Source and distribution of palynomorphs in bottom sediments, southern part of Gulf of California

Abstract

Detailed analyses are given of bottom surface sediments from about 150 cores in the southern half of the Gulf of California and preliminary analyses are given of another 100 bottom sediment samples from throughout the Gulf. Additional background data from the study of several hundred stream and surface sediment samples from surrounding regions in Mexico are drawn upon to aid in the interpretation of the distribution of the pollen and spores found. Conditions and characteristics of environment, plant distribution, structure of the basin, geomorphology of the surrounding area, air and water circulation, sedimentation and biology of the Gulf and adjacent regions which affect the dispersal of palynomorphs in the Gulf are reviewed. Sources of samples and methods of sampling bottom sediments and terrestrial sediments are given and preparation and study procedures are reviewed. Quantitative as well as qualitative analyses are made of all samples. About 120 sample localities along 9 transects across the southern part of the Gulf have been plotted on map-graphs to show, diagrammatically, the depth of samples, bottom profiles and sand-silt-clay ratios along the transects. On the same cross-sections, the absolute or relative frequencies of total pollen and spores, pine pollen, pollen of Chenopodiaceae and Amaranthaceae (Cheno-Ams), grass pollen, cuticles, tracheids (> 50 μ), fungal spores and dinoflagellates are plotted for direct comparison with bathymetrical, sedimentological and geographical features. Three maps show, by isopleths, generalized distribution of total pollen and spores, pine pollen and Cheno-Ams pollen for the entire Gulf; three similar maps show detailed areal distribution in the southern half of the Gulf of oak pollen, fungal spores, dinoflagellates, microforaminifera (those with acid-resistant inner membranes, only) and an unidentified palynomorph of wide distribution. Additional information given on maps includes bathymetry, wind and sedimentation patterns and regional distribution of pine forests (generally the same pattern defines oak distribution). A table lists the species or groups of entities, their agencies of pollination and the general abundance of the pollen and spores in the bottom sediments of the Gulf. Results of the present studies show different types or groups of palynomorphs per gram of dried bottom sediment vary from a few thousand to 80.000 for the pollen-spore sum; a few hundred to over 20,000 pine; a few to over 4,000 oak; a few to over 12,000 Cheno-Ams; up to 8,000 grass, 2,000 to 25,000 cuticle fragments, up to 100,000 tracheids larger than 50 μ and up to 3,000,000 tracheids between 5 and 50 μ; up to 22,500 fungal spores; up to 17,000 dinoflagellates and up to 6,250 microforams. Others less regular in their dispersal but significant in frequencies are Rhizophora, Buxaceae, Bursera, Fouquieriaceae, Yucca (up to 3,000 grains/g), Cyperaceae (abundant), Compositae (very abundant), Zea (up to 3,000 grains/g), juniper and alder. Comparisons made of several palynomorphs show high concentrations in silty clays, at delta mouths, and in some submarine channels. Low concentrations occur along some shores probably due to dilution by terrigenous sediment and winnowing from coarse sediments but high concentrations build-up immediately beyond zones of coarse sediments offshore. Some palynomorphs diminish in number with greater distance from shore (e.g., total spores and pollen, grass, Cheno-Ams, composites, Zea) whereas others become selectively increased in relative frequencies offshore (e.g., pine, mangrove) or in absolute frequency (e.g., dinoflagellates, O sp-1, an unknown entity). Cuticles and tracheids delineate shorelines and deltas and a striking parallelism between absolute frequency of dinoflagellates and relative frequency of fungi deserves further study. Dispersal of pollen and spores by streams and water currents (and in areas of upwelling and through submarine canyons to offshore deeps as submarine fans) appears to be far more important than wind distribution for the Gulf of California area.