Back to: Workshop to Integrate Subduction Factory and Seismogenic Zone Studies in Central America

Sabine Wulf¹, Robert A. Dull ², Paul Mann¹, Kirk D. McIntosh¹, Justin Funk¹ and James E. Gardner³

¹ Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, J.J. Pickle Research Campus, Bldg. 196, 10100 Burnet Rd., Austin, Texas, 78758, U.S.A.
² Department of Geography and the Environment, The University of Texas at Austin, 1 University Station, GRG 334, Austin, Texas 78712, U.S.A.
³ Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 1 University Station, Austin, Texas 78712, U.S.A.

A shallow coring program in Lake Nicaragua, the largest lake in Central America, was completed in May/June 2006 by the University of Texas (UT Department of Geography and UT Institute for Geophysics). A total of 35 sediment cores with lengths ranging between 12 cm and 100 cm along with five longer cores ( 5 m) were extracted from the lake using a gravity corer and a modified manual square rod piston corer, respectively. Core locations were sited using a grid of high-resolution seismic reflection and 3.5 kHz data collected in the weeks prior to the coring study. Analyses of lake sediments are in progress and have the following objectives: 1) to correlate the geophysical results with the core data to provide a stratigraphic framework for the shallow lake sediments; 2) to constrain past climate variability in this rather poorly investigated area; and 3) to establish a time series of explosive volcanic activity based on the identification and dating of tephra layers in the cores.

Initial results on core description, magnetic susceptibility, dry density and loss on ignition indicated a dominance of fine-grained homogeneous diatomaceous sediments cover most of the lake floor. Increasing values in magnetic susceptibility in the upper part of several short cores most likely reflect increased erosion caused by land-use changes during the Spanish colonial period (1522-1822). First results on the two longest cores from the northeastern (435 cm) and southwestern (496 cm) parts of the lake reveal complete Holocene paleoclimate records in both areas. A lithologic change from homogeneous gyttia (diatomaceous mud) to blue-grayish waxy clay at the bottom of these records marks the Late Pleistocene-Holocene transition as indicated by a radiocarbon dating on plant remains. The latter dense clay forms a distinctive stratigraphic marker in the lake basin. Tephra layers to date were detected in most gravity cores recovered west of Ometepe Island (Volcán Concepcíon), east of the Island of Zapatera and in a long record nearby the Solentiname Archipelago in 315 cm sediment depth (unknown source, ca. 8,100 interpolated cal 14C years BP).

Planned paleoclimate studies on the longer cores will include detailed analyses of their palynological content and stable isotopes analyses of ostracods and diatoms. Tephra layers deriving from southern Nicaraguan and northern Costa Rican active volcanoes will be used for dating and correlating of lake sediments and will allow correlation to other terrestrial and Pacific marine tephra records. Distal ashes furthermore will provide information about the eruptive histories, volumes of erupted magma, and eruptive intensities of local explosive volcanoes, with the focus on the more recent activity. These data in combination with geochemical analyses on volcanic ash layers will serve to develop models for the petrologic evolution of magmatic systems and better hazard indications of potential eruptive behavior.

Poster Session