Clay dispersal and the geochemistry of manganese in the Northern Gulf of California

The regional distribution of Fe and Mn in bulk sediments, and in different geochemical fractions, has been determined in surficial sediments from the Northern Gulf of California. The regional distribution of Fe (0.35-2.83%) and Mn (97-922 mug g(-1)) in the sediments is mainly controlled by the distribution of fine-grained particles. Unlike silt (4-62.5 mum), clay (< 4 μm) in the NGC is mainly derived from the eroding Colorado River Delta and transported southward adjacent to the Baja California coast, making up to 50% of the sediments. Normalisation of Fe and Mn against the relative abundance of clay indicates that the main source of these metals toward the Central Gulf of California is the re-suspension and re-deposition of the estuarine sediments derived from the Colorado River Delta. Iron and Mn correlate statistically; their concentrations are similar to other coastal sediments in Baja California and in seston from the Colorado River Delta. While the partition geochemistry of Fe suggests a dominant association of this element with the lithogenic phases throughout the NGC, Mn is significantly partitioned in the exchangeable (average 15%) and carbonate (average 32%) sediment phases, especially in clayey sediments derived from the estuarine region. The influence of Colorado River Delta-derived exchangeable Mn extends at least as far as the central Tiburon Basin, being probably responsible for the anomalously high Mn concentrations found in near-bottom waters in the Central Gulf of California. (C) 2002 Elsevier Science Ltd. All rights reserved.