Parsing the Oceanic Calcium Carbonate Cycle: a Net Atmospheric Carbon Dioxide Source, Or a Sink?

Parsing the Oceanic Calcium Carbonate Cycle: A Net Atmospheric Carbon Dioxide Source, or a Sink?

Stephen V. Smith

Typical environments and biota involved in oceanic CaCO3 production

Calcium carbonate in the ocean is produced in both shallow-water and deep-water environments. Most production in shallow environments is by benthic organisms, while deep-water production is dominated by planktonic organisms. The photographs presented here illustrate typical environments and biota; several of the specific environments are discussed in the text of the accompanying e-book.

The image shows estimated chlorophyll concentration along the northeast coast of Australia. The almost-white features ranging from about 50 to 300 km from the coast represent the Great Barrier Reef (GBR) itself. The GBR is composed of 3000 individual reefs and covering about 340,000 km2, stretching 2600 km along the northeastern coast of Australia (approximately 10° S, 144° E to 23°S, 153° E). The GBR is the largest reef structure on the planet and is considered to be the world’s largest structure made by living organisms. White areas immediately along the coast probably primarily represent chlorophyll from coastal plankton, although some fringing reefs also occur in that region. The white ring-shaped areas further offshore from the GBR represent oceanic reefs (atolls) of the Coral Sea. Dark gray areas are the Australian mainland as well as offshore islands.

Photo: Wikipedia.org

2 Lizard Island, towards the northern end of the Great Barrier Reef. (14.7°S, 145.5° E)

This is typical of coral reef fringing non-carbonate “high” islands within the Great Barrier Reef. Typically on coral reef flats, the dark areas represent hard substratum with benthic algae and/or corals as the dominant organisms covering the bottom, while the lighter colored areas are largely carbonate sediments. Lizard Island is the site of a research station operated by the Australian Museum since 1973.

3 Enewetak Atoll, Republic of the Marshall Islands. (11.5° N, 162.2° E)

A “typical” deep, open atoll, with numerous low islands formed from limestone detritus around the slightly submergent rim of the reef flats. Waves predominatly driven by the Northeast Trade winds drive a persistent flow of water across the eastern side of the atoll. There is deep pass on the southeast rim of the atoll and one wider and somewhat shallower passage on the south side, and a substantially shallower wide passage on the southwest side. Most active reef communities are the reef flats and oceanic fore-reef around the perimeter of the atoll. The fore-reef nominally extends to ~ 10 m although there are progressively deeper calcifying communities extending at least to the base of the photic zone. There are few coral pinnacles, or patch reefs, which reach the ocean surface within the lagoon. Most of the white areas seen aver the lagoon are clouds. Maximum lagoonal depth about 64 m, and mean depth is about 48 m. Lagoon area is about 1000 km2 (Atkinson et al. 1981). The patchwork squares within the lagoon are masks used for areas below detection by the satellite sensor.

4 Canton (or Kanton) Atoll, Phoenix Islands, Kiribati (2.8 °S, 171.7 °W).

Atolls such as this one, with an island around virtually the entire perimeter and a single pass (on the west side) exchanging lagoon water with open ocean water, are hydrographically simple systems which are useful for defining budgets of water, salt, and non-conservative materials (including TA, TCO2, nutrients). The community structure in this system varies from relatively rich coral reefs with high diversity of calcifying benthic organisms near the pass on the west side, to progressively more depauperate communities in the inner lagoon. When viewed on-site, the reticulated pattern seen through the lagoon illustrates a gradient from coral-dominated thickets to sediment-dominated ridges. The area of the atoll lagoon is about 50 km2, and the mean depth is 6 m (Smith and Henderson 1978).

5 Easter Group, Houtman Abrolhos Islands, Western Australia (28.4° S, 113.8° E).

The waters surrounding these islands include relatively unusual co-occurrences of tropical-water coral reef and temperate-water kelp communities, both of which include active calcifying organisms. (Smith 1981).

6 Underwater view of typical calcifying organisms on a coral reef: (Complex, diverse communities including (but not limited to) corals, coralline algae, mollusks, echinoderms, benthic foraminifera).

7 Halimeda, an important calcifying green algal genus common on coral reefs and tropical banks throughout the world ocean, and extending to depths exceeding 150m.

Photo: NOAA

8 The Bahama Banks (23.2° N, 78.0° W)

Dominated by the Great Bahama Bank is a large shoal area with fine-grained aragonite sediments. These deposits may be biogenic CaCO3 , or may be an inorganic precipitate; the matter has been hotly debated for many years(see Broecker and Takahashi 1966; Broecker et al. 2001, for a summary of this debate). These and other shoal-water CaCO3 environments in the Caribbean Sea have been studied for 100 years.

9 Shark Bay, Western Australia. (25.5° S, 113.5° E)

The bay covers an area of about 10,000 km2 and has an average depth of about 10 m. Extensive seagrass beds cover almost half the bay area, primarily in the northern portion of the bay. A wide variety of benthic calcifying organisms within these beds, deposits of pelecypod mollusks in areas such as Hamelin Pool (semi-enclosed southern end of the eastern arm of the bay), and abundant stromatolites (calcifying cyanobacteria) at the southern end of Hamelin Pool are found to the south of the seagrass beds. The water exchange time in this system is about a year, and both organic metabolism and CaCO3 production occur at very low rates (Smith and Atkinson 1983).

Photo: NASA

10 Oyster reef on Hunting Island, South Carolina, USA (32.4° N, 80.4° W).

Calcification on oyster reefs such as this one is dominated by the oysters (probably Crassostrea virginica), but include other less conspicuous calcifying organisms.

Photo: Wikipedia.org

11 Typical kelp bed calcifying community, Channel Islands, Central California

Kelp beds and other temperate climate hard-bottom communities such as this one are typified by a wide variety of calcifying organisms: coralline red algae, mollusks, echinoderms, barnacles, bryozoans, cold-water corals, etc. Near-vertical streamers of giant kelp can be seen in the background.

Photo: NOAA

12 The previous figures illustrate important aspects of benthic calcifying habitats, many of which have been discussed in the main text of this e-book. In addition to these figures, the following papers describe environments which are relatively difficult to represent well with photographs of calcifying organisms. They are not included in the main text of my manuscript, because their CaCO3 budgets have been estimated via biological (rather than geochemical) analyses.

1. Smith (1972) provides perhaps the most comprehensive analysis of benthic CaCO3 production in temperate shallow-water hard-bottom areas dominated by kelp and other macroalgae.

2. Lebrato et al. (2010) have estimated the contribution of various echinoderm communities to CaCO3 production and have extrapolated those estimates to the global ocean CaCO3 cycle, largely using methodology laid out by Smith (1972).

13 Coccolithophorid bloom in the Bering Sea, 1998. The light green water north of the Aleutian Islands represents a massive bloom of coccolithophorids algae, one of the major calcifying planktonic organisms found throughout much of the world oceans.

Photo: NASA

14 A single-celled planktonic calcifying coccolithophorid alga, Coccolithus pelagicus.

The individual plate-shaped segments are known as coccoliths. These fragments form extensive calcareous deposits through much of the world oceans.

15 Globigerina ooze (white, planktonic foraminifera), with one orange-colored elongated agglutinated benthic foraminifera. Gulf of Papua, in the northwestern portion of the Coral Sea: 10.6° S latitude, 146.4° E, 1668 m water depth. Along with coccolith deposits, planktonic foraminifera oozes are the dominating form of planktonic CaCO3 deposits.

Photo: Carlos Alvarez Zarikian

16 References

Atkinson, M., S. V. Smith, and E. D. Stroup. 1981. Circulation in Enewetak Atoll lagoon. Limnol. Oceanogr 26: 1074-1-83.

Broecker, W. S., C. Langdon, and T. Takahashi. 2001. Factors controlling the rate of CaCO3 precipitation on Great Bahama Bank. Glob. Biogeochem. Cycles. 15: 589-596.

Broecker, W. S., and T Takahashi. 1966. Calcium carbonate precipitation on the Bahama Banks. J. Geophys. Res. 71: 1575-1602.

Lebrato, M., D. Iglesias-Rodríguez, R. A. Feely, D. Greeley, D. O. B. Jones, N. Suarez-Bosche, R. S. Lampitt, J. E. Cartes, D. R. H. Green, and B. Alker. 2010. Global contribution of echinoderms to the marine carbon cycle: CaCO3 budget an benthic compartments. Ecol. Monogr. 80: 441-467.

Smith, S. V. 1972. Production of calcium carbonate on the mainland shelf of Southern California. Limnol. Oceanogr. 17: 28-41.

Smith, 1981. The Houtman Abrolhos Islands: carbon metabolism of coral reefs at high latitude. Limnol. Oceanogr. 26: 612-621.

Smith, S. V., and M. J. Atkinson. 1983. Mass balance of carbon and phosphorus in Shark Bay, Western Australia. Limnol. Oceanogr. 28: 625-639.

Smith, S. V., and R. S. Henderson. 1978. Phoenix Islands Report 1: An environmental survey of Canton Atoll Lagoon, 1973. Atoll Res. Bull. 221: 183 pp.