PII S0016-7037(01)00776-1 Sources of fine-sized organic matter in North Atlantic Heinrich Layers:  13 C and  15 N tracers SYLVAIN HUON, 1, *, FRANCIS E. GROUSSET, 2 DIDIER BURDLOFF 1,2 G´ ERARD BARDOUX, 1 and ANDR´ E MARIOTTI 1 1 UMR 7618 Bioge ´ochimie Isotopique (LBI), Universite ´ Pierre and Marie Curie, Case postale 120, 4 place Jussieu, 75252 Paris cedex 05, France 2 UMR 5805 EPOC, De ´partement de Ge ´ologie et Oce ´anographie (DGO), Universite ´ Bordeaux I, Avenue des Faculte ´s, 33405 Talence cedex, France (Received February 22, 2001; accepted in revised form July 12, 2001) Abstract—Organic carbon (OC) and total nitrogen (TN) concentrations and stable isotope ratios ( 13 C,  15 N) of fine (50 m) size fractions of deep-sea sediments from the central North Atlantic were employed to identify changes in sources of organic matter over the past 50 ka BP. Ambient glacial sediments are characterised by values that reflect mixtures of marine and terrestrial inputs (averages  1: OC/TN = 7.6  0.8;  13 C =-22.8  1.0‰;  15 N = 5.5  0.6‰).  13 C, OC, and TN concentrations shift to higher values during the Holocene, indicating a gradual decrease of fine terrigenous supply to the North Atlantic. The unchanged  15 N record between last glacial and Holocene stages indicates that the central North Atlantic region remained oligotrophic at least during the past 50 ka BP, but additional studies are required to support this result in terms of nitrogen oceanic budget. During the phases of enhanced ice-rafted detrital supply corresponding to prominent Heinrich events (HL 1 , HL 2 , HL 4 , and HL 5 ), fine-sized sedimentary organic matter has lower OC and TN concentrations, contrasting sharply with those of ambient glacial sediments. Lower  13 C (down to -28‰) and  15 N (down to 1.6‰) values and high OC:TN ratios (up to 14.7  1.1) are found for HL 1 , HL 2 , and with lesser extent for HL 4 . These values reflect enhanced detrital supply originating from poorly differentiated soil horizons that characterise periglacial climate conditions and from organic matter– bearing rock sources of the underlying geological basement. During HL 5 , only the  13 C offset records the input of fine size ice-rafted organic matter. Gradually changing soil development conditions during the time interval covering HL 5 to HL 1 (marine isotope stages 5 to 2), as well as varying erosion levels, have been hypothesized on the basis of constant  13 C, increasing OC/TN and decreasing  15 N values. Copyright © 2002 Elsevier Science Ltd 1. INTRODUCTION During the past 70,000 yr BP, marine sedimentation in the NE Atlantic is interrupted by six phases of enhanced ice-rafting detritus (IRD) supply called Heinrich events (HL = Heinrich layer; Heinrich, 1988; Broecker et al., 1992, Bond et al., 1992; Andrews and Tedesco, 1992; Andrews, 1998). There is now convincing evidence that these episodes are also observed in various climate records (Bond et al., 1993; Grimm et al., 1993; Porter and An, 1995; Lowell et al., 1995; Benson et al., 1996; Sanchez-Gon ˜i et al., 2000). Direct links between Heinrich events, oceanic circulation, and ice sheet dynamics have also been shown (Alley and McAyeal, 1994; Paillard and Labeyrie, 1994; Broecker, 1994; Maslin et al., 1995; Dowdeswell et al., 1995; Manabe and Stouffer, 1997; Cortijo et al., 1997; Vidal et al., 1997; Zahn et al., 1997; Elliot et al., 1998; McManus et al., 1998; Bard et al., 2000; Grousset et al., 2000; Van Kreveld et al., 2000). Each event appears to be an episode of very rapid sediment accumulation, associated with a drop in foraminifer concentration due either to a decrease of marine productivity or to a dilution of marine input by IRD supply (Bond et al., 1992; Grousset et al., 1993). Dilution or reduction of marine supply is a question of great interest because changes in the thermohaline circulation mode due to freshwater influx during Heinrich events have been hypothesized to account for some of the rapid climate oscillations observed in Greenland ice-core records (Broecker et al., 1990; Keigwin et al., 1991; Paillard and Labeyrie, 1994; Maslin et al., 1995; Zahn et al., 1997). Min- eralogical and isotopic studies of the detrital supply in Heinrich Layers point to continental source areas located at high lati- tudes, with contrasting differences during Heinrich events 3 and 6 that are attributed to reduced Laurentide ice-sheet con- tributions (Jantschik and Huon, 1992; Bond et al., 1992; Grous- set et al., 1993; Robinson et al., 1995; Revel et al., 1996; Gwiazda et al., 1996a, 1996b, 1998; Hemming et al., 1998). The enhanced supply of detrital organic matter of continental origin and its preservation under reduced bottom water oxygen content have also been shown by means of specific biomarkers (Rosell-Mele ´ et al., 1997; Madureira et al., 1997) and sediment mass accumulation studies (Manighetti and McCave, 1995) in various North Atlantic settings. In this study, we provide a high-resolution elemental (organ- ic carbon and total nitrogen) and isotopic ( 13 C and  15 N) record of fine-sized (50 m) particulate organic matter in North Atlantic deep-sea sediments covering the HL 1 to HL 5 time interval (back to 50 ka BP; Bond et al., 1993). This study was performed in order: (1) to estimate the contribution of terrestrial organic matter in glacial marine sediments due to enhanced IRD supply, and (2) to discuss some of the factors controlling the isotopic signature of preserved sedimentary organic matter in the North Atlantic during the last glaciation. Indeed, identification of terrigenous and marine supplies is necessary when estimating ocean paleo-productivity (or paleo- * Author to whom correspondence should be addressed (huon@ccr. jussieu.fr). Pergamon Geochimica et Cosmochimica Acta, Vol. 66, No. 2, pp. 223–239, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 + .00 223