The agroecological evolution of Cobweb Swamp, Belize

Abstract

The soils and sediments of Cobweb Swamp were investigated to adumbrate landscape evolution and the impact of the Maya on a palustrine wetland. Cobweb Swamp is adjacent to the archaeological site of Colha in northern Belize. Pedostratigraphic analysis of pits, trenches, and cores was used to define and delineate major stratigraphic units, which were pedologically and geochemically assayed. Paleobiotic remains (pollen, molluscs, ostracods, foraminifera) were compared with pedological inferences. Radiocarbon dates provided a chronological framework. Cores from the interior of Cobweb Swamp consisted of a 5-m sequence of black clay (the Basal Clay) overlain by alternating peat and marl layers. Analysis of these layers revealed a dynamic landscape from about 8000 ybp (years before present) to the present. The Basal Q ay formed as a cumulic wetland soil. The Lower Marl precipitated in a brackish lagoon after 6500 ybp, possibly the result of rising sea level. The Lower Peat formed in shallow waters after 5500 ybp. The Upper Marl precipitated in a freshwater lagoon between 3700 and 500 ybp, prior to formation of overlying peat. Evidence for deforestation and cultivation were found in the palynological record from 4500 ybp. Conformation of stratigraphy in mound-like features along the swamp margin suggested extensive human modification of buried soils, dating to A.D. 500 and possibly much earlier. The Maya may have ditched the paleo Cobweb Clay surface to enable crop production on the fertile soils of the swamp margin. Once drainage was effected, they encountered few limitations other than salinization. On the swamp margin the Maya Clay, dated to A.D. 900 or later, overlay Cobweb Clay (undated) on both ditched and non-ditched areas. The wedge-shaped geometry of this unit along the swamp margin suggested an erosional origin from the adjacent upland soils. Much of the Maya Clay found on the ditched fields may have been the result of canal dredging by the Maya in an attempt to maintain the ditched field system. The date obtained for the Maya Clay is highly suggestive of environmental degradation accompanying the collapse of the Maya at Colha.