Data Chem Paly 4kyrs La Paz Basin_100-48 samples
Description
Geochemical and palynological analyses of laminated sediments of samples corresponding to a core extracted at 745 m depth from La Paz Basin, allow recognition of centennial variations on rainfall and marine productivity from ~4487 to ~260 years BP. Concentrations of the paleoclimate proxies Ti and Fe (terrigenous input) and Mo (anoxia) were obtained by X-ray fluorescence analyses and total organic carbon concentrations (TOC, primary productivity) with a CO2 coulometer. Concentrations of marine (dinoflagellates) and continental palynomorphs (pollen and spores) indicate changes in marine productivity (PP) and rainfall (RF), with chemical results having significant statistical correlations and good graphic similarity with palynological data obtained from some of the same samples. Concentrations of TOC have a high statistical correlation with heterotrophic dinoflagellates and copepods, but only medium with continental palynomorphs, indicating the predominance of marine PP with an increasing trend through time. Values of proxies for PP differentiate four intervals: generally going from low to high, and reaching their highest values from 900-290 BP. Concentrations of Ti and Fe are statistically and graphically correlated with spores and autotrophic dinoflagellates, indicating their reliability as proxies for RF. Values of RF parameters are relatively stable, defining four intervals with differing values, mostly varying from low to high and reaching their highest values from 1050 to 600 BP. These proxies allow identification of regional conditions as an extreme drought period (1280-1070 BP) reported in southwestern North America, or global events like the Medieval Warm Period (1078-706 BP) and the Little Ice Age (706-260 BP).
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We processed and analyzed 100 samples from the upper 98.5 cm of the kasten core BAP94-9K, from the La Paz Basin (24°23.56’N and 110°04.16’W), at 745 m depth. A radiography was used to determine the thickness of the sampled intervals, which varied from 2 to 16 mm, with a mean of 10 mm. The sampled interval represents dates ranging from 261 to 4375 BP. The chronology is based on three radiocarbon (14C) dates made at Boulder University, Colorado (INSTAAR –AMS Radiocarbon Research Laboratory). The interval analyzed in this study contains dates from the intervals 7.88 cm (1070 ± 45 years), 73 cm (3850 ± 35 years) and 116 cm (5150 ± 50 years). Chemical methods: Concentrations of Ti, Fe, Mo and Al were determined by analyzing 1.5 g of finely ground dry sample by X-ray fluorescence (XRF), using a handheld XRF instrument model Olympus NDT Delta Premium in GEOCHEM mode. TOC = TC - TIC. The TC and TIC concentrations were measured with a CO2 Coulometer UIC, Inc. model CM5014 (Engleman et al., 1985) using 20 mg of dried sample and combustion (CM5120) and acidification (CM5230) modules. Palynological Method: 48 samples were processed following the conventional treatment (Wood et al., 1996) with cold HCl and HF, sodium polytungstate (Sp. Grav. = 2.0) and without oxidation. We added one Lycopodium spore tablet (Batch # 483216 with 18,583 spores per tablet and S= ± 3820) to each sample, as exotic markers. Mounts of each sample were prepared with the palynological residues sieved between 125 and 15 μm. Quantitative analyses were performed under a transmitted light microscope with magnifications of 450X and 1000X. The total number of palynomorphs per species/group at each level was recorded, as well as the total number of Lycopodium spores counted.