Multi-proxy lipid biomarker and compound-specific isotope analyses from Lake Harcha, Middle Atlas, Morocco

Lakes in the Middle Atlas of Morocco have been investigated for decades, mainly focusing on sedimentological and palynological analyses. They are great archives for paleoclimate and ‑environmental reconstruction, and Morocco is perfectly located to study past changes of the mid-latitude westerlies and the subtropical atmospheric circulation, respectively. Here we present first results from our study to explore the potential of lipid biomarkers preserved in Lake Harcha. The lake has a very small catchment (~0,6 km²), is only ~5000 m² big, but ~13 m deep. Two sediment cores, ~150 and ~130 cm in length, were retrieved. Age control is not available yet.

n-Alkanes (n-C₂₃ – n-C₃₃) are well preserved with odd over even predominances >7. n-C₂₉ and n-C₃₁ are particularly abundant and likely reflect input from terrestrial plants, whereas the shorter-chain n-alkanes (n-C₂₃ and n-C₂₅) are generally attributed to aquatic organisms. The Paq index increases to values >0.4 in the upper ~50 cm, indicating high aquatic productivity. At the same time dD₃₁ increases by 20-40‰, which we tentatively interpret as shift in atmospheric circulation and related changes in the isotopic composition of the precipitation (more Mediterranean-sourced waters?). dD₂₅ and also dD₂₇ are generally enriched compared to dD₃₁ (up to 50‰), which we attribute to evaporative enrichment of the lake water, but the difference becomes much less in the upper ~50 cm. We also analyzed polycyclic aromatic hydrocarbons (PAHs), biomarkers for fire, combustion, and soil erosion, as well as sterols, fecal biomarkers that are derived from humans and animals. They all reveal variable concentrations and thus past human impacts.