Geochemical Evidence for Palaeo-Environments of Algal Mats and Beat in Sri Lanka

V/Mn ratios with the organic carbon content and a C-29 sterol compound indicative of h~gher plants in a terrestrial environment have been used to demarcate the different palaeo*nvironments of algal mats and peat deposits of Sri Lanka. These chemical parameters were found to be especially use'ful in thedelineation of marine, tidal-flat, lagoonal and terrestrial environments of deposition.


Introduction
Algal mats and peat are now being studied for their metal distributions as they may provide useful clues to the formation of certain metal-bearing mineral deposits, even during Precambrian times.', 15. ' 6 The classic work of Renfrol* on the genesis of evaporite associated stratiform metalliferous deposits insabkha environments laid the foundation for further research.Both algal mats and peat, being organic materials, are the subject of studies on metal trapping properties by naturally occurring organic matter,.2.1 59 73 1.3, 14917 A fundamental necessity in all these studies is the reconstruction of the palaeoenvironment of the deposits and the conditions of deposition.It is theaim of this paper to make use of certain inorganic and organic constituents of the deposits to trace the source materials and the environments of deposition using two such deposits from Sri Lanka.

The Muthurajawela peat deposit
The Muthurajawela peat deposit(Figure I) is the largest in Sri Lanka, but up to the present, this deposit has not been utilized.The area studied consists of flat land perennially in a water-logged condition with the depth of water varying from place to place.Specialized vegetation characterized by brackish water plants is prevalent.The swamp area representing the southern extension of the Negombv !agoon, is bounded on the east by a belt of laterite(Figure 1)and on the south by a succession of laterite, peaty sand, and coastal sediments.Northward it is bounded by sandy peat and peaty sand.The peat deposit covers an area of approximately 21 km2; the average th~ckness being 3.7 m.The maximum thickness is in excess3 of 5 m.
Along the sea coast on the western slde of the swamp, a beach sand belt containing heavy minerals occurs.There are n o sharp contacts among the lithological variations, sand, peaty sand, and sandy peat passing gradually into each other.On the eastern side, peat is mixed with lateritic mud, and no obvious boundary exlsts.These intermediate areas are covered by paddy fields.

The algal mats of Mannar
These algal mats are located in the Mannar Island tidal flats off the northwest coast of Sri Lanka(Figure 2) :They are laminated blue-green algal mats formlng in the intertidal zone of a clastic (non-carbonate) tropical and lagoonal tidal flat.Gunatilaka6 studied these algal mats in detail and recognized three different types namely: (a) a smooth rounded-mat zone with discrete structures, (b) a crinkled and blistered zone, (c) a smooth flat-mat zone without any perceptible relief.
The mats were formed by the trapping and binding of sediment particles on to a sticky mucilaginous complex of algal filaments.All three zones are charactenzed by laminations produced by the alternation of thick sediment wlth thin algal-rich layers and show a n accretion rate of 5-15 mmlweek.For a deta~led account on the blology and sedimentology of the present-day Mannar algal mats, the reader is referred to Gunatilaka6

Material and methods
Peat samples from three bore-holes A, B and C (figure 1) were taken at various depths and air-dried.Each air-dried sample was crushed, ground and the minus 63 pm fraction retained for analysis.
Approximately 0.5 g of the sample was weighed in a teflon crucible and heated for 20-25 minutes with 15 ml conc H,,S04.until the organic matter was thoroughly decomposed.A 70% mixture of conc H N 0 3 and conc HCI were then added and carefully heated to dryness. 20ml of 60% H F was added to the residue t o dissolve any silica present.The final solution was then taken up in 25 ml of 3N HC1.The V and Mn contents were determined by Inductively Coupled Spectrometry (ICP) usinga Bausch The organic contents of the samples were determined by the pyrolysis and method of difference technique.The P-sitosterol contents of the peat samples were determined by ethyl acetate extraction and preparative thin layer chromatography (TLC).
The algal mat samples were collected from the inter-tidal zone and analyzed by the same procedure as mentioned above.

Results and discussion
Table 1 shows the analytical data obtained.In order to identify the nature of the source materials in the peat deposit, the compound 8-sitosterol was used.In recent advances in organic geochemistry certain specific compounds have\been used as environmental markers.As an example, the sterols are widely distributed as distinctive constituents of oceanic and terrestrial plants.The presence of branched side chains and asymmetric centres in the polycyclic sterol skeleton have allowed the development of a high specificity of certain sterols to certain types of plant^!^The qualitative differences in the sterol contents of sediments depend on the diversity of floral population and hence on the environmental and biological sources.Huang and Meinscheinlo have observed a relationship between C-27, C-28, C-29 sterol composition and the depositional environments (Figure 3).It is of interest to note that vascular plants, the primary producers of terrestrial organic matter, have a tendency to synthesize 1-sitosterol (a C-29 sterol) as the major ~t e r o l !~ It has been established that although 10'2 combinations of structural and optical isomers of C-27 to C-29 alcohols are theoretically possible, less than 10 distinctively structured C-27 to C-29 sterols comprise more than 90% by weight of biological sterols.It is the highly restricted number of structures which make it possible to use sterol analyses in ecological and environmental investigations?. l 1 These observations serve as guidelines in this study to evaluate the different environments that prevailed during the deposition of the Muthurajawela peat deposit.
Figure 3 illustrates the concentration ofj-sitosterol as a function of depth for the locations A, B and C. The concentration of 8-sitosterol is highest in the middle horizon whereas in the top and bottom horizons, its content is lower.It is clear that the anomalous increase in the sterol concentration coincides with the less decomposed tree trunk debris in the middle horizon.This indicates that the middle horizonis ofan origin different to the top and bottom horizons.
Figure 4 illustrates the variations in sitosterol concentration in the sample 'versus V / Mn ratios.The latter ratio was chosen in view of the fact that the V content-could be used to distinguish between terrestrial and non-terrestrial sources, V being more abundant in marine materials.From Figure 4 it is seen that the depositi~nal environments for the top, middle and the bottom horizons of the peat deposit can be distinguished.The negative correlation observed for the middle horizon is clearly a feature caused by abundant terrestrial input and a corresponding increase in the ,%sitosterol content.With the increasing influx of terrestrial material there is a corresponding lowering of the V/ Mn ratio due to the lower V contents of the terrestrial materials.A further point of interest is the higher gradient of the point clusters observed for the bottom horizon when compared to that of the top horizon.It appears that the bottom horizon constitutes partly terrestrial and partly marine source materials.The top.middle and bottom horizons of the peat deposit of Sri Lanka are again clearly distinguished and it is worthy of note that the more marine based samples ofthe lagoonal environments in the peat deposit plot closer to the marine samples.These saluples we) e also observed to contai~l 111arine pelecypods.As shown in Figure 5 .:he algal mats from the tidal flits of M a n n a r .9 1 Lanka all plot on the negative side of the lop V / Mn scale and are clearly distinguished from the other environments of deposition.

B
Sitosterol concentratl'on ( f o r extracts ---I ( a f f e r Huang 8 Meinschien 1979) l'igurc 3.Variation of B-sitosterol concentration as a function of depth in the peat deposit of Sri Lanka.The use of V / M n ratios to delineate terrestrial and marine,'semi-marine cnvi~onlnents can be illustrated further irl Figure 5.The different envilonnlents namely (a) geosynclinal (b) tidal flat (c) lagoonal (d) terrestrial -t h e top, middle and the bottom horizons of the peat deposit.( e ) marine are demarcated.

I:~SLI~-C 4 .
Plats of sitosterol concelrrrarion wilh V ' Mn ratior l o r the top.middle and botrorn hori7ons of the pea[ deposlt of Sri 1.anka (;eochem.lcalEvidence for Palaeo-fi71iro111i1~~11ts ~f'Algal Mats mid Pear 111 Sri I,ai~ka 345 log V/Mn ( Black shales I VINE and TOURTELOT -1970 I Plots of log V i M n with the organlc carbon c, f o r t he three hori7ons o f t h e peat d e p o s ~t of Sri Lanka and for other samples from different en\.ironment,v.

Table 1 -
Analytical data for the peat mid'ilgal mat samplCk from Sri Lanka