1. Introduction

The Anthropocene has been considered the geologic time period in which humans and their activities have become a global geophysical force, noticeable since 1800 CE (^{Steffen et al., 2007}). However, it is clear that human activities have been affecting ecosystems and environments since the Neolithic (^{Ruddiman, 2014}) in which fire was used to clear forests for cultivation (^{Dietre et al., 2017}). It is still a matter of debate to identify the exact time and stratigraphic interval for the start of the Anthropocene, ^{Steffen et al. (2007)} reported the first evidences of the Anthropocene from 8000 to 5000 yr BP while ^{Lewis and Maslin (2015)} stated that it is difficult to say when this epoch started because there is not an agreement in respect to a reference point.

There is several studies that evaluate changes in the Anthropocene, for instance, the record from El Valle (^{Bush and Colinvaux, 1990}), shows the responses of terrestrial ecosystems to variations in precipitation, while the records from La Yeguada (^{Piperno et al., 1991}) and Monte Oscuro (^{Piperno and Jones, 2003}) illustrate that human populations have played an important role in shaping terrestrial ecosystems in Panama since 11000 yr BP.

A record from El Darien, on the other hand (^{Bush and Colinvaux, 1994}), indicated that El Niño Southern Oscillation (ENSO) was not linked to major impacts on the aquatic ecosystem in this region. However, all the records mentioned above are from the midland and lowland areas and are mainly based on pollen and phytolith analyses. The recent record from San Carlos (^{Correa-Metrio et al., 2016}) has been the only one so far, from Panamanian middle elevations and the only one that has studied the evolution and dynamics of the aquatic-terrestrial ecosystem using a multiproxy approach. Thus, very little is known about environmental change in the highlands from Panama, and more records are necessary to understand the anthropogenic effect in natural systems. Paleorecords in the tropics have shown that climatic phenomena can vary and affect different regions in different ways, across the stretch of land and also in a vertical dimension (^{Bush and Colinvaux, 1994}; ^{Leyden, 1995}; ^{Behling, 2000}). Thus, dissociating from human and natural signals in climatic paleorecords in this region has significant challenges.

We analyzed diatoms, charcoal, and sediments from a core taken from Lake Boquete on March 13, 1996. This core was collected as part of a project led by Paul A. Colinvaux and Enrique Moreno, where a census of the lakes in Panama was made, as a first step towards the study of the Panama Quaternary flora after the last ice age. This project was part of one of the three global programs PEP (Pole-Equator-Pole) in which “PEP-1” was the transect for the Americas. Although this core was collected in 1996, it was not studied until 2010, when ^{Holmberg (2009}, ²⁰¹⁰, ²⁰¹⁶⁾ analyzed the tephra layers to explore their potential as a tool for archaeological dating.

The main aim of the present study is to contribute to the understanding of the long term anthropogenic effect in highland Lake Boquete and its watershed, a unique space where human occupation started ~7000 yr BP (^{García-Jordán, 1996}; ^{Palumbo, 2009}).

2. Study area

Lake Boquete is located at 8°52ʹ50.49ʺ N, 82°29ʹ47.96ʺ W in the Talamanca Mountain Range, within the boundaries of Bocas del Toro and Chiriquí provinces, Panama, at an altitude of 2116 m.a.s.l. The lake is located 9 km northeast of Barú Volcano (Figure 1), its maximum depth is 12 m. Since the lake lacks an official name, we use the name that Colinvaux used in the past.

Figure 1. Study Area. (Above) Panama within Central America. (Below) Location of Lake Boquete. 

The climate of the area is influenced by the Atlantic Ocean and modulated by different factors such as topography, subsidence, the cyclic migration of the Intertropical Convergence Zone (ITCZ), and the trade winds that lead to the Central American Monsoon (^{Giannini et al., 2000}). Other phenomena, such as ENSO, change precipitation rates and the length of seasonality in tropical regions, including Panama (^{Lachniet et al., 2004}). The nearest meteorological station in Bocas del Toro province, reports a mean annual temperature of ~14.7 °C and yearly rainfall of ~2650 mm (^{Contraloría Panama, 2003}).

3. Methodology

Sediment cores of Lake Boquete were taken with a Livingstone piston sampler in 1 m long and 60 mm diameter aluminum tubes. Three vertical cores were obtained from the deepest part of the lake (~12 m) and transferred to the laboratory where they were stored at 4 °C. The core was then sampled for radiocarbon dating, diatom counting and identification, charcoal analyses, and sediment description. Three samples of organic sediment were taken from the core for radiocarbon dating, pretreated with acid washes, and sent to Beta Analytic for analysis in October, 1996.

Radiocarbon dates (Table 1) were calibrated using IntCal13 calibration curve (^{Reimer et al., 2013}) and an age model (Figure 2a) was obtained through Bacon (^{Blaauw and Christen, 2011}).

Figure 2. Age model (a) and accumulation rates (b) of the core from Lake Boquete. 

Diatoms were sampled every 5 cm and prepared according to the methodology of ^{Battarbee (1986)}. Diatom slides were mounted using Zrax^® (RI ~ 1.7⁺), and at least 400 valves were counted for each sample using an Olympus CX41 microscope at 1000× magnification. Diatom diagram and cluster analysis were carried out using Tilia Graph and the CONISS program in Tilia (^{Grimm, 1987}, ¹⁹⁹²).

For charcoal analysis, sediment samples of 0,25 cm³ were extracted from the core, deflocculated with sodium pyrophosphate (Na₄P₂O₇·10H₂O) solution (ca. 10 %) and separated under a stereomicroscope according to the methodology proposed by ^{Clark (1988)}. Particles were analyzed with ImageJ (^{Rasband, 1997}). Different sizes of charcoal were directly correlated to distance of transport; i.e., the greater the distance, the smaller the particle size, and vice versa. Considering the longest axis, particles > 100 μm indicated that the fire events were local, and particles < 100 μm suggested regional or non-local fires (^{Withlock and Larsen, 2001}). In cases of having charcoal samples, with particle sizes of the two groups mentioned above, the occurrence of two or more simultaneous fire events is assumed (^{Patterson et al., 1987}).

A qualitative description of the sediment and organic matter content was done using a stereomicroscope. Sediment grain size was classified according to the grain size chart of ^{Stow (2005)} by analyzing the shape, roundness and sphericity of particles. Organic matter was counted as a percentage in each sample.

4. Results

The age model shows that the base of the core is 2840 cal yr BP (Figure 2a). Sediment accumulation rates (cm/5 yr) were obtained through Bacon; these show a gradual decrease from base values of 0.5 - 0.6 cm/5 yr to top values of 0.3 - 0.2 cm/5 yr (Figure 2b).

The sediment composition is mainly silt, with varying amounts of clay, sand, and organic matter (Figure 3). From 235 to 100 cm, the sediment is composed mainly of silt with relatively low organic content (5 - 20 %). Sand content is moderate to abundant. From 100 to 5 cm, the grain size decreases and the sediment is composed of silt, clay, and minor abundance of sand. The organic matter increased considerably with maxima of 50 %. A distinct tephra layer is present at 15 cm (Figure 3). Reworked tephra is observed at 107, 102, 92, 87, 7, and 5 cm.

Figure 3. Lithology, diatoms, and charcoal of the core from Lake Boquete. (c) clay, (s) silt, (fs) fine sand, (ms) mud stone. 

The highest concentrations of charcoal are recorded between 218 and 158 cm with a peak of 75.2 mm²/cm³ at 208 cm. From 158 to 5 cm some minor increases occurred at 127, 112, 81, 76, 71, 41, and 26 cm, with concentrations of 9.87, 8.99, 6.58, 13.14, 12.01, 7.29 and 5.86 mm²/cm³, respectively. At the charcoal peak at 208 cm, we found variation in sizes of charcoal, as approximately 70 % of particles were less than 0.01 mm in its longest side. However, there are also larger particles, about 1 mm, being the greatest particle 3.6 mm, indicating a possible fire event close to Lake Boquete.

Diatom analysis (Figure 3) shows that the core is dominated by benthic species, with small amounts of planktic species (less than 3 %). Benthic species are dominated by Encyonema evergladianum (^{Krammer, 1997}), and Gomphoema gracile (^{Ehrenberg, 1838}). Planktic species include Aulacoseira granulata (^{Simonsen, 1979}), and Dicostella stelligera (^{Houk and Klee, 2004}). Other benthic species common throughout the core include Pinnularia acrosphaeria (^{Krammer and Lange-Bertalot, 1986}), Encynopsis microcephala (^{Krammer, 1997}), Eunotia subherkiniensis (^{Krammer and Lange-Bertalot, 1991}), Frustulia rhomboides (^{De Toni, 1891}), and Caloneis silicula (^{Krammer and Lange-Bertalot, 1986}). The dominance of benthic species throughout the core suggests a relatively stable lake bottom. Two main zones were identified after a visual inspection of the core and from CONISS analysis:

5. Paleoenvironmental interpretation

The diatom assemblages from this core (Figure 3) suggest that for most of the time, the lake maintained clear waters with low nutrient concentrations and a changing pH from neutral to more basic, as indicated by the ecology of the dominant diatoms (^{Moro and Furestenberger, 1997}). From 2840 to 1740 cal yr BP (Zone 1) the dominance of benthic epiphytic species E. evergladianum and G. gracile indicates a stable lake bottom most possibly colonized by macrophytes. The high abundance of epipelic Psammothidium spp. (^{Spaulding and Edlund, 2008}) and aerophil species E. subherkiniensis (^{Lange-Bertalot, 2011}) and D. confervacea (^{Carvalho and Dos Santos, 2008}) suggests influence of littoral environments. E. evergladianum and G. gracile indicate that the pH was neutral; however, the appearance of P. acrosphaeria might be indicative of increased alkalinity towards the end of the period. Although planktic species are rare, A. granulata occurs in a few samples. This might be indicative of changes in turbulence.

From 1740 cal yr BP to present (Zone 2), conditions similar to Zone 1 are maintained as suggested by the dominant diatoms. However, it is possible that a decrease in pH toward acidic values occurred as suggested by the increase of Pinnularia spp, Eunotia spp, and E. microcephala. The marked increase in periphytic E. microcephala most likely indicates an increase in the littoral and/or in submerged vegetation; this is also supported by the increase of plant remains preserved in the sediment. The abundance of planktonic D. stelligera during this period could be indicative of periods of higher water levels and an increase in nutrients (^{Costa-Böddeker et al., 2012}).

At ~1490 cal yr BP (100 cm), the sediment grain size changes from generally coarser sand and silt below to finer silt and clay above (Figure 3). After this time, the organic matter increases overall, with up to 50 % content in some samples. In general, accumulation rates decrease, suggesting a change from higher to lower hydrologic energy, and possibly less erosion. There was also an increase in plant remains in the sediment indicative of more abundant vegetation. A volcanic event, most likely from Barú volcano, is recorded in our core at ~379 cal yr BP (15 cm). This event could be correlated with the Barú eruption dated at ~1550 AD reported by the Global Volcanism Program (2013), and with the tephra layers reported by ^{Holmberg (2010)}. This date also works as an independent age for our core, which validates the dates and age model obtained by Bacon.

The high abundance and great variability in charcoal sizes recorded at 2610 cal yr BP (208 cm) suggest that several fire events occurred around this time. The period from 2327 to 2140 cal yr BP (178 - 158 cm) seems to have been a period of frequent fires. After this time, fire frequency decreased.

The change in the diatom assemblages recorded at ~1740 cal yr BP predates the changes in lithology (grain size and organic matter content) and in sediment accumulation rates observed at ~1466. These changes, as well as the increase in water pH and in vegetation, suggest a switch in the system to quieter depositional conditions and less frequent fires.

6. Discussion and concluding remarks

The record obtained from Lake Boquete shows that a marked change in the surrounding environment occurred between 1740 and 1466 cal yr BP, correlated with the end of the Formative period. We hypothesize that coarser grain sizes, higher accumulation rates, and lower content of organic matter in the sediments recorded between 2840 and 1740 cal yr BP reflect erosion in the watershed, most likely as a by-product of deforestation. The change to finer grain sizes, lower accumulation rates, and higher content of organic matter recorded after 1740 cal yr BP reflect less erosion as a result of a stable soil cover by vegetation. The period before 1740 cal yr BP correlates with the Formative period of the Gran Chiriquí culture that witnessed an increase in efficiency of agricultural practices and corn cultivation, as a result of the need to feed a growing population now organized in bigger centers (^{Palumbo, 2009}). Thus, Zone 1 in our record reflects this period of demographic increase and agricultural development. Meanwhile Zone 2, correlates with the end of the Formative, when human migration from the highlands to the lowlands became more pronounced and agricultural practices in the highlands decreased.

According to ^{Taylor (2011)} the peak in agriculture occurred from about 1718 yr BP to 1400 yr BP. After this time it began to decline (^{Taylor et al., 2013}) and by 850 yr BP it had ceased completely. As a result of agricultural abandonment, forests recovered (^{Taylor 2011}), erosion decreased, and more stable conditions prevailed as inferred from period 2 of our record. It is suggested then, that from 2840 to 1740 cal yr BP the lake’s watershed was under intense anthropogenic activities including deforestation and cultivation, and that after 1740 cal yr BP anthropogenic activities decreased and a stable vegetation cover developed. The increase in organic matter arriving to the lake originated from the abundant vegetation, and could have been the cause of the more acidic waters inferred in period 2.

It is also important to consider the climatic scenario during these periods. At around 1400 yr BP important changes in precipitation affected Panama (^{Lachniet et al., 2004}). These include a dry period attributed to an intense “El Niño” event at 1300 yr BP (^{Linares et al., 1975}), and another dry period between 1050 and 640 yr BP (^{Lachniet et al., 2004}) or from 1180 to 940 yr BP according to ^{Webster (2002)}. The charcoal and diatom records show that these intense and frequent dry conditions did not affect Lake Boquete and its watershed, and thus our record suggest that the main stressors to the lake have not been the natural phenomena but anthropic activities.