Journal of Paleolimnology 1: 85-97, 1988 �9 Kluwer Academic Publishers, Dordrecht - Printed in the Netherlands 85

A sedimentary record of human disturbance from Lake Miragoane, Haiti

Mark Brenner 1 and Michael W. Binford 2 1Department of Natural Sciences, Florida State Museum, Gainesville, FL 32611, USA; 2Graduate School of Design, Harvard University, 48 Quincy Street, Cambridge, MA 02138, USA

Accepted 5 April 1988

Key words: land-water interactions, limnology, paleolimnology, 2mpb, sediment geochemistry, West Indies

Abstract

Lake Miragoane, Haiti is one of the largest, natural freshwater lakes in the Caribbean (A = 7.06 km 2, Zma x = 41.0 m, conductivity -- 350 #S c m - 1). Lake waters are dominated by calcium and bicarbonate ions. The lake was thermally stratified, and oxygen profiles were clinograde during summer visits in 1983 and 1985. A 72-cm mud-water interface core was taken near the center of the lake and dated with 21~ The local ZmPb fallout rate is low ( 0 . 0 9 p C i c m - 2 y r - 1 ) , about 20~o of the global average. Bulk sedimentation rates ranged from 0.008 to 0.030 g cm-2 y r - 1 during the past 130 years (0-8 cm depth). Sediment geochemistry and pollen have been analyzed in the topmost 58 cm of the section. Tentative ages were assigned to the core by extrapolation of 21~ dates. According to this preliminary chronology, the bottom part of the core (58-30 cm) records pre-Columbian sedimentation (1000-500 B.P.) and contains pollen evidence of intact, dry and mesic forest. Pre-Columbian deposits are rich in organic matter (2 = 3 0 ~ ) and relatively poor in carbonates (2 = 15~o as CO2). The top 30 cm of the core preserve the record since European contact (500 B.P. to present). Pollen data reveal two episodes of deforestation following European arrival. Consequent soil erosion is documented by a decrease in organic matter content (2 = 15~) and an increase in carbonates (2 = 2 7 ~ as CO2). Surficial sediments reflect the widespread deforestation and soil loss that characterize the watershed today.

Introduction

There are few natural, freshwater lakes on islands in the Caribbean (Candelas & Candelas, 1963). Limnological work in the region often focuses on flowing waters, reservoirs, or saline coastal lagoons. Paleolimnological information from the Caribbean is almost non-existent. To our knowl- edge, sediment stratigraphies from only three Caribbean lakes are now being studied: WaUy- wash Great Pond, Jamaica (Hales et al., 1987), a

crater lake on Grenada (J. H. McAndrews pers. comm.) and Lake (Etang) Miragoane in southern Haiti (Binford etaL, 1987, Higuera-Gundy in press, and this study).

Several inland lakes exist on Hispaniola, the second largest island in the Antilles. The early work of Bond (1935) reported three freshwater lakes (Trou Caiman, Laguna del Rincon, Lago Limon), and described them as shallow, swampy depressions. Limnological data, including water chemistry, were reported for saline lakes Bois

86

Neuf, Saumatre, and Enriquillo. Bond did not visit Lake Miragoane, but noted that it had a maximum depth greater than 35 m. The lake's size, depth, water chemistry, and the presence of several endemic fishes (Burgess & Franz in press) and snails (Clench & Aguayo, 1932; Eyerdam, 1961) suggested that the basin had been a lacustrine sediment trap for millennia.

This paper uses paleolimnological data from a core taken in Lake Miragoane to assess the effects of recent human disturbance on a tropical karst landscape. Paleolimnological studies elsewhere in the mainland American tropics (Deevey etal., 1979; Binford, 1983; Brenner, 1983, Rice etal., 1985; Vaughan et al., 1985; Binford et al., 1987) and subtropics (Deevey etaL, 1986; Binford et al., 1987) have documented the environmental consequences of long-term human disturbances on karst watersheds. Historical ecology studies in the tropics have focused on the Maya lowlands of northern Guatemala, where modem analogs of prehistoric human disturbances are lacking. Hu- man population densities throughout Haiti are very high. Deforestation and consequent soil erosion are serious problems in the Miragoane drainage, which may provide a modem analog of prehistoric Maya Guatemala.

We present morphometric and limnological data for Lake Miragoane and describe the local paleoecology based on geochemical and palyno- logical results from a sediment-water interface core. The sediment profile was collected from deep water near the center of the lake, in July 1985. Based on extrapolation of the 2~~ chro- nology, the bottom of the analyzed section (58 cm) was tentatively assigned an age of 1000 B.P. Thus, the section probably contains deposits of pre-Columbian age as well as a com- plete record since the arrival of Europeans.

The study site Lake Miragoane lies in a tectonic rift system at 18~ Lat, 73~ Long. The basin is in limestone terrain on the north side of Haiti's southern peninsula (Fig. 1). The lake lies about 5 km from the coast and the lake surface is at 20 m above m.s.l. Lowland coastal areas nearby

have mean annual temperatures of 26-27 ~ While much of central Haiti is very dry, the southern peninsula lies outside the rain shadow. Low elevation sites near the lake receive 1000-2000 mm of precipitation annually, and there are two rainy seasons, one in spring and one in fall. Spring rainfall peaks in May, while autumn rains achieve a maximum in October or November. July is the driest and usually the warmest summer month (Woodring et al., 1924).

Lake Miragoane lies in the tropical lowland dry forest life zone (Holdridge, 1945). More xeric con- ditions prevail at some coastal localities, and mesic forest is present at higher altitude sites nearby. The Miragoane drainage is largely de- forested today, and most extant trees represent economically important taxa (e.g. mango, coconut palm, calabash) or successional types (e.g. Cecropia).

Steep slopes overlook the southern shore of the lake, but to the east Miragoane is bounded by a marsh that separates it from another, as yet un- explored lake. Among the common marsh plants are sawgrass (Cladium), cat-tails (Typha), and water lily (Nymphaea). Common plants within the lake include pondweed (Potamogeton), American lotus (Nelumbo lutea) and the macroalga Chara.

Morphometry Depth data from nine transects were used to con- struct a bathymetric map of Lake Miragoane (Fig. 1). Morphometric parameters were calcu- lated according to H,~tkanson (1981) and are presented in Table 1. With a surface area of 7.06 km 2, Lake Miragoane is one of the largest, if not the largest, natural freshwater lakes in the Caribbean. The lake has a maximum depth of 41 m. As the surface lies at 20 m above m.s.l., the basin has a 21 m cryptodepression. The lake is unusually round, though the southwest bay con- tributes to shoreline development (F = 1.19). Water depth increases sharply along the southern and eastern shores of the lake. However, the lake contains a large, fiat bathyal region, and the area below 40 m depth is equal to 11 ~ of the lake surface area. Lake Miragoane has a concave (Cma) form.

LAKE MIRAGOANE, HAITI

87

H I S P A N I O L A

Dominican Haiti Republic

MIRAGOANE

CONTOUR INTERVAL 5 m

I N

25, 3 0 / 4 0 1a'_24' N

0 1 km T i i J t �9

73" 05'W Fig. 1. Bathymetric map of Lake Miragoane, Haiti, showing 5-m contour intervals. Cross hair symbol in the central, deep area of the lake denotes the location of mud-water interface core 17-VII-85-1. Inset map shows the island of Hispaniola and the

location of Lake Miragoane on the north side of Haiti's southern peninsula.

Hydrologic inputs to Lake Miragoane include direct precipitation, s lopewash, and inflow f rom springs and irrigation channels. There may be some water exchange with the marsh to the east,

Table 1. Morphometric data for Lake Miragoane, Haiti

Surface area (A), km 2 7.06 Maximum length (Lm~,), km 4.01 Maximum width (Bm~), km 2.8 Shoreline length (lo), km 11.2 Shoreline development (F), dimensionless 1.19 Maximum depth (Zm~x), m 41.0 Mean depth (2), m 25.9 Relative depth (zr), ~ 1.37 Volume (V), km 3 0.183 Lake form Concave (Cma)

but the direction of any flow is unknown. The lake possesses an outlet on the nor th shore. In July, 1985, the observed flow in the channel was about 1 m 3 sec - l Assuming that this est imated value is fairly cons tan t throughout the year and that evaporat ive loss is about 1.9 m y r - 1 (est imated by P e n m a n ' s Method, p. 106 in Dunne and Leopold , 1978), 3 0 ~ of the lake volume is lost annually to drainage and evaporat ion. I f seepage loss is negligible, the water residence t ime for the lake is about three years.

Prehistory and history The pre-Hispanic history o f the region is poorly known, but ceramic evidence indicates the pres- ence of A r a w a k sett lements in the area as early as

88

600 A.D. Two additional episodes of occupation date to between 900 and 1500A.D. and are identified by more recent pottery styles (Rouse & Moore, 1984). The incompleteness of the archaeo- logical record leaves unanswered questions con- cerning the proximity of Arawak settlements to the lake shore, population densities achieved dur- ing periods of occupation, or whether the area was settled continuously between 600 and 1500 A.D. Early inhabitants are believed to have subsisted by fishing and practicing swidden agri- culture. Principal crops probably included Mani- hot (cassava) and Zea mays (corn).

Since the arrival of Europeans, Haiti was sub- jected to two centuries of Spanish rule (1500-1700 A.D.), and then came under French control until 1804, when the country became the world's first black republic. Deforestation in Haiti is a serious problem that demands, and now receives attention. It is estimated that 60 percent of the country was forested in 1920, while only five percent remains forested today. Twenty-one of the country's 30 watersheds have been effec- tively deforested (Cobb, 1987).

Methods

Field methods Limnological data were collected during site visits in August 1983 and July 1985. Temperature and oxygen profiles were measured with a Hach Tem- perature/Oxygen meter. Water samples were col- lected with a Niskin bottle. Conductivity was measured with a Lab-Line Inc. Lectro Mho- Meter, and pH with an Instrumentation Labora- tory, Inc., Porto-matic Model 175 pH meter. Echo sounding of the lake bottom was done in 1985 with a Lowrance LRG-1510B Truline Recorder. The mud-water interface core was obtained with a 4-cm diameter piston corer. The core was extruded and sectioned in the field at 1-cm intervals down to 10 cm, at 2-cm intervals between 10 and 30 cm and at 4-cm intervals from 30 cm to the bottom of the section.

Laboratory methods Water chemistry: Cation concentrations in water samples were determined with a Jarrell-Ash ICP 9000 spectrometer. Sulfate was measured turbidi- metrically (APHA, 1975). Chlorides were measured using Technicon chloride color reagent.

Sediment chemistry: Percent dry material in sedi- ments was measured by weight loss on drying at 110 ~ C. Organic matter content was measured by weight loss on ignition at 550 ~ (H,~tkanson & Jansson, 1983). Carbonate content was deter- mined by weight loss on ignition between 600 ~ and 990 ~ (Dean, 1974). Total carbon and in- organic carbon were measured on selected levels with a Coulometrics, Inc. Model5011 CO2 coulometer in conjunction with Coulometrics System 120 and 140 preparation lines.

Cations, phosphorus, and sulfur in sediments were determined after digesting ashed (550 ~ material in 1NHCI (Andersen, 1976). The method was modified slightly, as greater volumes of acid were used to counteract the neutralizing capacity of sediment carbonates. Cations in solu- tion were read on the Jarrell-Ash ICP. Sulfate was read turbidimetrically (APHA, 1975). Phospho- rus was determined spectrophotometrically after blue color development with ammonium molyb- date-ascorbic acid (APHA, 1975).

21~ dates were obtained by measuring the 21~ content of sediments by extraction and plat- ing on copper planchettes (modified from Eakins & Morrison, 1978). 21~ dates were calculated by the constant-rate-of-supply (CRS) model (Goldberg, 1963; Appleby & Oldfield, 1978, 1983).

Results

Limnology While Lake Miragoane is thermally stratified dur- ing the summer, surface temperatures exceed bottom temperatures by only about 6 ~ C (Fig. 2). In 1985, the most precipitous temperature drop was detected between 20 and 25 m. However, temperature declined by only 0.28 ~ m - 1 in this interval.

89

LAKE MIRAGOANE, HAITI

E v

c~

Temperature

(~

20 22 24 26 28 0 I 1 I i I i I �9

10

20

30

Dissolved Oxygen

(mg L - 1 )

2 4 6

~ I I I I I I I

i

. F / ' ~ j

! ! I I

.I,

e - - - 4 2 7 August 1983

-- -- 28 July 1985

Conductivity (~IS cm -1 )

8 I

- 350

350

4OO

pH

7.75

7.30

40 4 400

Fig. 2. Temperature, oxygen, conductivity, and pH profiles, and Secchi disk depth for Lake Miragoane, Haiti. Temperature and oxygen data were gathered on 27 August 1983 and 28 July 1985. Conductivity, pH, and Secchi values are for 28 July 1985.

Summer oxygen curves for the lake were clino- grade. In 1985, oxygen concentration fell from 5 . 0 m g L - ' at 20m to 0 . 4 m g L - ' at 25m (Fig. 2). Bottom waters were virtually anoxic and smelled of H2S. Despite the lake's location near the coast and the deep cryptodepression, the con- ductivity profile did not indicate chemical meromixis. Surface waters were quite fresh (350 #S c m- '), and hypolimnetic waters showed only slightly higher conductivity (400 # S cm- ') (Fig. 2). pH values near neutrality are a con- sequence of buffering by the calcium bicarbonate system. Slightly higher pH values in the photic zone may reflect photosynthetic removal of CO2.

However, the algal standing crop was not dense as Secchi depth exceeded 7 m (Fig. 2).

Field visits were limited to summer months, and the seasonal mixing regime of the lake remains unknown. Based on studies of other low- elevation tropical lakes, we predict that Miragoane is a warm monomictic system. Circu- lation probably begins in late December or early January and stratification is reestablished in the spring, about April (Lewis 1973, 1983, pers. comln . ) .

Lake Miragoane's water is dominated by cal- cium and bicarbonate ions (Table 2). Concentra- tions of magnesium, potassium, sodium, chloride,

90

Table 2, Ionic composition of Lake Miragoane waters in meq L- 1. Bicarbonate concentrations were calculated by difference, assuming a balance between cation and anion charges.

Date Depth (m) Ca Mg K Na HCO3 SO4 C1 Sum

27-VIII-83 0 1.20 0.91 0.04 0.78 2.24 0.15 0.54 5.86 28-VII-85 0 1.75 0.91 0.04 0.78 2.80 0.14 0.54 6.96 28-VII-85 11 1.80 0.91 0.04 0.78 2.83 0.16 0.54 7.06 28-VII-85 25 2.04 0.91 0.04 0.78 3.04 0.15 0.54 7.46 28-VII-85 38 2.09 0.91 0.04 0.78 3.15 0.13 0.54 7.64

and sulfate in surface waters were the same in both 1983 and 1985. Calcium and bicarbonate concentrations increased by 32~o between visits. Calcium and bicarbonate are the only ions that display a change in concentration with depth. They are enriched in hypolimnetic waters relative to epilimnetic waters (Table 2), and the slight gradient is reflected in the conductivity measure- ments (Fig. 2).

21~ chronology The Miragoane mud-water interface core (17-VII-85-1) was dated twice by 2mpb assay. The first time, 2mpo was extracted from combined 2-cm intervals (e.g. 0 - 2 c m , 2 - 4 c m , etc.). Unsupported (fallout) 21~ was undetectable below 8-cm depth in the core. Following palyno- logical and chemical analysis of the uppermost deposits, sufficient dry material remained for a second 21~ determination, and extractions were done on 1-cm intervals. The concentration of un- supported 2mPb declines fairly regularly with depth (Fig. 3a). The two 2mpb procedures yielded dates at 8 cm of 133 (2-cm intervals) and 129 (1-cm intervals: Fig. 3b) years, equivalent to only a 3 ~ difference in basal age. Computed bulk sedimentation rates in oligotrophic Lake Mira- goane are slow, ranging from 0.008 g cm - 2 yr - 1 at 7 -8 cm depth (96-129 years) to 0.030 g c m - 2 y r - 1 at 0-1 cm depth (0-6 years) (see Fig. 3c.).

The total residual unsupported 21~ content of the Miragoane core is only 2.97 pCi cm - 2, and corresponds to a 2mpb fallout rate of 0.09 pCi c m - 2 y r - 1. The fallout rate derived from the sediment 21~ inventory in Miragoane

is low relative to the global average, which is about 0.5 pCi cm- 2 y r - 1 (Appleby & Oldfield, 1983; Turekian etal., 1977; Nozaki etal., 1978). Loss of sediments from erosional zones can pro- duce low total residual 21~ values, but this ex- planation does not apply as the core was collected from near maximum depth, in an extensive fiat area at the center of the lake. Indeed, the region might be a site for redeposition of resuspended, focused sediments. The low 21~ inventory may be a consequence of upward migration and solu- bilization of sedimentary 2X~ under conditions of deepwater anoxia (Guojang et al., 1984; Erten etaL, 1985; Benoit & Hemond, 1987). We believe that the total residual 2X~ value in Miragoane results from a low rate of 21~ fallout, which is a consequence of low rates of 222Rn emanation from the nearby sea surface and local soils.

Extrapolation of 21~ dates No 14C dates are yet available for the mud-water interface core, and as hard-water-lake errors (Deevey & Stuiver, 1964) are possible, pre- liminary downcore (below 8cm) ages were assigned to the short section by extrapolation of 21~ dates. Ages were computed by assuming a constant rate of dry mass accumulation below 8 cm, and applying the mean sediment accumula- tion rate for the topmost 8 cm of the core. At 8 cm, 1.484 g of dry material accumulated per cm- 2 during the past 133 years, yielding a mean accumulation rate of 0.011 g dry cm -2 yr - 1. We used the basal age of 133 years, obtained from the first 21~ measurements, to be consistent with downcore dates applied to the pollen sequence (Higuera-Gundy in press). As noted, use of the

LAKE MIRAGOANE, HAITI

MUD-WATER INTERFACE CORE 17-VI I -85-1

91

a.

0.1 0

3" E

4- o.

r 5-

U n s u p p o r t e d 2 1 0 P b b.

(pCi g-1 dry sediment) 1.0 10.0 0 20

I-IH

1-

=

1 �9 I

~H

~H

P~

2.

t'~ 5

6

7

8

40 i

Age (years) 60 80 100 120 140

a i 0

0.2

~ 0 .4

'0.6 ~E

0 .8 r

"o,... Q - 1.0

�9 1.2 ~

'1~...... "o ~

1.8 A~te Age vs. Cum.Mass �9 ...... .e ~ '~ 2.0

C.

0 L

, 0 I 60

1 2 0 ~

140 ~

Sediment Accumulation Rate

(g cm -2 yr- 1 )

0 0,01 0.02 0.03

i j 1 [

Fig. 3a. Unsupported 2]~ activity (pCi g- 1 dry sediment) plotted against depth in Miragoane mud-water inferface core 17-VII-85-1. The mean and standard deviation of the activity are plotted at the midpoint of each sample interval. 3b. Age vs. depth (solid line), and age vs. cumulative mass (dotted line) for the core, based on 21~ dating at 1-cm intervals. The age at 8 cm is 129 years. 3c. Bulk sediment accumulation rate (gcm -2 yr- s) vs. age in the core. Sediment accumulation rate can be

seen to increase during the past 85 years.

alternative date at 8 cm (129 years) for downcore extrapolation, causes only minor differences in estimated ages (about 3~o). Dates applied to levels below 8 cm rely on the dubious assumption ofoons tan t dry mass accumulation between 8 and 58 cm. Changes in proximate composition at 30 cm, and measured accumulation rate variation in the uppermost 8 cm, suggest some error in the assigned downcore dates. We consider the com- puted ages to be tentative. According to this cal- culation method, sediments between 26 and 28 cm are approximately 500 years old, and were deposited about the time of European arrival on Hispaniola. Basal deposits at 5 8 c m were assigned an age of 1000 years. Sediments from 58-72 cm were set aside for ]4C dating.

Sediment geochemistry Physical and chemical data were obtained for 22 stratigraphic levels in the core (Fig. 4). Dry den- sity (p) in the Miragoane short core varies from 0.119 to 0.460 g dry c m - 3 wet sediment. Values are higher in the top half of the core compared with the bot tom half of the section (Fig. 4). Orga- nic matter (L.O.I. 550 ~ constitutes between 11.0 and 35.6~o of the sediment dry weight. Sedi- ments below 30 cm are richer in organic matter than muds above 30 cm (Fig. 4). Dry density is negatively correlated with percent organic matter (r = - 0.71, P < 0.01).

Phosphorus concentrations range from 490 to 1 7 0 0 / ~ g P g - t dry, and sulfur makes up 0-15 mg S g - 1 dry weight (Fig. 4). Both phos- phorus and sulfur concentrations show strong negative correlations with carbonate (L.O.I.

92

LAKE MIRAGOANE, HAITI

MUD-WATER INTERFACE CORE 17-VII-85-1

0-

100-

A

500-

(D

<C

1000-

Dry Density L.O.I. 550"C

(g dry cm - 3 wet) (%)

0 0,2 0.4 0 10 30

0

!0 J = = w -~

,0 . 1 I

; 0 ' - - ?

I ;0"

P S

(l~g g - l ) (rag g - l )

0 4 0 0 1200 0 4 8 I

I

[ I

I

1

1 2

I - - S

L.O.I. 600-990~

(%)

1 0 2 0 3 0 ~

i

I ffJ

I

Ca

100 200

I I

IL" I

- - t \

Mg

(mg

5 1015

i I i

K Mn Fe

g - l ) )1

0.5 1,5 0 0.2 0.6 0 20 40

==q '~

I I

----h '1 "

I

Fig. 4. Chemical stratigraphy of Miragoane mud-water interface core 17-VII-85-1. Changes in sediment chemistry above 30 cm depth coincide with pollen evidence for deforestation (see Fig. 5b), and reflect activities of European settlers.

600-990 ~ content: r = -0.86, P < 0.01 and r = - 0.76, P < 0.01 respectively. Thus, there is probably little apatite-bound P, and inorganic phosphorus may be contained in minerals such as vivianite ( F e 3 ( P O 4 ) 2 . 8 H 2 0 ) o r adsorbed to clays, silts, and hydrated iron oxides. Indeed, P is positively correlated (P < 0.01) with Mg, K, Mn, and Fe. L.O.I. 550 ~ is positively correlated with phosphorus (r = 0.57, P < 0.01) and sulfur (r = 0.88, P < 0.01) concentrations, suggesting that some P and S is bound in the organic fraction. Positive correlation between Fe and S (r = 0.56, P < 0.01) may indicate formation of iron sulfides under conditions of deepwater anoxia.

Carbonates, reported as L.O.I. 600-990 ~ represent 12.6 to 33.2~o of the sediment dry weight. Uppermost sediments (0-30cm) are richer in carbonates than deeper (30-58 cm) de- posits (Fig. 4). Calcium content of the mud ranges from 90.5 to 292.2 mg C a g - 1 dry, and calcium concentration is highly correlated with L.O.I. 600-990 ~ (r --- 0.997, P < 0.01). It is probable that virtually all calcium is bound in carbonates. Magnesium ranges from 5.7 to 14.2 mg g - 1, and is uncorrelated with both Ca and L.O.I. 600-990 ~ C, suggesting that the Ca:Mg ratio of carbonates has varied through time or that not all magnesium is bound in carbonates.

Attempts to determine the source of sedi- mentary magnesium were equivocal. The inorga- nic carbon (I.C.) content of the sediment was computed from Ca and Mg equivalents, assuming the cations were bound exclusively in carbonates. I.C. was also computed for each stratigraphic level using the L.O.I. 600-990~ values (I.C. = 0.273 x L.O.I.). The ratio of I.C. deter- mined from loss on ignition to I.C. computed from Ca and Mg equivalents ranged from 0.91 to 1.09, with a mean and standard deviation of 1.01 +0.04 ( n = 2 2 levels). While the two methods yield similar independent estimates of I.C. and might suggest that magnesium is bound in carbonates, it must be noted that nearly all the I.C. figured from cations comes from calcium. High Ca: Mg ratios (by mass), ranging from 11.1 to 51.3 obscure the relationship between mag- nesium and the carbonate fraction.

Inorganic carbon was measured directly by coulometry on 16 sample levels for which suf- ficient dry material remained. The coulometric method consistently yielded lower values than those obtained using Ca and Mg equivalents or loss on ignition methods. Coulometer values averaged about 16~ lower than the other approaches, and discrepancies were most pronounced in the organic-rich, bottom half of the core. Ca and Mg equivalents probably over-

estimate inorganic carbon because some fraction of the cations is bound to organic matter, in clays, or to other non-carbonate sediment constituents. Loss on ignition may also overestimate the car- bonate fraction as some weight loss may be attributable to water of hydration in clays (Dean, 1974).

Potassium content in the sediment ranges from 0.34 to 1.81 mg K g - ~ dry. Manganese shows a narrower range of concentrations, from 0.47 to 0.78 mg Mn g - ~ dry. Iron represents 8.3 to 42.8 mg per g dry sediment (Fig. 4). Potassium and iron are both negatively correlated with L.O.I. 600-990 ~ r = -0 .75, P < 0 . 0 1 , and r = - 0.92, P < 0.01 respectively. Manganese is uncorrelated with the carbonate measure. Iron is positively correlated with organic content (r = 0.62, P < 0.01), while Mn and K are not correlated with the organic fraction.

Concentration trends for Mg, K, Mn, and Fe

93

are similar (Fig. 4). The four cations are all posi- tively intercorrelated at P < 0.01, and are proba- bly contained for the most part in siliceous clas- tics. Simple correlation can only hint at the source of the elements, some of which are undoubtedly present in several forms.

Discussion

The Miragoane short core is divisible into two principal stratigraphic zones based on physical and chemical properties (Figs. 4 & 5a). Below 30 cm depth, sediments are rich in organic matter (24.3-35.6Y/o L.O.I. 550 ~ iron oxide (4 .5 -5 .9~ as FezO3), and siliceous material (30.8-33.5 ~o as 'SiO2'). Carbonates comprise 25.2-37.6~o of the dry weight in the bottom half of the core (Fig. 5a). Above 30 cm depth in the core, carbonates increase to 44.5-75.1~o of the

LAKE MIRAGOANE, HAITI

MUD-WATER INTERFACE CORE 1 7 - V I I - 8 5 - 1

O)

0

100

500 -

a.

10

20

vo j : 30

o~

r.,, 40

50

b,

" , ; 7 . , . " : : . . . . . . . . L , LL' L . . . . . . . .

10-****~ " ~-~ " ..... "" " .........

20. ~ , ~ . . . . . . .

E ~ a ~ " , ..... " ,, '

3 0 - ~ . �9 . . . . . . . . . . . . . . . . .

4 0 . ~ r ,,,: . . . . . . . .

o o . . . . . ....

1000- ! I

60 0 10 20 30 40 50 60 70 80 90 100 60 0 210 ~ ; 0 60

SEDIMENT PROXIMATE COMPOSITION (%) POLLEN (%)

Fig. 5a. Proximate composition of the sediment in Miragoane mud-water interface core 17-VII-85-1, showing changes that accompanied European settlement beginning about 500 years ago. Organic matter values are L.O.I. 550 ~ C, carbonates represent CaCO 3 and MgCO 3 equivalents of Ca and Mg, and Fe20 3 is the iron oxide computed from the total iron values. 'SiOz' is the balance of the sedimentary matrix, and is probably composed of detrital silts and clays as well as some biogenic silica. 5b. Summary pollen diagram for Miragoane mud-water interface core 17-VII-85-1 (modified from Higuera-Gundy in press). 'Trees' include pollen of Moraceae, Celtis, Trema, Bursera, Alchornea, Phyllostylon, Sapindus, Caesalpinia, Sapotaceae, Myrtaceae, and Meliaceae. Pinus and Cecropia were excluded. 'Weeds' include grasses, Cyperaceae, Chenopodiaceae/Amaranthaceae, Ambrosia

and other composites, Solanaceae, Pilea, and other herbs.

94

sediment dry weight, while concentrations of organic matter, Fe203, and 'SiO 2' decline.

Based on extrapolation of the 21~ chrono- logy, changes in sediment composition at 30 cm appear to coincide with European intrusion in Hispaniola at the end of the 15th century A.D. Assuming constant mass accumulation below 8 cm, the arrival of Columbus would have occurred contemporaneously with the deposition of sediments from 26 to 28 cm. The slight dis- crepancy may result from the assumption of con- stant sediment accumulation rate over the length of the section. Sediments in the bottom half of the core (30-58cm) were deposited prior to European settlement and preserve at least five centuries of the pre-Columbian environmental record.

Geochemical shifts at 30 cm depth coincide with changes detected for pollen and carbonized fragments. These microfossil types are present in high concentrations in the bottom half of the core (Higuera-Gundy in press). High abundance of tree pollen relative to weed grains in the bottom half of the section suggests that dry and mesic forests prevailed during pre-Columbian times (Fig. 5b). While carbonized fragments and pollen of successional taxa ( Cecropia, Trema, Celtis) are present in the bottom half of the core, forest dis- turbances are believed to have been minimal, and due in part to natural events such as forest fires (Higuera-Gundy in press). When pollen spectra for deep, predisturbance levels in the Miragoane long (7.67 m) core are available, it may be possible to assess the environmental impact of the Arawak against the backdrop of baseline, precultural con- ditions. In any case, below 30 cm, relative stability of the sediment composition combined with high pollen concentrations dominated by arboreal taxa (Figs. 4 and 5), indicate a prolonged period of low-level human activity during which forests remained largely intact and soil erosion was mini- mal.

Above 30 cm, concentrations of total pollen and charcoal in the sediment decline by more than an order of magnitude (Higuera-Gundy in press), and weed pollen grains increase relative to arbo- real types (Fig. 5b). Forest clearance removed the

source of the arboreal pollen rain. Additionally, high erosion rates further diluted sediment pollen concentrations as clastic material was delivered to the lake shore at a higher rate. Deforestation accelerated the downwasting of riparian soils. Therefore, extrapolation of ages based on 2~~ dating of the uppermost 8 cm is somewhat questionable. Pre-Hispanic sedimentation rates may have been lower than values measured in the top 8 cm of the core. Thus, the 1000-year age assigned to the base of the section (58 cm) may be in error, and probably too young.

Sediments between 30 and 10 cm were proba- bly deposited between about 1500 and 1800 A.D., corresponding to the period of Spanish (1500-1700 A.D.) and French (1700-1800 A.D.) control of the country. The change in sediment composition at 30 cm reflects the consequences of land clearance and lakeward transport of riparian mineral soils. Carbonate concentrations average nearly twice as high in the upper versus the lower half of the core (Fig. 5a). Accelerated delivery of detrital carbonates is thought to account, in part, for the increase in the carbonate content of the sediments. However, as L.O.I. 600-990 ~ is not positively correlated with other sedimentary erosion indicators (e.g. Mg, K, Fe), some car- bonate may have been deposited biogenically, especially if nutrient delivery to the lake increased as a consequence of vegetation removal.

Sediments in the 6-8 cm horizon contain 53.3 ~o carbonates, the lowest value in the upper- most 24 cm of the section. Organic matter content reaches nearly 20~o in the interval (Fig. 5a). Total pollen concentrations increase (Higuera-Gundy in press) and arboreal pollen grains increase rela- tive to weed types in the sample level (Fig. 5b). Several lines of evidence point to temporary reestablishment of local forests and reduction of soil loss from watershed slopes in the latter half of the 19th century.

Historical evidence lends credence to the notion of forest regrowth in the late 1800's. When Haiti gained independence from France in 1804, large French-run plantations were supplanted by smaller subsistence plots. Small farms were estab- lished throughout the country and higher eleva-

95

tion sites were exploited for the first time (Holdridge, 1945; Woods, 1987). Re-establish- ment of natural forests may have been aided by the demise of the plantation system and a return to subsistence practices on smaller plots. Addi- tionaUy, local population densities near the lake may have declined as settlers moved to newly available high-elevation agricultural land.

Deposits accumulated in the last century (about 0-6 cm: Fig. 3b) contain evidence of in- creased human impact on watershed vegetation and soils. Total pollen concentrations decline (Binford et al., 1987; Higuera-Gundy, in press), and weeds increase relative to arboreal taxa in the pollen spectrum (Fig. 5b). Once again, carbonates

represent a greater fraction of the sediment dry weight, while organic matter content declines, reaching a low of 11.0~o at the top (0-2 cm) of the section (Fig. 5a).

The mean bulk accumulation rate in the top- most three cm (0.027 g cm - 2 y r - ~) is about twice as high as that calculated for the underlying 3-cm interval (3-6 cm: 0.014 g c m -2 y r - 1) (Fig. 3b), and indicates accelerated soil erosion in the past 25 years. However, recent sediment accumulation rates measured in Lake Miragoane are lower than modern values recorded for a suite of 34 Florida lakes, which display a range from 0.032-2.080 g c m - 2 y r - 1 (Binford & Brenner, 1986, unpublished data). Likewise, during Maya

Fig. 6. Photograph taken in 1985 looking northeast along the steep southern shores of the Lake Miragoane watershed. Contrast this view with W. J. Eyerdam's recollection of his 1927 visit to the lake (Eyerdam 1961: 74), 'Several times before I had come alone to the south side of the lake to collect specimens. There in the jungles many fine land snails were taken, including Bulimulus, Cepolis and Caracolus, and a colony of Liguus virgineus on the lignum vitae and logwood trees yielded several hundred fine

specimens.' The watershed is now largely deforested and soils are prone to erosion.

96

times (3000-400 B.P.), sedimentation rates in lakes of the Guatemalan lowlands (Deevey et al., 1979; Binford et al., 1987) were generally higher than those measured today in Miragoane. Never- theless, the pollen and geochemistry of Miragoane sediments deposited in the last quarter century (0 -3 cm) clearly reflect the widespread deforesta-

tion and exposed soil conditions that characterize the watershed today (Fig. 6).

Lake Miragoane mud-water interface core 17-VII-85-1 preserves an approximately one- thousand-year record of the historical ecology of the watershed. Pollen and geochemical evidence indicate that pre-Hispanic human disturbance in the drainage was minimal. Since the arrival of Europeans in Haiti at the end of the 15th century, the watershed has undergone two episodes of deforestation, giving rise to increased soil erosion. The data demonstrate that modern ecological conditions in the Lake Miragoane basin are a consequence o f human activities.

Acknowledgements

This work was supported by N S F grant B S R 85-00548 awarded to M. W. Binford and E. S. Deevey, and a Whitehall Foundat ion grant awarded to E. S. Deevey. J. Dan Skean assisted with field work, and collected and identified plant

specimens. Monsieur Reginal Ambroise was in- dispensable in the field. We thank Monsieur Gerson Cenee, his family, and the residents o f Chalon for field assistance. We are grateful to the people o f Miragoane who provided food and lodging. The University of Florida Institute of F o o d and Agricultural Sciences Soils Lab assisted with cation and chloride analyses. We thank Dr. Dan Engst rom and an anonymous reviewer for helpful comments on the manuscript.

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