Sources of fine-sized organic matter in North Atlantic Heinrich layers:: δ13C and δ15N tracers

Organic carbon (OC) and total nitrogen (TN) concentrations and stable isotope ratios (delta(13)C, delta(15)N) of fine (<50 mum) 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 +/- 1sigma: OC/TN = 7.6 +/- 0.8; delta(13)C = -22.8 +/- 1.0parts per thousand; delta(15)N = 5.5 +/- 0.6parts per thousand). delta(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 delta(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 (HL1, HL2, HL4, and HL5), fine-sized sedimentary organic matter has lower OC and TN concentrations, contrasting sharply with those of ambient glacial sediments. Lower delta(13)C (down to -28parts per thousand) and delta(15)N (down to 1.6parts per thousand) values and high OC:TN ratios (up to 14.7 +/- 1. 1) are found for HL1, HL2, and with lesser extent for HL4. 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 HL5, only the delta(13)C offset records the input of fine size ice-rafted organic matter. Gradually changing soil development conditions during the time interval covering HL5 to HL1 (marine isotope stages 5 to 2), as well as varying erosion levels, have been hypothesized on the basis of constant delta(13)C, increasing OC/TN and decreasing delta(15)N values. Copyright (C) 2002 Elsevier Science Ltd.