Recent benthic foraminifers from Tarut Bay, Arabian Gulf coast of Saudi Arabia

Recent bottom sediments from Tarut Bay were studied for their foraminiferal content. 13 of the 43 taxa comprise more than 90 percent of the total living and dead foraminifers. The bay sediments are characterized by a Miliolina-dominated assemblage, Rotaliina constitutes the next abundant suborder, with the Textulariina as a minor part of the fauna. Both living and dead populations exhibit low diversity (S=l–5) due to adverse hydrographic conditions in Tarut Bay. The foraminiferal species show significant variations between environments. The intertidal zone is dominated by Ammonia beccarii, Quinqueloculina spp., Elphidium, sp. aff. E. advena, Spirolina arietina and Peneroplis planatus. In the shallow subtidal zone (<3m), Ammonia beccarii becomes less important and other species of the intertidal zone show significant decreases in their concentrations. Trioculina spp. and Spiroloculina spp. become more abundant. The characteristic feature of the deeper subtidal zone (>3m) is the abundance of Eggerelloides scabra, which constitutes 18.5% of the total foraminifers. Quinqueloculina spp., is the only dominant intertidal taxa to be present as a dominant member of the deeper subtidal zone. Differences in hydrographic conditions and other ecological parameters are considered to be responsible for the species variation. The Quinqueloculina-abundant foraminiferal assemblage of Tarut Bay differs considerably from the foraminiferal assemblages of the southwest Gulf.


INTRODUCTION
The foraminifera of the southern and eastern parts of the Arabian Gu.lf were first investigated by Henson (1950), and subsequently by Houbolt (1957), Murray (196Sa, 1965b, 1966a, b and 1970a, Lutze et al. (1971) and Haake (1979). Further cursory notes about the foraminifers have also been included in the discussions on recent carbonate sedimentation in the Gulf. However the Saudi Arabian c o a s t l i n e is still "terra incognita" a s f a r a s its foraminiferal f a u n a is c o n c e r n e d . T h e only work on the northwestern Gulf is that of Anber (1974), who analysed the foraminiferal content of the sediments off Kuwait. This study was undertaken to complete the picture of the distribution of the foraminifers of the western part of the Arabian Gulf, and to compare these with those of other parts of the Gulf and the Red Sea.

STUDY AREA
Tarut Bay is located in the central part of the Gulf coast of Saudi Arabia. It represents a transitional region between the open northern coastline which trends roughly NW-SE and is more exposed to the waves generated by the prevailing northerly winds, and the southern coastline trending almost N-S and lying nearly parallel to the direction of the prevailing winds. Tarut Bay is a sheltered shallow, warm, hypersaline environment. It covers an area of about 440km2, including the small Tarut Island. Most of the bay is shallow with depths of less than 4m. However, the depth increases towards the eastern part where it reaches depths of 13 metres (Fig.1). Tarut Bay is surrounded by arid regions with high summer temperatures of up to 5OoC and with very low rainfall. T h u s the bay receives no significant terrigenous contribution of sediments. The bottom sediments are to a great extent locally produced skeletal and non-skeletal carbonates.
Three sub-environments were identified in Tarut Bay based on the nature of the substrate, vegetation and depth. The intertidal zone is made up of a soft greyish limestone and is generally covered by a veneer of coarse calcareous sediments. The high energy shallow subtidal zone (3m) is covered mostly with grey sandy sediments. The deeper subtidal zone (>3m) is characterized by relatively finer sediments. Seagrasses occur in irregular patches in both shallow and deeper subtidal zones.

HYDROGRAPHY
Tarut Bay experiences a diurnal tidal cycle with a range of about 2 metres. The bay is slightly influenced by the northerly winds prevailing in the Gulf. During the study period, the water temperatures in the bay varied from 233°C to 26.6"C without any significant difference between the bottom and surface waters. But Basson et al. (1976) report that the surface temperatures in the Saudi Arabian coastal waters range from 10°C in winter to 35°C in the summer. Probably the same temperature conditions exist in Tarut Bay. The salinity of the bay waters is higher than that of the open Gulf waters with values ranging from 45%0 to 75%0 (average ( 5 2 %~~) .
Maximum salinity was recorded in the inner parts of the northern region (Safwa Bay). In general, the western and northwestern parts of the bay have relatively higher salinities than the open bay waters in the east.

MATERIALS AND METHODS
During three field trips, spread over 6 months, a total of I2 I sediment samples were collected from Tarut Bay (Fig.2) using a tray of dimensions 10 x 10 x 12cm and a van Veen grab (22Scm). Out o f the total. 65 samples were recovered from the bottom with bare sediment and the rest from the areas covered with vegetation. At each location depth was measured by a weighted line subdivided into metres. lmmediately after the collection. the sediment samples around Tarut Island were preserved in 10% neutralized Formalin and the others in 95% alcohol.
For the foraminifera1 analysis. a representative sediment sample was washed over 10 and 200 mesh sieves to remove both crave1 and mud. The remaining sand-sized fraction was stained with Rose Bengal (Walton, 1952) to identify the living foraminifers and after one hour washed again over 200 mesh to remove excess stain. Foraminifers were concentrated from the dried sediment by flotation technique using carbon tetrachloride.
As most of the sediments contained high concentrations of foraminifers, a representative sample of 250mg was spread over a standard picking tray and various taxi1 of both living and dead foraminifers were identified a n d counted. The numher of percentages of the suborders and of the recognized tasa werc calculated for each s a m p l e . T h e data were also used f o r determining the living/dead ratios (Walton. 1955) and dikersit) indices (Murray. 1968).

RESULTS AND DISCUSSION
'The living/dead (LJD) ratios o f the foraminifers exhibit u idc variation from 8% to 100%. The ratios are randomly distributed in the bay and show no clear relationship with either thc nature 0 1 the substrate or water depth. Sediments from the seagrcisx bed\ have higher 215 well a h lower L/D ratios.
The random distrihution of the living and dend populationx in the bay suggests that prevailing hydrographic conditions 01-the rate of sedimentation are not controlliny the L/D ratios. F'erhaps Recent benthic foraminifers from Tarut Bay. Arabian Gulf coast of Saudi Arabia the organic matter and oxygen contents in the sediments play a 4ignificant role (Said, 1950: Bader, 1952, Even although these parameters were not measured, the generally lower IJD ratios in the organic-rich muddy sediments and high L/D ratios in the welloxygenated coarse sediments indicate the influence of these factors in determining the L/D populations. The diversity of the foraminifers in Tarut Bay as represented by the Fisher index (Fig.3) is generally low, varying between 1 and 5. These values are slightly lower than those (1-7.5) reported b y Murray ( I Y70b) in the A r a b i a n G u l f s e d i m e n t s , a n d remarkably lower than those ( 1 -13) recorded in Jeddah Bay (Elahafzallah, 1979). In Tarut Bay, higher diversity values are mostly confined to areas covered with seagrass beds and coarse c a l c a r e o u s sands. Even although there is no s y s t e m a t i c distribution trend in the diversity of the living and dead foraminifers in the sub-environments, the average number of Miliolina dominate the living foraminifers with 54.4% followed by Rotaliina (39.3%) and Textulariina (6.3%). An almost similar distribution of the suborders is also noticed in the dead population, except for a considerable drop in the Textulariina content (3.1%). The abundances of the three suborders in the living and dead foraminifers of the Tarut Bay sediments are shown in Fig.4. In more than 85% of the sediments, Rotaliina and Miliolina constitute 90% of the total foraminifers. Some of the s h a l l o w and d e e p subtidal s e d i m e n t s are enriched with Textulariina. This is particularly more significant in the living population than it is in the dead ones.
Miliolina dominant foraminiferal assemblages are typical of both hypersaline and normal marine tropical and subtropical shallow-water environments (Murray, 1970b, Brasier, 1 Y75b, Bahafzallah, 1979. Therefore Miliolina dominance by itself cannot be taken to indicate hypersaline conditions. The abundance of Miliolina i n Tarut Bay can be attributed to the physicochemical conditions, which favour the precipitation of unoriented calcite crystals for Miolina tests rather than oriented crystals for Rotalina tests (Greiner, 1969). The bay is shallow with high were considered to be responsible for the low diversity of the loraminifers in the Abu Dhabi region (Murray. 1970b).
The Tarut Bay sediments are characterized by high Miliolina foraminiferal assemblages typical of shallow water sub-tropical In the bottom sediin-ents of Tarut Bay. 43 taxa of forminifers h w e been recognized. However 1.3 of these taxa constitute 90% of the dead and 92% of the living populations. The relative abundances of the dominant taxa in the subenvironrnents of the h y are given in Table I .

P<.rrrr.op/is plutrtir.7 (Fitchel and Moll) aiid Petiei.op/i.t /wrtu.sirs
In the shalloa, subtidal zone. Qrriiiyrreloc~irliii~~ spp. dominate Attrniotriu hrc.c.ur-ii. There is a \ignificant decrease in the ahundance of E/phidiwtr sp. alf. E . udr,cnu and an increase in S,'~i,.o/~~(,ir/iiitr spp. and T/.i/~c.~r/iriu spp. compared to the intertidal Li.)ne. Another characteristic feature is the appearance of the foraminiferal species that are absent in the intertidal 7one. piriric/rrc,/oc,/r/i,ztr spp. dominate the total living foraminiferal population in the deeper subtidal zone. But Eg,qei.e//cr sc,uhrtr, Tt-ilocrrliiiu spp. and S /~i d~( , i~/ i~u spp.. which are either absent or Iciw in concentration in the intertidal zone and shallow subtidal Lone. form the next a b u n d a n t s p e c i e s . Further there is a reinarkable decrease in Animoiriu hccc~crr~ii. E/phidirrnr sp. aff. E . tr(/i.c,iictirrnr, L'jpii.o/iiitr ariatitla and PenPro/di.i p/utrutris i n the deeper subtidal zone. Although the percentages of individual species change to s o m e extent, the order of relative abundance of the dead foraminifers closely follows that of the living ones. Further. distribution patterns of the dead foraminiferal species in the subenvironments of Tarut Bay is almost identical to that of the living foraminifers described above.

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T h e varying ecological c o n d i t i o n s can be seen in the significant differences in the species assemblages in different parts of the Arabian Gulf. The southwestern Gulf is dominated by peneroplids ( P eneroplis per-tusus and Spiro(inu urirfinu), which are common in Tarut Bay, but which disappear northwards towards Kuwait. These peneroplids have not been recorded from the Iranian side of the Gulf. Ammoniu h~c.urii. which is abundant in Tarut Bay, becomes rare in the Abu Dhabi region, southwestern Gulf. The hydrological variations, which brought about the observed variance in the species assemblage in the Gulf, might have created micro-environments in Tarut Bay. As a consequence the foraminiferal assemblage is not uniform but varies from intertidal to subtidal environments.