palaeoclimatology and sea-level history...
TRANSCRIPT
92 MAR - APR 1999, VOL. 24 Nº 2
KEY WORDS / IGBP-PAGES / Climate change / Global Change / Palaeoclimatology / Proxy records / Modern analogues /
Valentí Rull is senior palynologist at PDVSA and co-ordinator of IGBP-PAGES in Ven-ezuela. Licenciate in Biological Sciences (University of Barcelona, Catalonia, Spain), M. Sc. and Ph. Sc. en Biology (Ecol-ogy) (Venezuelan Institute for Scientific Research, IVIC, Caracas, Venezuela). Professional interests: Palaeoecology, Evolu-tionary Ecology, Bioestratigraphy. Address: PDVSA Exploration and Production, Box 829, Caracas 1010-A, Venezuela,[email protected].
PALAEOCLIMATOLOGY ANDSEA-LEVEL HISTORY IN VENEZUELA.
NEW DATA, LAND-SEA CORRELATIONS, AND PROPOSALSFOR FUTURE STUDIES IN THE FRAMEWORK
OF THE IGBP-PAGES PROJECTVALENTÍ RULL
he increasing concernabout the possible con-sequences of rapid glo-
bal warming and sea level rise has led tothe creation of the International Geo-sphere-Biosphere Program (IGBP), in or-der to coordinate global change studiesworldwide. One of the IGBP programs isPAGES (Past Global Changes), estab-lished to record the climatic trends of thepast, and help to understand the natural(not anthropogenic) component of climatechange. Using information from long-term climate data, predictive models offuture climate change are more realisticthan those based solely on extrapolationof present trends. Several requirementsare necessary to produce palaeoclimaticrecords which can be considered valid forthis purpose. First of all, because instru-mental records are limited in time, pastclimatic parameters are derived from in-direct or proxy data, for which rigorouscalibration is essential. Such calibrationis usually done through modern analoguestudies, that quantitatively relate proxiesto specific climatic parameters. Second,records should be analyzed with hightemporal resolution. PAGES is concernedwith two main temporal domains: StreamI, the last two millenia, and Stream II,the two last glacial cycles. Annual todecadal resolution is required for StreamI studies, whereas Stream II time framesshould be resolved at a decadal to cen-tennial time scale. Finally, sampling andanalytical methods should be standard-
ized, to allow worldwide comparisonwith the results of other IGBP projects(Eddy, 1992).
In Venezuela, there is anoticeable interest from both the IGBPVenezuelan Committee and the NationalCouncil for Science and Technology (CO-NICIT) to officially initiate activities linkedto PAGES programs. So far, a number ofpalaeoclimatically-oriented studies havebeen already conducted on sedimentsfrom lakes and bogs in high-mountain re-gions, Precambrian plateaus in the low-lands, as well as in marine environments,showing the most important past climatictrends. A complete list of sites and theirmain characteristics can be found inTable I (see also Fig. 1), and a descrip-tion of the main present climatic andvegetation features of the zones studiedis in Rull (1996b) and Rull, et al. (inpress). This short review summarizes theprincipal palaeoclimatic results of thesestudies. The main topics reviewed are thecalibration of proxy data with modernanalogues, the last glacial cycle -withemphasis on the correlation between ma-rine and continental records-, the lastcenturies, and the Holocene sea-level his-tory. Finally, several propositions are sug-gested to improve and enhance palaeocli-matic records using new technology andadditional studies. The work intends toupdate the palaeoclimatic and palaeoenvi-ronmental information on which PAGESis to progress in Venezuela.
Modern analogues
Palaeoclimate estimatesare deduced from parameter estimatesbased on proxy data. The reliability ofthese estimates depends on calibration ofthese proxies with modern analogs. Usu-ally, this calibration is carried out throughmultivariate and multiple regression tech-niques, leading to transfer functions thatgive estimated values for temperature,moisture, etc., from quantitative fossildata. In Venezuela, the first study onmodern palynomorph distribution in sedi-ments was done by Muller (1959) in theOrinoco delta (Fig. 1). Muller’s aim wasto understand the relation between mod-ern palynomorph assemblages and deposi-tional environments, in order to extrapo-late from these results Tertiary palaeoenvi-ronments. Other studies of the same typehave been carried out in Playa Medina(Rull and Vegas-Vilarrúbia, in press)(Fig. 1). These analyses document thevariations of palynological assemblagesaccording to a sea-land gradient, from theshelf to the alluvial plains, and are usefulto interpret palaeoenvironments in sedi-mentary records. They are not oriented topalaeoclimatic reconstruction.
Studies on modern pol-len deposition and its relation to climatepurposes were developed on the Andes(Salgado-Labouriau, 1979) and the Guay-ana highlands and lowlands (Rull, 1991,1992, 1996a, 1988b). Salgado-Labouriau(1979) found a semi-quantitative relation-
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93MAR - APR 1999, VOL. 24 Nº 2
ship between several pollen types inmodern Andean sediments and elevation.According to her, the number of pollengrains of Podocarpus and Hedyosmumper milligram of sediment provide an ap-proximate estimate of the vertical dis-tance between the pollen deposition siteand the place where the trees grow.Since these plant genera occur near thetreeline, the elevation of the treeline withrespect to a given deposition site can beestimated using this relationship. In thisway, the past elevation of the treeline ata given time can be inferred and, usingthe known lapse rate -which in theMérida Andes is -0.6ºC/100 m- the ap-proximate average temperature can be es-timated. This procedure has been used inthe Mérida Andes to extrapolate from thepollen record palaeotemperatures for thePleniglacial, Late Glacial and Holocene(see following chapters). In the Guayanalowlands, differences in altitude andhence temperature are not significant, andthe modern pollen assemblages are re-lated to humidity parameters instead,such as the precipitation/evaporation ratio(Rull, 1991, 1992). However, the lack ofmeteorological stations in these remoteareas prevents the establishment of quan-titative relationships. Another attempt tocalibrate modern sedimentary characteris-tics with climate parameters was pro-posed by Weingarten, et al. (1990). Intheir study, the authors found that spe-cific geochemical parameters in lake sedi-ments related to elevation. For example,organically-bound iron was found todominate in the sediments of lakes fromthe uppermost altitudinal belts. Amor-phous iron is more abundant at interme-diate ones, and amorphous and crystallineiron are equally abundant in the lower-most levels. Similar observations weremade with clay composition, with illiteand chlorite dominating at high eleva-tions and decreasing downslope, whereasgibbsite and kaolinite are most abundantat low elevations. Geological differencesamong basins are not sufficient to ex-plain these trends. Instead, climate-depen-dent mechanisms such as weathering maybe the reason. Application of thisgeochemical approach, although qualita-tive, to past sediments, has been usefulas shown in the following sections.
In Lake Valencia, surfi-cial sediments were sampled for variouschemical and biological analyses, in or-der to relate them to present day environ-mental features, and establish a basis forthe interpretation of past climates andlake levels (Salgado-Labouriau, 1980;Bradbury, et al., 1981; Binford, 1982;Leyden, 1985). As an example, the de-tailed analysis of animal remains (includ-
Site Name Type Sediments Raciocarbon Studies References(altitude m a.s.l.) dates
1 Laguna Victoria (3250) HB Peat - Palynology Salgado-Labouriau & Schubert (1977)
2 Miranda (3920) OC Peat 2500 ± 70 Palynology Salgado-Labouriau et el. (1988)4240 ± 70
11,470 ± 170
3 Quebrada Mucubají OC Peat 11,960 ± 100 Palynology Salgado-Labouriau et al. (1977)(3600) 12,250 ± 150
12,390 ± 25012,570 ± 13012,650 ± 130
4 Laguna Mucubají (3540) PC Lacustrine 1555 ± 245 Geochemistry Bradley et al. (1985)2235 ± 380 Palynology Yuretich (1991)4745 ± 275 Salgado-Labouriau et al. (1992)6075 ± 2908300 ± 255
5 La Culata (3800) OC Peat 2530 ± 70 Palynology Salgado-Labouriau5250 ± 80 and Schubert (1976)5290 ± 806070 ± 806130 ± 806240 ± 906250 ± 1406350 ± 907530 ± 80
6 Piedras Blancas (4080) HB Peat 260 ± 70 Palynology Rull et al. (1987)1220 ± 80
7 El Caballo (3500) OC Peat 16,500 ± 290 Litostratigraphy Schubert & Rinaldi (1987)Lacustrine 19,080 ± 820 Palynology Rull (1997)
Palaeolimnology Rull & Vegas-Vilarrúbia (1996)
8 Tuñame (2490) OC Peat 43,050 ± 2340 Palynology Salgado-Labouriau (1984)50,640 ± 4000
9 Laguna Montón (3700) PC Lacustrine 1595 ± 275 Geochemistry Bradley et al. (1985)Yuretich (1991)
10 Laguna Saisay (3700) PC Lacustrine 1745 ± 300 Geochemistry Bradley et al. (1985)Yuretich (1991)
11 Laguna Negra-A (3460) PC Lacustrine 3165 ± 330 Geochemistry Bradley et al. (1985)Yuretich (1991)
12 Laguna Negra-T (1700) PC Lacustrine 1675 ± 190 Geochemistry Bradley et al. (1985)Yuretich (1991)
13 Laguna Brava (2380) PC Lacustrine 2010 ± 180 Geochemistry Bradley et al. (1985)Yuretich (1991)
14 Laguna Los Lirios (2300)PC Lacustrine 3395 ± 195 Geochemistry Bradley et al. (1985)5590 ± 170 Yuretich (1991)
16,840 ± 310
15 Laguna Blanca (1620) PC Lacustrine 1035 ± 170 Geochemistry Bradley et al. (1985)Yuretich (1991)
16 Laguna Urao (1100) PC Lacustrine 990 ± 170 Geochemistry Bradley et al. (1985)6300 ± 215 Yuretich (1991)
17 Lago Valencia (400) PC Lacustrine 425 ± 145 Geochemistry Salgado-Labouriau (1980)535 ± 180 Diatoms Bradbury et al. (1981)700 ± 200 Ostracods Binford (1982)880 ± 185 Copepods Leyden (1985)1675 ± 125 Palynology1820 ± 701895 ± 1452080 ± 1655105 ± 2605210 ± 1156060 ± 2906090 ± 210
TABLE ILIST OF SITES ANALYZED FOR PALAEOCLIMATES, WITH RADIOCARBON
DATES AND REFERENCES. HB = HILLER BORING, PC = PISTON CORE, OC =OUTCROP. OTHER DATES FROM NON-PALAEOCLIMATIC INVESTIGATIONS
ARE NOT INCLUDED.
94 MAR - APR 1999, VOL. 24 Nº 2
ing sponge spicules, ostracods and co-peods) is briefly presented below. Threemain animal assemblages were found, re-lated to habitat preferences. The firstgroup (sponges, ostracod IX, Potamocyp-ris, Alona) represented the near-shoreconditions, the second group (Cyther-idella, Leydigia, neorhabdocoel D) wasmore abundant at greathr water depths,and the third group (Cypria, Darwinula,Heterocypris, Bosmina, neorhabdocoel E)was abundant both near shore and inmoderately deep water. Discriminantanalysis showed that the main controllingparameters for these modern animal as-semblages were sediment type, depth andproximity to shoreline. This informationand the known autoecological require-ments of the taxa found were essential tosemi-quantitatively estimate past lake lev-els, which, together with the results ofother analyses such as diatoms and pol-len, allowed reconstruction of establish-ing the principal palaeoclimatic trends ofthe lake basin for late glacial and Ho-locene times.
In marine environments,modern analogs have been analyzed forliving and sedimented plankton. Over-peck, et al. (1989) and Peterson, et al.(1991) largely based their palaeoceano-graphic reconstructions on the fact thattoday the foraminifer Globigerina bul-loides dominates the plankton communi-ties during the upwelling season, whereasG. ruber is more abundant during thenon-upwelling season (de Miró, 1971).This finding is of a great climatic signifi-cance, because the upwelling season co-incides with the regional dry season,when the Intertropical Convergence Zone(ITCZ) is south of the Equator. The rainyseason is triggered by the subsequentnorthward displacement of the ITCZwhich reaches the Venezuelan coasts, andmitigates the trade winds, and conse-quently, the cessation of upwelling. Simi-larly, the observations of Lin, et al.(1997) on the 18O content in the shells ofliving G. bulloides and G. ruber havebeen useful to interpret past oxygen iso-tope records.
The record of past eu-static sea level changes also relies onmodern analogue information, in order toaccurately characterize the sediments thatrepresent the average position of the sealevel at a given time. According toKidson (1982), one of the most reliableindicators for sea-level position are or-ganic remains in growth position. Intropical regions, buried coral reefs and insitu mangrove sediments fulfill this con-dition (Ellison, 1989). In Venezuela, thestudies of Muller (1959) and Rull andVegas-Vilarrúbia. (in press), as well as
Site Name Type Sediments Raciocarbon Studies References(altitude m a.s.l.) dates
6350 ± 3506730 ± 3456820 ± 3507190 ± 3507245 ± 2758670 ± 2508800 ±390
9655 ± 31019840 ± 400
10,030 ± 540110,127 ± 36710,200 ± 35010,340 ± 35011,825 ± 360112,930 ± 500
18 Playa Medina (0) PC Peat, marine 6960 ± 70 Palynology Rull et al. (in press)
19 Mapaurí (940) HB Peat 8970 ± 90 Palynology Rinaldi et al. (1990)
20 Laguna Santa Teresa HB Peat 4040 ± 70 Palynology Rull (1991)(880) Rull (1992)
21 Laguna HB Peat 4260 ± 70 Palynology Rull (1991)Divina Pastora (800) Rull (1992)
22 Urué (940) HB Peat 1180 ± 70 Palynology Rull (1991)Rull (1992)
23 Laguna Carinapay HB Peat - Palynology Rull (1991)Palaeolimnology Rull (1992)
Rull & Vegas-Vilarrúbia (1991)Rull & Vegas-Vilarrúbia (1993a)
24 Arapán (900) HB Peat 2600 ± 70 Palynology Rull (1991)Rull (1992)
25 Toronó-teui (2100) HB Peat -4 ±70 Palynology Schubert et al. (1986)4040 ± 60 Rull (1991)
26 Acopán-tepui (1950) HB Peat 1150 ± 60 Palynology Schubert et al. (1986)3120 ± 80 Rull (1991)4380 ± 60 Rull (1996)
27 Amurí-tepui (2150) HB Peat 880 ± 40 Palynology Rull (1991)5260 ± 40 Rull (1996)
28 Churí-tepui (2250) OC Peat 1450 ± 60 Palynology Schubert & Fritz (1985)5740 ± 100 Rull (1991)
Rull (1996)
29 Guaiquinima (1350) HB Peat 330 ± 60 Palynology Schubert & Fritz (1985)350 ± 60 Rull (1991)5100 ± 90 Rull (1996)6000 ± 80
30 Auyán-tepui (1980) HB Peat 2900 ± 70 Palynology Rull (1991)3880 ± 804230 ± 80
31 V12-99 (-1005) PC Marine - Foraminifers Peterson et al. (1991)
32 V12-104 (-466) PC Marine 12 dates: Foraminifers Peterson et al. (1991)~1000 to ~17,000*
33 PL07-39PC (-790) PC Marine 860 ± 80 Oxygen- Hughen, et al. (1996)1400 ± 70 Isotops Lin et al. (1997)1840 ± 70 Laminations2580 ± 803500 ± 1004340 ± 704940 ± 705980 ± 60
10,630 ± 1607360 ± 607580 ± 708850 ± 909700 ± 809990 ± 60
10,070 ± 10010,730 ± 12010,760 ± 80
TABLE I (Continuación)
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others carried out along the coasts of theGuayanas (van der Hammen and Wijm-stra, 1964; Tissot, et al., 1988) have beenused for this purpose. According to them,palynological assemblages from sedi-ments under mangrove forests contain be-tween 45% and 90% of mangrove pollen(Rhizophora+Avicennia), whereas in themuds deposited in front of the coast line,the mangrove pollen accounts for 30% to50% of the total. Commonly, the pollenof Rhizophora is dominant, and Avicen-nia is under-represented. Indeed, in thesurface sediments of pure Avicennia for-ests, its pollen only attain 25%-40% ofthe total assemblage, and in Rhizophora-Avicennia forests it is under 10% (Rull,et al., in press).
Summarizing, modern ana-log studies have been developed for a va-riety of environments and proxies inVenezuela, but an accurate calibration ofproxies with climatic parameters is stilllacking. Fortunately, the available studiescan provide initial help. The MéridaAndes seem especially adequate for anextensive modern survey, because of theirdefinite temperature-elevation relation-ship. The results obtained so far show agreat potential for tree pollen as a proxyindicator. An enhanced systematic sam-pling program and the use of modern sta-tistical techniques needs to be encour-aged. Concerning the marine environ-ment, foraminifers have shown to be use-ful climatic indicators. A plan to quanti-tatively characterize modern coastal sedi-ments by their faunal content, in relationto the main oceanographic parameters,would be desirable.
The last glacial cycle
In this section, resultsrelated to the PAGES Stream II timeframe are summarized. They include bothcontinental and marine records, and em-brace only the last glacial cycle (Würm/Wisconsin), owing to the lack of palaeo-climatic analyses in older sediments. Theonly dated evidence of an earlier glacia-tion is the Timotes fluvial terrace com-plex, located in the Mérida Andes andinterpreted as a valley fill that occurredduring a glacial advance. Thermoluminis-cence dating gave an age of 147 to 170ka BP, potentially related to isotopicstage 6 and the Riss/Illinois glaciation(Schubert and Vaz, 1987). Within the lastglacial cycle, most palaeoclimatic datacome from the Pleniglacial, and from lateglacial and Holocene times. However, apreliminary pollen survey in 35 to 50 kaold Tuñame terrace (Salgado-Labouriau,1984) and the recent discovery of sedi-ments older than 35 ka BP (Mahaney, et
Site Name Type Sediments Raciocarbon Studies References(altitude m a.s.l.) dates
10,690 ± 8011,430 ± 15012,380 ± 10013,020 ± 22013,390 ± 20014,530 ± 31014,410 ± 26013,960 ± 30013,930 ± 10014,780 ± 14017,600 ± 15021,210 ± 210
34 PL07-43PC (-819) PC Marine 12,710 ± 110 Oxygen- Isotops Hughen et al. (1996)16,370 ± 140 Laminations Lin et al. (1997)20,460 ± 20025,220 ± 33028,350 ± 390
35 PL07-56PC (-810) PC Marine 8180 ± 120 Laminations Hughen et al. (1996)8320 ± 120 Hughen et al. (in press a, b)8440 ± 1408340 ± 1408350 ± 1408310 ± 1408460 ± 1208560 ± 1208610 ± 1208590 ± 1208920 ± 1408950 ± 1209070 ± 1209290 ± 1209380 ± 1409420 ± 1209490 ± 1209650 ± 1009550 ± 1409610 ± 1208260 ± 1209990 ± 120
10,010 ± 12010,140 ± 16010,070 ± 12010,240 ± 18010,160 ± 12010,310 ± 12010,320 ± 10010,400 ± 10010,390 ± 10010,540 ± 12010,550 ± 12010,530 ± 14010,710 ± 16010,700 ± 14010,970 ± 14011,190 ± 14011,100 ± 14011,370 ± 16011,470 ± 12011,360 ± 12011,430 ± 12011,700 ± 12011,770 ± 14011,730 ± 16011,970 ± 14012,080 ± 12012,360 ± 16012,560 ± 20012,720 ± 200
36 PL07-57PC (-815) PC Marine - Laminations Hughen et al. (1996)
37 PL07-58PC (-820) PC Marine - Laminations Hughen et al. (1996)
38 Lake Maracaibo (-20) PC Lacustrine 7370 ± 200 Sedimentology Sarmiento & Kirby (1962)8148 ± 2209250 ± 2509922 ± 250
39 Orinoco delta front (?) PC Marine 17,820 ± 600 Palynology Muller (1959)
TABLE I (Continuación)
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al., 1997) are encouraging. Concerningthe Pleniglacial-Holocene time interval,both marine and continental records areavailable, but a detailed correlation be-tween them has not been established sofar.
a. Continental record
Palaeoclimatological recordsfor the last 18 ka BP from continental locali-ties are obtained from lake and bog sedi-ments in high mountain regions (MéridaAndes and sandstone table mountains ortepuis), and in the lowlands (Lake Valen-cia, Lake Maracaibo and the Gran Sabana,in the Guayana region) (Fig. 1). Tempera-ture and moisture estimates from pollenanalysis of sediments from these sitescompare with the global trends (Fig. 2).Because a detailed revision of the evi-dence can be found in Rull (1996b), onlythe most significant palaeoclimate featuresand some new findings will be mentionedhere. The Pleniglacial has been studiedby geochemical and palynological analy-ses. Based on the clay mineral contentand metal analysis of the Laguna LosLirios sediments, and using modern ana-logues, Weingarten, et al. (1991) pro-posed that climates were cold to verycold between about 17 and 15 ka BP.New palynological data from the ElCaballo section support this interpretation(Rull, 1998a; Rull and Vegas-Vilarrúbia,1996). Using the altitudinal gradient pro-posed by Salgado-Labouriau (1979),these authors estimated that average tem-peratures were 7ºC below the presentones at 16.5 14C ka BP.
The late glacial wascharacterized worldwide by an alternationof short stadials (glacier readvances) andinterstadials (glacier retreats). One of themost widely studied stadials is theYounger Dryas (11 to 10 14C ka BP),represented in the Andes by the Mucubajíand the Páramo Miranda cold phases(Rinaldi, 1996a), when temperatures onaverage were around 3ºC below thepresent ones (Salgado-Labouriau, et al.,1977; 1988). At the same time, dry cli-mates were present in the lowlands, asreflected in the records from Lake Valen-cia (Salgado-Labouriau, 1980; Bradbury,et al., 1981; Leyden, 1985) and the GranSabana (Schubert and Fritz, 1985;Schubert, et al., 1986; Rull, 1991; 1992;1996a, b). With the onset of the Ho-locene, climatic conditions similar topresent day were established, but someminor fluctuations existed. One of themore important events recorded in LaCulata (Mérida Andes), consisted of acold and dry phase at about 6 ka BP(Salgado-Labouriau and Schubert, 1976),
and paralelled by dry climates in theGran Sabana (Rinaldi, et al., 1990; Rull,1996b).
The Andean glacier re-treat since the Last Glacial Maximumwas studied by Rull (1998a). The aver-age retreat velocity was 7.3 cm of eleva-tion/year, but it was not constant throughtime. Average retreat values of about 2.5-3 cm/y were recorded between thePleniglacial and 6 ka BP, when a glacierre-advance of the same magnitude oc-curred (La Culata Cold Phase). After theMiranda Warm Phase (3-4 ka BP) the re-treat was small (~1.5 cm/y) until thePiedras Blancas cold phase (coincidingwith the Little Ice Age), after which astrong acceleration took place (> 50 cm/y), coinciding with a general warmingtrend. As a consequence of glacial activi-ties in the Mérida Andes, sediments areyounger with increasing altitude, prob-ably due to the removal of older sedi-ments by glaciers during the glacialmaximum, and the sequential filling ofhigher elevation basins during the retreat(Rull, 1998a). This is important for fu-ture planning of sampling.
Most of the palaeocli-matic events shown have been recordedin high-mountain environments, but datafrom lowland sites are increasing. Basedon the available information, it seemsthat palaeotemperatures are best recordedin the mountains, whereas moisturetrends are reflected primarily in lowlandsediments. Future studies should consider
both environments, and use modern cor-ing and sampling techniques suitable torecover and analyze high-resolution datarecords (Colman, 1995). Emphasis shouldalso be on retrieval of palaeontological(especially palynological) material for thestudy of the sequences older than theLast Glacial Maximum. Based on theavailable information, two sites might bepromising: the Mucubají area, wheresediments older than 35 ka BP werefound; and Lake Valencia, from whichonly the uppermost portion of the sedi-mentary column (about 7.5 m of sedi-ment depth, 13 ka BP) has been ana-lyzed. The total thickness of Lake Valen-cia fill was estimated to be over 300 mwith a maximum basal age around 520ka BP (Schubert, 1980).
b. Marine record
Several marine coreshave been obtained in the anoxic CariacoBasin (Overpeck, et al., 1989; Peterson,et al., 1991; Hughen, et al., 1996a and b,in press a and b). Most of the informa-tion comes from cores V12-99, V12-104and P07-39PC (Fig. 1). The oldest sec-tion comes from core PL07-39PC, whichembraces the last 28 ka BP (Lin, et al.,1997). Detailed radiocarbon dating, fau-nal abundance patterns and high-resolu-tion oxygen-isotopic record of these coresshows the main climatic and oceano-graphic trends since the last glaciation.During the Pleniglacial, prior to 12.6 ka
Fig. 1. Location maps of the palaeoclimatic coring and outcrop sites from Venezuela (seeTable I for numerical codes).
97MAR - APR 1999, VOL. 24 Nº 2
BP, the Cariaco Basin was isolated fromthe Caribbean by lowered sea level but,despite of the restricted circulation, deepwaters were oxigenated. Sea surface tem-peratures (SST) were 4ºC lower than atpresent and the upwelling center was lo-cated northwards, beyond the TortugaBank (Fig. 1). The upwelling in Cariacobegan at 12.6 ka BP, after its connectionto the Caribbean sea which was relatedto rapid sea level rise. After this time,the basin became anoxic in its bottom,and laminated sediments began to accu-mulate reflecting seasonality. Dark layers(with high quantity of terrigenous mate-rial) were deposited during the continen-tal rainy season, when runoff is high, andlight layers with planktonic fossils,mainly diatoms, indicating an increase inthe water-column productivity, are depos-ited in the upwelling season. These an-nual laminations are more distinct before9.8 ka BP, probably due to more intenseupwelling and vertical mixing duringdeglaciation. Laminations also persistedlater in the Holocene, as demonstrated bymicroscopic and X-ray analysis of coresPL0-56PC and P07-57PC. Detailedcounting of laminae deposited between12.6 and 9 ka BP allowed calibration of14C dates and provided an accurate chro-nological basis for high-resolution palae-oclimatic and palaeoceanographic studies.Variations in the thickness of laminaerecord rapid deglacial climate changes,including the Older Dryas and theYounger Dryas coolings. Preliminary pol-len analysis of the core P07-39PC shows,between 11 and 13 ka BP, a decrease inarboreal pollen and a simultaneous in-crease of gramineae pollen (both in per-centage and concentration), which is in-terpreted as a cold and dry phase, andcorrelated with the Younger Dryas event(Rinaldi, 1996b and c). During the Ho-locene, a phase of enhanced seasonalcontrast (warmer summers) and/or in-creased regional precipitation and runoff,occurred between ~7 and 6 ka BP. Thestudy of additional climate proxies isneeded for a better understanding of theHolocene palaeoclimates.
c. Land-sea correlations
The available informa-tion from both continental and marinerecords of Venezuela is enough to at-tempt a preliminary correlation (Fig. 3).So far, no continental records are avail-able for the Last Glacial Maximum,which is recorded only in marine sedi-ments. The El Caballo Stadial, recordedin Andean sediments at 16.5 ka BP, fallswithin the full Pleniglacial and coincideswith minimum values in the oxygen iso-
tope curve from marine sediments. Thetemperature decrease for the Pleniglacialin the Mérida Andes has been estimatedto be 7ºC, and 4ºC for the surface watersin the Cariaco Basin, in agreement withcurrent estimates from the NorthernHemisphere (Flint, 1971). A consistentincrease of the estimated mean tempera-tures in the Mérida Andes parallels thetrends in the marine isotopic curve untilabout 12 ka BP, when the MucubajíWarm Phase was recorded, coeval withthe Northern Hemisphere Bölling intersta-dial. However, an earlier decrease, cor-relative to the Oldest Dryas (~ 13 14C kaBP), is recorded in both marine and con-tinental records. The warmer climate co-incides with the onset of upwelling in theCariaco Basin, after its connection to theCaribbean, owing to a sea-level rise. Thisis consistent with the worldwide tempera-ture increase.
Records from lowlandsthat began to sediment during Bölling/Alleröd times (Fig. 3) show arid or semi-arid climates, as reflected for instance bythe nonexistence of Lakes Valencia andMaracaibo (Sarmiento and Kirby, 1962),as well as the lack of peat accumulationsin Guayana. The Younger Dryas stadial(Mucubají Cold Phase II) was the last cli-matic event of the late glacial, its later
stages coinciding with an increase of hu-midity in the lowlands, and the filling ofLake Valencia. These continental changeswere recorded in the marine oxygen iso-tope curve of Cariaco Basin cores withhigh precision, due to the seasonal lamina-tions of its sediments. The oceanographicconditions, however, remained relativelyunchanged and the upwelling persisted,because its dominant forcing mechanism,i. e. the sea-level rise, was still acting asbefore. The initiation of the Holocene rep-resents a major environmental and eco-logical shift, reflected in all the Venezu-elan records. In the Andes, temperature es-timates approached present day values forthe first time, and Lake Valencia becamea freshwater lake, attaining a maximum inits depth. In the early Holocene, the oxy-gen curve from Cariaco shows a monoto-nous ascending trend, interrupted by threeshort and cool oscillations (I, II and III)and the sea-level was about 15 m belowits present position. The laminations ofmarine sediments became lighter abovethe Pleistocene-Holocene boundary and re-mained so to the present. Importantchanges occurred during the mid-Holocenebetween about 7 and 5 14C ka BP, and arealso recorded in both marine and conti-nental sequences. At that time, coincidingwith the Andean La Culata Dry Phase,
Fig. 2. Temperature estimates for the Mérida Andes since the Last Glacial Maximum,compared with the oxygen isotope curve from the Cap Century ice core (simplified fromRull, 1998a). Isotope stages according to Bowen, et al. (1986).
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tively. This is the onset of the PiedrasBlancas Cold Phase, which corresponds tothe Little Ice Age. Using the modern ap-proach of Salgado-Labouriau (1979), theestimated temperature decrease was 2ºCcompared to present. Around the middleof this phase (16th century) Europeanssettled in the vicinity of Lake Valencia,and deforestation and land use increased.The lake’s water level, however, remainedhigh until about the begining of 18th cen-tury, when the modern desiccation of thelake began. Indeed, nowadays the lake isan endorrheic basin, while at these timesthe water level was ~ 25 m above thepresent and it probably drained towardsthe southwest into the Orinoco Basin(Salgado-Labouriau, 1980). Several factorshave been invoked to explain this rapiddesiccation process, including climate andhuman activities. The climatic hypothesisis favored by evidence of increased dry-ness in both the Andes and the GranSabana records, during the Little Ice Age.However, it is also well-known that thedegree of human intervention of the LakeValencia catchment is high enough to pro-
Fig. 3. Land-sea palaeoclimatic correlations in Venezuela, during the last glacial cycle.
when temperatures were slightly lowerthan present ones, local and short termmoisture and water level changes tookplace in the lowlands. The marine recordsuggests an increased seasonality or run-off, and sea level was rising in an almostlinear way, showing its maximum rateknown so far (~ 0.5 cm.y-1). Around 3-4ka BP, a warmer climate was recorded inthe Andes (Miranda Warm Phase), coin-ciding with the begining of more humidclimates in the lowlands and the maxi-mum freshness in Lake Valencia. At thistime, the oxygen curve shows an increas-ing trend, and the sea level was less thanone meter below its present position. TheLittle Ice Age (AD 1450-1850) has beenrecorded only in the Mérida Andes (Rulland Schubert, 1989). The more recentevents are discussed in the next section.
In summary, there is areasonable agreement among Andean, low-land, coastal and marine Venezuelanrecords in the main palaeoclimatic and sealevel events of the last glacial cycle.Pleniglacial and Late Glacial shifts arebest reflected in the marine record, whileHolocene events are well expressed incontinental sediments. However, the reso-lution of continental records is compara-
tively low. Efforts need to be made to ob-tain high-resolution continental records, toprovide a better correlation frame.
The last millennium
This section deals withstudies at the Stream I temporal scale orthe last two millenia. The only palaeo-temperature record covering the last mil-lennium was provided by the Andean Pie-dras Blancas section (Rull, et al., 1987).Lowland sections provide moisture andlake-level estimates, and can be correlated.During the Northern Hemisphere ColdPhase that occurred between the 8th and11th centuries, the temperature at PiedrasBlancas was slightly lower than at present,and Lake Valencia water levels decreased(Fig. 4). At the same time, high fire inci-dence was recorded by high charcoalabundance at Urué, in the Gran Sabana(Rull, in press). In the Medieval WarmPeriod, a minor temperature increase oc-curred, but low water levels persisted inLake Valencia. Burning was notably re-duced in the Gran Sabana. Approximatelyduring the 13th-14th centuries, a drop intemperature and humidity was recorded inthe Andes and the Gran Sabana, respec-
99MAR - APR 1999, VOL. 24 Nº 2
Fig. 4. Palaeoclimatic trends for the last centuries in Venezuela, compared with the North-ern Hemisphere events.
duce a rapid desiccation like that docu-mented by Bradbury, et al. (1981). Afterthe 18th century, climates continued warmuntil the present, causing an intensedeglaciation process, historically docu-mented in the Mérida Andes (Schubert,1984; 1992).
Although several climaticphases of the last centuries can be recog-nized in the Venezuelan records, a high-resolution sequence is still lacking. Thesearch for laminated sediments and depos-its with high sedimentation rates shouldcontinue. The marine cores obtained in theCariaco Basin are promising, but no infor-mation has been published so far. Studieslike those developed on late glacial sedi-ments would be especially useful. On theother hand, several short lake cores em-bracing the last millenia have been takenin the Andes, but palaeontological analysisare not still available (Bradley, et al.,1985; Weingarthen, et al., 1990; Yuretich,1991). Finally, the Piedras Blancas peatbog has not been fully exploited. It seemsto have more potential value for detailedpalaeoclimatic reconstructions than hasbeen realized, owing to the time intervalcovered and the high sedimentation rates(average 25 cm/100 years; 1 cm = 4years). However, to fit with PAGESStream I requirements, it has to be re-sampled with high resolution techniquesand the time control has to be improvedwith more radiocarbon dates.
Holocene sea-levels
Another important issueis the present rising of sea-level, con-nected to the global warming. At present,in the absence of a sea-level curve forVenezuelan coasts, the more reliableworldwide estimations have been used tomake predictions. In this way, a presumedincrease of 0.66 m in the year 2100,might determine the flooding of about 78km2 and the erosion of some
35 km2 of
Venezuelan coasts, with the subsequentimpact on economically important indus-trial, tourist and urban centers (Olivo andPerdomo, 1996). However, no data areavailable to check this assumption and toseparate the effect of natural sea-levelvariability from the predictions based onthe ‘greenhouse effect’. A first attempt todevelop a Holocene sea-level curve forVenezuelan coasts based on buried coralsand mangrove sediments from severalsources is shown in Fig. 5. This curve isvery similar to others from the Caribbeanarea, and almost identical to those fromJamaica, Belize and Panama (Pirazzoli,1991). The main trends are briefly de-scribed in the following. At about 8 kaBP, sea-level was around 15 m below
Fig. 5. Variation of the Holocene sea-levels in the Caribbean Venezuelan coasts. Blacksymbols-mangrove sediments; white symbols-corals.
present. A rapid increase occurred subse-quently at an average rate of 0.5 cm/year.Between 6 and 4 ka BP the trend attenu-ated and after 4 ka BP the rates of sea-level increase were about 20 times lowerthan before (average 0.025 cm/year). Thispicture is based on too few data to ex-trapolate for prediction purposes; evidentlymore work is required in this respect toassess the natural component of the sea-level change. Several older dates (~10 to17 ka BP; Table I) exist in LakeMaracaibo and the Orinoco delta front(Muller, 1959; Sarmiento and Kirby,1962); however, the uncertainty of the en-vironments in which sediments were de-posited makes it difficult to include themin the proposed sea-level curve. In order
to select the coring sites, those with pos-sible buried corals and/or mangrove sedi-ments should be preferred, because theyare the more suitable to record past sea-levels (Kidson, 1982; Pirazzoli, 1991).
General conclusionsand some proposals
Palaeoclimatic recordsavailable in Venezuela consist of marineand continental sequences (peats and lakesediments) encompassing the Last GlacialMaximum (~18 ka BP) to the present, in-cluding some more detailed late glacial(~13-10 ka BP) and recent (last centuries)chronosequences. Palaeoclimate proxiesinclude both geochemical and palaeonto-
100 MAR - APR 1999, VOL. 24 Nº 2
logical analyses, such as oxygen isotopecomposition of forams, organic matter,minerals and metals, pollen, diatoms, fora-minifers, and remains of other planktonicorganisms. Few of these palaeoclimateproxies have been quantified, and moredetailed, extensive and systematic studiesare required for a reliable calibration.
No palaeoclimatic infor-mation is available for times prior to theLast Glacial Maximum. During the lateglacial, climatic oscillations have beenfound, namely the Oldest Dryas, OlderDryas and Younger Dryas stadials, as wellas the Bölling and Alleröd interstadials,based on pollen and laminated sedimentanalyses. The Holocene was characterizedby climates similar to the present one andincluded local and short-therm minor tem-perature and moisture oscillations. At theStream II temporal frame, correlations be-tween marine and continental records areacceptable, but more precision is neededfor predictive models. Studies embracingthe last millennium (Stream II) are prima-rily done on continental sediments. Theyhave shown the occurrence of the well-known Northern Hemisphere climaticphases as for example the Medieval WarmPeriod (11th to 14th centuries), and theLittle Ice Age (15th to 19th centuries).However, the precision of these records isnot enough for modelling of modern cli-mates. Finally, a tentative Holocene sea-level curve is proposed, showing a rapidincrease from 8 to about 6 ka BP, and astabilization before 4 ka BP.
These data need to beenhanced and improved with new projects,with special care on sampling methodsand dating, in order to produce high reso-lution records. Several possibilities havebeen identified, but new ideas and propos-als are welcome. As is known, highmountains are especially suitable for thestudy of recent climatic trends (Beniston,et al., 1997; Diaz and Bradley, 1997). Inthis sense, the Andean Piedras Blancasarea seems to be adequate for palaeocli-matic studies within the Stream I timeframe, whereas the Mucubají region andLake Valencia might be adequate forStream II research. For the establishmentof a detailed sea-level curve, sites withburied corals and mangrove sediments arepreferred. All these proposed sites followone of the most important recommenda-tions of the International Workshop onContinental Drilling for PalaeoclimaticRecords (Colman, 1995), i. e. the exist-ence of preliminary surveys, in order tocheck the type of sediment, the availabil-ity of climatic proxies and the dating reli-ability. Similarly, acoustic reflection pro-files are recommended for medium tolarge lakes like Lake Valencia, in which
an extensive geophysical study of this na-ture has been performed (Schubert, 1980).
The incorporation of newresearch areas to the palaeoclimatologicalstudy is needed. Until now, glaciology,geomorphology, palynology and isotopeanalysis have been almost the only fieldsfrom which former climates and sea-levelswere deduced in Venezuela. Other fossilsas for example diatoms, dinoflagellates,etc. have been successfully used elsewhereand should be incorporated (Rull and Ve-gas-Vilarrúbia, 1993b). The continued useof oxygen isotope records should be alsoencouraged. Archaeological and historicalresearch projects would be extremely use-ful to study the last centuries, and to abetter understanding of the man’s influ-ence on the environment. Finally, instru-mental records, although limited tempo-rally, should not be disregarded, becauseit could play an important role in the cali-bration of palaeoecologically-derivedproxy data.
Concerning the limita-tions, one of the main constraints is thelack of palaeoclimatological laboratoriesand in general of laboratories devoted toQuaternary palaeoecology. Only few indi-vidual researchers are currently developingsuch studies. Therefore, the success of thePAGES project in Venezuela depends, toa great extent, on international coopera-tion, and organizational structure andproject planning become especially impor-tant. This situation should benefit Venezu-elan researchers and students for their owntraining and improvement, and scientificauthorities (especially CONICIT), universi-ties and research institutions which shouldestablish Quaternary research units. Effortsto develop and support PAGES in Ven-ezuela are worth it, because the lack oflong instrumental records makes the roleof palaeoclimatology especially important,in placing recent environmental changes ina long-term perspective. Without such aperspective, estimates of future changeswill be highly speculative.
ACKNOWLEDGEMENTS
The author is very grate-ful to J. Patt Bradbury and two unknownreferees for the critical revision of themanuscript; and to Frank Audemard,Raymond Bradley, Jonathan Overpeck andMilagro Rinaldi for providing literature,including unpublished manuscripts.
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