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Venezuela

Lacustrine Records of Late Quaternary Water Balance Fluctuations in the Venezuelan Andes: Assessing the Interplay of Insolation and Ocean Circulation as Paleoclimatic Controls in the Tropics

The objective of this project is to produce new multi-proxy high-resolution paleoclimate records from the Venezuelan Andes and to test hypotheses concerning the causes of shifts in the precipitation-evaporation (P-E) balance from ~18 ka B.P. to the present.  Previous work provides only a broad paleoclimatic picture for this period: prolonged and intense aridity from the Last Glacial Maximum to the glacial-interglacial transition, and a complicated history of water balance shifts throughout the Holocene.  The proposed research has three key goals:

    (1) To produce well-dated records of several climatically-sensitive proxies at century-scale resolution, in order to identify regional patterns of P-E balance from ~18 ka to present.

    (2) To identify higher frequency hydrological fluctuations over the past 2000 years, using more detailed analyses of the same proxies at higher resolutions (multidecadal or better).

    (3) To compare these results with findings from our ongoing research on montane lake records in the Southern Hemisphere (Bolivian Andes). 

We propose to reconstruct the climate history of the region by applying an integrated methodology that combines seismic survey, sedimentology and sediment magnetic characteristics, elemental and isotopic geochemistry, diatoms, and pollen.  The primary proxies we propose to use (stable isotopes, diatoms, pollen) will be calibrated in climatic terms through modern collections along altitudinal gradients.  By integrating open- and closed-basin lake systems, we will eliminate the possibility of biasing from site-specific (edaphic) processes.  The ability to date environmental changes reliably is absolutely critical, and to this end we will rely upon state-of-the-art AMS 14C measurements (at least one date per 1000 yrs) on carefully screened organic constituents (macrofossils and pollen).  Recent sediment accumulation rates will be constrained by 137Cs and 210Pb measurements.

These records are needed to better understand the significance of shifts in the P-E balance in paleoclimatic terms and, notably, to address the following three hypotheses:

    (1)  The P-E balance of the Venezuelan Andes is driven primarily by insolation changes at the millennial timescale during the Holocene, when boundary conditions were no longer significantly affected by changes in continental ice volume or sea level.

    (2) Millennial-scale changes in the seasonal distribution of insolation across the Tropics of the Northern and Southern Hemispheres drive changes in the location and strength of convection during the summer wet season.  These effects are greater in regions that have a single wet season at the northern and southern limits of the annual migration of the Intertropical Convergence Zone (i.e. the Venezuelan and Bolivian Andes).

    (3) The P-E balance of the Venezuelan Andes has undergone decadal- to centennial-scale fluctuations during the Holocene, with periodicities that are too short to be driven by seasonal insolation variations.  This research seeks to produce time-series for these climatic shifts that can be used to explore the mechanism responsible for such changes, such as oceanic variability. 

The proposed research significantly broadens the scope of previous work in the Venezuelan Andes by providing the most detailed and best-dated terrestrial paleoenvironmental records to date from this region.  Initial results from a companion study in the Andes of the southern hemisphere (Bolivia) suggest a high probability that the current proposal will succeed.  For example, measurements of 18O, 2 H, and ion concentrations from a modern transect of lake water samples show systematic trends between glacially-fed overflow lakes to closed basin systems.  We are presently calibrating isotopic and biological ( cf. diatoms) proxies so that down-core analyses may be interpreted in terms of moisture balance fluctuations.  By comparing the records from Venezuela to those we are developing in Bolivia, we anticipate the possibility of synthesizing spatially, for the first time, the broad patterns of tropical climatic change from the perspectives of both hemispheres.
 

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