Changes in organic matter–mineral interactions for marine sediments with varying oxygen exposure times

Abstract

Density fractionation, X-ray photoelectron spectroscopy (XPS) and amino acid analyses were used to evaluate the physical form of preserved organic carbon (OC) in sediments from transects in the north east Pacific Ocean off the Mexican (Mazatlan) and Washington coasts. Low density (i.e. mineral-free) organic material dominated the OC in sediments with very short oxygen exposure times (OET <1 yr; total wt% OC 7.9–9.3, ∼70% in ρ < 1.9 fractions). Quantities of low density OC decreased with increasing OET, suggesting that organic-rich debris was oxygen-sensitive. As OET increased from years to decades, OC was found predominately in the form of organic–mineral aggregates (58–77% of total OC was in the 1.9–2.2 ρ mesodensity fraction). Aggregated OC then decreased in abundance as a function of OET, suggesting that long-term oxygen exposure leads to destruction of organic–mineral aggregates. At OET values of centuries to millennia, the dominant forms of organic matter were OC sorbed to mineral surfaces (∼50% of total) and OC locked within biogenic diatom frustules (∼22% of total). These two physical forms of OC in marine sediments may provide the best long-term protection for OC.