Reconstruction of Holocene Optimum paleoclimatic variations using long-chain n-alkanes and alkenones in sediments from Dabusu Lake, northeastern China

Abstract

Long-chain n-alkanes and alkenones were analyzed in sediments from Dabusu Lake, northeastern China, from ca. 7.2–4.5 kyr BP (the Holocene Optimum) and the present. Long-chain n-alkanes are derived from leaf waxes of terrestrial vascular plants and aquatic macrophytes. Using n-alkane proxies such as Paq and average chain length (ACL) indices, variations in paleovegetation were reconstructed. The Paq values were generally low, indicating that aquatic macrophytes were a minor floral component in Dabusu Lake. The ACL data indicated that forest/grassland mixed vegetation occurred around the lake. From the alkenone distribution patterns (C37/C38, C40/C37, and %C37:4 ratios), the main alkenone producers were inferred to be Group II producers, especially Ruttnera lamellosa. We estimated water temperatures based on alkenone unsaturation indices (UK37 and UK’’37) using the temperature calibrations obtained from culture strains of R. lamellosa. The variation pattern of the UK’’37-based water temperatures was almost similar to that of the UK37-based pattern, although some spikes differed between the temperatures based on the two indices. The UK’’37-based water temperatures were lower from 7.2 to 6.2 kyr BP, and subsequently increased after 6.2 kyr BP. The alkenone-based temperatures were highest from 5.5 to 5.3 kyr BP, indicating that this maximal stage was the Holocene Thermal Maximum in the Dabusu region. The markedly decreasing spikes of UK’’37-based water temperatures in our study likely corresponded to the cooling events during the Holocene Optimum, as previously reported, especially Asian monsoon events (AM) 1 and Bond event 4 (BE4). Also, the increasing spike (7.2 kyr BP) in temperatures was possibly associated with the warming regime just after AM2. Moreover, the rapid increasing spikes of alkenone-based temperatures corresponded to those of Paq at 7.2 kyr BP (just after AM2) and 6.4 kyr BP. These results imply that the rapid warming reconstructed using alkenone-based temperatures might have resulted in increased precipitation around the lake at these ages, especially just after the AM2 cooling.