Late Pleistocene paleoceanography of the Subarctic Pacific : derived from the diatom record

Abstract

Paleoceanography and climate variability of the Late Pleistocene is of vital importance to understand the mechanisms of glacial-interglacial cycles and abrupt climate changes during deglaciations. In relation to the paleoceanography studies carried out in the North Atlantic, rather few recent research has been done in the North Pacific. Developments of sea surface temperature and sea ice in the past provide crucial information on paleoceanographic variability. The aim of this thesis is to establish a diatom-based transfer function for sea surface temperature and to reconstruct the Late Pleistocene paleoceanographic history based on diatom assemblages, in order to improve the understanding of the North Pacific s role in the global glacial-interglacial climate changes. The thesis consists of three studies, of which the results are presented in three manuscripts. In the first study, a new diatom data set was generated from 422 surface sediments, covering a broad range of environmental variables of the northern North Pacific, the Sea of Okhotsk and the Bering Sea. The diatom biogeographic distribution patterns of 38 species and species groups were mapped. Statistical analysis discloses that 32 species and species groups have strong correspondence with the pattern of summer sea surface temperature (SSST). This close relationship between diatom assemblages and the SSST is useful in deriving a transfer function in the northern North Pacific for the quantitative paleoceanographic studies. In addition, the relative abundance of the sea ice indicator diatoms Fragilariopsis cylindrus and F. oceanica of >20% in the diatom composition is used to represent the winter sea ice maximum extent in the Bering Sea. In the second research project, a new diatom-based transfer function for SSST reconstructions was established based on 206 surface sediment samples with 32 species and species groups recovered in the northern North Pacific and its marginal seas. The SSST was estimated by three different techniques: the Imbrie and Kipp Method (IKM), the Modern Analog Technique (MAT) and the Artificial Neural Network technique (ANN). All the SSST estimates for surface samples derived from three methods show high correlations with the modern observations. The transfer functions based on IKM, MAT and ANN were tested on a sediment core retrieved from the western North Pacific for the last 180 ka (ka=103 yr). The SSST reconstruction, although with low glacial-interglacial temperature amplitudes of 1.5-2 °C, displays clear climate variability patterns. The reconstructed SSST is consistent with the local planktonic à ´18O record and the NGRIP à ´18O profile, indicating its potential for paleoceanographic reconstruction in the region. In the third study, diatom records from both the eastern and western North Pacific demonstrate glacial-interglacial variability for the last 160 ka. The diatom-derived SSSTs show uninterrupted temperature reconstructions in the northern North Pacific, covering the whole last glacial-interglacial cycle for the first time in the region. The sea ice diatom and cold water species imply ice-free environment for the last 160 ka in the eastern and western North Pacific open ocean. A possible sea ice expansion during Heinrich Event 1 is indicated by high abundance of cold water diatoms as well as sea ice biomarker from previous studies. An ocean basin-wide east-lead-and-west-lag phenomenon has been discovered in the eastern and western North Pacific during the Termination I and II. It is likely that the mountain glaciers in northwestern America responded faster to the increased summer insolation during the terminations, and discharged huge amount of melting water into the nearby open ocean, which consequently changed the surface environment earlier in the eastern North Pacific The studies of this thesis document the diatom distribution in the northern North Pacific surface sediments and discover a close relationship between the diatom assemblages and the SSST, which is fundamental for a quantitative paleoceanographic study based on diatom fossils. Accordingly, a new diatom-based transfer function for SSST reconstructions is established and applied to sediment cores, providing continuous SSST records for the first time in the northern North Pacific for the last 160 ka. The reconstructed SSST records show similar patterns with the global climate variability records elsewhere. During the terminations, diatom records also display a basin wide lead-and-lag phenomenon, implying possible local imprints on global climate signals.