Pliocene‐Pleistocene evolution of the North Pacific Ocean‐Atmosphere system, interpreted from fossil diatoms

Abstract

The relative abundance of diatom taxa in 216 core tops has been quantified and compared, using cluster analysis, with assemblages in six cores located beneath different water masses of the North Pacific Ocean. All but one sequence span at least 1.5 m.y., (early Pleistocene to Holocene), and one extends to 2.6 Ma. The quantitative abundance of each taxon and the space‐time occurrence of each cluster are interpreted as tracers of oceanographic conditions. In addition to high‐frequency (glacial‐interglacial) oscillations, the records display rapid steplike changes at 2.4 Ma, 1.6–1.5 Ma, 1.0 Ma, 0.8–0.7 Ma, and 0.35 Ma. Two secondary transitions, recorded only in the northwestern cores, occur at 2.0 Ma and 1.1 Ma. We conclude that both the high‐frequency oscillations and the stepwise transitions reflect variations in extent of subarctic water masses, under the influence of the Siberian high‐pressure and Aleutian low‐pressure atmospheric cells. We further conclude that (1) the subarctic water mass has developed progressively and attained its modern structure only in the late Pleistocene; (2) some stages of this evolution may have occurred earlier in the eastern subarctic; (3) the Holocene is an atypical “warm” extreme, so that the present is not a very good key to the past; (4) there were several glaciations between 2.4 and 2.0 Ma; and (5) the Subtropical Gyre is more conservative than the Subarctic Gyre, and major changes in the region have been due to forcing from the north. We propose a conceptual model of oscillating metastable climatic modes, maintained by internal feedback; and suggest that steplike changes may occur when one set of boundary conditions is temporarily exceeded.

Expansion of subarctic water masses in the North Atlantic and Pacific oceans and implications for mid‐Pleistocene ice sheet growth

“…Although insolation at 65°N peaks during MIS 25, circa 0.94 Ma [Berger and Loutre, 1991] because of high eccentricity, the rising limb of %C 37:4 occurs during the previous eccentricity peak, suggesting that North Pacific water mass distribution is not controlled by insolation. At present, the distinctive salinity maximum of the subarctic gyre is linked to the excess precipitation that results from the strong Aleutian low-pressure system and the resulting winter storms and reduced cloud cover [Sancetta and Silvestri, 1986]. A weaker East Asian winter monsoon between MIS 22-16 is consistent with weaker Aleutian low-pressure and Siberian high-pressure systems [Heslop et al, 2002], but occurs after the decline in %C 37:4 and thus cannot be considered as a forcing mechanism.…”

“…However, the complex extratropical response to ENSO variability within our present climate system limits acceptance of this mechanism. Analysis of diatom assemblages across the North Pacific has shown that between 1.0 and 0.8 Ma, in parallel with the reduction in %C 37:4 shown in Figure 3, the western Pacific experienced enhanced climate variability [Sancetta and Silvestri, 1986]. This variability included periodic migrations of the Subarctic Front over several degrees of latitude and an enhanced variability in the subarctic Pacific between intervals of stronger and weaker seasonal contrasts [Sancetta and Silvestri, 1986].…”

“…In the northwest, increased ice rafting at 1.0 Ma occurred to the south of Site 882, but not at Site 882 itself [St. John and Krissek, 1999]. This has been linked to a more southerly position of the subarctic front and thus the zone of iceberg melting and deposition [Sancetta and Silvestri, 1986;St. John and Krissek, 1999].…”

“…Analysis of diatom assemblages across the North Pacific has shown that between 1.0 and 0.8 Ma, in parallel with the reduction in %C 37:4 shown in Figure 3, the western Pacific experienced enhanced climate variability [Sancetta and Silvestri, 1986]. This variability included periodic migrations of the Subarctic Front over several degrees of latitude and an enhanced variability in the subarctic Pacific between intervals of stronger and weaker seasonal contrasts [Sancetta and Silvestri, 1986]. We propose here that this increased variability most likely accounts for the partial decrease in %C 37:4 after 1.0 Ma, as the cool and fresh conditions were no longer sustained.…”

“…We propose here that this increased variability most likely accounts for the partial decrease in %C 37:4 after 1.0 Ma, as the cool and fresh conditions were no longer sustained. Sancetta and Silvestri [1986] attributed this variability to east-west shifts between the Siberian high-pressure and Aleutian low-pressure cells, but the cause is unclear.…”