Where and What is Blenheim Reef?

Blenheim Reef is a coral atoll which now appears to be largely submerged but some parts are above water at low tide. It is situated in the northern part of the Chagos Archipelago approximately 11 nautical miles from the nearest land on Salomon Atoll.

From space the shallow extent of the Reef is apparent (Fig 2).

Is it an island or a submerged reef which breaks the surface at low tide?

Examining past evidence for the status of Blenheim Reef, the earliest report is from 1841, when Horsburgh described Blenheim Reef as “a large lagoon reef” where “the rocks are generally covered at high water, excepting some large blocks of coral and sand-stone on its eastern side”, and that “the rocks … which are always above water, are supposed by Captain Moresby to be the remains of the three low Sandy Islands next described”. Horsburgh’s then gives the co-ordinates for these ‘Sandy Islands’ as reported by mariners in 1749 and Lieutenant Blair of the East India Company in 1786 (Blair 1788) which place these slightly further away (18 nm) and to the south (5^o 17’ S) of Blenheim Reef (Horsburgh 1841) thus casting some doubt whether these are indeed the same features.

Darwin (1842) in his treatise on coral reefs described Blenheim Reef as “a water-washed annular reef, thirteen miles in circumference, surrounding a lagoon ten fathoms deep: on its surface there were a few worn patches of conglomerate coral-rock, of about the size of hovels; and these Captain Moresby considered as being, without doubt, the last remnants of islets; so that here an atoll has been converted into an atoll-formed reef”. Similarly, Stanley Gardiner who visited the Chagos with the Percy Sladen Expedition in 1905 noted that Blenheim Reef was “a similar atoll to Salomon, but without land” (Gardiner 1906).

Later scientific expeditions and scientists who have visited Blenheim include Drew (1980) who describes it as one of the “shallow, submerged banks”, and the most prolific of Chagos scientists, Charles Sheppard has consistently reported it to be entirely submerged at high tide (Sheppard and Sheppard 2019). From his first surveys of the reef with the Joint Service Chagos Expedition in 1978/9, Sheppard described it as “the completely awash atoll of Blenheim” and “Blenheim has no islands” (Sheppard 1980) whilst later acknowledging that it “dries at low tide” (Sheppard 2007). From his own historical researches he concluded that “When first seen in the late 1700s, Blenheim reef apparently had three vegetated islands on it; now it is entirely awash” (Sheppard 2012) and “Blenheim atoll in the north used to have three low sandy islands, with low bushes (report by Archibald Blair, 1787) but this atoll is now submerged, with only some sections of its western rim emergent at low tide” (Sheppard et al. 2012). Hamylton and Andréfouët (2013) come to a similar view “There is also an atoll whose islands disappeared and which became awash in the past 250 years (Blenheim)” although they did not visit the Reef themselves.

Surveys by the Chagos Science Expedition in 2014 (Koldewey et al. 2014) and 2015 (Turner et al. 2015) and the Global Reef Expedition in 2015 (Bruckner 2015) (https://www.livingoceansfoundation.org/wp-content/uploads/2021/02/Chagos-Archipelago-Final-Report.pdf) also consistently describe it as a submerged reef albeit with emergent parts at low tide.

The US Government Sailing Directions seem therefore to be erroneous “it covers nearly everywhere at HW [High Water] except at its S extremity” (US Government 2001,2013), as is the entry in Wikipedia (https://en.wikipedia.org/wiki/Blenheim_Reef) which states that it is “partly submerged” whilst attributing the atoll with 4 islands, one of which is said to be 200 x 70 metres in extent.

Thus whilst it seems fairly clear that Blenheim Reef in the 21^st century is now completely submerged at high tide, part of the stated purpose of a recent Mauritian visit The Guardian 13 Feb 2022 is to ascertain whether any part of the reef remains above water at all states of the tide in order to establish whether it is indeed only a ‘low tide elevation’ or whether it can be classified as an ‘island’ for the purposes of the UN Convention on the Law of the Sea.

REFERENCES

Blair A (1788) Remarks and Observations in a Survey of the Chagos Archipelago, by Lieutenant Archibald Blair, 1786 and 1787, Published fromthe MSS at the charge of the East India Company,by A. Dalrymple. 1788. George Bigg, London

Bruckner AW (2015) Global Reef Expedition: British Indian Ocean Territory. Field Report 19. Kahled bin Sultan Living Oceans Foundation, Annapolis, MD 36

Darwin C (1842) The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. Smith Elder & Co, London

Drew EA (1980) Seagrasses in the Chagos Archipelago. Aquat Bot 9:179-184

Gardiner JS (1906) The Indian Ocean. Being Results Largely Based on the Work of the Percy Sladen Expedition in H. M. S. "Sealark," Commander B. T. Somerville, 1905. The Geographical Journal 28:313-332

Hamylton S, Andréfouët S (2013) An Appraisal of the Extent and Geomorphological Diversity of the Coral Reefs of the United Kingdom Dependent Territories. In: Sheppard CRC (ed) Coral Reefs of the United Kingdom Overseas Territories. Springer Netherlands, pp1-11

Horsburgh J (1841) The India Directory, or, Directions for Sailing to and from the East Indies, China, Australia, and the interjacent ports of Africa and South America: compiled chiefly from original journals of the Honourable Company's ships, and from observations and remarks, resulting from the experience of twenty-one years in the navigation of those seas. Wm H Allen & Co, London

Koldewey H, Sheppard C, Turner J, Couch C, Fenner D, Widman E, Bailey J, Slayer J, Samoilys M, Morgan S, Curnick D, Sheppard A, Carr P (2014) Chagos Science Expedition Report: March 24th - April 15th 2014 47

Sheppard C (2012) Seawater inundations in the Chagos archipelago at high tides, and shoreline erosion.

Sheppard C, Sheppard A (2019) Chapter 11 - British Indian Ocean Territory (Chagos Archipelago). In: Sheppard C (ed) World Seas: an Environmental Evaluation (Second Edition). Academic Press, pp237-252

Sheppard CRC (1980) Coral fauna of Diego Garcia lagoon, following harbour construction. Mar Pollut Bull 11:227-230

Sheppard CRC (2007) Effects of the Tsunami in the Chagos Archipelago. Atoll Res Bull 544:135-148

Sheppard CRC, Ateweberhan M, Bowen BW, Carr P, Chen CA, Clubbe C, Craig MT, Ebinghaus R, Eble J, Fitzsimmons N, Gaither MR, Gan CH, Gollock M, Guzman N, Graham NAJ, Harris A, Jones R, Keshavmurthy S, Koldewey H, Lundin CG, Mortimer JA, Obura D, Pfeiffer M, Price ARG, Purkis S, Raines P, Readman JW, Riegl B, Rogers A, Schleyer M, Seaward MRD, Sheppard ALS, Tamelander J, Turner JR, Visram S, Vogler C, Vogt S, Wolschke H, Yang JM-C, Yang SY, Yesson C (2012) Reefs and islands of the Chagos Archipelago, Indian Ocean: why it is the world's largest no-take marine protected area. Aquat Conserv: Mar Freshwat Ecosyst 22:232-261

Turner J, Sheppard C, Sheppard A, Roche R, Couch C, Perry C, Murphy G, Head C, Graham N, Wilson S, Carr P (2015) Darwin Initiative Science and Conservation Expedition, Chagos Archipelago, 16 March to 14 April 2015 44

US Government (2001) Sector 8 - Islands in the West Indian Ocean - The Seychelles Islands to the Chagos Archipelago Sailing Directions Indian Ocean Pilot, pp99-107

US Government (2013) East Africa and the South Indian Ocean: Sailing Directions Pub 171. National Geospatial-Intelligence Agency, Springfield, Virginai, USA 241