CO Z':TRIB UTIONS FROM THE CUSHMAN FOUN DATION FOR FORAMIN IFE!RAL RESEARCH 123 CONTRIBUTIONS FROM THE CUSHMAN FOUNDATION FOR FORAMINIFERAL RESEARCH VOLUM E XXI, PART 4, OCTOBER 1970 389. DISTRIBUTION AND ECOLOGY OF BENTHONIC FORAMINIFERA IN THE SEDIMENTS OF THE ANDAMAN SEA W. E. FRERICHS University of Wyoming, Laramie, Wyoming ABSTRACT the extreme southern and southwestern parts of the Fo raminiferal a..<Jtiemblages in sediments of the Andaman sea (text fig. 1). Sea characterize five fauna l provinces. each of w hich Is de fined by ecologic factors, S lightly euryhallne conditions Cores were split routinely in the laboratory, and and a rela tively coarse grained substrate chal'acteri ze the upper 5 cm of each was sampled for the faunal the delta-front faunal province. Extremely hig h ra tes of analyses. These core sections and representative sedimentation. euryhaline co ndition~. and clay substl'ate are typical of the Gulf o f Mal'taban province. Extre me ly fractions of the grab samples were dried and s low rates of sedimentation a nd a coarse-grained s ub­ weighed and then washed on a 250-mesh Tyler strate characterize the Mergul platform province. Normal screen (0.061 mm openings). salinities and average rates of sedimentation characterize the Andaman-N lcobar R idge faunal province, Sediments Samples used to determine the rel ative abun­ having a h igh organic content and Indicating active solu­ dance of species at the tops of the cores and in the tion of calcium carbonate occur in the basin fa unal PI'ovlnce. grab samples were split to a convenient size with a Foraminiferal numbe rs increase from less than 100 to modified Otto microsplitter. Frequency counts tlimost 10, 000 with Inc reasing depth to 1. 800 mete rs. Be­ low 1.800 mete rs solution of calcareous spec imens re'::IUIts were made on a representative split of the ben­ in g reatly reduced fOI'a miniferal numbe rs (less than 100), thonic foraminifers; insofar as practicable at least Fora m iniferal numbers in excess of a million w e l'e noted 200 specimens were counted. Benthonic species are for saml}les from the Mergu! platform; these refiect very reported as percentages of the total benthonic s low rates of deposition. Species numbers Increase direct, popUlation. ly with depth from less than 10 at 20 mete l's to more than 50 at uppe r ba thyal depths a nd then they decline s lightly Only those samples collected by the Pioneer, a or remain cons tant until solution of calcareous species at small minority of the samples studied, were stained. depths greater than 1. 800 meters results in lower num­ Consequently, no studies were made of the distri­ bers. Radiolarian numbers increase with Increasing depth from less than 10 at the shelf edge to slightly less than bution of living foraminifers. 10 ,000 at middle and lower bathya l depths, The distribu­ Hydrographic data are available from the 1961 tion of radiolarian numbers In the central basin suggests that the rate of deposition is greatest In the weatern half and 1963 cruises of the U.S. Navy Oceanographic of the Andaman Sea. Ship Serrano and the 1964 cruise of the U.S. Coast and Geodetic Ship Pioneer. Sediment parameters INTRODUCTION were provided by K. S. Rodolfo, Allan Hancock This investigation consists of an analysis of the Foundation, who concurrently studied the marine areal and bathymetric distribution of benthonic geology of the Andaman Sea. foraminifers in surface sediments of the Andaman Recent foraminifers in the general region of the Sea. A comparable analysis of planktonic foramin­ Andaman Sea have been recorded by a few authors. ifers in the samples is in progress. The area is MiJlett (1898-1904) described foraminifers from bounded approximately by latitudes 16°N. and samples taken along a traverse extending from the 6°N. and longitudes 93 °E. and 98 °E. The northern north coast of Australia to the Malay Peninsula, boundary of the sea is Burma, the eastern boundary Cushman (1921) reported on the foraminifers of is the Malay Peninsula, the southern boundary is the Philippine Sea, and LeRoy (1938) studied for­ the island of Sumatra, and the western boundary is aminifers along a traverse across Peper Bay on the the Andaman-Nicobar Ridge (text fig. 1) . west coast of Java . Bel yaeva (1964) studied plank­ The sediment samples used in this study were col­ tonic foraminifers of the Indian Ocean and in­ lected during the 1961 and 1963 cruises of the cluded a few samples from the Bay of Bengal. The U.S. Navy Oceanographic Ship Serrano and the distribution and ecology of foraminifers on the 1964 cruise of the U.S. Coast and Geodetic Ship southeastern continental shelf of Asia has been Pioneer. These materials include 41 Phleger cores, reported by Haga (1964) for the Gulf of Thailand 39 Kullenberg cores, 9 two-inch piston cores, and and by Waller (1960) off the south China coast. 37 grab samples. The samples are from water In addition, the monographic studies of Recent depths of 13 to 3,778 meters (appendix A), and foraminifers in the Pacific Ocean (Cushman 1910- the bathymetric distribution generally is adequate. 17 ) and in the Atlantic Ocean (Cushman 1918-31) Areal distribution of the sediment samples is most­ have been invaluable general references. The tax­ ly adequate, although coverage is very scattered in onomic work by Barker (1960) on species of for- 124 FRERICH8-ANDAiUAN SEA BENTHONIC FORAMINIFERA 9 0 96" I BAY / I ( ANDAMAN I /1 BASIN OF \ I I ( ( )2: 7 ~~~~~~f o 40 80 \ ~ . 2 40 f 264 269 BENGAL . 2 37 • • 271 K ILOMETERS ) • e 235 . 246 o'ter K. 5. Rodolfo 249 · 250 · CONTOURS IN METERS 2~1 . I ( l> \ r 34 • . 33 » -< .46 . ,6 221 l 4. • "\ . 39 II" ~ \ 192 _ ~ . '91 ( e '90 ( 186 • • 219 · 220 \ ~ 18 0 197 <... · ) ~ . 6 7 ~ - 66 ) ~ ' 68 ( 69 • / 21 4 . ,,--,i 2OZ..,04 4. t9g •••203 . 21 2 • PCIS . 2: 9 •• 370 . 2361 . 920 LOCATION OF SAMPLES TEXT FIGURE 1 Stations in the Andaman Sea from which samples we re analyzed (after Rodolfo, 1967) . Water depths are shown by contour lines. The location, water depth, and source of the samples are tabulated in appendix A. CON TRI B U TI O~S f' RO.M THE CL·S HMA N FOUN DATION F OR FORAMI N IF ERAL RES E ARCH 125 aminifers illustrated in the Challenger Reports also , , has been belpful. BA Y AN OAMAN BAS LN OF ACKNOWLEDGEMENTS The present study, carried on largely at the University of Southern California, has been aided " sUbstantially by many individuals and organiza­ tions. Dr. O. L. Bandy of the University of South­ " ern California suggested tbe problem and provided " valued advice and counsel throughout the investi­ >­ gation; Drs. D. S. Gorsline and R. L. Zimmer, also -''" " -' of the University of Southern California, made >'" helpful suggestions and criticized the manuscript; o ,,' : ~ the National Science Foundation supported the in­ vestigation (GB 8628, GP 2530 and GA 730) ; the , , " U.S. Navy Hydographic Office made available -' ::> ' cores from the 1961 and 1963 cruises of the U.S. '" ' ", " >-'" , ..,'" Navy Oceanographic Ship Serrano; the U.S. Coast ..,z u " and Geodetic Survey provided cores from the 1964 " cruise of their Ship Piolleer; and the Allan Hancock Foundation furnished laboratory and other facili­ " ties. Guidance in revising the original manuscript .... .. was given by R. M. Jeffords of Esso Production ,. Research Company. " • ANDAMAN SEA GENERAL 1-:l",:---,:r":---:1,~::----:~c:,--::,T.,:--":r,~-:1::----:,C.."- -::,:o;;,,,,::-.L,, The Andaman Sea is located in what has been PHYSIOGRAPHIC PROVI NCES and still is one of the most tectonically active re­ TEXT FIGURE 2 gions of the world (Van Bemmelen, 1949 ; Jacob, Physiographic provinces recognized in the Anda­ 1954). Topographically the sea is a silled basin man Sea. with a sill depth of 1,800 meters. The basin is divisible into three distinct physiographic provinces are prevalent, and strong southwesterly flowing cur­ (text fig. 2): the eastern Mergui platform, which is rents are developed. During March to May the related genetically to the adjacent Malay Peninsula northeast monsoons begin to dissipate. South­ (Alexander, 1962); the Central Rift and Valley westerly currents remain prevalent, but easterly province, a region of north-trending linear basins flowing currents enter north of the Andaman and numerous seamounts; and the Andaman-Nico­ Islands and south of the Nicobar Islands. bar Ridge, which represents a eugeosynclinal com­ Surface salinities are related indirectly to the plex that has been traced northward into the monsoons. During the southwest monsoons, mois­ Arakan Range of Burma and southeastward ture-laden air moving inland results in extremely through Indonesia (Jacob, 1954). heavy rainfall on land areas drained by the O CEANOGRAPHY ANn SEDIMENTOLOGY Irrawaddy and Salween Rivers which empty into the In the Andaman Sea, as in the northern Indian head of the Gulf of Martaban at the northern end Ocean, surface currents reflect the monsoons which, of the sea (Von Arx, 1962). During the northeast therefore, have an important effect on the local monsoons, however, rainfall is at a minimum on oceanography. From June through August south­ the land drained by the Irrawaddy and Salween west monsoons are prevalent, and eastward-flowing Rivers, and the amount of fresh water introduced currents enter the sea across the Andaman-Nicobar at the northern end of the sea is greatly curtailed. Ridge and exit through the Malacca Strait (text fig. Salinities within the Andaman Sea (text fig. 3A) 3B). A small countercurrent is developed along reflect this monsoonal regime. From June to No­ the northern coast of Sumatra.