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G. & Sonntag, C.) 275–292 (Wiley, New York, 1995). 29. Kieber, R. J., Zhou, X. & Mopper, K. Formation of carbonyl compounds from UV-induced photodegradation of humic substances in natural waters; fate of riverine carbon in the sea. Limnol. Oceanogr. 35, 1503–1515 (1990). 30. Provencher, S. W. A constrained regularization method for inverting data represented by linear algebraic or integral equations. Comput. Phys. Commun. 27, 213–227 (1982). Acknowledgements. We thank M. J. Perry, P. Jumars and J. Hedges for their comments and suggestions. This work was supported by the US NSF (Polar Program) and the Royalty Research Fund (University of Washington). Correspondence shouldbe addressed to P.V. Influence of oxygen exposure time on organic carbon preservation in continental margin sediments Hilairy E. Hartnett, Richard G. Keil, John I. Hedges & Allan H. Devol School of Oceanography, Box 357940, University of Washington, Seattle, Washington 98195-7940, USA ......................................................................................................................... Today, over 90% of all organic carbon burial in the ocean occurs in continental margin sediments 1 . This burial is intrinsically linked to the cycling of biogeochemically important elements (such as N, P, S, Fe and Mn) and, on geological timescales, largely controls the oxygen content of the atmosphere 2–4 . Currently there is a volatile debate over which processes govern sedimentary organic carbon preservation 5–8 . In spite of numerous studies demonstrating empirical relationships between organic carbon burial and such factors as primary productivity 9 , the flux of organic carbon through the water column 10 , sedimentation rate 11,12 , organic carbon degradation rate 13 , and bottom-water oxygen concentration 8,14 , the mechanisms directly controlling sedimentary organic carbon preservation remain unclear. Furthermore, as organic carbon burial is the process that, along with pyrite burial 15 , balances O 2 concentrations in the atmosphere, it is desirable that any mech- anism proposed to control organic carbon preservation include a feedback buffering atmospheric oxygen concentrations over geo- logical time. Here we compare analyses of sediments underlying two regions of the eastern North Pacific Ocean, one which has oxygen-depleted bottom waters and one with typical oxygen distributions. Organic carbon burial efficiency is strongly corre- lated with the length of time accumulating particles are exposed to molecular oxygen in sediment pore waters. Oxygen exposure time effectively incorporates other proposed environmental vari- ables 8–14 , and may exert a direct control on sedimentary organic carbon preservation and atmospheric oxygen concentrations. Marine sediments and overlying waters were sampled during three cruises off Washington State and two cruises off the north- western coast of Mexico. There is a prominent O 2 -deficient zone over the Mexican margin between 150 and 600 m water depth (Fig. 1a) in which dissolved O 2 concentrations are often too low to detect by conventional techniques. In contrast, the Washington margin has a weaker O 2 -minimum zone with concentrations decreasing to 20 mol l -1 at 800 m. Both margins have a range in sediment accumulation rates (see Table 1). Ocean-colour satellite images from the Coastal Zone Color Scanner suggest that through- out the year, the Mexican margin study-site has lower pigment concentrations (and by inference, lower primary production rates) than the Washington coast site. The sharp contrasts between these two regions help to distinguish factors affecting sedimentary organic carbon (OC) preservation. For example, if primary produc- tion is the controlling factor, the Washington margin should have a higher degree of OC preservation. Alternatively, if sediment accumulation rate is the controlling factor, deposits with similar accumulation rates should have comparable degrees of OC pre- servation in both regions. Finally, if length of oxygen exposure dictates the extent of preservation, then the Mexican margin should have a greater degree of OC preservation. Sediment samples were taken along transects that ran perpendi- cular to the shore from depths of 100–1,000 m. Organic carbon contents in Washington margin sediments were 2.0 wt%, whereas those on the Mexican margin were as high as 12.0 wt% (Fig. 1b). This difference alone suggests the strongly oxygen-deficient zone off the coast of Mexico increases OC preservation. Mexican slope sediments, which had the lowest overlying water oxygen concentra- tions, had higher OC burial rates and lower rates of OC oxidation than did inshore shelf sediments (Table 1). Washington slope sediments had lower rates of both OC oxidation and burial than did Washington shelf sediments. Although OC burial rates for the Washington and Mexican margins were roughly similar, OC oxida- tion rates on the Washington margin were up to a factor of five higher than for the Mexican margin. Organic-carbon burial efficiency has been used as an indicator of the extent of OC preservation in sediments 10,12,16,17 . We define burial efficiency as the burial rate of OC below 15 cm, expressed as a Figure 1 Water column O 2 and sediment carbon contents for study sites from the Washington and Mexican margins. a, Dissolved O 2 concentration as a function of water depth for the continental margins of Washington State (squares) and northwestern Mexico (circles). Note that the Mexican O 2 concentrations between 150 and 600 m depth are indistinguishable from zero. b, Weight per cent organic carbon as a function of depth in sediments for representative stations from the Washington (empty squares, 630 m; filled squares, 120 m) and Mexican continental margins (empty circles, 620 m; filled circles,150 m).