We apply multivariate statistics to explore the large data sets encountered from Fourier transform ion cyclotron resonance mass spectra of dissolved organic matter (DOM). Molecular formula assignments for the individual constituents of DOM are examined by hierarchal cluster analysis (HCA) and principal component analysis (PCA), to measure the relationships between numerous DOM samples. We compare two approaches: (1) using averages of elemental ratios and double bond equivalents calculated from the formulas, and (2) employing individual formulas and either their presence/absence or relative magnitude in each sample. With approach 2, PCA deciphers which of the thousands of formulas are significant to particular samples, and then a van Krevelen diagram highlights what types of compounds are molecular signatures to the samples. Our dual approach, especially approach 2, allows for complex data sets to be more easily interpreted, aiding in the characterization of DOM from various sources. By applying this methodology, clear trends can be delineated, trends that are not apparent from currently employed methods. Terrestrial DOM contains various lignin-derived compounds, tannins, and condensed aromatics. Marine DOM contains aliphatic compounds with heteroatom functionalities, as well as lignin-like molecules.

Continental margin systems are important contributors to global nutrient and carbon budgets. Effort is needed to quantify this contribution and how it will be modified under changing patterns of climate and land use. Coupled models will be used to provide projections of future states of continental margin systems. Thus, it is appropriate to consider the limitations that impede the development of realistic models. Here, we provide an overview of the current state of modeling carbon cycling on continental margins as well as the processes and issues that provide the next challenges to such models. Our overview is done within the context of a coupled circulation-biogeochemical model developed for the northeastern North American continental shelf region. Particular choices of forcing and initial fields and process parameterizations are used to illustrate the consequences for simulated distributions, as revealed by comparisons to observations using quantitative statistical metrics.

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Settling particles were collected from the Ligurian Sea in the northwestern Mediterranean Sea in May 2003 and separated by elutriation into different settling velocity classes (.230, 115-230, 58-115, and ,58 m d 21 ). Particles of the different classes were incubated for 5 d to study their biodegradability. Particulate opal content and organic compound composition (amino acids, pigments, lipids, and carbohydrates) were analyzed initially and at regular time intervals during the incubation period. Most particles (48-67% of total mass) sank at greater than 230 m d 21 and were dominated by large diatom-derived aggregates produced during the spring bloom period. The initial organic composition and the biological lability of these particles varied with settling velocity. The strong phytoplankton signal was visible in all settling velocity classes, while slower settling particles carried with them a greater zooplankton and bacterial signature. As the different class particles decomposed, their compositions changed and became more similar with time, with a dominance of compounds that suggests a more degraded state: the amino acids c-aminobutyric acid and b-alanine, the pigments pyropheophorbide and pheophytin, the deoxysugars fucose and rhamnose, and lipid metabolites (diglycerides and monoglycerides, alcohols, and free fatty acids). Biogenic opal in the particles dissolved faster in more degraded particles than in fresher particles, suggesting that loss of organic matter may expose opal to dissolution. The coupling of settling velocity and decomposition rate measurements shows quantitatively that slower settling particles are quickly degraded and AcknowledgmentsThis research was part of the MedFlux and PECHE (Production and Export of Carbon: Control by Heterotrophs at small temporal scale) programs and was supported by the U.S. National Science Foundation Chemical Oceanography Program (OCE-0136370, OCE-0136318, and OCE-0113687) and the French CNRS (Centre National de la Recherche Scientifique), respectively. Participation of B.M. was funded by ORFOIS (Origin and Fate of Biogetic Particle Fluxes in the ocean) (EVK2-CT2001-00100). We thank Michael Peterson, Lynn Abramson, Jenni Szlosek, Meaghan Askea, and Isabell Putnam for shipboard and laboratory help; David Hirschberg and Michael Peterson for CHN analysis; Claude Mante for help with statistical data treatment; and the captain and crew of the RV Seward Johnson II. We wish to acknowledge the associate editor and two anonymous reviewers for very helpful comments and suggestions on the manuscript. This is MedFlux contribution 7 and MSRC contribution 1318.