Sedimentary geochemistry of hydrothermal vents in Guaymas Basin, Gulf of California, Mexico

Surface sediments from 4 contrasting environmental settings in the Southern Trough of Guaymas Basin were analysed to determine their texture, total organic matter (TOM), P (TP), and N (TN) content as well as the spatial variation in their C:N:P atomic ratios in hydrothermally altered sediments. Particle size analysis revealed a marked spatial heterogeneity among sites. Cores taken at Beggiatoa bacterial mats and at clam fields contained significant silt and clay content, while those obtained near venting sites had sand and gravel as the main components because no fine sedimentary materials are deposited at these sites. Fine grain sediments were common farther away from vents. High levels of organic C (3.4 to 12.4%) were recorded in the upper 8 cm but no clear trends of decreasing TOC with depth were noted. The presence of hydrothermally generated petroleum in some cores increased the organic C content between 8 and 12 cm. TN was generally high (456.3–4356 μg/g) and sustained a significant correlation with TOM (r=0.834, P<0.005). TP is essentially of lithogenic origin and reached concentrations greater than those normally reported in productive coastal areas (1165 μg/g) in the Gulf of California. A marked increase with depth was noted for this parameter. High C:N:P atomic ratios were estimated. These had vertical and horizontal gradients decreasing in the upper 4 cm, and then slightly increasing in intermediate strata. Organic matter enrichment at venting sites results in high C:N:P ratios (2130:25:1–2692:57:1) which are 5-fold greater than the values estimated at sedimentary environments where Beggiatoa or vesicomyd clams are predominant.