Pacific Science (1975), Vol. 29, No.1, p. 99-115 Printed in Great Britain On the Use of Benthic Foraminifera as Sediment Tracers in a Hawaiian BayI WILLIAM T. COULBOURN2 AND JOHANNA M. RESIG3 ABSTRACT: Populations of foraminifera were investigated in regard to the suitability of the various species as indicators of sand transport. Of 53 species recorded in the sediments of Kahana Bay, Oahu, 16 showed distribution patterns that give evidence of the direction of sand transport within the littoral cell. IN THE COURSE of a study to map and calculate eastern side ofthe bay; however, after unusually offshore sediment volume around Hawaii heavy rainfall a channel opens throughthecenter (Campbell et al. 1970), a detailed sampling of of the beach. Kahana Bay, Oahu, was undertaken for the Along the east and west shores of the bay, purpose of defining the sources of the sand as the submarine topography is bounded by well as paths of transport within a littoral cell. fringing reefs whose surfaces range from a little A cursory examination of the microfaunas of more than 10 feet below sea level to blocks qf the samples showed that foraminifera were coral that are exposed during low tides (Figure dominant and that the foraminiferal populations 2). The outer edges of the reef drop vertically varied specifically and numerically from sample 30 feet or more to a sandy area in the center of to sample. Since data relevant to the physical the bay that forms a channel sloping gently oceanographyandsedimentologyofthebaywere seaward. The channel sands are rippled uni­ available, analyses of the foraminiferal popu­ formly to depths of at least 40 feet. A small lations of 53 selected samples were made to patch reefis located in the northwestern corner determine whether a systematic variation exis­ of the channel. ted that could provide an additional means of determining the direction of sand transport. METHOD OF STUDY The behavior offoraminiferal tests as sediment particles has been investigated by Grabert Sediment samples and in situ water salinity (1971). and temperature measurements were taken in Kahana Bay, Stream, and Valley from Septem­ THE ENVIRONMENT ber 1970 to August 1971. Of the 136 surface sediment samples collected, only 42 (those with Kahana Bay, located on the northeast (wind­ the suffix F, Figure 3) were collected by hand ward) coast of Oahu, Hawaii (Figure 1), is a and treated with a solution of rose bengal and submarine extension of Kahana Valley. Abun­ ethyl alcohol to preserve the living protoplasm dant rainfall in the valley, more than 250 inches while staining it red. The remaining deep-water a year in the higher elevations (Mink, Lee, and samples were obtained with a van Veen grab Watson 1963), produces a discharge of 29.5 sampler and were not stained, because they mgd by Kahana Stream (Cox and Gordon were initially collected solely for sedimento­ 1970). The stream usually discharges at the logical analysis. Thus, the reported distribution of live specimens is only an approximation I Hawaii Institute of Geophysics contribution no. because some of the samples were not stained 637. Manuscript received 1 March 1974; revised and returned 21 June 1974. to detect those individuals, and because seasonal 2 University of Hawaii: Hawaii Institute of Geo­ variation was not sampled systematically. physics and Department of Geology and Geophysics, As an independent means of determining Honolulu, Hawaii 96822. paths of sediment transport, direct measure­ 3 University of Hawaii: Hawaii Institute of Geophy­ sics and Department of Geology and Geophysics, ments of sand movement were made through Honolulu, Hawaii 96822. a sand tracer experiment as described by 99 100 PACIFIC SCIENCE, Volume 29, January 1975 PACIFIC OCEAN i .------------------------------~~~~'>~ ~BI:TIKY:~~~:~~:::~~ tAT '.30.00.>00 . ""'. PACIFIC OCEAN TOP<lliflUMICCOfIlTOU'" AT 110. 400' o , ~t o t.:-.. 5 \." ':":~=S.'-A1l1l'tlOT ~~_.~. , SANO-e<JfTOWD ~I.$ • rnfI"EII~[L.S._UIA(OnIOM Q.DCMUlT$ANOlfJl"S r~'UT1JI'U'lII01lUSGSfCPo­ (;II........c_f70AHU,1"4EDITlOH .," ~ .IOO ...·lt$lII.....,IO'IlOM·12dCOf'lCulll) .-. , .- SHOULIII[ .. H~:~U:rull[s. lISGS I9ll'EClTICIl ~\._ ... ... S.-s. fIOIC)S. Mel UIIO ''''f(;I1[$ FIGURE 1. Location map for Kahana Bay (after Moberly 1968). Obdyke (1966). (See Coulbourn 1971 for full elsewhere (Coulbourn 1971), are summarized description of method of study.) here. The course taken by stream water entering Grain-size analyses were performed on the the bay is marked by reduced salinity and samples by the standard methods. Statistical temperature, and high turbidity relative to size-parameters were determined graphically water flowing off the flanking reef flats. At the according to the formulas presented by Folk stream mouth, the temperature and salinity (1968). Approximations of the weight percent gradients are large but vary in position depend­ of calcareous, organic, and terrigenous frac­ ing on the ebb and flow of the tide. Near shore, tions in each sample were made by dissolving clear and warmer water flowing off the eastern the calcareous fraction of 2- to 5-gram samples reef flat adjacent to Huilua fishpond displaces in hydrochloric acid, reweighing, oxidizing the the flow of the stream toward the western side organic fraction with hydrogen peroxide, and of the bay. Farther offshore, high coral blocks finally weighing the terrigenous residue. along the eastern reef margin prevent any Foraminifera wereconcentratedfrom selected significant flow of marine water into the chan­ samples through flotation in perchloroethylene. nel. Just to the southeast of the patch reef, About 300 individuals were then tallied from a line of floating debris trending northeast­ a split of the concentrate. Tests that were so southwest marks the convergence of water abraded as to render species identification flowing off the northern and western reefs and impossible were ignored. the mixture of stream and marine waters flow­ ing seaward in the channel. Sand-tracer experi­ ments performed at grids 1 and 2 (Figure 3) demonstrated a southeasterly sediment drift. OCEANOGRAPHY The seaward flow of water within the bay The results of the in situ measurements of results primarily from contributions offfringing salinity and temperature, reported in detail reefs flanking both sides of the bay and from ; ¢IJ$ lllil! !Ii ! @!iii¥¥$i5SW Benthic Foraminifera as Sediment Tracers-CouLBOURN AND RESIG 101 the stream discharge. Freshwater springs located along the base of the reef contribute volume to the flow, but have little effect on the environment, since they are rapidly mixed with the seaward flow. The overall flow pattern closely resembles that proposed by Inman, Gayman, and Cox (1963) for a littoral cell at --------- Kapaa, Kauai. SEDIMENTS Values' of mean grain size and percent by weight of the terrigenous fraction of each sample are given at the top of Table 1. Com­ parison of plots of these values for the 136 samples reveals that areas oflow mean ¢ values (larger grain sizes) are also characterized by high percentages of calcareous constituents. It is of particular interest that these areas are highly reflective in aerial photographs, irr"e-. spective of bathymetry. Conversely, samples with high mean ¢ values and with high percentages ofterrigenous material (determined by microscopic examination to be mostly grains of weathered basalt rock) are associated 3 with dark areas in aerial photographs, again ----- 2 irrespective ofbathymetry. Thus, where sample distribution was sparse, allowing several pos­ sible interpretations, the contours were biased to trend along the borders between light and dark areas. The interrelation of the properties of sedi­ ment grain size and composition provides an insight into sediment transport in the bay. The middle of the bay is characterized by finer grain size and high percentages of terrigenous material, suggesting that sediment introduced from Kahana Stream settles out from suspen­ sion in the area seaward of the surf zone. It would appear that calcareous, coarse-grained sand is being introduced into the channel from localized points along the edges of the fringing reefs. Substantiation of direction of transport came from the southeasterly movement of dyed sand monitored at two test stations, and o 300 from the distribution of the terrigenous com­ II !, , ! METERS ponent in the sand. Although the sand tracer experiment provided conclusive results, it was FIGURE 2. Bathymetry of Kahana Bay. costly and extremely time-consuming. Another approach, utilizing biological components as tracers, is proposed as a desirable alternative. 102 PACIFIC SCIENCE, Volume 29, January 1975 / ,/ / / e600-F / I e404-F /' / e306-F e401-F e1301-F ----- e1302-F REEF - /'---- e1300-F / ---- 302-F / -- e119 ­ GRID No.l-j+ -108 ..- ------...., ,---- ........ (- 'e700-F /-, REEF e130l-I~../ /REEF I i e1305-F 1201-Fj 701-~_/ e -114 / 157'52'00" 30l-F /_/ + 21'3400... GRIDN~.27 / 1304-F / 'I el06 I I el05 I e402-F _1306-F I e1102-F / e104/ / /- - ell00-F . / / I (/:, '~H ."0'" / ~ _I el00l-F --203 _ e1200- ~/ ~ e903-F 1l00 F 901-F 10OO-F e902-F _907-F e ~06-F 3~9 ~ e904_Fe905-F 90l- F e910-F e911-F KAHANA BAY OAHU,HAWAII .. SAMPLE LOCATION -l •.. = .. ,." :~i··~ ~. I- ,I• ~ . .. C 01.. ,- lIit, .... -,,~ ... ...! -;y. ." .J j1. l. '" , 0 1000 .- J I :''', .~I. .... •• 01,. -.ill•• l FEET ... ~ l .;11.. '" li ? ...t 300 l;" .\ 0 ,II;. J II I ;,.. &t.... I'. i ill. ~, /Il. METERS .j), J. '" .IL '" ., FIGURE 3. Location ofsample stations in Kahana Bay, Stream, and Valley, and of grids 1 and 2. A compilation of all foraminiferal species and significant representation of brackish recorded in the samples and grouped according water species. An anomalous sample, 1303 to the constraints of mean grain size of the (group lIb) is placed with group IIa on the substrate, weight percent terrigenous material, basis of its microfaunal population alone. and location is shown in Table 1. The samples Groups III and IV represent mixed environ­ of group I belong to an estuarine environment ments subject to varying input from terrigenous that strongly influences the microfaunal popu­ and reefsources.