Characteristics, dynamics and significance of marine snow

The composition of any environment or object is determined by a particular balance between material transport processes and chemical reactions within and around it. In the case of marine sedimentary deposits, the dominant agents of mass transport are often large bottom-dwelling animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation. Such biogenic material transport has major direct and indirect effects on the composition of sediments and their overlying waters. In this chapter I review some of what is presently known about these effects, their implications for both chemical and biological properties of a deposit, and how they can be conceptualized in quantitative models.

The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water

Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-Atlantic-western Tethyan region. More than 1,000 closely spaced samples were analyzed during this study. At least seven major ^dgr13C excursions can be correlated from section to section. The most important "heavy events" occur near the Aptian-Albian and Cenomanian-Turonian boundaries, whereas "light events" are near the Jurassic-Cretaceous, Albian-Cenomanian, Turonian-Coniacian, and Cretaceous-Tertiary boundaries. The association of "events" with stage boundaries and the consistent correlation of "events" between stratigraphic sections provides a significant new tool for time-rock correlation independent of stan ard biostratigraphic techniques. The temporal association of these carbon isotope events with stage boundaries (faunal and floral events), global eustatic sea-level variations, and oceanic "anoxic events" demonstrates the potential usefulness of carbon isotope studies in interpreting variations in paleo-oceanic circulation. Furthermore, the association of carbon isotope variations with anoxic events is potentially useful for evaluation of the precise timing and the magnitude of preservation of organic matter in deep-sea and continental-margin sediments. Thus, isotopic studies may aid in estimating potential hydrocarbon resources in largely unexplored oceanic basins or along continental margins.

Carbon Isotope Fluctuations in Cretaceous Pelagic Limestones: Potential Stratigraphic and Petroleum Exploration Tool

Today, over 90% of all organic carbon burial in the ocean occurs in continental margin sediments1. 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 atmosphere2,3,4. Currently there is a volatile debate over which processes govern sedimentary organic carbon preservation5,6,7,8. In spite of numerous studies demonstrating empirical relationships between organic carbon burial and such factors as primary productivity9, the flux of organic carbon through the water column10, sedimentation rate11,12, organic carbon degradation rate13, and bottom-water oxygen concentration8,14, the mechanisms directly controlling sedimentary organic carbon preservation remain unclear. Furthermore, as organic carbon burial is the process that, along with pyrite burial15, balances O2 concentrations in the atmosphere, it is desirable that any mechanism proposed to control organic carbon preservation include a feedback buffering atmospheric oxygen concentrations over geological 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 correlated with the length of time accumulating particles are exposed to molecular oxygen in sediment pore waters. Oxygen exposure time effectively incorporates other proposed environmental variables8,9,10,11,12,13,14, and may exert a direct control on sedimentary organic carbon preservation and atmospheric oxygen concentrations.

Influence of oxygen exposure time on organic carbon preservation in continental margin sediments

Work of the last 10 years has demonstrated that oceanic particle size distribution by volume tends to be flat at mid-water depths (equivalent to a cumulative particle number distribution with a slope of −3) and is peaked in nepheloid layers with active resuspension and in surface waters with active biological production. The observed loss of fine peaks from the suspensions to yield flat distributions requires aggregation of the material, as the fines settle slowly. Mechanisms leading to particle collision are examined; for interactions between particles of similar size, Brownian motion dominates below 1.5 to 8 μm. However, if large particles (such as ‘marine snow’) are present at realistic concentrations, they become important in the removal of fine particles by shear-controlled coagulation. The coagulation times calculated for shear are too long for steady state to be presumed while the size distributions evolve under the influence of coagulation mechanisms. Therefore suggestions that the flat size distributions are quasi-stationary results of shear-controlled coagulation are rejected, and the notion that there is sub-equal production of particles at different points in the spectrum is favoured. Such production and the subsequent scavenging of small particles by large settling ones confers great importance on components of biological origin in both providing elements of the total size spectrum and determining the distribution and sedimentation of others of lithogenic origin. In surface waters, filtration rates by zooplankton indicate that aggregation rates of particles above submicron sizes are biologically determined.

Size spectra and aggregation of suspended particles in the deep ocean

The approach of this book to geochemistry can be summarized in the question: What happens, and how fast does it happen, when waters, solids, and gases interact in the earths surface environment? The environment of the earths surface is made of solids and fluids, and theinteractions among them are responsible for much of what is taking place in the physical world around us. The dissolved load of natural waters and the materials of which sediments are made are the products of reactions taking place practically everywhere on land, in the atmosphere, and in the hydrosphere. Thus the term water and sediment environments applies effectivelly to much of the surface environment of the earth, including the zone of up to a few kilometers above and below the land and ocean surface. Evolution present itself to us as a more or less complex variety of processes-geological, physical, chemical, and biological. To this end, the inclusive title Geochemical Processes was chosen for the book, to introduce a text that emphasizes processes and time-dependent phenomena.

Abraham Lerman

Papers

Geochemical Processes: Water and Sediment Environments