Abstract:

The California Current System (CCS) is a very important boundary current in the Northeast Pacific Ocean and it plays an important role in the global air-ocean interactions. On the interannual and decadal timescales, the ocean-climate change and activity of upwelling in the CCS are principally affected by the Pacific Decadal Oscillation (PDO, with variable periods ranging from 20 to 30 a). To understand the variation of upwelling activity in the northern region of the CCS since the Early Pleistocene and its main control mechanism, we analyzed the data of calcareous nannofossils (fossils of coccolithophores) and diatom fossils from the International Ocean Drilling Program Holes 1020B (southern site, at 41° N) and 1245B (northern site, at 44° N). Our results suggested that abundance variations in these two fossil groups were correlated with most glacial-interglacial cycles, by which high abundances of diatom fossils and nannofosills were associated with glacial and interglacial stages, respectively. Three ca.0.5 Ma long-term stages of fossil abundance variations were recognized: the Early Pleistocene Stage (1.5-1.1 Ma) featured by abundant nannofossils in both holes, the Middle Pleistocene Stage (1.1-0.65 Ma) by relatively abundant diatoms and nannofossils with opposite variation trends in the two holes, and the Late Pleistocene Stage (since 0.65 Ma) by relatively abundant diatoms with very low and sporadic nannofossils in Hole 1245B and contrarily, abundant nannofossils with less abundant diatoms in Hole 1020B. We proposed a mode of long-term dominance of “Pacific Decadal Oscillation (PDO-like)” in combination with “North Pacific Gyre Oscillation (NPGO-like)” mode to explain the possible mechanisms for these records. The northern region of the CCS had been affected by a long-term dominance of +PDO-like phase that led to decreased upwelling during the Early Pleistocene, and by an altering of -PDO (intensified upwelling ) and +PDO phases over the Middle Pleistocene Transition period (MPT). And for the last 0.65 Ma, the region was affected by an altering of strong -NPGO and weak -PDO phases for Hole 1020B, but only by -PDO phase for Hole 1245B. An abrupt increase in diatom abundance (by three times in Hole 1020B and five times in Hole 1245B) marked the setup of MPT at 1.1 Ma, and a significant change in abundances of calcareous nannofossils (3-fold increase and reduction at the southern and northern sites, respectively) indicated the end of MPT at 0.65 Ma. This altering of microfossil abundances was seen as the results of abrupt palaeoceanographic changes. It further implied that the shift from a positive to a negative phase of long-term PDO-like dominance period may be abrupt too. The opposite changing trends of these two fossil groups at the northern and southern sites during and after MPT indicated possible geographical differentiation of ocean-climate conditions in the northern region of the CCS. It provided the Pleistocene records for the study of modern air-ocean processes and biological distribution with the regional boundary at 40° N.

Key words: calcareous nannofossils, diatom fossils, upwelling, California Current System, PDO-like, Middle Pleistocene Transition