The balance between photosynthetic organic carbon production and respiration controls atmospheric composition and climate 1,2. The majority of organic carbon is respired back to carbon dioxide in the biosphere, but a small fraction escapes remineralization and is preserved over geologic timescales 3. By removing reduced carbon from Earth's surface, this sequestration process promotes atmospheric oxygen accumulation 2 and carbon dioxide removal 1. Two major mechanisms have been proposed to explain organic carbon preservation: selective preservation of biochemically unreactive compounds 4,5 and protection resulting from interactions with a mineral matrix 6,7. While both mechanisms can play a role across a range of environments and timescales, their global relative importance on 10 3-to 10 5-year timescales remains uncertain 4. Here we present a global dataset of the distributions of organic carbon activation energy and corresponding radiocarbon ages in soils, sediments, and dissolved organic carbon; we find that activation energy distributions broaden over time in all mineral-containing samples. This result requires increasing bondstrength diversity, consistent with the formation of organo-mineral bonds 8 but inconsistent with selective preservation. Radiocarbon ages further reveal that high-energy, mineralbound organic carbon persists for millennia relative to low-energy, unbound organic carbon. Our results provide globally coherent evidence for the proposed 7 importance of mineral protection in promoting organic carbon preservation. We suggest that similar studies of bond-strength diversity in ancient sediments may elucidate how and why organic carbon preservation-and thus atmospheric composition and climate-has varied over geologic time. Two classes of mechanisms-selectivity and protection-have been proposed to explain why some organic carbon (OC) escapes remineralization in soils and sediments 4-7. Biochemical selectivity hypotheses state that intrinsically bioavailable compounds such as sugars and amino acids are rapidly respired, whereas "recalcitrant" (macro)molecules such as lignin are selectively preserved due to their low energy yield, large size, and/or a lack of enzymes that can decompose them 4,5. Selective preservation has been extensively documented in dissolved OC (DOC) 9 , decaying woody tissue 10 , and sapropel sediments containing almost exclusively organic matter 5. In contrast, protection hypotheses state that particles shield OC from respiration regardless of intrinsic recalcitrance, potentially due to occlusion within pore spaces that are inaccessible to microbes and their extracellular enzymes 4,8,11-14. Specifically, protection often involves inspiration was always invaluable. We thank the National Ocean Sciences Accelerator Mass Spectrometer staff, especially A

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This paper describes the application of a novel, practical approach for isolation of individual compounds from complex organic matrices for natural abundance radiocarbon measurement. This is achieved through the use of automated preparative capillary gas chromatography (PCGC) to separate and recover sufficient quantities of individual target compounds for 14 C analysis by accelerator mass spectrometry (AMS). We developed and tested this approach using a suite of samples (plant lipids, petroleums) whose ages spanned the 14 C time scale and which contained a variety of compound types (fatty acids, sterols, hydrocarbons). Comparison of individual compound and bulk radiocarbon signatures for the isotopically homogeneous samples studied revealed that ∆ 14 C values generally agreed well ((10%). Background contamination was assessed at each stage of the isolation procedure, and incomplete solvent removal prior to combustion was the only significant source of additional carbon. Isotope fractionation was addressed through compound-specific stable carbon isotopic analyses. Fractionation of isotopes during isolation of individual compounds was minimal (<5‰ for δ 13 C), provided the entire peak was collected during PCGC. Trapping of partially coeluting peaks did cause errors, and these results highlight the importance of conducting stable carbon isotopic measurements of each trapped compound in concert with AMS for reliable radiocarbon measurements. The addition of carbon accompanying derivatization of functionalized compounds (e.g., fatty acids and sterols) prior to chromatographic separation represents a further source of potential error. This contribution can be removed using a simple isotopic mass balance approach. Based on these preliminary results, the PCGC-based approach holds promise for accurately determining 14 C ages on compounds specific to a given source within complex, heterogeneous samples.

Wildfires and incomplete combustion of fossil fuel produce large amounts of black carbon. Black carbon production and transport are essential components of the carbon cycle. Constraining estimates of black carbon exported from land to ocean is critical, given ongoing changes in land use and climate, which affect fire occurrence and black carbon dynamics. Here, we present an inventory of the concentration and radiocarbon content (Δ 14C) of particulate black carbon for 18 rivers around the globe. We find that particulate black carbon accounts for about 15.8 ± 0.9% of river particulate organic carbon, and that fluxes of particulate black carbon co-vary with river-suspended sediment, indicating that particulate black carbon export is primarily controlled by erosion. River particulate black carbon is not exclusively from modern sources but is also aged in intermediate terrestrial carbon pools in several high-latitude rivers, with ages of up to 17,000 14C years. The flux-weighted 14C average age of particulate black carbon exported to oceans is 3,700 ± 400 14C years. We estimate that the annual global flux of particulate black carbon to the ocean is 0.017 to 0.037 Pg, accounting for 4 to 32% of the annually produced black carbon. When buried in marine sediments, particulate black carbon is sequestered to form a long-term sink for CO2.