Environmental & Earth Science

St. Peter and St. Paul Rocks

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St. Peter and St. Paul Rocks. @ Associação Brasileira de Radioexpedicionários

St. Peter and St. Paul Rocks are a set of small rocky islands far out in the Atlantic Ocean between Brazil and the coast of West Africa. In particular, there are five islets, five very large rocks and a series of smaller skerries that comprise this formation. (Thompson. 1981) The rock type of this formation, also known as St. Peter and St. Paul Archipelago,  is ultramafic and not volcanic. The archipelago is claimed as a national territory of the nation of Brazil, who maintains a permanent manned station here.

caption Three dimensional view of St.Peter and Paul Rocks megamullion. @ A. Motaki

 

This is one of the few places on Earth where an underwater oceanic ridge breaks through the surface of the sea. This formation can be viewed as the second tallest deep sea mountain, also known as a deep-ocean-complex or megamullion. These isolated rocks function as a type of oasis in the deep ocean, providing a prime niche for marine life to prosper nearer the ocean’s surface. While the islands are virtually devoid of terrestrial vegetation, the rich marine flora and fauna provides a food source to seabirds that reside and breed here. Isolation from the mainland provides a habitat of significant ecological and biogeographical interest.

Location and general depiction

caption Source: Limongi/ Creative Commons More than 800 kilometers (km) from South America, St. Peter and St. Paul Rocks (0°56’N, 29°21’W) are a small group of islands in the equatorial Atlantic Ocean. The island group is 250 metres across, and their highest point is 19.5 m. The land area is estimated to be about 15,000 square metres. Composed of mylonitic peridotite, the submarine mountain of which these rocks are the pinnacles extends 4000 metres into the ocean depths. St. Peter and St. Paul Rocks were  visited by Charles Darwin in 1832 on the first Beagle expedition. Darwin noted that the Rocks appeared to have a pearly luster, even viewed from a distance. Upon close inspection of the rocks, he attributed the whitish sheen to a combination of bird guano and a surface mineralization he called "phosphate of lime".

The structure of the islets is a rugged serpentine surface, consisting of numerous fissures, pinnacles and ridgelines. The terrain is almost devoid of vegetation, except for mosses, algae, fungi and marine grasses.

caption St. Peter and St. Paul Archipelago. Periscope view from USS Triton. 1960. @ Commander Joseph Baynor Roberts, US Navy Off the islets the ocean depth takes a steep descent, especially to the north and south. The sea level attains a depth of three km at a distance from shore of approximately eight km. The sea bottom structure in the vicinity of the formation is very irregular as well as steep and rocky; silt bottoms do not occur to the north and south until attaining a depth of approximately three km.

 St. Peter and St. Paul Archipelago lies at the top of a 90 km long, 4000 m high complex in the Atlantic Ocean.  It is one of the few places where sea floor mantle rocks are visible above sea level.  The only source of freshwater on the St. Peter and St. Paul Rocks is from rainfall, accounting for the outcome of almost no vegetation. A lack of vascular vegetation is also attributed to the incessant sea-spray bombarding the islets and rocks. The meager terrestrial vegetation that does occur includes simplistic marine grasses, mosses, fungus and algae. In 1971, Smith et al. identified a filamentous blue-green algae as Lyngbya spp. and a minute green algae as Stichococcus bacillaris. (Smith et al. 1974)

Biodiversity aspects

caption Brown booby (Sula leucogaster), St. Peter and St. Paul Rocks. @ Associação Brasileira de Radioexpedicionários

While terrestrial flora is scarce, the isolated islands provide habitat for a rich benthic and littoral marine biota. This food source supports many seabirds, which are the only vertebrate wildlife found on the islands. Breeding seabirds found on the Rocks during the 1971 survey included Brown booby (Sula leucogaster), Brown noddy (Anous stolidus), and Black noddy (Anous minutus). All life-cycle stages of the booby were found during this survey, suggesting that their breeding was aseasonal. The bird eggs often fall prey to Sally Lightfoot Crabs (Grapsus grapsus), a marine invertebrate that is present in large numbers on the islets. The invertebrate element of the Rocks’ food chain primarily consists of microbial feeders. These include protozoa, nematodes (Acrobeloides, Diploscapter and Panagrolaimus genera), bdellodes rotifers, and certain mites (Scheloribates spp.).

Ecological status

Scientists and military personnel are virtually the only human visitors to these islands and pose very few direct threats. St. Peter and St. Paul Rocks are in the center of the migratory ranges for the seabirds found there. The entire archipelago became ecologically protected by Brazil in 1986.

Old accounts of visits to the island reported very large numbers of birds on the Rocks. For example, Darwin stated that he observed "a vast multitude of sea-fowl", and Moseley noted "birds hovering in thousands". These accounts are not consistent with more current numbers observed, and it is therefore suspected that the seabird population of St. Peter and St. Paul Rocks are declining. It is suspected that this is due to human disturbance or possibly to the gradual erosion of the Rocks into the sea. Excluding this possible decline, the ecology of St. Peter and St. Paul Rocks is relatively stable.

Type and severity of threats

Ecological threats are not significant for these isolated island rocks; however, if the world climate tends toward higher temperature, a possible subsequent increase in ocean levels could eventually inundate some lower portions of this rock ecoregion. Of considerable concern is the decreasing seabird populations and the lack of knowledge to identify the cause of this decline.

Justification of ecoregion delineation

St. Peter and St. Paul Rocks, far offshore in the equatorial Atlantic Ocean, represent one of the few places on Earth where an underwater oceanic ridge breaks through the surface of the sea. Isolation from the mainland also provides a habitat of significant ecological and biogeographic interest. The rocks function as an oasis in the deep ocean, providing a niche for marine life nearer the ocean’s surface. While the islands are virtually devoid of terrestrial vegetation, the rich marine flora and fauna provides a food source to the unique assemblage of seabirds that reside here. This islet assemblage is designated by the World Wildlife Fund as ecoregion NT1318.

References

  • Edward L. Beach. 1962. Around the world submerged: the voyage of the Triton. New York: Holt, Rinehart and Winston. ISBN 1-55750-215-3
  • E. Bonatti. 1990. Subcontinental mantle exposed in the Atlantic Ocean on St Peter-Paul islets. Nature, 345, 800-802.
  • Charles Darwin. 1839. Voyages of the Adventure and Beagle, Volume IIIJournal and remarks. 1832–1836. London: Henry Colburn Publisher
  • S. E. Sichel, Esperança, S., Motoki, A., Maia, M., Horan, M.F., Szatmari, P., Alves, E.C., Mello, S.L.M. 2008. Geophysical and geochemical evidence for cold upper mantle beneath the Equatorial Atlantic Ocean. Revista Brasileira de Geofísica, 26-1, 69-86.
  • H. N. Moseley. 1879. Notes by a naturalist on the "Challenger". Macmillan, London.
  • A. Motoki, S. E. Sichel, T. F. C. Campos, N. K. Srivastava, R. S. Soares. 2009. Taxa de soerguimento atual do Arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial. Revista Escola de Minas, 62-3. 331-342.
  • H. G. Smith, P. Hardy, I.M. Leith, V.W. Spaull, and E.L. Twelves. 1974. A biological survey of St. Paul's Rocks in the equatorial Atlantic Ocean. Biol. J. Linn. Soc. 6:89-96.
  • D. K. Smith and J. Escartin. 2006. Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge. Nature, v. 442 pp. 440-443
  • Geoffrey Thompson. 1981. St. Peter and St. Paul's Rocks (Equatorial Atlantic) and the Surrounding Sea Floor, Woods Hole, Woods Hole Oceanographic Institution (Technical Report) (Woods Hole Oceanog. Inst. Tech. Rept. WHOI-81 -98)
  • Willis L. Tressler. 1956. Rochedos São Pedro e São Paulo (St. Peter and St. Paul Rocks), Washington, U. S. Navy Hydrographic Office (Technical Report, TR-31).

 

Disclaimer: This article contains certain information that was originally published by the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added substantial new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

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Citation

Hogan, C., & Fund, W. (2014). St. Peter and St. Paul Rocks. Retrieved from http://www.eoearth.org/view/article/51cbeef27896bb431f69b5d6

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