Natural Variability and Climatic Change in the Delta of the Colorado River region
Juan Contreras, Arturo Martín-Barajas, and Juan Carlos Herguera Centro de Investigación Científica y de Educación Superior de Ensenada
Ana Luisa Carreño Universidad Nacional Autónoma de México
The Salton Trough region of southern California and the Mexicali valley in north Mexico conforms the Delta of the Colorado. The delta of the Colorado provides with water to a population of more than 2.5 million inhabitants. The delta is so fertile that its agricultural output is the largest in Mexico; it also hosts the largest settlement of industrial complexes in the country. The combined production of those two economical sectors accounts for 1.5% of the gross domestic product of Mexico. Understanding of past climatic changes in the region is of vital importance because it can give us an idea of how the hydrological balance, specially the dynamics of the Colorado River, will be affected by global warming.
Figure 1 – Panoramic view of the drilling operations on the barren lakebed of Laguna Salada. The lake disappeared after the building and filling of the Hoover Dam in the 30’s.
Drilling in this basin also opens a window into the tectonic processes acting in the northern Gulf of California. Recovering sediment samples from the Laguna Salada basin will also help to characterize the mechanical response of the soil to shaking during earthquakes.
The delta of the Colorado River is an area of rapid subsidence due to extension along the San Andreas-Imperial fault system and high flux of sediments transported by the Colorado River. These areas, therefore, have a high preservation potential to store information of past climate events. In January 2004 DOSECC recovered 92 m of lacustrine sediments from two shallow boreholes drilled in Laguna Salada, and active sedimentary basin in northern Baja California, México. Laguna Salada occupies a semiclosed depression between the Sierra the Cucapá on the west and the Sierra de Juárez on the east. Our goal for drilling in this basin was threefold: (i) to document past climatic changes during the last glacial age and its transition to the present warmer climate, (ii) to document climate changes during the last two glacial cycles, and (iii) to document the vertical slip component of the Laguna Salada fault, which bounds the eastern margin of the basin.
Figure 2 – Drillers emplacing downhole logging geophysical instruments. This picture, as well as brief note about the drilling operations, made into the Spanish edition of National Geographic Magazine.
Boreholes were drilled at the toe of the alluvial fans adjacent to the Sierra de Juárez and on dry lakebed close to the Sierra de Cucapá and the Laguna Salada fault. Recovery was in excess of 95%. Equipment used during the drilling operation included DOSECC Lake System (DLS) suite of coring tools and a modified CS-500 rig. The main tool employed for coring was a hydraulic piston core. The quality of the cores is excellent, being found millimeter-scale primary sedimentary structures preserved in the sequence.
Ten C14 dates in charred organic matter and plant remnants indicate the core spans ~50 Ka and sedimentation rates are in the order of ~0.7 mm/yr. We can resolve in the core, therefore, climatic variability at timescales ranging from Milankovitch forcing up to millennial to centennial periodicities imprinted at scales ranging from centimeters to meters. We have found, for example, that 0.7-2 mm-thick lamina group in bundles of 6, 25 and 50 cm. Intercalations of massive clay, gypsum and sand tend to form cycles of 50 cm and 100-120 cm. We also have identified evidences of abrupt climate changes.
At Milankovitch time scales we have identified cycles based on sedimentary facies in the core, color, granulometry, mineralogical composition and primary structures such as laminae, dissecation cracks, and bioturbation. Additionally, we obtained reflectivity of sediments every 5mm to 1 cm depending on the scale of the primary structures. The recovered stratigraphy consists of three sedimentary successions. The base of the core is characterized by laminae of silt and mud 5mm-1cm thick deposited during glacial stage 2 through the last glacial maximum. This ancient paleolake was characterized by moist conditions in which a water table prevailed year-round. Good preservation of laminae suggests that bottom anoxia was frequent phenomenon.
Figure 3 – stratigraphic column of the LS04-1 core. It contains a description of the sedimentary facies recovered by the cores. Additionally, it shows the major climatic changes experienced in the region during the last 60 kyr.
During the last glacial maximum, moisture conditions changed drastically. Laminations are replaced by finely stratified sand and further upsection by repetitive packages 50cm-thick composed of coarse sand, brown mud, greenish silt and mud, caped by 5-10 cm of evaporites. These sediments were deposited in a continental sabka environment with intermittent freshwater input, as evidenced by the clear dissecation cycles.
The Holocene climate in the region also experienced major shifts. For instance, the Holocene thermal maximum is characterized by deposition of well-classified sand deposits with granulometry similar to that of modern dune fields. This is indicative of hyper-arid conditions. The second half of the Holocene, on the other hand, experienced a return to relatively more moist conditions. However, the hydraulic balance probably is close to null given the presence of evaporitic deposits intercalated with laminated mud and fine sand.
Laguna Salada project, located in Mexico, was a 2004 project that was CICESE-funded.