Oceanographic controls on sedimentary and geochemical facies on the Peru outer shelf and upper slope

Concentrations and characteristics of organic matter in surface sediments deposited under an intense oxygen-minimum zone (OMZ) on the Peru margin were mapped and studied in samples from deck-deployed box cores and push cores acquired by submersible on two east-west transects spanning depths of 75 to 1,000 meters (m) at 12°S and 13.5°S. On the basis of sampling and analyses of the top 1–2 centimeters (cm) of available cores, three main belts of sediments were identified on each transect with increasing depth: (1) muds rich in organic carbon (OC); (2) authigenic phosphatic mineral crusts and pavements; and (3) glaucony facies.

Sediments rich in OC on the 12°S transect were mainly located on the outer shelf and upper slope (150–350 m), but they occurred in much shallower water (approximately 100 m) on the 13.5°S transect. The organic matter is almost entirely marine as confirmed by Rock-Eval pyrolysis and isotopic composition of OC. Concentrations of OC are highest (up to 18 percent) in sediments within the OMZ where dissolved oxygen (DO) concentrations are