Late Quaternary climate-induced lake level variations in Lake Petén Itzá, Guatemala, inferred from seismic stratigraphic analysis

Abstract

We used seismic images and sedimentary data from piston cores to conduct a sequence stratigraphic analysis of sediments in Lake Petén Itzá, northern Guatemala. Our results document lake level fluctuations in this lowland Neotropical region that were related to glacial-to-interglacial climate change during the Late Pleistocene. A bathymetric survey of Lake Petén Itzá (area = 100 km²) revealed a maximum water depth of ∼160 m and the existence of a deep cryptodepression that extends 50 m below modern sea level. The great depth suggests that the basin held water even during arid conditions associated with full glacial periods. Lake Petén Itzá may thus possess the only long continuous lacustrine sediment record of Late Pleistocene environmental and climate change in the lowland Neotropics. Two seismic reflection campaigns imaged the subsurface basin sediments that overlie basement. The sediment package was divided into four major seismic sequences (T, G, R, and B). Sequences are separated by unconformities that represent depositional cycles related to lake level fluctuations. Sediments of the uppermost sequence (T) were recovered and radiocarbon-dated in Kullenberg piston cores taken along a water depth transect. Seismic profiles reveal a basin-wide paleoshoreline just below sequence T at ∼56 m below present lake level. This constructional feature formed during a lowstand of the last glacial period when the lake was reduced to only ∼13% of its present volume. In cores taken landward of the paleoshoreline, Late Glacial-age deposits consist of paleosols, indicating subaerial exposure. Basinward of the shoreline, sediments are composed of dense gypsum sands and interbedded silty clays, reflecting authigenic gypsum formation under arid climate conditions. The top of the soil horizon and cessation of gypsum precipitation are represented by a strong seismic reflection (t). It marks the base of the uppermost seismic sequence T and is dated in several cores between ∼11.1 and 10.2 cal kyr BP. Lake level rose quickly at this time in response to a shift from arid-to-humid climate conditions at the Late Glacial/Early Holocene transition. We infer a similar sediment response to climate variations in the older stratigraphic sequences (G, R, and B), related to earlier glacial-to-interglacial and stadial-to-interstadial cycles. Older sequences are also distinguished from one another by erosional unconformities that probably represent major lake level falls. Future recovery of the older stratigraphic record by drilling in Lake Petén Itzá will provide ages for these older units and enable us to test the depositional model inferred from seismic stratigraphy.

Introduction

Lake sediments are valuable archives of past environmental change on the continents. They can provide a continuous, sensitive record of changing conditions and processes that occur within the lake itself and in the surrounding catchment. Geophysical reflection seismic surveys provide quantitative data on sediment geometry, type, and stratigraphy, as well as depositional processes (Scholz, 2001, and references therein). In addition, seismic profiles are invaluable for choosing optimum locations for lake coring or drilling, especially in lakes with large lateral differences in sedimentation pattern. In closed basins that lack surface outflows, seismic stratigraphic analysis can be used to reconstruct past lake level fluctuations because such fluctuations control the lateral extent of seismic sequences, onlap relationships, and the occurrence of (subaerial) erosional surfaces and paleoshorelines (Scholz and Rosendahl, 1988, De Batist et al., 1996, Seltzer et al., 1998, Ariztegui et al., 2000, Gilli et al., 2001, Gilli et al., 2005). Water level fluctuations in closed basins are mostly a function of the balance between precipitation and evaporation and thus leave a record of past climate changes. In addition to being regulated by the precipitation / evaporation ratio, water level fluctuations can be controlled by changes in subsurface inflows and outflows, particularly in karst areas. Wet/dry climatic alternations may also be recorded in the sediments of closed-basin lakes by varying amounts of evaporitic salts that are deposited as the lake volume expands and contracts (Piovano et al., 2002, Fedotov et al., 2004).

The objectives of seismic reflection studies dictate the choice of the seismic source to be used. High-frequency sources provide high-resolution data, but limit the depth of penetration. In contrast, lower-frequency sources penetrate to greater depth, but spatial resolution is compromised. Here we present results from a lacustrine seismic stratigraphic survey that used two seismic systems coupled with analyses of sedimentary piston cores that were collected along depth transects sited on seismic lines. The dual sources provided both high-resolution images of shallow subsurface deposits, and sufficient information on deep sediment layers, for sequence stratigraphic analysis. Integration of core analyses and seismic information permitted chronostratigraphic dating and correlation of lithologic changes with specific seismic reflections, and enabled us to develop a conceptual model for interpreting lacustrine depositional processes.

We applied a seismic stratigraphic approach to the sediments of Lake Petén Itzá, a large water body in the lowland Neotropics of northern Guatemala (Fig. 1), on the southernmost part of the Yucatan Peninsula. The lake has a surface area of 100 km² and lies at ∼16°55′N, 89°50′W in the Department of Petén within the Petén Basin, an intra-cratonic basin located on the Maya block of the North American Plate (Fig. 2). The underlying stratigraphic section includes an up to 5-km thick sequence of Cretaceous to Tertiary deposits consisting of marine carbonates underlain by Jurassic continental sediments. A 1970 map of the geology of Guatemala [Mapa geológico de Guatemala, 1970, Instituto Geográfico Nacional (Guatemala)] indicates that Paleocene–Eocene marine carbonates are exposed at the surface surrounding the northern basin of Lake Petén Itzá, whereas Cretaceous carbonates characterize surface exposures to the south (Fig. 2). The lake basin occupies a large karst depression that follows a series of east–west aligned faults (Vinson, 1962). The northern shore of the lake is bounded by an inactive normal fault and marked by a steep karst ridge that follows the strike of the fault system, whereas the south shore is shelving, and in places rimmed by poorly drained seasonal swamps (bajos). Presently, Lake Petén Itzá's water is dilute (11.22 meq l^− 1) and dominated by calcium and bicarbonate, with magnesium and sulfate following closely in concentration. Lake water pH is high (∼8.0) and is saturated for calcium carbonate. There are abundant shells of carbonate microfossils (ostracods and gastropods) in the lake sediments (Covich, 1976, Curtis et al., 1998).

We produced the first bathymetric map for Lake Petén Itzá that shows a maximum water depth of ∼160 m. The bathymetry is complex, reflecting the karst nature of the basement (Fig. 2). The modern lake surface is ∼110 masl, which means the deepest part of the Petén Itzá basin is a cryptodepression that is ∼50 m below present sea level. Today the lake is divided into a large main basin in the north (∼30 km long and ∼3–4 km wide) and a much smaller and shallower southern basin (14 km long and up to 1.5 km wide). A shallow sill connects the two. The north basin consists of several subbasins that may have been isolated during lowest lake level stands. Despite the lake's large size, water level has fluctuated appreciably (5–6 m) in the past few decades, flooding lakeside homes and businesses. Although Lake Petén Itzá is affected by subsurface leakage through the karstic bedrock, we believe that the climate-controlled precipitation / evaporation ratio has been more variable through time and thus exerts an important influence on lake level changes. Most recent lake level fluctuations can be attributed to changes in the precipitation pattern of the 20th century (Deevey et al., 1980). Furthermore, variations in surface inflow as well as variations in subsurface in- and outflow are also strongly affected by climatically influenced surface water recharge. Consequently, climate acted as a major control on past lake level fluctuations.

Along with marine-based paleoclimate records from the Caribbean, terrestrial archives of past climate variations in Central America are of great interest for understanding the role of the tropics in the global climate system. Previous evidence from lake sediment records in the lowland Neotropics demonstrated that climate was drier during the last ice age than during the Holocene, and that most shallow lake basins in the region were dry in the Late Pleistocene (Deevey et al., 1980, Brenner et al., 2002). The termination of the Late Glacial and the transition into the Holocene has been studied in cores from Lake Miragoane, Haiti, and Lake Valencia, Venezuela, located on the northern and southern rim of the Caribbean, respectively. Both lakes were sufficiently deep to have held water and preserved sediment records during those time periods (Bradbury et al., 1981, Hodell et al., 1991, Curtis and Hodell, 1993, Brenner et al., 1994, Curtis et al., 1999). Older glacial-age lacustrine sediments from the region are rare and have been found in only a few deep basins in the Petén Lake District (e.g., Lake Quexil) of northern Guatemala (Deevey et al., 1983, Leyden et al., 1993, Leyden et al., 1994). Lake Petén Itzá's great water depth made it resistant to desiccation. Thus, sediments in the deep north basin probably contain a long, continuous archive of latest Pleistocene paleoenvironmental change.

Based on seismic and core data, we present a conceptual model that relates changing sedimentation processes and sediment characteristics to presumed climate-induced lake level fluctuations in Lake Petén Itzá. The piston corer only retrieved the upper 6 m of Late Glacial and Holocene sediment, but the cores were sufficiently long to interpret the uppermost seismic sequence (Hillesheim et al., 2005). Drilling will be required to recover deeper, older deposits. Because of its great water depth (∼160 m) and sediment thickness (> 100 m), Lake Petén Itzá has been targeted for deep drilling by the International Continental Drilling Program (ICDP). Long cores from deep water will provide the stratigraphic sequences needed to recover the long paleoenvironmental archives that will be used to test the conceptual depositional model developed using cores and seismic stratigraphy.