doi:10.1016/S0016-7037(00)00263-1 Changes in the molecular-level characteristics of sinking marine particles with water column depth ELIZABETH C. MINOR, 1, *STUART G. WAKEHAM, 2 and CINDY LEE 3 1 Department of Chemistry and Biochemistry, Alfriend Chemistry Building, Old Dominion University, Norfolk, VA 23529, USA 2 Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, USA 3 Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794-5000, USA (Received October 30, 2002; revised 16 April 2003; accepted in revised form April 16, 2003) Abstract—Over the past decade, sinking particulate organic matter (POM) samples from depth profiles in the equatorial Pacific have been analyzed by multiple techniques to evaluate the organic matter preservation mechanisms most dominant in the oceanic water column. How the samples were analyzed strongly influenced which organic matter preservation scheme appeared to dominate. Bulk functional group analysis by solid-state 13 C-NMR showed that organic matter composition varied very little in light of the extreme degree of remineralization (98%) that occurred with water column depth. This indicates preservation by a physical mechanism, such as sorption to mineral grains or protection within a mineral aggregate. However, detailed lipid studies of the characterizable fraction showed that selective preservation was important, with lipid structure being correlated with preservation over depth. However, the characterizable fraction decreases greatly with depth. Therefore, in this paper, direct temperature-resolved mass spectrometry (DT-MS), was used to further characterize POM, with the assumption that this approach could “see” a substantial proportion of the “uncharacterized” organic matter. DT-MS, which provides compositional information at an intermediate level between the detailed wet chemical studies and one-dimensional solid-state C 13 -NMR, also indicates an intermediate view between the mechanistic extremes of selective preservation and physical protection. Copyright © 2003 Elsevier Ltd 1. INTRODUCTION The sinking of particulate organic matter (POM) is a major mechanism transporting organic carbon from the surface ocean to depth. Therefore, it sequesters carbon from exchange with the atmosphere and provides energy (in the form of organic carbon) to heterotrophic organisms living below the euphotic zone. Much of the organic matter within sinking particles is remineralized or converted to dissolved organic carbon during transport to depth so that the flux of POM reaching the seafloor is generally 1% of primary production in the euphotic zone (Wakeham and Lee, 1993). In addition to the very strong attenuation of organic matter flux with depth, there is also a shift in the composition of the remaining organic matter as determined by extraction and/or hydrolysis and chromato- graphic analyses of individual compounds (Wakeham et al., 1997a). When these techniques were applied to the sinking POM collected from depth profiles in the Equatorial Pacific (EqPac), most of the organic carbon (80%) in surface POM was identifiable at the molecular level as being in amino acids, sugars, or lipids. At depth ( 4000 m), the proportion of characterizable organic carbon decreased to only 24% of total organic carbon (Wakeham et al., 1997a). Changes in composition that lead to poor molecular charac- terization of POM with water-column depth could result from three possible scenarios: (1) Macromolecular organic material is created in the eu- photic zone, perhaps consisting of phytoplankton or bacterial cell wall components (e.g., Philp and Calvin, 1976; Hatcher et al., 1983; Largeau et al., 1984, 1986; Gelin et al., 1996) and/or complex compounds resulting from photochemical crosslink- ing reactions (Harvey et al., 1983; Kieber et al., 1997). This upper ocean-derived material is not susceptible to degradation and thus is “selectively” preserved as particles sink out of the euphotic zone into the interior of the ocean. (2) Unidentifiable and resistant POM is created de novo throughout the water column via physicochemical processes, such as melanoidin-type condensations (Maillard, 1917; Hedges, 1978), or by biologic processes, such as production of recalcitrant bacterial membrane material (De Leeuw and Largeau, 1993). The production of these resistant reaction products would lead to their accumulation in older, usually deeper, waters. This preferential accumulation coupled with “selective” remineralization of the more labile organic matter over time would cause a shift in POM composition toward an increasing proportion of molecularly uncharacterizable compo- nents. (3) The molecularly uncharacterizable portion is preserved through bulk organic matter-mineral interaction or through encapsulation within refractory organic material. Potentially labile material is physically protected from attack by organisms or enzymes in situ (Knicker et al., 1996; Hedges et al., 2000, 2001) and from extractions and hydrolyses within a laboratory setting (Knicker et al., 2001). With such a protection mecha- nism, there is the potential for “nonselective preservation of organic matter” (Hedges et al., 2001), so that normally labile compounds would be protected to the extent that that there would be no discernible change in the composition of bulk POM as a function of depth in the water column. However, the encapsulated material may be a “selected” portion of the labile OM that is then “nonselectively” preserved. For example, nor- * Author to whom correspondence should be addressed (eminor@ odu.edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 67, No. 22, pp. 4277– 4288, 2003 Copyright © 2003 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/03 $30.00 + .00 4277