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A contribution of diatom analysis to Lateglacial and Holocene environmental reconstructionsof Colombian lowland and montane ecosystems

Velez, M.A.

Link to publication

Citation for published version (APA):Velez, M. A. (2003). A contribution of diatom analysis to Lateglacial and Holocene environmental reconstructionsof Colombian lowland and montane ecosystems. IBED, Universiteit van Amsterdam.

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Download date: 13 Mar 2021

Chapterr 2

Latee Holocene environmental history of southern Chocóó region, Pacific Colombia; sediment, diatom and

pollenn analysis of core £1 Caimito

Mari aa Isabel Velez, Michael Wille, Henry Hooghiernstra, Sarah Metcalfe, Jeff Vandenberghe and Klaas van der Borg

Abstract t

Wee present a multi-prox y study of pollen, diatoms, sediment characteristicss and major elements of a 610-cm sediment core from lake Ell Caimito, located in the humid rain forest of southern Chocó, Pacific Colombia,, We propose an integrated reconstruction of the local basin developmentt and of the regional vegetation which is possibly related to thee tectonic activity of an unstable coastal area. Time control is based on

77 AMS 14C dates that show that the record represents the last 3850 cal yr BP.. From 3850-2700 cal yr BP sandy deposits, low carbon content, absencee of diatoms, and low diversity of the pollen spectra indicate that thee site was under the influence of the fluvial system. Erosive event(s) removedd part of the sediment record and we observed a hiatus representingg 700 years. After that, the basin became more isolated from thee river drainage system From 2010-1430 cal yr BP mainly clay was depositedd and repeatedly interrupted by river pulses that left sandy and siltyy horizons in the record. Benthic and littoral-benthi c diatom species indicatee a shallow water body and a stable water chemistry. Mangrove forestt was close to the lake, apparently growing along the close-by inlets. Regionally,, the main vegetation elements were palms, and taxa in the familiess of Moraceae-Urticaceae, Melastomataceae, Leguminoseae and fromfrom a number of other families and genera, characteristic of tropical lowlandd rain forest. Fromm 1430-810 cal yr BP the river impact gradually diminished. Each fluviall event that affected the local forest is shown in the pollen record by ann expansion of Cecropia dominated pioneer forest. Decreasing intensity

7 7

off forest disturbance coincides with an increase in the diversity of fossil pollenn taxa, possibly reflecting an increasing plant diversity of the forest. Mangrovee pollen declined, indicating that the coastline moved seaward andd suggesting tectonic uplif t of the coastal area. Between 810 and 580 call yr BP mangrove forest was closer to the lake again, reflecting an inlandd migration of the coastline, suggesting tectonic subsidence. From 580-3000 cal yr BP the last fluvial events were recorded. Diatom associationss indicate oligotrophic and acidic water. The mangrove belt movedd seaward again, suggesting tectonic uplift . Palms and Cecropia becamee more abundant, suggesting increased human impact in the near shoree lowland forest. During the last 300 years, stable lacustrine conditionss and lowland rain forest with the highest floral diversity is registered. .

1.. Introductio n

Thee strip of rain forest along the Pacific coast of Colombia, known as Chocóó Biogeographic Area, is known for its high precipitation values, up too 12,000 mm/year, and high plant diversity. The geological history of thiss area since the Miocene has been discussed among others by Hooghiemstraa and Van der Hammen (1998) and Hoorn et al. (1995). Theree is substantial evidence from records in the Amazonian rain forest thatt its present extension was reduced during dry intervals of the last glaciall period, in particular during the last glacial maximum (Hooghiemstraa and Van der Hammen, 1998; Van der Hammen and Absy, 1994;; Van der Hammen and Hooghiemstra, 2000), but this evidence is debatedd by others (Bush et al., 2001; Colinvaux et aL, 2000). There is insufficientt understanding how high plant diversity in the neotropical rain forestss was generated during the Neogene and Quaternary timee (Bush and Colinvaux,, 1988; Colinvaux, 1997; Colinvaux, 1998; Colinvaux et al., 2000;; Schneider et al., 1999). According to Colinvaux (1997; Colinvaux ett al., 2000) speciation can be understood in terms of an 'engine model' or aa 'museum model'. From Amazonia there is important evidence that showss high internal dynamics in the rain forest, caused e.g. by migrating river ss or precipitation variabilit y related e.g. to ENSO or even perhaps to orbitall yy induced precession changes. As the setting of Chocó rain forest differ ss from Amazonia, it is unclear to which degree environmental

8 8

instabilityy is also characteristic of the rain forests in Chocó. To explore thiss question further, sediment cores in three lakes were collected in 1997. Previouss palaeoenvironmental records from southern Chocó (Laguna Piusbi,, Behling et aL, 1998), and from northern Chocó (Laguna Jotaordó, Berrioo et aL, 2000) and the San Juan Delta (Ramirez and Urrego, 1999) documentedd for the first time in Chocó environmental change during the lastt c. 4500 years. From those data a stable forest history was inferred. Thee present study presents at a multi-proxy analyses of the core from Lagunaa El Caimito, integrating data from pollen (analysed by M. Wille), diatomss (analysed by M. Velez) and sediments (analysed by M. Velez and J.. Vandenberghe) in order to provide a maximum understanding of regionall environmental change. The use of different proxies enables us to testt the palaeoclimatic interpretation of a single proxy, as is the case in thee studies of Lagunas Piusbi (Behling et aL, 1998) and Jotaordó (Berrio ett aL, 2000), and provides the opportunity to validate climatic calibration data.. We compare the environmental history of Chocó to palaeoclimatic changess in the Colombian savannas (Behling and Hooghiemstra, 2000; Behlingg et al., in prep., Berrio et aL, 2002).

2.. Environmental setting

2.12.1 Geography

Thee Pacific coast of Colombia, where the Chocó biogeographic area is located,, is a tectonically active area. Earthquakes are relatively frequent andd have caused subsidence of portions of the Pacific coastline (Herd et aL,, 1981). Oceanic Cretaceous limestones are overlain by Tertiary sedimentss represented by fluvial terraces, and which are covered by Quaternaryy pyroclastic and alluvial deposits (Galvis et aL, 1993).

Lakee El Caimito (2°32' N, 77°36' W) is located at 50 m elevation in the southwesternn part of the Department of Cauca, southwest Colombia (Fig. 1).. It lies between the towns of Buenaventura in the north and Tumaco in thee south. The lake has a maximum depth of c. 4 m, a diameter of about

2000 m, and covers an area of c. 0.5 km . The lake has steep slopes which aree breached by the outlet creek. The main inlets to the lake come from thee east and northeast. In the western part the El Caimito creek forms the

9 9

outlett which flows from southeast to northwest. After about 6 km this creekk meets the Rio Guapi which reaches the Pacific Ocean near the town off Guapi. The Pacific coast, made up by many relatively small islands separatedd by smaller and larger inlets, is located c. 40 km to the northwest off the lake. About 30 km to the east of the lake the Western Cordiller a (Serranfaa de San Juan) rises up to a maximum altitude of some 1500 m.

2.22.2 Vegetation

Thee vegetation around Laguna El Caimito consists of tropical evergreen rainn forest. The steep slopes prevent growth of a shore vegetation. For the vegetationn around Laguna Piusbi (Fig. 1; Behling et al., 1998) Moraceae, Melastomataceae,, Lauraceae, Fabaceae, Caesalpiniaceae, Mimosaceae, Rubiaceae,, Myrtaceae and Arecaceae were identified as the most importantt families representing rain forest vegetation. Along the coast theree is a c. 10-20 km wide belt with mangrove forest. The forest in the hinterlandd of Guapi is named 'guandal' (del Valle, 1996), a floristically diversee forest in which the following taxa are important for the pollen record:: Campnosperma panamensis, Socratea exorrhiza, Otoba gracilipesgracilipes and Ocotea oblingifolia.

2.32.3 Climate

Thee Pacific coast of Colombia receives some of the highest precipitation amountss in the world. Mean annual precipitation ranges from 2000 mm in thee south to 12,700 mm in the north. Seasonality in precipitation is related too the latitudinal movement of the Intertropica l Convergence Zone (ITCZ) .. Maximum precipitation occurs from Apri l to June, when the ITCZZ shifts north, and from September to November, when the ITCZ shiftss to the south. Periods of marked decrease in precipitation are rare andd short (one week) and may occur during El Niflo events (West, 1957). Inn 1983, a strong El Niflo year, the weather station in Guapi recorded the followingg reductions in precipitation relative to the same period in 1982: Januaryy -37%, February -50% anèMarch -43% (IDEAM , pers. comm.). Meann annual precipitation between 1981 and 1999 is c. 6500 mm, and meann annual temperature 26°C (IDEAM , pers. comm.). The diurnal temperaturee range in Tumaco and Buenaventura is less than 5QC (Del Valle,, 1994).

10 0

Figuree 1. Map showing the location of Laguna El Caimito in the Chocó Biogeographicc area

2.42.4 Human occupation of Chocó

Thee Pacific coast of Colombia and the area of Guapi have been affected byy human settlements since the second century AD. According to Patifioo (1988) early populations lived in the coastal area and did not reachh further inland. However, the pollen record of Laguna Piusbi

recordedd human impact since about 3460 C yr BP based on an increasee of palms and grassy vegetation (Behling et al., 1998). Agriculturall activities in the area of Laguna Piusbi, shown by the

14 4 presencee of Zea mais, were recorded since about 1700 CyrBP.

3.. Methods

3.1.3.1. Core recovery, pollen and diatom analysis

Thee 610-cm long core was drilled in 100-cm increments by H. Behling andd H. Hooghiemstra in the centre of the lake with a modified Livingston

11 1

corer.. Presence of sand prevented further sediment recovery. The 100 cm sedimentt intervals, collected in aluminium tubes, were transported to the laboratoryy in Amsterdam and stored under cold (4°C) conditions. Followingg lengthwise opening of the tubes in the laboratory separate sampless of 1 cm-* were taken for diatom and pollen analysis at 5 cm intervalss along the core. Diatomss were prepared according to Van der Werffand Huls (1963). The followingg literatur e was used for diatom identification and ecology: De Oliveir aa and Steinitz-Kannan (1992), Fürstenberger (1997), Germain (1981),, Hickel and Hakansson (1991), Hustedt (1930, 1959, 1961-66), Krammerr and Lange-Bertalot (1986, 1991, 1997), Moro (1998), Patrick andd Reimer (1966), and Steinitz-Kannan et al. (1982, 1986). In each samplee a minimum of 400 valves were counted; only the interval from 5044 to 475 cm contained insufficient diatoms to reach that sum. Diatoms aree grouped into the following habitat preferences, 1) planktonic, 2) benthic,, 3) littora l and 4) aerophylous. Pollenn samples were prepared using standard treatment, including sodium pyrophosphate,, acetolysis and heavy liquid separation by bromoform (Faegrii and Iversen, 1989). Before treatment, tablets with exotic LycopodiumLycopodium spores were added to each sample for calculation of pollen concentrationn and pollen influx values. Pollen samples were mounted in a glycerinn gelatin medium. A minimum of 300 pollen grains was counted. Thee pollen sum consists of taxa representing the regional forest; aquatic taxa,, fern spores, fungal spores and algae colonies are excluded from the pollenn sum. For pollen and spore identification we used publications by Behlingg (1993), Graf (1992), Herrera and Urrego (1996), Hooghiemstra (1984),, Murill o and Bless (1974, 1978), Roubik and Moreno (1991), and thee reference collection of modern pollen and spores of the Hugo de Vries-Laboratory .. The software EXCEL, TILIA , TILIAGRAP H and CONISSS was used for calculations, graphing the diagrams, and cluster analysiss of the diatom and pollen diagrams.

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0-1122 cm soft dark greenish/grey fine detritus mud with leaf fragments;fragments; horizons with melastomataceous leaves att 15 and 84.5 cm

112-182.55 cm peaty, dark grey detritus mud; horizons with leaves att 112,140 and 162 cm; wood fragments at 167 cm

182.5-183.55 cm light grey clay 183.5-215.55 cm dark greenish/grey fine detritus mud; horizon with

leavess at 215 cm 215.5-5088 cm dark grey organic rich clay.; few leave remains;

horizonss with leaves at 367 and 451 cm 508-6077 cm light brown/grey silt 607-6100 cm hard brown sand

Tablee 1. Stratigraphy of the core El Caimito

3.23.2 Time control of the sediments

33 1A

Sevenn bulk samples of 1 cm were collected for AMS C dating at depthss where significant changes in the pollen record occurred. The samplee at 546 cm had a volume of 4 cm due to low organic content. Sampless were cleaned of roots and dated at the Van der Graaff-Laboratoryy of the University of Utrecht (Van der Borg et aL, 1987). The 14CC ages were calibrated with the Cal20 software (Van der Plicht, 1993). Thee calibrated ages were determined after visual inspection of the probabilit yy distribution s (2s) along the dendro-calibration curve. We tried too fix the calibrated dates relatively in the middle of the probabilit y distributio nn curves and there at highest probabilit y values. Due to this proceduree we give no standard deviation or time intervals for the calibratedd ages.

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Depth h (cm) ) 11 1 151 1 266 6 398 8 498 8 546 6 598 8

14 4

CyrB P P

-16211 8 5222 0 8799 1 15166 1 20400 0

4 4 34922 5

calB P P

modern n 535 5 775 5 1405 5 1975 5 3175 5 3760 0

MCC (p.mll)

-32.4 4 -33.0 0 -32.1 1 -33.8 8 -32.9 9 -25.6 6 -28.3 3

UtCC No.

8365 5 8366 6 8367 7 8368 8 5790 0 8369 9 5789 9

Coll No.

1186 6 1187 7 1188 8 1189 9 1107 7 1190 0 1106 6

Tablee 2. Radiocarbon ages of samples from core El Caimito

3.33 Sediment analysis

Sampless for grain size analysis were taken every 5 cm at depths adjacent too those used for pollen and diatoms samples; from 500 to 610 cm core depthh we sampled at 10 cm increments. Samples were prepared for sedimentologicall analysis according to Konert and Vandenberghe (1997) andd analysed by a FRITSCH A22 'Laser Particle Sizer*. Sand is >63nm, clayy is <5.5 urn, and silt represents the range from 5.5 to 63 um. The sedimentss were 'described with, a Munsell colour chart. To analyse the totall organic content of the sediments, samples for loss on'ignition (LOI) weree taken every 10 cm and processed after Speranza et'M? (2000). Major elementt analysis was carried out using atomic emission spectrometry for sampless taken at 20 cm increments along the core.

4.. Results

4.14.1 Stratigraphy and time control

Tablee 1 shows the major sedimentological units of the core. Fig. 2 shows sedimentt characteristics including grain size, LOI and major elements. Thiss diagram includes 57 samples for LOI, 106 samples for grain size and 355 samples for major elements.

Thee seven AMS 14C ages (Table 2) of core El Caimito show that the sedimentt core represents the last c. 3850 calendar years before present (call yr BP). The depth versus age graph (Fig. 3) shows a sedimentary gap betweenn 546 and 498 cm. In the sedimentary record there is a distinct

14 4

changee at 504 cm; this core depth has a calculated age ranging from 2700 too 2010 cal yr BP. We conclude that during that period sediments were erodedd and sedimentation started again around 2010 cal yr BP; as a consequence,, there is a hiatus in the record of almost 700 years. From 498 cmm to the top sedimentation rate continuosly decrease. We suggest that thiss decrease is related to the gradually decreasing availabilit y of sedimentt in the small catchment area after the lake came into existence. Thiss interpretation is supported by the decrease in sand content in the upperr part of the sediment column (Fig. 2).

Downcoree changes in the diatom content are shown in Fig. 4 based on 99 samples.. A total of 68 diatom species were identified. Species with less thann 1% representation were not graphed and not used for the interpretation ;; most represented benthic and periphytic taxa. Planktonic diatomss were limited to only a few species. Downcoree changes in the pollen spectra are shown in Fig. 5 including 119 samples.. Below 605 cm no pollen was preserved; between 605 and 540 cmm pollen was absent or the pollen content was too low to reach a meaningfull sum. In total 168 different fossil pollen and spore types were identified.. Additional 25 pollen and spore types remained unknown. Pollenn grains represent mainly arboreal taxa (about 90%). Between 605 andd 500 cm herbaceous taxa reach about 20% of the total pollen sum. Aquaticc pollen was almost absent.

15 5

Scal ee facto r 0.10: MgO/AI£> j Sea l * facto r 0.5: P£fc , TiCfe F e ^ V A I j p ,

Figuree 2. Downcore changes of grain size and major elements of the core Ell Caimito. Major elements are in percentages of total sample weight.

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100 0

200 0

Isco o

f f a a

0 0

500 0

600 0

/l\ \

JLJM M 15000 2000 2500

w w

3000 0

f f 455 J 4 0 | |

3 0 , ,

2 S | | 2 0 | | 155 g. 100 J 55 V)

35000 4000

Call BP

Figuree 3. Depth vs age graph and sand content of the core El Caimito

4.24.2 Description of the zonation of the pollen and diatom records

Fig.. 6 shows a selection of the most important pollen and diatom taxa. Thee zonation is based on combining results from cluster analysis of the pollenn and diatom diagrams, the grain size analysis, and visual inspection off the CONISS dendrogram. Six different zones, CAI-1 to CAI-6, can be recognised. .

Inn zone CAI-1 (605-504 cm) sediments are mainly composed of sandy siltss (25% sand and 44% silt) and a smaller proportion of clay (30-35%). Thee colour of the sediments (2.5Y 4/4) and the low LOI values (5-10%) indicatee a low organic matter content. The percentages of the major elementss are low; only Si02 is abundant (64%). No diatoms are preserved inn this core interval. It is likely that the high porosity of the sandy depositss prevented preservation. Also pollen grains are badly preserved.

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Especiallyy in the lower part of this zone (605-540 cm) pollen content is loww and mainly grains of Fabaceae (32%-50%), Moraceae/Urticaceae (40%),, and Melastomataceae (15%-30%) are found. Between 540 and 5044 cm the representation of Leguminosae decreases to 35-10%, whereas thee percentages of Moraceae/Urticaceae and Melastomataceae increase slightly.. Selaginella (15%), Cyatheaceae and other fern spores and Asteraceaee (30%) reach maximum values. This interval includes the radiocarbonn age of 5 14C yr BP (3760 cal yr BP) at 598 cm, and

44 14C yr BP (3175 cal yr BP) at 545 cm.

Thee onset of zone CAI-2 (504-402 cm) is defined by a change from non-organicc silty sediments in CAI-1 to a humic clay (10YR 4/2)* The stratigraphi cc contact between both types of sediments is sharp. This abruptt change is also recorded by the LOI which rises suddenly from 8.3%% to 29%. The sharp contact and the shift in the depth vs. age graph (Fig.. 3) points to a hiatus at 504 cm. In the lower part of zone CAI-2 the sandd fraction is high (15%) but it gradually decreases towards the top of thee zone (< 4%). The contribution of clay varies between 30% to 74%, whereass the silt fraction is relatively stable around 30%. Most of the elementss show a marked increase at the bottom of the zone, except for Si022 (average 64%-35%), Ti02 (average 0,9%-0,7%) and the MgO/Al2033 ratio (average 0,09%-0,055%) which decrease. From this depthh to the upper part of the ^ore, the percentages of Fe2Ü3, AI2O3, Ti022 are relatively constant (Fig. 2).

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?/?/ At/ / /&/\#B*tHc/&/\#B*tHc Planktonk: Littoral

200 IQ X

Figuree 4. Diatom percentage diagram of the core El Caimito. Zones basedd on CONISS cluster analysis

19 9

f\ \ '«« »M«MIWIB«BW««nBB»sarMB3E3gWHW»WBB 5 \\ BiSSIBWaSMBWWWBWtMIBSSWWHISfflSSSIBS =S

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! \\ s

ss \ V .. %fiil'i!ily.rriiiini„.i,... i

^ * Ü ^ ^

>>> h .,... .

Si Si V r r

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HH l|nll feil i,, i iftlll lil illlu. t« i U I uil i iuu

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6-/V1M M

*%i ;IL i i

\ \\ \UUËlÉÉ!tt

11 il ; I: i Jül i.,

I II ftilll lilillli

ii I k ,

ii ëiwt . ü . i . i . i . i l l i ï a

ïMlIli S S

dkL L

,..,1... .1.1 , i

„i,L L

l f f

11133 «

t>°° s s g s s s s s ss s

Figuree 5. Pollen percentage diagram of the core El Caimito showing the mostt important taxa. Pollen sum values and CONISS cluster analysis is shownn at the right.

20 0

Diatomss are scarce and badly preserved at the bottom of zone CAI-2; theyy appear at 475 cm. Initially , the assemblage is dominated by FrustuliaFrustulia rhomboides (55%), Pinnularia spp. (50% in the bottom to 15% inn the middle part) (bad preservation prevented further identification), CymbellaCymbella gracilis (15%) and C. spicula (15%). The middle and upper partt of the zone is dominated by Frustulia rhomboides (30%-45%), AulacoseiraAulacoseira cf herzogii (c.30%), Anomoeoneis brachysira (20%-30%). CymbellaCymbella and Pinnularia species are reduced to about 5% or less. Speciess of Eunotia, Navicula, Achanthes subatomoides and Actinella guianensis,guianensis, appear for the first time (Fig. 4). Thee pollen record shows at the bottom of CAI-2 a sharp increase of RhizophoraRhizophora (5%-10%) and Cecropia (5%-15%). Towards the upper part off this zone representation of Rhizophora is increasing from 10% to 25%. SelaginellaSelaginella disappears, and representation of Asteraceae and Fabaceae dropp to low values (5%). The contribution of Moraceae/Urticaceae is stablee in the whole pollen zone (50%), whereas percentages of Melastomataceaee decrease from 30% to 15%. This interval includes the radiocarbonn age of 204G£60 14C yr BP (1975 cal yr BP) at 498 cm.

Inn zone CAI-3 (402-274 cm) sediments are dominated by clay (around 70%).. The silt fraction is relatively stable (around 28%) and the sand fractionfraction is low (< 2%). At 294 cm and 341 cm there are marked increases inn sand (15% and 5%, respectively). LOI values are relatively constant betweenn 25% and 30%. P2O5 is relatively high (0,65%) but decreases to thee top of the zone to 0,51%. The CaO/Al203 ratio is low (average 0,0144%)) with maxima at the bottom (0,0071%) and in the upper part of thee zone (0,0061%), At 340 cm most of the major elements have a sharp increasee (Fig.3) except for phosphorous. Thee diatom assemblage is dominated by Anomoeoneis brachysira (55%-60%)) and Frustulia rhomboides (25%). Other main taxa are Eunotia intermedia,intermedia, Cymbella gracilis, C. spicula, Aulacoseira cf. herzogii and PinnulariaPinnularia spp. In this zone Eunotia species become more common and PinnulariaPinnularia spp. are less common than in the previous zone (Figs. 4 and 5). . Inn the pollen record Moraceae/Urticaceae and Melastomataceae (both aroundd 20%) are most abundant. Rhizophora decreases from 30% at the bottomm of the zone to 20% at the top. Cecropia values increase from 5%

21 1

att the bottom to 15% at the top. This interval includes the radiocarbon age off 1 14C yr BP (1405 cal yr BP) at 398 cm.

Inn zone CAI-4 (274-174 cm) sediments continue to be dominated by clay (aroundd 70%). The content of silt and sand is similar to the previous zone. Thee proportion of sand increases sharply at 273 cm (up to 10%) and 219 cmm (up to 7%), and at these horizons the contribution of clay decreases to 45%% and 49%, respectively. LOI values decline from 27% at the bottom too 18% at the top of the zone, probably related to the increasing sand content.. P2O5 values decrease from 0,47% to 34%, values of other elementss are stable during this interval. Thee diatom assemblage is dominated by Anomoeomis brachysira (45%-55%)) and Frustulia rhomboides (c. 40%). Important taxa, in decreasing orderr of abundance, are: Aulacoseira cf. herzogii, Eunotia intermedia, CymbellaCymbella gracilis, C. spicula, Eunotia aff. glacialis and Pinnularia spp. A.A. brachysira tends to decrease towards the top, whereas F. rhomboides andd A cf herzogii tend to increase. Common elements are Anomoeomis seriansserians and Actinella guianensis. Eunotia septentrionalis disappears fromfrom the record. Thee pollen assemblages continue with Moraceae/Urticaceae (20-30%) andd Melastomataceae (15%). The contribution of Cecropia increases to 15%% at the middle of the zone, falls to 3% around 200 cm, and increases too 13% at the top of CAI-4. The values of Rhizophora range between 10% andd 30% and representation is contrary to the representation of Cecropia. Thiss interval includes a radiocarbon age of 1 14C yr BP (775 cal yr BP)) at 266 cm core depth.

Inn zone CAI-5 (174-91 cm) sediments are composed mainly of clay (aroundd 80%). At 119, 124, 167, 173 cm core depth, there is a marked increasee in sand content (12%, 6%, 20%, and 4% respectively) and a concomitantt decrease in clay. At 119 cm there is a marked increase in the grainn size as there are sand particles of 500 urn diameter. Values of LOI havee a minimum of 13% at 165 cm, and maxima of 30% (125 cm) and 32%% (115 cm); during the remaining part of this interval values are low (17-18%).. At 120 cm depth all major elements show a marked decrease: P2O55 from 0.35% to 0.24%, Ti02, from 0,9% to 0,7%, Fe203/Al203 rati oo from 0,4% to 0,3%, and S1O2 from 46,5% to 39%. Only the ratio

22 2

CaO/Al2033 increases from 0,01% to 0,004%. In the remaining upper part off the core average values of the previous zone are reached again. Thee diatom assemblages are dominated by Anomoeoneis brachysira (20%-45%),, Frustulia rhomboides (c. 35%-40%) and Aulacoseira cf. herzogiiherzogii (10%-30%). Common species are Anomoeoneis serians, Eunotia intermedia,intermedia, Cymbella gracilis, C. spicula, Eunotia aff glacialis, E. subarcuatoides,subarcuatoides, E. minor, E. trigibba, Actinella guianensis. Pinnularia spp.. are rare and tend to disappear. Thee pollen assemblages show average values of Melastomataceae of 15-20%% with two maxima at 150 cm and 110 cm; at the top of the zone only 10%% is noted. The percentages of Moraceae/Urticaceae decrease to 10%. RhizophoraRhizophora values decrease to 3% in the middle of the zone, but increase too 10% at the top. The representation of Cecropia increases from 7% to 10%.. The representation of Arecaceae, mostly with values of 3-4% in the core,, reach in the middle of the zone relatively high values of 10%. This intervall includes a radiocarbon age of 0 14C yr BP (535 cal yr BP) att 151 cm.

Inn zone CAI-6 (91-0 cm) sediments are dominated by clay (around 75%). Att 73 cm the clay content decreases from 70% to 44% coinciding with an increasee in silt of 28% to 55%. LOI values are stable around 17% . The contributionn of most elements are stable, except for the increase in the ratioss of MgO/Al203 (average 0,068%-0,078%) and Fe203/Al203 (averagee 0,3%-0,47%). Thee diatom assemblages are dominated by Anomoeoneis brachysira (c. 40%),, Frustulia rhomboides (25%) and Aulacoseira cf. herzogii (10-20%).. This zone shows high representation of Eunotia intermedia (increasee from 5% to 10%), E. praerupta (c. 5%), Cymbella gracilis (c. 5%),, and Eunotia subacuartoides. Pinnularia braunii appeared again in thiss interval.

23 3

JBHHHHL..... . . V M t t k l t i M i i É t t

II i I I l i l Figuree 6. Percentage diagram of dominant pollen and spore taxa (left) andd diatom taxa (right) of the core El Caimito. Radiocarbon ages, extrapolatedd time frame (in cal yr), depth scale, lithological column! zoness and CONISS cluster analysis are shown.

Thee pollen assemblages are characterised by increasing values of Moraceae/Urticaceaee from 14% to 25% and decreasing percentages of Melastomataceaee from 15% to 10% minimum. Rhizophora values show

24 4

ann average around 10% with two spikes of 15%. Cecropia percentages increasee from 10% to 15% at the top of the zone. This interval includes a radiocarbonn age of 8 14C yr BP (modern) at 11 cm.

5.. Reconstruction of environmental change in the study area

Zonee CAI-1 (estimated period from 3840-2700 cal BP; 605-504 cm). The highh proportions of sand and silt, and the low organic matter content of thee sediments indicate that during this period the site was a depression influencedd by fluvially transported material. Abundant coarse sediments aree indicative of a high-energy environmental setting. Reducing conditionss in the sediments might be indicated by relatively low values of Fe203.. The coarse deposits provided unfavourable conditions for preservationn of diatoms and pollen grains and, as a consequence, the pollenn and diatom association may be biased. The presence of Selaginella fernss (common in active drainage systems) is indicative of a high energy environment,, as well as abandoned parts of a drainage system. Presence off Fabaceae, Asteraceae and Melastomataceae may be indicative of an arboreall vegetation that occurs in coastal areas out of the reach of brackishh water input (del Valle, 1996). Mangrove forest (Rhizophora) is absentt in the catchment area of the basin. Arboreal taxa, such as Piper, Acalypha,Acalypha, Alchornea, Solanaceae, Hedyosmum and Cyatheaceae reflect inlandd lowland forest.

Thee transition from zone CAI-1 to CAI-2 coincides with a gap in the sedimentaryy record of about 700 years. Our study area is well known for itss tectonic activity and episodes of subsidence. In the coastal area near thee town of Guapi an earthquake in 1979 caused a subsidence event of aboutt 1.4 m (Herd et al., 1981). Gonzalez (2001) also recorded for the periodd of about 500 years ago subsidence of the coastaal area near the deltaa of the San Juan river of about 1.3 m. It is possible that a tectonic eventt caused a change in the river system allowing the formation of El Caimitoo lake.

Zonee CAI-2 (estimated period from 2010-1430 cal yr BP; 504-402 cm). Thee increase in clay and total carbon content of the sediments suggest thatt sedimentation occurred under quiet lacustrine conditions where

25 5

organicc material from the surrounding vegetation could accumulate. The amountt of silt and sand at the beginning of the period indicates that the sedimentaryy basin was not fully isolated from the regional drainage systemm and at least periodically experienced input of coarse sediment underr higher energy conditions. However, the coarse sediment fraction diminishedd rapidly and indicates that the basin became isolated within aboutt 200 years. Shallow conditions are indicated by the abundance of benthicc and littoral-benthi c diatom species, such as Frustulia rhomboides,rhomboides, Anomoeoneis brachysira, Pinnularia spp. Cymbella spicula, C.C. pseudogracilis and species of Eunotia. This diatom association suggestss the lake was acidic (pH range between c. 4 and 6), oligotrophia andd the water had low conductivity. Riverine influence during the second halff of this period is supported by the presence of Aulacoseira cf herzogiiherzogii which requires turbulence (Van Landingham, 1964) and some turbidit yy (Jewson et al., 1993). The diatom association, grain size characteristics,, LOI values, and major elements indicate that sedimentationn in a proper lacustrine environment started in this period. Mostt conspicuous in the pollen record is the presence of mangrove forest.. Marin e palynological studies (Muller , 1959; Hooghiemstra and Agwu,, 1986) have shown that significant representation of mangrove pollenn is only found at a short distance from the source area and there is nono evidence that wind transport is an important factor in the distributio n off Rhizophora pollea We therefore assume that the mangrove forest was closee to the lake. At present day brackish water and mangrove forest penetratee several kilometres inland. Given the mangrove forest proportions,, we infer that the coastline was closer to the lake. This situationn is very plausible as tectonic uplif t and/or fluvial deposition of materiall from the Western Cordiller a might have caused shifts of the coastline.. In the regional forest Moraceae/Urticaceae, Melastomataceae, Arecaceaee (palms), Fabaceae, Piper, Acalypha, Solanaceae, Alchornea, Sapotaceaee and Sapindaceae were important taxa. Significant representationn of Weinmannia may be related to regional forest at higher elevation,, but Gentry (1986) noted presence of 'montane trees' in the lowlandd forests of Chocó under extreme precipitation values (see the discussionn in Van der Hammen and Hooghiemstra, 2000). Cecropia is a typicall pioneer tree which occupies barren land after disturbance. Highest representationn of Cecropia coincides with peaks of silt and sand fractions,fractions, suggesting that expansion of Cecropia forest follows

26 6

disturbancee of local forest by river action. In between these events of forestt disturbance mature forest, dominated by Melastomataceae, Fabaceaee and Arecaceae, was common.

Zonee CAI-3 (estimated period from 1430-810 cal yr BP; 402-274 cm/ Sedimentss consist almost entirely of clay and silt. Apparently, the fluvial inputt decreased. Events with increased sand content, and changes in majorr elements, indicate that there were two main pulses of increased riverin ee energy. The first one occurred between 1160 and 1130 cal yr BP (340-3466 cm). The marked change in major elements, in particular the increasee in Ti02 and Fe203, indicates more soil erosion followed by an increasedd input of heavy minerals and detrital material into the lake. The secondd pulse occurred around 900 cal yr BP (295 cm). Cecropia respondss to these events and pioneer forest expanded. Also the high representationn of 'indeterminate pollen grains' (Fig. 5) may be related to riverin ee supply of a wider variety of pollen taxa from the watershed area too the basin. Benthic species dominate the diatom assemblages; it is plausiblee that the lake reached its present setting during this period with a waterr depth of 4 m. The Rhizophora record reaches highest values in the beginningg of this zone (around 1430 cal yr BP) and mangrove forest, possiblyy representing a narrow zone along the river system, must have =beenn close to the lake. During this period the proportion of Rhizophora decreased;; apparently the belt with mangrove forest shifted seaward betweenn 1430 and 810 cal yr BP. We infer a westward migration of the coastline,, possibly related to regional tectonic uplift . Compared to the previouss zone, the regional forest hardly changed in floral composition. Moraceae/Urticaceae,, Melastomataceae, Arecaceae (palms), Fabaceae, Acalypha,Acalypha, Solanaceae, Alchornea, Sapotaceae and Sapindaceae were importantt taxa, but Piper and Acalypha became less common. The proportionn of Weinmannia remained unchanged compared to the previouss zone.

Zonee CAI-4 (estimated period from 810-580 cai yr BP; 274-174 cm). Thiss period shows alternating peaks of clay and silt, pointing to a stable lacustrinee setting. Three spikes of the coarse fraction (sand and silt) at 2755 cm, 225 cm and 174 cm document occasional riverine input of sediments.. The first two events are, as in the previous events, followed

27 7

byy expansion of Cecropia dominated pioneer forest. Also the increase of AulacoseiraAulacoseira cf. herzogii might indicate an increase of deposition̂ energy,, as this diatom species requires turbulence and turbidit y to float (Vann Landingham, 1964; JewsonetaL, 1993). Thee representation of Rhizophora first increased (until about 720 cal yr BP),, but diminished between about 640 and 580 cal yr BP and remained att a low level up to recent time. We interpret the second major expansion off mangrove forest in this record either as an inland migration of the coastlinee (in the direction to lake El Caimito) related to tectonic activity, orr a significant re-organisation of the system of water ways in the coastal areaa near Guapi. The latter may also be related to tectonic activity. The pollenn record shows littl e evidence for changes in the floral composition off the regional lowland forest, compared to the previous zone. Palms (Jriartea(Jriartea in particular) , Sapotaceae and Protium became more common in thee forest, whereas the gradually increasing values of 'mdeterminate pollenn grains1 (not occurring as spikes as previously; Fig. 5) is considered too reflect a floristically more diverse forest.

Zonee CAI-5 (estimated period from 580-300 cal yr BP; 174-91 cm). Grainn size shows a continuation of the conditions in the previous zone. Theree are two main spikes of coarse sand and silt which represent occasionall pulses of high riverin e energy. The oldest peak occurred at c. 5700 cal yr BP and the other at c. 420 cal yr BP. Both events relate to abruptt maxima in the total organic content of the sediments. It is plausiblee that this organic matter was supplied by the river and caused a moree productive lake. Considering the total record, this zone reflects a loww energy environment with maximum accumulation of clay, and an inputt of organic matter that reflects the autochthonous production in and aroundd the lake (high energy riverin e events supply coarse fraction as welll as allochthonous organic matter). The diatom association, AnomoeoneisAnomoeoneis brachysira, Eunotia spp. and Frustulia rhomboides in particular ,, indicates oligotropic and acidic water with low conductivity. Thee peak of Aulacoseira cf. herzogii and the concomitant decrease in benthicc diatoms during the last high energy riverin e event support a more energeticc environment, a disturbed bottom lake and an increased in turbidit yy and water turbulence. Thee pollen record from this zone onward (the last 580 years BP) shows loww representation of Rhizophora. The belt with mangrove forest

28 8

apparentlyy reached the greatest distance from lake El Caimito recorded in thee last 2000 years. This may be indicative of further tectonic uplif t of the coastall area. From this zone onward, the representation of Cecropia and palmm trees (Arecaceae) increased markedly suggesting that more inland areass were used and disturbed by human activities. Palms are intensively usedd in human settlements and a response of the palm record to increased humann disturbance was also noted in lake Piusbi (southern Chocó; Behlingg et al., 1998), Amazonia (Berrio et al., 1999), and in the savannas off the Llanos Orientales (Behling and Hooghiemstra, 1998, 1999). The proportionn of 'indeterminated pollen grains' has higher values reflecting ann increase of the floral composition of the coastal forest.

Zonee CAI-6 (estimated period from 300 cal yr BP to present day). Sedimentss are dominated by clay and silt indicative of stable lacustrine conditionss without riverin e influence. Low LOI values indicate that the levell of carbon input by the local ecosystem in and around the lake was low.. The diatom assemblages indicate acidic (estimated pH 4-6) and oligotrophyy water with low conductivity. The increase of Eunotia praeruptapraerupta and P. braunii indicative of iron (Moro and Fürstenberger, 1997)) matches the increased values of Fe2<>3. Therefore, we infer the waterr was richer in iron. Thee pollen record shows a regional forest type in which Moraceae/Urticaceae,, Melastomataceae, Arecaceae (palms), Fabaceae, Piper,Piper, Acalypha, Sapotaceae, Protium, and Ficus are most important. Thee proportion of'indeterminate pollen grains' reaches the highest values reflectingg higher floral diversity of the forest. The relatively high proportionn of Cecropia and palms are considered as (partly) a reflection off increased human disturbance. Indeed, the Spanish Conquest in Colombia,, provoked migration of populations from the Andes to remote areas,, including Chocó. During our visit to the forest we noted at remote placess shelters for fishermen and hunters totally constructed by palm trees.. The contribution of potential 'montane trees' to the pollen spectra {Weinmannia,{Weinmannia, Hedyosmum, Ilex) is low and may reflect the natural proportionn of these taxa to super-humid lowland rain forest. We expect melastomataceouss trees to be most important in the forest close to the mangrovee belt (see zone CAM) ; the decrease reflects the present day environmentall stability in the study area.

29 9

6.. Discussion

Ass demonstrated in the previous section, integration of pollen, diatom, grainn size, and major element proxies appears to enhance our reconstructionn of the development of a dynamic environment. Thee record demonstrates a development from: (1) a high energy (coarse sediments)) river-influenced basin, at close distance to the coastal mangrovee forest, from 3850 to 2700 cal yr BP (zone CAI-1), to (2) a periodd of fluvial erosion in the depression, causing a hiatus in the record fromfrom 2700 to 2010 cal yr BP to (3) a lake system that is periodically affectedd by events of high energy by the regional drainage system (repeatedd abrupt input of coarse sediments in a matrix of clay and silt, followedd by an expansion of Cecropia dominated pioneer forest in riverinee disturbed areas) and at very close distance to mangrove forest alongg the drainage system from 2010 to 580 cal yr BP (zones CAI-2 to CAI-4),, to (4) a lake isolated from regional riverine system disturbance (highh content of fine grained sediments, a total carbon content of the sedimentt that reflect only the local input and production in the lake) surroundedd by a floristically diverse forest with probable impact of humann activities (during the last 580 cal yr BP, but especially during the lastt 300 years). The decrease in P2O5 indicate that the lake become graduallyy less productive and depleted of nutrients. The increasing MgO/Al2033 ratio indicate a gradual increase in the input of manganese intoo the lake. CaO content is positively correlated to the increase of silt. Thereforee calcium seems to be allochthonous and probably derived from thee Cretaceous limestones. Elements such as aluminium and iron have beenn relatively stable in the lake.

Duringg zones CAI-2 to CAI-5 (2010 to 300 cal yr BP), six distinct horizonss with a high content of sand and silt (430-420 cm, 1593-1536 cal yrr BP; 310-290 cm, 990-885 cal yr BP; 275 cm, 808 cal yr BP; 223-220 cm,, 685-677 cal yr BP; 173-167 cm, 580-568 cal yr BP; 120-124 cm, 432-4133 cal yr BP) indicate periodic events of high energy of the river system.. The increase in riverine energy led to grater erosion of the sandy fluviall terraces and lake slopes and the subsequent increase in sand deposition.. The intensity of these events (based on the sand and silt record)) seems to decrease and matches the decreasing content of total

30 0

organicc matter. During zones CAI-2 and CAI-3 the basin received allochtonouss carbon but during zones CAI-5 and CAI-6 only locally producedd carbon reached the sediments. The floristi c composition of the forestt responds clearly to the periodic fluvial disturbance: expansion of thee Cecropia dominated pioneer forest follows each event of high energy andd forest rich in melastomataceous taxa prevail during the stable periods whenn mainly clay is deposited. Decreasing riverine also matches with the observedd succession in the pollen record: periodically damaged lowland rainn forest at close distance to the mangrove belt (zones CAI-2 to CAI-4) becomess gradually floristically richer (increase of unidentified pollen types)) when soils are better drained and the distance from the study site to thee belt with mangrove forest increases.

Wee observed that periods of higher riverin e energy interrupted more quiet lakee deposition (clay deposition and less than 0.4% sand) periodically afterr AD 1570. By using linear interpolation of the calibrated radiocarbon agess we calculated 57, 119, 9, 24, 21, 27, 77, 39, 19 and 210 years time betweenn these high energy events. The occurrence of those periods could bee related to the El Nino/Southern Oscillation (ENSO) frequency, in additionn to the present day recurrence intervals of 3 to 7 years. According too Anderson et al., (1982) long term ENSO variabilit y is expressed in cycless of 50, possibly 22 and between 80 to 100 years.

Thee diatom associations from lake El Caimito are dominated by AnomoeoneisAnomoeoneis brachysira, Frustulia rhomboides, Aulacoseira cf. herzogii andd species of Eunotia, and Cymbella. This association points to acidic andd oligotrophic conditions in a relatively shallow and stable water body duringg the last 1840 years. Decreasing abundance of A. brachysira and thee P2O5 content in the sediments, might indicate that this species was favouredd during the first part of the record by the relatively high nutrient contentt of the water. Maximum abundance of Aulacoseira cf. herzogii, indicativee of increased turbidit y and turbulence, coincided and followed episodess of higher energy in the ecosystem.

Thee diatom assemblages found in the El Caimito sediments are very similarr to those found in modern lakes in Brazilian and Ecuadorian Amazoniaa (De Oliveira and Steinitz-Kannan, 1992; Moro and

31 1

Fürstenbergerr 1997; Steinitz-Kannan et al., 1982, 1988). They are characterisedd by being poor in planktonic, and rich in periphytic and benthicc species. Such water bodies are also frequently dominated by a

.. Eunotia-Frustulia assemblage (Uherkovich and Franken, 1980).

Figuree 7. Map showing the tentative changes of the geographical position off the coastline and the mangrove belt in the Caimito area

Togetherr with the pollen records of Laguna Piusbi (Behling et al., 1998) andd Laguna Jotaordó (Berrio et al. 2000) (Fig. 1) the pollen record of El Caimitoo is the third document of environmental dynamics in the lowland rainn forests of Chocó. All sedimentary basins originate from a dynamic andd high energy fluvial system and developed into a lake system more of lesss isolated from the regional drainage pattern. Continuous lake sedimentationn was registered from 4300 14C yr BP in Piusbi, around 20100 14C yr BP in El Caimito, and around 1440 14C yr BP in Jotaordó. Lakess Piusbi and El Caimito are both located in the coastal plain and registerr the presence of mangrove forest. Migrations of the belt with mangrovee forest near the coast were inferred from the pollen record (Fig. 7).. The movement of the coastline can be explained by the tectonic instabilityy (uplift and subsidence) (Herd et al., 1981) and/or fluvial depositionn of material coming from the Western Cordillera. Shifting coastliness inferred from pollen records were earlier documented for Guyanaa and Surinam by Wijmstra (1971).

Thee floral composition of the regional forest near Piusbi and El Caimito iss similar, but differs from Jotaordó. This is due to the different setting of

32 2

thee Jotaordó site which has no contact to the coastal area and is located in aa broad river valley with a meandering drainage system.

7.. Acknowledgements

Diatomm research was funded by NWO-WOTRO grant WB 75368 to H. Hooghiemstraa / M.I . Velez. Pollen research was funded by NWO-GOA grantt 750.197.08 to H. Hooghiemstra / M. Wille. The core was collected underr exiting, but logistically and physically difficul t conditions by H. Behling,, A. Negret and H. Hooghiemstra during a NWO-GOA funded post-doctorall project (grant 750.195.10 to H. Hooghiemstra / H. Behling).. We thank the director of the 'Corporación de Cauca' in Guapi forr providing transport and the local population along the Guapi River for supportt with carrying our coring équipement through the rain forest. We thankk the Tropenbos-Colombia office in Bogota (J. Saldarriaga and C. Rodriguez)) for logistic support during our expeditions in one of the wettestt areas on the globe. We thank Jody dos Santos for invaluable supportt from our laboratory in Amsterdam. Nico de Wilde (Amsterdam) iss acknowledged for major mineral analysis, Annemarie Philip for preparingg the pollen samples, Kay Beets (Vrij e Universiteit) for supportingg the interpretation of mineral records. We thank Martin a Hagenn and Marti n Konert (Vrij e Universiteit) for their support in the sedimentt analysis. We thank Kur t Graf and an anonymous reviewer for constructivee comment which improved the manusript substantially.

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