Hemipelagic sediment is a distinct category of marine deposits, characterized by a unique blend of materials from both land and ocean biological activity. Their composition offers a detailed record of geological and environmental changes, helping scientists reconstruct Earth’s past conditions and processes.
Defining Hemipelagic Sediments
Hemipelagic sediments are marine deposits primarily composed of a mixture of terrigenous, or land-derived, material and biogenous material, which comes from the remains of marine organisms. These sediments accumulate in marine environments relatively close to continental landmasses, such as continental slopes, continental rises, and marginal basins, where contributions from both land runoff and marine biological productivity are substantial.
Land-derived sediments, such as fine-grained clays and silts, are carried to the ocean by rivers, wind, and even glaciers. Concurrently, the remains of marine organisms, particularly microscopic ones, settle from the water column to the seafloor.
Hemipelagic sediments are distinct from purely terrigenous sediments, which dominate nearshore environments, and purely pelagic sediments, which accumulate in the deep ocean and are predominantly biogenous or formed in situ. Pelagic sediments, for instance, are fine-grained and accumulate far from land, often consisting of microscopic shells or clay. In contrast, hemipelagic sediments maintain a balance between land-sourced and marine-sourced components, typically with biogenic material making up between 5% and 75% of the total volume.
The Makeup of Hemipelagic Deposits
Hemipelagic sediments are composed of diverse materials, reflecting their dual origin from both continental and marine sources. The terrigenous components include fine-grained clastic sediments like clays, silts, and fine sands. These materials, along with organic matter, are transported from continents to the ocean by river discharge, wind, and glacial activity. Minerals such as feldspar and quartz, derived from the lithosphere, are common terrigenous constituents.
Marine life contributes a substantial biogenous fraction to these deposits, mainly from microscopic organisms. Examples include calcareous oozes, formed from the calcium carbonate shells of organisms such as foraminifera and coccolithophores. Siliceous oozes, derived from the silica skeletons of diatoms and radiolarians, also form a significant part of the biogenous component. The relative proportions of calcareous and siliceous material vary depending on factors like marine productivity and water depth.
Other minor constituents can also be present in hemipelagic sediments. These include volcanogenic particles, such as volcanic ash, transported from volcanic eruptions on land or from underwater volcanic activity. Additionally, authigenic minerals, which are minerals that form directly within the sediment on the seafloor, can contribute to the overall composition.
Insights from Hemipelagic Research
Studying hemipelagic sediments offers insights into Earth’s past climate and oceanography. These layered deposits function as natural archives, preserving records of past climate, ocean circulation patterns, and continental erosion over geological timescales. Scientists analyze variations in sediment composition, grain size, and accumulation rates to infer historical environmental shifts. For example, changes in the ratio of terrigenous to biogenous material can indicate shifts in continental weathering or marine productivity.
Analysis of hemipelagic sediments can reveal evidence of past glacial-interglacial cycles, changes in river discharge, and even major paleoclimatic events like Heinrich Stadial 1 or the Younger Dryas. Examining oxygen isotopes provides information about ancient ocean temperatures and salinity. Changes in grain size and mineral composition can also reflect variations in ocean currents and the input of land-derived sediments.
They also contribute to understanding modern ocean processes. Their distribution and characteristics help scientists trace present-day ocean currents and sediment transport pathways. These deep-sea deposits provide habitats for various deep-sea organisms and play a role in nutrient cycling within the marine environment, connecting geological processes with biological systems.