marine, carbonate-poor sediments, because it was relatively easy to isolate in sufficient amounts and consistent regarding reservoir effects. An increasing relative age of the terrestrial biomarker towards recent times pointed towards an ongoing aging of the soil in the fjord environs, due to the ongoing accumulation of the refractory soil organic carbon after deglaciation of the area. The results suggest that the terrigenous organic carbon pool in temperate regions may still be increasing as a long-term response to the end of the last ice-age. A 400 year sedimentary record of the anoxic Kyllaren Fjord (West Norway) was investigated using organic geochemical, geochemical and sedimentological approaches in order to reconstruct environmental changes. The results show that in the first half of the 19th century an environmental change occurred, likely caused by a change in the precipitation regime, vegetation and/or land use. In the last century a distinct increase in primary productivity is observed, likely caused by 'natural' eutrophication due to the transformation of the fjord from an open to a confined system by the building of a partially open dam in 1954, possibly combined with anthropogenic eutrophication. Decreased delta-13C values of several biomarkers point towards enhanced carbon recycling in the fjord over the last century. The results of a similar biogeochemical study on the South Norwegian Drammensfjord, covering the last millenium, show that eutrophication already started halfway the 19th century. This was concurrent with the industrialization of sawmills and papermills along the river Drammen and upstream water bodies, which could be traced by 'coniferous' biomarkers in the sediment. The concurrent disappearance of a suite of (bi)cyclobotryococcenes from the upper fjord sediments also revealed that eutrophication likely caused the demise and eventual disappearance of the alga Botryococcus braunii from the fjord. Because of the preference of this alga for oligotrophic and slightly brackish conditions, and the possibility to easily trace these specific biomarkers, it is argued that the disappearance of this alga from a sedimentary record can serve as an palaeoenvironmental indicator for early eutrophication.