Pollen record of the past 60ka BP in the Middle Okinawa Trough: Terrestrial provenance and reconstruction of the paleoenvironment

摘要

Core DGKS9602 (931cm in length) was retrieved from the Middle Okinawa Trough. The age model was correlated with the adjacent core, DGKS9603 (Li et al., 2001), for the upper part and to the standard SPECMAP chronology down to ~60ka BP using marine oxygen isotope control points. Pollen analysis was carried out, and seven pollen zones were identified. The results showed that the pollen spectra in MIS 3 were very different to MIS 2 but comparable with the Holocene. The saccate pollen, principally Pinus, predominated in most parts of the core, except for the interval of the Last Glacial Maximum (LGM) and the late MIS 3 when Artemisia and other herbs increased prominently. The ratios of P/A (Pinus/Artemisia) and of PB/NAP (pollen with bladders versus non-arboreal pollen) were used as a proxy for sea-level change and for estimating distances from the coastline to the study site. The provenance study, using a modern analog technique based on the East Asian Modern Pollen Database and fossil pollen assemblages, revealed that the pollen were derived from diverse source areas during different stages since ~60ka BP. The pollen zones equivalent to MIS 1 and 3 were characterized by uncertain and disparate areas of provenance, and the surrounding continents, including Eastern Mainland China, Korea and Southern Japan, possibly made equivalent contributions, which may be the result of enhanced pollen transport by the northwesterly wind. However, the LGM represented in core DGKS9602 was distinguished by restricted sources from the exposed East China shelf, where the vegetation was quite similar with the herb coverage in the present-day North China Plain. The pollen assemblages sensitively reflected the coastline changes, marked by transport patterns between fluvial runoff and northwesterly winds plus ocean currents. Some pollen changes and limits of pollen zones coincided with the timing of Heinrich events, implying that the rapid climate fluctuations during MIS 3 and MIS 2 may have partially resulted in the forest changes in Eastern Asia. The pattern of relative abundance of subtropical and total arboreal taxa is consistent with that of δ~(18)O from Hulu Cave, implying monsoonal climatic control over vegetation change. The estimated temperature (MAT) by the means of pollen ratios for the LGM was 5-8°C lower than that in the Changjiang River mouth today.