GEOLOGY | July 2014 | www.gsapubs.org 579 INTRODUCTION The hydrological cycle in northern South America was drastically affected by latitudinal migrations of the insolation-driven Intertropical Convergence Zone (ITCZ) and monsoon activ- ity during the late Pleistocene and Holocene (e.g., Arbuszewski et al., 2013; Cruz et al., 2009; Haug et al., 2001; Peterson et al., 2000). How- ever, ocean-atmosphere interactions also play a major role, in particular the immediate propa- gation of high northern latitude climate signals into the tropics via disturbances in the Atlantic Meridional Overturning Circulation (AMOC) (e.g., Hüls and Zahn, 2000; Rühlemann et al., 1999; Schmidt et al., 2004). In particular, the fallback into glacial climate conditions during the Younger Dryas (YD, 12.9–11.6 ka) has been related to a reduction in the AMOC, consistent with a decrease in northward heat transport and subsequent warming of the tropical Atlantic (McManus et al., 2004; Rühlemann et al., 2004). The resultant anomalously warm sea-surface temperatures (SSTs) in the tropical Atlantic are an important factor controlling ITCZ position and regional climate (Arbuszewski et al., 2013; Fensterer et al., 2013). According to Arbuszew- ski et al. (2013), this led to a mean annual ITCZ position of 2°S during the Last Glacial Maximum (LGM) and YD, whereas it shifted to northern South America (10°–12°N) during the early Holocene. The aim of our study is (1) to determine the roles of changes in AMOC and insolation on the northern South American hy- drology during the deglaciation and early Ho- locene, (2) to constrain the geographic extent of these changes, and (3) to explore their dy- namics. For this approach, a sediment core off the Orinoco River mouth is used to reconstruct source regions, SST, and salinity. Generally, northern South America is strong- ly influenced by the Amazon and Orinoco Riv- ers; they contain the largest and third-largest amount of river freshwater, respectively, and make up 25% of the total world’s river discharge (Meade, 1994; Müller-Karger et al., 1989). Here we focus on the Orinoco catchment because of its location at the northernmost position of the summer ITCZ (Fig. 1A), which makes it highly sensitive to hydrological changes. This sensi- tivity is reflected by the effect of the seasonal migration of the ITCZ, which induces wet con- ditions in northern South America during boreal summer and dry conditions during boreal win- ter (Poore et al., 2004). These variations cause a maximum Orinoco River outflow of 7 × 10 4 m 3 s –1 in August and a minimum discharge of 1 × 10 4 m 3 s –1 in March (Bidigare et al., 1993; Corredor and Morell, 2001; Müller-Karger et al., 1989). To examine the influence of hydrological changes in the Orinoco catchment area, we produced a high-resolution Ba/Ca record of the *E-mails: ju.hoffmann@em.uni-frankfurt.de; a.bahr @em.uni-frankfurt.de; s.voigt@em.uni-frankfurt.de; jschoenfeld@geomar.de; dnuernberg@geomar.de; janet .rethemeyer@uni-koeln.de. Disentangling abrupt deglacial hydrological changes in northern South America: Insolation versus oceanic forcing J. Hoffmann 1 *, A. Bahr 1 *, S. Voigt 1 *, J. Schönfeld 2 *, D. Nürnberg 2 *, and J. Rethemeyer 3 * 1 Institute of Geosciences, University of Frankfurt, 60438 Frankfurt, Germany 2 GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany 3 Institute for Geology and Mineralogy, University of Cologne, 50937 Cologne, Germany ABSTRACT Paleoenvironmental studies and climate models demonstrate that fluvial runoff and mois- ture availability in the Caribbean hinterland react very sensitively to climatic variations. Late Pleistocene and Holocene climate records document pronounced dry and wet periods over tropical South America mainly caused by shifts of the Intertropical Convergence Zone (ITCZ). However, forcing mechanisms for changes in the ITCZ position remain controver- sial. Here we present high-resolution foraminiferal Ba/Ca and δ 18 O seawater records from a core located within the Orinoco River outflow documenting abrupt hydrological changes in the Orinoco catchment area during the deglacial and Holocene. Our data, obtained from the surface-dwelling foraminifera Globigerinoides ruber (pink), show an abrupt increase in Ba/ Ca ratios in the early Holocene, starting ~600 yr after the end of the Younger Dryas (YD) cold interval at ca. 10.8 ka and suggesting a massive reorganization of moisture sources in northern South America. In contrast, the salinity dependent δ 18 O seawater from the same samples shows a gradual decrease starting at the end of the YD. The offset of our Ba/Ca peak excludes meltwa- ter release in conjunction with the northern Andean glacier retreat well before the end of the YD as a forcing mechanism. We suggest that the Ba/Ca record documents an abrupt increase in Ba-rich waters of a northern Andean source caused by the insolation-driven shift of the ITCZ and/or enhanced monsoon activity. GEOLOGY, July 2014; v. 42; no. 7; p. 579–582; Data Repository item 2014203 | doi:10.1130/G35562.1 | Published online 16 May 2014 © 2014 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. elevated Ba elevated Ba elevated Ca elevated Ca Salinity (PSU) Ba/Ca (μmol/mol) B ORINOCO ORINOCO Capanaparo Cinaruco COLUMBIA VENEZUELA BRAZIL BRAZIL TRINIDAD Orinoco Delta Lake Valencia Cariaco Basin Caracas CARIBBEAN SEA LLANOS GUAYANA SHIELD M78/1- 235-1 today‘s mean Summer ITCZ proposed early Holocene mean Summer ITCZ Gran Sabana LLANOS 200 km 70°W 65°W 60°W 10°N 5°N 35 33 31 29 27 25 4 6 8 10 12 14 16 A left bank tributaries: Ba 2+ = 0.13 μmol/L right bank tributaries: Ba 2+ = 0.09 μmol/L 300 m 1000 m 300 m 1000 m ANDEAN AREA reference reference hypothetical Ba/Ca ratios hypothetical Ba/Ca ratios Figure 1. A: Map of Orinoco River catchment area (South America); three different source re- gions are Andean and Caribbean coastal ranges (brown), Llanos areas (green), and Guayana shield (turquoise) (after Edmond et al., 1995, 1996). Present mean summer position of the Intertropical Convergence Zone (ITCZ) (e.g., Arbuszewski et al., 2013) and proposed early Holocene mean summer position (after Arbuszewski et al., 2013) are also shown. B: Hypo- thetical Ba/Ca (μmol/mol) ratios for varying freshwater end members. An end member with elevated Ba (brown) results in distinctly elevated Ba/Ca ratios, while one with elevated Ca (gray) has a negligible effect in comparison to reference Ba/Ca ratio (black).