BIOGEOCHEMISTRY AND ORIGIN OF A PHOSPHORITIZED COPROLITE FROM ANOXIC SEDIMENT OF THE SANTA-BARBARA BASIN

We recovered a single, calcite-coated phosphorite nodule, 8-10 cm in d iameter, buried 70-80 cm deep in a box core of anoxic, organic-rich, v arved sediment from the central Santa Barbara Basin (SBB) off southern California. Varve chronology enables us to assign an age of 240 +/- 5 0 yr to the nodule. On the basis of its preserved pattern of sterol bi omarkers, the exclusively small size (< 1 cm long) of fractured fish b ones embedded within it, and other evidence, we identify this nodule a s a coprolite of possible human origin. Historical sources confirm tha t native Americans with predominantly marine diets canoed routinely ac ross the SBB from surrounding villages during the 18th Century. If thi s geologically modern coprolite truly originated as human feces, then it represents the first such fossil ever identified in the marine reco rd. The nodule also provides unique and valuable insight about the ini tial stages of phosphoritization in an organic-rich microenvironment t hat acted as a sink for phosphorus. Mineralogically, the coprolite con sists mainly of a very poorly crystallized variety of apatite with a c arbonate-rich and fluorine-deficient composition similar to that of da hllite, rather than typical marine sedimentary francolite. This substa nce probably would transform eventually to well-crystallized francolit e. We contend that lenticular inclusions of nearly pure francolite in certain ancient sedimentary deposits, such as the Monterey Formation o f California, might have originated as coprolites.