Modeling the Vertical Flux of Organic Carbon in the Global Ocean.

The oceans play a fundamental role in the global carbon cycle, providing a sink for atmospheric carbon. Key to this role is the vertical transport of organic carbon from the surface to the deep ocean. This transport is a product of a diverse range of physical and biogeochemical processes that determine the formation and fate of this material, and in particular how much carbon is sequestered in the deep ocean. Models can be used to both diagnose biogeochemical processes and predict how the various processes will change in the future. Global biogeochemical models use simplified representations of food webs and processes but are converging on values for the export of organic carbon from the surface ocean. Other models concentrate on understanding specific processes and can be used to develop parameterizations for global models. Model development is continuing by adding representations and parameterizations of higher trophic levels and mesopelagic processes, and these are expected to improve model performance.

From Nano-Gels to Marine Snow: A Synthesis of Gel Formation Processes and Modeling Efforts Involved with Particle Flux in the Ocean.

Marine gels (nano-, micro-, macro-) and marine snow play important roles in regulating global and basin-scale ocean biogeochemical cycling. Exopolymeric substances (EPS) including transparent exopolymer particles (TEP) that form from nano-gel precursors are abundant materials in the ocean, accounting for an estimated 700 Gt of carbon in seawater. This supports local microbial communities that play a critical role in the cycling of carbon and other macro- and micro-elements in the ocean. Recent studies have furthered our understanding of the formation and properties of these materials, but the relationship between the microbial polymers released into the ocean and marine snow remains unclear. Recent studies suggest developing a (relatively) simple model that is tractable and related to the available data will enable us to step forward into new research by following marine snow formation under different conditions. In this review, we synthesize the chemical and physical processes. We emphasize where these connections may lead to a predictive, mechanistic understanding of the role of gels in marine snow formation and the biogeochemical functioning of the ocean.

Integrating marine oil snow and MOSSFA into oil spill response and damage assessment.

Marine snow formation and vertical transport are naturally occurring processes that carry organic matter from the surface to deeper waters, providing food and sequestering carbon. During the Deepwater Horizon well blowout, oil was incorporated with marine snow aggregates, triggering a Marine Oil Snow (MOS) Sedimentation and Flocculent Accumulation (MOSSFA) event, that transferred a significant percentage of the total released oil to the seafloor. An improved understanding of processes controlling MOS formation and MOSSFA events is necessary for evaluating their impacts on the fate of spilled oil. Numerical models and predictive tools capable of providing scientific support for oil spill planning, response, and Natural Resource Damage Assessment are being developed to provide information for weighing the ecological trade-offs of response options. Here we offer considerations for oil spill response and recovery when assessing the potential for a MOSSFA event and provide tools to enhance decision-making.

Terrestrial and marine perspectives on modeling organic matter degradation pathways.

Organic matter (OM) plays a major role in both terrestrial and oceanic biogeochemical cycles. The amount of carbon stored in these systems is far greater than that of carbon dioxide (CO2 ) in the atmosphere, and annual fluxes of CO2 from these pools to the atmosphere exceed those from fossil fuel combustion. Understanding the processes that determine the fate of detrital material is important for predicting the effects that climate change will have on feedbacks to the global carbon cycle. However, Earth System Models (ESMs) typically utilize very simple formulations of processes affecting the mineralization and storage of detrital OM. Recent changes in our view of the nature of this material and the factors controlling its transformation have yet to find their way into models. In this review, we highlight the current understanding of the role and cycling of detrital OM in terrestrial and marine systems and examine how this pool of material is represented in ESMs. We include a discussion of the different mineralization pathways available as organic matter moves from soils, through inland waters to coastal systems and ultimately into open ocean environments. We argue that there is strong commonality between aspects of OM transformation in both terrestrial and marine systems and that our respective scientific communities would benefit from closer collaboration.

Particle aggregation.

A basic problem in marine biogeochemistry is understanding material and elemental distributions and fluxes in the oceans, and a key part of this problem is understanding the processes that affect particulate material in the ocean. Aggregation of particulate material is a primary process because it alters the transport properties of particulate material and provides a mechanism for transferring material from the dissolved into the particulate pools. Aggregation theory not only provides a framework for understanding these processes, but it also provides a means for making predictions and has been successfully used to predict maximum particle concentrations in the oceans and the fate of diatom blooms (including those from iron fertilization), the size spectra of particles in the oceans, and the size distributions of trace metals. Here we review the basic theory involved, summarize recent developments, and explore unresolved issues.

The effect of inhomogeneous stickiness on polymer aggregation.

The aggregation of polymers is important in the formation of marine aggregates and the vertical transport of material in the ocean. A polymer may be inhomogeneous along its length, with associating groups at some points along its length where bonds are more likely to form. In this paper we investigate the effects of inhomogeneous 'stickiness' along the polymer length. We describe the results of three-dimensional off-lattice simulations of polymer-polymer aggregation for four different types of polymer: polymers which are sticky along their entire length, polymers which are sticky at the ends only and two types of polymer which are slightly sticky along their entire length. We examine the mean radius of gyration and the fractal dimension of the resulting aggregates and the dynamics of aggregation. The slightly sticky polymers and the polymers which are sticky only at the ends form aggregates with a higher fractal dimension than the polymers which are sticky along their entire length. However, the mean radius of gyration of the aggregates formed by polymers which are sticky only at the ends is significantly larger than that of the aggregates formed from slightly sticky polymers. The aggregation dynamics are also different for the polymers which are sticky only at the ends compared to the slightly sticky polymers. A single 'stickiness value' is therefore likely to be inadequate to describe a polymer. We also examine the effect of polymer rigidity; it seems that the effect of inhomogeneous stickiness is greater for almost-straight polymers than for coiled chains.

Modeling steady-state particle size spectra.

Fractal dimensions of marine aggregates are often estimated from the measured slopes of particle size spectra. One technique uses dimensional analysis to determine the dependence of the spectrum's slope with fractal dimension. In this paper, we use numerical simulations to examine the assumptions underlying the dimensional analysis approach to particle size spectra. We find that the slopes of numerically computed steady-state particle size spectra disagree with those predicted by dimensional analysis. The assumptions underlying the dimensional analysis approach that are responsible for the disagreement are as follows: only one coagulation mechanism operates in each particular particle size range, particle loss through sedimentation occurs at particle sizes larger than those for which differential sedimentation dominates, particles only interact with like-sized particles. Including disaggregation steepens the slope of the particle size spectrum for both large and small particles and changes the shape of the spectrum. These results indicate that one should exercise caution when using the measured slopes of particle size spectra to estimate aggregate fractal dimension.