L10 - Ocean Sediments

Overview of Earth's Oceanic Sediments

  • Journey to the Open Ocean: Understanding oceanic sediments is vital as they play critical roles in various biogeochemical cycles, particularly in the sequestering of carbon and nutrient cycling. Biogenic sediments, made from the remains of marine organisms, are essential for studying both present ecosystems and past environmental conditions.

  • Significance of Deep Sea Sediments:

    • Serve as major reservoirs of organic and inorganic carbon, influencing long-term carbon storage and stabilization, thus affecting global climate and carbon cycling.

    • These sediments significantly influence carbon distribution between the atmosphere and ocean, which has implications for global warming and climate change models.

    • Possess the potential to act as archives for environmental changes, including historical ocean circulation patterns, past temperatures, and nutrient dynamics, making them invaluable for paleoclimatology.

    • Their capacity to maintain a continuous, undisturbed stratigraphy is crucial for understanding geological history, contrasting with more dynamic environments like floodplains, which often undergo significant disturbance.

International Ocean Drilling Program

  • For decades, international ocean drilling platforms have been used to collect deep-sea sediment samples, allowing scientists to piece together the history of the Earth's climate and geological events. The continuous sediment record obtained from these drills is fundamental for understanding past environments, changes in sea level, and tectonic activity.

Types of Sediments in Deep Ocean

  • Classes of Sediments:

    • Terrigenous Sediments: Originating from terrestrial sources via rivers, wind, and glaciers, these sediments are primarily made up of siliciclastic minerals and are typically found near continental margins.

    • Pelagic Sediments: Composed of biogenic materials, these are categorized into two major types: silicious (produced mostly from diatoms and radiolarians which are silica-based) and calcareous (originating from marine organisms like foraminifera, which are calcium carbonate-based).

Key Distribution Features

  • Observable patterns of sediment distribution highlight notable features, such as the three primary bands of siliceous sediments seen across various oceanic regions, particularly prominent in the Pacific Ocean. Granular terrigenous sediments are predominantly found on continental shelves, influenced by river discharge and coastal erosion.

Sediment Accumulation Rates

  • Sediment accumulation rates vary significantly depending on proximity to land and the nature of the sediment. Typically, higher sedimentation rates are observed near river deltas and continental margins due to increased terrestrial input, whereas deep-sea areas showcase much lower rates of sediment accumulation, influenced by factors like water depth and sediment type.

Transport of Terrigenous Sediment

  • The mechanisms helping terrigenous material reach the deep ocean are multifaceted:

    • Estuarine and Deltaic Processes: These processes contribute significantly to sediment transport as rivers flow into the ocean, dispersing sediments into the open sea.

    • Turbidity Currents: These undercurrents enable sediment movement from continental shelves down to deep ocean basins, often leading to the creation of graded beds.

    • Glacial Processes: Icebergs act as mobile transporters, carrying sediments as they drift and subsequently melt in warmer waters.

    • Ocean Circulation: Fine clay particles are distributed throughout ocean currents, which can result in gradual settling on the ocean floor over extended periods.

Characteristics of Biogenic Pelagic Sediments

Calcareous Oozes

  • Calcareous oozes are primarily produced by:

    • Phytoplankton: Key contributors include coccolithophores, both critical for forming calcareous sediments in the oceanic environment.

    • Zooplankton: This group encompasses organisms such as pteropods and foraminifera also add valuable biogenic material to sediment layers.

  • An understanding of biostratigraphy derived from calcareous sediment records is crucial for reconstructing past marine environments and assessing changes in ocean chemistry.

Siliceous Oozes

  • Siliceous oozes are mainly produced through the activities of:

    • Phytoplankton: These algae, such as diatoms, utilize silica present in seawater, thriving particularly in nutrient-rich waters, thus contributing significantly to the sediment record in these regions.

    • Zooplankton: Similarly, these microorganisms, including radiolaria, contribute silica-based structures, being abundant particularly in low latitude regions.

  • Dissolution and Preservation: Both the production of siliceous material and its preservation can vary significantly with depth and environmental conditions, thus affecting the overall sediment composition found at various oceanic locations.

Factors Influencing Production and Preservation of Silicates/Carbonates

  • Production: Highly influenced by nutrient availability, light penetration, and water temperature, leading organisms like diatoms to thrive in nutrient-rich waters, while calcareous organisms prefer warmer areas that offer optimal growth conditions.

  • Preservation:

    • Calcite Compensation Depth (CCD): This depth is crucial because it determines where carbonate sediments begin to dissolve rather than accumulate. Above this depth, carbonate sediments can accumulate; below this depth, dissolution dominates, resulting in the loss of carbonate material from the sediment record.

    • Aragonite Compensation Depth: Important to note, aragonite dissolves at shallower depths compared to calcite, influencing the sorting of calcium carbonate sediments and overall sediment characteristics found in different oceanic regions.

  • Equivalently, SiO2 is undersaturated everywhere in the ocean, and opal easily dissolves especially in warmer waters. The solubility of biogenic silica decreases with temperature, so only 1-10% of siliceous material is not dissolved, meaning supply rate must be high to accumulate ‘siliceous ooze’

  • 75% of organic matter which leaves the photic zone is decomposed and recycled within the upper 500-1000m of the water column, and only 1% of the organic matter actually reaches the sea floor.

Marine Snow and Organic Matter

  • Marine Snow: This consists of organic materials like fecal pellets and detritus that sink from the upper ocean to the seafloor, playing a critical role in the transfer of carbon and nutrients to deeper waters.

  • These faecal pellets come from consumers and are consumed by other organisms which recycles the nutrients and carbon within the system. However an advantage of faeces in preserving the carbon is that it packages the fine grained material together, reducing time spent sinking and allows sediment to reach the sea floor.

  • Blooms of phytoplankton can lead to extremely rapid accumulation of marine snow, even at depths of over 4000m

  • An astonishingly small percentage (approximately 1%) of organic carbon produced in the surface ocean manages to reach the seafloor due to significant recycling processes enacted by marine organisms, indicating the efficiency of biological systems in nutrient cycling.

Unique Sedimentary Deposits

  • Iron-Manganese Nodules: These nodules form directly from seawater and are significant due to their role in climate studies and the fact they contain various trace metals, which are important for understanding oceanic chemistry. They are found in areas of very low sedimentation rate, and grow extremely slowly for extended (Myr) periods of time

  • Contourites: These deposits, formed through the action of deep-sea currents, often contain mixed sediment compositions, offering insights into past current dynamics and sediment transport processes. These sediments can be terrigeonous, calcareous or volcanoclastic in origin

Paleoclimatology and Rock Records

  • The preservation of sediments can provide critical insights into past ocean chemistry, as biogenic materials transform through diagenesis into sedimentary rocks such as limestone and chert.

  • Over time, opaline silica (amorphous form of silica) turns into chert as it is unstable.

  • Techniques analyzing foraminifera and other microorganisms are integral for reconstructing historical ocean temperatures, ice sheet volumes, and past climate conditions.

Conclusion

  • Distribution of Oceanic Sediments: This distribution is a complex interplay between distance from land, biological productivity levels, and seawater chemistry, all of which fundamentally influence what sediment material is ultimately preserved on the ocean floor and how it can inform us about historical and current oceanographic and climatic conditions.