Surface-water freshening: A cause for the onset of North Pacific stratification from 2.75 Ma onward? Junsheng Nie a,b,c, ⁎, John King a , Zhengyu Liu d , Steve Clemens e , Warren Prell e , Xiaomin Fang c,b a Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, RI 02882, USA b Key Laboratory of Western China's Environment System, Ministry of Education, Lanzhou University, Lanzhou Gansu 730000, China c Center for Basin Research, Institute of Tibetan Plateau Research, Chinese Academy of Science, P. O. Box 2871, Beijing 100085, China d Center for Climatic Research, University of Wisconsin, Madison, WI 53706, USA e Department of Geological Sciences, Brown University, Providence, RI 02912, USA abstract article info Article history: Received 29 February 2008 Accepted 31 August 2008 Available online 7 September 2008 Keywords: North Pacific ocean stratification Pliocene monsoon Paleoceanographic data indicate a sudden stratification in the North Pacific Ocean around 2.75 Ma. Understanding the triggering mechanism for this sudden North Pacific stratification is one key to resolving forcing mechanisms for the Northern Hemisphere glaciation around 2.75 Ma, one of the most perplexing problems in paleoclimate research. The trigger for this sudden stratification has not been fully explored. Changes in deep-water or surface-water temperature and/or salinity can cause stratification. Here we re- examine paleoceanography data from the North Pacific Ocean and find evidence that the stratification was preceded by a surface-water salinity decrease between 2.83 and 2.75 Ma. This paleoceanographic result, in combination with recent model simulation results, suggests that fresh water input to the North Pacific via an intensified East Asian summer monsoon and/or an intensified Polar Westerly winter jet stream associated with a phase of accelerated uplift of the northeastern Tibetan Plateau from 4–2.6 Ma might be one important forcing for the onset of North Pacific stratification. Onset of North Pacific stratification around 2.75 Ma is closely related to the intensification of Northern Hemisphere glaciation around the same time via its effects on atmospheric CO 2 concentration and sea surface temperature seasonality. Forcing the onset of the North Pacific stratification and Northern Hemisphere glaciation via surface-water freshening has not been emphasized previously. © 2008 Elsevier B.V. All rights reserved. 1. Introduction An abrupt decrease in biogenic opal mass accumulation rates and increased seasonality of sea surface temperatures, recorded simulta- neously by an increase in the saturation index of alkenones and δ 18 O values of planktonic foraminifer G. bulloides, indicate the sudden development of a halocline and stratification in the North Pacific Ocean between 2.75 and 2.72 Ma (Haug et al., 1999, 2005)(Fig. 1). The sudden stratification of the North Pacific Ocean (Haug et al., 1999, 2005) may be important in the initiation of Northern Hemisphere glaciation for two reasons (Haug et al., 2005). First, it would have trapped more CO 2 in the deep ocean, thereby reducing greenhouse effects (Sigman et al., 2004). Second, an increase in the sea surface- water temperature seasonality would increase summer evaporation resulting in the increased supply of necessary moisture for ice sheet growth (Haug et al., 2005). Researchers argue that the warmer early Pliocene climate during 5–3 Ma and the consequent rapid intensifica- tion of Northern Hemisphere glaciation starting from ∼ 2.75 Ma may be two consecutive analogues for future climate change if atmospheric CO 2 concentration continues increasing in response to the burning of fossil fuels and forests (Raymo, 1994; Haywood et al., 2005, 2000; Fedorov et al., 2006). Therefore, a thorough understanding of the triggering mechanisms for this stratification is emergent and timely. The mechanism that caused the sudden development of a halocline and stratification has not been fully explored (Haug et al., 1999). Recent studies argue that gradual deep-water cooling could trigger stratification by weakening the role of temperature in the polar ocean density structure and reducing its opposition to the stratifying effect of the vertical salinity distribution (Sigman et al., 2004; Haug et al., 2005). Alternatively, freshwater addition to the surface ocean can decrease surface-water salinity and density and thus can trigger stratification in a more direct way (Manabe and Stouffer, 1995). Previous work emphasizes the role of deep-water cooling on the onset of the North Pacific stratification. In this paper, we propose that the onset of North Pacific stratification from 2.75 Ma onward might be a result of both deep-water cooling and surface-water freshening. Global and Planetary Change 64 (2008) 49–52 ⁎ Corresponding author. Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, RI 02882, USA. Tel.: +1 401 874 6182; fax: +1 401 874 6811. E-mail address: niejunsheng@gmail.com (J. Nie). 0921-8181/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2008.08.003 Contents lists available at ScienceDirect Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha