PALEOCEANOGRAPHY, VOL. 21, PA4202, doi:10.1029/2006PA001278, 2006 Changes in the vertical profile of the Indonesian Throughflow during Termination II: Evidence from the Timor Sea Jian Xu,1 Wolfgang Kuhnt,1 Ann Holbourn,1 Nils Andersen,2 and Gretta Bartoli1 Received 26 January 2006; revised 31 May 2006; accepted 30 June 2006; published 13 October 2006. [1] We use a multiproxy approach to monitor changes in the vertical profile of the Indonesian Throughflow as well as monsoonal wind and precipitation patterns in the Timor Sea on glacial-interglacial, precessional, and suborbital timescales. We focus on an interval of extreme climate change and sea level variation: marine isotope (MIS) 6 to MIS 5e. Paleoproductivity fluctuations in the Timor Sea follow a precessional beat related to the intensity of the Australian (NW) monsoon. Paired Mg/Ca and d18O measurements of surface- and thermocline- dwelling planktonic foraminifers (G. ruber and P. obliquiloculata) indicate an increase of >4°C in both surface and thermocline water temperatures during Termination II. Tropical sea surface temperature changed synchronously with ice volume (benthic d18O) during deglaciation, implying a direct coupling of high- and low-latitude climate via atmospheric and/or upper ocean circulation. Substantial cooling and freshening of thermocline waters occurred toward the end of Termination II and during MIS 5e, indicating a change in the vertical profile of the Indonesian Throughflow from surface- to thermocline-dominated flow. Citation: Xu, J., W. Kuhnt, A. Holbourn, N. Andersen, and G. Bartoli (2006), Changes in the vertical profile of the Indonesian Throughflow during Termination II: Evidence from the Timor Sea, Paleoceanography, 21, PA4202, doi:10.1029/2006PA001278. 1. Introduction surface temperature (SST) difference between the WPWP and the eastern tropical Pacific cold tongue [Lee et al., [2] It has been increasingly recognized that tropical 2002]. oceans, and in particular the Western Pacific Warm Pool [3] Today, the ITF is dominated by low-salinity, well- (WPWP), play a fundamental role in modulating global ventilated upper thermocline North Pacific water. North climate change on interannual, millennial and orbital time- Pacific water mainly flows southward through the Sulawesi scales [e.g., Godfrey, 1996; Cane and Clement, 1999; Lea et Sea into the Makassar Strait, and then follows two ways: al., 2000; Koutavas et al., 2002; Visser et al., 2003; Medina- one branch enters the Indian Ocean via the Lombok Strait Elizalde and Lea, 2005]. The Indonesian Throughflow (sill depth: 350 m) while the other branch mixes into the (ITF) connects the upper water masses of the Pacific and Flores Sea. Besides the Lombok Strait, there are two other Indian Oceans and substantially influences the salinity and main exit passages: the Ombai Strait (sill depth: 3250 m) heat exchange between these oceans [Gordon and Fine, and Timor Passage (sill depth: 1890 m). The mean trans- 1996]. The annual mean heat transport of the ITF (about ports through the three pathways obtained from mooring 1.15 1015 W) represents a heat sink for the Pacific Ocean  measurements are respectively 1.7 Sv [Murray and Arief, and a major heat source for the Indian Ocean [Schiller et al., 1988], 5.0 ± 1 Sv [Molcard et al., 2001] and 7.0 Sv 1998]. Today, the ITF transports an annual average 16 Sv [Cresswell et al., 1993]. The modern ITF transport mainly (1 Sv = 106 m3 sÀ1) of warm, low-salinity water from the occurs within the thermocline rather than at the sea surface. WPWP and Indonesian-Malaysian archipelago into the It has large seasonal and interannual variability with max- eastern Indian Ocean [You and Tomczak, 1993; Gordon imum annual average flow speeds between 50 and 100 m and Fine, 1996; Schiller et al., 1998; Gordon et al., 2003] and important net flow down to 400 m water depth in the (Figure 1). Warm water transported by the ITF amounts to Makassar and Timor Straits [Gordon et al., 1999; Potemra 5.3 Sv of the total transport of 17.8 Sv in the upper branch et al., 2003]. Recent modeling work suggested that changes of the global thermohaline Conveyor Belt at 20°N in the in the vertical profile of the ITF are more important to alter North Atlantic [Speich et al., 2001]. Thus the ITF forms an the stratification and surface heat fluxes of the Indian Ocean important component of the global thermohaline circulation, than mean transport flux [Song and Gordon, 2004] and that and exerts a strong influence on regional as well as global during glacials, thermocline flow was significantly reduced, climate. For instance, blockage of the ITF would depress the whereas surface flow remained virtually unchanged [Kuhnt mean thermocline of the tropical Pacific and reduce the sea et al., 2004; Zˇuvela, 2005]. [4] The hydrography of the WPWP region is strongly 1Institute of Geosciences, Christian-Albrechts-University, Kiel, influenced by the semiannually reversed wind regime of the Germany. Asian-Australian monsoon and associated seasonal precip- 2 Leibniz Laboratory for Radiometric Dating and Stable Isotope itation patterns. The Asian-Australian monsoon is a huge Research, Christian-Albrechts-University, Kiel, Germany. thermal circulation system driven by seasonal surface tem- Copyright 2006 by the American Geophysical Union. perature differences between broadly central Asia and 0883-8305/06/2006PA001278 Australia [Tapper, 2002] (Figure 2). The east Asian boreal PA4202 1of14 PA4202 XU ET AL.: TERMINATION II INDONESIAN THROUGHFLOW PA4202 summer. The winds are relatively light in the monsoon transition periods of March–April and October–November, as the Intertropical Convergence Zone (ITCZ) moves alter- natively north and south across the region [Tapper, 2002]. The ITCZ shift has substantial impact on the seasonal precipitation pattern over Australasia. While precipitation focuses in the north (South China Sea) and east (west Pacific) during boreal summer, a large part of the ITF area, including the eastern part of the Timor Sea and Timor Strait, receives high precipitation during the austral summer mon- soonal season (December to March) [Tapper, 2002]. How- ever, during glacial times, the southward shift of the ITCZ in boreal winter may have been considerably restricted, and/ or the Australian summer monsoon may have been drier because of cooler Indian Ocean water masses (Figure 2). [5] The main objectives of this work are to track changes in the vertical profile of the ITF outflow in the Timor Sea during a period of extreme climate change and sea level variation (Termination II) and to determine leads and lags in climatic, hydrographic and paleoproductivity proxies. We use a multiproxy approach to reconstruct temperature and salinity of surface and thermocline waters, and to investi- gate changes in the productivity driven particle flux to the deep ocean. Our work is based on high-resolution sediment Figure 1. (a) Flow paths of present-day Indonesian Throughflow [after Kuhnt et al., 2004] and Indian Ocean annual (b) mean temperature (°C) and (c) salinity in upper thermocline on isopycnal surface 25.7, located in the depth range 150–200 m [after You and Tomczak, 1993]. Arrows in Figures 1b and 1c indicate movement of Indian Central Water (black) and Australasian Mediterranean Water (red) derived from Indonesian Throughflow (ITF). Solid circles Figure 2. Modern and inferred glacial Australasian denote the locations of MD98-2162 and MD01-2378. Core monsoon system. Main air pressure cells and wind-driven MD01-2378 is situated in the frontal area between the two surface circulation are after Wang et al. [2005] and Tapper water masses. [2002]. Modifications for the glacial state include increased SE Asian winter monsoon and decreased SE Asian summer winter (northeast) monsoon, which is in phase with the monsoon strength [Jian et al., 2001], position of winter and Australian austral summer (northwest) monsoon, is charac- summer Intertropical Convergence Zone closer to the terized by cold, dry air flowing southward across the South equator with reduced precipitation [De Deckker et al., China Sea. In contrast, a moist northwest monsoonal flow 2002], and absence of surface water flow from South China becomes established over northern Australia during austral Sea into Java Sea because of an exposed Sunda Shelf. 2of14 PA4202 XU ET AL.: TERMINATION II INDONESIAN THROUGHFLOW PA4202 samples from IMAGES Core MD01-2378, recovered mundulus was analyzed). In a few samples, where benthic during the WEPAMA cruise in 2001, and complements an foraminiferal density was low, a smaller number (1–2) of earlier investigation of the 40.73 m core in a time resolution specimens was analyzed. The initial isotopic data of of 1–2 kyr [Holbourn et al., 2005]. This previous study Holbourn et al. [2005] were included. 16 replicate samples indicated that productivity fluctuations in the Timor Sea of G. ruber and P. obliquiloculata indicate that the mean over the last 460 kyr were strongly influenced by monsoonal reproducibility (1s) is ±0.12% for d18O and ±0.13% for wind patterns offshore NW Australia (23 and 19 kyr d13C. The mean reproducibility of 17 paired samples of variability), but were also modulated by sea-level-related P. wuellerstorfi is better than ±0.08% for d18O and d13C. variations in the intensity of the ITF (100 kyr variability) Paired measurements on P. wuellerstorfi and C. mundulus over glacial-interglacial cycles. indicate no significant offset between these two species. [9] All tests were checked for cement encrustations and 2. Material and Methods infillings before being broken into large fragments, then cleaned in alcohol in an ultrasonic bath and dried at 40°C. 0 0 [6] IMAGES Site MD01-2378 (13°4.95 S, 121°47.27 E; Stable carbon and oxygen isotope measurements were made water depth: 1783 m) is located at the southern margin of with the Finnigan MAT 251 mass spectrometer at the the main outflow of the ITF in the Timor Sea (Figure 1).