Upper Quaternary vegetation dynamics and palaeoclimatology of the La Chonta bog area (Cordillera de Talamanca, Costa Rica)

Abstract

Two pollen records from La Chonta bog (2310 m altitude) and one pollen record of a soil profile (2430 m altitude) at a short distance from the bog permit the reconstruction of the vegetational history and climatic sequence of probably the last ca. 80 000 yr of the Cordillera de Talamanca, Costa Rica. The three pollen records can be correlated on the basis of three radiocarbon-dated horizons (> 39 900 yr BP, 34 850 yr BP and 1390 yr BP) and palynostratigraphy. Pollen concentration data provide indirect time control between radiocarbon-dated horizons of the La Chonta 2 core. During the Early Glacial successive fluctuations of the upper forest line occurred between ca. 2300 m and 2800 m altitude and the bog is alternately situated close to the subalpine rainforest belt and in the uppermontane rainforest belt. During the Pleniglacial (ca. 50 000-1 3 000 yr BP) the forest line fell to ca. 2000 m altitude and paramo vegetation surrounded the La Chonta site. During the Late Glacial the forest line shifted rapidly in ca. 270 yr to 2700-2800 m altitude and the lake became surrounded by Quercus-dominated upper montane rainforest during a period of ca. 1655 yr. A distinct climate cooling (by 2-2.5"C) followed during ca. 580 yr and the upper forest line dropped to ca. 2400 m altitude. This temperature rebound is probably equivalent to the Younger Dryas event. In the depression extensive Alnus carr and swamp vegetation developed. Finally, during the Holocene, the forest line shifted to 3300-3500 m elevation and the lower montane rainforest reached close to ca. 2300 m altitude. In the later part of the Holocene subparamo scrub with Hypericum, Puya, Compositae, Escallonia and Ericaceae largely replaced Alnus carr and the regional and local vegetation closely resembles the presentday composition. During the Last Glacial to Holocene transition a shift of the upper forest line of ca. 1400 m is inferred, corresponding to a glacial temperature depression at 2300 m altitude of ca. 8°C. journal of QuaterGry Science

A cooling event during the Younger Dryas Chron in Costa Rica

“…The radiocarbon dates ace close to the earlier estimated ages of 11,080-10,500 yr B.P. for this interval, based on interpolation and pollen concentration rates (Hooghiemstra et al, 1992). The downslope shift of the upper forest line indicates an estimated temperature drop of 2-3°C during the La Chonta stadial.…”

“…Only a few palaeoecological studies are available, namely Martin (1964), Hooghiemstra et al (1992) and Horn (1993). A cold Late Glacial climatic oscillation like the one to be outlined here, was earlier reported by Hooghiemstra et al (1992), but poor pollen recovery and lack of direct time control made the chronostratigraphical position and amplitude of this climatic oscillation tentative.…”

“…Only a few palaeoecological studies are available, namely Martin (1964), Hooghiemstra et al (1992) and Horn (1993). A cold Late Glacial climatic oscillation like the one to be outlined here, was earlier reported by Hooghiemstra et al (1992), but poor pollen recovery and lack of direct time control made the chronostratigraphical position and amplitude of this climatic oscillation tentative. More extra-regional evidence of a cooling event of Younger Dryas Chron age comes from Colombia, where the E1 Abra stadial is clearly evidenced in pollen records of E1 Abra-II (Schreve-Brinkman, 1978) and Ftiquene-II (Van Geel and Van der Hammen, 1973).…”

“…The oval-shaped La Chonta bog (330 x 110 m, Hooghiemstra et al, 1992) is located on the boundary between lower and upper montane oak forest in the Costa Rican Cordillera de Talamanca (9°41'N, 83°57'W, 2310 m alt., Fig. 1).…”

“…The objectives of this paper are to document in detail the AMS radiocarbon-dated environmental change in core La Chonta-1, to infer the sequence of climatic change (in terms of temperature and precipitation) and to correlate this sequence with the pollen record La Chonta-2 of the same bog. Hooghiemstra et al (1992) reported insufficient pollen for pollen zone 5 of the La Chonta-2 core, interpreted to represent a cooling event at the Late Glacial to Holocene transition. Based on the available radiocarbon ages and age interpolation based on pollen concentration rates the age of the cooling event was between 11,080 and 10,500 yr B.P.…”