Accumulation of radiocarbon in ancient terrestrial landscapes: A small but significant input of unknown origin

Abstract

The persistence of soil organic carbon (C) in soil, defined as the mean residence time of organic C compounds in soils, is a critical measure for understanding the capacity of terrestrial ecosystems to regulate biogeochemical cycles. The persistence of organic carbon in soil is most often considered at timescales ranging from tens to thousands of years, but the study of organic C in paleosols (i.e., ancient, buried soils) suggests that buried soils may have the capacity to preserve organic compounds for tens of millions of years. A quantitative assessment of C sources and sinks from these ancient terrestrial landscapes is complicated by additions of modern organic C, primarily due to the infiltration of dissolved organic carbon. In this study, we quantify total organic C content and radiocarbon activity in samples collected from 28- to 33-million-year-old paleosols that are naturally exposed as unvegetated badland outcrops near eastern Oregon’s “Painted Hills”. The study site is part of a well-mapped ~400-meter-thick sequence of Eocene-Oligocene (45-28 Ma) volcaniclastic paleosols, and thus we expected to find “radiocarbon dead” samples preserved in deep layers of the lithified, brick-like exposed outcrops. Total organic C, measured in three individual profiles spanning depth transects from the outcrop surface to a 1-meter depth, range from 0.01 - 0.8 wt. % with no clear C-concentration or age-depth profile. Ten radiocarbon dates from the same profiles reveal calibrated radiocarbon ages of ~11,000 – 30,000 years BP that unexpectedly indicate additions of recent and /or modern organic C. A two-endmember mixing model for radiocarbon activity suggests that modern C may compose ~0.5-3.5% of the total organic carbon pool preserved in these ancient landscapes. We discuss several mechanisms by which modern organic C could have infiltrated into the lithified, brick-like paleosol surfaces and discuss potential implications for future research of ancient soils.

Eocene (50-55 Ma) greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA

“…Alternatively, there could have been late diagenetic groundwater alteration with acidic fluids, but we find this hypothesis less likely due to the dearth of evidence representing early diagenetic intense weathering conditions. Diagenetic additions of recent/modern organic C can inflate the so-called "preserved" organic C (Broz et al, 2023), but enrichments of TOC in uppermost horizons of paleosols are consistent with preservation of endogenous organic C (Broz et al, 2022). Thus, it is possible that organic C is preserved in the La Jolla profile, though additions of small amounts of geologically recent/ modern carbon are possible and perhaps likely.…”