Sea sediments and climate change
Sea sediments, which accumulate on the ocean floor, are composed of materials from various sources, including weathered continental material, the remains of microscopic organisms, and minerals from hydrothermal vents. They are classified into three main types: terrigenous, biogenic, and metalliferous. Terrigenous sediments arise from land processes and are often transported by rivers and wind. Biogenic sediments, predominantly made of plankton remains, are significant in regions rich in nutrients, while metalliferous sediments are associated with mid-ocean ridges. The rate of sediment accumulation varies considerably, ranging from rapid deposition near continental margins to much slower accumulation on abyssal plains.
The study of sea sediments is crucial for understanding climate change, as their distribution and composition reflect historical environmental conditions. Sediment types can indicate shifts in climate; for instance, during colder periods, preservation of calcareous sediments increases due to lower sea levels and heightened carbonate ion concentrations in deep waters. Conversely, warmer climates tend to enhance terrigenous sediment deposition near coastlines. Additionally, isotopic analysis of these sediments can reveal changes in global temperatures and ocean chemistry, providing insights into past climate dynamics.
Subject Terms
Sea sediments and climate change
Definition
Sea sediments collect on the sea bottom, constituting the upper layer of the ocean floor. They consist of materials weathered from the continents, the remains of planktonic organisms, and minerals that are deposited by hydrothermal vents. Marine sediments are generally classified according to their source as terrigenous, metalliferous, or biogenic.
Terrigenous sediments are derived from the weathering of continental material, and are transported to the oceans by rivers, wind, and even glaciers. Terrigenous sediments are found most often near the continental margins, where rivers deposit much of their load. Very fine grained terrigenous sediments (clays) are transported to the central abyssal plains by wind and are the dominant sediment type under surface waters characterized by low levels of plankton productivity. Local accumulations of terrigenous sediments deposited by calved icebergs after they melt are known as “ice-rafted debris.” These terrigenous sediments are typically larger than are sediments transported by rivers or winds and accumulate near the polar regions.
Biogenic sediments, which dominate most regions of the seafloor, are the skeletal remains of microscopic plankton. Biogenic sediments are classified as either calcareous or siliceous depending on their chemical composition. Coccolithophores, foramaniferans, and pteropods produce calcareous skeletons composed of carbonate minerals, while diatoms and radiolarians produce siliceous skeletons composed of opaline silica.
Metalliferous sediments are found near the crests of mid-ocean ridges, where mixing of hot hydrothermal vent fluids with deep ocean waters causes the precipitation of metal (iron, zinc, copper) sulfide and oxide minerals. Sea sediments accumulate on the seafloor at very different rates. Accumulation of sediments near the continental margins can be as rapid as 1-10 centimeters every thousand years, while wind-blown terrigenous dust accumulates on the abyssal plains at rates of 1-2 millimeters per thousand years.
Significance for Climate Change
The distribution of marine sediments is a function of the location of the sediment source, the chemical reactions that occur in the water column and on the seafloor as the sediments accumulate, and the input rate of terrigenous sediments. Biogenic sediments are produced in surface waters in regions that contain sufficient concentrations of nutrients such as nitrogen, phosphorus, and silica. Production of siliceous plankton dominates in surface waters of the Southern Ocean, the Arctic Ocean, along continental margins where upwelling occurs, and along the Equatorial Divergence. Production of calcareous plankton dominates in the remaining regions.
When both calcareous and siliceous plankton settle through the water column, they are subject to chemical dissolution. Calcareous sediments are preserved where the ocean floor is generally less than approximately 4,000 meters deep and large inputs of terrigenous material are absent. Below 4,000 meters, the carbonate minerals that make up calcareous sediments are dissolved before they can be preserved, because there are low concentrations of dissolved carbonate ions in deep waters.
While the ocean is everywhere undersaturated with respect to silica, the rate of dissolution decreases with temperature, and thus with depth, in the ocean. Siliceous sediments are therefore preserved in deep regions of the ocean, under areas of high radiolarian or diatom productivity. The preservation of large amounts of terrigenous sediments in the deep abyssal plains, far removed from the continents, reflects the low levels of productivity found in the central gyres: There are no biogenic sediments produced in these waters to dilute the amount of terrigenous sediments delivered by the wind.
Changes in sea sediments preserved over geologic time reflect climate change in a variety of ways. For example, along a transect from a mid-ocean ridge toward a continental margin, the expected distribution of sediments will be as follows: metalferous sediments near the ridge crest, calcareous sediments, siliceous sediments as the ocean floor deepens below 4,000 meters, and then terrigenous clay. During times of colder climate, when sea level drops in response to glacier formation, a larger region of the ocean floor will be shallow enough to preserve calcareous sediments. When the climate is colder, the atmospheric concentration of decreases. This, in turn, leads to an increase in the concentration of carbonate ions in the deep ocean and a deepening of the depth at which calcareous sediments can be preserved. Thus, a wider swath of seafloor on either side of a mid-ocean ridge will preserve calcareous sediments.
Because siliceous oozes accumulate under highly productive surface waters in generally cool regions, colder climates can favor the expansion of areas on the seafloor where these sediments are found. Generally drier conditions during glacial times, moreover, contribute to an increase in the of terrigenous sediments to the deep abyssal plains. Warmer, wetter climates can be recorded in sea sediments by an increase in the deposition of terrigenous sediments near the continental margins. Finally, global temperature changes are recorded in the isotopic composition of oxygen preserved in calcareous or siliceous sediments, and of carbon found in calcareous sediments. Changes in the of the ocean—which reflect atmospheric concentrations of CO2 and temperature—are recorded in the isotopic composition of boron that is preserved in some biogenic sediments.
In 2022, marine sediment samples gave scientists an increasingly accurate view of how the Earth's climate has changed throughout much of its history, particularly the Late Pliocene and the Pliocene-Pleistocene transition. This information gives climate scientists a better perspective on how different parts of the world might react to the current incidence of global climate change, including allowing scientists to better predict what populations might be at the greatest risk for climate-related hardships. Analysis of the sediment samples showed that prior climate shifts were not globally synchronous. Instead, ocean temperature changes occurred earlier than predicted in some regions, while other regions were not at all impacted by growing ice sheets in the Northern Hemisphere. If modern climate change follows a similar pattern, climate scientists expect that global climate change may advance rapidly in some parts of the world, while other regions may experience few visible changes.
Bibliography
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