Abstract
Long chain n-alkanes are terrestrial higher plant biomarkers analysed in marine sedimentary archives to reconstruct continental palaeoclimatic and palaeohydrological conditions. Latitudinal variation in their concentration and distribution in marine sediments relatively close to the continent has been widely studied, but little is known on the extent to which this continental signal extends to the ocean. Furthermore, no studies have examined the seasonal variation in the deposition of these biomarkers in marine sediments. Here we studied longitudinal variation in the composition of long chain n-alkanes and two other terrestrial higher plant biomarkers (long chain n-alkanols and long chain fatty acids) in atmospheric particles, as well as longitudinal and seasonal variation in long chain n-alkanes in sinking particles in the ocean at different water depths and in surface sediments, all collected along a 12°N transect across the tropical North Atlantic Ocean. The highest abundance of all three biomarker classes was closest to the African coast, as expected, because they are transported with Saharan dust and the largest part of the dust is deposited close to the source. At this proximal location, the seasonal variability in long chain n-alkane flux and the chain length distribution of the n-alkanes in sinking particles was most pronounced, due to seasonal change in the dust source or to change in vegetation composition in the source area, related to the position of the Intertropical Convergence Zone (ITCZ). In contrast, in the open ocean the seasonal variability in both the long chain n-alkane flux and chain length distribution of the n-alkanes was low. The abundance of the alkanes was also lower, as expected because of the larger source-to-sink distance. At the western part of the transect, close to South America, we found an additional source of the alkanes in the sinking particles during spring and autumn in the year 2013. The δ13C values of the alkanes in the surface sediment closest to the South American continent indicated that the isotope signal was likely derived from C3 vegetation from the Amazon, implying an input from the Amazon River, as there is no significant aeolian input from South America there since the prevailing wind direction is from the east. Finally, the concentration of the alkanes was similar in the material collected from the atmosphere, the particles collected while settling through the marine water column, and in the surface sediments, providing evidence that degradation of long chain n-alkanes from the atmosphere to settling at the sediment–water interface at deep open ocean sites is minimal.
Highlights
The epicuticular wax of vascular plant leaves contains several series of long chain n-alkyl compounds, including n-alkanes, with⇑ Corresponding author at: NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Microbiology and Biogeochemistry, and Utrecht University, P.O
We focus here on a longitudinal transect at 12°N across the tropical North Atlantic Ocean (Fig. 1), where on average 182 Mt of dust from the Sahara are transported over the ocean every year, of which 140 Mt are deposited into the Atlantic (Yu et al, 2015)
The concentration of long chain n-alkanes (C25–C33), long chain n-alkanols (C28–C30) and long chain fatty acids (FAs) (C28– C30) in air decreased with increasing distance from the African coast (Fig. 2A)
Summary
Typical chain length ranging from C25 to C35, and n-alkanols and fatty acids (FAs) ranging from C16 to C36 (Eglinton and Hamilton, 1967). The long chain n-alkanes in particular are relatively resistant to degradation (Cranwell, 1981), making them useful as higher plant biomarkers in sediments. They typically have a strong odd/ even predominance, while the long chain n-alkanols and FAs have a strong even/odd predominance (Eglinton and Hamilton, 1963).
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