GEOLOGY, September 2010 783 INTRODUCTION Tropical-subtropical precipitation patterns track seasonal migration of the Intertropical Convergence Zone (ITCZ), a narrow latitudinal zone of wind convergence that oscillates across the equator. Today, the maximum amplitude in the seasonal shift of the ITCZ occurs between southern China and northern Australia with a latitudinal displacement of ~35° (Fig. 1), though this may have differed substantially in the past. Precipitation, continental runoff, and marine salinity proxies have shown that marked varia- tions in the ITCZ position occurred during the late Pleistocene and Holocene, variations that appear to be linked to Northern Hemisphere climate variability, net heat transports associ- ated with thermohaline circulation, and differ- ential hemispheric heating due to changes in the Earth’s radiation budget (Haug et al., 2001; Benway et al., 2006; Leduc et al., 2007; Partin et al., 2007; Sachs et al., 2009). The Pleistocene scenario of an extensive and strongly fluctuating Northern Hemisphere ice cap exerting a major influence on global climate is relatively unusual in Earth’s history. During warmer climate phases of the Cenozoic, charac- terized by predominantly unipolar glaciations in Antarctica, the dynamics of the ITCZ may have been different. Understanding the past evolution of this variable climate feature in the absence of large Northern Hemisphere ice sheets pro- vides a perspective relevant to future variabil- ity on a warmer Earth. Of particular interest is the Middle Miocene interval of stepwise high- latitude (and likely subsurface ocean) cooling, and Antarctic ice sheet expansion, which led to the inception of continuous icehouse conditions in Antarctica (Flower and Kennett, 1993; Hol- bourn et al., 2005; Lewis et al., 2007; Shevenell et al., 2008). Ice expansion in the Northern Hemisphere is thought to have been small rela- tive to Antarctica, although perhaps not negli- gible (DeConto et al., 2008). Here we present upper ocean temperature and salinity records from Ocean Drilling Program (ODP) Site 1146 (19°27.40′N, 116°16.37′E, 2092 m water depth), where a continuous clay-rich Middle Miocene sedimentary archive (15.7–12.7 Ma) was recovered. This site is situ- ated on Eurasian continental crust within a small rift basin on the mid-continental slope of the northern South China Sea (Fig. 1). During the Miocene, this site was at approximately the same latitude as today and fully open to the western Pacific Ocean (Wang et al., 2000). Today, ODP Site 1146 is close to the northernmost position of the ITCZ during boreal summer, and is thus ideally situated to closely track changes in the summer position of the ITCZ rain belt during inception of Miocene icehouse conditions. SURFACE HYDROLOGY OF SUBTROPICAL NORTHWESTERN PACIFIC Our upper ocean temperature estimates are based on Mg/Ca ratios in foraminiferal cal- cite, which vary exponentially with tempera- ture. Mg/Ca paleothermometry, combined with stable oxygen isotopes, allows us to extract the temperature component of calcite δ 18 O and to calculate past seawater δ 18 O (δ 18 O sw ). The salin- ity-related δ 18 O sw component is then estimated by removing the global glacioeustatic δ 18 O sw component, as approximated from benthic δ 18 O. Detailed methods are provided in the GSA Data Repository. 1 Application of these proxies relies on a number of assumptions concerning in par- ticular the use of modern calibration equations for extinct species, the Mg/Ca composition of seawater, and the isotopic effects of ice volume fluctuations during the Miocene. While uncer- tainties in temperature and salinity estimates may be higher for the Miocene than for the Plio- cene and Pleistocene, our interpretations and conclusions are based on robust trends extend- ing beyond individual error ranges. The sea-surface planktic δ 18 O and tempera- ture time series in ODP Site 1146 (Fig. 2) are characterized by high-frequency oscillations (~20–40 k.y.) that are superposed on lower fre- quency variations (~10 2 –10 3 k.y.). Although the two records show a high degree of coherency at main Milankovitch frequencies, sea-surface temperature (SST from Mg/Ca) exhibits signifi- cantly lower amplitude variability than planktic δ 18 O, when plotted on a temperature-equivalent Geology, September 2010; v. 38; no. 9; p. 783–786; doi: 10.1130/G31043.1; 4 figures; Data Repository item 2010221. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. 1 GSA Data Repository item 2010221, methods, is available online at www.geosociety.org/pubs/ft2010 .htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. Does Antarctic glaciation force migration of the tropical rain belt? Ann Holbourn 1 , Wolfgang Kuhnt 1 , Marcus Regenberg 1 , Michael Schulz 2 , Alan Mix 3 , and Nils Andersen 4 1 Institute of Geosciences, Christian-Albrechts-University, D-24118 Kiel, Germany 2 MARUM—Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, D-28334 Bremen, Germany 3 College of Oceanic and Atmospheric Sciences (COAS), Administration Building 104, Oregon State University, Corvallis, Oregon 97331-5503, USA 4 Leibniz Laboratory for Radiometric Dating and Stable Isotope Research, Christian-Albrechts-University, D-24118 Kiel, Germany ABSTRACT High-resolution (~3–6 k.y.) upper ocean temperature and salinity estimates derived from planktic foraminiferal δ 18 O and Mg/Ca in Ocean Drilling Program (ODP) Site 1146 reveal stepwise changes in the precipitation-evaporation balance of the subtropical northwestern Pacific during the Middle Miocene (15.7 to 12.7 Ma). We attribute the punctuated pattern of surface warming and freshening following Antarctic ice growth episodes at 14.6, 14.2, 13.9, and 13.1 Ma to successive northward movements of the Intertropical Convergence Zone, implying high sensitivity of tropical rain belts to the interhemispheric temperature gradient driven by high-latitude climate. This dynamic interaction has implications for future warmer climate regimes with differential warming of the Northern Hemisphere, as it may lead to changes in the latitudinal penetration of tropical Pacific moisture over Southeast Asia. 1146 60N 30N 0 30S 60S 60N 30N 0 30S 60S MSU/Legates precipitation (mm month -1 ) 0 40 80 120 160 200 240 280 320 360 400 440 480 520 60E 90E 120E 150E 180 150W 60E 90E 120E 150E 180 150W 1146 Figure 1. Mean monthly precipitation dur- ing boreal winter (January, southernmost position of Intertropical Convergence Zone, ITCZ) and boreal summer (July, northern- most position of ITCZ). Precipitation (in mm/ month) from Wallace et al. (1995). Ocean Drilling Program Site 1146 (19°27.40′N, 116°16.37′E) was drilled at 2092 m water depth in northern South China Sea. MSU— Microwave sounding unit. on September 13, 2010 geology.gsapubs.org Downloaded from