Chairman Mike. Fuller and colleagues in the Department of ... Palaeoclimatol.., Palaeoecol., 74: 265-278. Abell, P.I., Awramik, S.M., Osborne, R.H. and Tomellini,.
Chemical
Geology (Isotope Geoscience Section), 80 ( 1990) 26 l-279
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Elsevier SciencePublishers B.V., Amsterdam
Review Paper
A review of the palaeohydrological interpretation of carbon and oxygen isotopic ratios in primary lacustrine carbonates M.R. Talbot Geological Institute, University of Bergen, Allbgt. 41, N-5007 Bergen (Norway)
(ReceivedOctober 24, 1989; acceptedfor publication April 23, 1990)
ABSTRACT Talbot, M.R., 1990. A review of the palaeohydrologicalinterpretation of carbon and oxygen isotopic ratios in primary lacustrinecarbonates.Chem. Geol. (Isot. Geosci.Sect.), 80: 261-279. Primary carbonatesare a common feature of many modem and ancient lacustrine deposits.Carbonatesfrom hydrologically open lakesshow little or no correlation between6t3C and 6’*0. In short-residence-timeopen lakes,carbonateoxygen isotopic composition is relatively invariant and typically is closelyrelated to the bulk isotopiccomposition of inflow waters to the lake. Suitesof carbonateswhich display covatying 613Cand St80 compositionsprecipitate from waterbodieshaving relatively long residencetimes. Where the correlation between carbon and oxygenisotopic variations is high (r>, 0.7), the carbonateshave normally precipitated from a closedlake. In addition, becauseof large changesin water balance,the 6’*0 of closed-lakecarbonatesusuallyvariesover a rangeof several?&I.Therefore, the combination of degreeof covarianceand spreadof 6 1*O-valuescan be used to discriminate between carbonatesproduced in hydrologicallyopen and closedbasins. Within individual basins,covariant trendsmay haveremarkablelong-termpersistencedespitemajor environmental changes, indicating considerablestability in basin hydrology. Eachclosedlake hasa unique isotopic identity defined by its covariant trend, which is a function of the basin’sgeographicaland climatic setting, its hydrology, and the history of the waterbody. Any major interruption or realignment of this trend reflects a fundamental change in basin hydrology. Isotopic trends based upon the carbon and oxygen isotopic composition of primary lacustrine carbonateshave several applications in palaeolimnology.The oxygenisotopic composition of open-lakecarbonatesmay, with caution, be usedasa proxy indicator of the composition of regional rainfall. Covariant trends can be used to trace the hydrological history of a basin, the evolution of individual water masses,and to correlatecarbonate-bearingsedimentsfrom different parts of a basin.
1. Introduction Carbon and oxygen isotopic analyses of lacustrine carbonateshave become establishedas a routine technique in palaeolimnological research,and have made some important contributions to our understanding of lacustrine systems (see, e.g., review in Kelts and Talbot, 1989). Nevertheless, the impact of stable isotope studies has been relatively restrained in comparison to their influence upon investigations of marine palaeoenvironments, where 0168-9622/90/$03.50
isotope analyses have had a truly revolutionary effect. Lakes lack calcareous microfossils of the sort which provide the principal source of carbonate for analysis in palaeoceanographic research,although ostracods may preserve a valuable record of benthic environments (Gasse et al., 1987; Lister, 1988). However, other types of carbonate are often present in lake sediments; it is thus not an absenceof suitable material for analysis that has inhibited the widespread application of iso-
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tope techniques in palaeolimnology. A major reason for the lesser impact of isotope studies is the diversity of lacustrine systems, and the problems this causes in interpreting isotopic data from lake carbonates (see also Cerling et al, 1988). No two lakes are the same, each responds differently to externally-forced environmental change,and will thereforerecord the effects of this change in a slightly different fashion. Distinguishing and interpreting changesthat are due to wholly local causesfrom those of regional significance may often be problematic. However, the growing number of published analyses now allow the identification of isotopic relationships which are apparently common to particular sortsof lakes. Some of those relationships which are of palaeoenvironmental significance will form the main focus of this paper. A striking feature of many sets of isotopic analyses of lacustrine carbonates is the tendency for carbon and oxygen isotopic ratios to covary. Several other workers have noted this covariance (Stuiver, 1970; Fritz et al., 1975, 1987; Either and Siegenthaler, 1976; Abel1 et al., 1982; Turner et al., 1983; Spencer et al., 1984; Stiller and Kaufman, 1985; Siegenthaler and Either, 1986; Gasseet al,. 1987; HillaireMarcel and Casanova, 1987;Gasseand Fontes, 1989). A comprehensive review of the various explanations that have been advanced for this relationship is beyond the scopeof’the present paper and will be given in a separatepublication (Talbot and Kelts, in prep.). This paper reviews carbon and oxygen isotopic variations in lacustrine carbonates, and shows that isotopic covariance is a characteristic feature of particular lacustrine settings.It is proposedthat the recognition of covariance has valuable applications in several branchesof palaeolimnological research.The absenceof any discussion of the causesof covariance invalidates neither the observation that certain sorts of lakes precipitate carbonates with distinctive isotopic signatures,nor the utility of thesesignaturesas palaeolimnological tracers.
M.R. TALBOT
2. Data base
Analytical data from all available stable isotopic studiesof lacustrine carbonateshave been compiled and examined for covariance between carbon and oxygen isotopic ratios (expressedin the conventional 6 notation). These data have been produced at a number of laboratories using standard mass-spectrometric techniques (Craig, 1957); results are expressed with reference to the PDB standard. Although carbonates of a variety of origins have beenutilised by different workers, this review will be confined to two types of material: ( 1) Carbonates that were produced as primary precipitates in the surfacewaters of lakes and settled as a pelagic rain to form accumulations of discrete, typically silt-sized crystals which may comprise the dominant component of the sediment (lake marl, “Seekreide”, mitrite, of various authors), or occur as laminae within other types of sediment (Kelts and Hsii, 1978; Eugster and Kelts, 1983; Kelts and Talbot, 1989). (2) Carbonate produced as a result of the photosynthetic activities of stromatolite- and oncolite-forming blue-green algae and associated organisms. This carbonate forms dense encrustations of interlocking, laminated crystal aggregates.Unlike some types of stromatolite, trapping and binding of sediment particles is of little importance to the growth of these structures; therefore, elastic carbonate and noncarbonate materials is normally absent,or present in only minor amounts. Stromatolites and oncolites of this sort grow in areasof lakes that are essentially free of aquatic macrophytes, receivelittle or no elastic sediment, and are often associated with rocky or stabilised substrates (Dean, 1981; Casanova, 1986a). Benthic carbonatesare almost exclusively of biogenic origin (principally mollusc and ostracod shells), and although their S’80-values may parallel those of the precipitates described above, 6i3C-values typically vary more erratically, due to the influence of local habitat
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effects upon the dissolved inorganic carbon (DIC) pool from which the shell carbonatewas formed [see, e.g., discussions in Fritz et al. ( 1975), and Abel1 and Williams ( 1989) ] and possible incorporation of metabolic carbon into the shell (Tanaka et al., 1986). For these reasons, shell carbonates have been excluded from the present review. For the pelagic carbonates [point ( 1) 1, analyseshave ideally been carried out on monomineralic samples containing little or no clastic carbonate. However, the quality of sampling varies and in some casesis difficult to assess,due to an absenceof accompanying sediment descriptions. Where bulk sampling has been carried out, some elastic or shell carbonate may have been present. However, most workers have been aware of this problem, and have taken stepsto minimise or correct for the effects of contamination. Linear relationships to be shown here reflect variations in the composition of the primary phase, rather than varying mixtures of two end-member carbonates. Samples analysed by the author were obtained by hand-picking or scraping from individual carbonate-rich laminae. Stromatolite samples have been drilled from a single or a few growth laminae using a microdrill. Before analysis any organic matter present was removed by treatment with commercial grade sodium hypochlorite solution, by roasting, or by plasma ashing. Most freshwater lakes produce only one carbonate, most commonly calcite but others oscillate between calcite, high-Mg calcite or aragonite precipitation, depending upon salinity (Miiller et al., 1972; Kelts and Hsti, 1978; Eugster and Kelts, 1983). Although there are few experimental data, qualitative evidence suggeststhat these carbonatesare precipitated in isotopic equilibrium with the lake water (Turner et al., 1983; McKenzie, 1985; Fritz et al., 1987; Gasseet al., 1987). Becauseof continuing uncertainty about the formation of primary dolomite in lacustrine environments (Talbot and Kelts, 1986), analytical data from
263
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dolomitic samples have not been included in this compilation. 3. Isotopic characteristics of primary lacustrine carbonates Isotopic covariance is apparently most typical of carbonatesfrom lakes that are hydrologically closed. Fig. 1. summa&es data from modem closed lakes. In these examples the correlation coefficient for covariance between 613Cand 6180 is > 0.8. Such highly correlated covariant trends do not occur in carbonatesof benthic origin, even from closed waterbodies where other sorts of of carbonate display a distinct trend (see, e.g., ostracod analyses from Lake Turkana, Kenya, by Halfman et al., 1989) . In general the inflow-evaporation bal10.0
CLOSED
7
LAKES
Fig. 1. Covariant carbon and oxygen isotopic trends in carbonates from modem closed lakes. Lake Natron data from stromatolites, Lake Bosumtwi data from stromatolite and primary carbonate, rest from primary carbonates only (r = regression cqeffrcient; n = number of samples) . Sources:Great Salt Lake (Utah, U.S.A.) - Spencer et al., 1984, McKenzie, 1985; Van (Turkey) - Schoell, 1978; Natron-Magaadi (Kenya-Tanzania) - Hillaire-Marcel and Casanova, 1987; Turkana (Kenya - Halfman et al., 1989; Bosumtwi (Ghana) - Talbot and Kelts, 1986, and M.R. Talbot, unpublished data, 1988; and Rukwa (Tanzania) - M.R. Talbot, unpublished data, 1988.
264
M.R.TALBOT
ante dominates the isotopic evolution of closed lakes; covariant trends in setsof isotopic analyses from pelagic and stromatolitic lacustrine carbonates must thus reflect hydrological changesto the lake from which they precipitated. Temperature effects, which influence both the isotopic composition of rainfall (and thus that of lake inflow waters), and isotopic fractionation during carbonate precipitation, are apparently of secondary importance in closed lakes, being generally masked by evaporative- and residence-relatedeffects (Stuiver, 1970; Fontes and Gonfiantini, 1967; Gat, I98 1; Gonfiantini, 1986;Fritz et al., 1987). For comparison, Fig. 2 shows isotopic data for carbonatesfrom some hydrologically open lakes. These have no single characteristic relationship in common, but three main types may be identilied: ( 1) Carbonates showing only small variations in both 6i80 and 613C,e.g. Greifenseen, Switzerland (McKenzie, 1985). (2 ) Lakes where the carbonatesdisplay only rl5C
OPEN LAKES
i
OOOOOHENDERSON eM-M HIJLEH -W GREWENSEE LDBSIGENSEE (r=O.65) e&Qm UTTLE (r=0.51)
L-15.0
Fig. 2. Isotopic data for primary calcites from some typical modern open lakes: Henderson (U.S.A.) - Stuiver ( 1970); Huleh (Israel) - Stiller and Hutchinson ( 1980); Greifensee (Switzerland) -McKenzie ( 1985); Lobsigensee (Switzerland) - Siegenthaler and Either ( 1986); and Little (Canada) - Turner et al. ( 1983).
small variations in 6’*0 but relatively large changesin 6r3C, e.g. lakes Henderson, Maine, U.S.A. (Quiver, 1970) and Huleh, Israel (Stiller and Hutchinson, 1980). ( 3 ) Carbonates showing a tendency to covariance, but correlation of the regression is poorer than in closed lakes, e.g. Lobsigensee, Switzerland (Siegenthaler and Either, 1986 and Little Lake, Ontario, Canada (Turner et al., 1983). Lakes such as these probably have, like most closed lakes, relatively long residencetimes. One feature common to open-lake carbonates, and a contrast to those from most closed lakes,is the small variation in 6 ‘*O within each basin. Relatively invariant carbonate oxygen isotopic values suggest relatively fixed lake water compositions; the small variations that do occur probably reflect minor oscillations in temperature and inflow-evaporation balance between the periods of carbonate precipitation. Studies of modern lake and river systems indicate that short residence time lakes have comparatively little impact upon the isotopic composition of the rivers that traverse them (Fritz, 1981)) so fixed or vertical oxygen isotopic trends (i.e. parallel to the 6i3C axis) of the sort shown in Fig. 2 are probably typical of waterbodies with relatively rapid throughflow. Since the isotopic composition of many rivers is a weighted average of the composition of precipitation over their catchment (Fritz, 1981), the oxygenisotopic composition of primary carbonates from hydrologically open, short-residence lakes may, with caution, be used as a guide to the composition of regional precipitation. The combination of relatively little or no variation in 6’*0, with a covariance between 6 ‘*O and 6 ’ 3Cthat is < 0.7, generally seemsto discriminate carbonates formed in open lakes from those precipitated in closed lakes. Carbonates from the latter typically display relatively large changesin a’*0 and 613C,and the covariance shows a strong positive correlation (raO.7 - seeFig. 1). These differences sug-
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gestthat the isotopic characteristics of primary carbonatesmay be used to distinguish between open and closed lakes in the ancient lacustrine record. Fig. 3 shows stable isotopic data from presumed primary carbonates from some ancient continental basins. Using the criteria just outlined, carbonates from the Ries Crater (Bayern, F.R.G.) and the Orcadian (Scotland, U.K. ) and Cenajo basins (southeasternSpain) are identified as closed-lake deposits. With respect to the Orcadian, such a conclusion is in complete accord with abundant evidence suggesting that much of the Middle Devonian of northern Scotland accumulated in one or more major continental waterbodies. This lake (or lakes) oscillated considerably in level, was at times concentrated enough to precipitate r 15.0
d13C 1
-1
.G
ANCIENT
LAKES
I
Fig. 3. Isotopic analysesof presumed primary carbonates from some ancient lacustrine basins: Ries Crater (Miocene, Bayem, F.R.G.) - Rothe and Hoefs ( 1977); Orcadian (Devonian, Scotland, U.K.) - Janaway and Pamell ( 1989); Hula (Miocene-Pleistocene, Israel) - Bein ( 1986); Fundy (Triassic, Nova Scotia-New Brunswick, Canada) - Suchecki et al. ( 1988); Caithness Flags (Devonian, Scotland, U.K.) - Duncan and Hamilton ( 1988); and Cenajo (Miocene, Spain) - Bellanca et al. ( 1989).
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evaporites, and low enough for the lake floor to be subject to desiccation and aeolian processes(Donovan, 1975, 1980; Parnell, 1986; Rogers and Astin, 1988; Hamilton and Trewin, 1988; Janaway and Parnell, 1989). Periodic hydrological closure is almost certain to have occurred in a basin of this sort. Hamilton and Trewin ( 1988) have assembled a considerable body of sedimentological and palaeoecological evidence to indicate that parts of the famous fish-bearing sequencesof the Orcadian basin must have accumulated in a closedbasin. The Cenajo carbonatesare associatedwith significant accumulations of evaporites (Bellanca et al., 1989) , again indicating prolonged periods of negative water balance, and highly suggestive of closed-basin conditions. Independent evidence doesnot seemto exist for the Ries Crater, but hydrological closure is a characteristic feature of impact craters, becauseof the restricted sizeof their catchment. The lower covariance of the Hula basin (Israel) carbonates suggestsprecipitation in an open lake. This, too, is in agreement with other sorts of geological evidence. The sediments accumulated in a Neogene precursor of Lake Huleh (Bein, 1986), an open lake until it was drained in 1958 (Stiller and Hutchinson, 1980). Continuity between this lake and the waterbody within which the Hula basin carbonatesaccumulated is confirmed by the fact that the compositional field of the latter (Fig. 3) overlaps completely that of the Lake Huleh sediments (Fig. 2). The moderately high covariance (r= 0.67) of the Hula carbonates suggestsperiods of relatively extended residencetime for the lake waters and that the basin probably tended towards hydrological closure from time to time. This is to be expected in an area like the Dead Sea Rift, Israel, where evaporation rates are high, and rainfall seasonalin character. Identification of distinct covariant trends in these ancient examples indicates that primary isotopic signatures can be preserved in ancient lacustrine sedimentsat least as far back as the Palaeozoic.
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M.R.TALBOT
The value of covariant trends as a means of analysing isotopic data from lacustrine carbonatescan be further emphasisedby illustrating two examples from different depositional settings. Fig. 4a shows isotopic data from Late Pleistocene and Early Holocene stromatolites from the Natron-Magadi (Kenya-Tanzania) basin (Casanova, 1986b; Hillaire-Marcel and Casanova, 1987). The Late Pleistocene data show considerable scatter, and the correlation is only moderate. Nevertheless, the regression line produced by thesedata is identical to that obtained from the Holocene stromatolite, thus demonstrating that the two formed in the same waterbody. In Fig. 4b, data from calcite micrites in two Late Holocene cores from Lake Turkana,
NATRON-MAGADI STROMATOLITES
00~.QP A&&AA lM *z**r 0.0
1, 0.0
Kenya, are presented (Halfman et al., 1989). Analyses from core 7P define a satisfactory regression line, but those from core 2P are scattered and produce a poorly-correlated line that lies at a high angle to those from any of the lakes shown in Fig. 1. If the data from 2P and 7P are computed together, however, a regressionline is obtained that is essentiallythe same as that defined by the 7P data alone (Fig. 4b), confirming that the two sets of analyses do in fact belong to the same population. (Some of the scatter in the 2P data is probably due to the presenceof small amounts of biogenie carbonate; Halfman, pers. commun., 1988.) Lake Bosuwtwi, Ghana, presently provides
o ,I %,,
>200 ka (r=0.84) ca.140 ka (r=O.i'8) Late Pleistocene (r=0.53) Early Holocene (r=0.82)
2.0
4.0
6.0
6180 Fig. 4. a. Analyses of stromatolite carbonate from Pleistocene-Holocene high stands of the Natron-Magadi Lake (data replotted from Casanova, 1986b; Hillaire-Marcel and Casanova, 1987). Regression line for Late Pleistocene analyses is coincident with line for Early Holocene samples,confuming that the stromatolites from which the samples were taken all grew in the same waterbody. Combined regressionline (not shown) for Late Pleistoceneand Early Holocene stromatolites has a correlation coefftcient of 0.72. Difference between the pre-0.2 Ma and Late Pleistocene-Holocene regressionlines is significant at 99% level (see text for discussion).
b. Isotopic compositions of primary calcite micrites from cores 2P and 7P, Lake Turkana, Kenya (replotted from Halfman et al., 1989). Upper broken line is regression for 2P data alone (t-=0.23), lower broken line (m0.86) is for 7P data alone, solid line (t-=0.83) is for the two cores combined. The near-coincidence between the 7P and 7P+2P covariant trends confirms that despite the poor correlation and abnormal trend of the 2P data, the carbonates at both sites precipitated from the same waterbody. Samplesfrom core 2P show generally heavier &valuesbecausethis site is far from the Omo inflow; the waters have evidently evolved isotopically as they moved down the lake. Also shown (dotted line) is the covariant trend identified by Abel1 et al. ( 1982) from analyses of PlioPleistocenealgal carbonates.
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the only examples of isotopic analyses from pelagic carbonatesand contemporaneous stromatolites. The data lie on the same trend, indicating that the latter are responding to the same isotopic effects as the former (Fig. 5 ). Both pelagic and stromatolitic carbonate clearly precipitate from well-mixed lake waters. Therefore, it can be concluded that covariant trends obtained from both sorts of carbonate reflect variations in the isotopic composition of the lake water and its DIC pool. The trends indicate temporal or spatial changes in lake composition, or both. In the caseof Lake Turkana, for example, the fact that data from two widely separated core sites (Halfman et al,.
LAKE Primary l
l
l
l
l
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1989) fall on the same isotopic trend confirms that waters throughout Lake Turkana have evolved from a single parent water-the Omo inflow. Waters - and thus the carbonatesprecipitated from them - at the 2P core site, which is far from the Omo delta, are isotopitally more enriched than those at the 7P site, due to progressive isotopic evolution as the original inflow migrates southward along the lake. Plots of data from lakes Turkana, NatronMagadi, and those shown in Fig. 1, also summarise variations through time, and indicate that despite major climatically-induced changes in lake level and salinity, there has
BOSUMTWI Carbonates CALCITE
APPROX AGE YEARS B.P
Fig. 5. Covariant trend for primary carbonates from Lake Bosumtwi, Ghana (r= 0.97). (Compositions normalised to calcite to allow for differing fractionation effects in Mg-calcite and aragonite - Rubinson and Clayton, 1969; Tarutani et al., 1969). The single covariant trend has persisted for at least the last 27.5 ka, despite major changes in lake level and at least one possible period of overflow (Talbot et al., 1984; Kelts and Talbot, 1986; Talbot and Kelts, 1986). Stromatolite carbonate falls on same trends as primary carbonates.
268
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been an underlying continuity in the isotopic identity of eachwaterbody. Some of the trends indicate a remarkable persistence in the isotopic characteristicsof certain lakes. The Great Salt Lake (Utah, U.S.A.) record, for example, spans the last - 15 ka of the lake’s history, a
period which saw this waterbody change from the deep,brackish Lake Bonneville, to its present hypersaline state (Spencer et al., 1984; McKenzie, 1985;Currey and Oviatt, 1985;Fig. 6). The record from Lake Bosumtwi is even longer. During the N 27.5 ka covered by the
0 00
GREAT SALT LAKE LATE PLEISTOCENE
GREA;ES$MTT
LAKE
I 15
AGE ( k a
10
5
B.P.)
Fig. 6. Downcore and covariant variations in isotopic composition of Great Salt Lake aragonite muds [isotopic data replotted from Spencer et al. (1984) and McKenzie ( 1985); lake level curve redrawn from Spencer et al. ( 1984) and Currey and Oviatt ( 1985 ) 1. Note how Recent and Late Pleistocene analysesfall on same regression (r= 0.87 ), despite the dramatic environmental changesthat have occurred in the basin over the last 18 ha.
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core from which the samplesaretaken, the lake has oscillated considerably, varying from a deep, dilute, probably open lake, to a shallow, alkaline, closed waterbody (Talbot et al., 1984; Kelts and Talbot 1986;Talbot and Kelts, 1986; Fig. 5 ). Despite these drastic changes,the primary carbonates still define a single isotopic trend common to the whole period of record (Fig. 5 ) . However, theserecords from existing lakes are short compared to those from some ancient lake basins (Fig. 3). The data upon which the covariant trend for the Ries Crater is based probably span at least 1 Ma (Ftichtbauer et al., 1977; Rothe and Hoefs, 1977), while the Orcadian basin samples may cover an even longer period (Janaway and Parnell, 1989). Covariance is nevertheless as clear in thesedata as it is in the Quaternary examples. The existence of similar compositional trends in lakes of very different origins and climatic setting suggeststhat the underlying cause of isotopic covariance is common to most closed lakes. Each lake nevertheless seems to possessa unique covariant trend, hence the varying slopes and origins of the regression lines shown in Figs. 1 and 3. The form of the compositional trend (origin, length, slope of regressionline) must be a function of the nature and history of the waterbody in which the carbonates formed. Although highly variable, the isotopic composition of individual closed lakes varies in a way that is systematic, predictable, and apparently tightly constrained by the lake’s particular hydrological and climatic setting. The isotopic composition of any primary carbonates moves along the covariant trend in response to changes in hydrological balance and water residencetimes. Carbonates precipitated during periods of high lake level will in generalplot towards the negative end of the trend, those precipitated during lake level lows towards the positive end. In detail, however, the relative positions of such samplesmay not be quite so straightforward, due to gradual isotopic evolution of the waterbody through time.
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4. Some applications of covariant trends 4.1. Generalfeatures It is apparent from Figs. 1 and 3-6 that each lake basin has a unique covariant trend. Differences in the trend obtained from existing basins suggestthat it should be possible to extract useful palaeoenvironmental and palaeogeographical information from covariant trends in ancient lacustrine sequences.A variety of factors influence the nature of the trend. The origin (i.e. isotopically most negative point) on the trend is defined by carbonate precipitated from the least evolved lake water, which, in turn, has a composition closest of all those representedon the trend line to that of the inflow water. As indicated on p. 264, inflow waters will, to a first approximation, reflect the mean oxygen isotopic composition of catchment precipitation, which is primarily a function of latitude, modified by altitude and continentality (Craig, 1961; Dansgaard, 1964; Siegenthaler and Oeschger, 1980). These effects are readily apparent in Fig. 1, the relatively high-latitude Great Salt Lake and Lake Van (Turkey) having significantly more negative origins than the tropical African lakes. Local influences can modify these global effects. The Natron-Magadi trend has the most 180rich origin, reflecting the arid, as well as equatorial setting of this basin, while Lake Turkana, which is also located in a near-equatorial, arid basin, receives the bulk of its inflow from the Ethiopian highlands. Its covariant trend thus has a more negative origin. No latitude effect is apparent in the 6 13Cvalue of the origin, nor is there any evidence that the efficiency of internal cycling of remineralised organic matter has any influence on the origin. Lakes Bosumtwi and Turkana have similar origins, yet Bosumtwi is meromictic and Turkana monomictic, while Lake Rukwa (Tanganyika) has probably oscillated between these two extremes (Talbot and Livingstone, 1989). Data are presently insufficient to make
270
any definitive statement, but the primary control here may be climatic, through its influence upon vegetation cover. The change from negative to positive 613C-valuescould reflect the difference between the humid, forested catchments of basins like Bosumtwi, and the Omo River which feeds Lake Turkana, as opposed to the semi-arid, more sparselyvegetatedGreat Salt Lake, Van and Natron-Magadi basins. Groundwaters in well-vegetated catchments are more influenced by the decomposition of isotopically-light plant material than those from drier, thinly vegetated catchments. The former thus tend to produce runoff with more negative 6’3Cr,Ic than the latter. Climaticallyinduced floral differences, specifically variations in the relative proportions of C3 and C, plants, may also influence the carbon isotopic composition of the inflow; if important, these, too, would produce isotopically less negative inflow from the hotter, drier catchments (Salomons et al., 1978;Cerling, 1984; Quade et al., 1989). At present, there is no satisfactory explanation for the considerable differences in slopeof the various covariant trends, but they may, in a qualitative way, reflect the morphology of the waterbody. Lakes like Great Salt Lake, with large surface area/depth ratios, are probably more sensitive to P-E (precipitation-evaporation) variations than relatively deep lakes, like Lake Bosumtwi. The impact of changesin evaporation rate is therefore likely to be greater in shallower, pan-like lakes than in the more bowl-like basins. The steepestslope so far encountered is that defined by the Natron-Magadi carbonates. Lake levels at times of stromatolite development seem to have been determined by the height of the water table within the very extensive area of fractured basalts that surrounds this basin (Hillaire-Marccl and Casanova, 1987). A groundwaterdominated system of this sort would obviously provide a buffer against the extreme effects of evaporation, and thus react less dramatically to changesin evaporation rate. A summary of
M.R. TALBOT EVAPORATION
/
.
RESIDENCE
LATITUDE/ALTITUDE/CONTlNENTALiTY .
Fig. 7. Summary of principal environmental controls upon nature of covariant trend in lakesof different morphology and different geographical setting. Increasing evaporation or residence time causes lake waters to evolve towards more positive 6’*0-values; increasing latitude, altitude or continentality moves the origin to more negative 6r*Ovalues. Short residence times or humid, well-vegetated catchments favour a negative 6r3C position for the origin. Lakes in dry catchments have covariant trends with more positive 613Corigins. Primary production of organic matter (McKenzie, 1985 ) or nonequilibrium outgassingof “C to the atmosphere (Talbot and Kelts, in prep.) in longresidence lakes may be responsible for the shift to more positive S13Cwith increasing evaporative evolution of the waterbody.
the principal factors that may influence covariant trends is given in Fig. 7. The example of the groundwater-controlled Natron-Magadi palaeolake, indicates that caution will clearly be required in the interpretation of covariant trends until we understand more fully what combination of factors actually determines the slope of each trend line. Nevertheless, we can note that the shallow slope of the Orcadian basin trend (Fig. 3 ) implies a pan-like lake, a conclusion that is in agreement with sedimentological investigations of this basin (Donovan, 1975,198O;Parnell, 1986; Rogers and Astin, 1988; Janaway and Parnell, 1989) . The steepslopeof the Ries covariant trend, on other hand, suggestsa relatively deep basin. This lake formed in a me-
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teorite-impact crater (Ftichtbauer et al., 1977), so its morphology was probably similar to that of present-day Lake Bosumtwi, which also occupies an impact crater. Furthermore, the nature of the country rocks (mainly Jurassic limestones and marls) in which the Ries Crater was excavated may well have favoured a significant groundwater input to the lake. Persistenceof covariant trends through time indicates considerablelong-term stability in the isotopic composition of the inflow to the lake, and in the responseof each basin to changing P-E conditions. The behaviour of closed basins, at least in terms of their stable isotope hydrology, is obviously tightly constrained by each basin’s regional setting (climate, topography, catchment area) and the morphometries of the lake itself. Becauseof their stability, covariant trends provide a unique isotopic “fingerprint” for each lake. Since major climatic changes apparently do not affect the trend (seeexamples from Bosumtwi and Great Salt Lake described on p. 268), any changein the covariant trend must reflect some fundamental changein basin hydrology, e.g.changes in water supply due to drainage capture or diversion, or shifts in atmospheric circulation bringing new moisture sources.The use of covariant trends to detect these and other limnological changeswill be illustrated with specific examples from a number of lake basins. 4.2. Natron-Magadi (Kenya-Tanzania) The earliest generation of stromatolites ( 3 0.2 Ma B.P. ) identified in the Natron-Magadi basin ( Hillaire-Marcel and Casanova, 1987) defines a covariant trend that is signiticantly different from those shown by later stromatolites (Fig. 4a). The origins of thesetrends are virtually the same, so there is unlikely to have been any major change in the composition of the inflow. These differences in trend probably record a permanent change in basin hydrology between the periods of formation of the first and later stromatolite generations.
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Morphological changesor river capture, either of which would have permanently affected the P-E balance, seem altogether probable in the tectonically active sector of the Gregory Rift where this basin is located. 4.3. Great Salt Lake (Utah, U.S.A.) Late Pleistoceneand Recent carbonatesfrom Great Salt Lake define a single covariant trend which spansthe changein this waterbody from deep, brackish Lake Bonneville to the present shallow, hypersaline lake. Such clearly defined continuity implies that no major hydrological changeshave occurred in the Great Salt Lake basin during the past - 17ka. If correct, this is a conclusion of considerable interest. A number of previous discussions of the PleistoceneHolocene climatic history of the Great Basin have suggestedthat some variations in the level of Great Salt Lake may have been caused by changesin precipitation related to shifts in atmospheric circulation that allowed air masses of different origin to reach the region (Currey and James, 1982; McKenzie and Eberli, 1987; Oviatt, 1988; Currey, 1990). Since these air masses have very different origins, the rains they produce have distinctive oxygen isotopic compositions. Fritz ( 1981) records differencesof several permil between winter (N.E. Pacific Ocean source) and summer (Gulf of Mexico source) precipitation in the north-central U.S.A.-south-central Canada region. However, continuity of the covariant trend suggeststhat there have been no major changes in the oxygen isotopic composition of the inflow to the Lake Bonneville-Great Salt Lake basin, so the principal airmass responsible for maintaining the lake has probably remained unchanged throughout the period of record summarised in Fig. 6. 4.4. Lake Turkana (Kenya) Carbonatesfrom two widely separatedcores from this lake define a single covariant trend
272
(Fig. 4b). The water budget of Lake Turkana is dominated by the Omo River, which enters at the northern end of the lake and provides 80-90% of the inflow (Barton and Torgersen, 1988) . One of the two cores (2P) comes from the extreme southern basin, and the only other streams of any significance, the Turkwell and Kerio, enter the lake betweenthe two core sites. The existence of a single covariant trend demonstrates that the chemistry of the southern basin is nevertheless dominated by Omo inflow. Lake Turkana, despite its extreme elongation is isotopically a single hydrological entity, and has been so throughout the period representedby the cores. In similar fashion to the stromatolite record from the Natron-Magadi basin, the covariant trend of the Holocene micrites from Lake Turkana differs markedly from that identified by analyses of Plio-Pleistocene algal carbonates from the same basin (Abel1 et al., 1982; Fig. 4b). This suggeststhat the earlier carbonates were not formed in a waterbody that was a direct ancestor of modern Lake Turkana, a conclusion that is supported by other work on the basin which indicates that the present lake is probably a rather youthful waterbody (Cerling, 1986; Barton and Torgersen, 1988). 4.5. The Dead Sea Rift (Israel) The Dead Sea Rift provides a particularly interesting example of a closed drainage system, as the Jordan rivers passesthrough open lakes en route to the Dead Sea. Isotopic data are available from this terminal lake, as well as the open basins upstream. Lake Kinneret (the Sea of Galilee) is the secondof two open lakes on the Jordan River system. Stiller and Kaufman ( 1985) have analyzed a large number of samples from coresof the lake’s Holocene sedimentary record. No trends are apparent in the analysesas a whole, but Stiller and Kaufman note that the isotopic record can be divided into two distinct periods (5100-2750 B.P. and 1150-150 B.P.), with a
M.R. TALBOT
DEAD
SEA
RIFT
613C
Fig. 8. Isotopic data from basins in the Dead Sea Rift, Israel. Lake Huleh analyses(Holocene) fall mainly within area covered by the Hula basin data (Miocene-Pleistocene), proving that the lake was a direct descendant of the waterbody within which the Hula carbonates accumulated. Covariant trend for Kinneret 2 analyses( 1150-l 50 B.P.) coincides with trend for Hula basin, indicating that during this period Lake Kinneret was dependent upon water carried by the River Jordan from Lake Huleh. (Kinneret I - 5 loo-2750 B.P. ). Although formed in a closedlake that was part of the same drainage system, the Lisan carbonates show no covariant trend and no obvious compositional relationship to those from the upstream basins (see text for discussion). (Data sources:Hula basin - Bein, 1986; Huleh - Stiller and Hutchinson, 1980; Kinneret - Stiller and Kaufman, 1985; Lisan - Katz et al., 1977; modem Dead Sea aragonite - Friedman, 1965; Stiller et al., 1985).
transitional interval between. Despite the lake being hydrologically open, the two groups of carbonatesdisplay highly correlated covariant trends (Fig. 8 ) . Becausethe lake is open, however, the rangein 6i80 is small ( < 2%0), much lower than in most closed lakes. The high covariance of the Kinneret carbonates is probably due to the nature of the inflow. Lake Kinneret today receivesthe bulk of its inflow from the River Jordan, which formerly flowed directly from Lake Huleh, N 15 km to the north (Stiller and Hutchinson, 1980). Lake Huleh thus acted as a preconcentration basin for Lake Kinneret. Compositional dependence upon
CARBON
AND
OXYGEN
ISOTOPIC
RATIOS
IN PRIMARY
LACUSTRINE
Lake Huleh is confirmed by the fact that the covariant trend for the youngest carbonates from Lake Kinneret is coincident with that for the Hula basin sediments (Kinneret 2, Fig. 8 ). These carbonates clearly precipitated from isotopically more evolved, but otherwise characteristic Jordan river waters derived directly from the Hula basin. They demonstrate that the isotopic composition of the carbonates was in part determined by the history of the inflow water beforeit reached the lake. It is noteworthy that the pre-2750 B.P. Lake Kinneret carbonates do not fall on the Hula basin trend. Stiller and Kaufman ( 1985) explained the two compositional groups in terms of the combined effects of productivity and temperature change. As an alternative or additional explanation, it may be speculatedthat the post-l 150 B.P. group also reflects a change in basin hydrology, which saw increasedinfluence of Hula basin waters upon the isotopic composition of Lake Kinneret. Examination of isotopic data from the intervening period (2750-l 150 B.P.) in relation to the covariant trends for the two groups (Kinneret I and 2) suggeststhat this change occurred during the interval 2500-2000 B.P. No well-defined trends are apparent in the large number of isotopic analysesthat Katz et al. ( 1977) performed on primary aragonites from Lake Lisan (Fig. 8 ) . If anything, the wide scatter in 613C combined with relatively limited 6 ‘*O variations is more characteristic of open lakes. There is little doubt, however, that Lake Lisan occupied a closed basin. Katz et al. suggest that the aragonite precipitated from low-salinity surfacewaters formed during each winter rainy period, when a layer of relatively uniform composition, with no distinct isotopic gradients, may have covered the lake. This low-salinity layer is thought to have disappearedas the dry seasonprogressed,a completely new one forming during the next rainy season.At present,such low-salinity layers vary considerably in composition, depending upon their origin (Gat, 1984)) and may be the rea-
CARBONATES
213
son for varying composition of modem aragonite from the lake (Friedman, 1965; Stiller et al., 1985;Fig. 8). In addition to surface runoff, springs made significant contributions to the Pleistocene lake (Katz et al. 1977; Druckman et al., 1987) and these, too, show a considerable range in isotopic composition (Gat, 1984). There is thus unlikely to have been any compositional continuity from one year to the next; each primary carbonate lamina would primarily reflect the particular low-salinity event, a variable combination of lake surface water, Jordan inflow, spring discharge and local flood waters, from which it precipitated. Clearly, a carbonate-producing system of this sort will have little isotopic memory and hence no covariant trend. 4.6. The Orcadian Basin (northern Scotland, U.K.) As a final illustration of the use of covariant trends, isotopic data from the Orcadian basin (Devonian) will be examined. Unlike the Neogene examples discussed in Sections 4.24.5, no traces of the drainage system in this basin remain, and the palaeogeography is known only in its broadest outline. By drawing upon experience gained from the Quaternary basins, however, interesting insights can nevertheless be gained into the hydrology of parts of the Orcadian drainage network. Recent reviews of the Orcadian basin are given by Mykura (1983), Trewin (1986), Hamilton and Trewin ( 1988), and Janaway and Pamell ( 1989) . The basin lay at sub-tropical latitudes, and developed at the margin of the Old Red Sandstone continent. Lacustrine sediments, including carbonates, organic-rich shales and fish-beds, are most notably developed within Middle Devonian sequences,at which time it has been postulated that a single largelake may have occupiedmuch of the basin (Rayner, 1963;Donovan, 1980;Trewin, 1986; Hamilton and Trewin, 1988). Isotopic analysesfrom a number of well-preserved micritic
274
M.R. TALBOT ORCADIAN BASIN CARBONATES
*ttt* a A A6 A KFZC •~a 9 9 1. +++++ 0 00 00 l
l
l
l
l
l
3.0 ^
Trinkie-Harrow Harbour (r=0.93, n=12) Sandwick Robbie’s (r=0.77, n=48) Achanarras centre) (r=0.95, n=22) Achanarras top) Achanarras bottom) Achanarras dolomite - centre) Achanarras - bottom) I dolomite
L-5.0
Fig. 9. Isotopic analysesof primary calcite laminites and micritic dolomites from Middle Devonian lacustrine sediments of the Orcadian basin, Scotland, U.K. (data replotted from Hamilton and Trewin, 1988; Duncan and Hamilton, 1988; Janaway and Parnell, 1989). Laminites from the centre of the Achanarras Fish Bed and Robbie’s Fish Bed show highly correlated 6r3C/6’*0 covariance, suggesting closed-lake conditions. Laminites from Trinkie-Harrow Harbour lie on a single, highly correlated covariant trend, suggestingthey formed in the same closed lake, or within the same hydrological network (cf. Dead Sea Rift). Samples from Harrow Harbour are isotopically more evolved, and thus probably precipitated from waters that were further from source than those responsible for the Trinkie laminites. Carbonates from Sandwick and the top and bottom of the Achanarras Fish Bed all show poor covariance, indicating either that they precipitated under open-lake conditions or, in the caseof the Achanarras samples, from the waters of a very large closed lake. Seetext for further discussion.
calcite laminites of inferred primary origin are shown in Fig. 9. As has been demonstrated in Fig. 3, some of the data sets show highly correlated covariance, indicating closed-basin conditions. The existence of closed lakes in the Orcadian basin had been deduced previously from sedimentological and palaeoecological studies. Reassessmentof the isotopic data in part confirms these earlier conclusions; what emerges of particular significance from the identification of covariant relationships, is that there appear to have been at least three separate sub-basins which each held it sown isolated waterbody (Trinkie-Harrow Harbour, Achanarras, Robbies; Fig. 9), rather than one major closedbasin. Isotopic evidencefor closed basin conditions at the time the central part of the Achanarras fish bed accumulated conflicts with the interpretation of Hamilton and Trewin (1988, p. 594), who suggestthat this was the period when
“... the lake was most extensive and the climate more pluvial”.
However, the high degree of correlation between 613Cand S180 (r=0.95) makes closedbasin conditions particularly likely at this time. More speculatively, the isotopic data can also be interpreted to infer occasional hydrological connection between the Achanarras and some of the other basins, possibly in the form of a larger lake, which existed for limited periods of time, presumably during intervals of maximum humidity. Sets of analysesfrom the top and bottom of the Achanarras Fish Bed (Fig. 9) show only poor covariance which, together with the small spread in S’*O-values, suggest carbonate precipitation from a hydrologically open lake. What is additionally striking is that these data mainly lie along the Trinkie-Harrow Harbour trend, rather than that defined by the carbonatesfrom the middle of the Fish Bed, and indicate possible hydrological connection
CARBON
AND OXYGEN
ISOTOPIC
RATIOS
IN PRIMARY
LACUSTRINE
between the Achanarras and Trinkie-Harrow Harbour basins during the early and late stages of fish bed accumulation. At thesetimes waters of Trinkie-Harrow Harbour type dominated the lake. When the basin was closed, as during accumulation of the central part of the fish bed, waters of quite different origin occupied the basin. The Robbie’s carbonates seem to have precipitated from a waterbody that was hydrologically quite separatefrom any of the other systems. 5. Final remarks The occurrence of clear and persistent covariance in many closed-lake basins suggests that the Lake Lisan, Israel (Fig. 8 ) data are atypical. Normally, carbonates precipitate from the main body of a lake (or at least its epilimnion) and their changing isotopic composition records the evolution of the waterbody as a whole. Since closed basins may have very long and distinctive isotopic memories, characterising these through the identification of covariant isotopic trends promises to provide a powerful palaeolimnological tool that can yield important new insights into the hydrological and palaeoclimatic history of individual basins and, in some situations, whole drainage systems as well. Some hypothetical examples of possible applications of covariant trends in basin analysis, based upon interpretations of isotopic data discussedin this paper, are shown in Fig. 10. Features of more general significance have also emerged from this compilation. It is, for example, clear that the stable isotopic composition of lacustrine carbonates shows a very wide spreadofboth 613C-and 6’80-values, and include compositions that would previously have been regarded as typically marine (e.g. Keith and Weber, 1964; Hudson, 1977; Hoefs, 1987). Within the area of overlap, isotopic determinations cannot discriminate carbonatesof marine or non-marine origin. However, since lakes containing water with an isotopic com-
275
CARBONATES
FLOW
CORRELATION
SAME BASIN
DIFFERENT BASINS
*
DIRECTION
FLOW FROM S TO A
HYDROLOGICAL
SEPARATION
Fig. 10. Some potential applications of covariant trends. A and B are two hypothetical sections of carbonate-bearing lacustrine sediments from different localities; the symbolsrepresent the stratigraphic position of carbon and oxygen isotopic analyses. i. Analyses define a single covariant trend - the two sections were deposited in the same waterbody. The continuity may be spatial or temporal, cf. data from the contemporaneous cores 2P and 7P, Lake Turkana (Fig. 4b), which provide an example of spatial continuity, or Fig. 6 (Great Salt Lake), which shows an example of a single covariant trend for two setsof samples of different age. ii. The analysesdefine two separate covariant trends - the two sections accumulated in different waterbodies, cf. different generations of Natron-Magadi stromatolites, Fig. 4a. iii. Samples from site A are on average isotopically more evolved than those from site B, the water mass was moving from B to A. Cores 2P and 7Pfrom Lake Turkana (Fig. 4b) provide an example of this sort of isotopic evolution. iv. Older samples fall on a single covariant trend, but later carbonates define two different trends - while the earlier carbonates were accumulating the two localities were in hydrological connection, either within the same lake (cf. Lake Turkana) or becauseof direct river connection (cf. Lakes Huleh and Kinneret , Fig. 8). Hydrological separation then occurred and the two waterbodies evolved separately.
position similar to seawaterare likely to have been low-latitude or evaporatively concentrated waterbodies subject to periods of closure, it is probable that any primary calcium carbonate they produce would display isotopic covariance, a property not likely to be present in normal marine carbonates. Finally, it is worth noting that isotopic covariance has also been noted in some nonmarine dolomites (Hamilton and Trewin, 1988; Rosen et al.,
216
1988;Fairchild and Spiro, 1990)) in somecases plotting on extensions of covariant trends defined by coexisting primary calcites (e.g., Fig. 9). Since lacustrine dolomites of undoubted diagenetic origin do not display this property - their 6’3C-values typically diverge significantly from those of the primary carbonate phases(Talbot and Kelts, 1990) - it is possible that covariance can be used to identify primary dolomites. 6. Conclusions ( 1) Covariance between 613C- and 6l*Ovalues is characteristic of primary and stromatolitic carbonates formed in closed lakes. Each lake has a unique covariant trend which is a function of the lake’s morphology, and its climatic and geographic setting. In contrast, carbonates from hydrologically open lakes show no, or only poorly developed isotopic covariance and display only a limited spread of 6r*O-values, which typically are closely related to the composition of the inflow water. (2) The persistenceof individual covariant trends through time indicates considerable long-term stability in the inflow composition and hydrological characteristicsof many closed lakes. Any marked realignment in covariant trend reflects a fundamental change in basin hydrology. ( 3 ) Identification of covariant trends in ancient lacustrine carbonatescan be used to confirm hydrological closure, and in addition provide information upon basin morphology, palaeogeographyand climatic setting, its evaporative history, and the isotopic composition of inflow to the lake. (4) Covariant trends have considerable potential as tools for basin analysis. In particular they may be used for correlation purposes, to confirm hydrological connection between basins, and segmentation of basins for tectonic or other reasons.
M.R. TALBOT
Acknowledgments Financial support from the Royal Norwegian Research Council (NAVF) is gratefully acknowledged.Isotopic analyseswhose source is not otherwise acknowledged, were carried out at the National GMS Laboratory, Geological Institute, University of Bergen, and at GeologischesInstitut, ETH, Zurich. The GMS Lab., Bergen, is supported by NAVF. John Halfman (Notre Dame) and Tom Johnson (Duke) are thanked for their (at the time) unpublished isotopic analyses from Lake Turkana, and M. Stiller for the original data from her work at Lake Kinneret. Kerry Kelts (EAWAG/ETH, Zurich) has been an inspiring sourceof advice and discussion on a multitude of aspectsof lacustrine systems.Tom Johnson, Mariana Stiller, Nigel Trewin, Peter Fritz and an anonymous referee made valuable suggestions for improvements to earlier versions of this paper, which was written while the author was on sabbatical leave at the University of California Santa Barbara. Chairman Mike Fuller and colleagues in the Department of Geological Sciencesthere are thanked for their generoushospitality. References Abell, P.I. and Williams, M.A.J., 1989. Oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in the Afar region of Ethiopia. Palaeogeogr., Palaeoclimatol.., Palaeoecol., 74: 265-278. Abell, P.I., Awramik, S.M., Osborne, R.H. and Tomellini, S., 1982. Plio-Pleistocenelacustrine stromatolites from Lake Turkana, Kenya: morphology, stratigraphy and stable isotopes. Sediment Geol., 32: l-26. Barton, C.E. and Torgersen, T., 1988. Palaeomagnetic and ‘iOPbestimates of sedimentation in Lake Turkana, East Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 68: 53-60. Bein, A., 1986. Stable isotopes, iron and phosphorus in a sequence of lacustrine carbonates - paleolimnic implications. Chem. Geol. (Isot. Geosci. Sect.), 59: 305 313. Bellanca, A., Calvo, J.P., Censi, P., Elizaga, E. and Neri, R., 1989. Evolution of lacustrine diatomite carbonate cyclesof Miocene age, southeastern Spain: pe-
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