Can benthic foraminiferal morpho-groups be used as indicators of paleomonsoonal precipitation?

Estuarine, Coastal and Shelf Science (1992) 34, 533-542

Can Benthic Foraminiferal M o r p h o - G r o u p s be u s e d as I n d i c a t o r s o f Paleomonsoonal Precipitation?

Rajiv N i g a m , N e l o y K h a r e a n d D. V. B o r o l e National Institute of Oceonography, Dona Paula, Goa 403004 {India] Received 16 August 1991 and in revisedform 31 December 1991

Keywords: benthic foraminifera; paleomonsoons; turbulence; morpho-groups; shelf; river mouth micro-environment; Indian coast (west); technique A technique is proposed for the quick and easy assessment of paleomonsoonal precipitation through the study of morphological groups of foraminifera in a shallow water (20 m water depth) sediment core collected off Karwar, near Kali river mouth, central west coast of India. The percentage distribution of two morpho-groups (i.e. rounded-symmetrical and angular-asymmetrical morphogroups) showed considerable fluctuations which correlate well with the 7 years average rainfall over a period of 116 years of the catchment area of Kali river. The dominari.ce of rounded-symmetrical forms indicates a period of higher rainfall whereas angular-asymmetrical forms tend to increase in abundance during dry periods (poor monsoons). The inferences drawn shown high potential in generating proxy data for paleomonsoonal precipitation required to develop predictive climatic models.

Introduction T h e recent years have witnessed an increase in awareness of m a n towards the environment. And now economic planners are in quest of climatic models to make available the future trend of the climate. T o predict the future we need to understand the past with fine time resolution. T h e reliability of such models depends upon the length of the time data. T h e available reliable climatic data do not go beyond 100-125 years which, in m a n y cases is not sufficient for modelling purposes. H o w e v e r records of sufficient length are contained in coastal marine sediments. T h e s e areas, which have high sediment accumulation rate, retain fine resolution climatic signals. Moreover, benthic foraminifera, which are abundant in coastal regions and sensitive to the environmental conditions, are obvious candidates to be used as proxy indicators. An inmlense quantity of work to evaluate the marine environment and past climates has already been done on foraminifera1 species using different parameters. Such as, the isotopic composition o f the carbonate test (Curry et al., 1988; Pedersen et al., 1988; O p p o & Fairbanks, 1989), morphological characters like, size (Loubere et al., 1988), shape (Collins, 1989), chemical composition of the test (Boyle & Keigwin, 1985; Delany & Boyle, 1986; Izuka, 1988), n u m b e r of chambers (Malmgren, 1984), porosity (Gary et al., 1989), ecology of various species (Sejrup et al., 0272-7714/92/060533 + 10 803.00/0

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1981; Petterson & Lohman, 1982; Katz & Thunell, 1984), dimorphic ratios (Nigam, 1986, 1988; Nigam & Rao, 1987) and coiling direction of the species (Kalia & Chowdhury, 1983). Foraminiferal species may serve as excellent indicators, but when any species parameter is considered as a tool for climatic inferences the following factors cannot be ignored: (1) Species are not identified consistently by different authors; (2) Intraspecific variants (morpho-types) show different results (Williams et al., 1988), thus limiting the scope of isotopic studies; (3) Same species may show different responses in different geographic provinces (Bandy, 1960); (4) Species level identification often takes a considerable amount o f time. Such problems could be minimized if species level identification is avoided and a search made for a quicker and easier method to decipher paleoclimates. T h e study of morphological groups, based on external test morphology, may serve to illuminate the paleoclimatic record. Earlier work on foraminiferal test morphologies established their value in environmental assessment. Severin (1983) proposed that morpho-groups show marked changes with depth and Bernhard (1986) and Kaiho (1991) used morpho-groups to discriminate between oxygenated and anoxic conditions. On the other hand Corliss and co-workers have recognized morphological differences between epifaunal and infaunal foraminiferal assemblages in deep sea sediments and used them as paleoproductivity indicators (Corliss, 1985; Corliss & Chert, 1988; Corliss & Emerson, 1990. These studies indicate the sensitivity of external morphologies of foraminiferal forms to environmental conditions. T h e aim of the present study is to apply benthic foraminiferal morpho-groups analysis to unravel the variations in paleomonsoonal precipitation. Severin (1983) observed that sediment turbulence may govern the overall morphology and geometry of the epibenthic foraminiferal species. He further concluded that during periods of higher turbulence species tend to develop more symmetry. In our study area turbulence is linked with monsoonal precipitation. Sediment trap studies in Arabian sea (Nair et al., 1989) showed that increased wind speed, lithogenic flux and precipitation are synchronous during monsoon period (June-September). Moreover, in the coastal, shallow water area off Karwar, precipitation and wind speed both are high during monsoons (unpubl. data, meteorological observatory, Karwar). These factors create turbulence in coastal waters. In this study we particularly considered fresh water runoff from rivers as an indicator of monsoonal precipitation. This can be best studied from the shelf areas adjoining river mouth. T h e aerial extent of river mouth micro-environments on the shelf depends upon the amount o f fresh water discharge which is proportional to the intensity of monsoons. Variations in the monsoons during the last few hundred years could be deduced by examination of fluctuations of the river mouth micro-environment through the study of sediment cores taken from the adjoining continental shelf using foraminiferal morpho-groups as a tool. T h e resulting pattern can be correlated with the available rainfall data of the catchment area of the river. This concept is tested using foraminiferal data obtained from a core taken near the mouth o f Kali river, central west coast of India.

P h y s i o g r a p h i c setting o f the study area T h e continental shelf off Karwar, a monsoon dominated one, is bordered by the coastal mountain range known as western ghats comprising gneisses and schists. T h e climate is typically tropical and the western ghats play a prominent role in determining it. T h e

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southwest monsoon ( J u n e - S e p t e m b e r ) sheds m u c h of its moisture in the region west of the ghats. T h e seasonal heavy precipitation is carried into the Arabian sea through numerous streams and rivers in addition to the land runoff. T h e major discharge in this region is through Kali river whose approximate length and annual average discharge are 68kin and 2 0 7 m 3 s - I respectively. T h e 2~°pb method estimated the sediment accumulation rate in this region as 2-6 m m y e a r - 1for the top portion of the core (Nigam & Nair, 1989).

Materials and methods A 1-16 m long box core (3713) was collected during the 156th cruise o f the R . 17. Gaveshani from a water depth of 20 m. T h e sampling site was located in the inner shelf region off Karwar, near the m o u t h of the Kali river (latitude 14°53. I ' N and longitude 73°57.9'E) (Figure 1). T h i s site was selected because Kali river is by far the largest river in the central west coast of India (Rao, 1979). T h e core was sub-sampled at 2 cm intervals. T h e topmost (0-2 cm) sample was too loose to be utilized for analysis and therefore, was excluded. We analysed the top 13 samples (up to 28 cm core depth) only. T h e selection of these samples

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TABLE1. Showing list of genera clubed into morphological groups Rounded symmetrical morho-group Ammonia soberina Ammonia tepida Asterorotalia sp.

Cavarotalia annectens (Parker and Jones) Elpln'dium sp. Florilus sp.

Angular-asymmetrical rnorpho-group Ammobaculites sp. Bolivina sp. Bulimina exilis Bulimina marginata Reusella sp. Virgulinella sp.

Nonion sp. Nonionella sp. Pararotalia sp. Pseudoeponidesequatoriana Trochammina sp.

is made because the rainfall data for the meteorological subdivision no. 31 (the catchment area of Kali river) is available for the last 116 years only. All samples were washed through a 63 ~tm sieve and oven dried. About 300-500 foraminifera were picked from weighed sediment samples. T h e 7 years average rainfall data for the last 116 years of the meteorological subdivision no. 31 is calculated from the earlier published record (Parthasarathy et al., 1987). A seven year average is considered because in each sample 2 cm interval covers approximately this period (based on 21°pb method).

Results

T h e most abundant foraminiferal species could be placed broadly into two distinct morphological groups, following the criteria of Severin (1983). On the basis of external test morphology, these morpho-groups are termed angular-asymmetrical and roundedsymmetrical irrespective of the taxonomic level (Table 1; Plate 1). A ngular-asymmetrical morpho-group This group incorporates elongated flattened forms which are oval to compressed in apertural view and have either parallel or subparallel sides, straight cylindrical forms which are rounded in apertural view and have generally parallel sides in side view, and tapered forms which are either rounded or angular in apertural view and are tapered throughout their length in side view. Rounded-symmetrical morpho-group This group includes trochospiral forms which have both a flattened and a more rounded side when viewed in profile (the flat side may be convex to some degree) and also those trochospiral and planispiral forms which do not have readily distinguished spiral and umblical sides in apertural view (Plate 1). Examination of the distribution pattern of the morpho-groups in the core revealed that rounded-symmetrical morpho-group varied from 81-80 to 99.67 % of the total foraminiferal assemblage, whereas angular-asymmetrical morpho-group fluctuated between

Benthic f oraminif eral as indicators

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Plate I. Showing representative species of different morpho-groups. Angular asymme-

trical morpho-group: (a) Bolivina striatula (X170); (b) Bulimina marginata (X160); (c) Bolivina persiensis (X150); (j) Virgulinella gunteri (X140); (k) Reusella sp. (X150); (1) Ammobaculites sp. (XI70). Rounded symmetrical morpho-group: (d) C. annectens (Parker & Jones) (X 110); (e) Asterorotalia dentata (X 180); (f) Pseudoeponides equatoriana (X100); (g) Nonion bouanum (X100); (h) Elphidium indicum (X140); (i) Nonion elongatum (X160).

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0.29 and 17-83%. T h e rainfall shows positive relationship with rounded-symmetrical forms and negative relationship with angular-asymmetrical forms (Figures 2, 3). Discussion

In coastal shallow water environment fluctuations in the intensity of turbulence is mainly ascribed to wind velocity, lithogenous flux and suspended load through river discharge. Detailed studies carried out by Naik and Neelakantan (1989) and Naik et al. (1990) suggested that the coastal region off Karwar had higher suspended load (1-70 to 2-06grn1-1) during monsoon period compared to non-monsoon periods (0-120.26 gm 1-1). Gazetteer (Record of Government of India) of Goa Daman and Diu (1979) also reports that monsoon periods are associated with strong winds. Moreover, in the Karwar area, the minimum and maximum wind speed recorded during the southwest monsoon were 2-5 m s-1 and 4-4 m s-1 respectively whereas, wind speed in premonsoon and postmonsoon period varied from 0-5 to 2-4 m s -1 and 0-5 to 1-4 m s -1 respectively during October 1975 to October 1976 [National Institute of Oceanography report, 1976].

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Benthic f oraminiferal as indicators

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Figure 3. Showing relationship of averagerainfall with angular asymmetricalmorphogroup. T h e above facts suggest that stronger winds together with increased rainfall over river catchment areas result in a higher suspended load during monsoon period (Neelakantan et al., 1988). This is confirmed by Rivonker and Reddy (1990) who found a high value (0.8) of extinction coefficient ' K ' (a turbidity index) during monsoons in coastal areas near Karwar and attributed it to inland drainage brought by adjoining rivers. Therefore, monsoon periods apparently are associated with an increase in the turbulence which should favour the rounded-symmetrical forms of foraminifera and have an adverse effect over angular-asymmetrical forms. This suggests that foraminiferal morpho-groups show a correlation with the strength o f fresh water (river) discharge and thus reflect the monsoonal precipitation in the catchment area. Comparison o f the above mentioned relationship with the average rainfall data (in 7 years) in meteorological subdivision n u m b e r 31 (which is the catchment area of Kali river) for the past 116 years (1871-1986) reveals art apparent relationship. T h e comparison indicates that rounded and symmetrical morpho-group shows a direct relationship with more fresh water discharge (Figure 2) and thus good monsoons, while the abundance of the angular-asymmetrical morpho-group is inversely related (Figure 3).

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T h u s during periods of good rainfall, rounded symmetrical forms flourished whereas angular-asymmetrical forms became rare. This sort of relationship may be explained by the fact that in undisturbed sediment an individual can easily occupy a preferred microhabitat. On the other hand, in turbulent conditions, its position and orientation would be controlled largely by the motion of the sediment. Foraminifera with symmetrical tests are able to move more easily through the sediment and hence are able to regain their prefered microhabitat following disturbance of the sediment by turbulence (Severin, 1983). However, it is interesting to note that angular-asymmetrical morpho-group includes most of the infaunal morphologies (Corliss, 1985; Corliss & Emerson, 1990). Infaunal foraminiferal species are also associated with low oxygen conditions (Mackensen & Douglas, 1989; Corliss & Emerson, 1990). Bolivina and Bulimina (both are members of angular-asymmetrical morpho-group) are classic low oxygen taxa. This could provide an alternative explanation for fluctuations in the abundance of these species. Perhaps increased rainfall leads to increased oxygenation and hence a diminution in abundance of taxa such as Bolivina and Bulimina. Our speculation is in agreement with Neelakantan et al. (1988) who reported that dissolved oxygen was generally low during premonsoon and postmonsoon seasons and moderately high during monsoon period with the mean values of 4-02, 4.09 and 4.36 ml 1-~ respectively, possibly on account of the favourable conditions like low temperature and salinity for higher photosynthesis during monsoons periods (Annigeri, 1972).

Conclusions T h e present study carried out on a shallow water core provided a wealth of information on the sensitivity of foraminifera towards environmental changes and their role for the analysis of the marine environment and paleo monsoons. This study shows that besides other methods, frequency of distribution of the morphogroups may be used as an additional tool for quick, easy and preliminary estimates of paleoclimates due to: (1) Species taxonomy need not be determined, much more time can be saved in the sorting of the samples. (2) Morpho-groups are independent o f taxonomic differences that arises among the authors regarding synonymy. (3) Morpho-groups should not be sensitive to changes in the habitat of a single species. (4) No modern counterpart of such groups is required as in case of species. (5) Data handling is also simplified because number of morpho-groups is much more smaller than n u m b e r of species.

Acknowledgements This research was supported by the Department of Science and Technology Government of India, New Delhi vide their grant no. SP5/YS/M14/87. T h e authors are grateful to D r B. N. Desai, Director National Institute of Oceanography for his encouragement and permission for publication. Thanks are also due to Shri. R. R. Nair and D r N. H. Hashimi for their suggestions. We have greatly benefited from the constructive comments of D r A. J. Gooday, Institute of Oceanographic Sciences, Deacon Laboratory Wormley Godalming (U.K.) and would also like to thank him for reviewing this manuscript.

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