Great Calcite Belt
The Great Calcite Belt (GCB) refers to a region of the ocean where there are high concentrations of calcite, a mineral form of calcium carbonate. The belt extends over a large area of the Southern Ocean surrounding Antarctica. The calcite in the Great Calcite Belt is formed by tiny marine organisms called coccolithophores, which build their shells out of calcium carbonate. When these organisms die, their shells sink to the bottom of the ocean, and over time, they accumulate to form a thick layer of calcite sediment.
The Great Calcite Belt occurs in areas of the Southern ocean where the
The Great Calcite Belt plays a significant role regulating the global carbon cycle. Calcite is a form of carbon that is removed from the atmosphere and stored in the ocean, which helps to reduce the amount of carbon dioxide in the atmosphere and mitigate the effects of climate change. Recent studies suggest the belt sequesters something between 15 and 30 million tonnes of carbon per year.[1][2]
Scientists have further interest in the calcite sediments in the belt, which contain valuable information about past climate, ocean currents, ocean chemistry, and marine ecosystems. For example, variations in the CCD depth over time can indicate changes in the amount of carbon dioxide in the atmosphere and the ocean's ability to absorb it. The belt is also home to a diverse range of contemporary marine life, including
Overview
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Plankton |
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The Great Calcite Belt can be defined as an elevated particulate inorganic carbon (PIC) feature occurring alongside seasonally elevated chlorophyll a in austral spring and summer in the Southern Ocean.[4] It plays an important role in climate fluctuations,[5][6] accounting for over 60% of the Southern Ocean area (30–60° S).[7] The region between 30° and 50° S has the highest uptake of anthropogenic carbon dioxide (CO2) alongside the North Atlantic and North Pacific oceans.[8] Knowledge of the impact of interacting environmental influences on phytoplankton distribution in the Southern Ocean is limited. For example, more understanding is needed of how light and iron availability or temperature and pH interact to control phytoplankton biogeography.[9][10][11] Hence, if model parameterizations are to improve to provide accurate predictions of biogeochemical change, a multivariate understanding of the full suite of environmental drivers is required.[12][3]
The Southern Ocean has often been considered as a
Diatoms are present throughout the GCB, with the
The Great Calcite Belt spans the major Southern Ocean circumpolar fronts: the Subantarctic front, the polar front, the Southern Antarctic Circumpolar Current front, and occasionally the southern boundary of the Antarctic Circumpolar Current.[28][29][30] The subtropical front (at approximately 10 °C) acts as the northern boundary of the GCB and is associated with a sharp increase in PIC southwards.[7] These fronts divide distinct environmental and biogeochemical zones, making the GCB an ideal study area to examine controls on phytoplankton communities in the open ocean.[15][9] A high PIC concentration observed in the GCB (1 µmol PIC L−1) compared to the global average (0.2 µmol PIC L−1) and significant quantities of detached E. huxleyi coccoliths (in concentrations > 20,000 coccoliths mL−1)[7] both characterize the GCB. The GCB is clearly observed in satellite imagery [4] spanning from the Patagonian Shelf [31][32] across the Atlantic, Indian, and Pacific oceans and completing Antarctic circumnavigation via the Drake Passage.[3]
Coccolithophores versus the diatom
The
The ocean is changing at an unprecedented rate as a consequence of increasing anthropogenic CO2 emissions and related climate change. Changes in
Calcifying coccolithophores and silicifying diatoms are globally ubiquitous phytoplankton functional groups.[41][42] Diatoms are a major contributor to global phytoplankton biomass [43] and annual net primary production.[44] In comparison, coccolithophores contribute less to biomass [43] and to global NPP.[45][46][47][48][33]
However, coccolithophores are the major phytoplanktonic calcifier.
Top-down and bottom-up approaches
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Coccolithophore biomass is controlled by a combination of bottom-up (physical–biogeochemical environment) and top-down factors (
However, phytoplankton growth rates do not necessarily covary with biomass accumulation rates. Using satellite data from the North Atlantic, Behrenfeld stressed in 2014 the importance of simultaneously considering bottom-up and top-down factors when assessing seasonal phytoplankton biomass dynamics and the succession of different phytoplankton types owing to the spatially and temporally varying relative importance of the physical–biogeochemical and the biological environment.[66][33]
In the Southern Ocean, previous studies have shown zooplankton grazing to control total phytoplankton biomass,[67] phytoplankton community composition,[68] and ecosystem structure,[69][70] suggesting that top-down control might also be an important driver for the relative abundance of coccolithophores and diatoms. But the role of zooplankton grazing in current Earth system models is not well considered,[71][72] and the impact of different grazing formulations on phytoplankton biogeography and diversity is subject to ongoing research.[73][74][33]
The diagram on the left shows the spatial distribution of different types of marine sediments in the Southern Ocean. The greenish area south of the Polar Front shows the extension of the subpolar opal belt where sediments have a significant portion of silicous plankton frustules. Sediments near Antarctica mainly consist of glacial debris in any grain size eroded and delivered by the Antarctic Ice.[75][76]
See also
References
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