Barbara W. Leyden

Ecosystems, paleoecology and human disturbance in subtropical and tropical america

Abstract Human disturbances of ecosystems last a long time and have quantifiable influences on the structure and function of the systems. If long records (e.g. paleoecological) of both disturbances and the responses are available, the array of disturbances provides quasi-experimental treatments useful for the study of factors which govern ecosystems. This paper examines the paleoecology of a series of lake-drainage basin ecosystems that have been subject to disturbances which vary through time and space. In all cases studied, it has been demonstrated that human activities have increased the movement of materials from the catchment to the lake. Examples in Guatemala, Haiti and Florida demonstrate that the flow of macronutrients (nitrogen and phosphorus) is proportional to human population sizes, and that the flow of inorganic particulates is related to the nature of both the disturbances and the catchment. Lake eutrophication is driven by growing human populations, but the rate of increase can be slowed by activities such as urbanization, which increases siltation. Several tropical ecosystems have recovered from severe disturbances, but the rate of recovery was related to the severity and temporal extent of the disturbances.

POLLEN EVIDENCE FOR CLIMATIC VARIABILITY AND CULTURAL DISTURBANCE IN THE MAYA LOWLANDS

Palynology provides a record of past environmental change in the Maya Lowlands. The underlying principles are simple, but, as with all proxies, there are limitations. During the late glacial period, environmental change was governed by climate, which was cooler and much drier and supported sparse temperate vegetation. The early Holocene epoch was warmer and very wet in the southern Lowlands when mesic tropical forests predominated, while the northern Yucatan Peninsula was edaphically dry until eustatic sea level rose. The modern distributions of plant associations and climatic gradients were established at the end of the early Holocene. Climatic variability continued throughout the Holocene. However, the ability of palynology to identify climatic events is hindered after the Maya became numerous in the Lowlands. Then, multidisciplinary studies provide a better interpretation of events, especially during the late Holocene. Pollen records poorly reflect cyclical droughts seen in isotopic records, as natural vegetation has adapted to these fluctuations.

Orbital and internal forcing of climate on the Yucatan Peninsula for the past ca. 36 ka

Abstract A 19.6 m sedimentary sequence from Lake Quexil, Guatemala, is presented as a proxy record of climatic change on the low-lying Yucatan Peninsula over the last ca. 36 ka. Long-term climatic fluctuations are attributed to orbital forcing of insolation, while abrupt changes during the late glacial period are the result of internal forcing mechanisms that remain imperfectly modelled. GCMs simulating meltwater cooling of the Gulf of Mexico may predict some but not all climatic responses in the Circum-Caribbean. Lake Quexil did not experience greater aridity than predicted by orbital forcing during late glacial zones Q-LG 1 and 2, which were drier only in comparison with Holocene conditions. Q-LG 1 appears to have been colder than expected based on insolation alone, and is contemporary with the Younger Dryas event between meltwater pulses 1A and 1B. Thus, other feedbacks also may have cooled surface temperatures in the Gulf of Mexico, such as unique late glacial circulation patterns, increased albedo for the Florida and Yucatan Peninsulas, or surface temperatures on the North American continent. Additional terrestrial sequences are needed to improve the chronology and confirm the distribution of sub-regional climatic variability in the Circum-Caribbean.

Holocene climatic and human influences on lakes of the Yucatan Peninsula, Mexico: an interdisciplinary, palaeolimnological approach

We used palaeolimnological techniques to examine effects of Holocene climate change and human influence on lakes of the Yucatan Peninsula, Mexico. The three study lakes are located along a west-east transect that represents a gradient of increasing modern precipitation and density of former Maya settlement. At Lake Coba, an 880-cm sediment core yields a complete record of lacustrine sedimentation that began when the lake first filled ∼8000 BP as groundwater level rose in response to rising sea level and increased precipi tation. Diatom, ostracod, and δ 18O evidence indicate that Lake Coba was initially shallow and saline. Coba, presently in the region of greatest rainfall, showed more episodic water-level changes than Lake San Jose Chulchaca or Lake Sayaucil. High lake level and fresh water were evident at 440 cm (∼2600 BP), followed by a decline in water levels and an increase in total ionic salinity to the present time. In a 613-cm core from Sayaucil, in the intermediate precipitation zone, total salinity was high between 600 and 400 cm (∼3050 and 2000 BP), followed by consistently higher water levels. Salinity was high in the lower portion of a 110-cm San Jose Chulchaca core (beginning ∼1860 BP), followed by a gradual and consistent freshening of water to the present time. Trophic state changes and human influence on lakes were evaluated using diatom, δ13C, total P, sedimentary organic matter, and preliminary pollen data. Maximal human disturbance at Lake Coba, a densely settled Maya urban site, occurred during a deep-water event at 440 cm, followed by a decline in human influence and trophic state to the present time. Trophic state and linear sedimentation rates in Sayaucil increased significantly above 400 cm (after ∼2000 BP), probably associated with initial Maya settlement near Xtojil and subsequent small- scale farming. Limnological disturbance may have preceded the period of maximal human occupation because initial land clearance and consequent soil erosion probably affected water quality substantially. San Jose Chul chaca lacks archaeological evidence of human occupation in the drainage, and shows gradual changes in trophic state not caused by human disturbance. With the exception of a 14C date on wood from the base of the Coba core, 14C dates and chronologies may be artificially old as a consequence of hard-water-lake error. Trophic state changes in the study lakes were generally consistent with known patterns of human settlement and popu lation change. Late-Holocene water-level fluctuations were most pronounced in Lake Coba in eastern Yucatan, where modern rainfall is currently greatest, but lake level is generally lower than during much of the past. Lake level was relatively constant in Sayaucil in the central peninsula, whereas lake level in San Jose Chulchaca in the arid western portion of the peninsula increased gradually over time.

Climate change in lowland Central America during the late deglacial and early Holocene

The transition from arid glacial to moist early Holocene conditions represented a pro- found change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition ( � 11 250 to 7500 cal. yr BP) inferred from sediment cores retrieved in Lake Peten Itza ´, northern Guatemala. Pollen assemblages demon- strate that a mesic forest had been largely established by � 11 250 cal. yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage. From 11 250 to 10 350 cal. yr BP, during the Preboreal period, lithologic changes in sediments from deep-water cores (>50 m below modern water level) indicate several wet-dry cycles that suggest distinct changes in effective moisture. Four dry events (designated PBE1-4) occurred centred at 11 200, 10 900, 10 700 and 10 400 cal. yr BP and correlate with similar variability observed in the Cariaco Basin titanium record and glacial meltwater pulses into the Gulf of Mexico. After 10 350 cal. yr BP, multiple sediment proxies suggest a shift to a more persistently moist early Holocene climate. Comparison of results from Lake Peten Itzawith other records from the circum-Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period appears to be tightly linked to climate change in the high- latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores-Bermuda high-pressure system. This mechanism operated on millennial-to-submillennial timescales and may have responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and the Atlantic meridional overturning circulation (MOC). Copyright 2005 John Wiley & Sons, Ltd.

Recent sedimentary histories of shallow lakes in the guatemalan savannas

Shallow basins in the savannas of Peten, Guatemala filled with water after 305±55 BP (calibrated age+1430–1660 AD). Aguadas Chimaj and Chilonche possess dilute waters and iron-rich, clayey sediments that are poor in Ca and Mg, reflecting the highly weathered nature of riparian soils. Low 210Pb flux rates to Chimaj (0.085 pCi cm-2 yr-1) and Chilonche (0.134 pCi cm-2 yr-1) are attributed to low 222Rn emission rates from the nearby Caribbean Sea. Mean sediment accumulation rates in Chimaj and Chilonche for the past 150 years are 0.015 g cm-2 yr-1 and 0.047 g cm-2 yr-1 respectively. Forest expansion after 305 BP is documented in pollen profiles from the small aguadas and larger Lake Oquevix. Regional reforestation postdates the 9th century Classic Maya collapse and coincides with indigenous depopulation that was a consequence of European intrusion that began in the early 1500s. The timing of forest regrowth indicates the importance of historical anthropogenic factors in controlling Petens vegetation. Nevertheless, other sedimentological lines of evidence (e.g. lithology, algal remains and charcoal particles) suggest that changing climate and/or local hydrology may have played a role in the reforestation process.

Evidence of the younger dryas in Central America

Abstract Three sites provide records that address whether the Younger Dryas was manifest in Central America. Marine records from Barbados and the Cariaco Basin are discussed, as well as possible climatic forcing mechanisms. Sequences from montane La Chonta Bog, Costa Rica, and lowland Lake Quexil, Guatemala, record a temperature decline of 1.5–2.5°C dated between 11,070–10,400 14 C years BP and ca. 12,000–10,300 14 C years BP, respectively. The record from Lake La Yeguada, Panama, does not include cooling, although significant reorganization of local communities occurred during the Younger Dryas chronozone. Colder tropical SSTs throughout deglaciation contributed to sub-regional climatic variation, more intense monsoonal activity, and cooler air temperatures. However, the temperature reversals are not associated with the coldest tropical SSTs which occurred during meltwater pulses. The temperature reversals appear to be a response to extra-tropical conditions, presumably in the North Atlantic, which suggests that Younger Dryas cooling extended as far south as Costa Rica. In Panama, monsoonal conditions may have blocked cooler temperatures. Precipitation continued to increase throughout deglaciation due to strong forcing by seasonal solar insolation in Central America while desiccation events occurred to the north and south.