THE ORGANIC MEMORY OF SEDIMENTARY SYSTEMS AS AN INDICATOR OF THE PAST LIVES OF PARTICLES

Reconstruction of ancient sedimentary environments employs a diverse set of tools, including mineralogy, texture, fossil content, sedimentary structures, and geochemistry. Typically we presume that these parameters reflect processes that operated shortly prior to, during or soon after the sediments were deposited. We know, however, that up to 90% of particles in Recent sedimentary systems are derived from pre-existing sedimentary rocks. The impacts of recycling on mineralogical and textural maturity of siliciclastic sediments have long been recognized. Less well known, however, is the impact that recycling may have on sedimentary organic matter.

At present, small mountainous rivers carry more than 40% of the sediment discharged to the oceans, and similar rivers are likely responsible for the thick, ancient siliciclastic successions that have accumulated in tectonically active settings throughout the Phanerozoic Eon. Investigation of a suite of such rivers in the western U.S. and New Zealand reveals that up to 70% of the particulate organic carbon currently transported to the adjacent margins is derived from the ancient sedimentary rocks that underlie the watersheds. Rates of erosion in these watersheds appear to control the extent to which rock-derived carbon (kerogen) survives to reach the river channels. Similarly, marine sediment dispersal processes control the extent to which modern marine organic carbon replaces terrestrial organic matter or is added to the sedimentary particles prior to reburial. In these settings, the isotopic composition of organic carbon in shelf and slope sediments reflects not only the length of time that sedimentary particles are exposed to contemporary surficial processes, but their history in depositional systems that existed 10’s of millions of years ago and perhaps earlier. Study of kerogen recycling in modern sedimentary systems provides a note of caution for interpretations of the organic geochemistry of ancient successions, but also offers the exciting possibility of more thoroughly decoding the complex histories of sedimentary particles.