Biogeochemistry and Origin of a Phosphoritized Coprolite from Anoxic Sediment of the Santa Barbara Basin

Abstract

ABSTRACT We recovered a single, calcite-coated phosphorite nodule, 8-10 cm in diameter, buried 70-80 cm deep in a box core of anoxic, organic-rich, varved sediment from the central Santa Barbara Basin (SBB) off southern California. Varve chronology enables us to assign an age of 240 ± 50 yr to the nodule. On the basis of its preserved pattern of sterol biomarkers, the exclusively small size (< 1 cm long) of fractured fish bones embedded within it, and other evidence, we identify this nodule as a coprolite of possible human origin. Historical sources confirm that native Americans with predominantly marine diets canoed routinely across the SBB from surrounding villages during the 18th Century. If this geologically modern coprolite truly originated as human feces, then it represents the first such fossil ever identified in the marine record. The nodule also provides unique and valuable insight about the initial stages of phosphoritization in an organic-rich microenvironment that acted as a sink for phosphorus. Mineralogically, the coprolite consists mainly of a very poorly crystallized variety of apatite with a carbonate-rich and fluorine-deficient composition similar to that of dahllite, rather than typical marine sedimentary francolite. This substance probably would transform eventually to well-crystallized francolite. We contend that lenticular inclusions of nearly pure francolite in certain ancient sedimentary deposits, such as the Monterey Formation of California, might have originated as coprolites.